SPITZER/IRAC OBSERVATIONS OF GALAXIES AT Z > 2

Giovanni G. FazioJiasheng Huang

Harvard-Smithsonian Center for AstrophysicsCambaridge, 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 rest-frame 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.

starlightPAH

dustre-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 , 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)

IRAC 3.6 µm IRAC 8.0 µm

5’x5’

z = 0

z = 6

z = 3

z = 2[3.6

] AB -

[8.

0]A

B

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, lower-mass stars galaxy stellar mass

LBGs in Q1700 FieldP. Barmby et al. 2004

• Field around z=2.7 QSO, chosen for high ecliptic latitude.

• Deep optical imaging allowed selection of few hundred LBGs.

• Portion of field has deep K-band data.

• ~20 LBG candidates have spectra.

3.6

4.5

5.8

8.0

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

• Solar-metallicity, Salpeter IMF models from B&C 2003

• Range of ages, star formation timescale, E(B-V), 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

G R K 3.6 4.5 5.8 8.0

Galaxies at z =3LGB 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 HDFSLabbe 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 (JsKs > 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)

Ages 1 2 GyrAv 1 2.5 mag

SFR 15 150 Msolar/yr Stellar Masses 1011 Msolar

*SED modeling: Bruzual Charlot 03 models, Salpeter IMF, Calzetti 2000 dust law, solar metalicity

Lyman break GalaxiesJsKs galaxies

single burstconstant SF + Av=1.5

LGBs

Js - Ks > 2.3

Single Burst SF

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 ~ 23

• 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 HUDFHaojing 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 HUDFHaojing 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 CDFSEyles 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- 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 CDFSEyles 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 CDFSEyles 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 exponentially-declining SFR ( ~ 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/CDFSHaojing 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 ()

Reddening E (B - V)

Metallicity (Z)

z~5 galaxies

Model parameters:

Redshift (z)

Age (T)

Stellar Mass (M)

Star formation history ()

Reddening E (B - V)

Metallicity (Z)

1 5 10

1 5 10

z~6 galaxies

Model parameters:

Redshift (z)

Age (T)

Stellar Mass (M)

Star formation history ()

Reddening E (B - V)

Metallicity (Z)

LYMAN- EMITTERS AT Z ~ 6J. Huang , L. Cowie, G. Fazio

GOODS HDFN

Spectroscopically identified Ly emitters.

Single Burst Model Template

B - C Model

Salpeter IMF

E (B - V) = 0

z = 5.634

z = 5.671

z = 5.634

LYMAN- EMITTERS AT Z ~ 6J. Huang , L. Cowie, G. Fazio

GOODS HDFN

Spectroscopically identified Ly 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 MyrSteeply rising UV continuum toward 1216A

low extinction and/or low metallicity (degenerate) Stellar mass: ~ 109 M⊙

of z=3-4 LBG

stellar massSFR: ~ 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 CDM 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.