## Statistical Science

### Finding the Most Distant Quasars Using Bayesian Selection Methods

Daniel Mortlock

#### Abstract

Quasars, the brightly glowing disks of material that can form around the super-massive black holes at the centres of large galaxies, are amongst the most luminous astronomical objects known and so can be seen at great distances. The most distant known quasars are seen as they were when the Universe was less than a billion years old (i.e., $\sim\!7\%$ of its current age). Such distant quasars are, however, very rare, and so are difficult to distinguish from the billions of other comparably-bright sources in the night sky. In searching for the most distant quasars in a recent astronomical sky survey (the UKIRT Infrared Deep Sky Survey, UKIDSS), there were $\sim\!10^{3}$ apparently plausible candidates for each expected quasar, far too many to reobserve with other telescopes. The solution to this problem was to apply Bayesian model comparison, making models of the quasar population and the dominant contaminating population (Galactic stars) to utilise the information content in the survey measurements. The result was an extremely efficient selection procedure that was used to quickly identify the most promising UKIDSS candidates, one of which was subsequently confirmed as the most distant quasar known to date.

#### Article information

Source
Statist. Sci., Volume 29, Number 1 (2014), 50-57.

Dates
First available in Project Euclid: 9 May 2014

https://projecteuclid.org/euclid.ss/1399645728

Digital Object Identifier
doi:10.1214/13-STS432

Mathematical Reviews number (MathSciNet)
MR3201846

Zentralblatt MATH identifier
1332.85006

#### Citation

Mortlock, Daniel. Finding the Most Distant Quasars Using Bayesian Selection Methods. Statist. Sci. 29 (2014), no. 1, 50--57. doi:10.1214/13-STS432. https://projecteuclid.org/euclid.ss/1399645728

#### References

• Dye, S. et al. (2006). The UKIRT Infrared Deep Sky Survey early data release. Monthly Notices of the Royal Astronomical Society 372 1227–1252.
• Fan, X. et al. (2006a). Constraining the evolution of the ionizing background and the epoch of reionization with $z\sim 6$ quasars. II. A sample of 19 quasars. The Astronomical Journal 132 117–136.
• Fan, X., Narayanan, V. K., Lupton, R. H. et al. (2001). A survey of $z>5.8$ quasars in the Sloan Digital Sky Survey. I. Discovery of three new quasars and the spatial density of luminous quasars at $z\sim 6$. The Astronomical Journal 122 2833–2849.
• Fan, X., Strauss, M. A., Schneider, D. P. et al. (2003). A survey of $z>5.7$ quasars in the Sloan Digital Sky Survey. II. Discovery of three additional quasars at $z>6$. The Astronomical Journal 125 1649–1659.
• Fan, X., Strauss, M. A., Richards, G. T. et al. (2006b). A survey of $z>5.7$ quasars in the Sloan Digital Sky Survey. IV. Discovery of seven additional quasars. The Astronomical Journal 131 1203–1209.
• Glikman, E., Eigenbrod, A., Djorgovski, S. G., Meylan, G., Thompson, D., Mahabal, A. and Courbin, F. (2008). An exploratory search for $z>6$ quasars in the UKIDSS Early Data Release. The Astronomical Journal 136 954–962.
• Lawrence, A. et al. (2007). The UKIRT Infrared Deep Sky Survey (UKIDSS). Monthly Notices of the Royal Astronomical Society 379 1599–1617.
• Mortlock, D. J. et al. (2011). A luminous quasar at a redshift of $z=7.085$. Nature 474 616–619.
• Mortlock, D. J., Patel, M., Warren, S. J., Hewett, P. C., Venemans, B. P., McMahon, R. G. and Simpson, C. (2012). Probabilistic selection of high-redshift quasars. Monthly Notices of the Royal Astronomical Society 419 390–410.
• Rees, M. J. (1984). Black hole models for active galactic nuclei. Annual Review of Astronomy and Astrophysics 22 471–506.
• Schmidt, M. (1963). 3C 273: A star-like object with large red-shift. Nature 197 1040–1041.
• Warren, S. J., Hewett, P. C. and Osmer, P. S. (1994). A wide-field multicolor survey for high-redshift quasars, $z\geq2.2$. 3: The luminosity function. Astrophysical Journal, Part 1 421 412–433.
• Willott, C. J. et al. (2007). Four quasars above redshift 6 discovered by the Canada-France High-z Quasar Survey. The Astronomical Journal 134 2435–2450.
• Willott, C. J. et al. (2010). The Canada-France High-z Quasar Survey: Nine new quasars and the luminosity function at redshift 6. The Astronomical Journal 139 906–918.
• York, D. G. et al. (2000). The Sloan Digital Sky Survey: Technical summary. The Astronomical Journal 120 1579–1587.