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Stellar Populations of Galaxies at 6.3 < z < 8.6Mauro
GiavaliscoPSU June 6-10, 2010Collaborators:Steve FinkelsteinEros
VanzellaAdriano FontanaCasey PapovichNaveen ReddyHarry
FergusonAnton KoekemoerMark Dickinson & GOODS teamUMass
Amherst
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WFC3 Sample SelectionUsing the WFC3 HUDF data, detected 3000
objects in a J+H-band detection image.Computed photometric
redshifts of all objects detected at 3.5 ( 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 SelectionDeep UBVRIZYJK imaging with VTL/FORS
and VLT/Hawk-I in 3 fields (including GOODS-S).Total of 15 z-band
dropouts at 6.5
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Comparison to LBG CriteriaCircles: 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 = 0.18All of our 31
objects are resolved.
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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?
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Stellar PopulationsWe 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
Ly 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.
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UV Colors: z~7 +0.20SED fit to the HawK-I stack: E(B-V) = 0.05
-0.15
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Rest-UV ColorsHawk-I: 16 galaxies
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UV colors at z~8Galaxies seem to have redder UV color at
z~8These probably reflects true scatter of UV SEDLya in H, which
would make the colors bluer.But as we will see later, these
galaxies do not seem to have much LyaFinkelstein et al. 2010
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Constraints on Metallcityfrom SED fittingMetallicity is probably
lower than that of z~3 LBG
No evidence for zero metallicity
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LBG at z~4, 5 same UV spectral types as at z~3Vanzella, MG, et
al. 2009
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Absorbers more dust-obscured than emitters; effect of the
outflows (Shapley et al. 2003)Vanzella et al. 2009
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At z~6, only one type identified, but huge selection effects
against other typesAbsorption 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]
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Lya up to z~6.5 (GOODS-S)Vanzella, MG et al.
2009EmittersAbsorbers
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Lya observed up to z~6.5in i-dropoutsThe highest-z GOODS i-band
dropout (GOODS-N);Also observed by PEARS
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Lya stronger at fainter UV luminosity and higher redshiftAmong
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)
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We obtained very deep Lya spectroscopy for 7 of our z~7 galaxies
(z-band dropouts), with no detectionsAll we found is one possible
detection at z=6.972All the other galaxies have no detection down
to ~1x1018 erg/s/cm2 (3s)Interesting! Confirmed i-band dropouts
have strong Lya emissionFontana et al. 2010
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Fontana, Vanzella+10Spectroscopy of z>6.5 candidates20hr 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 ( 16 F = 3.0 +/- 0.3
10-18 erg s-1cm-2 Stark+10
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Expected detection rateProbability to detect Lya emission (10s)
in N galaxies in our observations
Fontana et al. 2010Lya distrib. consistent with Stanway+07,
Vanzella+09, Stark+10
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Why no Ly emission?
Expected line flux > 1 x 10-18 erg s-1 cm-2.This is the 3s
flux limit of our VLT observationsWhy all of a sudden we dont 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
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Evolution of the UV Luminosity FunctionCastellano et al.
2010
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Mass Evolution of L* Galaxies
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Stellar Masses at z > 7Stellar masses 108 - 109 M (109 at
L*), compared to M 1010 at z 3Solid 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.
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Mass Evolution of L* GalaxiesTypical (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 Ly emitters (LAEs)z
7-8 galaxies look similar LAEs at all redshifts than LBGs at any
redshift (cf. S. Malhotras talk).
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Cosmic ReionizationBy 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
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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 starsThis can be done
with high spatial sampling by replacing QSO with galaxies to study
absorption systemsHere is an example of LBG MgII absorption
systemsIntervening gas in an overdensity at z~1.61There are no
galaxies with d
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Detected PoP III gas?Fe II trough not detected in the stacked
spectrum of the four MgII absorbersIt likely implies that the
intra-overdensity gas is chemically more pristine than that in the
IGM (outflows) of star-forming galaxies at high-zWith more
observing power (TMT, EELT), galaxy absorption systems will be a
very powerful tool to explore the early universe MG, EV+ 10
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SummaryStudy 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 26.5 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).