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SPITZER/IRAC
OBSERVATIONS OF
GALAXIES AT Z > 2
Giovanni G. Fazio
Jiasheng Huang
Harvard-Smithsonian Center for Astrophysics
Cambaridge, MA 02138, USA
IRAC EXTRAGALACTIC GTO
PROGRAMS
•
One of the principal IRAC GTO
programs is the study of the formation
and evolution of galaxies from z = 0 to
z > 3.
•
Core of the program is to measure from
z = 0 to z > 3:
– Luminosity function of galaxies
– Galaxy mass function
– Star formation rate
– Mass-to-Light ratios
•
Measurement of the rest-frame near-IR
flux is crucial to determining the nature
of galaxies; IRAC bands trace the restframe NIR luminosities for 0.5 < z < 5
 bulk of stellar mass
•
Observations include IRAC/MIPS
surveys carried out at a range of areas
and sensitivities.
PAH
starlight
dust
re-radiation
DEEP SURVEYS
• Ultra-deep:
– Q1700, 5’x 8’, 10h/pointing
– HDFS, 5’x 5’, 5h/pointing
• Deep:
– Extended Groth Strip (EGS), 2 deg x 10’, 3h/pointing for
IRAC, 500-800s/pointing for MIPS
• GOODS Fields
– HDFN, CDFS, HUDF
Extended Groth Strip Survey
• Optical imaging (CFHT B, R, and I with
R_limit=25.5) by N. Kaise.
• K-band imaging (K=20.5) by K. Bundy and R.
Ellis.
• IRAC imaging at 3.6, 4.5, 5.8, and 8.0 microns;
22.5 mag, 5 s, at 3.6 microns;
• DEEP spectroscopic survey (R=24); M. Davis and
S. Faber.
• Imaging at other wavelengths in progress (Subaru
R-band, HST ACS (120 orbits)).
Extended Groth Strip (EGS)
5’x5’
IRAC 3.6 µm
IRAC 8.0 µm
[3.6]AB - [8.0]AB
z=0
z=6
z=3
z=2
IRAC
OBSERVATIONS OF GALAXIES AT
Z=2-3
Lyman-Break Galaxies (LBGs)
z~3
• Redshift ~3 galaxies selected by UV-dropout
technique (Steidel et al.)
• Optical observations sample the rest-frame UV:
are these really massive galaxies or just extreme
star-formers?
• IRAC bands sample the rest-frame near-IR: less
affected by extinction, dominated by old, lowermass stars  galaxy stellar mass
LBGs in Q1700 Field
P. Barmby et al. 2004
3.6
5.8
4.5
• Field around z=2.7 QSO,
chosen for high ecliptic
latitude.
• Deep optical imaging
allowed selection of few
hundred LBGs.
8.0 • Portion of field has deep
K-band data.
• ~20 LBG candidates have
spectra.
IRAC Detections of LGBs
• Most LBGs detected at 3.6, 4.5 m
• About half detected at 5.8, 8.0 m: SEDs
are fairly flat, sensitivity is lower at longer
wavelengths
LBGs: SED Model Fitting
GR
K
3.6 4.5 5.8
8.0
• Solar-metallicity, Salpeter
IMF models from B&C
2003
• Range of ages, star
formation timescale, E(BV), mass normalization
• Massive stellar systems
with recent star formation:
– M*=1.5-4 x 1010 Msun)
– SFR = 7-33 Msun/yr
– Age: 100-300 Myr
Galaxies at z =3
LGB Galaxies in EGS
J. Huang et al. (2005)
• Among 334 LBGs in the EGS area, 188 with spectroscopic redshifts
at z = 3 (Steidel et al. 2003).
• 253 LGBs are in the Spitzer EGS field.
– 211 are detected in the 3.6 um band.
– 199 at 4.5 um; 53 at 5.8 um; and 44 at 8.0 um
• 11 LGBs are detected in the MIPS 24um band (> 60 Jy).
– Includes 3 quasars and 1 AGN (Steidel et al. 2003).
– All 11of the 24 um LGBs are detected in all IRAC bands.
• Define 24 um LGBs as: Infrared-Luminous LBGs (ILLBGs).
– 5% of sample of LGBs.
R
3.6
4.5
5.8
8.0
24
INFRARED PROPERTIES OF LBGs
Infrared colors of
LGBs exhibit a wide
range of flux
densities in IRAC
bands.
8 um (rest frame 2
um) fluxes, which are
proportional to stellar
mass, extend over 1.5
mag.
COLOR - MAGNITUDE DIAGRAM FOR LBGS
Most LGBs in sample are
only detected at 3.6 and 4.5
um and have faint mid-IR
luminosities and blue R -[3.6]
colors.
Bright LGBs are red; dim
LGBs are blue;
LBGs with 8 um fluxes are
brigther; LBGs with 24 um
fluxes are still brighter.
LGBs are very diverse in
terms of both mass and dust
content.
Origin of the 24 m Emission from
ILLGBs
• QSOs and warm SCUBA
sources have rest J - Ks > 1.5
(Vega), indicative of an AGN;
Cutri et al. 2001; Ivison et al.
2004).
• ILLBGs have rest J - Ks < 1.5.
• ILLBGs are much redder,
consistent with starburst galaxy.
• ILLBGs are most likely
starburst galaxies with strong
dust emission.
•
ILLBGs, like cold SCUBA sources,
are starburst galaxies.
Dust Emission in Blue LGBs:
Stacking of LBGs Without 24 m Emission
LGB with z
LGB total
Characterization of the Galaxy
Population at z = 3
• There is a significant difference
between LBGs with and
without 8 micron emission.
• LGBs without 8 micron
detection have Mk (Vega) = 21.5, in the range of local dwarf
galaxies, suggesting they have
the same stellar mass, dust, and
star formation history.
• LBGs with 8 micron detection
are only slightly fainter and
bluer than ILLBGs; may be
similar.
• 8 micron sample is massive:
> 4 x 1010 M(sun); ILLBG: >
1011 M(sun), Rigopoulou et al.
(2005).
DISTANT RED GALAXIES (DRGs) AT
Z>2
Franx et al. (2000); Forster Screiber et al. (2004)
• Faint Infrared Extragalactic Survey (FIRES)
– HDFS (2.5’ x 2.5’)
– MS 1054-03 (5’ x 5’)
• Deep imaging
• UBVI (HST/WFPC2)
• Js, H, Ks (VLT/ISAAC); Ks < 22.5
• Selected galaxies with Js - Ks > 2.3
• Isolates galaxies at z > 2 with red rest-frame
optical colors.
IRAC Imaging of Distant Red Galaxies (z >
2)
in HDFS
Labbe et al. (2005)
• FOV = 5 arcmin2
• Deep imaging
• UBVI (HST/WFPC2)
• Js, H, Ks (VLT/ISAAC); Ks < 22.5
• 3.6, 4.5, 5.8, 8.0 micron (Spitzer/IRAC)
• Deepest Ks band image used to resolve galaxies;
confusion not issue
HUBBLE DEEP FIELD SOUTH
Central 2.5’ x 2.5’
region
Composite image
of K-band (blue),
3.6 (green) and 4.5
(red) microns.
DRGs circled.
Typical Properties DRG (JsKs > 2.3) Galaxies
Redshift
z = 2.6 ± 0.5
Number density
0.0014 ± 0.0004 h3/Mpc3 (40% of LBGs)
(approx. 3 per arcmin sq)
1  2 Gyr
1  2.5 mag
15  150 Msolar/yr
1011 Msolar
Ages
Av
SFR
Stellar Masses
*SED modeling:
Bruzual Charlot 03 models, Salpeter IMF, Calzetti 2000 dust law, solar metalicity
Single
Burst SF
Js - Ks > 2.3
LGBs
Lyman break Galaxies
JsKs galaxies
single burst
constant SF + Av=1.5
70% constant SF + dust
30% single burst
IRAC Imaging of DRG Galaxies in HDFS
•
Galaxies are massive (1011) and evolved (high M/LK)
May dominate stellar mass density at z ~ 23
•
Have high surface density (~ 1 arcmin2 to K = 22.5).
•
Space densities about one-half LGBs.
•
IRAC colors can distinguish between DRGs that are dusty star
forming galaxies (70%) and maximally old ``dead’’ galaxies
(30%).
•
The most massive galaxies are the oldest and have the highest
mass-to-light ratio.
HIGH-Z EXTREMELY RED OBJECTS IN HUDF
Haojing Yan et al. (2004)
• HUDF
– HST ACS and NICMOS imaging
– VLT Ks-band imaging
– GOODS IRAC 3.6, 4.5, 5.8 and 8.0 micron bands
• Sample of 17 infrared extremely red objects
(IEROs) with fv (3.6 m)/ fv (z850) > 20.
HIGH-Z EXTREMELY RED OBJECTS IN HUDF
Haojing Yan et al. (2004)
• IERO color criterea picks up the fainter, higher redshift EROs.
• All IEROs satisfy (J - K) > 2.3, and have similar surface density.
•
Median redshift in this sample is ~2.4.
• SEDs indicate presence of an old (~2 Gyr) stellar populations.
• Stellar mass ~ 0.1 to 1.6 x 1011 M(solar); mounting evidence for a
significant population of red, evolved galaxies at z > 2.
• May be direct progenitors for at least 14 to 51% of the local population
of massive, early-type galaxies.
Z = 2 - 3 Summary
• Earlier results have indicated that galaxies with masses ~ 1010 M(sun)
are already common at z ~ 3 (Papovich et al. 2001; Shapley et al.
2001).
– Well evolved stellar populations, implying formation at z > 5.
• Recent IR observations have identified galaxies with more massive,
evolved galaxies at z ~ 2 - 3.
– Stellar masses can exceed 1011 M(sun).
– Evidence for old stellar populations with ages 1.5 - 2.5 Gyr
• However, they are a diverse group in terms of mass and dust content.
• The 8um/24um bright LBGs (ILLBGs) are very massive (~ 1011 Msun)
and dusty, and may be the bridge between LBGs and cold SCUBA
sources.
• These massive galaxies may be the progenitors of today’s giant
elliptical galaxies.
• These results indicate that massive galaxies formed by z = 5 and
possibly by z = 15 - 20, favoring numerical simulation models with
rapid mass accumulation (Nagamine et al. 2004).
IRAC OBSERVATIONS OF GALAXIES AT
Z=5-7
Galaxies at z = 5 - 6 in CDFS
Eyles et al. (2005)
• GOODS Legacy Science Program in CDFS
–
–
–
–
GOODS HST/ACS/NICMOS
VLT/ISAAC
GOODS/IRAC (23.9 hr exposure); [3.6] = 26.5 (AB, 3 sigma)
Ground-based spectroscopic redshifts based on Ly-a emission
• Criteria: (i’ - z’)AB >1.5 mag (i - dropouts)
• z = 6 region important; indicates the end of the reionization of
Universe
• IRAC samples wavelengths longwards of age sensitive Balmer & 4000
A breaks at z = 6.
• Four z ~ 6 galaxies confirmed; two robust detections by IRAC.
• First Spitzer/IRAC detection of population at z ~ 6.
Galaxies at z = 5 - 6 in CDFS
Eyles et al. (2005)
SBM03#1 (z = 5.83)
Exponential decay SFR with t = 100 Myr at 320 Myr
M = 2.3 x 1010 M(sun)
Galaxies at z = 5 - 6 in CDFS
Eyles et al. (2005)
• From Ly-a emission, SFR > 6 M(sun)/yr
• Significant Balmer/4000A break, indicating a prominent older stellar
population which dominates stellar mass.
• Average stellar age > 100 Myr (250 - 650 Myr) for an exponentiallydeclining SFR (t ~ 70 -500 Myr).
• Best fit stellar masses are > 1010 M(sun)
• Indicates that at least some galaxies with stellar masses > 20% of mass
of L* galaxies today were already assembled within the first Gyr.
• May have played an important role in reionizing the Universe.
Galaxies at z = 5 - 6 in
HUDF/CDFS
Haojing Yan et al. (2005)
• GOODS Legacy Science Program in HUDF
– HST ACS/NICMOS ([z] < 30.0 (AB))
– VLT/ISAAC (K-band)
– GOODS/IRAC (23.2 hr, [3.6] = 26.4 (AB, 5 sigma)
• Candidates from Yan & Windhorst (2004)
– Z ~ 6 criteria: (i775 - z850) > 1.3, and non-detection at B and V
• Detected 3 objects at z ~ 6 and 11 objects at z ~ 5
– Verified IRAC (3.6 and 4.5 micron) can probe galaxies to such high
redshifts
– All IRAC objects reasonably isolatedd to avoid confusion
z~5 galaxies
BC03 Model
parameters:
Redshift (z)
Age (T)
Stellar Mass (M)
Star formation
history (t)
Reddening
E (B - V)
Metallicity (Z)
z~5 galaxies
Model
parameters:
Redshift (z)
Age (T)
Stellar Mass
(M)
Star formation
history (t)
1
Reddening
E (B - V)
Metallicity (Z)
1
5
10
5
10
z~6 galaxies
Model parameters:
Redshift (z)
Age (T)
Stellar Mass (M)
Star formation history
(t)
Reddening
E (B - V)
Metallicity (Z)
LYMAN-a EMITTERS AT Z ~ 6
J. Huang , L. Cowie, G. Fazio
GOODS HDFN
Spectroscopically
identified Ly a emitters.
Single Burst Model
Template
B - C Model
Salpeter IMF
E (B - V) = 0
z = 5.634
z = 5.634
z = 5.671
LYMAN-a EMITTERS AT Z ~ 6
J. Huang , L. Cowie, G. Fazio
GOODS HDFN
Spectroscopically identified
Ly a emitters
Single Burst Model Template
B - C Model
Salpeter IMF
E (B - V) = 0
z = 5.671
STARBURST GALAXY (6L*) AT Z = 5.5
Dow-Hygelund et al. (2005)
HST ACS
- red color: i775 - z850 = 1.5
VLT FORS2 UV
- continuum spectrum
- spectral features: LBG
- SFR = 142 M(sun)/yr
IRAC
- 3.6 mm: 23.3 (AB)
- 4.5 mm: 23.2 (AB)
- Mass: 1 - 6 x 1010 M(sun)
BD 38 in field of cluster RDCS 1252.9 -2927;
magnification: 0.3
Open circles: measurement; Dots: B-C theory
IRAC/HST Images of z~7 Lensed
Galaxy
Egami et al. 2004
SED Model Fitting
Bruzual-Charlot model
(GALAXEV); Redshift: 6.6-6.8
Significant Balmer Break
 Age > 50 Myr, quite
possibly a few/several
hundred Myr
Steeply rising UV continuum
toward 1216A
low extinction and/or low
metallicity (degenerate)
Stellar mass: ~ 109 M⊙
of z=3-4 LBG
stellar mass
SFR: ~ 0.1-5 M⊙yr-1
●
Summary
Haojing Yan et al. (2005)
• Galaxies as massive as ~ 1010 M(sun) already existed when
the Universe was about a billion years old.
– Stellar masses are similar to a typical LBG at z = 3
– Lower limits on space density at these stellar masses
consistent with recent LCDM models.
• Photometry shows pronounced Balmer break that results
from the dominant presence of stars with ages of a few
hundred Myr.
– strongly indicates that the Universe was already forming
massive galaxies at z > 7, consistent with WMAP
reionization results.
• All high-z galaxies are consistent with solar metallicities.
• Best fit models have no dust reddening and low extinction
values.