Stellar Populations of Galaxies at 6.3 < z < 8.6

1. Stellar Populations of Galaxies at 6.3 < z < 8.6 Mauro Giavalisco Collaborators: Steve Finkelstein Eros Vanzella Adriano Fontana Casey Papovich Naveen Reddy Harry Ferguson Anton Koekemoer Mark Dickinson & GOODS team UMass Amherst PSU – June 6-10, 2010

2. zphot=7.55 ± 0.35 WFC3 Sample Selection • Using the WFC3 HUDF data, detected ∼ 3000 objects in a J+H-band detection image. • Computed photometric redshifts of all objects detected at ≥ 3.5 σ (∼ 2500) in both J125 and H160 using EAZY (Brammer+08). • Sample consists of 31 galaxies with 6.3 < zphot < 8.6. • 23 at 6.3 < zphot ≤ 7.5, and 8 and 7.5 < zphot < 8.6.

3. VLT/Hawk-I Sample Selection • Deep UBVRIZYJK imaging with VTL/FORS and VLT/Hawk-I in 3 fields (including GOODS-S). • Total of 15 z-band dropouts at 6.5<z<7.5 (LBG), no Y-band dropouts, down to Y<26.7 (AB) • Spectroscopic follow-up for 7 galaxies (3 Hawk-I and 4 WFC3); no detections, except for a possible one • Figures show the stacked images and photo-z of the 15 galaxies Castellano et al. 2010

4. Comparison to LBG Criteria • Circles: Gray = z < 6.3, Cyan = 6.3 < z < 7.5, Red = 7.5 < z < 8.6. • Brown symbols represent synthesized colors of brown dwarfs, from empirical spectra. • Using stars in the WFC3 image, we measured FWHMPSF = 0.18” • All of our 31 objects are resolved.

5. Goals • Study their colors, compare UV spectra to those at z~3-5 • Characterize the evolution • Can we rule out the null hypothesis that these objects are no different than galaxies at z ∼ 3-6? • If so, can we say anything about more exotic stellar populations? • UV (SFR), Optical (stellar mass) LF • Examine the remaining stellar population properties? • What can we confidently say, and what is guesswork?

6. Stellar Populations • We investigated the stellar populations in these galaxies by comparing our observations to CB07 models. • We fit data from ACS, WFC3 and Spitzer, assuming that z = zphot. • Included Lyα emission in the models (see Finkelstein+07,08,09), as it can significantly affect the Y105/J125 fluxes at these redshifts, as the line EW increases as (1+z). • Results imply that current NIR NB surveys are not deep enough to see majority of objects. • We computed 68% confidence ranges on each fitted parameter simulations. • We included the uncertainty on the photometric redshift in these simulations - increases final uncertainties significantly! • Age and dust are not well constrained with WFC3 fluxes alone - average of 200 Myr and AV = 0.3 mag - though majority are consistent with very young ages and low/zero extinction. • While metallicity is traditionally poorly constrained, the blue colors of these objects rule out Z > Z⨀ at 95% confidence, and Z > 0.05 Z⨀ at 68% confidence.

7. UV Colors: z~7 +0.20 SED fit to the HawK-I stack: E(B-V) = 0.05 -0.15

8. 23 z ~ 7 objects Rest-UV Colors Hawk-I: 16 galaxies

9. UV colors at z~8 • Galaxies seem to have redder UV color at z~8 • These probably reflects true scatter of UV SED • Lya in H, which would make the colors bluer. • But as we will see later, these galaxies do not seem to have much Lya Finkelstein et al. 2010

10. Constraints on Metallcityfrom SED fitting • Metallicity is probably lower than that of z~3 LBG • No evidence for zero metallicity

11. LBG at z~4, 5 same “UV spectral types” as at z~3 Vanzella, MG, et al. 2009

12. Absorbers more dust-obscured than emitters; effect of the outflows (Shapley et al. 2003) Vanzella et al. 2009

13. At z~6, only one type identified, but huge selection effects against other types • Absorption lines weaker in composite spectrum, most likely diluted because of wavelength shifts due to winds (spectra registered to Lya) • We do not know how to stack “absorbers” [see Vanzella, MG, et al. 2009]

14. Lya up to z~6.5 (GOODS-S) Emitters Absorbers Vanzella, MG et al. 2009

15. Lya observed up to z~6.5in i-dropouts The highest-z GOODS i-band dropout (GOODS-N); Also observed by PEARS

16. Lya stronger at fainter UV luminosity and higher redshift • Among LBG with Lya emission, there is a trend of increasing Ew with both increasing redshift and decreasing LUV • (Vanzella, MG, 2009; see also Stark+10)

17. We obtained very deep Lya spectroscopy for 7 of our z~7 galaxies (z-band dropouts), with no detections • All we found is one possible detection at z=6.972 • All the other galaxies have no detection down to ~1x1018 erg/s/cm2 (3s) • Interesting! Confirmed i-band dropouts have strong Lya emission Fontana et al. 2010

18. Spectroscopy of z>6.5 candidates 20hr of FORS2 on Hawk-I + WFC3 candidates on GOODS-S: only one “likely” detection (3s) systematic follow-up of z>7 candidates with 8m telescope expensive --> LF untested until JWST/TMT/ELT Monte Carlo simulations of expected detection rate suggest that the likelihood of such negative result is low (<10%) If confirmed: either most candidates not at z~7 or... something at z~7 is very efficient at absorbing Lyα (gas infall from PGM) z(Lya) = 6.972 ? Wavelength (Å) Wr > 16 Å F = 3.0 +/- 0.3 10-18 erg s-1cm-2 Stark+10 Fontana, Vanzella+10

19. Expected detection rateProbability to detect Lya emission (10s) in N galaxies in our observations Lya distrib. consistent with Stanway+07, Vanzella+09, Stark+10 Fontana et al. 2010

20. Why no Lyα emission? • Expected line flux >∼ 1 x 10-18 erg s-1 cm-2. • This is the 3s flux limit of our VLT observations • Why all of a sudden we don’t see Lya any more? • Unphysical that LBG selection only works up to z~6.5 • What is the reason for the non-detections: • Galaxies are more dust-free…? • The ISM? (e.g. gas accretion) • Cosmic opacity? Our flux limit

21. Evolution of the UV Luminosity Function Castellano et al. 2010

22. Stellar Masses at z > 7 • Stellar masses ∼ 108 - 109 M⨀(∼109 at L*), compared to M ∼ 1010 at z ∼ 3 • Solid line is the joint probability distribution of mass from the bootstrap simulations. • Overall uncertainty on mass is a factor of ∼ 10, BUT is a factor of only +2 and -5. • The upper limit on mass appears to be well constrained. • Test this by fitting a two-population model, where 90% of the mass is forced to form at z = 20. • The young age of the Universe at these redshifts limits the amount of mass in old stars - more so than Spitzer. • At tUni ∼ 500-800 Myr, M/L ratio dominated by stars with M > 3M⨀, or O, B and early A stars.

23. Mass Evolution of L* Galaxies • Typical (L*) galaxies are lower mass at higher redshift. • We confirm a drop in typical galaxy stellar mass, first hinted at, at z ∼ 5-6. • Gray dots are Lyα emitters (LAEs) • z ∼ 7-8 galaxies look similar LAEs at all redshifts than LBGs at any redshift (cf. S. Malhotra’s talk).

24. Cosmic Reionization • By adding up the rest-UV fluxes of our objects, we can examine how they would impact reionization. • We computed the specific luminosity density for the z ∼ 7 and 8 samples. • With no correction for dust or incompleteness, our objects come within a factor of a few of sustaining reionization for high escape fractions (∼ 50%). • Assumes low clumping factors (Pawlik+09, Finlator+09). • Accounting for unseen faint galaxies brings us a factor of ∼ 2-3 closer. • Escape fractions of ~50% might be reasonable (e.g., Siana+10; Vanzella+10). • Check out poster (#5) by Vanzella: detected ion. rad. from LBG at z~3.8 + new f_esc estimates

25. How else can we observe early star formation?By looking at things that absorb (not emit) light • i.e. by finding the gas released by early stars • This can be done with high spatial sampling by replacing QSO with galaxies to study absorption systems • Here is an example of LBG MgII absorption systems • Intervening gas in an overdensity at z~1.61 • There are no galaxies with d<210 kpc from the LOS of these LBG MG, EV+ 2010

26. Detected “PoP III” gas? • Fe II trough not detected in the stacked spectrum of the four MgII absorbers • It likely implies that the “intra-overdensity” gas is chemically more pristine than that in the IGM (outflows) of star-forming galaxies at high-z • With more observing power (TMT, EELT), galaxy absorption systems will be a very powerful tool to explore the early universe MG, EV+ 10

27. Summary • Study the evolution of star-forming galaxies at z>6.5 within reach (HST+WFC3, 8-m telescopes). • Expect significant progress from WFC3 GOODS+AEGIS MCTP. • These early galaxies appear bluer than at 2<z<6, consistent with normal, young, low-dust populations ( > 4σ result). • Little evidence for exotic stellar populations, i.e. Z=0, top-heavy IMF. • Their stellar populations are consistent with z ∼ 3-6 Lyman alpha emitters. • Apparently fast evolution of the Lya properties: despite previous increasing trend with redshift, at z>6.5 Lya seems to suddenly disappear from these galaxies. Why? • UV luminosity function (bright end) evolves rapidly, faster than previous measures. • Stellar mass relatively small, growing rapidly. • If escape fractions are high, galaxies at these redshifts may be able to sustain reionization (you MUST read E. Valnzella poster here: detected ion. Rad at z~3.8).