Galaxy Formation
in the Early Universe
(z≥7)
Haojing Yan
Center for Cosmology & AstroParticle Physics
Ohio State University
CCAPP Symposium 2009
October 14, 2009
Based on our recent paper submitted to
Astrophysical Journal (see arXiv:0910.0077)
“Galaxy Formation in the Reionization Epoch as Hinted by
Wide Field Camera 3 Observations of the Hubble Ultra Deep Field”
Collaborators:
Rogier Windhorst (Arizona State University)
Nimish Hathi (UC Riverside)
Seth Cohen (Arizona State University)
Russell Ryan (UC Davis)
Robert O’Connell (University of Virginia)
Patrick McCarthy (Carnegie Observatories)
“Dropout” Search for High-z Galaxies
• Line-of-sight neutral H
absorption (Lyman limit +
Lyforest) creates strong
Lyman-break signature in
SEDs of galaxies at z3
(Steidel & Hamilton 1992)
i'
z'
Result from ACS HUDF: galaxy luminosity function
at z  6 has a very step faint-end slope  = -1.8— -1.9
108 i’-dropouts (z~6 galaxy candidates) to
~ 30 mag in the Hubble Ultra Deep Field
taken by the Advanced Camera for Surveys
(Yan & Windhorst 2004b; ApJ, 612, L93)
(Universal) Schechter Formalism of
Luminosity Function of Galaxies
Luminosity domain
Absolute magnitude domain
Apparent magnitude domain
Cumulative surface density
(credit: Binggeli)
Why it’s a big deal: Low-luminosity Galaxies Could
Be Major Contributors of Ionizing Photons at z  6
Cumulative contribution from galaxies
(with different LF faint-end slopes)
•
Complete Gunn-Peterson
trough detected in SDSS
quasar spectra only at z6.3
and beyond: universe still fully
ionized until looking-back to
z6.3
•
“Normal” star-forming galaxies
can do the job at z6 as their
LF is step enough
•
Q: Could they be the major
source of Reionization?
Critical
Value
Yan & Windhorst 2004a; ApJ, 600, L1
Let’s push to higher redshifts
and find out!
Moderate Success (up to 2008)
•
Dropout selection using HST NICMOS+ACS in field
•
— Yan & Windhorst (2004b) in HUDF
•
— Bouwens & Illingworth (2006), Bouwens et al. (2008) using
archival NICMOS data (including HUDF)
•
Dropout selection using HST NICMOS+ACS around clusters
(gravitational lensing)
•
— Bradley et al. (2008): the best z~7 candidate (zph=7.4)
•
Direct Slit-spectrosopy around clusters
•
— Stark et al. (2007): 6 possible Lya-emitter at z~8.5-10.4
•
Ground-based Lya-emitter search
•
— Iye et al. (2006): z=6.96 (Record holder)
Disturbing Results
• All observations seem to suggest a decreasing
number density of galaxies at higher redshifts
(dimmer M* and lower *)
• But more active star-forming activities (reads:
more star-forming galaxies) at higher redshifts
are needed to explain:
• — Reionization
•
— “Matured”, high-mass galaxies observed
at z~6
New Opportunity Offered by HST WFC3 (UVIS + IR)
May 14, 2009
Deepest Optical + Deepest NIR
HUDF WFC3,  4.7 arcmin2
 29 mag from 0.9—1.7m
(36% more data to come next year)
HUDF ACS,  11 arcmin2
 30-31 mag from 0.4—0.9m
HST Cycle-17 GO 11563, PI: G. Illigworth
Fast Papers!
•
Data taken Aug. 26 - Sept. 6, released to public mid-night Sept. 9
•
Two papers from the GO team appeared at arXiv on Sept. 10:
• — Oesch et al. (0909.1806): 16 candidates at z~7
• — Bouwens et al. (0909.1803): 5 candidates at z~8
•
Two more papers appeared on Sept. 14:
• — Bunker et al. (0909.2255): 10 candidates at z~7 and (didn’t even
mention in their abstract) 7 at z~8
• — McClure et al. (0909.2437): 4 additional candidates (w.r.t. Oesch +
Bouwens) at z>7
Reiterating the same thing: decreasing SFR at higher redshifts
We decided that SCIENCE could be done
differently …
•
Starting from zero photon (and zero character) at mid-night
Sept. 9, doing a better data reduction/analysis to take full
advantage of these precious data
•
— reducing data from scratch (rather than relying on
pipeline)
•
— using in-flight calibration files (rather than ground-test
files)
•
— extra treatment to remove instrumental signatures
•
— extreme care in alignment while mosaicking
•
We were able to perform a much more complete search at
the faintest level (but did not go beyond what data allow),
20 Galaxy Candidates at z  7
15 Galaxy Candidates at z  8
20 Galaxy Candidates at z  10
LF & GSFRD @ Very High-z
Cumulative
Surface
Density
LUV
SFR

Volume
Global Star
Formation
Rate Density
Extrapolate to
M  -15.0 mag
(AB  32 mag)
to AB~29 mag
(observed)
(Data points at z<6 taken from compilation of Hopkins & Beacom 2006)
While totally unexpected, this result could solve
many (every?) problems
• Not a problem in producing high-mass
galaxies at z~6
• Not a problem in producing
reionization photons
(In)direct Supporting Evidence from GRB-based SFRD
Estimate
Kistler et al. (arXiv:0906:0590)
GRB 090423
@ z=8.26
Salvaterra et al. (arXiv:0906.1578)
Tanvir et al. (arXiv:0906.1577)
Summary
• After careful analysis, the deepest IR data reveal
a large number of galaxy candidates at z  7, 8
and 10
• Earlier estimate of z  7 galaxy luminosity
function consistent with new data, but a sudden,
strong change in LF seems inevitable at z  8
and beyond
• Star-formation Rate Density could rise sharply
from z>7 to z  10
•  First direct evidence that the Universe must
be actively forming galaxies in the reionization