Extragalactic surveys are, like a wedding cake, layered in increasing depth but decreasing area. The Chandra COSMOS Survey hits a sweet spot

among the wedding cake layers: deep enough to find obscured AGN with optical galaxy continua and wide enough to have large samples and to find unusual, rare objects. Yet, the Chandra COSMOS Survey sources are bright enough that virtually all X-ray sources can be identified and followed up across their spectral energy distributions, especially with optical or near-IR spectroscopy, using the vast COSMOS multiwavelength photometric and spectroscopic database.

Highlights from the Chandra COSMOS Survey include:

COSMOS is a pan-chromatic - radio to X-ray - survey of a patch of the sky both large enough (2 sq.deg) and deep enough (AB~26 at the optical wavelength) to study galaxy and quasar evolution up to high redshifts in typical environments, with minimal 'cosmic bias' . Almost all major telescopes have observed this field deeply. From space - Hubble, Spitzer, Herschel, GALEX, XMM-Newton. Ground-based - VLA, Subaru, CTIO, KPNO, CFHT, Magellan, VLT. The central region (~1sq.deg) has been targeted with even deeper surveys by Chandra, VLT, HST (CANDELS), VISTA. The location of COSMOS near the equator (10h +02deg) allows access by all future facilities (esp. JVLA, ALMA).

Chandra is observing the entire field to a depth of ~2e-16 cgs with ACIS in the 0.5 - 2keV band, spending nearly 2 days at each location (~180 ksec exposure). The first phase was the Chandra COSMOS Survey (C-COSMOS), a 1.8 Megasecond GO program (PI: Martin Elvis, Cycle 8) covering the central 0.9 sq. deg. C-COSMOS found 1761 X-ray sources, 97% of them having optical or infrared counterparts. Only 2 sources are truly empty fields. The second phase was approved in Chandra Cycle 14: the 2.8 Megasecond 'COSMOS Legacy' survey (PI: Francesca Civano), the second largest Chandra GO program awarded in 14 years. COSMOS-Legacy will complete the Chandra coverage of the full COSMOS field to the same depth, during 2013-2014.

• COSMOS (Caltech)
• COSMOS IRSA
• Local C-COSMOS Site (Password Access Only)

Martin Elvis, Francesca Civano, Tom Aldcroft Antonella Fruscione