Characterizing the WISE-selected Heavily Obscured Quasar Population with Optical Spectroscopy from the Southern African Large Telescope

• Published in: arXiv.org 2017-11
• Main Authors: Hviding, R E, Hickox, R C, Hainline, K N, Carroll, C M, DiPompeo, M A, Yan, W, Jones, M L
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Photometry; Quasars; Red shift; Sky surveys (astronomy); Spectroscopy; Spectrum analysis; Wide-field Infrared Survey Explorer
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1711.01269

We present the results of an optical spectroscopic survey of 46 heavily obscured quasar candidates. Objects are selected using their mid-infrared (mid-IR) colours and magnitudes from the Wide-Field Infrared Survey Explorer (WISE) and their optical magnitudes from the Sloan Digital Sky Survey (SDSS). Candidate Active Galactic Nuclei (AGNs) are selected to have mid-IR colours indicative of quasar activity and lie in a region of mid-IR colour space outside previously published X-ray based selection regions. We obtain optical spectra for our sample using the Robert Stobie Spectrograph on the Southern African Large Telescope. Thirty objects (65%) have identifiable emission lines, allowing for the determination of spectroscopic redshifts. Other than one object at \(z\sim2.6\), candidates have moderate redshifts ranging from \(z=0.1\) to \(0.8\) with a median of 0.3. Twenty-one (70%) of our objects with identified redshift (46% of the whole sample) are identified as AGNs through common optical diagnostics. We model the spectral energy distributions of our sample and found that all require a strong AGN component, with an average intrinsic AGN fraction at 8\(\,\mu\)m of 0.91. Additionally, the fits require large extinction coefficients with an average \(E(B-V)_\textrm{AGN} = 17.8\) (average \(A(V)_\textrm{AGN} = 53.4\)). By focusing on the area outside traditional mid-IR photometric cuts, we are able to capture and characterise a population of deeply buried quasars that were previously unattainable through X-ray surveys alone.