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SPITZER/IRAC OBSERVATIONS OF GALAXIES AT Z > 2

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.

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SPITZER/IRAC OBSERVATIONS OF GALAXIES AT Z > 2

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  1. SPITZER/IRAC OBSERVATIONS OF GALAXIES AT Z > 2 Giovanni G. Fazio Jiasheng Huang Harvard-Smithsonian Center for Astrophysics Cambaridge, MA 02138, USA

  2. 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. PAH starlight dust re-radiation

  3. 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

  4. 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)).

  5. Extended Groth Strip (EGS) 5’x5’ IRAC 3.6 µm IRAC 8.0 µm

  6. z = 0 z = 6 [3.6]AB - [8.0]AB z = 3 z = 2

  7. IRAC OBSERVATIONS OF GALAXIES AT Z = 2 - 3

  8. 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

  9. LBGs in Q1700 FieldP. Barmby et al. 2004 3.6 4.5 • 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. 5.8 8.0

  10. 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

  11. LBGs: SED Model Fitting G R K3.6 4.5 5.8 8.0 • 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 = 7-33Msun/yr • Age: 100-300 Myr

  12. Galaxies at z =3LGB Galaxies in EGSJ. 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 mJy). • 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.

  13. R 3.6 4.5 5.8 8.0 24

  14. 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.

  15. 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.

  16. Origin of the 24 mm 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.

  17. Dust Emission in Blue LGBs:Stacking of LBGs Without 24 mm Emission LGB with z LGB total

  18. 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).

  19. DISTANT RED GALAXIES (DRGs) AT Z > 2Franx 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.

  20. 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

  21. 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.

  22. 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) • Ages 1  2 Gyr • Av 1  2.5mag • SFR 15  150 Msolar/yr • Stellar Masses 1011 Msolar • *SED modeling: • Bruzual Charlot 03 models, Salpeter IMF, Calzetti 2000 dust law, solar metalicity

  23. Single Burst SF Js - Ks > 2.3 LGBs Lyman break Galaxies JsKs galaxies single burst constant SF + Av=1.5

  24. 70% constant SF + dust 30% single burst

  25. 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.

  26. 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 mm)/ fv (z850) > 20.

  27. 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.

  28. 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).

  29. IRAC OBSERVATIONS OF GALAXIES AT Z = 5 - 7

  30. 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-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.

  31. 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)

  32. 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 (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.

  33. 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

  34. z~5 galaxies BC03 Model parameters: Redshift (z) Age (T) Stellar Mass (M) Star formation history (t) Reddening E (B - V) Metallicity (Z)

  35. z~5 galaxies Model parameters: Redshift (z) Age (T) Stellar Mass (M) Star formation history (t) Reddening E (B - V) Metallicity (Z) 1 5 10 1 5 10

  36. z~6 galaxies Model parameters: Redshift (z) Age (T) Stellar Mass (M) Star formation history (t) Reddening E (B - V) Metallicity (Z)

  37. 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

  38. 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

  39. 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

  40. IRAC/HST Images of z~7 Lensed GalaxyEgami et al. 2004

  41. 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

  42. SummaryHaojing 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.

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