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arXiv2014

SDSS J013127.34$-$032100.1: A newly discovered radio-loud quasar at $z=5.18$ with extremely high luminosity

Only very few z>5 quasars discovered to date are radio-loud, with a radio-to-optical flux ratio (radio-loudness parameter) higher than 10. Here we report the discovery of an optically luminous radio-loud quasar, SDSS J013127.34-032100.1 (J0131-0321 in short), at z=5.18+-0.01 using the Lijiang 2.4m and Magellan telescopes. J0131-0321 has a spectral energy distribution consistent with that of radio-loud quasars. With an i-band magnitude of 18.47 and radio flux density of 33 mJy, its radio-loudness parameter is ~100. The optical and near-infrared spectra taken by Magellan enable us to estimate its bolometric luminosity to be L_bol ~ 1.1E48 erg/s, approximately 4.5 times greater than that of the most distant quasar known to date. The black hole mass of J0131-0321 is estimated to be 2.7E9 solar masses, with an uncertainty up to 0.4 dex. Detailed physical properties of this high-redshift, radio-loud, potentially super-Eddington quasar can be probed in the future with more dedicated and intensive follow-up observations using multi-wavelength facilities.

astro-ph.GA↗

arXiv2013

Discovering bright quasars at intermediate redshifts based on the optical/near-IR colors

Identifications of quasars at intermediate redshifts (2.2<z<3.5) are inefficient in most previous quasar surveys as their optical colors are similar to those of stars. The near-IR K-band excess technique has been suggested to overcome this difficulty. Our study also proposed to use optical/near-IR colors for selecting z<4 quasars. To this method, we selected 105 unidentified bright targets with i<18.5 from the quasar candidates of SDSS DR6 with both SDSS ugriz optical and UKIDSS YJHK near-IR photometric data, which satisfy our proposed Y-K/g-z criterion and have photometric redshifts between 2.2 and 3.5 estimated from the 9-band SDSS-UKIDSS data. 43 of them were observed with the 2.16m telescope of NAOC in 2012. 36 of them were identified as quasars at 2.1<z<3.4. High success rate of discovering these quasars in the SDSS spectroscopic surveyed area demonstrates the robustness of both the Y-K/g-z selection criterion and the photometric redshift estimation technique. We also used the above criterion to investigate the possible star contamination rate to the quasar candidates of SDSS DR6, and found that it is much higher in selecting 3<z<3.5 quasar candidates than selecting lower redshift ones (z<2.2). The significant improvement in the photometric redshift estimation by using the 9-band SDSS-UKIDSS data than using the 5-band SDSS data is demonstrated and a catalog of 7,727 unidentified quasar candidates with photometric redshifts between 2.2 and 3.5 is provided. We also tested the Y-K/g-z selection criterion with the SDSS-III/DR9 quasar catalog, and found 96.2% of 17,999 DR9 quasars with UKIDSS Y and K-band data satisfy our criterion. With some samples of red and type II quasars, we found that 88% and 96.5% of them can be selected by the Y-K/g-z criterion respectively, which supports that using the Y-K/g-z criterion we can efficiently select both unobscured and obscured quasars. (abridged)

astro-ph.CO↗