AAS 207th Meeting, 8-12 January 2006
Session 80 Evolution of Galaxies, Galaxies Surveys II
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

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[80.11] IRAC and K-band Colors of Galaxies as a Function of Redshift

S. M. Faber (UCO/Lick Observatory), J.-S. Huang (Harvard-Smithsonian Center for Astrophysics), K. G. Noeske (UCO/Lick Observatory), K. Bundy (California Institute of Technology), DEEP2 Team, IRAC GTO Team, Palomar K-band Team

We have compiled a sample of 1500 galaxies in the Extended Groth Strip (EGS) having DEEP2 redshifts and complete photometry in R, K, and IRAC Channels 1-4. Restframe U-B colors are available, and the sample is magnitude-limited at R ~24. IRAC colors are sensitive to the 1.6 \mu bump and yield accurate photo-z's for galaxies in the range z = 1 to 1.5. Adding K extends the reach down to z = 0. Objects beyond z = 1.5 are clearly distinguishable from lower-redshift objects, but the exact redshift calibration is uncertain due to the paucity of objects with known redshifts. Color bimodality is apparent in IRAC colors out to z = 1.4, generally paralleling the division seen in rest-frame U-B colors. However, some optically red galaxies are found in the IRAC blue-galaxy region and are likely to be dust-reddened spirals or diffuse red galaxies. IRAC may thus offer a new way of weeding out dusty star-forming galaxies from bona-fide spheroidal galaxies on the red sequence. Blue-galaxy SEDs are similar to spheroidal SEDs in K, Ch1, and Ch2 but are brighter in Ch4 (and Ch3 at low z) due to PAH emission and/or hot dust. A clump of galaxies stands out in the IRAC Ch1-Ch4 vs.Ch1-Ch2 two-color diagram. Model SEDs predict that these objects lie in the range z = 1.5 to 2.5. If this is confirmed with more spectroscopic redshifts, this diagram would offer a way to identify large samples of galaxies near z = 2 that are more nearly mass-limited, independent of optical color.

This work was supported by NSF grants AST~00-71198 and AST~00-71048 to the DEEP2 team and utilized observations made with the Keck, Palomar, NOAO, and Spitzer Observatories.

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