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Register a new userThe Infrared Medium-Deep Survey. V. A New Selection Strategy for Quasars at z > 5 based on Medium-Band Observation with SQUEAN
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Authors
Yiseul Jeon
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Multiple color selection techniques have been successful in identifying quasars from wide-field broad-band imaging survey data. Among the quasars that have been discovered so far, however, there is a redshift gap at $5 lesssim {rm z} lesssim 5.7$ due to the limitations of filter sets in previous studies. In this work, we present a new selection technique of high redshift quasars using a sequence of medium-band filters: nine filters with central wavelengths from 625 to 1025 nm and bandwidths of 50 nm. Photometry with these medium-bands traces the spectral energy distribution (SED) of a source, similar to spectroscopy with resolution R $sim$ 15. By conducting medium-band observations of high redshift quasars at 4.7 $leq$ z $leq$ 6.0 and brown dwarfs (the main contaminants in high redshift quasar selection) using the SED camera for QUasars in EArly uNiverse (SQUEAN) on the 2.1-m telescope at the McDonald Observatory, we show that these medium-band filters are superior to multi-color broad-band color section in separating high redshift quasars from brown dwarfs. In addition, we show that redshifts of high redshift quasars can be determined to an accuracy of $Delta{rm z}/(1+{rm z}) = 0.002$ -- $0.026$. The selection technique can be extended to z $sim$ 7, suggesting that the medium-band observation can be powerful in identifying quasars even at the re-ionization epoch.
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The intergalactic medium (IGM) at $zsim$ 5 to 6 is largely ionized, and yet the main source for the IGM ionization in the early universe is uncertain. Of the possible contributors are faint quasars with $-26 lesssim M_{rm 1450} lesssim -23$, but their number density is poorly constrained at $zsim5$. In this paper, we present our survey of faint quasars at $zsim5$ in the European Large-Area {it ISO} Survey-North 1 (ELAIS-N1) field over a survey area of 6.51 deg$^2$ and examine if such quasars can be the dominant source of the IGM ionization. We use the deep optical/near-infrared data of the ELAIS-N1 field as well as the additional medium-band observations to find $z sim 5$ quasars through a two-step approach using the broadband color selection, and SED fitting with the medium-band information included. Adopting Bayesian information criterion, we identify ten promising quasar candidates. Spectra of three of the candidates are obtained, confirming all of them to be quasars at $zsim5$ and supporting the reliability of the quasar selection. Using the promising candidates, we derive the $zsim5$ quasar luminosity function at $-26 lesssim M_{rm 1450} lesssim -23$. The number density of faint $zsim5$ quasars in the ELAIS-N1 field is consistent with several previous results that quasars are not the main contributors to the IGM-ionizing photons at $zsim5$
Faint $zsim5$ quasars with $M_{1450}sim-23$ mag are known to be the potentially important contributors to the ultraviolet ionizing background in the post-reionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our $zsim5$ quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg$^{2}$. From our spectroscopic observations with the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South 8 m Telescope, we discovered eight new quasars at $zsim5$ with $-26.1leq M_{1450} leq -23.3$. Combining our IMS faint quasars ($M_{1450}>-27$ mag) with the brighter Sloan Digital Sky Survey (SDSS) quasars ($M_{1450}<-27$ mag), we derive the $zsim5$ quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of $M_{1450}=-23$ mag. We find that the faint-end slope of the QLF is very flat ($alpha=-1.2^{+1.4}_{-0.6}$), with a characteristic luminosity of $M^{*}_{1450}=-25.8^{+1.4}_{-1.1}$ mag. The number density of $zsim5$ quasars from the QLF gives an ionizing emissivity at 912 $unicode{x212B}$ of $epsilon_{912}=(3.7$--$7.1)times10^{23}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$ and an ionizing photon density of $dot{n}_{rm ion}=(3.0$--$5.7)times10^{49}$ Mpc$^{-3}$ s$^{-1}$. These results imply that quasars are responsible for only 10-20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at $zsim5$, disfavoring the idea that quasars alone could have ionized the IGM at $zsim5$.
We investigate whether stellar dust sources i.e. asymptotic giant branch (AGB) stars and supernovae (SNe) can account for dust detected in 5<z<6.5 quasars (QSOs). We calculate the required dust yields per AGB star and per SN using the dust masses of QSOs inferred from their millimeter emission and stellar masses approximated as the difference between the dynamical and the H_2 gas masses of these objects. We find that AGB stars are not efficient enough to form dust in the majority of the z>5 QSOs, whereas SNe may be able to account for dust in some QSOs. However, they require very high dust yields even for a top-heavy initial mass function. This suggests additional non-stellar dust formation mechanism e.g. significant dust grain growth in the interstellar medium of at least three out of nine z>5 QSOs. SNe (but not AGB stars) may deliver enough heavy elements to fuel this growth.
Deep near-infrared photometric surveys are efficient in identifying high-redshift galaxies, however they can be prone to systematic errors in photometric redshift. This is particularly salient when there is limited sampling of key spectral features of a galaxys spectral energy distribution (SED), such as for quiescent galaxies where the expected age-sensitive Balmer/4000 A break enter the $K$-band at $z>4$. With single filter sampling of this spectral feature, degeneracies between SED models and redshift emerge. A potential solution to this comes from splitting the $K$-band into multiple filters. We use simulations to show an optimal solution is to add two medium-band filters, $K_mathrm{blue}$ ($lambda_mathrm{cen}$=2.06 $mu$m, $Deltalambda$=0.25 $mu$m) and $K_mathrm{red}$ ($lambda_mathrm{cen}$=2.31 $mu$m, $Deltalambda$=0.27 $mu$m), that are complementary to the existing $K_mathrm{s}$ filter. We test the impact of the $K$-band filters with simulated catalogues comprised of galaxies with varying ages and signal-to-noise. The results suggest that the $K$-band filters do improve photometric redshift constraints on $z>4$ quiescent galaxies, increasing precision and reducing outliers by up to 90$%$. We find that the impact from the $K$-band filters depends on the signal-to-noise, the redshift and the SED of the galaxy. The filters we designed were built and used to conduct a pilot of the FLAMINGOS-2 Extra-galactic Near-Infrared $K$-band Split (FENIKS) survey. While no new $z>4$ quiescent galaxies are identified in the limited area pilot, the $K_mathrm{blue}$ and $K_mathrm{red}$ filters indicate strong Balmer/4000 A breaks in existing candidates. Additionally we identify galaxies with strong nebular emission lines, for which the $K$-band filters increase photometric redshift precision and in some cases indicate extreme star-formation.
We present a study of the galaxy environment of 9 strong HI+CIV absorption line systems ($16.2<{rm log}(N({rm HI}))<21.2$) spanning a wide range in metallicity at $zsim4-5$, using MUSE integral field and X-Shooter spectroscopic data collected in a $zapprox 5.26$ quasar field. We identify galaxies within a 250 kpc and $pm1000$ km s$^{-1}$ window for 6 out of the 9 absorption systems, with 2 of the absorption line systems showing multiple associated galaxies within the MUSE field of view. The space density of Ly$alpha$ emitting galaxies (LAEs) around the HI and CIV systems is $approx10-20$ times the average sky density of LAEs given the flux limit of our survey, showing a clear correlation between the absorption and galaxy populations. Further, we find that the strongest CIV systems in our sample are those that are most closely aligned with galaxies in velocity space, i.e. within velocities of $pm500$ km s$^{-1}$. The two most metal poor systems lie in the most dense galaxy environments, implying we are potentially tracing gas that is infalling for the first time into star-forming groups at high redshift. Finally, we detect an extended Ly$alpha$ nebula around the $zapprox 5.26$ quasar, which extends up to $approx50$ kpc at the surface brightness limit of $3.8 times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. After scaling for surface brightness dimming, we find that this nebula is centrally brighter, having a steeper radial profile than the average for nebulae studied at $zsim3$ and is consistent with the mild redshift evolution seen from $zapprox 2$.
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