FIRST LIGHT IN THE UNIVERSE Richard Ellis, Caltech 1.Role of Observations in Cosmology & Galaxy Formation 2.Galaxies & the Hubble Sequence 3.Cosmic Star Formation Histories 4.Stellar Mass Assembly 5.Witnessing the End of Cosmic Reionization 6.Into the Dark Ages: Lyman Dropouts 7.Gravitational Lensing & Lyman Alpha Emitters 8.Cosmic Infrared Background 9.Future Observational Prospects Saas-Fee, April 2006

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, concerned with separating 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 Recent reviews: Leinert et al (1998, A&A Suppl, 127, 1) - good technical intro Hauser & Dwek (2001, Ann Rev Astr. Astr. 39, 249) - post- COBE Kashlinksy (2005, Phys. Reports, 409, 361) - pre-Spitzer

Galaxy Counts Differential counts in magnitudes where  is count slope e.g. deg -2 Integral counts to mag limit m deg -2 Contribution to surface brightness Units: Jansky = W m -2 Hz -1 ; 1 nW m -2 sr -1 = 3000/ (  m) MJy sr -1 Note: 1. EBL diverges unless  < Contribution of resolved sources peaks where  ~ Interesting quantity is difference between measured EBL and integrated contribution from faintest resolved sources Integrated surface brightness (EBL) Bolometric background (EBL)

Galaxy Counts Madau & Pozzetti (2000) MNRAS 312, L9 Pre-Spitzer counts from U thru K extend well into  <0.4 regime N(m) deg mag -1 i cgs Hz -1 sr -1  =0.4

Contributions to integrated counts Extrapolated counts enables contribution to integrated light from known populations to be evaluated as function of wavelength Find EBL =  I d = 55 nW m -2 sr -1 - largest contribution in near-IR Observational issues: isophotal losses and (1+z) 4 redshift dimming nW m- 2 sr -1

Digression: sanity check on integrated light Bolometric output from stellar evolution should be consistent with present mass density of stars:  * = Simple stellar population For reasonable SF histories find agreement between EBL and  * EBL

EBL Measurements Key issue is extent to which measured value (or limits) on EBL lie in excess of integrated counts as this would provide evidence for additional populations (e.g. Pop III) Tentative evidence for excess lies in 0.3< <10  m region only Problem is these EBL 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

EBL Measurements Dole et al astro-ph/ Bernstein HST DIRBE/2MASSDIRBE IRAC

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 EBL 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

Airglow and Zodiacal Light 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

Contaminating Optical/IR Foregrounds Kashlinsky 2005 Zodi Cirrus Airglow

Case Study - I: Bernstein et al (2002) 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 V AB ~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:  0.3 (  cgs sr -1 Å -1 ) Zodi background:  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

Contribution of resolved sources Nominal EBL is quoted excluding V AB <23 sources but the likely contribution of 23<V AB <28.5 sources is estimated WFPC-2 imageWFPC-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

Case Study - II: COBE detections DIRBE: 10-channel photometer 1.25< <240  m with 0.7° resolution chopping at 32Hz onto an internal zero flux surface. Many independent analyses with various assumptions FIRAS: Absolute spectrometer with 7° resolution in 100  m< < 5mm

DIRBE detections Early detections (>3  ) in selected fields at high Galactic and ecliptic latitudes at 140 and 240  m only, using elaborate time- dependent Zodi model (Kelsall et al 1998, Hauser et al 1998). Schlegel et al (1998) combined with higher resolution IRAS 100  m maps to improve removal of Galactic cirrus and dust emission, 100/240  m 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 100  m Wright (2000) used 2MASS to improve Galactic source removal and adopted the DIRBE Zodi model to claim the first detection at 2.2  m Key issue in all of the above is the reliability of the Zodi model

COBE Results IRTSDIRBE FIRAS Galaxy counts Residual DIRBE map at 240  m (  m) DIRBE dust DIRBE+IRAS

Case Study - III: Fluctuation Analyses Kashlinsky et al (2002, 2005) argue that the difficulties inherent in extracting the absolute EBL signal may be alleviated by considering the fluctuation spectrum  F(  ) and its 2-D Fourier transform P 2 (q) The key issues are whether: (i)contributions to P 2 (q) can be readily distinguished from one another (ii) isotropic (or D.C.) measures of EBL can be easily compared with those based on fluctuation analyses

Masking Deep IRAC Images Procedure: Extract sources to flux limit ~0.3  Jy (m AB ~22-25) Zero all pixels exceeding N cut  along with the N mask  N mask surrounding pixels Signal evaluated as a function of N cut and N mask. Image further filtered to eliminate artefacts & large scale gradients Signal differentiated from noise by adding and differencing two subsets S= A+B; N=A-B Plot P S (q) - P N (q) N cut =4 N mask =3 N cut =2 N mask =3 ~11 arcmin

Power spectra of IRAC EBL Shot noise from galaxy counts fits fluctuations on small scales but there is a significant excess on large scales Zodi tested via time-dependent observations; Cirrus via low latitude field cirrus?

Inter-band correlations Estimators of IRAC channel 1/2 color as function of scale Energy spectrum of EBL flat in I ruling out instrumental effects and Galactic sources as source of claimed signal

Interpretation of IRAC fluctuations Possible causes for excess power on arcsec scales: Instrumental noise/stellar sources - ruled out by tests and color of signal Galactic emission - unlikely given the trends seen in low latitude field Zodiacal light - no analysis of fluctuation spectrum and `6 month test’ not quantitatively assessed Faint galaxies beyond 0.3  Jy: hard to predict P 2 (q) without detailed model including N(z). However, extrapolation of deep IRAC counts yields < 0.2 nW m -2 sr -1 so sources would have to be remarkably strongly clustered to explain excess power Pop III sources at z~10: Can model signal expected depending on time interval  t over which clustered sources lie. Limber’s equation (relating spatial P 3 to angular P 2 ) indicates P 2 (q) can be amplified if  t is short. Bottom line: result is intriguing, unexplained but needs confirmation

Dole et al astro-ph/ Summary: EBL Measurements Excess over counts only significant in optical (marginal) & near-IR. Recent stacked MIPS data now resolve >70% of mid-IR EBL

Stacked MIPS images arranged by 24  m flux By stacking MIPS images centered at deep 24  m sources, the contribution of otherwise inaccessibly faint sources at 70 and 160  m to the background can be evaluated (Dole et al astro-ph/ ) 24  m 70  m

Modeling the Near-IR EBL 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 (MNRAS 359, 37, 2005) calculate stellar mass produced by the associated star formation Find  * = M  Mpc -3, corresponding to  * =  b. i.e. almost all stars need to be produced by z~9 to explain the EBL! Likewise, the ionizing flux associated with such SF would be in excess of that required to explain the WMAP optical depth  E Salvaterra & Ferrara (astro-ph/ ) 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 New EBL Constraints from TeV Gamma Rays Gamma ray ~ 10 km Particle shower ~ 1 o Cherenkov light ~ 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  m 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)

New Results from HESS Team - I HESS team assume 3 model EBLs and see if any are consistent with their TeV spectra of z~0.2 blazars ----HESS----

New Results from HESS Team - I observed corrected (z=0.186) acceptable 

Summary of Lecture #8 EBL 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 1-10  m 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 1-10  m