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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). The observations - PowerPoint PPT Presentation
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Spitzer observations of Lyman Break Spitzer observations of Lyman Break GalaxiesGalaxies
Dimitra Rigopoulou (Oxford,UK)J.-S. Huang, G. Fazio,(CfA, Harvard),& the IRAC team
C. Papovich, E. Egami (Arizona, MIPS)
Extreme Starbursts: Near and Far, 14-20 August, Lijiang, China
The observations
Deep IRAC & MIPS imaging (part of the IRAC GTOprogram)
IRAC: 3 hrs/pointingMIPS: 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
Extended Groth Strip (EGS)
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
R [3.6] [4.5] [5.8] [8.0] [24]
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
U G R J K IRAC MIPS
Mag
(A
B)
Rest wavelength (microns)
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
Constant vs. exponentially decaying SF models
Huang et al. 2005
Huang et al. 2005
BC03 model fits
Rigopoulou et al. 2005
Blue LBGRed LBG
Constant vs. Exponentially decaying Mass estimates
Large scatter in [3.6] fluxes
4 magnitudes in [3.6]
1.5 magnitude in [K](Shapley et al. 2001)
[5.8] rest frame H-band
[3.6] rest frame I-band
Mid-IR colours of LBGs
Mid-IR colours of LBGs
[8.0] rest—frame K-band
Best tracer for mass
Massive LBGs have R-[3.6] >=3
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 MyrE(B - V) : 0.17 E(B - V): 0.35
Does extinction matter?
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
missing link to SCUBA galaxies?
SCUBA galaxies
Cold SED: Arp220
80—90% of sampleStarburst-like
Warm SED: Mrk 231
Egami et al. (2004)
Origin of the [24] emission
@ z=3 [3.6] — [8.0] [z’] – [K]blue for SBred for AGN
[8.0] – [24] [K] – [6.0]red for SBblue for AGN
Ivison et al. 2004
Saracco 04
Drory 04
Cole et al 00
This work
Number density as a function of redshift
= 0.7 , m = 0.3, H0 = 70)
Kauffmann99
Baugh03
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…..)