Lecture Thirteen:

Tuesday 29th March

High redshift observations (contd) !

Longair chap. 17, 19

Daddi et al 2004 Ap J 617, 746

apparently less-biased technique for finding all

galaxies 1.4<z<2.5

sBzK: star forming galaxies

pBzK: quiescent galaxies

(z-K)

(B-z) WHERE DO THESE FIT IN?

`BzK’ selection of passive & SF z>1.4 galaxies!B, z & K photometry

Reddy et al 2005 Ap J 633, 248

Stellar mass distributions overlap indicating primary difference is current SF

log stellar mass

K

Passive 1.6<z<2.9 LBG 1.5<z<2.9

Are SF and passive z"2 populations distinct?! The Spitzer Space Telescope Revolution A modest 60cm cooled telescope can see the most distant known objects and provide crucial data on their assembled stellar masses!

IRAC camera has 4 channels at 3.6, 4.5, 5.8 and 8 µm corresponding to 0.5-1µm at z~7!

•! Egami et al (2005) - characterization of a lensed z~6.8 galaxy

•! Eyles et al (2005) - old stars at z~6

•! Yan et al (2005) - masses at z~5 and z~6

•! Mobasher et al (2005) - a galaxy > 1011 M! at z~7?

Spitzer detections of i-drops at z=6 #1 z=5.83 #3 z=5.78

•!4 i-drops in GOODS-S confirmed spectroscopically at Keck

•! Ly ! emission consistent with SFR > 6 M!

yr-1

•! IRAC detections from GOODS Super-Deep Legacy Program

Eyles et al (2005) MNRAS 364, 443

Spectral Energy Distributions of i-drops #1 z=5.83 #3 z=5.78

Spitzer + Ly! emission constrains present & past star formation

Ages > 100 Myr, probable 250-650 Myr (7.5<zF<13.5)

Stellar masses: 2-4 "1010 M! (>20% L*)

VLT K

VLT K

Independent z"6 UDF Spitzer analysis

Yan et al (2005) ApJ 634, 109

(J – H)AB > 1.3 plus no detection in combined ACS

strong K/3.6µm break # potential high mass z~7 source

Mobasher et al (2005) ApJ 635, 832

Spitzer detection of resolved J-drop in UDF!

Spitzer Studies of Massive Red Galaxies (J-K>2.3)

Papovich et al. (2006) ApJ, 640, 92

K-selected sample of 153 DRGs z<3; many with M>1011 M! ; 25% with AGN

Specific SFR (including IR dust emission) ~2.4 Gyr-1; >> than for z<1 galaxies

Witnessing bulk of SF in massive galaxies over 1.5<z<3

Stellar mass Specific SFR (/mass) Cosmic mean density of haloes vs. halo mass Abundance of

massive galaxies at

z~6 with $CDM in

terms of their

implied halo masses,

assuming

•! Scalo IMF

•! SF efficiency 20%

Find a 1013 M! halo

in the tiny UDF is a

problem!

Yan et al

Eyles et al

Barkana & Loeb (2006) MNRAS, 371, 395

z = 5.8

Mobasher et al

z = 15

z= 15 10 8 6.5 5.8

Summary

•! Great progress using v,i,z,J-band drop outs to probe abundance of SF galaxies from 3<z<10: Bouwens et al (2006) ApJ, 653, 53 discuss the properties of 506 I-band dropouts!

•! In practice, these samples are contaminated by foreground stars, z~2 galaxies etc to an extent which remains controversial. We are unlikely to resolve this definitively with spectroscopy until era of ELTs.

•! Comoving SF rate declines from z~3 to z~6 (and probably beyond) suggesting insufficient 6<z<10 luminous galaxies to reionize Universe

•! Contribution of lower luminosity systems less clear

•! Spitzer’s IRAC can detect large numbers of z~5-6 galaxies and it seems many have high masses (one spectacularly so!) and signatures of mature stellar populations - implies earlier activity

•! Reconciling mature galaxies at z~6 with little evidence for SF systems with 7<z<10 may turn out to be a very interesting result

Cosmic Star Formation history

Cosmic Star Formation History Various probes of the global SF rate: %

* (z) M

! yr-1 comoving Mpc-3

•! UV continuum (GALEX, LBGs)

•! H! and [O II] emission in spectroscopic surveys

•! mid-IR dust emission

•! 1.4GHz radio emission

No simple `best method’: each has pros and cons (dust extinction, sample depth, z range and physical calibration uncertainties)

Each has different time-sensitivity to main sequence activity so if SFR not uniform do not expect same answers for the same sources

Would expect the integral of the past activity to agree with locally-determined stellar density (Fukugita & Peebles 2004)

Can also determine the stellar growth rate for comparison with the stellar mass assembly history (next lecture)

e.g., Hopkins 2004 Ap 615, 209

Cosmic SFH: Calibration Kennicutt (1998) ARAA, 36, 189

1.! UV continuum (1250-2500 Å) : Pro: Extensive datasets over 0<z<6: easily calibrated via MS models

M>5M!, timescales >108 yr, calibration largely independent of l Con: dust! (A < 3 mag); IMF-dependent

2. Line emission (H!, [O II] : Pro: Very sensitive probe, available to z~2: M>10M!timescales <106 yr, Con: uncertain fesc of ionizing photons; strong IMF-dependence ("3), excitation uncertainties [OII]

3. Far IR emission (10-300 µm) : Pro: Independent method, available for obscured sources to high z: Con: uncertain source of dust heating (AGN/SF?); age of stellar popn,

primarily applicable in starbursts, bolometric FIR flux required

Some Popular Dust Extinction Laws Evolution of SFR density with redshift - standardized all measures to same IMF, cosmology, extinction law

-! integrated LF over standardized range for each diagnostic (except at v high z)

%*&(1+z)3.1 (z<2)

Fossil record

decline?

Star formation rate per unit comoving volume

Hopkins 2004 Ap J 615, 209

Colour coded by rest-frame wavelength: Blue UV; green [O II]; red H!, H'; pink X-ray, FIR, submillimeter, and radio

Radio LF

Implications of Cosmic SFH

Hopkins & Beacom (2006) ApJ, 651, 142

GALEX, SDSS UV

Spitzer FIR

ACS dropouts

Fitting parametric SFH can predict %* (z) in absolute units

Cole et al 2dF

•! Satisfactory agreement with local 2dF/2MASS mass density

•! Data suggests half the local mass in stars is in place at z~2 ± 0.2

•! Major uncertainties are IMF and luminosity-dependent extinction

Star formation history Mass assembly history

Spitzer!

SFH measurements

Hopkins (2006) astro-ph/0611283

The level of (unobscured) SFH required for reionisation (Madau et al. 1999).

•! How effective are the various high z selection methods?

- L*(z=6) # i~26 where spectroscopy is hard

- spectroscopic samples biased to include strong L!

- great reliance on photometric redshifts

•! Is there a decline in the UV luminosity density 3<z<6?

- results are in some disagreement

- differing trends in continuum drops & L! emitters

•! Is the observed %UV at z>6 sufficient for reionization?

- contribution from (unobserved) faint end of LF?

- unusual popns: intense EW(L!), steep UV continua?

•! Significant stellar masses for post-burst z~6 galaxies

- how reliable are the stellar masses?

- inconsistent with declining SF observed 6<z<10?

- does this imply an early intense period of activity?

Some Key Issues !

Backgrounds

Role of Extragalactic Background The study of cosmic backgrounds have played a key role in astrophysics:

-! Identifying redshift distribution of active galactic nuclei dominating X-ray background

-! Identifying contribution of bright, resolved SCUBA sources to sub-mm background

In each case, have to separate contribution of counts of resolved sources to some detectability limit with the measured integrated extragalactic background.

Key issues:

-! contribution of resolved sources

-! removal of spurious foregrounds

some reviews:

Leinert et al (1998) A&AS, 127, 1 - good technical intro Hauser & Dwek (2001) ARAA, 39, 249 - post-COBE Kashlinksy (2005) Phys. Rep, 409, 361 - pre-Spitzer Soifer, Helou & Werner (2008) ARAA, 46, 201 - general Spitzer review

Background Radiation

Oldest problem in cosmology: why is the sky dark at night? Olber’s paradox

Background provides us with general information about large scale distribution of matter & radiation in the Universe.

The relation between observed source counts and background population of sources with differential source count dN &S-('+1) dS is:

There is thus a critical value '=1 for the slope of the integral source counts

Most background radiation originates from '=1

In real world models '=1 at z~0.5, which means the bulk of background radiation in sources originates from z<<1 (for a uniform population).

A uniform population in Euclidean space has '=1.5

Evaluating background due to discrete sources Assume power-law spectra, S & (-!

flux density-luminosity relation The number of sources per steradian in the increment of comoving radial distance coordinate dr for a uniform source distribution

Thus the background intensity I((0) due to this uniform distribution of sources is:

For Friedman world models:

always finite integral (converges ! > -1.5 )

I(ν0) =L(νo)N0

4π

∫ ∞

0dr

Background intensity

For a critical model ()0 =1; )$ = 0; ! =1), the background intensity out to redshift z is:

Half the background intensity originates at z * 0.31

Thus somewhat contrary to what might be thought background radiation does not probe the vey early universe, but most of it origniates from z<0.5. This makes it relatively easy to identify the sources.

This is not true if the properties of the sources have evolved strongly with z.

Radio Background The effects of evolution

Evolution of co-moving luminosity function

Simplest forms of evolution of luminosity function which give a reasonable fit to the observed number counts and the red-shift data

The integrated background emission from a population of sources which locally has luminosity L0 and space density N0

Performing the integral:

This agrees with observations - if the effects of evolution were neglected, the background brightness temperature would be extremely small.

Contributions to background from redshifts 0 to zm and from zm

to infinity are in the ratio 5[1-(1+zm)-1/2] : 1

zm= 2 means the background intensity is 5.4 times greater than no evolution case

In this case the bulk of the background comes from z~2 because of very strong effects of evolution.

If the LF of sources evolved, the background intensity is:

(1 + z)1+α

I(ν0) =25

cH0

(N0L0

π

)L(z) = L0

X-ray Background Background Measurements

Measurements •! Key issue is extent to which measured value (or limits) on background lie in excess of integrated counts as this provides evidence for additional populations (e.g. Pop III)

•! Tentative evidence for excess lies in 0.3< +<10µm region only

•! Problem is these background measures are notoriously difficult requiring absolute calibration and removal of spurious foregrounds; claims remain controversial

•! Early work in optical as example:

Mattila (1976): dark Galactic cloud as zero level

Dube et al (1979): first attempt to remove foregrounds

Bernstein et al (2002): HST + ground-based spectra

Contaminating Optical/IR Foregrounds •! Airglow: emission in upper atmosphere (90km) which has time dependent structure on fine scales due to `seeing’

•! Zodiacal light: scattering of sunlight by interplanetary dust, varies due to motion of Earth on ecliptic plane, extends to '~30˚

•! Faint galactic stars: although counts rapidly decline in region of interest, difficult to remove for mid-IR experiments with low angular resolution

•! Diffuse Galactic light (`cirrus’): gas clouds illuminated by starlight

Can attempt to remove by careful design of experiment:

-! Use of HST avoids airglow

-! Zodi background should show seasonal variation & its spectrum in optical/near-IR is consistent with solar

-! Galactic cirrus can be minimized/monitored via Galactic latitude of fields

Ecliptic

•! Airglow varies continuously so simultaneous measures crucial

•! Zodi light requires both a 3-D Interplanetary model and a scattering model so that time dependence can be evaluated

Airglow and Zodiacal Light! Contaminating Optical/IR Foregrounds

Kashlinsky (2005) Phys.Rep, 409, 361

Zodi

Cirrus

Airglow

Case Study OPTICAL DETECTION Claim a significant optical detection in excess of known sources at

+ = 300, 550 and 800nm

Experiment design:

-! Fields observed with HST WFPC-2 and FOS (avoiding airglow)

-! Sources removed to VAB~23 (Galactic stars easily identified by HST)

-! Zodi spectrum measured simultaneously with ground-based telescope & iteratively subtracted according to model assuming solar spectrum

Illustration of the problem at + = 550nm

Total WFPC-2 background: 105.7 ± 0.3 (" 10-9 cgs sr-1 Å-1) Zodi background: 102.2 ± 0.6

Diffuse Galactic: 0.8 Galaxies V<23: 0.5

Claimed excess: 2.7 ± 1.4 (1,) Lower limit in 23<V<27: 0.89

Bernstein et al (2002a,b,c)

Contribution of resolved sources

Nominal background is quoted excluding VAB<23 sources but the likely contribution of 23<VAB<28.5 sources is estimated

WFPC-2 image WFPC-2 counts

Estimating Zodiacal Contribution

Simultaneous long-slit spectrum in WFPC field

Solar spectrum

Cross-correlation of night sky spectrum with solar template used to estimate absolute Zodi contribution.

Method disputed due to unclear correction for extinction and atmospheric scattering

DIRBE detections •! Early detections (>3,) in selected fields at high Galactic and ecliptic latitudes at 140 and 240µm only, using elaborate time dependent Zodi model (Kelsall et al 1998, Hauser et al 1998).

•! Schlegel et al (1998) combined with higher resolution IRAS 100µm maps to improve removal of Galactic cirrus and dust emission, 100/240µm ratio used to give spatial distribution of dust temp.

•! Finkbeiner et al (2000) extended the DIRBE Zodi model to provide first detections at 60 and 100µm

•! Wright (2000) used 2MASS to improve Galactic source removal and adopted the DIRBE Zodi model to claim the first detection

at 2.2µm

Key issue in all of the above is the reliability of the Zodi model

DIRBE Results IRTS DIRBE DIRBE

FIRAS

Galaxy counts

Residual DIRBE map at 240µm

+ (µm)

DIRBE dust

DIRBE+IRAS

Dole et al (2006) A&A, 451, 417

Summary: background Measurements

Excess over counts only significant in optical (marginal) & near-IR. Recent stacked MIPS data now resolve >70% of mid-IR EBL

Dole et al (2006) A&A, 451, 417

Optical Background (COB)

Infra-Red Background (COB)

II5 IR photons emitted for every optical one

A model based on very massive Pop III stars forming until z=8.8 can fit this excess (Salvaterra & Ferrara 2002).

BUT – this model then over predicts the number of J-drop out galaxies that should be detected (Salvaterra & Ferrara 2006).

Dole et al (2006) A&A, 451, 417

brightness nW m-2 sr-1

Galaxy formation & evolution provides 5% in brightness to the EM content of Universe: half in starlight (optical) and half as dust reprocessed starlight (IR)

Radio, UV, X-ray, --ray background 10-30 times less than optical/IR Modeling the Near-IR Excess

Suppose DIRBE/2MASS near-IR excess is real: is it consistent with likely Pop III predictions and high z observations?

•! For the J-band EBL > 2.5 nW m-2 sr-1 (Cambresy et al 2001)

•! Assuming J-band flux arises from z~9 Ly! emission, Madau & Silk (2005 MNRAS 359, 37) calculate stellar mass produced by the associated star formation

•! Find %* = 2.7 108 M!

Mpc-3, corresponding to )* = 0.045 )b. i.e. almost all stars need to be produced by z~9 to explain the background!

•! Likewise, the ionizing flux associated with such SF would be in excess of that required to explain the WMAP optical depth .E

•! Salvaterra & Ferrara (2006, MNRAS, 367,11) fail to find SF examples in high z J-dropouts and Ly! emitters and argue that if the excess came from z>8 sources, it would have strong implications for deep IRAC counts.

Bottom line: seems hard to account for such a strong excess

High Energy Stereoscopic System (HESS) Windhoek, Namibia

Gamma ray

~ 10 km

Particle shower

~ 1o

~ 250 m

Array of 4 telescopes detecting Cerenkov radiation

High energy gamma rays are absorbed and converted into secondary particles forming an ‘air shower’. Cerenkov light is generated, a faint beam of blue light, which on the ground illuminates an area of about 250 m in diameter. The faint flash last a few billionths of a second.

Gamma rays interact with 1-10 µm IR photons via pair creation process producing absorption features in distant sources (e.g. blazars). Strength of the absorption indicates the ambient IR photon background (EBL) – upper limit to EBL gives lower limit to transparency of Universe to very high energy --rays.

New Constraints from TeV Gamma Rays! Summary

•! background measurements in principle offer an important constraint on undiscovered populations

•! However, the observations remain challenging and controversial because of instrumental effects and dominant foregrounds

•! Only in the 1-10µm region have positive detections been claimed,

•! IRAC is a particular promising instrument for the redshift range 10<z<20 - therefore worth continuing to improve situation

•! Fluctuation analyses minimize some contaminating components (but it is hard to interpret the signals)

•! Ultra high energy TeV spectra of distant blazars may provide a sensitive upper limit to the infrared background 1-10µm