arXiv subjects

Yoshiki Toba

Publications and source records attributed to Yoshiki Toba.

At least 163 records · Page 9Linked to original sources

arXiv2018

SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) II: Structural Properties and Near-Infrared Morphologies of Faint Submillimeter Galaxies

We present structural parameters and morphological properties of faint 450-um selected submillimeter galaxies (SMGs) from the JCMT Large Program, STUDIES, in the COSMOS-CANDELS region. Their properties are compared to an 850um selected and a matched star-forming samples. We investigate stellar structures of 169 faint 450-um sources (S450=2.8-29.6mJy; S/N>4) at z<3 using HST near-infrared observations. Based on our spectral energy distribution fitting, half of such faint SMGs (LIR=10^11.65+-0.98Lsun) lie above the star-formation rate (SFR)/stellar mass plane. The size-mass relation shows that these SMGs are generally similar to less-luminous star-forming galaxies selected by NUV-r vs. r-J colors. Because of the intrinsic luminosity of the sample, their rest-frame optical emission is less extended than the 850um sources (S850>2mJy), and more extended than the star-forming galaxies in the same redshift range. For the stellar mass and SFR matched sample at z~=1 and z~=2, the size differences are marginal between faint SMGs and the matched galaxies. Moreover, faint SMGs have similar Sersic indices and projected axis ratios as star-forming galaxies with the same stellar mass and SFR. Both SMGs and the matched galaxies show high fractions (~70%) of disturbed features at z~=2, and the fractions depend on the SFRs. These suggest that their star formation activity is related to galaxy merging, and the stellar structures of SMGs are similar to those of star-forming galaxies. We show that the depths of submillimeter surveys are approaching the lower luminosity end of star-forming galaxies, allowing us to detect galaxies on the main sequence.

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arXiv2018

Imaging Extended Emission-Line Regions of Obscured AGN with the Subaru Hyper Suprime-Cam Survey

Narrow-line regions excited by active galactic nuclei (AGN) are important for studying AGN photoionization and feedback. Their strong [O III] lines can be detected with broadband images, allowing morphological studies of these systems with large-area imaging surveys. We develop a new technique to reconstruct the [O III] images using the Subaru Hyper Suprime-Cam (HSC) Survey aided with spectra from the Sloan Digital Sky Survey (SDSS). The technique involves a careful subtraction of the galactic continuum to isolate emission from the [O III]$λ$5007 and [O III]$λ$4959 lines. Compared to traditional targeted observations, this technique is more efficient at covering larger samples with less dedicated observational resources. We apply this technique to an SDSS spectroscopically selected sample of 300 obscured AGN at redshifts 0.1 - 0.7, uncovering extended emission-line region candidates with sizes up to tens of kpc. With the largest sample of uniformly derived narrow-line region sizes, we revisit the narrow-line region size-luminosity relation. The area and radii of the [O III] emission-line regions are strongly correlated with the AGN luminosity inferred from the mid-infrared (15 $μ$m rest-frame) with a power-law slope of $0.62^{+0.05}_{-0.06} \pm 0.10$ (statistical and systemic errors), consistent with previous spectroscopic findings. We discuss the implications for the physics of AGN emission-line region and future applications of this technique, which should be useful for current and next-generation imaging surveys to study AGN photoionization and feedback with large statistical samples.

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arXiv2018

Discovery of an extremely-luminous dust-obscured galaxy observed with SDSS, WISE, JCMT, and SMA

We present the discovery of an extremely-luminous dust-obscured galaxy (DOG) at $z_{\rm spec}$ = 3.703, WISE J101326.25+611220.1. This DOG is selected as a candidate of extremely-luminous infrared (IR) galaxies based on the photometry from the Sloan Digital Sky Survey and Wide-field Infrared Survey Explorer. In order to derive its accurate IR luminosity, we perform follow-up observations at 450 and 850 $μ$m using the Submillimetre Common User Bolometer Array 2 on the James Clerk Maxwell Telescope, and at 870 and 1300 $μ$m using the Submillimeter Array, which enable us to pin down its IR Spectral Energy Distribution (SED). We perform SED fitting using 14 photometric data (0.4 - 1300 $μ$m) and estimate its IR luminosity, $L_{\rm IR}$ (8-1000 $μ$m), to be $2.2^{+1.5}_{-1.0}$ $\times 10^{14}$ $L_{\odot}$, making it one of the most luminous IR galaxies in the Universe. The energy contribution from an active galactic nucleus (AGN) to the IR luminosity is $94^{+6}_{-20}$%, which indicates it is an AGN-dominated DOG. On the other hand, its stellar mass ($M_*$) and star formation rate (SFR) are $\log \,(M_\ast/M_{\odot})$ = $11.2^{+0.6}_{-0.2}$ and $\log \,({\rm SFR}/M_{\odot}\,{\rm yr}^{-1}$) = $3.1^{+0.2}_{-0.1}$, respectively, which means that this DOG can be considered as a starburst galaxy in $M_*$--SFR plane. This extremely-luminous DOG shows significant AGN and star forming activity that provides us an important laboratory to probe the maximum phase of the co-evolution of galaxies and supermassive black holes.

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arXiv2018

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) III. Star formation properties of the host galaxies at $z \gtrsim 6$ studied with ALMA

We present our ALMA Cycle 4 measurements of the [CII] emission line and the underlying far-infrared (FIR) continuum emission from four optically low-luminosity ($M_{\rm 1450} > -25$) quasars at $z \gtrsim 6$ discovered by the Subaru Hyper Suprime Cam (HSC) survey. The [CII] line and FIR continuum luminosities lie in the ranges $L_{\rm [CII]} = (3.8-10.2) \times 10^8~L_\odot$ and $L_{\rm FIR} = (1.2-2.0) \times 10^{11}~L_\odot$, which are at least one order of magnitude smaller than those of optically-luminous quasars at $z \gtrsim 6$. We estimate the star formation rates (SFR) of our targets as $\simeq 23-40~M_\odot ~{\rm yr}^{-1}$. Their line and continuum-emitting regions are marginally resolved, and found to be comparable in size to those of optically luminous quasars, indicating that their SFR or likely gas mass surface densities (key controlling parameter of mass accretion) are accordingly different. The $L_{\rm [CII]}/L_{\rm FIR}$ ratios of the hosts, $\simeq (2.2-8.7) \times 10^{-3}$, are fully consistent with local star-forming galaxies. Using the [CII] dynamics, we derived their dynamical masses within a radius of 1.5-2.5 kpc as $\simeq (1.4-8.2) \times 10^{10}~M_\odot$. By interpreting these masses as stellar ones, we suggest that these faint quasar hosts are on or even below the star-forming main sequence at $z \sim 6$, i.e., they appear to be transforming into quiescent galaxies. This is in contrast to the optically luminous quasars at those redshifts, which show starburst-like properties. Finally, we find that the ratios of black hole mass to host galaxy dynamical mass of the most of low-luminosity quasars including the HSC ones are consistent with the local value. The mass ratios of the HSC quasars can be reproduced by a semi-analytical model that assumes merger-induced black hole-host galaxy evolution.

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arXiv2017

Cosmic star formation history revealed by AKARI and Hyper Suprime-Cam

Understanding infrared (IR) luminosity is fundamental to understanding the cosmic star formation history and AGN evolution. Japanese infrared satellite, AKARI, provided unique data sets to probe this both at low and high redshift; the AKARI all sky survey in 6 bands (9-160 $μ$m), and the AKARI NEP survey in 9 bands (2-24$μ$m). The AKARI performed all sky survey in 6 IR bands (9, 18, 65, 90, 140, and 160 $μ$m) with 3-10 times better sensitivity than IRAS, covering the crucial far-IR wavelengths across the peak of the dust emission. Combined with a better spatial resolution, we measure the total infrared luminosity ($L_{TIR}$) of individual galaxies, and thus, the total infrared luminosity density of the local Universe much more precisely than previous work. In the AKARI NEP wide field, AKARI has obtained deep images in the mid-infrared (IR), covering 5.4 deg$^2$. However, our previous work was limited to the central area of 0.25 deg$^2$ due to the lack of deep optical coverage. To rectify the situation, we used the newly advent Subaru telescope's Hyper Suprime-Cam to obtain deep optical images over the entire 5.4 deg$^2$ of the AKARI NEP wide field. With this deep and wide optical data, we, for the first time, can use the entire AKARI NEP wide data to construct restframe 8$μ$m, 12$μ$m, and total infrared (TIR) luminosity functions (LFs) at 0.15$<z<$2.2. A continuous 9-band filter coverage in the mid-IR wavelength (2.4, 3.2, 4.1, 7, 9, 11, 15, 18, and 24$μ$m) by the AKARI satellite allowed us to estimate restframe 8$μ$m and 12$μ$m luminosities without using a large extrapolation based on a SED fit, which was the largest uncertainty in previous work. By combining these two results, we reveal dust-hidden cosmic star formation history and AGN evolution from z=0 to z=2.2, all probed by the AKARI satellite.

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arXiv2017

North Ecliptic Pole multi-wavelength survey : new optical data with Hyper Suprime-Cam and near-future prospects with eROSITA

The AKARI North Ecliptic Pole (NEP) survey consists of two survey projects: NEP-Deep (0.5 sq.deg) and NEP-Wide (5.4 sq.deg), providing with tens of thousands of galaxies. A continuous filter coverage in the mid-infrared wavelengths (7, 9, 11, 15, 18 and 24 $μ$m) is unique to diagnose the contributions from dusty star-formation activity and AGNs. Here we present current status focused on the newly obtained optical images and near-future prospects with a new X-ray telescope. Hyper Suprime-Cam on Subaru telescope is a gigantic optical camera with huge Field of View (FoV). Thanks to the wide FoV, we successfully obtained deep optical images at g, r, i, z and Y-bands covering most of the NEP-Wide field. Using the deep optical images, we identified over 5000 optical counterparts of the mid-IR sources, presumably deeply obscured galaxies in NEP-Wide field. We also investigated properties of these infrared sources with SED-fitting. eROSITA, to be launched early 2018, is a new all-sky X-ray survey telescope, and expected to conduct ultra deep 2-10 keV imaging toward NEP. We expect unprecedentedly numerous Compton-thick AGN candidates when combined with the multi-wavelength data in NEP region.

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arXiv2017

No sign of strong molecular gas outflow in an infrared-bright dust-obscured galaxy with strong ionized-gas outflow

We report the discovery of an infrared (IR)-bright dust-obscured galaxy (DOG) that shows a strong ionized-gas outflow but no significant molecular gas outflow. Based on detail analysis of their optical spectra, we found some peculiar IR-bright DOGs that show strong ionized-gas outflow ([OIII]$λ$5007) from the central active galactic nucleus (AGN). For one of these DOGs (WISE J102905.90+050132.4) at $z_{\rm spec} = 0.493$, we performed follow-up observations using ALMA to investigate their CO molecular gas properties. As a result, we successfully detected $^{12}$CO($J$=2-1) and $^{12}$CO($J$=4-3) lines, and the continuum of this DOG. The intensity-weighted velocity map of both lines shows a gradient, and the line profile of those CO lines is well-fitted by a single narrow Gaussian, meaning that this DOG has no sign of strong molecular gas outflow. The IR luminosity of this object is $\log\,(L_{\rm IR}/L_{\odot})$ = 12.40 that is classified as ultraluminous IR galaxy (ULIRG). We found that (i) the stellar mass and star-formation rate relation and (ii) the CO luminosity and far-IR luminosity relation are consistent with those of typical ULIRGs at similar redshifts. These results indicate that the molecular gas properties of this DOG are normal despite that its optical spectrum showing a powerful AGN outflow. We conclude that a powerful ionized-gas outflow caused by the AGN does not necessarily affect the cold interstellar medium in the host galaxy at least for this DOG.

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arXiv2017

Infrared Selection of Obscured Active Galactic Nuclei in the COSMOS Field

We present a study of the connection between black hole accretion, star formation, and galaxy morphology at z<=2.5. We focus on active galactic nuclei (AGNs) selected by their mid-IR power-law emission. By fitting optical to far-IR photometry with state-of-the-art spectral energy distribution (SED) techniques, we derive stellar masses, star formation rates, dust properties, and AGN contributions in galaxies over the whole COSMOS field. We find that obscured AGNs lie within or slightly above the star-forming sequence. We confirm our previous finding about compact host galaxies of obscured AGNs at z~1, and find that galaxies with 20-50% AGN contributions tend to have smaller sizes, by ~25-50%, compared to galaxies without AGNs. Furthermore, we find that a high merger fraction of up to 0.5 is appropriate for the most luminous (log (LIR/Lsun) ~ 12.5) AGN hosts and non-AGN galaxies, but not for the whole obscured AGN sample. Moreover, merger fraction depends on the total and star-forming infrared luminosity, rather than the decomposed AGN infrared luminosity. Our results suggest that major mergers are not the main driver of AGN activity, and therefore obscured AGNs might be triggered by internal mechanisms, such as secular processes, disk instabilities, and compaction in a particular evolutionary stage. We make the SED modeling results publicly available.

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arXiv2017

SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES): Faint-End Counts at 450 um

The SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) is a three-year JCMT Large Program aiming at reaching the 450 $μ$m confusion limit in the COSMOS-CANDELS region, to study a representative sample of the high-redshift far-infrared galaxy population that gives rise to the bulk of the far-infrared background. We present the first-year data from STUDIES. We have reached a 450 $μ$m noise level of 0.91~mJy for point sources at the map center, covered an area of 151 arcmin$^2$, and detected 98 and 141 sources at 4.0 and 3.5 $σ$, respectively. Our derived counts are best constrained in the 3.5-25 mJy regime using directly detected sources. Below the detection limits, our fluctuation analysis further constrains the slope of the counts down to 1 mJy. The resulting counts at 1-25 mJy are consistent with a power law having a slope of $-2.59$ ($\pm0.10$ for 3.5-25 mJy, and $^{+0.4}_{-0.7}$ for 1-3.5 mJy). There is no evidence of a faint-end termination or turn-over of the counts in this flux density range. Our counts are also consistent with previous SCUBA-2 blank-field and lensing cluster surveys. The integrated surface brightness from our counts down to 1 mJy is $90.0\pm17.2$ Jy deg$^{-2}$, which can account for up to $83^{+15}_{-16}\%$ of the COBE 450 $μ$m background. We show that Herschel counts at 350 and 500 $μ$m are significantly higher than our 450 $μ$m counts, likely caused by its large beam and source clustering. High-angular resolution instruments like SCUBA-2 at 450 $μ$m are therefore highly beneficial for measuring the luminosity and spatial density of high-redshift dusty galaxies.

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arXiv2017

Ionized gas outflows in infrared-bright dust-obscured galaxies selected with WISE and SDSS

We present the ionized gas properties of infrared (IR)-bright dust-obscured galaxies (DOGs) that show an extreme optical/IR color, $(i - [22])_{\rm AB} > 7.0$, selected with the Sloan Digital Sky Survey (SDSS) and Wide-field Infrared Survey Explorer (WISE). For 36 IR-bright DOGs that show [OIII]$λ$5007 emission in the SDSS spectra, we performed a detailed spectral analysis to investigate their ionized gas properties. In particular, we measured the velocity offset (the velocity with respect to the systemic velocity measured from the stellar absorption lines) and the velocity dispersion of the [OIII] line. We found that the derived velocity offset and dispersion of most IR-bright DOGs are larger than those of Seyfert 2 galaxies (Sy2s) at $z < 0.3$, meaning that the IR-bright DOGs show relatively strong outflows compared to Sy2s. This can be explained by the difference of IR luminosity contributed from active galactic nucleus, $L_{\rm IR}$ (AGN), because we found that (i) $L_{\rm IR}$ (AGN) correlates with the velocity offset and dispersion of [OIII] and (ii) our IR-bright DOGs sample has larger $L_{\rm IR}$ (AGN) than Sy2s. Nevertheless, the fact that about 75% IR-bright DOGs have a large ($>$ 300 km s$^{-1}$) velocity dispersion, which is a larger fraction compared to other AGN populations, suggests that IR-bright DOGs are good laboratories to investigate AGN feedback. The velocity offset and dispersion of [OIII] and [NeIII]$λ$3869 are larger than those of [OII]$λ$3727, which indicates that the highly ionized gas tends to show more stronger outflows.

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arXiv2017

X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey

We construct a sample of X-ray bright optically faint active galactic nuclei by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source catalogs. 53 X-ray sources satisfying i band magnitude fainter than 23.5 mag and X-ray counts with EPIC-PN detector larger than 70 are selected from 9.1 deg^2, and their spectral energy distributions (SEDs) and X-ray spectra are analyzed. 44 objects with an X-ray to i-band flux ratio F_X/F_i>10 are classified as extreme X-ray-to-optical flux sources. SEDs of 48 among 53 are represented by templates of type 2 AGNs or starforming galaxies and show signature of stellar emission from host galaxies in the optical in the source rest frame. Infrared/optical SEDs indicate significant contribution of emission from dust to infrared fluxes and that the central AGN is dust obscured. Photometric redshifts determined from the SEDs are in the range of 0.6-2.5. X-ray spectra are fitted by an absorbed power law model, and the intrinsic absorption column densities are modest (best-fit log N_H = 20.5-23.5 cm^-2 in most cases). The absorption corrected X-ray luminosities are in the range of 6x10^42 - 2x10^45 erg s^-1. 20 objects are classified as type 2 quasars based on X-ray luminsosity and N_H. The optical faintness is explained by a combination of redshifts (mostly z>1.0), strong dust extinction, and in part a large ratio of dust/gas.

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arXiv2017

Clustering of galaxies around AGN in the HSC Wide survey

We have measured the clustering of galaxies around active galactic nuclei (AGN) for which single-epoch virial masses of the super-massive black hole (SMBH) are available to investigate the relation between the large scale environment of AGNs and the evolution of SMBHs. The AGN samples used in this work were derived from the Sloan Digital Sky Survey (SDSS) observations and the galaxy samples were from 240~deg$^{2}$ S15b data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The investigated redshift range is 0.6--3.0, and the masses of the SMBHs lie in the range $10^{7.5}$--$10^{10}$~M$_{\odot}$. The absolute magnitude of the galaxy samples reaches to $M_{λ310}$ $\sim$ $-$18 at rest frame wavelength 310~nm for the low-redshift end of the samples. More than 70\% of the galaxies in the analysis are blue. We found a significant dependence of the cross-correlation length on redshift, which primarily reflects the brightness dependence of the galaxy clustering. At the lowest redshifts the cross-correlation length increases from 7~$h^{-1}$~Mpc around $M_{λ310}$$ = $$-19$~mag to $>$10~$h^{-1}$~Mpc beyond $M_{λ310}$$ = $$-20$~mag. No significant dependence of the cross-correlation length on BH mass was found for whole galaxy samples dominated by blue galaxies, while there was an indication of BH mass dependence in the cross-correlation with red galaxies. These results provides a picture of the environment of AGNs studied in this paper being enriched with blue starforming galaxies, and a fraction of the galaxies are being evolved to red galaxies along with the evolution of SMBHs in that system.

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arXiv2017

Enhancement of Galaxy Overdensity around Quasar Pairs at z<3.6 based on the Hyper Suprime-Cam Subaru Strategic Program Survey

We investigate the galaxy overdensity around proto-cluster scale quasar pairs at high (z>3) and low (z~1) redshift based on the unprecedentedly wide and deep optical survey of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using the first-year survey data covering effectively ~121 deg^2 with the 5sigma depth of i~26.4 and the SDSS DR12Q catalog, we find two luminous pairs at z~3.3 and 3.6 which reside in >5sigma overdense regions of g-dropout galaxies at i<25. The projected separations of the two pairs are R_perp=1.75 and 1.04 proper Mpc, and their velocity offsets are Delta V=692 and 1448 km s^{-1}, respectively. This result is in clear contrast to the average z~4 quasar environments as discussed in Uchiyama et al. (2017) and implies that the quasar activities of the pair members are triggered via major mergers in proto-clusters, unlike the vast majority of isolated quasars in general fields that may turn on via non-merger events such as bar and disk instabilities. At z~1, we find 37 pairs with R_perp<2 pMpc and Delta V<2300 km s^{-1} in the current HSC-Wide coverage, including four from Hennawi et al. (2006). The distribution of the peak overdensity significance within two arcminutes around the pairs has a long tail toward high density (>4sigma) regions. Thanks to the large sample size, we find a statistical evidence that this excess is unique to the pair environments when compared to single quasar and randomly selected galaxy environments at the same redshift range. Moreover, there are nine small-scale (R_perp<1 pMpc) pairs, two of which are found to reside in cluster fields. Our results demonstrate that <2 pMpc-scale quasar pairs at both redshift range tend to occur in massive haloes, although perhaps not the most massive ones, and that they are useful to search for rare density peaks.

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arXiv2017

The nature of H$α$-selected galaxies along the large-scale structure at z=0.4 revealed by Subaru Hyper Suprime-Cam survey

We present the environmental dependence of colour, stellar mass, and star formation (SF) activity in H-alpha-selected galaxies along the large-scale structure at z=0.4 hosting twin clusters in DEEP2-3 field, discovered by Subaru Strategic Programme of Hyper Suprime-Cam (HSC SSP). By combining photo-z selected galaxies and H-alpha emitters selected with broad-band and narrow-band (NB) data of the recent data release of HSC SSP (DR1), we confirm that galaxies in higher-density environments or galaxies in the cluster central regions show redder colours. We find that there still remains a possible colour-density and colour-radius correlation even if we restrict the sample to H-alpha-selected galaxies, likely due to the presence of massive H-alpha emitters in denser regions. We also find a hint of increased star formation rates (SFR) amongst H-alpha emitters towards the highest-density environment, again primarily driven by the excess of red/massive H-alpha emitters in high-density environment, while their specific SFR does not significantly change with environment. This work demonstrates the power of the HSC SSP NB data to study SF galaxies across environment in the distant universe.

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arXiv2017

Active Galactic Nuclei with a Low-Metallicity Narrow-Line Region

Low-metallicity active galactic nuclei (AGNs) are interesting to study the early phase of the AGN evolution. However most AGNs are chemically matured and accordingly low-metallicity AGNs are extremely rare. One approach to search for low-metallicity AGNs systematically is utilizing the so-called BPT diagram that consists of the [O III]$λ$5007/H$β$$\lambda4861$ and [N II]$\lambda6584$/H$α$$\lambda6563$ flux ratios. Specifically, photoionization models predict that low-metallicity AGNs show a high [O III]$λ$5007/H$β$$λ$4861 ratio and a relatively low [N II]$λ$6584/H$α$$λ$6563 ratio, that corresponds to the location between the sequence of star-forming galaxies and that of usual AGNs on the BPT diagram (hereafter "the BPT valley"). However, other populations of galaxies such as star-forming galaxies and AGNs with a high electron density or a high ionization parameter could be also located in the BPT valley, not only low-metallicity AGNs. In this paper, we examine whether most of emission-line galaxies at the BPT valley are low-metallicity AGNs or not. We select 70 BPT-valley objects from 212,866 emission line galaxies obtained by the Sloan Digital Sky Survey. Among the 70 BPT-valley objects, 43 objects show firm evidence of the AGN activity; i.e., the He II$λ$4686 emission and/or weak but significant broad H$α$ emission. Our analysis shows that those 43 BPT-valley AGNs are not characterized by a very high gas density nor ionization parameter, inferring that at least 43 among 70 BPT-valley objects (i.e., $>60$ %) are low-metallicity AGNs. This suggests that the BPT diagram is an efficient tool to search for low-metallicity AGNs.

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arXiv2017

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8

We present spectroscopic identification of 32 new quasars and luminous galaxies discovered at 5.7 < z < 6.8. This is the second in a series of papers presenting the results of the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The photometric candidates were selected by a Bayesian probabilistic algorithm, and then observed with spectrographs on the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous paper, we have now identified 64 HSC sources over about 430 deg2, which include 33 high-z quasars, 14 high-z luminous galaxies, 2 [O III] emitters at z ~ 0.8, and 15 Galactic brown dwarfs. The new quasars have considerably lower luminosity (M1450 ~ -25 to -22 mag) than most of the previously known high-z quasars. Several of these quasars have luminous (> 10^(43) erg/s) and narrow (< 500 km/s) Ly alpha lines, and also a possible mini broad absorption line system of N V 1240 in the composite spectrum, which clearly separate them from typical quasars. On the other hand, the high-z galaxies have extremely high luminosity (M1450 ~ -24 to -22 mag) compared to other galaxies found at similar redshift. With the discovery of these new classes of objects, we are opening up new parameter spaces in the high-z Universe. Further survey observations and follow-up studies of the identified objects, including the construction of the quasar luminosity function at z ~ 6, are ongoing.

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arXiv2017

Clustering of quasars in a wide luminosity range at redshift 4 with Subaru Hyper Suprime-Cam wide field imaging

We examine the clustering of quasars over a wide luminosity range, by utilizing 901 quasars at $\overline{z}_{\rm phot}\sim3.8$ with $-24.73<M_{\rm 1450}<-22.23$ photometrically selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S16A Wide2 date release and 342 more luminous quasars at $3.4<z_{\rm spec}<4.6$ having $-28.0<M_{\rm 1450}<-23.95$ from the Sloan Digital Sky Survey (SDSS) that fall in the HSC survey fields. We measure the bias factors of two quasar samples by evaluating the cross-correlation functions (CCFs) between the quasar samples and 25790 bright $z\sim4$ Lyman Break Galaxies (LBGs) in $M_{\rm 1450}<-21.25$ photometrically selected from the HSC dataset. Over an angular scale of \timeform{10.0"} to \timeform{1000.0"}, the bias factors are $5.93^{+1.34}_{-1.43}$ and $2.73^{+2.44}_{-2.55}$ for the low and high luminosity quasars, respectively, indicating no luminosity dependence of quasar clustering at $z\sim4$. It is noted that the bias factor of the luminous quasars estimated by the CCF is smaller than that estimated by the auto-correlation function (ACF) over a similar redshift range, especially on scales below \timeform{40.0"}. Moreover, the bias factor of the less-luminous quasars implies the minimal mass of their host dark matter halos (DMHs) is $0.3$-$2\times10^{12}h^{-1}M_{\odot}$, corresponding to a quasar duty cycle of $0.001$-$0.06$.

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arXiv2017

The Quasar Luminosity Function at Redshift 4 with Hyper Suprime-Cam Wide Survey

We present the luminosity function of z=4 quasars based on the Hyper Suprime-Cam Subaru Strategic Program Wide layer imaging data in the g, r, i, z, and y bands covering 339.8 deg^2. From stellar objects, 1666 z~4 quasar candidates are selected by the g-dropout selection down to i=24.0 mag. Their photometric redshifts cover the redshift range between 3.6 and 4.3 with an average of 3.9. In combination with the quasar sample from the Sloan Digital Sky Survey in the same redshift range, the quasar luminosity function covering the wide luminosity range of M1450=-22 to -29 mag is constructed. It is well described by a double power-law model with a knee at M1450=-25.36+-0.13 mag and a flat faint-end slope with a power-law index of -1.30+-0.05. The knee and faint-end slope show no clear evidence of redshift evolution from those at z~2. The flat slope implies that the UV luminosity density of the quasar population is dominated by the quasars around the knee, and does not support the steeper faint-end slope at higher redshifts reported at z>5. If we convert the M1450 luminosity function to the hard X-ray 2-10keV luminosity function using the relation between UV and X-ray luminosity of quasars and its scatter, the number density of UV-selected quasars matches well with that of the X-ray-selected AGNs above the knee of the luminosity function. Below the knee, the UV-selected quasars show a deficiency compared to the hard X-ray luminosity function. The deficiency can be explained by the lack of obscured AGNs among the UV-selected quasars.

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