The Infrared Medium-deep Survey. VI. Discovery of Faint Quasars at z ∼ 5 with a Medium-band-based Approach

Abstract

The faint quasars with M 1450 > −24 mag are known to hold the key to the determination of the ultraviolet emissivity for the cosmic re-ionization. But only a few have been identified so far because of the limitations on the survey data. Here, we present the first results of the z ∼ 5 faint quasar survey with the Infrared Medium-deep Survey (IMS), which covers ∼ 100 deg 2 areas in J-band to the depths of J AB ∼ 23 mag. To improve selection methods, the medium-band follow-up imaging has been carried out using the SED camera for QUasars in Early uNiverse (SQUEAN) on the Otto Struve 2.1 m Telescope. The optical spectra of the candidates were obtained with 8-m class telescopes. We newly discovered 10 quasars with −25 < M 1450 < −23 at z ∼ 5, among which three have been missed in a previous survey using the same optical data over the same area, implying the necessity for improvements in high redshift faint quasars selection. We derived photometric redshifts from the medium-band data, and find that they have high accuracies of |∆z|/(1 + z) = 0.016. The mediumband-based approach allows us to rule out many of the interlopers that contaminate 20 % of the broad-band-selected quasar candidates. These results suggest that the medium-band-based approach is a powerful way to identify z ∼ 5 quasars and measure their redshifts at high accuracy (1-2 %). It is also a cost-effective way to understand the contribution of quasars to the cosmic re-ionization history. dreds of them being at high redshift of z 5 (Fan et al.1 The medium-band filters are named as m (initial of the medium-band) + the central wavelength of the filter in nm.

The rapid transition from star formation to AGN-dominated rest-frame ultraviolet light at z ≃ 4

“…Warren et al 1994;Richards et al 2006;Masters et al 2012;Ikeda et al 2012), recent surveys have been able to select larger samples over a wider luminosity range, thus providing greater precision. In particular, there have been several successful campaigns to identify fainter sources at ≥ 4 with surveys such as the Subaru High-Exploration of Low-Luminosity Quasars (SHELLQs: Matsuoka et al 2018a), the Infrared Medium Survey (IMS: Kim et al 2019) and the Hyper-SuprimeCam Stragetic Survey Program (HSC-SSP; Akiyama et al 2018). These studies have been able to constrain the faint-end 2 of the AGN LF for the first time at 4, however there remain large discrepancies in the derived faint-end slope which ranges from −1.3 (Matsuoka et al 2018b;Akiyama et al 2018) to as steep as −2 (McGreer et al 2018;Giallongo et al 2019;Shin et al 2020).…”

The Infrared Medium-deep Survey. \Romannum{7}. Faint Quasars at $z \sim 5$ in the ELAIS-N1 Field

“…The quasar candidates were first selected using broadband colors. We employed broadband color cuts in riz and riJ color-color diagrams (CCD), similarly to that described in McGreer et al (2013) and Kim et al (2019), with the color-cuts adjusted for the HSC magnitude system. First, we selected point sources by using the difference between CModel magnitude (m CModel ) and pointspread function (PSF) magnitudes (m psf ) (Matsuoka et al 2018).…”

“…For M-dwarf models, we obtained T eff , log(g), [M/H], [α/M], and the normalization parameter. The SED-fitting followed the procedure as described in Kim et al (2019), and it takes into account the upper limits as described in by Sawicki (2012). The results of the SED fitting are shown in Figure 5.…”

“…Previous studies have suggested either star-forming galaxies or quasars as candidates to explain the majority of the required photon budget (Fontanot et al 2012;Giallongo et al 2015, hereafter G15;Madau & Haardt 2015;Bouwens et al 2016;D'Aloisio et al 2017;Ricci et al 2017;Kakiichi et al 2018). In some works, galaxies are considered as the primary contributors to IGM reionization (Alvarez et al 2012;Paardekooper et al 2013;Kashikawa et al 2015;Kim et al 2015Kim et al , 2019Matsuoka et al 2018;Smith et al 2018;Fletcher et al 2019). In contrary, the escape fraction of the hydrogen-ionizing Lyman-continuum (LyC) photons of star-forming galaxies is found to be only few percent (Bridge et al 2010;Rutkowski et al 2016;Vasei et al 2016;Japelj et al 2017;Iwata et al 2019), and this makes it challenging for galaxies to maintain the ionized state of the IGM (Finkelstein et al 2015;Grazian et al 2017;Naidu et al 2018;Steidel et al 2018;Lam et al 2019).…”

“…Given the spectral shape and the escape fraction of LyC photons of quasars, the contribution of quasars to the hydrogen reionizing photons can be calculated from the quasar luminosity function (LF). Therefore, many observational studies have tried to determine the LF of quasars at z > 4 (Barger et al 2003;Fan et al 2006;Jiang et al 2008;Willott et al 2010;McGreer et al 2013;Bañaods et al 2016;Yang et al 2016;Jeon et al 2017;Kim et al 2019).…”