arXiv subjects

Yutaka Komiyama

Publications and source records attributed to Yutaka Komiyama.

At least 91 records · Page 5Linked to original sources

arXiv2018

The Missing Satellite Problem Outside of the Local Group: I -- Pilot Observation

We present results from a pilot observation of nearby (~20 Mpc) galaxies with mass similar to that of the Milky Way (MW) to address the missing satellite problem. This is the first paper from an on-going project to address the problem with a statistical sample of galaxies outside of the Local Group (LG) without employing an assumption that the LG is a typical halo in the Universe. Thanks to the close distances of our targets, dwarf galaxies around them can be identified as extended, diffuse galaxies. By applying a surface brightness cut together with a careful visual screening to remove artifacts and background contamination, we construct a sample of dwarf galaxies. The luminosity function (LF) of one of the targets is broadly consistent with that of the MW, but the other has a more abundant dwarf population. Numerical simulations by Okamoto (2013) seem to overpredict the number of dwarfs on average, while more recent predictions from Copernicus Complexio are in a better agreement. In both observations and simulations, there is a large diversity in the LFs, demonstrating the importance of addressing the missing satellite problem with a statistically representative sample. We also characterize the projected spatial distributions of the satellites and do not observe strong evidence for alignments around the central galaxies. Based on this successful pilot observation, we are carrying out further observations to increase the sample of nearby galaxies, which we plan to report in our future paper.

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arXiv2018

Tidal Features at 0.05<z<0.45 in the Hyper Suprime-Cam Subaru Strategic Program: Properties and Formation Channels

We present 1,201 galaxies at $0.05<z<0.45$ that host tidal features, detected from the first $\sim\! 200$ deg$^2$ of imaging from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). All galaxies in the present sample have spectroscopic observations from the Sloan Digital Sky Survey (SDSS) spectroscopic campaigns, generating a sample of 21208 galaxies. Of these galaxies, we identify 214 shell systems and 987 stream systems. For 575 of these systems, we are additionally able to measure the $(g-i)$ colors of the tidal features. We find evidence for star formation in a subset of the streams, with the exception of streams around massive ellipticals, and find that stream host galaxies span the full range of stellar masses in our sample. Galaxies which host shells are predominantly red and massive: we find that observable shells form more frequently around ellipticals than around disc galaxies of the same stellar mass. Although the majority of the shells in our sample are consistent with being formed by minor mergers, $15\% \pm 4.4\%$ of shell host galaxies have $(g-i)$ colors as red as their host galaxy, consistent with being formed by major mergers. These "red shells" are additionally preferentially aligned with the major axis of the host galaxy, as previously predicted from simulations. We suggest that although the bulk of the observable shell population originates from fairly minor mergers, which preferentially form shells that are not aligned with the major axis of the galaxy, major mergers produce a significant number of observable shells.

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arXiv2018

Illuminating Low-Surface-Brightness Galaxies with the Hyper Suprime-Cam Survey

We present a catalog of extended low-surface-brightness galaxies (LSBGs) identified in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using the first ${\sim}$200 deg$^2$ of the survey, we have uncovered 781 LSBGs, spanning red ($g-i\geq0.64$) and blue ($g-i<0.64$) colors and a wide range of morphologies. Since we focus on extended galaxies ($r_\mathrm{eff}=2.5$-$14^{\prime\prime}$), our sample is likely dominated by low-redshift objects. We define LSBGs to have mean surface brightnesses $\barμ_\mathrm{eff}(g)>24.3$ mag arcsec$^{-2}$, which allows nucleated galaxies into our sample. As a result, the central surface brightness distribution spans a wide range of $μ_0(g)=18$-$27.4$ mag arcsec$^{-2}$, with 50% and 95% of galaxies fainter than 24.3 and 22 mag arcsec$^{-2}$, respectively. Furthermore, the surface brightness distribution is a strong function of color, with the red distribution being much broader and generally fainter than that of the blue LSBGs, and this trend shows a clear correlation with galaxy morphology. Red LSBGs typically have smooth light profiles that are well-characterized by single-component Sérsic functions. In contrast, blue LSBGs tend to have irregular morphologies and show evidence for ongoing star formation. We crossmatch our sample with existing optical, HI, and ultraviolet catalogs to gain insight into the physical nature of the LSBGs. We find that our sample is diverse, ranging from dwarf spheroidals and ultra-diffuse galaxies in nearby groups to gas-rich irregulars to giant LSB spirals, demonstrating the potential of the HSC-SSP to provide a truly unprecedented view of the LSBG population.

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arXiv2018

Structure of the Milky Way stellar halo out to its outer boundary with blue horizontal-branch stars

We present the structure of the Milky Way stellar halo beyond Galactocentric distances of $r = 50$ kpc traced by blue horizontal-branch (BHB) stars, which are extracted from the survey data in the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We select BHB candidates based on $(g,r,i,z)$ photometry, where the $z$-band is on the Paschen series and the colors that involve the $z$-band are sensitive to surface gravity. About 450 BHB candidates are identified between $r = 50$ kpc and 300 kpc, most of which are beyond the reach of previous large surveys including the Sloan Digital Sky Survey. We find that the global structure of the stellar halo in this range has substructures, which are especially remarkable in the GAMA15H and XMM-LSS fields in the HSC-SSP. We find that the stellar halo can be fitted to a single power-law density profile with an index of $α\simeq 3.3$ ($3.5$) with (without) these fields and its global axial ratio is $q \simeq 2.2$ ($1.3$). Thus, the stellar halo may be significantly disturbed and be made in a prolate form by halo substructures, perhaps associated with the Sagittarius stream in its extension beyond $r \sim 100$ kpc. For a broken power-law model allowing different power-law indices inside/outside a break radius, we obtain a steep power-law slope of $α\sim 5$ outside a break radius of $\simeq 100$ kpc ($200$ kpc) for the case with (without) GAMA15H and XMM-LSS. This radius of $200$ kpc might be as close as a halo boundary if there is any, although larger BHB sample is required from further HSC-SSP survey to increase its statistical significance.

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arXiv2018

A Detection of the Environmental Dependence of the Sizes and Stellar Haloes of Massive Central Galaxies

We use ~100 square deg of deep (>28.5 mag arcsec$^{-2}$ in i-band), high-quality (median 0.6 arcsec seeing) imaging data from the Hyper Suprime-Cam (HSC) survey to reveal the halo mass dependence of the surface mass density profiles and outer stellar envelopes of massive galaxies. The i-band images from the HSC survey reach ~4 magnitudes deeper than Sloan Digital Sky Survey and enable us to directly trace stellar mass distributions to 100 kpc without requiring stacking. We conclusively show that, at fixed stellar mass, the stellar profiles of massive galaxies depend on the masses of their dark matter haloes. On average, massive central galaxies with $\log M_{\star, 100\ \mathrm{kpc}}>11.6$ in more massive haloes at 0.3 < z < 0.5 have shallower inner stellar mass density profiles (within ~10-20 kpc) and more prominent outer envelopes. These differences translate into a halo mass dependence of the mass-size relation. Central galaxies in haloes with $\log M_{\rm{Halo}}>14.0$ are ~20% larger in $R_{\mathrm{50}}$ at fixed stellar mass. Such dependence is also reflected in the relationship between the stellar mass within 10 and 100 kpc. Comparing to the mass--size relation, the $\log M_{\star, 100\ \rm{kpc}}$-$\log M_{\star, 10\ \rm{kpc}}$ relation avoids the ambiguity in the definition of size, and can be straightforwardly compared with simulations. Our results demonstrate that, with deep images from HSC, we can quantify the connection between halo mass and the outer stellar halo, which may provide new constraints on the formation and assembly of massive central galaxies.

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arXiv2018

A large sample of shear selected clusters from the Hyper Suprime-Cam Subaru Strategic Program S16A wide field mass maps

We present the result of searching for clusters of galaxies based on weak gravitational lensing analysis of the $\sim 160$~deg$^2$ area surveyed by Hyper Suprime-Cam (HSC) as a Subaru Strategic Program. HSC is a new prime focus optical imager with a 1.5 diameter field of view on the 8.2-meter Subaru telescope. The superb median seeing on the HSC $i$-band images of $0.56$ arcsec allows the reconstruction of high angular resolution mass maps via weak lensing, which is crucial for the weak lensing cluster search. We identify 65 mass map peaks with signal-to-noise (SN) ratio larger than 4.7, and carefully examine their properties by cross-matching the clusters with optical and X-ray cluster catalogs. We find that all the 39 peaks with SN$>5.1$ have counterparts in the optical cluster catalogs, and only 2 out of the 65 peaks are probably false positives. The upper limits of X-ray luminosities from ROSAT All Sky Survey (RASS) imply the existence of an X-ray under-luminous cluster population. We show that the X-rays from the shear selected clusters can be statistically detected by stacking the RASS images. The inferred average X-ray luminosity is about half that of the X-ray selected clusters of the same mass. The radial profile of the dark matter distribution derived from the stacking analysis is well modeled by the Navarro-Frenk-White profile with a small concentration parameter value of $c_{500}\sim 2.5$, which suggests that the selection bias on the orientation or the internal structure for our shear selected cluster sample is not strong.

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arXiv2018

Planck Sunyaev-Zel'dovich Cluster Mass Calibration using Hyper Suprime-Cam Weak Lensing

Using $\sim$140 deg$^2$ Subaru Hyper Suprime-Cam (HSC) survey data, we stack the weak lensing (WL) signal around five Planck clusters found within the footprint. This yields a 15$σ$ detection of the mean Planck cluster mass density profile. The five Planck clusters span a relatively wide mass range, $M_{\rm WL,500c} = (2-30)\times10^{14}\,M_\odot/h$ with a mean mass of $M_{\rm WL,500c} = (4.15\pm0.61)\times10^{14}\,M_\odot/h$. The ratio of the stacked Planck Sunyaev-Zel'dovich (SZ) mass to the stacked WL mass is $ \langle M_{\rm SZ}\rangle/\langle M_{\rm WL}\rangle = 1-b = 0.80\pm0.14$. This mass bias is consistent with previous WL mass calibrations of Planck clusters within the errors. We discuss the implications of our findings for the calibration of SZ cluster counts and the much discussed tension between Planck SZ cluster counts and Planck $Λ$CDM cosmology.

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arXiv2018

Source Selection for Cluster Weak Lensing Measurements in the Hyper Suprime-Cam Survey

We present optimized source galaxy selection schemes for measuring cluster weak lensing (WL) mass profiles unaffected by cluster member dilution from the Subaru Hyper Suprime-Cam Strategic Survey Program (HSC-SSP). The ongoing HSC-SSP survey will uncover thousands of galaxy clusters to $z\lesssim1.5$. In deriving cluster masses via WL, a critical source of systematics is contamination and dilution of the lensing signal by cluster {members, and by foreground galaxies whose photometric redshifts are biased}. Using the first-year CAMIRA catalog of $\sim$900 clusters with richness larger than 20 found in $\sim$140 deg$^2$ of HSC-SSP data, we devise and compare several source selection methods, including selection in color-color space (CC-cut), and selection of robust photometric redshifts by applying constraints on their cumulative probability distribution function (PDF; P-cut). We examine the dependence of the contamination on the chosen limits adopted for each method. Using the proper limits, these methods give mass profiles with minimal dilution in agreement with one another. We find that not adopting either the CC-cut or P-cut methods results in an underestimation of the total cluster mass ($13\pm4\%$) and the concentration of the profile ($24\pm11\%$). The level of cluster contamination can reach as high as $\sim10\%$ at $R\approx 0.24$ Mpc/$h$ for low-z clusters without cuts, while employing either the P-cut or CC-cut results in cluster contamination consistent with zero to within the 0.5% uncertainties. Our robust methods yield a $\sim60σ$ detection of the stacked CAMIRA surface mass density profile, with a mean mass of $M_\mathrm{200c} = (1.67\pm0.05({\rm {stat}}))\times 10^{14}\,M_\odot/h$.

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arXiv2018

SILVERRUSH. VII. Subaru/HSC Identifications of 42 Protocluster Candidates at z~6-7 with the Spectroscopic Redshifts up to z=6.574: Implications for Cosmic Reionization

We report fourteen and twenty-eight protocluster candidates at z=5.7 and 6.6 over 14 and 19 deg^2 areas, respectively, selected from 2,230 (259) Lya emitters (LAEs) photometrically (spectroscopically) identified with Subaru/Hyper Suprime-Cam (HSC) deep images (Keck, Subaru, and Magellan spectra and the literature data). Six out of the 42 protocluster candidates include 1-12 spectroscopically confirmed LAEs at redshifts up to z=6.574. By the comparisons with the cosmological Lya radiative transfer (RT) model reproducing LAEs with the reionization effects, we find that more than a half of these protocluster candidates are progenitors of the present-day clusters with a mass of > 10^14 M_sun. We then investigate the correlation between LAE overdensity delta and Lya rest-frame equivalent width EW_Lya^rest, because the cosmological Lya RT model suggests that a slope of EW_Lya^rest-delta relation is steepened towards the epoch of cosmic reionization (EoR), due to the existence of the ionized bubbles around galaxy overdensities easing the escape of Lya emission from the partly neutral intergalactic medium (IGM). The available HSC data suggest that the slope of the EW_Lya^rest-delta correlation does not evolve from the post-reionization epoch z=5.7 to the EoR z=6.6 beyond the moderately large statistical errors. There is a possibility that we would detect the evolution of the EW_Lya^rest - delta relation from z=5.7 to 7.3 by the upcoming HSC observations providing large samples of LAEs at z=6.6-7.3.

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arXiv2017

The first-year shear catalog of the Subaru Hyper Suprime-Cam SSP Survey

We present and characterize the catalog of galaxy shape measurements that will be used for cosmological weak lensing measurements in the Wide layer of the first year of the Hyper Suprime-Cam (HSC) survey. The catalog covers an area of 136.9 deg$^2$ split into six fields, with a mean $i$-band seeing of $0.58$ arcsec and $5σ$ point-source depth of $i\sim 26$. Given conservative galaxy selection criteria for first year science, the depth and excellent image quality results in unweighted and weighted source number densities of 24.6 and 21.8 arcmin$^{-2}$, respectively. We define the requirements for cosmological weak lensing science with this catalog, then focus on characterizing potential systematics in the catalog using a series of internal null tests for problems with point-spread function (PSF) modeling, shear estimation, and other aspects of the image processing. We find that the PSF models narrowly meet requirements for weak lensing science with this catalog, with fractional PSF model size residuals of approximately $0.003$ (requirement: 0.004) and the PSF model shape correlation function $ρ_1<3\times 10^{-7}$ (requirement: $4\times 10^{-7}$) at 0.5$^\circ$ scales. A variety of galaxy shape-related null tests are statistically consistent with zero, but star-galaxy shape correlations reveal additive systematics on $>1^\circ$ scales that are sufficiently large as to require mitigation in cosmic shear measurements. Finally, we discuss the dominant systematics and the planned algorithmic changes to reduce them in future data reductions.

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arXiv2017

Stellar Stream and Halo Structure in the Andromeda Galaxy From a Subaru/Hyper Suprime-Cam Survey

We present wide and deep photometry of the northwest part of the halo of the Andromeda galaxy (M31) using Hyper Suprime-Cam on the Subaru Telescope. The survey covers 9.2 deg$^{2}$ field in the $g$, $i$, and $NB515$ bands and shows a clear red giant branch (RGB) of M31's halo stars and a pronounced red clump (RC) feature. The spatial distribution of RC stars shows a prominent stream feature, the North Western (NW) Stream, and a diffuse substructure in the south part of our survey field. We estimate the distances based on the RC method and obtain $(m-M)$ = 24.63$\pm 0.191$(random)$\pm0.057$(systematic) and 24.29$\pm 0.211$(random)$\pm0.057$(systematic) mag for the NW stream and diffuse substructure, respectively, implying that the NW Stream is located behind M31, whereas the diffuse substructure is located in front. We also estimate line-of-sight distances along the NW Stream and find that the south part of the stream is $\sim$20 kpc closer to us relative to the north part. The distance to the NW Stream inferred from the isochrone fitting to the color-magnitude diagram favors the RC-based distance, but the TRGB-based distance estimated for $NB515$-selected RGB stars does not agree with it. The surface number density distribution of RC stars across the NW Stream is found to be approximately Gaussian with a FWHM of $\sim$25 arcmin (5.7 kpc), with a slight skew to the south-west side. That along the NW Stream shows a complicated structure including variations in number density and a significant gap in the stream.

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arXiv2017

Survey of Gravitationally-lensed Objects in HSC Imaging (SuGOHI). I. Automatic search for galaxy-scale strong lenses

The Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) is an excellent survey for the search for strong lenses, thanks to its area, image quality and depth. We use three different methods to look for lenses among 43,000 luminous red galaxies from the Baryon Oscillation Spectroscopic Survey (BOSS) sample with photometry from the S16A internal data release of the HSC SSP. The first method is a newly developed algorithm, named YATTALENS, which looks for arc-like features around massive galaxies and then estimates the likelihood of an object being a lens by performing a lens model fit. The second method, CHITAH, is a modeling-based algorithm originally developed to look for lensed quasars. The third method makes use of spectroscopic data to look for emission lines from objects at a different redshift from that of the main galaxy. We find 15 definite lenses, 36 highly probable lenses and 282 possible lenses. Among the three methods, YATTALENS, which was developed specifically for this problem, performs best in terms of both completeness and purity. Nevertheless five highly probable lenses were missed by YATTALENS but found by the other two methods, indicating that the three methods are highly complementary. Based on these numbers we expect to find $\sim$300 definite or probable lenses by the end of the HSC SSP.

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arXiv2017

SILVERRUSH. IV. Ly$α$ Luminosity Functions at $z = 5.7$ and $6.6$ Studied with $\sim$ 1,300 LAEs on the $14-21$ deg$^2$ Sky

We present the Ly$α$ luminosity functions (LFs) at $z=$5.7 and 6.6 derived from a new large sample of 1,266 Ly$α$ emitters (LAEs) identified in total areas of 14 and 21 deg$^2$, respectively, based on the early narrowband data of the Subaru/Hyper Suprime-Cam (HSC) survey. Together with careful Monte-Carlo simulations that account for the incompleteness of the LAE selection and the flux estimate systematics in the narrowband imaging, we have determined the Ly$α$ LFs with the unprecedentedly small statistical and systematic uncertainties in a wide Ly$α$ luminosity range of $10^{42.8-43.8}$ erg s$^{-1}$. We obtain the best-fit Schechter parameters of $L^{*}_{\rm Lya}=1.6^{+2.2}_{-0.6} (1.7^{+0.3}_{-0.7}) \times10^{43}$ erg s$^{-1}$, $ϕ^{*}_{\rm Lya}=0.85^{+1.87}_{-0.77}\ (0.47^{+1.44}_{-0.44})\times10^{-4}$ Mpc$^{-3}$, and $α=-2.6^{+0.6}_{-0.4}\ (-2.5^{+0.5}_{-0.5})$ at $z=5.7$ ($6.6$). We confirm that our best-estimate Ly$α$ LFs are consistent with the majority of the previous studies, but find that our Ly$α$ LFs do not agree with the high number densities of LAEs recently claimed by Matthee/Santos et al.'s studies that may overcorrect the incompleteness and the flux systematics. Our Ly$α$ LFs at $z=5.7$ and $6.6$ show an indication that the faint-end slope is very steep ($α\simeq -2.5$), although it is also possible that the bright-end LF results are enhanced by systematic effects such as the contribution from AGNs, blended merging galaxies, and/or large ionized bubbles around bright LAEs. Comparing our Ly$α$ LF measurements with four independent reionization models, we estimate the neutral hydrogen fraction of the IGM to be $x_{\rm HI}=0.3\pm0.2$ at $z=$6.6 that is consistent with the small Thomson scattering optical depth obtained by Planck 2016.

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arXiv2017

Multiwavelength study of X-ray Luminous Clusters in the Hyper Suprime-Cam Subaru Strategic Program S16A field

We present a joint X-ray, optical and weak-lensing analysis for X-ray luminous galaxy clusters selected from the MCXC (Meta-Catalog of X-Ray Detected Clusters of Galaxies) cluster catalog in the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) survey field with S16A data, As a pilot study of our planned series papers, we measure hydrostatic equilibrium (H.E.) masses using XMM-Newton data for four clusters in the current coverage area out of a sample of 22 MCXC clusters. We additionally analyze a non-MCXC cluster associated with one MCXC cluster. We show that H.E. masses for the MCXC clusters are correlated with cluster richness from the CAMIRA catalog (Oguri et al. 2017), while that for the non-MCXC cluster deviates from the scaling relation. The mass normalization of the relationship between the cluster richness and H.E. mass is compatible with one inferred by matching CAMIRA cluster abundance with a theoretical halo mass function. The mean gas mass fraction based on H.E. masses for the MCXC clusters is $\langle f_{\rm gas} \rangle = 0.125\pm0.012$ at spherical overdensity $Δ=500$, which is $\sim80-90$ percent of the cosmic mean baryon fraction, $Ω_b/Ω_m$, measured by cosmic microwave background experiments. We find that the mean baryon fraction estimated from X-ray and HSC-SSP optical data is comparable to $Ω_b/Ω_m$. A weak-lensing shear catalog of background galaxies, combined with photometric redshifts, is currently available only for three clusters in our sample. Hydrostatic equilibrium masses roughly agree with weak-lensing masses, albeit with large uncertainty. This study demonstrates that further multiwavelength study for a large sample of clusters using X-ray, HSC-SSP optical and weak lensing data will enable us to understand cluster physics and utilize cluster-based cosmology.

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arXiv2017

SILVERRUSH. II. First Catalogs and Properties of ~2,000 Lya Emitters and Blobs at z~6-7 Identified over the 14-21 deg2 Sky

We present an unprecedentedly large catalog consisting of 2,230 > L* Lya emitters (LAEs) at z=5.7 and 6.6 on the 13.8 and 21.2 deg2 sky, respectively, that are identified by the SILVERRUSH program with the first narrowband imaging data of the Hyper Suprime-Cam (HSC) survey. We confirm that the LAE catalog is reliable on the basis of 96 LAEs whose spectroscopic redshifts are already determined by this program and the previous studies. This catalogue is also available on-line. Based on this catalogue, we derive the rest-frame Lya equivalent-width distributions of LAEs at z~5.7-6.6 that are reasonably explained by the exponential profiles with the scale lengths of ~120-170A, showing no significant evolution from z~5.7 to z~6.6. We find that 275 LAEs with a large equivalent width (LEW) of >240A are candidates of young-metal poor galaxies and AGNs. We also find that the fraction of LEW LAEs to all ones is 4% and 21% at z~5.7 and z~6.6, respectively. Our LAE catalog includes 11 Lya blobs (LABs) that are LAEs with spatially extended Lya emission whose profile is clearly distinguished from those of stellar objects at the >~ 3sigma level. The number density of the LABs at z=6-7 is ~ 10^-7-10^-6 Mpc^-3, being ~ 10-100 times lower than those claimed for LABs at z~ 2-3, suggestive of disappearing LABs at z>~6, albeit with the different selection methods and criteria for the low and high-z LABs.

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arXiv2017

SILVERRUSH. III. Deep Optical and Near-Infrared Spectroscopy for Lya and UV-Nebular Lines of Bright Lya Emitters at z=6-7

We present Lya and UV-nebular emission line properties of bright Lya emitters (LAEs) at z=6-7 with a luminosity of log L_Lya/[erg s-1] = 43-44 identified in the 21-deg2 area of the SILVERRUSH early sample developed with the Subaru Hyper Suprime-Cam (HSC) survey data. Our optical spectroscopy newly confirm 21 bright LAEs with clear Lya emission, and contribute to make a spectroscopic sample of 96 LAEs at z=6-7 in SILVERRUSH. From the spectroscopic sample, we select 7 remarkable LAEs as bright as Himiko and CR7 objects, and perform deep Keck/MOSFIRE and Subaru/nuMOIRCS near-infrared spectroscopy reaching the 3sigma-flux limit of ~ 2x10^{-18} erg s-1 for the UV-nebular emission lines of He II1640, C IV1548,1550, and O III]1661,1666. Except for one tentative detection of C IV, we find no strong UV-nebular lines down to the flux limit, placing the upper limits of the rest-frame equivalent widths (EW_0) of ~2-4 A for He II, C IV, and O III] lines. Here we also investigate the VLT/X-SHOOTER spectrum of CR7 whose 6 sigma detection of He II is claimed by Sobral et al. Although two individuals and the ESO-archive service carefully re-analyze the X-SHOOTER data that are used in the study of Sobral et al., no He II signal of CR7 is detected, supportive of weak UV-nebular lines of the bright LAEs even for CR7. Spectral properties of these bright LAEs are thus clearly different from those of faint dropouts at z~7 that have strong UV-nebular lines shown in the various studies. Comparing these bright LAEs and the faint dropouts, we find anti-correlations between the UV-nebular line EW_0 and UV-continuum luminosity, which are similar to those found at z~2-3.

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arXiv2017

Luminous Quasars Do Not Live in the Most Overdense Regions of Galaxies at z~4

We present the cross-correlation between 151 luminous quasars ($M_{ \mathrm{UV}} < -26$) and 179 protocluster candidates at $z \sim 3.8$, extracted from the Wide imaging survey ($ \sim 121~ $deg$^2$) performed with a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We find that only two out of 151 quasars reside in regions that are more overdense compared to the average field at $ > 4 σ$. The distributions of the distance between quasars and the nearest protoclusters and the significance of the overdensity at the position of quasars are statistically identical to those found for $g$-dropout galaxies, suggesting that quasars tend to reside in almost the same environment as star-forming galaxies at this redshift. Using stacking analysis, we find that the average density of $g$-dropout galaxies around quasars is slightly higher than that around $g$-dropout galaxies on $1.0 - 2.5$ pMpc scales, while at $ < 0.5$ pMpc that around quasars tends to be lower. We also find that quasars with higher UV-luminosity or with more massive black holes tend to avoid the most overdense regions, and that the quasar near zone sizes are anti-correlated with overdensity. These findings are consistent with a scenario in which the luminous quasar at $z \sim4 $ resides in structures that are less massive than those expected for the progenitors of today's rich clusters of galaxies, and possibly that luminous quasars may be suppressing star formation in their close vicinity.

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arXiv2017

First results on the cluster galaxy population from the Subaru Hyper Suprime-Cam survey. II. Faint end color-magnitude diagrams and radial profiles of red and blue galaxies at $0.1<z<1.1$

We present a statistical study of the redshift evolution of the cluster galaxy population over a wide redshift range from 0.1 to 1.1, using $\sim 1900$ optically-selected CAMIRA clusters from $\sim 232$~deg$^2$ of the Hyper Suprime-Cam (HSC) Wide S16A data. Our stacking technique with a statistical background subtraction reveals color-magnitude diagrams of red-sequence and blue cluster galaxies down to faint magnitudes of $m_z\sim 24$. We find that the linear relation of red-sequence galaxies in the color-magnitude diagram extends down to the faintest magnitudes we explore with a small intrinsic scatter $σ_{\rm int}(g-r)<0.1$. The scatter does not evolve significantly with redshift. The stacked color-magnitude diagrams are used to define red and blue galaxies in clusters for studying their radial number density profiles without resorting to photometric redshifts of individual galaxies. We find that red galaxies are significantly more concentrated toward cluster centers and blue galaxies dominate the outskirt of clusters. We explore the fraction of red galaxies in clusters as a function of redshift, and find that the red fraction decreases with increasing distances from cluster centers. The red fraction exhibits a moderate decrease with increasing redshift. The radial number density profiles of cluster member galaxies are also used to infer the location of the steepest slope in the three dimensional galaxy density profiles. For a fixed threshold in richness, we find little redshift evolution in this location.

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