Abstract
Being observed only one billion years after the Big Bang,z∼ 7 quasars are a unique opportunity for exploring the early Universe. However, only twoz∼ 7 quasars have been discovered in near-infrared surveys: the quasars ULAS J1120+0641 and ULAS J1342+0928 atz= 7.09 andz= 7.54, respectively. The rarity of these distant objects, combined with the difficulty of distinguishing them from the much more numerous population of Galactic low-mass stars, requires using efficient selection procedures. The Canada-France High-zQuasar Survey in the Near Infrared (CFHQSIR) has been carried out to search forz∼ 7 quasars using near-infrared and optical imaging from the Canada-France Hawaii Telescope (CFHT). Our data consist of ∼130 deg2of Wide-field Infrared Camera (WIRCam)Y-band images up to a 5σlimit ofYAB∼ 22.4 distributed over the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) Wide fields. After follow-up observations inJband, a first photometric selection based on simple colour criteria led us to identify 36 sources with measured high-redshift quasar colours. However, we expect to detect only ∼2 quasars in the redshift range 6.8 <z< 7.5 down to a rest-frame absolute magnitude ofM1450= −24.6. With the motivation of ranking our high-redshift quasar candidates in the best possible way, we developed an advanced classification method based on Bayesian formalism in which we model the high-redshift quasars and low-mass star populations. The model includes the colour diversity of the two populations and the variation in space density of the low-mass stars with Galactic latitude, and it is combined with our observational data. For each candidate, we compute the probability of being a high-redshift quasar rather than a low-mass star. This results in a refined list of the most promising candidates. Our Bayesian selection procedure has proven to be a powerful technique for identifying the best candidates of any photometrically selected sample of objects, and it is easily extendable to other surveys.
Highlights
Quasars reside at the centres of active galactic nuclei (AGNs) and are believed to be powered by mass accretion onto a supermassive black hole (SMBH)
Several scenarios for the formation of SMBHs seeds, including, for instance, remnants of Population III stars (e.g. Madau & Rees 2001; Volonteri et al 2003) or a direct collapse of gas in atomic cooling halos (DCBH, e.g. Visbal et al 2014; Smidt et al 2017) have been proposed, but they are widely debated, partly because known bright high-z quasars are likely to be the tip of an iceberg that is mainly composed of fainter quasars
Ongoing NIR surveys such as the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007), the Visible and Infrared Survey Telescope for Astronomy (VISTA) Kilodegree Infrared Galaxy (VIKING; Emerson et al 2004), the Panoramic Survey Telescope & Rapid Response System (PanSTARRS; Kaiser et al 2010), the Dark Energy Survey (DES; Dark Energy Survey Collaboration 2016), the VISTA Hemisphere Survey (VHS; McMahon et al 2013), and the Subaru HSC-SSP Survey with the Subaru High-z Exploration of LowLuminosity Quasars (SHELLQs) project (Miyazaki et al 2012; Matsuoka et al 2016) have started to increase the number of known z 6.5 quasars by employing filters centred on 1 μm
Summary
Quasars reside at the centres of active galactic nuclei (AGNs) and are believed to be powered by mass accretion onto a supermassive black hole (SMBH). Ongoing NIR surveys such as the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007), the Visible and Infrared Survey Telescope for Astronomy (VISTA) Kilodegree Infrared Galaxy (VIKING; Emerson et al 2004), the Panoramic Survey Telescope & Rapid Response System (PanSTARRS; Kaiser et al 2010), the Dark Energy Survey (DES; Dark Energy Survey Collaboration 2016), the VISTA Hemisphere Survey (VHS; McMahon et al 2013), and the Subaru HSC-SSP Survey with the Subaru High-z Exploration of LowLuminosity Quasars (SHELLQs) project (Miyazaki et al 2012; Matsuoka et al 2016) have started to increase the number of known z 6.5 quasars by employing filters centred on 1 μm The use of such data allowed Mortlock et al (2011) to discover the previous redshift record holder, ULAS J1120+0641 at z = 7.09, before it was superceded by ULAS J1342+0928 at z = 7.54, for which Bañados et al (2018) mined data from the UKIDSS Large Area Survey (Lawrence et al 2007), the Wide-field Infrared Survey Explorer (ALLWISE; Wright et al 2010), and the DECam Legacy Survey (DECaLS1).
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