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Spectrophotom etric O bservations ofB lue C om pact D warf

G alaxies: M rk 370

LuzM .Cair�os

luzma@uni-sw.gwdg.de

Universit�ats-Sternwarte G�ottingen,Geism arlandstr. 11,37083,G�ottingen,Germ any.Instituto de

Astrof��sica de Canarias ,E-38200 La Laguna,Tenerife,Canary Islands,Spain.Departam ento de

Astronom��a,Universidad de Chile,Casilla 36-D,Santiago,Chile

Nicola Caon

ncaon@ll.iac.es

Instituto de Astrof��sica de Canarias,E-38200 La Laguna,Tenerife,Canary Islands,Spain

Bego~na Garc��a Lorenzo

bgarcia@ing.iac.es

Isaac Newton Group ofTelescopes (ING),E-38780 Santa Cruzde La Palm a,La Palm a,Canary Islands,

Spain

Jos�eM .V��lchez

jvm@iaa.es

Instituto de Astrof��sica de Andaluc��a,CSIC,Apdo. 3004,18080 Granada,Spain

Casiana M u~noz-Tu~n�on

cmt@ll.iac.es

Instituto de Astrof��sica de Canarias,E-38200 La Laguna,Tenerife,Canary Islands,Spain

A B ST R A C T

W e present results from a detailed spectrophotom etric analysis ofthe blue com pact dwarf

galaxy (BCD)M rk 370,based on deep U B V RI broad-band and H� narrow-band observations,

and long-slitand two-dim ensionalspectroscopy ofitsbrightestknots.

The spectroscopic data are used to derive the internalextinction,and to com pute m etallic-

ities,electronic density and tem perature in the knots. By subtracting the contribution ofthe

underlying older stellar population,m odeled by an exponentialfunction,rem oving the contri-

bution from em ission lines,and correcting forextinction,we can m easure the true colorsofthe

young star-form ing knots. W e show that the colors obtained this way di�er signi�cantly from

thosederived withouttheabovecorrections,and lead to di�erentestim atesoftheagesand star-

form ing history ofthe knots. Using predictions ofevolutionary synthesis m odels,we estim ate

the agesofboth the starburstregionsand the underlying stellarcom ponent.W e found thatwe

can reproduce the colorsofallthe knotswith an instantaneousburstofstarform ation and the

Salpeter initialm ass function (imf) with an upper m ass lim it of100 M � . The resulting ages

rangebetween 3 and 6 M yrs.Thecolorsofthe low surfacebrightnesscom ponentareconsistent

with ages largerthan 5 G yr. The kinem atic results suggestordered m otion around the m ajor

axisofthe galaxy.

Subjectheadings: galaxies:dwarfgalaxies-galaxies:starburst-galaxies:com pact-galaxies:kinem atics

and dynam ics-galaxies:evolution

1

1. Introduction

Blue Com pactDwarfgalaxies(BCD) are low-

lum inosity (M B � � 18 m ag),com pact(starburst

diam eter � 1 kpc) objects, which have optical

spectra sim ilarto those presented by H IIregions

in spiralgalaxies(Sargent& Searle 1970;Thuan

& M artin 1981).They arem etalde�cientgalaxies

| them etallicitiesoftheirionized gasrangingbe-

tween Z� =50 and Z� =2 | thatform starsathigh

rates,able to exhausttheir available gascontent

in a tim e m uch sm aller than the age ofthe Uni-

verse.Thisfactim pliesthateitherthese galaxies

are young system s,experiencing their �rst burst

ofstarform ation,orthatstarform ation occursin

short bursts separated by long quiescent periods

(Sargent& Searle1970).Nowadays,thelatterex-

planation isthem orewidely accepted,atleastfor

the m ajority ofBCDs(Thuan 1991).

In the recent years BCDs have attracted a

great dealofinterest,and have becom e key ob-

jectsforunderstanding fundam entalastrophysical

problem s. M uch e�ort has been devoted to the

study of these galaxies, driven by the following

fundam entalquestions:1)whatisthem echanism

that triggers the starburst activity and is there

a unique one in allBCD subtypes? 2) what are

thepropertiesoftheunderlying stellarpopulation

(age,m etallicity,dynam icalstatus)? Answering

the above questions would allow to form a gen-

eralview ofdwarfgalaxy form ation and evolution,

and in particular to test those evolutionary sce-

narioslinking the di�erentclassesofdwarfgalax-

ies,nam ely,dwarfirregulars(dIs),dwarfellipticals

(dEs)and BCDs(Thuan 1985;Davies& Phillipps

1988;Papaderosetal.1996b;M arloweetal.1997;

M arlowe,M eurer,& Heckm an 1999),or the dif-

ferent m orphologicalclasses ofBCDs (Noeske et

al. 2000). However, m ost of the work carried

out so far has focused on a statistical analysis

ofBCD sam plesby m eansofsurface photom etry

(Papaderoset al.1996a;Telles 1995;Doublier et

al.1997;Doublier,Caulet,& Com te 1999;�O stlin

1998),and hasratherdem onstrated the com plex-

ity ofthe topic than given �rm answers.

Prom pted by thislack ofconclusiveresults,we

started an extensive study of a large sam ple of

BCDs. The �rst step was the analysis ofbroad-

band observations in B ,V ,R and I for a sam -

ple of28 BCDs.In Cair�osetal.(2001a,hereafter

Paper I) we presented deep contour m aps in the

B band,and surfacebrightnessand colorpro�les,

which wereused toexam ineand discusstheirm or-

phology and structure. In Cair�os etal.(2001b,

hereafterPaperII)weenlarged thedatasetby in-

cluding U and H� observations;weprovided inte-

grated photom etry ofthe galaxies and produced

an atlasofdetailed colorand H� m aps.Thequal-

ity ofthe data allowed usto identify the di�erent

star-form ing regions,and discrim inate them from

the regionswhere the starform ation has already

ceased.

O ne of our m ain conclusions was that opti-

calbroad-band photom etry alone does not allow

to disentangle the distinct stellar com ponents in

BCDs and derive their evolutionary status. The

m easured broad-band colorsare strongly a�ected

byinterstellarreddeningand gaseousem ission;be-

sides,we need to separate the lightcom ing from

youngstarsfrom theem ission ofthoseborn in pre-

viousstar-form ation episodes| a m ethodological

aspect often overlooked. Spectroscopic inform a-

tion isrequired in orderto correctforinterstellar

reddening and em ission line contam ination,and

U -band and H� m easurem ents are fundam ental,

since they are m uch m ore sensitive to the star-

form ation properties. Nevertheless,such kind of

analysisisscarcein theliterature,m ainly because

itrequiresa lotofobserving tim e. In particular,

not enough e�ort has been devoted so far to the

characterization ofthe stellar population hosting

the actualburst,indeed a challenging task.

Forthesereasons,wedecided toperform am ore

detailed study of a num ber of individualgalax-

ies,based on a new strategy thatcom binesbroad-

and narrow-band photom etry with long-slitand 2-

D spectroscopy.The objectswereselected am ong

thosepresented in PapersIand II,so asto form a

sam pleincluding thedi�erentsubtypesofobjects

classi�ed as BCDs | a not so wellde�ned class

which spans a wide range oflum inosities,m etal-

licities and m orphologies (K unth & �O stlin 2000;

Cair�os2000;Cair�osetal.2001b).

Thispaperisthe�rstin a serieswhich present

theresultsobtained from theuseofthistechnique

to analyzetheselected galaxies.Herewedescribe

the technique and its application to the galaxy

M rk 370,a lum inous BCD,with M B = � 17:20

(Paper II), located at a distance of 12.9 M pc

2

(Thuan & M artin 1981)1.

W e have chosen this galaxy in order to study

a prototype ofthe group ofBCDspopulating the

higherlum inosity and m etallicity rangewithin the

m ostcom m on BCD m orphologicalsubclass,theiE

type(Loose& Thuan 1986, hereafterLT86).This

typeischaracterized by an underlying,low bright-

ness ellipticalcom ponent which hosts num erous

star-form ing regions.

In M rk370,thebrighterknotsofstar-form ation

are located in the inner part ofthe galaxy,from

wheresm allerstar-form ingknotsem ergeand form

a structure resem bling spiralarm s (see Papers I

and II).The choice ofM rk 370 restsalso on the

fact that the com parison ofthe properties ofits

m any individualstarburstknotsprovidesvaluable

inform ation on the m echanism s that trigger the

star form ation, allowing to study how it prop-

agates within the galaxy, and is a possible test

ofthe stochastic self-propagating star form ation

m echanism (G erola, Seiden, & Schulm an 1980),

which hasbeen suggested to explain how thestar-

form ation processin dwarfsism aintained (Coziol

1996).

In addition, M rk 370 m ay be seen as a pro-

totype of the BCDs classi�ed as m ultiple nu-

cleusstarburstgalaxies.Thepresenceoftwo cen-

tralstar-form ing knotsled M azzarella & Boroson

(1993)to labelthisgalaxy asa "double nucleus"

BCD,prom pting furthersuggestionsofitsm erger

origin.Aswewillshow in thiswork,thekinem atic

and spectrophotom etricresultsobtained castseri-

ousdoubtson the m ergerscenario.

A com plete collection of color and H� m aps

ofthisgalaxy can be found atthe following W eb

page: http://www.iac.es/proyect/G EFE/BCDs.

The basicdata ofM rk 370 areshown in Table1.

2. O bservations and D ata R eduction

2.1. Long-slit spectra

Long-slitspectroscopyofM rk 370wasobtained

in Septem ber 1998,with the Isaac Newton Tele-

1For consistency with Papers Iand II,we use here the dis-

tance from Thuan & M artin (1981), which is based on

the radial velocity with respect to the Local G roup and

H 0 = 75 km sec� 1 M pc� 1; applying the correction for

LG infallinto V irgo would give D = 10:9 M pc,and total

lum inosities,absolute m agnitudes and sizes would change

accordingly.

scope(INT)attheO bservatoriodelRoquedeLos

M uchachos (O RM , La Palm a). The data were

taken using the Interm ediate Dispersion Spectro-

graph (IDS),equipped with aCCD.A slit3:08long

and 1:002 wide wasused.The position angleofthe

slitwassetto 125�,so asto passthrough thetwo

brightestcentralknots.Thespectralsam plingwas

1.85�A perpixel,thewavelength range3600� 7300�A,and the spectralresolution � 5:5 �A (FW HM );

thespatialsam plingwas0.39arcsecperpixel.The

seeing wasabout1:005. The log ofthe observation

isshown in Table2.

Data reduction was perform ed using IRAF

standard tasks: after subtracting the bias and

at-�elding the spectra,they were calibrated in

wavelength. The sky spectrum was derived by

averaging the signalin two windows 30{40 pixel

wide outside the region where the object is still

detectable,and subtracted out.The spectra were

then corrected foratm osphericextinction and cal-

ibrated in ux by m eans ofobservationsofspec-

trophotom etricstandards.

2.2. T w o-dim ensionalspectroscopy

The 4.2m W illiam HerschelTelescope (W HT,

atthe O RM ,La Palm a),equipped with the �ber

system INTEG RAL (Arribasetal.1998)and the

W YFFO S spectrograph (Bingham et al. 1994),

wasused to observethecentralregion ofM rk 370.

INTEG RAL currently has three �ber bundles

with di�erent spatialcon�gurations on the focal

plane. These three bundles can be interchanged

during the observations depending on the scien-

ti�c program orthe seeing conditions.O n the fo-

calplane,the�bersofeach bundlearearranged in

twogroups:oneform sa rectanglewhich m apsthe

object,the otherform san outerring which sam -

ples the sky-background. M rk 370 observations

were done with the standard bundle 2 (hereafter

SB2). Itconsistsof219 �bers,each 0:009 in diam -

eter;the centralrectangle isform ed by 189 �bers

covering an area of1600� 12:003, while the other

30 �bers are evenly spaced on a ring 9000 in di-

am eter.Figure 1 showsthe actualdistribution of

those�bersin the focalplane.Atthe entranceof

theW YFFO S spectrograph,the�bersarealigned

so asto form a pseudo-slit.

W eobserved M rk 370 on August22 1999,with

aseeingof� 1:003.Thespectrograph wasequipped

3

with agratingof600groove/m m ,and aTek6CCD

array of1124x112424-m icron pixels.Thespectral

rangecoveragewas4400� 7150�A,with a spectral

resolution ofabout6�A;itincludestheH�,[O III],

H�,[N II],and [S II]em ission lines(seeAppendix

A).

Datareduction wasperform ed within theIRAF

environm ent,and includesbiasand scattered light

subtraction,�berextraction,�berthroughputcor-

rection,wavelengthcalibration,rejectionofcosm ic

ray events,and correction from di�erentialatm o-

spheric refraction e�ects(fordetailson reduction

of�berdata see e.g. G arc��a-Lorenzo,M ediavilla,

& Arribas 1999,and referencestherein).

In Appendix A wepresentthenuclearspectrum

ofM rk 370 in the fullwavelength range observed

with the �bersystem . W e also show the individ-

ualspectra corresponding to each ofthe 189 ob-

served positions(�bers)in a selected spectralin-

tervalwhich includesthem ostim portantem ission

lines.

2.3. H � im aging

Narrow-band im ages centered on the H� line

and on the adjacent continuum were taken in

1997 Decem ber at the 2.2-m telescope of the

G erm an-Spanish Astronom ical O bservatory on

CalarAlto(Alm er��a,Spain).Theinstrum entation

consisted ofthe Calar Alto FaintObjectSpectro-

graph,CAFO S,and a 2048� 2048SiTeCCD chip.

W ith a pixelsizeof0.53 arcsec,itprovidesa �eld

16 arcm in wide;however,due to the physicalsize

oftheavailable�lters,only thecentralround area

ofabout 11 arcm in is free from vignetting. The

averageseeing was1.5 arcsec.

The im age reduction was carried out using

IRAF.Each im age was corrected for bias,using

an averagebiasfram e,and was attened by divid-

ing by a m ean twilight at�eld im age.Thefram es

werethen registered(foreach�lterwetookasetof

dithered exposures)and com bined toobtain the�-

nalfram e,with cosm icrayeventsrem ovedandbad

pixelscleaned out.Theaveragesky levelwasesti-

m ated bycom putingthem ean valuewithin several

boxessurrounding theobject,and subtracted out

asa constant.Flux calibration wasdone through

the observation ofspectrophotom etric starsfrom

the listsin O ke(1990).

The com plete log ofthe observationsisshown

in Table2.

2.4. B roadband Im aging

UBVRIim aging ofM rk 370 wascarried outin

O ctober 1998 at the 1-m Jacobus K apteyn Tele-

scope,JK T (O RM ,LaPalm a).W ecollected CCD

im agesatthe f/15 Cassegrain focus,using a Tek

1024� 1024pixelchip,yielding a pixelsizeof0.33

arcsecand a �eld ofview of5:6� 5:6 arcm in.The

averageseeing was1:005.

A log ofthe observations is given in Table 2.

The im age processing was perform ed using stan-

dard proceduresavailable in IRAF.Flux calibra-

tion was done through the observation ofphoto-

m etric starsfrom Landolt(1992)throughoutthe

night.

3. Spectroscopic A nalysis

3.1. Long-slit spectroscopy

The position of the slit we used is shown in

Figure 2, overplotted on a contour m ap of the

continuum -subtracted H� im age. (The m eaning

ofthelabelsisexplained in Section 4).Thespatial

run oftheintensity ofthebrightestem ission lines

(H�,H�,[O III])hasbeen used asabaseforde�n-

ing the regions from where the one-dim ensional

spectra ofeach knotwereextracted by integrating

the signalwithin them . Two distinct starburst

regions have been identi�ed,m arked a and b in

Figure2.

Figure3 showsthe sum m ed spectra ofthe two

knots,and thespectrum integrated overthewhole

galaxy (which we shallrefer to as s). The spec-

trum ofregion a shows a high continuum level.

Absorption wings are resolved in H� and H ,

H� being visible in absorption only. The Balm er

discontinuity,togetherwith these absorption fea-

tures,can be interpreted asthe productofa sub-

stantialpopulation ofolder stars,with ages con-

siderably larger than 10 m illion years. Region b

shows an alm ost at spectrum , characteristic of

a dom inant O B population,with no evident ab-

sorption features. The integrated spectrum ,s,is

sim ilarto thatofregion a.

Fluxes and equivalent widths ofthe em ission

lineswerem easured using thegaussian pro�le�t-

ting option in the iraftask splot (a directinte-

gration ofthe ux insideeach linegavevery sim i-

4

larresults).Itisknown thatthese m easurem ents

arean underestim ateofthereal ux oftheBalm er

lines,becauseoftheunderlyingabsorptioncom po-

nent.In thepresentspectra wecould notdeblend

thetwocom ponents.Tocorrectfortheunderlying

stellarabsorption,som e authors(M cCall,Rybski

& Shields1985;Skillm an & K ennicutt1993)adopt

a constantequivalentwidth (1:5� 2 �A)forallthe

hydrogen absorption lines. However,the actual

value oftheir equivalent width is uncertain and

depends on the age of the star form ation burst

(D��az 1988;Cananzi,Augarde,& Lequeux 1993;

O lofsson 1995).Thuswepreferred to leaveitasa

freeparam eter,which wasdeterm ined asfollows.

W eassum ethatthe equivalentwidth,W abs,of

the absorption line isthe sam e forallthe Balm er

lines,but can vary from knot to knot. W e then

start with an initialguess for W abs, correct the

m easured uxes,and determ inetheextinction co-

e�cient at � = 4861 �A,C (H�), and its uncer-

tainty,�C (H�),througha�ttotheBalm erdecre-

m ent,given by the equation:

F (�)

F (H �)=

I(�)

I(H �)� 10C (H �)�f(�) (1)

whereF (�)

F (H �)isthe line ux corrected forabsorp-

tion and norm alizedtoH�;I(�)

I(H �)isthetheoretical

valueforcaseB recom bination,from Brocklehurst

(1971),and f(�) is the reddening curve norm al-

ized to H� which wetook from W hitford (1958).

W ethen search,foreach knot,thevalueofW abs

thatprovidesthebestm atch | thatis,thelowest

�C (H�) | between the corrected and the theo-

reticalline ratios.

Reddening-corrected intensity ratiosand equiv-

alentwidthsarequoted in Table3.

The [O III]� 4363 line could not be detected

in the spectra,so the oxygen abundance for the

di�erentregionsofM rk 370 wasderived using the

em pirical12+ log O /H vs R 23 relation, �rst de-

rived by Pageletal.(1979).W ehaveused theR 23

calibration according to M cG augh (1991). The

ratio [N II]/[O II]was used to break the degen-

eracy in the calibration in order to select either

the upper or the lower branch (M cG augh 1991).

Sincethevaluesof[N II]/[O II]forthethreespec-

tra analyzed are always above the 0.1 threshold

(log�6584=�3727 = � 0:59,� 0:81,� 0:75 for re-

gionsa,b and s),theupperbranch ofthecalibra-

tion has been used for the determ ination ofthe

abundance.W ehavederived thefollowing values:

12 + logO =H = 8:7,8.6 and 8.7 for the spectra

ofregions a,b and s respectively,for which the

corresponding valuesforthelogarithm ofelectron

density are2.07,2.2 and 2.3.

The average value for the nitrogen to oxygen

ratio is logN =O = � 1:1. The derived average

oxygen abundance is approxim ately Z� /2. The

abundancesare equalforregionsa and b,within

the uncertainties.

3.2. T w o-dim ensionalspectroscopy

Two-dim ensional spectroscopy with optical

�bersallowsto collectsim ultaneously the spectra

ofm any di�erent regions ofan extended object,

com bining photom etry and spectroscopy in the

sam edataset.

From the individualm easurem ents(line uxes

and ratios,continuum level,radialvelocity,etc.)

obtained at each �ber, we can build 2-D m aps

by using a two-dim ensional interpolation tech-

nique,such asthe Renka & Cline algorithm (im -

plem ented in the E01SAF and E01SBF routines

in theNAG Fortran Library).Thisrequiresa pre-

ciseknowledgeofthe exactlocation ofeach �ber,

which isdeterm ined by a m etrology m achine. In

thisway webuiltup im agesof95� 95 pixelswith

a scale � 0:002 pixel�1 . W hile the spatialsam -

pling (thatis,the �berdiam eter)ofINTEG RAL

(STD2)is0.9 arcsec,the centroid ofany peak in

our m aps can be m easured with an accuracy of

around 0.2 arcsec(thatis 1=5 ofthe�berdiam e-

ter;see forinstanceM ediavilla etal.1998)

3.2.1. Continuum and Line-Intensity M aps

Because of non-photom etric conditions dur-

ing INTEG RAL observations,we could not ux-

calibratedirectly ourspectra.The ux calibration

was done by using the long-slit data as a refer-

ence.W e integrated the signalin our2-D spectra

within tworegionsm atchingregionsa and b in the

long-slitspectra. Com paring the uxes ofdi�er-

entlines,we could com pute the conversion factor

with an accuracy ofabout20% .Thisisthe error

associated to the uxesderived from INTEG RAL

data using thiscalibration.

Figure 4(a) shows a continuum m ap (within

5

a spectral region where no em ission lines are

present) of M rk 370 from INTEG RAL spectra.

W e also present in Figure 4(b) a V band im age

ofthe galaxy,in order to evidence the excellent

agreem ent between the continuum m ap and the

broad-band im ages.

TheINTEG RAL datashow thattheposition of

thepeak in thecontinuum m apsdoesnotdepend

on what spectralrange was used to build them .

Therefore,we can safely de�ne this point as the

"opticalnucleus"ofthegalaxy,and wesetitasthe

origin ofthe coordinatesystem in allourspectral

m aps.

By�ttingasingle-gaussiantoeachem issionline

in ourspectra,wehaveobtained em ission line ux

m aps. Figure 4 (c) shows the H� em ission m ap.

W ealso�tted an absorption com ponenttoBalm er

lines in those spatialpositionswhere the absorp-

tion wingswereclear.Forcom parison,an H�nar-

row band �lter im age ofM rk 370 is displayed in

Figure4(d).Theareacovered by INTEG RAL in-

cludesfourofthem any star-form ingknotspresent

in the galaxy. The opticalnucleusdoesnotcoin-

cide with any ofthem .

Figure4 (e)isthem ap ofthe[O III]/H� ratio,

which iscom m only used asan indicatoroftheex-

citation.Them orphologyofthe[O III]/H� m ap is

sim ilarto the em ission linesintensity m aps. The

regions with the highest ionization coincide with

the knotpeaks,and the levelofexcitation isnor-

m alfor star-form ing regions. Figure 4 (f) is the

m ap oftheextinction coe�cientC (H�)com puted

from the observed H�/H� ratio.

Thepeaksin theextinction coe�cientm ap are

closeto,butnotcoincidentwith,thepeaksofthe

H� knots.Theaverageo�setforthefourknotsin

ourobserved area is0:26� 0:11arcsec.W hilethis

displacem ent is sm all| in fact,it is at our de-

tection lim it| ,itm ay suggesta realspatialshift

between thecenteroftheHIIregionsand theholes

in theC(H�)distribution.A sim ilarcasehasbeen

reported beforeby M a��z-Apellanizetal.(1998)for

thestarburstknotsofthegalaxyNG C 4214.They

�nd thedustto beconcentrated attheboundaries

ofthe ionized regions.The explanation thatthey

propose is that stellar winds have contributed to

di�use the dustinto the inter-clusterm edium .

3.2.2. Kinem aticalPattern

The kinem atic analysisis lim ited to the em is-

sion features,asabsorption linesare too weak to

be ofsom eutility.

IndividualRadialVelocities

Radialvelocitieswereobtained by �tting a sin-

gle gaussian to each em ission line present in our

spectra. Although the spectralresolution ofthe

IntegralField Spectroscopy is � 275 km /s, the

centroid of the em ission features could be m ea-

sured with an accuracy of� 10 km /s,asindicated

bythescatteram ongtheradialvelocitiesobtained

from di�erentlines. The �nalradialvelocitiesof

theionized gaswereobtained by averagingthere-

sults derived from individualem ission lines. W e

com pared the results for the two di�erent indi-

vidualexposures ofM rk 370. The m ean di�er-

enceam ongtheindividualvalueswas0� 12km /s.

Therefore,by adding the erroron the wavelength

calibration (� 7 km /s),we estim ate that the �-

naluncertaintieson thevelocitym easurem entsare

. 20 km /s.

The M ean Ionized GasVelocity Field

Figure5showsthatthevelocity�eld appearsto

be regularand sim ilarto thatofa rotating disk.

The west region is receding,the east part is ap-

proaching. In order to determ ine the location of

the kinem atic center (k hereafter),we com puted

the "di�erential" ofthe velocity �eld,averaging

theabsolutedi�erencesbetween onepointand its

nearestneighbors.Thisprocedureem phasizesre-

gions in which the velocity is changing rapidly.

The position derived in this way is placed � 2:004

SE (� 2:0036,0:0037)ofthe opticalnucleus. k is lo-

cated between regionsa and b,in an apparently

non-signi�cant location in the galaxy,and has a

velocityvalueof766� 20km /s,consistentwith the

system ic velocity ofthe galaxy,783� 16 km /s,re-

ported by Falco etal.(2000).

k isnearly coinciding with thegeom etricalcen-

ter ofthe externalisophotes,obtained from our

broad band im ages. Thisfactm ightsuggestthat

the star-form ing knots in the central region of

M rk 370 could sharethesam evelocity �eld ofthe

m ain body.

Velocity Field Param eters

To quantify therelevantparam etersoftheion-

ized gasvelocity �eld,we adopted a sim ple kine-

6

m atic m odel(M ihalas& Binney 1981):

Vr(R;�)= Vsys+ Z(R)cosi+ (R)cos[�� �(R)]sini

(2)

whereV sys isthesystem icvelocity,(R)isthe

strength ofthe velocity �eld projection onto the

galactic plane,and �(R) is the PA ofthe m ajor

kinem atic axis. R and � are the galactocentric

polarcoordinatesin the planeofthe galaxy.

W e�tthism odelto thedata,keeping �xed the

position ofthecenteratk,and assum ingan initial

estim ate ofthe position angle ofthe kinem atical

m ajor axis �0 � 90�, and an inclination of the

galaxy i� 40� (Nordgren etal.1995).Theresults

do notdepend on �0,and areratherinsensitiveto

reasonable changes in i. In Figure 6 we present

the dependence on galactocentric distance ofthe

param eters resulting from the �t. The system ic

velocity we derived is761 km /s,with an internal

errorof� 2 km /s.

4. P hotom etric A nalysis

4.1. H � photom etry ofthe starburstknots

The left panelofFigure 7 shows a grey-scale

H� im ageofM rk 370.The galaxy revealsa com -

plex structureconsisting ofseveralresolved knots

located atop an extended low surface brightness

(LSB)com ponenthaving regularappearance(see

theR band m ap in Figure7,rightpanel)and sub-

stantially reddercolors(PaperIand PaperII).

In the centralpartofthe galaxy are two large

star-form ing regions (labeled 11 and 12 in Fig-

ure2),� 7 arcsec(450 pc)apart,which appearto

beconnected byafaintbridge-likestructure;these

twostar-form ingregionshavebeen previously cat-

aloged as"doublenucleus"by M azzarella& Boro-

son (1993).Sm allerstar-form ingknotsextend out

in thenorth-eastand south-westdirections,to ap-

proxim ately 40 arcsec (2.5 kpc) from the optical

center(i.e.,thecenteroftheouterisophotes).The

star-form ation activity isconcentrated along this

direction,with the exception ofregion 11).

W e have used the focas package to identify

and analyzethedi�erentknotsin the continuum -

subtracted H� im ages. The focas task looksfor

localm axim a and m inim a in the adu counts of

each pixelin the im age. W e have adopted the

criteria that,for a knot to be detected,its area

m ust be larger than 12 pixels (corresponding to

an equivalentdiam eter of2 arcsec or125 pc),to

ensurethatthediam eteroftheknotislargerthan

the psf,and the countsofeach ofitspixelsm ust

be higherthan 3 tim esthe standard deviation of

the sky background. Using these criteria,a to-

talof16 knotswere detected;they are labeled in

Figure2.2

Dataon theH� ux,lum inosity,num berofion-

izing photons,ionized hydrogen m ass and equiv-

alent width of each knot were calculated; these

results are presented in Table 4. The num ber of

ionizing photons and the ionized hydrogen m ass

were calculated following K ennicutt (1988). The

electron tem peratureand density needed to calcu-

latethem assofthehydrogen ionized gasand the

num ber of ionizing photons were obtained from

the long-slitspectra.

The resulting param eters have been corrected

forinterstellarextinction and forthecontribution

ofthe [N II]lines. For the K nots 11 and 12,we

used the spectroscopicparam etersderived forthe

subregions b and a (see Figure 2). W e used the

bi-dim ensionaldata to correctknotsinsidethein-

tegral �eld. Forthose knotsoutside the region

covered by our2-D spectra,theabovecorrections

have been applied by using averaged values de-

rived from the centralregion.

The lum inosity ofthe brightestknots(11 and

12) are com parable to those ofindividualgiant

H II regions(typically 1039{1040 erg sec�1 ;K en-

nicutt1991).The range in lum inositiesissim ilar

to thatfound fortheH IIregionscataloged in the

M agellanicIrregularNG C 4449(Fuentes-M asip et

al.2000).

Theequivalentwidth W (H�)wascom puted as:

W (H�)= C H�=Ccont� �W H� (3)

where CH� is the totalnum ber ofnet counts of

theknot,Ccont isthecorresponding totalnum ber

ofcounts in the rescaled continuum im age,and

�W H� isthe width ofthe H� �ltertransm ission

curve.

TheH�equivalentwidth hasbeen corrected for

thecontribution oftheunderlying stellarem ission

to the continuum level; it m eans that Ccont are

the totalcountsm easured in the continuum band

2The Integralcontinuum m aps show another fainter knot,

notdetected by Focas as itdoes notsatisfy these criteria.

7

after subtracting the counts due to the em ission

ofthe olderstars.In orderto estim ate thislatter

contribution,weassum ed thattheunderlyingpop-

ulation can bedescribed by an exponentialm odel

(this procedure is explained in the next section).

As the exposures in the continuum narrow-band

�lterwerenotdeep enoughtoderivetheproperties

ofthe underlying disk,the structuralparam eters

derived in R band havebeen used.Thecontinuum

ux m ap through the H� �lter has been derived

from theR fram eafterthelatterhasbeen rescaled

so asto m atch the continuum in the form er. W e

followed the sam eprocedure,based on �eld stars,

com m only em ployed tom atch H�and itsadjacent

continuum (see Paper IIfor a description ofthis

technique).

In Table4 weshow thevaluesofW (H�)ofthe

di�erentstar-form ing regionsin M rk 370;wehave

listed thevaluesbeforeand afterapplyingthecor-

rection forthe contribution ofthe older stars,to

provide a direct estim ation ofthe im portance of

such contribution. W e found the di�erence to be

signi�cantin m ostcases.

4.2. B road-band P hotom etry

4.2.1. The Starburst

Theyoung starburstregionshavebeen isolated

in each broad-band �lterim age.Using focas,we

builta m ask wheretheem ission regionsdelim ited

in the H� fram e have the constantvalue one and

therestaresetto zero.Each broad-band im ageis

then m ultiplied by thism ask.Finally we perform

aperture photom etry on these im ages,to derive

m agnitudesand colorsofeach knot.

Table 5 shows the broad-band photom etry of

the sixteen knots detected in M rk 370;the m ea-

surem ents have been corrected from G alactic ex-

tinction using the data published by Burstein &

Heiles (1984). The extinction coe�cient in the

otherbandswererelated to the B value following

Rieke& Lebofsky (1985).

In order to characterize the stellar population

ofthe individualregions,we com pare their inte-

grated properties with the predictions of evolu-

tionary synthesis m odels. This task is especially

di�cultin thiscase,because atleastthree di�er-

entcontributorsareproducing theobserved light:

the young stars,the gaseous nebula surrounding

them ,and the olderstarsunderlying the ongoing

burst.Broad-band colorsm ustbe corrected from

the contribution ofthese di�erent factors before

they can becom pared with them odelpredictions.

In order to derive the "true" colors ofthe star-

burst,weproceeded in the following way.

First,colorswerecorrected forinterstellarred-

dening,usingthevalueofC (H�)derived from our

long slitspectra.Forthoseknotslacking spectro-

scopicdata,weused theC (H�)averaged overthe

whole long-slit spectrum (which is probably our

bestpossibleguess).

Second,we havecorrected forthe contribution

ofem ission lines. In gasrich objects,like BCDs,

the presence ofstrong em ission linesin the spec-

tracan severelya�ectbroad-bandcolors;theexact

am ountofthecontribution ofthelinesto the ux

observedin agiven�lterdependson theequivalent

width ofthelineand on itspreciselocation under

the �ltertransm ission pro�le. W e used ourspec-

troscopic inform ation to com pute the fraction of

ux in the observed broad-band �ltersaccounted

for by each em ission line,by using the task cal-

cphotin the synphotpackageofSTSDAS.

Last,we subtracted the em ission ofthe under-

lying population ofstars. W e assum ed that the

galaxy iscom posed ofthestarburstregionsdelim -

ited by the focas task,and ofan underlying low

surface brightnesscom ponent,which isdescribed

by an exponentialfunction (seenextsection).

Theintegrated m agnitudeofa knot,in a given

band,�,isgiven by:

m � = � 2:5� log�CSB� + C

D�

�+ K � (4)

where C SB�

is the num ber ofcounts in the knot

com ing from the starburst; C D�

is the contribu-

tion oftheunderlying disk within theknotregion,

and K � isthe corresponding calibration constant

(which incorporatesexposuretim e,airm ass,etc.).

C D�iseasily com puted from thecentralsurface

brightness and scale length ofthe best-�t expo-

nentialm odel,and subtracted outin theprevious

form ula,yielding thecorrected m agnitudeofeach

knot:

m �(SB )= � 2:5� log�CSB�

�+ K � (5)

This processhasbeen applied to the B ,V ,R

and I bands. The im ages in U were not deep

8

enough to allow a reliable �tto the lightpro�les;

however,by using typicalvalues ofU � B for an

evolvedstellarpopulation (Bruzual1993;Vazdekis

etal.1996),wehaveestim ated thecontribution of

thehostgalaxy in theU band.W efound that,in

thispass-band,the oldercom ponentaccountsfor

only � 1% of the knot ux, so we can neglect

thiscorrection.Theresulting corrected colorsare

listed in Table5.

W e would like to stress that uncorrected and

corrected knot colors di�er signi�cantly. In the

case ofthe M rk 370,while em ission lines do not

change m uch the broad band colors (shifts in

B � V are � 0:05 or less),the internalextinction

m ay change B � V by up to 0.25 m ag, and the

e�ectofthe contribution ofthe LSB stellarcom -

ponentisequally im portant.The �nalcolorshift

is clearly function ofthe position in the galaxy;

thism eansthatitisdangerousto interpretcolor

gradientsasgenuinevariationsoftheknotproper-

ties(forinstancetheirage)iftheabovecorrections

havenotbeen applied.

4.2.2. The Underlying Stellar Population

Surface brightnesspro�les(SBP)in B V RI,as

wellas color pro�les ofM rk 370,have been pre-

sented in PaperI.TheSBP ofthegalaxywasused

to decom pose the galaxy in two com ponents:the

starburst,H� em itting regions,plusan olderLSB

stellarhost.By �tting an exponentialfunction to

the faint, outer parts ofthe galaxy light pro�le

(where we expect no or only negligible contribu-

tion from thestarburst),wedeterm ined thestruc-

turalparam eters(scale length � and centralsur-

facebrightness�0)oftheLSB host.Theseparam -

eters were used to calculate its totalm agnitudes

in the di�erentbands.

The structural param eters in the di�erent

bands,togetherwith the totallum inositiesofthe

starburst and of the old stellar com ponent, are

given in Table6.

Colors ofthe underlying com ponent were de-

rived from the best-�t exponential m odel; we

found (B � V )host = 0:80,(V � R)host = 0:50 and

(V � I)host = 1:46.

5. Evolutionary properties ofM rk 370

5.1. T he Starburst

In orderto constrain thepropertiesoftheindi-

vidualH II regions, we com pare their observed

properties with the predictions of evolutionary

population synthesism odels.Thesem odelsassum e

that stars are born with m asses distributed ac-

cording to the initialm assfunction (imf),with a

star form ation rate (sfr). The stars are evolved

according to theoreticalevolutionary tracks and

�nally,using em piricalortheoreticalcalibrations,

the predicted colors or spectra ofthe com posite

population are obtained. In the last years,peo-

ple have been doing a lotofwork in evolutionary

synthesism odels,see forinstance Leithereretal.

(1996). Depending on the hypothesis on which

the di�erentm odelsare based,the predicted val-

uesfortheobservablescan beslightlydi�erent(for

exam ple,theoreticalm odeling ofthe evolution of

m assivestarisstillundergoingsigni�cantchanges,

M aeder& Conti1994;and revisionofthesem odels

often leadsto new generationsofsynthesiscalcu-

lations).Thism eansthatitisim portantto exam -

inethecharacteristicsoftheavailablem odelsand

selectthe one m ostsuitable to each speci�c case.

W e have com pared the derived observables of

the starburstwith the predictionsofLeithereret

al.(1999,hereafter SB99) evolutionary synthesis

m odels,known asStarburst99.Thesem odelspro-

vide a wide setofpredictionsforthe spectropho-

tom etricpropertiesofstar-form inggalaxies.They

are an im proved version of the ones previously

published by Leitherer & Heckm an (1995): the

latest set im plem ents the stellar evolution m od-

elsofthe G eneva group,aswellasthe m odelat-

m osphere grid com piled by Lejeune,Cuisinier,&

Buser(1997).

For each of the sixteen delim ited knots, we

searched forthem odelwhich bestm atchestheob-

served param eters: W (H�),U � B ,B � V ,V � R

and V � I. In order to break the age and m etal-

licity degeneration,wesetthem etallicity equalto

thatderived from the em ission line uxes(which

isareasonableapproxim ation to them etallicity of

a young population).

W e found that, for allthe knots, we can re-

producetheobserved quantitieswith an instanta-

neousburst(IB)ofstarform ationand theSalpeter

9

imf with an upper m ass lim it of 100 M � . No

restriction can be put on the lower m ass,given

its poorsensitivity to the young burstevolution,

which isdom inated by thecontribution ofm assive

stars(M > 8M � ). The derived agesfrom U � B

and H� are quoted in Table 7;both observables

giveusually agesconsistentwith each other.

The agesderived using B � V ,V � R and V � I

areslightly larger,butin generalcom patiblewith

the form er. W e notice that these colors are not

very sensitiveto the evolution ofthe young stars,

as they vary by about 0.3 to 0.4 m ag when the

age increases from 0 to 100 M yr (SB99). Also,

thesecolorsarestrongly contam inated by thedif-

ferent factors described in Section 4.2.1 (redden-

ing,contribution ofem ission linesand em ission of

the olderstars).

Although wehaveattem pted tocorrectforsuch

factors, the results m ight be im precise due to

the various hypotheses we m ade to work them

out.First,weadopted a constantC (H�)in those

zone having no spectralinform ation,while there

m ay bespatialvariationsin thedustdistribution,

and C (H�) can vary from knot to knot. That

this is likely the case is shown by the results we

obtained for the two centralknots: C (H�)A =

0:31 and C (H�)B = 0:39, which translates into

E (B � V )A = 0:22,and E (B � V )B = 0:27,and

E (V � I)A = 0:34 and E (V � I)B = 0:43. If

such variationsare detected in two knotsso close

to each other,wecan expectlargerdi�erencesfor

them oreperipheralknots.Thesam eresultisob-

tained from 2D spectroscopy:the2D distribution

of C (H�) do not present an hom ogeneous m or-

phology (see Figure4),butlargerC (H�)closeto

the centerofthe star-form ing knots.

The sam e applies to the contribution ofem is-

sion lines,though,asm entioned earlier,they play

a secondary role.

Furtherm ore,theuncertaintiesin thestructural

param etersobtained by the exponential�tofthe

LSB com ponent also translate into uncertainties

in the resultantcolors.

Allin all,the com bined errorson theknotcol-

orsduetothevariousfactorsherediscussed can be

easily aslargeas0:2� 0:3 m ag;thisfact,together

with theirlow sensitivity to the starburstproper-

ties,im ply thatB � V ,V � R and V � I alonecan

not constrain the properties ofthe young stellar

population.

O n the other hand, U � B and H� are m uch

m oresensitiveto theevolution oftheyoung stars.

They drastically change during the �rst m illion

yearsofthe starslife (see Figures55,56,83 and

84 in SB99),which convertsthem in reliableindi-

catorsofthe propertiesofstar-form ing regions.

W erem ark thatnoneoftheobservableparam -

eterswillconstrain accurately the propertiesofa

star-form ingregion,unlesswecorrectproperly for

thefactorsdescribed above.Theparam etersm ea-

sured directly (i.e.without correction) usually do

not �t the m odels,and,ifthey do,they tend to

give m uch olderages. Thispointisillustrated in

Figures8 and 9.

5.2. A ge ofthe older stars

The V � I colorappearsto be m uch too red to

becom patiblewith a norm alolderstellarpopula-

tion.Thedata in theI band arenotasgood and

deep as in the other bands,and the large uncer-

taintieson the I-band structuralparam etersm ay

be responsible for this discrepancy. Thus,the �-

nalageshavebeen derived by using theB � V and

V � R colors.Thesecolorshavenotbeen corrected

from interstellarextinction,becausetheextinction

coe�cientswederived werebased on thegasem is-

sion lines,and they m ay notapply to the regions

outside the area occupied by the starburst. W e

have assum ed that the m etallicity ofthe stellar

population is lower than the m etallicity derived

forthe gas.

W e havecom pared the B � V and V � R colors

derived for the LSB com ponent with the predic-

tionsoftwo di�erentgroupsofevolutionary syn-

thesism odels:

The Galaxy Isochrone Synthesis SpectralEvo-

lutionary Library, GISSEL 96 (Leitherer et al.

1996;Bruzual& Charlot1993),which includesim -

ple stellarpopulations(SSP)orIB with di�erent

m etallicities from Z = 0:0004 to Z = 0:10. The

agevariesfrom 0to20G yr,andSalpeterand Scalo

imfs are considered. W e have also used the pre-

viousversion ofthe m odels(G ISSEL 95)in order

to include a constant star form ing rate (CSFR)

| only availableforsolarm etallicity.

The Spectrophotom etric Population Synthesis

Library for Old Stellar System s (Vazdekis et al.

1996),a set ofstellar population synthesis m od-

10

els designed to study old stellar system s. They

synthesize SSP or follow the galaxy through its

evolution from an initialgascloud to the present

tim e,assum ing an analyticalfunctionalform for

the SFR (seeSection 3.1 in Vazdekisetal.1996),

and including the chem icalevolution.Two di�er-

entimfs are considered: a unim odalimf,with a

power law form whose slope is a free param eter,

and a bim odalimf, equalto the unim odalimf

for stars with m asses above 0.6 M � ,but with a

reduced in uence ofthe starswith lowerm asses.

W hen com paringwith thepredictionsoftheIB

approxim ation,both m odelsgiveagesof� 5 G yr

forthe olderstellarcom ponentofM rk 370.

However,when changing the starform ing his-

tory,we obtained di�erentresults.the B � V and

V � R colors could not be reproduced by G IS-

SEL95 with a CSFR.Next,we applied Vazdekis

etal.(1996)m odels,which includechem icalevolu-

tion. W e considered Salpeterimf,and the di�er-

ent� (� isa constant�xing the tim escale ofstar

form ation,see Sect 3.1 in Vazdekis et al.1996),

provided by the m odels. W e found the following

results:when � = 1 theobserved colorsareincon-

sistent with the predictions ofthe m odels; with

� = 5 colors agree with ages between 9 and 11

G yr;and in the restofthe cases(� = 10;20 and

50)colorsareconsistentwith agesbetween 4 and

7 G yrs.

O pticalcolorsarenotvery sensitiveto theevo-

lution ofthe older stars;for instance,B � V in-

crease by � 0.1 m ag when ages varies from 5 to

17 G yrand the sam eappliesto V � R.Such vari-

ationsare ofthe orderofthe uncertaintiesin our

m easured colors. Although no �rm constraints

could be put on the age ofthe underlying stel-

lar population,the above com parison with m od-

els point to a m inim um age of5 G yrs;but it is

clearthat in orderto better constrain the age of

the LSB host,itisindispensable to include near-

infrared colorsin the analysis.

6. D iscussion

Itturnsoutfrom theaboveresultsthatthekind

ofspectrophotom etric study we have carried out

isfundam entalin orderto disentangle the stellar

populationsin BCDsand derivetheirstarform ing

histories. However,as we have already pointed

outin the introduction,m ostofthe work carried

out so far has focused on statisticalanalysis of

BCDssam ples,whileonly a few papershavebeen

devoted to exam ining in detailthe characteristic

ofindividualobjects:

Thetwoextrem elycom pactand low-m etallicity

BCDs,SBS 0335-052and Tololo65,both classi�ed

asi0 (LT86),havebeen thesubjectofrecentspec-

trophotom etricstudiesby Papaderosetal.(1998,

1999). They found in both cases that the prop-

ertiesofthe underlying com ponentare consistent

with those ofa stellar population not older than

oneG yr.

Noeske et al.(2000) report on results from a

spectrophotom etricanalysisoftwoiI,C ("Com etary

BCDs",LT86)M rk59andM rk71.Spectralpopu-

lation synthesism odels,in com bination with color

m agnitudes diagram s and color pro�les, yield a

m ost probable form ation age of� 2 G yr for the

old stars in both galaxies. Two other com etary

BCDs have been studied by Fricke et al.(2001)

| Tololo 1214-277| and G useva etal.(2001)|

SBS0940+ 544. In the latter case,no com pelling

evidencesfavoreithera young oran old age.

DespitethefactthatnE/iEsarethem ostcom -

m on typeofBCD,therearevery few studies,with

thesam equality,ofobjectsbelongingtothisclass.

Steeletal.(1996)and M �endez etal.(1999)have

perform ed spectrophotom etricobservationsofthe

iE BCDs Haro 3 and II Zw 33 respectively. Al-

though in both casesan underlying com ponentof

older stars was detected,their data did not per-

m it to determ ine its properties, and both stud-

ies lim ited the analysis to the young starburst.

The ages and star-form ing scenarios they found

forthesetwo objectsareconsistentwith our�nd-

ings.They also found m etallicitiescom parableto

them etallicity wehavederived forM rk 370.M ore

recently,a com prehensiveanalysisofthe iE BCD

galaxy M rk 86 hasbeen published by G ilde Paz

etal.(2000)and G ildePaz,Zam orano,& G allego

(2000).They studied thepropertiesof46 individ-

ualstar-form ing knots as wellas the underlying

population ofolderstars. By applying evolution-

ary synthesis m odels,they found that three well

de�ned stellarpopulationsarepresentin M rk 86:

thestar-form ing regions,with agesbetween 5 and

13 M yr,and no signi�cantage orm etallicity gra-

dient,acentralstarburstwith an interm ediateage

of30 M yr,and an underlying population ofstars,

with a surface brightness pro�le that can be de-

11

scribed by an exponential,with no colorgradients

and an agebetween 5 and 13 G yrs.

In allthe above studiesthe star-form ing knots

presentasim ilarrangeofages,consistentwith our

�ndings for M rk 370. However,the galaxies ap-

pearto bedi�erentwith respectto theproperties

oftheir underlying population ofstars: while i0

and com etary BCDs seem to be lessevolved sys-

tem s,the structuralproperties and ages derived

forthe LSB ofM rk 370 arequite sim ilarto those

presented by the iE BCD M rk 86. Thisfactm ay

im ply thatBCDsare objectswhich have in com -

m on the presence of an active starburst, which

dom inates their opticalproperties but,when the

characteristicsoftheunderlyingstellarcom ponent

aretaken intoaccount,they form am oreheteroge-

neousclass. Thispicture could naturally explain

the di�erent evolutionary stages derived for the

galaxies.

W earguethatconsideringBCD asaclassm ight

be then inappropriate.Therefore,looking forab-

soluteunifying scenarios| forinstance,trying to

link the whole BCD classwith the di�erenttypes

ofdwarfgalaxies (when only a fraction ofthem

could be actually connected) | m ay be specula-

tiveatthistim e.

O n the other hand,Noeske et al.(2000) sug-

gested that an evolutionary sequence m ight ex-

ist connecting the di�erent BCD subtypes, the

iI,C galaxiesbeing a possiblelink between theex-

trem ely young galaxy candidates (iO class) and

the m oreevolved iE/nE BCDs.

The inform ation available at the present tim e

doesnotperm ittodiscard anyofthesehypothesis.

Com prehensivestudiesoflargersam plesofBCDs

arerequired in orderto �nd outwhich scenario is

the correctone.

O ur kinem atic results suggest ordered m otion

around thenorth-south axisofM rk 370,thekine-

m atic center being located close to the center of

the externalisophotes;this could im ply that the

gas is rotating coupled with the m ain body of

thegalaxy (theunderlying population hosting the

starburst).However,we should take into account

that, because of our lim ited spectralresolution,

m ore com plex gas m otions could escape detec-

tion.In fact,�O stlin etal.(1999,2001),analyzing

high-resolution (velocity sam pling ’ 5 km sec�1 )

Fabry-PerotobservationsofBCD galaxies,found

that,in general,the velocity �eldsofthe studied

galaxiesareirregularand distorted.Spectroscopic

observationsofM rk 370 with high velocity resolu-

tion areneeded to bettertraceitsgaskinem atics.

Besides,M rk 370 present som e other puzzling

peculiarities:thepeak in thecontinuum m ap does

not coincide with the kinem atic center,nor with

the centerofthe outerisophotes,norwith one of

the knots;and the position angle ofthe outerre-

gions,seen in the R-band grey-scale m ap,di�ers

by about 30 degreesfrom the position angle dis-

played by the H� m ap.

7. Sum m ary and C onclusions

Broad-and narrow-band im ages ofthe BCD

M rk370,togetherwith long-slitandtwo-dim ensional

spectra, have been analyzed in order to derive

the propertiesofthe di�erentcom ponents ofthe

galaxy and to constrain its evolutionary status.

O urresultscan be sum m arized asfollow:

� Two di�erentstellarcom ponentsareclearly

distinguished in thegalaxy:thepresentstar-

burst and an underlying older population.

The current star form ation activity takes

placein num erousknots,aligned in a north-

eastsouth-westdirection.An extended LSB

com ponent,with regular appearance,hosts

thisstar-form ing area.

� The oxygen abundance derived from the

spectra isZ ’ Z� =2,a valuerelatively high

fortheBCD class;thehigh bluecontinuum ,

theabsorption wingsin theBalm erlinesand

the pronounced Balm er discontinuity in its

spectrum areclearindicatorsofthepresence

ofan evolved stellarpopulation.

� The starburst region is resolved into sm all

and com pactstar-form ingregions.W eiden-

ti�ed a totalofsixteen knots,and derived

their individual properties. The brightest

knots have lum inosities sim ilar to those of

giantH IIregionsin M 33 and in the LM C.

The photom etric param etersofthe individ-

ualstar-form ing knots were corrected from

the contribution ofem ission lines,internal

extinction and em ission from theunderlying

LSB com ponent. The colorsbefore and af-

terthecorrectionsaresigni�cantly di�erent.

12

In M rk 370 the internalextinction and the

contribution ofolderstars are m ore im por-

tant than em ission lines in shifting the ob-

served broad-band colors ofthe knots The

color shifts are function of the position of

theknotin thegalaxy,and westrongly cau-

tion againstinterpreting observed colorgra-

dientsasagegradientsbefore applying such

corrections.

W e found thatwe can reproduce the colors

ofallthe knots with an IB ofstar form a-

tion and the Salpeter initialm ass function

(imf)with an upperm asslim itof100 M � .

The agesofthe knotsrange between 3 and

6 M yrs.

� Theageoftheunderlying stellarpopulation

hasbeen estim ated by com paring itsoptical

colorswith twodi�erentgroupsofevolution-

ary synthesism odels. Colorsare consistent

with ageslargerthan 5 G yr.

� The inner, brightest star form ing-regions,

previously cataloged as double nucleus,are

norm al H II regions. W e would like to

stressthatthe"m ultiplenuclei"oftenseen in

broad-band im agesofBCD galaxiesm ay be

justthesuperposition ofo�-centerlum inous

H II regions. IfM rk 370 is not a double-

nucleus galaxy,then there m ay be no com -

pelling evidence that ithas experienced re-

centinteractions.Indeed,thereareno other

signs of interactions; the regularity of the

velocity �eld,and the fact that the central

knotssharethesam ekinem aticsofthem ain

body point against a recent m erger or in-

teraction event. Also,Noeske et al.(2001)

reportthatM rk 370 isan isolated galaxy.

� W e obtained the m ean ionized gas velocity

�eld in the centralpartofthe galaxy. The

velocity �eld seem s to be regular and sim -

ilar to that ofa rotating disk (west region

approaching,eastregion receding).

Allin all,spectrophotom etric studiesofBCDs

are ofparam ount im portance in order to derive

theirevolutionary status.Theseanalysesm ustin-

clude, in addition to the whole opticaldataset,

near-infrared photom etry (NIR) | colors in the

NIR bettertracethepropertiesoftheolderstellar

com ponent| aswellasdeep opticalspectropho-

tom etry ofthe host galaxy. Besides,high reso-

lution spectroscopy ofBCDs willhelp trace the

kinem aticsofthe galaxy and provide valuable in-

form ation on the m echanism s triggering the star

form ation.

Based on observationswith the JK T,INT and

W HT,operated on the island ofLa Palm a by the

RoyalG reenwich O bservatory in the Spanish O b-

servatorio delRoque de losM uchachosofthe In-

stituto de Astrof��sica de Canarias. Based also on

observationstaken atthe G erm an-Spanish Astro-

nom icalCenter, Calar Alto, Spain, operated by

the M ax-Planck-Institutf�urAstronom ie (M PIA),

Heidelberg, jointly with the spanish "Com ision

Nacionalde Astronom ia". W e thank the sta� of

both observatories.Thisresearch hasm adeuseof

the NASA/IPAC Extragalactic Database (NED),

which is operated by the Jet Propulsion Labora-

tory, Caltech, under contract with the National

Aeronauticsand SpaceAdm inistration.

W ethankJ.Iglesias-P�aram oandJ.N.G onz�alez-

P�erez for their help in the initialstages of this

project.W ethank A.Vazdekis,P.Papaderosand

K .Noeskeforvaluablecom m entsand discussions.

W e also acknowledge the anonym ous referee for

his/her helpful com m ents which helped us im -

provethispaper.

This work has been partially funded by the

spanish \M inisterio de Ciencia y Tecnologia"

(grants AYA2001-3939 and PB97-0158). L.M .

Cair�os acknowledges support by the EC grant

HPM F-CT-2000-00774.

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15

Table 1

Basic data of M rk 370

G alaxy O therdesignations R.A. (1950) Dec. M B D (M pc) M 1H I

M 1T

M rk 370 NG C 1036, 02h 37m 40s 19� 050 0100 � 17:20 12.9 0.36� 109 4:2� 109

UG C 02160

N ote.| (1)Neutralhydrogen m assM H I and totalm assM T in unitsofM � ;both from Thuan and M artin

(1981).

Table 2

Log of the observations

Date Telescope Instrum ent Filter/grism Exposuretim e (s)

O ct.98 JK T 1.0m Cass.focus U 2400

O ct.98 JK T 1.0m Cass.focus B 1500

O ct.98 JK T 1.0m Cass.focus V 1000

O ct.98 JK T 1.0m Cass.focus R 800

O ct.98 JK T 1.0m Cass.focus I 1200

Dec.97 CAHA 2.2m CAFO S 6569 (113) 5400

Dec.97 CAHA 2.2m CAFO S 6462 (98) 5400

Sep.98 INT 2.5m IDS R300V 1800

Aug.99 W HT 4.2m INTEG RAL 600g/m m 3600

N ote.| JK T = Jacobous K apteyn Telescope, O RM (La Palm a),Spain

(detector:Tek1024� 1024,0:0033/pixel).CAHA = CentroAstron�om icoHispano

Alem �an,Alm er��a,Spain(Site-1D 2048� 2048,0:0053/pixel)/INT = IsaacNewton

Telescope,O RM (La Palm a),Spain (EEV10 4100� 2048 13:5� pixels).W HT

= W illiam HerschelTelescope,O RM (La Palm a),Spain (Tex 1024� 1024 24�

pixels).

16

Table 3

R eddening corrected line intensity ratios

Line Ion f(�) A B S

(�A) F� � W � F� � W � F� � W �

3727 [O II] 0.26 2:74� 0:06 35:1� 0:9 3:22� 0:09 136 � 10 2:68 � 0:04 48 � 1

3869 [NeIII] 0.23 0:50� 0:07 5:1� 0:8 1:1 � 0:2 330 � 166 0:49 � 0:06 7 � 1

4101 H� 0.18 0:28� 0:03 2:1� 0:4 0:24� 0:04 7 � 1 0:26 � 0:02 2:5� 0:3

4340 H 0.14 0:45� 0:02 3:0� 0:2 0:50� 0:02 14:0� 0:8 0:49 � 0:07 5 � 1

4861 H� 0 1:00� 0:01 12:6� 0:1 1:00� 0:02 26:9� 0:8 1:00 � 0:01 16:0� 0:3

4959 [O III] � 0:02 0:53� 0:01 6:1� 0:2 0:60� 0:02 15:5� 0:6 0:52 � 0:01 7:6� 0:1

5007 [O III] � 0:03 1:60� 0:02 19:2� 0:2 1:93� 0:03 51:2� 0:6 1:63 � 0:02 25:0� 0:2

5876 HeI � 0:23 0:06� 0:01 0:8� 0:1 0:10� 0:01 3:5� 0:3 0:069� 0:006 1:3� 0:1

6548 [N II] � 0:34 0:14� 0:01 2:5� 0:2 0:11� 0:01 4:2� 0:6 0:149� 0:008 3:4� 0:2

6563 H� � 0:34 2:85� 0:05 56:8� 0:6 2:93� 0:09 112 � 2 2:86 � 0:03 68:1� 0:7

6584 [N II] � 0:34 0:50� 0:02 10:1� 0:5 0:49� 0:02 24 � 2 0:476� 0:009 11:6� 0:3

6678 HeI � 0:35 0:04� 0:02 0:8� 0:4 0:06� 0:03 3 � 1 0:030� 0:008 0:7� 0:2

6717 [S II] � 0:36 0:50� 0:01 10:0� 0:2 0:45� 0:02 21:3� 0:9 0:471� 0:008 11:5� 0:2

6731 [S II] � 0:36 0:38� 0:01 7:4� 0:3 0:34� 0:02 17 � 2 0:365� 0:007 9:0� 0:2

7135 [ArIII] � 0:41 0:11� 0:01 2:4� 0:4 0:10� 0:01 4:9� 0:8 0:093� 0:006 2:5� 0:2

C (H�) 0.31� 0.02 0.39� 0.03 0.285� 0.004

W abs(�A) 1.3 0.0 1.4

F (H�) 2.67� 0.0014 1.55� 0.12 3:87� 0:06

N ote.| Reddening-corrected lineintensities(norm alized to H� = 1)fortheregionsextracted from thelong slit

spectrum ofM rk 370.Theintegrated spectrum S ispresented in therightm ostcolum ns.Balm erlinesarecorrected

from underlying stellarabsorption.Thereddening coe�cient,C (H�),thevalueoftheabsorption correction,W abs,

and the m easured H� ux,F (H�)(� 10�14 erg cm �2 sec�1 )arealso included.

17

Table 4

H � photometry of the individual knots in M rk 370

K not Flux(H�) logL(H�) logN L ym log(M H II=M � ) � W (H�) � W (H�)corr

1 5.70 38.05 49.92 3.13 201 364

2 1.96 37.59 49.46 2.67 170 534

3 5.08 38.00 49.87 3.08 138 260

4 8.01 38.20 50.01 3.28 173 329

5 4.13 37.91 49.78 2.99 45 71

6 0.68 37.13 49.00 2.20 94 � � �

7 1.64 37.51 49.38 2.59 38 116

8 37.30 38.87 50.74 3.95 58 63

9 7.29 38.16 50.03 3.24 81 119

10 4.56 37.96 49.82 3.03 114 578

11 147.07 39.47 51.33 4.64 373 533

12 232.17 39.66 51.53 4.96 192 226

13 2.32 37.66 49.53 2.74 71 � � �

14 33.31 38.82 50.69 3.90 237 579

15 4.01 37.90 49.77 2.98 73 223

16 5.39 38.03 49.90 3.11 220 656

N ote.| The data are corrected from interstellarextinction and from [N II]em ission. The

equivalentwidthsareshown beforeand aftercorrection from thecontribution oftheunderlying

stellarem ission to the underlying continuum (exceptknots6 and 13). H� uxesare in 10�15

erg cm �2 sec�1 units;H� lum inositiesin erg sec�1 units.W (H�)in Am strongs

18

Table 5

Broad-band photometry of the individual knots detected in M rk 370.

K not B U � B B � V V � R V � I B c (U � B )c (B � V )c (V � R)c (V � I)c

1 20.19 � 0:57 0.39 0.25 0.88 19.67 � 0:94 0.05 0.04 0.24

2 21.01 � 0:56 0.33 0.38 1.01 20.70 � 1:05 � 0:14 0.23 0.26

3 20.15 � 0:45 0.47 0.39 1.00 19.69 � 0:88 0.12 0.21 0.47

4 19.88 � 0:54 0.42 0.44 1.02 19.43 � 0:97 0.05 0.30 0.47

5 19.28 � 0:20 0.53 0.38 0.96 18.76 � 0:57 0.24 0.17 0.45

6 21.82 � 0:23 0.67 0.31 1.41 22.24 � 1:54 0.14 � 0.04 1.20

7 20.11 � 0:09 0.60 0.41 1.07 19.88 � 0:74 0.24 0.17 0.46

8 16.80 � 0:42 0.28 0.29 0.66 16.04 � 0:55 0.06 0.10 0.33

9 19.17 � 0:20 0.50 0.40 0.94 18.56 � 0:47 0.24 0.18 0.55

10 20.00 � 0:24 0.57 0.47 1.13 19.73 � 0:85 0.21 0.24 0.66

11 17.46 � 0:65 0.33 0.39 0.69 17.37 � 0:79 0.19 0.28 0.46

12 15.77 � 0:45 0.31 0.33 0.66 14.80 � 0:60 0.02 0.05 0.21

13 20.13 � 0:11 0.56 0.48 1.06 19.97 � 0:84 0.12 0.10 0.43

14 18.43 � 0:56 0.35 0.41 0.82 17.90 � 0:92 � 0:01 0.14 0.17

15 19.81 � 0:45 0.42 0.41 1.25 19.61 � 1:14 � 0:15 0.26 0.76

16 20.50 � 0:48 0.32 0.47 0.84 20.21 � 1:08 � 0:28 0.45 � 1.68

N ote.| Colum ns 2 to 6: only the G alactic extinction correction has been applied;colum ns 7 to 11: values

corrected from interstellarextinction,contribution ofem ission lineand em ission from theunderlying hostgalaxy.

19

Table 6

Structural parameters characterizing the LSB host.

Band �0 � M SB M host R SB =host

B 22.56 1.06 � 16:69 � 16:13 1.68

V 21.74 1.05 � 16:91 � 16:93 0.98

R 21.04 0.96 � 17:32 � 17:44 0.89

I 20.57 1.14 � 17:23 � 18:28 0.38

N ote.| Colum ns 2,3: centrals surface brightness

(m ag arcsec�2 ) and scale-length (kpc); colum n 3,4:

absolute m agnitude ofthe starburst and ofthe host

LSB in each band; colum n 5: the ratio ofthe lum i-

nosity ofthe starburstoverthe lum inosity ofthe un-

derlying com ponent. As m entioned in the text, the

U -band im age wastoo shallow to reach outthe outer,

starburst-freeregions.

Table 7

D erived ages (in M yr) for the individual star-forming knots in M rk 370

K not AgeU �B AgeE W (H�) K not AgeU �B Age

E W (H�)

1 3.2{4.8 4.8{5.1 9 5.6{6.4 5.9{6.3

2 3.1{3.8 4.6{4.8 10 4.5{5.0 4.6{4.8

3 3.5{5.0 5.0{5.5 11 4.6{5.1 4.6{4.8

4 3.2{4.7 4.8{5.2 12 5.1{5.9 5.1{5.7

5 5.1{6.0 6.4{7.0 13 4.5{5.1 |

6 | 6.1{6.6 14 3.2{4.8 4.6{4.8

7 4.8{5.3 5.9{6.3 15 2.2{3.3 5.1{5.7

8 5.3{6.2 6.4{7.0 16 2.9{3.5 4.3{4.7

20

Fig.1.| (left)Im ageofM rk 370 through theH�

�lter, taken with the 2.2m telescope at CAHA.

Thespatialdistribution ofthe�bers(SB2)on the

focalplane has been overlaid. (right) Fibers in

the centralarray. Num bers indicate the actual

position ofthe �bersatthe slit.

Fig. 2.| Contour plot of the continuum sub-

tracted H� im age ofM rk 370. The slit position

is indicated, and the two subregions selected in

the long-slitspectrum are m arked a and b. The

individualstar-form ing regionsarelabeled.North

isatthe top and Eastison the left.

Fig.3.| Spectraofregionsa and b,and thetotal

integrated spectrum (region s).

21

Fig.4.| [from leftto right,top to bottom ](a)M rk 370 continuum m ap obtained from the2D spectroscopic

data by integrating the signalin the indicated spectralinterval;(b)V �lterim age;(c)Intensity m ap ofthe

H� em ission lineobtained by �tting a singlegaussian to 2D spectroscopy data;(d)H� �lterim agefrom the

2.2m CAHA;(e)Two-dim ensionaldistribution of[O III]/H� ratio;(f)Two-dim ensionaldistribution ofthe

extinction coe�cientC (H�).A crossm arkstheopticalnucleusin each m ap.Thegeom etricalcenterofthe

outerisophotesisindicated by a star.

22

Fig. 5.| Velocity �eld ofthe ionized gas in the

centralregion ofM rk 370,determ ined by averag-

ing m easurem entsfrom di�erentem ission lines.A

black crossm arks the opticalnucleus. The kine-

m atic center is indicated by an X.The center of

the outerisophotesisindicated by a black star.

Fig.6.| Radialdependenceofthesystem icveloc-

ity V sys,am plitudeofthevelocity �eld projection

on the galacticplane,and position angleofthe

m ajorkinem aticalaxis�. Errorbarsonly re ect

the localrm s in the m easurem ents,and do not

re ect the actualuncertainties, which are likely

a�ected by system aticerrors.

Ηα ΝΕΤ R BAND

Fig. 7.| left panel: Continuum -subtracted H�

grey-scale m ap ofM rk 370;right panel: R-band

im age.North isatthetop and Eastison theleft;

axisunitsarearcsec.

(U-B)

EQ

W(H

)

α

Fig. 8.| Equivalent width ofthe H� line ver-

sus (U � B ). W e also plot the tracks ofan in-

stantaneous burst ofz = 0:008,aged from 1 to

30 M yr (Leitherer et al. 1999),for three imfs:

solid line, �= 2.35, M up= 100M � ; dotted line:

�= 3.30, M up= 100M � ; short-dash line, �= 2.35,

M up= 30M � . Triangles represent the observed

(uncorrected)param etersofthe individualknots;

squares representthe knots param eters after ap-

plying the corrections from reddening and from

the contribution ofem ission linesand olderstars.

23

(B-V)

(U -

B)

Fig. 9.| (U � B )versus (B � V ). The track of

an instantaneousburstofz = 0:008,aged from 1

to 30 M yr (Leitherer et al. 1999),are also plot-

ted,for three di�erent imfs: solid line,�= 2.35,

M up= 100M � ;dotted line:�= 3.30,M up= 100M � ;

short-dash line, �= 2.35, M up= 30M � . Sym bols

arethe sam easin the previous�gure

24

8. A P P EN D IX A

8.1. A tlas ofSpectra

Thenuclearspectrum ofM rk 370 derived from

theINTEG RAL data isshown in �gure10,in the

whole wavelength range observed. W e can easily

recognizeseveralem ission lines.Hereweonly plot

som e ofselected em ission as spectrum diagram s

(�gures11 through 14). Spectrum diagram srep-

resentlinepro�lesfrom individualspectra ateach

pointin theobservedregionand in asm allspectral

range. The spectra at each location are norm al-

ized by the localpeak so asto betterdisplay the

pro�le shape (linesnearerto the brightknotsare

brighterthan thosefartherout).Fig.10.| Nuclearspectrum ofM rk 370,obtained

by averaging the7 �bersclosestto thecontinuum

m axim um (r< 100:2).

Fig. 11.| Spectra distribution diagram s ofthe

central1600� 1200 ofM rk 370. Two-dim ensional

distribution oftheH� lines(theabsorption wings

are visible). The spectralrange plotted is 4795{

4950 �A.

25

Fig. 12.| Two-dim ensional distribution of

[O III]�4959+ [O III]�5007 (4950{5045 �A).

Fig. 13.| Two-dim ensional distribution of

H�+ [N II]��6548;6584 (6550{6640 �A).

Fig. 14.| Two-dim ensional distribution of

[S II]��6716;6731 (6720{6785) �A).

26