Catalogs of Lyman-α emitting candidates
Lyman-α (Lyα) emission (at rest-frame wavelength of 1215.67Å) is the strongest line feature in astrophysical spectra (e.g. Selsing et al. 2016). Due to its efficient coupling with neutral hydrogen and the strong ionizing fluxes required to its production (e.g. Dijkstra M. 2017), Lyα radiation is used as a probe for the distribution of gas surrounding sources and to study several astrophysical processes, such as recent star formation events and accretion of matter onto Black-Holes. Lyα emission is redshifted into the optical band at z>2, hence it offers the possibility of studying these processes as they took place in the very young Universe by exploiting ground-based optical observations. One of the main tools used to study the population and properties of Lyα-emitting sources is their so-called Lyα luminosity function (LF), namely their volume number-density as a function of Lyα luminosity.
The four catalogs reported in this page are composed of Lyα-emitting candidates selected on the basis of their strong and reliable photometric excess in either of the J0395, J0410, J0430 and J0515 J-PLUS NBs. The four lists presented in this page contain 2547, 5556, 4994 and 1467 candidates respectively for J0395, J0410, J0430 and J0515 NBs. We measure NB excess in each NB by exploiting the method developed and described by Vilella-Rojo et al. (2015). We then define a series of photometric cuts and cross-matches with external databases to both select sources showing a reliable NB excess and remove known interlopers from our selection. The latter are defined as sources with a secure identification (either spectroscopic, photometric or astrometric) which pinpoint their nature as low-redshift (z<2) sources. The main classes of contaminants for our selection are z<2 QSOs (whose C-IV emission falls in the same NB used for candidates selection), low-z galaxies (such as [OII]3727 and [OIII]4959+5007 emitters) and stars. All the details of this candidate selection are extensively explained in Spinoso et al. (2020). Figure 1 summarizes the photometric cuts performed to select sources with a reliable NB excess (see the figure caption for details).
The candidate selection performance was assessed by means of two different photometric programs at the GTC telescope. These observed a total of 45 sources selected within the list of candidates with photometric excess in the J0395 narrow-band. The programs confirmed 65% of the sources as genuine z=2.2 Lyα-emitting quasars (QSOs) and allowed to reduce from 17% to 5% the contamination from stars in our selection. In addition, the contamination of our selection was measured by exploiting the cross-matches with the spectroscopic sample of SDSS stars (Majewski et al. 2017), galaxies (Bundy et al. 2015) and QSOs (Paris et al. 2018), as well as with Gaia DR2 data (Gaia collaboration et al. 2018) and additional external databases (all the details are extensively discussed in Spinoso et al. 2020). This analysis shows that the contamination of our NB-excess selection is relatively high (>50%) and mostly due to interloping Galaxies, QSOs and stars. Figure 2 shows the measurements of purity (1-contamination) as a function of rJAVA magnitude for each NB filter, along with the average purity of the GTC spectroscopic results. The catalogs reported in this page do not include contaminants, hence their contamination is expected to be sensitively lower than 50% and closer to the one measured on the GTC data (i.e. ∼35%). Nevertheless a precise measurement of the samples contamination should be performed via secure spectroscopic confirmation of all candidates.
Finally, the spectroscopic follow-up confirmed that our NB-excess selection efficiently points-out high-redshift line emitters. Indeed, a total of 37 sources (82%) were confirmed as either z=2.2 or z=1.5 QSOs (respectively emitting Lyα and CIV lines). This implies that our catalogs are composed of high-z line-emitting candidates, with the relatively high purity of ∼80%, as suggested by our spectroscopic follow-up results. In addition, both the crossmatch with external databases and the follow-up results suggest that 65% of these high-z candidates are genuine Lyα-emitting QSOs without previous spectroscopic identification. This would translate into roughly 1300, 3200, 2900 and 900 newly-identified z>2 QSOs. Nevertheless, we stress that the nature of the candidates presented in this page should be confirmed by a rigorous and extensive spectroscopic follow-up program.
The J0395, J0410, J0430 and J0515 J-PLUS NBs can probe Lyα emission respectively at z=2.2, z=2.4, z=2.5 and z=3.2 and hence allow to build four different determinations of the Lyα LF. This is the final goal of the source selection which led to the construction of the candidates samples reported in this page. The details of the LFs computation and results are presented in Spinoso et al. (2020). Figure 3 shows the final determination of the four LFs (left panel) and the confidence regions fpr the Schechter parameters of the LFs fits (right panel). As already commented, the J-PLUS data allows to probe very bright Lyα-luminosity regimes (LLyα > 1044 erg s-1) and very low number densities (down to 10-8 Mpc-3). These values are unprecedented for analog studies and show for the first time that the extremely-bright end of the Lyα LF is composed by a population of Lyα-emitting AGN/QSOs, whose LF is structurally different from that of Lyα-emitting galaxies (LAEs). In particular, AGN/QSOs appear to be roughly 100 times more luminous and 1000 times less dense than galaxies, namely: L*(QSOs) ∼ 100 L*(LAEs) and Phi*(QSOs) ∼ 0.001 Phi*(LAEs).
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