Download FIRST LIGHT IN THE UNIVERSE

FIRST LIGHT IN THE UNIVERSE
Saas-Fee, April 2006
Richard Ellis, Caltech
1. Role of Observations in Cosmology & Galaxy Formation
2. Galaxies & the Hubble Sequence
3. Cosmic Star Formation Histories
4. Stellar Mass Assembly
5. Witnessing the End of Cosmic Reionization
6. Into the Dark Ages: Lyman Dropouts
7. Gravitational Lensing & Lyman Alpha Emitters
8. Cosmic Infrared Background
9. Future Observational Prospects
z > 6 Surveys Represent the Current Frontier
Motivation:
- census of earliest galaxies (z=6, =0.95 Gyr)
- contribution of SF to cosmic reionization
- constraints on early mass assembly
- planning effective use of future facilities (ELTs,
JWST)
Developing complementary optical/IR techniques:
- continuum dropouts: reducing contamination
- HST ACS grism, SEDs with low background
- Ly LF: sensitivity to physical state of IGM
- strong lensing: extends capabilities at faint end of LF
- Spitzer detections: masses & early SF histories
Some Key Issues
• How effective are the various high z selection methods?
- L*(z=6)  i~26 where spectroscopy is hard
- spectroscopic samples biased to include strong L
- great reliance on photometric redshifts
• Is there a decline in the UV luminosity density 3<z<6?
- results are in some disagreement
- differing trends in continuum drops & L emitters
• Is the observed UV at z>6 sufficient for reionization?
- contribution from (unobserved) faint end of LF?
- unusual popns: intense EW(L), steep UV continua?
• Significant stellar masses for post-burst z~6 galaxies
- how reliable are the stellar masses?
- inconsistent with declining SF observed 6<z<10?
- does this imply an early intense period of activity?
Continuum sources probed via dropout technique
z-dropout
Stanway et al (2003)
Traditional dropout technique poorly-suited for z>6 galaxies:
- significant contamination (cool stars, z~2 passive galaxies)
- spectroscopic verification impractical below ~few L*
i-drop volumes: UDF (2.6 104), GOODS-N/S (5.105), Subaru (106) Mpc3
flux limits: UDF z<28.5, GOODS z<25.6, Subaru z<25.4
Reducing Contamination from z~2 Passive Galaxies
Addition of a precise
optical-infrared color
(z - J) can, in
addition to the (i - z)
dropout cut, assist in
rejecting z~2 passive
galaxy contaminants.
(i – z)
5.7 < z < 6.5
(Stanway et al 2004)
This contamination is
~10% at z~25.6 but
is negligible at UDF
limit (z~28.5)
z~2
passive
galaxies
(z – J)
Contamination by cool Galactic dwarfs - more worrisome
UDF z<25.6 (Stanway et al 2004)
L dwarfs
E/S0
HST half-light radius Rh more effective than broad-band colors
Contamination at bright end (z<25.6) is significant (30-40%)
Keck spectroscopy of i-drops: 10.5 hrs zAB < 25.6
z=5.83 L
L-dwarfs contaminate at bright end
Counts for i-band drops (GOODS+UDF)
GOODS
Spec
limit
UDF
6
from
z~3
GOODS/UDF data to zAB=28.5 consistent with z=3 LBG LF but  6
Bunker, Stanway, Ellis & McMahon MNRAS 355, 374 (2004)
ACS dropouts: Luminosity Dependent Evolution?
z=3
Bouwens et al (2006, astro-ph/0509641) propose L-dependent evolution
- decline in abundance over 3<z<6 mostly for luminous sources
If correct, this affects z-dependent integrated SF density measures
corrected to some fiducial luminosity
How much steeper could LF slope be at early times?
i-drop counts
N(28.5) = 54
N(30.0) = 108
Yan & Windhorst (2004) extend UDF i-drop search to z(AB)=30.0 (c.f.
Bunker et al z(AB)=28.5) and claim  -1.9 after correcting for
incompleteness implying a significant increase in integrated density at z~6
Decline in UV over 3<z<6 has been controversial
Giavalisco et al 2004
Bunker et al 2004
Poisson errors fail to account for dispersion in claimed number of z~6
i-drops, because of varying ways of accounting for contamination plus
cosmic variance (10% in GOODS; 40% in UDF)
Bouwens et al 2005 Ap J 624, L5
Results from Subaru
• HST offers superior photometry & resolution (important for stellar
contamination) but SuPrimeCam has much bigger field (each pointing
= 2  GOODS-N+S)
• Additional photometric bands developed to sort stellar contamination
• Shioya et al (2005): used intermediate band filters @ 709nm, 826nm
to estimate stellar contamination in z~5 and z~6 samples respectively
• Shimasaku et al (2005) split z-band into two intermediate filters
zB, zR - to measure UV continuum slope
These studies confirm decline indicated via HST studies
z~6 dropouts from Subaru - the upshot
• SDF dataset > 2  GOODS N+S; cosmic variance ~ 25%
• Confirm 5 abundance drop from z~3 to 6 (c.f. Bunker et al, HST)
• Luminosity dependent trends - more evolution in massive galaxies?
Remember: this is observed number not dust-corrected SFR
Further confirmation of i-drop technique
star
z~2 passive
Malhotra et al (2005) use 40 orbit ACS UDF grism exposures
& consider utility of (i - z) color cut vs. contamination
Traditionally use (i - z) > 1.3 (e.g. UDF)
Of 29 z<27.5 candidates with (i - z) > 0.9, 23 are at z~6
Determining Abundance Necessary for Reionization
e.g. Madau, Haardt & Rees (1999):
Input details depend on considerable imponderables:
• TIGM  10,000 – 20,000 K ?
• Teff & Z of stellar population (IMF) ?
• C = <HI2 > / <HI>2 simulations suggest C  30 ?
• fesc (=1 implies no HI absorption) ?
Leading to factors of 10 uncertainty!
Cosmic Star Formation History
The combined
uncertainties in
the data (plus
uncertain input
physics
necessary for
the predictions)
mean we cannot
yet convincingly
claim (or reject)
that the
abundance of
z>5 sources is
sufficient for
reionization
Bouwens et al astro-ph/0509641)
The Spitzer Space Telescope Revolution
A modest 60cm cooled telescope can see the
most distant known objects and provide crucial
data on their assembled stellar masses!
IRAC camera has 4 channels at 3.6, 4.5, 5.8
and 8 m corresponding to 0.5-1m at z~7!
• Egami et al (2005) - characterization of a lensed z~6.8 galaxy
• Eyles et al (2005) - old stars at z~6
• Yan et al (2005) - masses at z~5 and z~6
• Mobasher et al (2005) - a galaxy > 1011 M at z~6?
Spitzer detections of i-drops at z=6
#1 z=5.83
#3 z=5.78
Eyles et al (2005)
MNRAS 364, 443
• 4 i-drops in GOODS-S confirmed spectroscopically at Keck
• Ly  emission consistent with SFR > 6 M yr-1
• IRAC detections from GOODS Super-Deep Legacy Program
Spectral Energy Distributions of i-drops
#1 z=5.83
#3 z=5.78
VLT K
VLT K
Spitzer + Ly emission constrains present & past star formation
Ages > 100 Myr, probable 250-650 Myr (7.5<zF<13.5)
Stellar masses: 2-4 1010 M (>20% L*)
Independent z~5-6 UDF Spitzer analysis
Yan et al Ap J 634, 109 (2005)
Evidence for Unusually Strong UV Continua
Several UDF
objects at z~6
show unusually
blue (z-J) colors
which lie
outside the
predictions of
standard
Bruzual Charlot
models; a point
first noted by
Stanway et al
(2004) - unclear
what this
means!
Yan et al Ap J 634, 109 (2005)
Spitzer detection of resolved J-drop in UDF
Criterion: (J – H)AB > 1.3 plus no detection in combined ACS
JD2: strong K/3.6m break  potential high mass z~7 source
Mobasher et al (2005) Ap J 635, 832
High mass resolved J-drop in UDF + two breaks
Mobasher et al (2005)
STARBURST99: z=6.6; EB-V =0.0; Z=0.02, zF>9
BC03: z=6.5; EB-V =0.0; Z=0.004, zF>9
Stellar Mass: 2-7  1011 M dependent on AGN contamination
Uncertainty in Redshift and Stellar Mass
~ 25% chance of being z~2.5
`Double-Break’ Objects Seem Fairly Common
Abundance of Massive Galaxies at z~6: A Crisis?
Abundance of
massive galaxies at
z~6 with CDM in
terms of their
implied halo masses,
assuming
• Scalo IMF
• SF efficiency 20%
Find a 1013 M halo
in the tiny UDF is a
problem!
Barkana & Loeb (2005)
z = 5.8
Mobasher et al
z = 15
Yan et al
Eyles et al
Measured z~5-6 Mass Density
all
SFH only accounts for
mass assembled in SF
galaxies at z~5, so still a
lower limit
Stark, Bunker, Eyles, Ellis & Lacy (2006)
Luminous
SF galaxies
Summary of Lecture #6
• Great progress using v,i,z,J-band drop outs to probe abundance of SF
galaxies from 3<z<10: Bouwens et al discuss the properties of 506 Iband dropouts to z~29.5!
• In practice, these samples are contaminated by foreground stars, z~2
galaxies etc to an extent which remains controversial. We are unlikely to
resolve this definitively with spectroscopy until era of ELTs.
• Comoving SF rate declines from z~3 to z~6 (and probably beyond)
suggesting insufficient 6<z<10 luminous galaxies to reionize Universe
• Contribution of lower luminosity systems less clear (lensing: Lecture 7)
• Spitzer’s IRAC can detect large numbers of z~5-6 galaxies and it
seems many have high masses (one spectacularly so!) and signatures
of mature stellar populations - implies earlier activity
• Reconciling mature galaxies at z~6 with little evidence for SF systems
with 7<z<10 may turn out to be a very interesting result
Combining intermediate & broad-bands (Shioya et al)
Stars
Starburst
candidates
Passive
SDF example (for z~5 R-drops) shows how high z galaxies can be
separated from interlopers by demanding strong intermediate
band depressions (709-i), in addition to a regular (R-i) color cut
Method reduces number of `traditional’ R-drops from 3519
Similar technique using 826nm filter reduces i-drops from 96
Intermediate + broad-bands contd.. (Shimasaku et al)
z~6 gals
stars
z~2 passive
Breaking z-band into two components (zR, zB) can insist on blue
continuum slope within z-band for z~6 galaxies.
Reduces conventional i-band drops z < 25.4 from 22  12
Low Abundance of z~6 Sources
No extinction
GOODS+UDF
Bunker et al (2004)
Can rescue situation appealing to:
• cosmic variance (unlikely given independent datasets)
• steep faint end LF (Yan & Windhorst 2004)
• low Z populations (Stiavelli et al 2004)
Necessary for
reionization
6<z<9 (Stiavelli
et al 2003)
Ages and Stellar Masses
#1 z=5.83
#3 z=5.78
Explored range of BC models including dust:
Ages > 100 Myr, probable range 250-650 Myr (7.5<zF<13.5)
Stellar masses: 2-4 1010 M (>20% L*)