stellar populations of galaxies at 6 3 z 8 6 n.
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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. z phot =7.55 ± 0.35.

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stellar populations of galaxies at 6 3 z 8 6
Stellar Populations of Galaxies at 6.3 < z < 8.6

Mauro Giavalisco


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

wfc3 sample selection

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.
vlt hawk i sample selection
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

comparison to lbg criteria
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.


  • 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?
stellar populations
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.
uv colors z 7
UV Colors: z~7


SED fit to the HawK-I stack: E(B-V) = 0.05


uv colors at z 8
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

constraints on metallcity from sed fitting
Constraints on Metallcityfrom SED fitting
  • Metallicity is probably lower than that of z~3 LBG
  • No evidence for zero metallicity
absorbers more dust obscured than emitters effect of the outflows shapley et al 2003
Absorbers more dust-obscured than emitters; effect of the outflows (Shapley et al. 2003)

Vanzella et al. 2009

at z 6 only one type identified but huge selection effects against other types
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]

ly a up to z 6 5 goods s
Lya up to z~6.5 (GOODS-S)



Vanzella, MG et al. 2009

ly a observed up to z 6 5 in i dropouts
Lya observed up to z~6.5in i-dropouts

The highest-z GOODS i-band

dropout (GOODS-N);

Also observed by PEARS

ly a stronger at fainter uv luminosity and higher redshift
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)
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


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


Fontana, Vanzella+10

expected detection rate probability to detect lya emission 10 s in n galaxies in our observations
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

why no ly emission
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

stellar masses at z 7
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.
mass evolution of l galaxies
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).
cosmic reionization
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
how else can we observe early star formation by looking at things that absorb not emit light
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

detected pop iii gas
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

  • 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).