FarInfrared Stacking of LIRGs in the COSMOS Field

Luminous Infrared Galaxies n n n Le Floc’h 2005 LIR > 1012 Lsun Rare

Problem 1 - Poor sensitivity n Detector sensitivity and resolution is poor at longer

Solution - Stacking! n n n Average the signal from many nondetections to find

Data & Results n Use MIPS 24 µm source list (Le Floc’h et al.

Evolution of Mid/Far-IR Color n n No trends in 70/24 µm color 160/24 µm

Comparison to Models n n Compare to models of Arp 220, Mrk 231, and

Model Fits n n 2 Best-fits found using Le. Phare (Ilbert & Arnouts) Fit

Average IR Luminosities n n Derive average LIR from average of best-fit models Create

Summary n n n 24 µm sources have average flux densities consistent with normal

n 35, 797 sources detected at 24 microns with photometric redshifts n n 1,

A Median Stacking Analysis n n Median stacking is robust to small numbers of

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Far-Infrared Stacking of LIRGs in the COSMOS Field Nicholas Lee COSMOS Team Meeting - 6/8/10 ar. Xiv: 1005. 3852 Emeric Le Floc’h, David Sanders, Dave Frayer, Stephane Arnouts, Olivier Ilbert, Hervé Aussel, Mara Salvato, Nick Scoville, Jeyhan Kartaltepe

Luminous Infrared Galaxies n n n Le Floc’h 2005 LIR > 1012 Lsun Rare in present universe, dominate star formation by z~1 Play important role in evolution of L > L* galaxies

Problem 1 - Poor sensitivity n Detector sensitivity and resolution is poor at longer wavelengths n n Hard to identify sources Only the brightest sources are detected MIPS 160 24 70 µm µm

Problem 2 - Characterization

Solution - Stacking! n n n Average the signal from many nondetections to find average properties Noise will drop as N 1/2 (perfect for COSMOS) Use median (not mean) stacking analysis

Data & Results n Use MIPS 24 µm source list (Le Floc’h et al. 2009) n n 0. 6 1. 0<<zz<<0. 8 1. 2; ; 0. 20 0. 06 m. Jy<<SS 2424<<0. 50 0. 08 m. Jy Bin in 56 bins of photometric redshift and S 24 0 < z < 2. 5 0. 06 m. Jy < S 24 < 3. 0 m. Jy Stack at 70 µm and 160 µm

Evolution of Mid/Far-IR Color n n No trends in 70/24 µm color 160/24 µm color shows warmer colors (lower ratios) n Evidence for increasing AGN fraction in more luminous sources

Comparison to Models n n Compare to models of Arp 220, Mrk 231, and “normal” starforming galaxies (Dale & Helou 2002) Inconsistent with Arp 220 n 24 µm biased against heavily obscured objects

Model Fits n n 2 Best-fits found using Le. Phare (Ilbert & Arnouts) Fit to 4 libraries n n Dale & Helou (2002) Chary & Elbaz (2001) Lagache (2003) Siebenmorgen & Krugel (2007)

Average IR Luminosities n n Derive average LIR from average of best-fit models Create new method of estimating LIR that takes into account long wavelength data

Summary n n n 24 µm sources have average flux densities consistent with normal star-forming galaxies and Mrk 231, a galaxy known to host an AGN. 24 µm sources have average flux densities inconsistent with heavily obscured galaxies like Arp 220 Sources with brighter S 24 have warmer S 160/S 24 colors, possibly suggesting a higher AGN fraction. We provide a new table to estimate LIR using our stacked 70 and 160 µm data. Previous methods of estimating LIR using only 24 µm overestimate the true LIR at high redshifts.

The End!

n 35, 797 sources detected at 24 microns with photometric redshifts n n 1, 512 sources detected at 70 microns 499 sources detected at 160 microns

A Median Stacking Analysis n n Median stacking is robust to small numbers of bright sources (not dominated by objects with high flux) Does not require a maximum flux density cutoff prior to stacking Mean stack Median stack