Spitzer observations of Lyman Break Galaxies

1. Extreme Starbursts: Near and Far, 14-20 August, Lijiang, China Spitzer observations of Lyman Break Galaxies Dimitra Rigopoulou (Oxford,UK) J.-S. Huang, G. Fazio,(CfA, Harvard),& the IRAC team C. Papovich, E. Egami (Arizona, MIPS)

2. The observations Deep IRAC & MIPS imaging (part of the IRAC GTO program) IRAC: 3 hrs/pointing MIPS: slow scanning 2 deg x 10 arcmin Limiting flux densities (5 sigma) (3.6 µm) : 0.5 µJy (4.5 µm) : 0.5 µJy (5.8 µm) : 2.7 µJy (8.0 µm) : 2.7 µJy (24 µm) : 60 µJy

3. Extended Groth Strip (EGS)

4. The EGS Lyman Break Galaxies sample • Among 334 LBGs in the EGS area, 193 with spectroscopic redshfit (Steidel et al 2003) • 244 are in the Spitzer EGS field • ~200 are detected in the 3.6/4.5µm band • ~50 are detected in the 5.8/8.0µm band • 6 are detected in the 24µm band

5. R [3.6] [4.5] [5.8] [8.0] [24]

6. U G R J K IRAC MIPS Mag (AB) Rest wavelength (microns) Rest-frame shifted and R-normalised SEDs • Spitzer reveals • the diversity of LBGs • IRAC-bright LBGs • have more dust? • more massive? • MIPS detections • probes the rest—frame • mid-infrared

7. Estimating stellar masses • Use Bruzual and Charlot (2003) code with • Padova 1994 tracks • Exponentially decaying SFR α exp (-t / τ) where • τ=0.05, 0.1,0.5,2.0, 5.0 Gyr (e-folding times) • t = age • Constant SFR • Solar metallicity • Calzetti (2000) extinction law

8. Constant vs. exponentially decaying SF models Huang et al. 2005

9. Huang et al. 2005

10. BC03 model fits Rigopoulou et al. 2005

11. Red LBG Blue LBG Constant vs. Exponentially decaying Mass estimates

12. Large scatter in [3.6] fluxes 4 magnitudes in [3.6] 1.5 magnitude in [K] (Shapley et al. 2001)

13. Mid-IR colours of LBGs [3.6]  rest frame I-band [5.8]  rest frame H-band

14. Mid-IR colours of LBGs [8.0]  rest—frame K-band Best tracer for mass

15. Massive LBGs have R-[3.6] >=3

16. A test – case: Westphal D49 In K-selected LBG sample of Shapley et al. (2001) Constant SFR model U G R J K U G R J K IRAC Shapley et al. (1991) Our model: Age: 1139 Myr Age: 1350 Myr E(B - V) : 0.17 E(B - V): 0.35

17. Does extinction matter?

18. missing link to SCUBA galaxies? • IRAC has “discovered” a new class of LBGs: • luminous at [8.0] microns • faint in the optical bands • significant(?) dust extinction • slightly older stellar pops • …. massive, M > 1011 M

19. SCUBA galaxies Cold SED: Arp220 80—90% of sample Starburst-like Warm SED: Mrk 231 Egami et al. (2004)

20. Origin of the [24] emission @ z=3 [3.6] — [8.0]  [z’] – [K] blue for SB red for AGN [8.0] – [24]  [K] – [6.0] red for SB blue for AGN

21. Ivison et al. 2004

22. Number density as a function of redshift Cole et al 00 Drory 04 Saracco 04 This work Kauffmann99 Baugh03  = 0.7 , m = 0.3, H0 = 70)

23. Summary • LBGs are detected in the mid-IR. Optical does not reveal full complexity. • Large scatter in IRAC fluxes  variations in mass, age, extinction • Average SED  young populations • We detect more massive LBGs at z=3 than predicted by hierarchical models (downsizing…..)