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Constraining Star Formation at z>7
Dan Stark (Caltech)
Collaborators: Richard Ellis, Johan Richard, Avi Loeb, Eiichi Egami,
Graham Smith, Andy Bunker, Jean-Paul Kneib, Mike Santos
A Century of Cosmology, San Servolo, Italy, 27 August 2007
Characterizing Star Formation at z>7: the
Current Frontier
Motivation:
• Probing some of the earliest galaxies (t<750 Myr)
 how does the global star formation rate density evolve?
 what is the density and luminosity distribution of earliest star-forming systems?
 what are physical properties (e.g. metallicity, stellar mass) of galaxies at the
highest redshifts?
• contribution of early star formation to reionization
 what were the objects that reionized the IGM?
 what is the redshift distribution of the reionizing systems?
 if star-forming galaxies, were the galaxies primarily low luminosity?
• note: current mode of observational study of z>7 universe is exploratory in nature
Decline in Star Formation Rate Density?
Bouwens et al., 2007
arXiv:0707:2080
Does decline in SFRD extend to higher redshifts?
Old stars at z~5-6 Implies Star Formation at z>6
z = 5.554
1.1x1011 M
Measurement of stellar mass density
allows constraints to be placed on
required SF at z>5-6 to assemble
galaxies (Stark & Ellis 2005, Stark et al.
2006, Yan et al. 2006, Eyles et al. 2006)
Stark et al. (2007a) ApJ, 659, 84
Assembled stellar mass density at z~5-6 requires greater star
formation rate density at z>6 than has been observed thus far.
How do we directly observe missing sources at z>6?
Evidence for Luminosity-Dependent Evolution
Bouwens et al. 2006, ApJ, 653, 53
Bouwens & Illingworth, 2006, Nature
Bouwens et al (2006, 2007) and Yoshida et al. (2006) propose L-dependent
evolution - decline in abundance over 3<z<6 mostly for luminous sources
Observations suggest star formation increasingly dominated by low luminosity
sources for z>6?
Beyond z~7 with Strong Gravitational Lensing
z=5.6
z = 6.8
Utilizing strong magnification (10-30) of clusters, probe much
fainter than other methods in small areas (<0.1 arcmin2 /cluster)
Spectroscopic Surveys for Lensed Lyman-alpha Emitters
From arclet
spectroscopy
the location of
the “critical
lines” is known
precisely for
z=1
and for
z=5
A Keck survey in the optical discovered 11 candidate LAEs
between 2.2 < z < 5.6 -- Santos et al. ApJ 606, 683 (2004)
Low Luminosity z~10 Ly Emitters: Critical Line
Mapping With Keck NIRSPEC
Cluster critical line for zS > 7
Wavelength sensitivity (1.5hr)
J-band
10-17 cgs
NIRSPEC slit positions
• 9 clusters completed between 04-05 (Stark et al. 2007b, ApJ, 663,10)
• Clusters have well-defined mass models & deep ACS imaging
• Obs. sensitivity ~ 0.6-2 x10-17 cgs; magn. > 10-30 throughout
• Sky area observed: 0.3 arcmin2; V(comoving)~30 Mpc3
• 6 promising lensed emitter candidates (>5)
• 8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 -5 M yr-1
Candidate Ly Emitters
8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 - 5 M yr-1
Recognize burden of proof that these are z~10 emitters is high!
(see discussion in Stern et al. 2000ab, Bremer et al. 2004, G. Smith et al. 2006)
Each detection is > 5, seen in independent exposures/visits
Spectroscopic Elimination of Interlopers
If J-band emission is H…
z~0.9
Ly, [OII], and [OIII] observable with optical spectroscopy
Spectroscopic Elimination of Interlopers
• Archival LRIS spectroscopy (Santos et al 2004) from 4000-9400Å available
for all candidates
• No emission lines detected: candidates are probably not H or [OII]
• H-band spectroscopy obtained for 3 candidates with NIRSPEC
• No emission lines detected: 2/6 candidates are probably not H, [O III]
•J-band spectroscopy available for all candidates with NIRSPEC
• No additional emission lines detected: candidates are most likely not
[OIII] 4959
• optical broadband? one candidate has marginal z-band detection
• remove from high-z sample
At least 2 candidates are most easily explained as z~10 sources, and
an additional 3 candidates have strong case to be at z~10.
Confirming Galaxies at z>8
1. Asymmetric Line Profile?
Long integrations needed. Only look for in best candidates.
Should be potentially feasible with MOSFIRE.
2. Lensed Counterimage?
Could confirm redshift, but difficult with longslit spectrograph.
Stern et al. 2005, ApJ
Contribution to Reionization?
# density required for reionizaton
Consider range:
fesc ~ 0.02-0.5
t ~250-575 Myr
None are real
All are real
If even 1-3 of the 6
candidates is at z~10,
low luminosity
galaxies may play a
dominant role in
cosmic reionization
Key uncertainty: if candidates are at z~10, are observed
densities characteristic given large cosmic variance?
A surprisingly large abundance of z~10 Lya Emitters?
Stark, Loeb, & Ellis, 2007, ApJ, in press
Observed densities of z~10 LAEs
model for z~10 LF assuming
LLya-M mapping unchanged
from z~5.7/6.6
model for z~10 LF assuming
increase in star formation efficiency
• abundances of z~10 Lya emitters not consistent with extrapolation from z~6
• require more Lya photons per unit halo mass, achievable through 1)
increase in star formation efficiency or 2) change in IMF
• possibilities tantalizing but more work needed to confirm candidates and
additional survey volume needed to confirm densities.
Predictions for future LBG surveys at z>7
Stark, Loeb, & Ellis 2007, ApJ, in press
Ground-based
telescope
regime
• Upcoming surveys (e.g. VISTA / WFC3) should be able to detect large number
of LBGs at z~7-8.
• If increase in density suggested by z~9-10 lensed LAE candidates proves false,
probing to z~10 may be much more difficult, especially from the ground
Summary
• Strong lensing surveys are finding an abundant population of
candidate faint Ly emitters at z~8-10 with SFR <1 M yr-1 - a
population which may contribute significantly to reionization.
• Exhaustive spectroscopic and imaging follow-up supports
hypothesis that many of lensed Lya emitters are at z~10 but
additional follow-up still required. Final confirmation is especially
difficult at z>7!
• Implied densities of z~9 LAEs still very uncertain, but if correct,
would require increase in Lya photon output per unit halo mass from
z~6.
• Even with conservative assumptions, new instruments should result in
reasonably large samples of galaxies at z~7-8 in the next few years. If
increase in density implied in lensed LAE survey proves false, z~10
objects may be difficult to find before JWST, especially for (nonlensing) ground-based imaging surveys
Lensed z>7 Lyman-Break Galaxies
UDF flux
limit
Bouwens & Illingworth, 2006
Only most luminous systems probed at z~7.5!
Predictions for Lya Emitters at z>7
z~7-8
z~10
• Upcoming/ongoing surveys should be able to detect LAEs at z~7-8
• Expected Poisson and cosmic variance will complicate conclusions about
reionization.
• Probing to z~10 may be much more difficult, unless there is upward evolution in
LAE LF (as implied in lensed LAE searches).
HST+Spitzer Survey for Lensed z>7 LBGs
MS1358 - H
Critical line
Magnification
> 2 mags
Richard, Stark, Ellis et al, in prep.
•
•
•
•
11 NICMOS pointings in 6 lensing clusters
(4 orbits J/F110W, 5 orbits H/F160W)
ACS/F850LP imaging of all clusters
K-band ground based imaging with Keck/NIRC + Subaru/MOIRCS
Deep IRAC imaging for all clusters
Candidate z~7.5 LBGs
Bouwens
et al
Lensed
candidates
z-J
J
“effective”
magnitudes
•10 candidate z-drops in the 6 clusters surveyed with H ~ 26 - 26.8
• Implied SFR ~ 0.1 - 2 M yr-1 (unlensed)
• Time allocated to follow up these spectroscopically with NIRSPEC
Source Plane and Image Plane
Position of
source
relative to
lens is crucial!
The Real Sky
No rings:
giant arcs
with counter
images
What Observer Sees
In the general case (here elliptical lens) as background source moves
closer in alignment, multiple images, some highly magnified appear.
Lines where magnification is maximized are known as `caustics’ in the
source plane, `critical lines’ in the image plane.
Bulk of candidates unlikely to be z~2 interlopers
z ~ 2 quiescent
Current
limit
z ~ 8 star-forming
Possible
limit in
warm
mission
Stacked IRAC limit for 8 unconfused candidates gives upper limit at
3.6 microns rejecting passive z~2 population as primary population
Implied Luminosity Function z~7.5
All 10 z-drops are real
None is real
Bouwens &
Illingworth z~7.4
UDF
Lensed LBG survey probes faintward of UDF