Improvement of Quasar Surveys Utilizing Infrared and Optical Data: II Spectroscopic Results

Astronomy and Astrophysics – Astronomy

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Peak quasar activity is found to occur between redshifts of z=2 to z=3, although, selection of quasars in this range of redshifts is incomplete in most optical surveys because quasar colors mimic those of stars at these redshifts. We have utilized infrared data to better search for quasars at redshifts z=2 to 3 by cross matching optical data from the Sloan Digital Sky Survey (SDSS) and near infrared (NIR) data from the 2 Micron All Sky Survey (2MASS). We have computed colors for each matched object and plotted the matched objects in various color spaces. We have also computed the expected colors of synthetic quasar spectra in the SDSS and 2MASS pass bands to identify the expected optical/NIR colors of quasars at z=2 to 3 as well as plot known SDSS quasars of redshift z < 3 to identify where known quasars lie in various color spaces. We then selected objects as quasar candidates whose colors diverge from the locus of stars, with colors in the same region of color space as our synthetic quasar spectra. We checked these candidate objects against the SDSS quasar catalog and those lacking SDSS spectroscopy were considered as candidates for follow-up. We obtained spectroscopy of 110 candidates in September 2007 at the 2.1 m telescope at Kitt Peak National Observatory. Here we present this spectroscopic data and the resulting discovered quasars, including their properties as compared to the known SDSS quasars. We also discuss how the addition of infrared imaging to optical surveys improves the selection of quasars at redshifts z=2 to 3.

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