Dwarf LSB galaxiesDwarf LSB galaxiesin the Virgo clusterin the Virgo cluster

Jonathan Davies

CDM and Dwarf galaxiesCDM and Dwarf galaxies

Simulationof Dark Matter

ObservationsofBaryons

Dwarf galaxy - 1 fluctuation of 106 M

collapsing at z=6 (13 Gyr)

Virgo cluster- 1 fluctuation of 1014 M

collapsing at z=0

Cardiff and the sub-structure problemCardiff and the sub-structure problem

Does there exist a large population of dwarf galaxies that have gone undetected ?

Optical Surveys HI surveys

Dwarfs are ‘cursed’1. Low luminosity (-14<MB<-10)2. Low surface brightness (23<o<26 B )3. Low HI mass (dE/dSph) (<107 M)

Image Detection and SelectionImage Detection and Selection

The algorithm we have developed is a Fourier convolutionmethod using matched filters to enhance faint structures

Numerical simulation of Virgo and the backgroundto derive the selection criteria

-14<MB<-1023<o<26 B

Virgo and other EnvironmentsVirgo and other Environments

Virgo

EquatorialStrip

Sabatini et al., (2003)MNRAS, 341, 981

Roberts et al., (2004)MNRAS, in press

Roberts et al., (2004)in prep

The Virgo ClusterThe Virgo ClusterWe detect 20 gal/sq/deg in the range -14<MB<-1023<o<26 B

Surface density v radialdistance from M87

The selection criteria pick out the cluster population

The Luminosity functionThe Luminosity function

Virgo luminosityfunction

Norberg et al., 2002Sabatini et al., 2003

The resultThe result

Virgo - 20:1 (Fornax - 20:1)Field < 5:1 (Ursa Major - 5:1)

Local Group - 5:1Field Luminosity function - 7:1

CDM – 350 (=-1.6), 8000 (=-2.0)

1. There is no LSB field dwarf galaxy population that has been missed by the redshift surveys. 2. There is a large dwarf galaxy population in nearby rich clusters.

The dwarf to giant ratio.N(MB<-19)/N(-14<MB<-10)

An HI survey of the Virgo ClusterAn HI survey of the Virgo ClusterAutomated galaxy detection in HIJASS dataAutomated galaxy detection in HIJASS data

Survey limitsMass 5x107 M

Column Density 6x1018 atoms cm2

Virgo HIdetections

Davies et al., (2004)

The HI mass functionThe HI mass function

Davies et al., 2004

Why are the luminosity function and HI mass functionWhy are the luminosity function and HI mass functionso different in different environments ?so different in different environments ?

1. Dwarfs unable to form after re-ionization, z20 (13 Gyr)!

2. A feedback mechanism.

3. Galaxy ‘Harassment’.

4. Change the initial conditions or the nature of the dark matter particles.

‘Squelching’

Suppressesdwarf

formation everywhere

Ad hocadditionto CDM

?

CommentsCommentsLots of DM halos around (no gas or stars) ?Difficult physics to model (from large scale structure to details)

Investigating the nature of dwarf Investigating the nature of dwarf galaxies in different environmentsgalaxies in different environments

Typical examples

1. Cluster galaxies typical of Local Group dSph.2. Field galaxies typical of Local Group dI.

Galaxy colour and environmentGalaxy colour and environment

Galaxy colour changessystematically with

environment

(B-I) against number

Field

Ursa Major

Virgo

Fornax The UKIDSS survey

Arecibo observations ofArecibo observations ofdwarf galaxy detectionsdwarf galaxy detections

1. Virgo cluster 3 detections out of 107 observed to a mass limit of 2x107 M. Consistent with them being mostly dE/dSph galaxies.2. Field 4 detections out of 14 to a mass limit of 2x107 M. Consistent with them being mostly dI galaxies.

Sabatini et al. submitted

Dark Matter dominated ?Dark Matter dominated ?

Draco has (M/LB)=440 !

Kleyna et al., 2002, MNRAS, 330, 792

Draco

Cluster galaxy evolutionCluster galaxy evolution

1. Ram pressure stripping )8.26(4.02

10

gal

ICM

B

dyn v

L

M

Destroyed if they enterthe cluster core ?

2. Tidal interactions (Roche limit)

3. Supernovae driven winds

3/2

2 3

dwarf

gal

M

Mr

vtnN

1010 M galaxy 30 times more likely to interact with another 1010 M than with a 108 M galaxy. Dwarf galaxy tidal stripping interactions are rare

A viable gas strippingmechanism ?

Pressure confinement ?(Babul and Rees, 1992,

MNRAS, 255, 346) 5

3

)8.26(4.03

10

102

1010

SN

B

dynW

B

dynW

T

L

MT

rL

MT

Gas consumed by accelerate evolution? – need star formation histories

What about Harassment ?What about Harassment ?

Cluster

GalaxyCRr

r 7 kpc much largerthan the galaxies wedetect (r 1 kpc).

Moore et al., 1998, ApJ, 495, 139

Arecibo observations ofArecibo observations ofHIJASS detections withHIJASS detections withno (obvious) opticalno (obvious) opticalcounterpartcounterpart

VIRGOHI21 VIRGOHI27

The AGES survey

GMRT and INT observationsGMRT and INT observations

Faint optical source found forVIRGOHI27

butVIRGOHI21 ?

1. Redshift surveys have not missed a large population of LSB field dwarf galaxies.

2. There are a large number of LSB dwarf galaxies in rich clusters.

3. Does not appear to be a large population of HI clouds with no optical counterparts.

4. Dwarf (mainly dE/dSph) galaxies with red colours found in large numbers in the cluster. They are gas poor, but are they stripped of their gas?

5. Low numbers of dwarf (dI) galaxies with blue colours and gas rich found in the field - morphology density relation.

6. The cluster cannot have just been made up of in-falling ‘Local groups’ the dwarf to giant ratio is too large – there are ‘cluster’ (tidal?) dwarfs.

7. Cluster luminosity function steep, HI mass function shallow compared to the field - gas more efficiently converted into stars ?

8. The galaxies we detect are too small to have been the result of harassment.

Summary