The <i>Chandra</i> Multiwavelength Project: Optical Follow‐up of Serendipitous <i>Chandra</i> Sources

Abstract

We present followup optical g', r', and i', imaging and spectroscopy of\nserendipitous X-ray sources detected in 6 archival Chandra, images included in\nthe Chandra, Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected\nbetween 3e-16 and 2e-13 (with a median flux of 3e-15 erg cm-2 s-1, we find\noptical counterparts for 377 (78%), or 335 (68%) counting only unique\ncounterparts. We present spectroscopic classifications for 125 objects,\nrepresenting 75% of sources with r&lt;21 optical counterparts (63% to r=22). Of\nall classified objects, 63 (50%) are broad line AGN, which tend to be blue in\ng-r colors. X-ray information efficiently segregates these quasars from stars,\nwhich otherwise strongly overlap in these SDSS colors until z&gt;3.5. We identify\n28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%)\nare absorption line galaxies. Eight galaxies lacking broad line emission have\nX-ray luminosities that require they host an AGN (logL_X&gt;43). Half of these\nhave hard X-ray emission suggesting that high gas columns obscure both the\nX-ray continuum and the broad emission line regions. We find objects in our\nsample that show signs of X-ray or optical absorption, or both, but with no\nstrong evidence that these properties are coupled. ChaMP's deep X-ray and\noptical imaging enable multiband selection of small and/or high-redshift groups\nand clusters. In these 6 fields we have discovered 3 new clusters of galaxies,\ntwo with z&gt;0.4, and one with photometric evidence that it is at a similar\nredshift.\n