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P.S. Smith, E.M. Huff, G.D. Schmidt, D.C. Hines (Steward Observatory), R.M. Cutri, B.O. Nelson (IPAC, Caltech), B.J. Wilkes (Harvard-Smithsonian CfA)
The Two-Micron All Sky Survey (2MASS) has found a large population of previously unknown Active Galactic Nuclei (AGN). These objects more than double the space density of AGN at low redshift (z < 0.6) and were found from their ``red'' near-IR colors (J - Ks > 2). This new AGN sample includes many objects with near-IR luminosities comparable to those of traditional ultraviolet-excess QSOs. Luminous 2MASS AGN (objects having MK_s < -23) have a median MK_s = -26.6 that is nearly identical to the Palomar-Green QSOs (median MK_s = -26.7). Despite this similarity, the optical properties of 2MASS and UV-excess QSOs are vastly different with the near-IR--selected objects being optically much fainter. The median B - Ks\/ color index of the 2MASS QSOs is nearly two magnitudes redder than that of PG QSOs in the same redshift range. In addition, more than half of the 2MASS sample is composed of narrow-line (Type~2) AGN and objects of intermediate optical spectral type (Type~1.5--1.9). Together with higher levels of optical polarization observed for 2MASS QSOs, these properties suggest that 2MASS has revealed a large number of AGN that are partially obscured by dust associated with the nucleus. Also, their X-ray faintness relative to other AGN indicates N\rm H ~1021--1023~cm-2. We present preliminary results from UV spectroscopy of a small sample of X-ray--observed 2MASS QSOs obtained with the Space Telescope Imaging Spectrograph\/ (STIS\/). 2MASS QSOs, if detected in the UV by STIS\/, all show very steep UV spectra and are extremely faint in the ultraviolet compared to PG QSOs of similar near-IR luminosity. The Hubble Space Telescope\/ observations and contemporaneous ground-based optical spectroscopy obtained to date are consistent with dust extinction and reddening of an otherwise typical AGN spectrum. Simple models like the unified scheme for AGN adequately explain the range in infrared, optical polarization, and X-ray/UV absorption properties of the 2MASS QSOs. Support for this work was provided by NASA through grant number GO-09161.01 from the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy Incorporated, under NASA contract NAS5-26555. 2MASS is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and NSF.
The author(s) of this abstract have provided an email address for comments about the abstract: psmith@as.arizona.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.