arXiv subjects

Nobunari Kashikawa

Publications and source records attributed to Nobunari Kashikawa.

At least 91 records · Page 5Linked to original sources

arXiv2020

Dual supermassive black holes at close separation revealed by the Hyper Suprime-Cam Subaru Strategic Program

The unique combination of superb spatial resolution, wide-area coverage, and deep depth of the optical imaging from the Hyper Suprime-Cam (HSC) Subaru Strategic Program is utilized to search for dual quasar candidates. Using an automated image analysis routine on 34,476 known SDSS quasars, we identify those with two (or more) distinct optical point sources in HSC images covering 796 deg^2. We find 421 candidates out to a redshift of 4.5 of which one hundred or so are more likely after filtering out contaminating stars. Angular separations of 0.6 - 4.0" correspond to projected separations of 3 - 30 kpc, a range relatively unexplored for population studies of luminous dual quasars. Using Keck-I/LRIS and Gemini-N/NIFS, we spectroscopically confirm three dual quasar systems at z < 1, two of which are previously unknown out of eight observed, based on the presence of characteristic broad emission lines in each component, while highlighting that the continuum of one object in one of the pairs is reddened. In all cases, the [OIII]5007 emission lines have mild velocity offsets, thus the joint [OIII] line profile is not double-peaked. We find a dual quasar fraction of 0.26+/-0.18% and no evidence for evolution. A comparison with the Horizon-AGN simulation seems to support the case of no evolution in the dual quasar fraction when broadly matching the quasar selection. These results may indicate a scenario in which the frequency of the simultaneous triggering of luminous quasars is not as sensitive as expected to the cosmic evolution of the merger rate or gas content of galaxies.

astro-ph.GA↗

arXiv2020

The UV Luminosity Function of Protocluster Galaxies at $z\sim4$: the Bright-end Excess and the Enhanced Star Formation Rate Density

We report the rest-frame ultraviolet luminosity function of $g$-dropout galaxies in 177 protocluster candidates (PC UVLF) at $z\sim4$ selected in the Hyper Suprime-Cam Subaru Strategic Program. Comparing with the UVLF of field galaxies at the same redshift, we find that the PC UVLF shows a significant excess towards the bright-end. This excess can not be explained by the contribution of only active galactic nuclei, and we also find that this is more significant in higher dense regions. Assuming that all protocluster members are located on the star formation main sequence, the PC UVLF can be converted into a stellar mass function. Consequently, our protocluster members are inferred to have a 2.8 times more massive characteristic stellar mass than that of the field Lyman break galaxies at the same redshift. This study, for the first time, clearly shows that the enhancement in star formation or stellar mass in overdense regions can generally be seen as early as at $z\sim4$. We also estimate the star formation rate density (SFRD) in protocluster regions as $\simeq 6-20\%$ of the cosmic SFRD, based on the measured PC UVLF after correcting for the selection incompleteness in our protocluster sample. This high value suggests that protoclusters make a non-negligible contribution to the cosmic SFRD at $z\sim4$, as previously suggested by simulations. Our results suggest that protoclusters are essential components for the galaxy evolution at $z\sim4$.

astro-ph.GA↗

arXiv2020

Possible evolution of the circum-galactic medium around QSOs with QSO age and cosmic time revealed by Ly$α$ halos

We first present new Subaru narrow-band observations of the Ly$α$ halo around the quasi-stellar object (QSO) CFHQ J232908$-$030158 at $z=6.42$, which appears the most luminous and extended halo at $z>5$ ($L_{Lyα}=9.8\times10^{43}$ erg s$^{-1}$ within $37$ pkpc diameter). Then, combining these measurements with available data in the literature, we find two different evolutions of QSOs' Ly$α$ halos. First is a possible short-term evolution with QSO age seen in four $z>6$ QSOs. We find the anti-correlation between the Ly$α$ halo scales with QSOs' IR luminosity, with J2329-0301's halo being the brightest and largest. It indicates that ionizing photons escape more easily out to circum-galactic regions when host galaxies are less dusty. We also find a positive correlation between IR luminosity and black hole mass ($M_\text{BH}$). Given $M_\text{BH}$ as an indicator of QSO age, we propose a hypothesis that a large Ly$α$ halo mainly exists around QSOs in the young phase of their activity due to a small amount of dust. The second is an evolution with cosmic time seen over $z\sim2-5$. We find the increase of surface brightness toward lower-redshift with a similar growth rate to that of dark matter halos (DHs) which evolve to $M_\text{DH}=10^{12}-10^{13}$ M$_\odot$ at $z=2$. The extent of Ly$α$ halos is also found to increase at a rate scaling with the virial radius of growing DHs, $r_\text{vir} \propto M_\text{DH}^{1/3}$($1+z$)$^{-1}$. These increases are consistent with a scenario that the CGM around QSOs evolves in mass and size keeping pace with hosting DHs.

astro-ph.GA↗

arXiv2020

ALMA band 8 observations of DLA2233+131 at z=3.150

We present our ALMA Band 8 observations of a damped Ly$α$ absorption (DLA) system at $z$=3.150 observed in the spectrum of the quasar Q2233+131 at $z$=3.295. The optical counterpart of this DLA has been identified and it shows a double-peaked Ly$α$ emission line. Since one possible origin of DLAs at high redshift is an outflowing gas from star-forming galaxies, DLA2233+131 provides a good laboratory to investigate the nature of high-$z$ DLAs. Motivated by this, we have carried out ALMA band 8 observations to study the [C II] line in this system. However, we do not detect any significant emission line in the observed pass bands. Instead, we have serendipitously found three submm continuum sources in the observed sky area. One appears to be the quasar Q2233+131 itself while the other two sources are newly identified submm galaxies (SMGs), called SMG1 and SMG2 in this paper. They are located at a separation of 400.7 and 800.1 from Q2233+131, respectively. Their 646 $μ$m fluxes are 6.35 mJy and 6.43 mJy, respectively, being higher than that of Q2233+131, 3.62 mJy. Since these two SMGs are not detected in the optical images obtained with the Hubble Space Telescope and the Subaru Telescope, they have a very red spectral energy distribution. It is, therefore,suggested that they are high-redshift galaxies or very dusty galaxies at intermediate redshift although we cannot rule out the possibility that they are optically very faint SMG analogs at low redshift. Follow up observations will be necessary to explore the nature of this interesting region.

astro-ph.GA↗

arXiv2020

Evidence for a highly opaque large-scale galaxy void at the end of reionization

We present evidence that a region of high effective Ly$α$ optical depth at $z\sim5.7$ is associated with an underdense region at the tail end of cosmic reionization. We carried out a survey of Lyman-break Galaxies (LBGs) using Subaru Hyper Suprime-Cam in the field of the $z=5.98$ quasar J0148+0600, whose spectrum presents an unusually long ($\sim160 \mathrm{cMpc}$) and opaque ($τ\gtrsim7$) Ly$α$ trough at $5.5\le z\le 5.9$. LBG candidates were selected to lie within the redshift range of the trough, and the projected number densities were measured within 90~cMpc of the quasar sightline. The region within $8'$ (or $\approx 19~\mathrm{cMpc}$) of the quasar position is the most underdense of the whole field. The significance of the presence of the void is estimated to be $99\%$. This is consistent with the significant deficit of Ly$α$ emitters (LAEs) at $z=5.72$ reported by Becker et al. and suggests that the paucity of LAEs is not purely due to the removal of the Ly$α$ emission by the high opacity but reflects a real coherent underdensity of galaxies across the entire redshift range of the trough. These observations are consistent with scenarios in which large optical depth fluctuations arise due to fluctuations in the galaxy-dominant UV background or due to residual neutral islands that are expected from reionization that is completed at redshifts as low as $z\lesssim5.5$.

astro-ph.GA↗

arXiv2019

The Subaru HSC Galaxy Clustering with Photometric Redshift I: Dark Halo Masses Versus Baryonic Properties of Galaxies at 0.3<z<1.4

We present the clustering properties of low-$z$ $(z\leq1.4)$ galaxies selected by the Hyper Suprime-Cam Subaru Strategic Program Wide layer over $145$ deg$^{2}$. The wide-field and multi-wavelength observation yields $5,064,770$ galaxies at $0.3\leq z\leq1.4$ with photometric redshifts and physical properties. This enables the accurate measurement of angular correlation functions and subsequent halo occupation distribution (HOD) analysis allows the connection between baryonic properties and dark halo properties. The fraction of less-massive satellite galaxies at $z\lesssim1$ is found to be almost constant at $\sim20\%$, but it gradually decreases beyond $M_{\star} \sim 10^{10.4}h^{-2}M_{\odot}$. However, the abundance of satellite galaxies at $z>1$ is quite small even for less-massive galaxies due to the rarity of massive centrals at high-$z$. This decreasing trend is connected to the small satellite fraction of Lyman break galaxies at $z>3$. The stellar-to-halo mass ratios at $0.3\leq z\leq1.4$ are almost consistent with the predictions obtained using the latest empirical model; however, we identify small excesses from the theoretical model at the massive end. The pivot halo mass is found to be unchanged at $10^{11.9-12.1}h^{-1}M_{\odot}$ at $0.3\leq z\leq1.4$, and we systematically show that $10^{12}h^{-1}M_{\odot}$ is a universal pivot halo mass up to $z\sim5$ that is derived using only the clustering/HOD analyses. Nevertheless, halo masses with peaked instantaneous baryon conversion efficiencies are much smaller than the pivot halo mass regardless of a redshift, and the most efficient stellar-mass assembly is thought to be in progress in $10^{11.0-11.5}h^{-1}M_{\odot}$ dark haloes.

astro-ph.GA↗

arXiv2019

Discovery of Protoclusters at z~3.7 & 4.9: Embedded in Primordial Superclusters

We have carried out follow-up spectroscopy on three overdense regions of $g$- and $r$-dropout galaxies in the Canada-France-Hawaii Telescope Legacy Survey Deep Fields, finding two new protoclusters at $z=4.898$, 3.721 and a possible protocluster at $z=3.834$. The $z=3.721$ protocluster overlaps with a previously identified protocluster at $z=3.675$. The redshift separation between these two protoclusters is $Δz=0.05$, which is slightly larger than the size of typical protoclusters. Therefore, if they are not the progenitors of a $>10^{15}\,\mathrm{M_\odot}$ halo, they would grow into closely-located independent halos like a supercluster. The other protocluster at $z=4.898$ is also surrounded by smaller galaxy groups. These systems including protoclusters and neighboring groups are regarded as the early phase of superclusters. We quantify the spatial distribution of member galaxies of the protoclusters at $z=3.675$ and 3.721 by fitting triaxial ellipsoids, finding a tentative difference: one has a pancake-like shape while the other is filamentary. This could indicate that these two protoclusters are in different stages of formation. We investigate the relation between redshift and the velocity dispersion of protoclusters, including other protoclusters from the literature, in order to compare their dynamical states. Although there is no significant systematic trend in the velocity dispersions of protoclusters with redshift, the distribution is skewed to higher velocity dispersion over the redshift range of $z=2\mathrm{-}6$. This could be interpreted as two phases of cluster formation, one dominated by the steady accretion of galaxies, and the other by the merging between group-size halos, perhaps depending on the surrounding large-scale environments.

astro-ph.GA↗

arXiv2019

Planck far-infrared detection of Hyper Suprime-Cam protoclusters at $\bf z\sim4$: hidden AGN and star formation activity

We perform a stacking analysis of {\it Planck}, {\it AKARI}, Infrared Astronomical Satellite ($IRAS$), Wide-field Infrared Survey Eplorer ($WISE$), and {\it Herschel} images of the largest number of (candidate) protoclusters at $z\sim3.8$ selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Stacking the images of the $179$ candidate protoclusters, the combined infrared (IR) emission of the protocluster galaxies in the observed $12-850~μ$m wavelength range is successfully detected with $>5σ$ significance (at $Planck$). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at $z\sim4$. The observed IR SEDs of the protoclusters exhibit significant excess emission in the mid-IR compared to that expected from typical star-forming galaxies (SFGs). They are reproduced well using SED models of intense starburst galaxies with warm/hot dust heated by young stars, or by a population of active galactic nuclei (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8 to 1000 $μ$m) luminosity of $19.3_{-4.2}^{+0.6}\times10^{13}~L_{\odot}$ and a star formation rate (SFR) of $16.3_{-7.8}^{+1.0}\times10^3~M_{\odot}$ yr$^{-1}$ are found whereas for the AGN/SFG composite model, $5.1_{-2.5}^{+2.5}\times10^{13}~L_{\odot}$ and $2.1^{+6.3}_{-1.7}\times10^3~M_{\odot}$ yr$^{-1}$ are found. Uncertainty remaining in the total SFRs; however, the IR luminosities of the most massive protoclusters are likely to continue increasing up to $z\sim4$. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the $z\sim4$ protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical.

astro-ph.GA↗

arXiv2019

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) VIII. A less biased view of the early co-evolution of black holes and host galaxies

We present ALMA [CII] line and far-infrared (FIR) continuum observations of three $z > 6$ low-luminosity quasars ($M_{\rm 1450} > -25$) discovered by our Subaru Hyper Suprime-Cam (HSC) survey. The [CII] line was detected in all three targets with luminosities of $(2.4 - 9.5) \times 10^8~L_\odot$, about one order of magnitude smaller than optically luminous ($M_{\rm 1450} \lesssim -25$) quasars. The FIR continuum luminosities range from $< 9 \times 10^{10}~L_\odot$ (3$σ$ limit) to $\sim 2 \times 10^{12}~L_\odot$, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show [CII]/FIR luminosity ratios similar to local star-forming galaxies. Using the [CII]-based dynamical mass ($M_{\rm dyn}$) as a surrogate for bulge stellar mass ($M_{\rm bulge}$), we find that a significant fraction of low-luminosity quasars are located on or even below the local $M_{\rm BH} - M_{\rm bulge}$ relation, particularly at the massive end of the galaxy mass distribution. In contrast, previous studies of optically luminous quasars have found that black holes are overmassive relative to the local relation. Given the low luminosities of our targets, we are exploring the nature of the early co-evolution of supermassive black holes and their hosts in a less biased way. Almost all of the quasars presented in this work are growing their black hole mass at much higher pace at $z \sim 6$ than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local $M_{\rm BH} - M_{\rm bulge}$ relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at $z \sim 6$, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.

astro-ph.GA↗

arXiv2019

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). X. Discovery of 35 Quasars and Luminous Galaxies at 5.7 $\le$ z $\le$ 7.0

We report the discovery of 28 quasars and 7 luminous galaxies at 5.7 $\le$ z $\le$ 7.0. This is the tenth in a series of papers from 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 total number of spectroscopically identified objects in SHELLQs has now grown to 93 high-z quasars, 31 high-z luminous galaxies, 16 [O III] emitters at z ~ 0.8, and 65 Galactic cool dwarfs (low-mass stars and brown dwarfs). These objects were found over 900 deg2, surveyed by HSC between 2014 March and 2018 January. The full quasar sample includes 18 objects with very strong and narrow Ly alpha emission, whose stacked spectrum is clearly different from that of other quasars or galaxies. While the stacked spectrum shows N V 1240 emission and resembles that of lower-z narrow-line quasars, the small Ly alpha width may suggest a significant contribution from the host galaxies. Thus these objects may be composites of quasars and star-forming galaxies.

astro-ph.GA↗

arXiv2019

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). VI. Black Hole Mass Measurements of Six Quasars at 6.1<z<6.7

We present deep near-infrared spectroscopy of six quasars at 6.1 5.8 quasars with measured black hole masses. From single-epoch mass measurements based on MgII2798, we find a wide range in black hole masses, from M_BH=10^7.6 to 10^9.3 Msun. The Eddington ratios L_bol/L_Edd range from 0.16 to 1.1, but the majority of the HSC quasars are powered by M_BH=10^9 Msun supermassive black holes (SMBHs) accreting at sub-Eddington rates. The Eddington ratio distribution of the HSC quasars is inclined to lower accretion rates than those of Willott et al. (2010a), who measured the black hole masses for similarly faint z=6 quasars. This suggests that the global Eddington ratio distribution is wider than has previously been thought. The presence of M_BH=10^9 Msun SMBHs at z=6 cannot be explained with constant sub-Eddington accretion from stellar remnant seed black holes. Therefore, we may be witnessing the first buildup of the most massive black holes in the first billion years of the universe, the accretion activity of which is transforming from active growth to a quiescent phase. Measurements of a larger complete sample of z>6 low-luminosity quasars, as well as deeper observations with future facilities will enable us to better understand the early SMBH growth in the reionization epoch.

astro-ph.GA↗

arXiv2019

SILVERRUSH. VIII. Spectroscopic Identifications of Early Large Scale Structures with Protoclusters Over 200 Mpc at z~6-7: Strong Associations of Dusty Star-Forming Galaxies

We have obtained three-dimensional maps of the universe in $\sim200\times200\times80$ comoving Mpc$^3$ (cMpc$^3$) volumes each at $z=5.7$ and $6.6$ based on a spectroscopic sample of 179 galaxies that achieves $\gtrsim80$\% completeness down to the Ly$α$ luminosity of $\log(L_{\rm Lyα}/[\mathrm{erg\ s^{-1}}])=43.0$, based on our Keck and Gemini observations and the literature. The maps reveal filamentary large-scale structures and two remarkable overdensities made out of at least 44 and 12 galaxies at $z=5.692$ (z57OD) and $z=6.585$ (z66OD), respectively, making z66OD the most distant overdensity spectroscopically confirmed to date with $>10$ spectroscopically confirmed galaxies. We compare spatial distributions of submillimeter galaxies at $z\simeq 4-6$ with our $z=5.7$ galaxies forming the large-scale structures, and detect a $99.97\%$ signal of cross correlation, indicative of a clear coincidence of dusty star-forming galaxy and dust unobscured galaxy formation at this early epoch. The galaxies in z57OD and z66OD are actively forming stars with star formation rates (SFRs) $\gtrsim5$ times higher than the main sequence, and particularly the SFR density in z57OD is 10 times higher than the cosmic average at the redshift (a.k.a. the Madau-Lilly plot). Comparisons with numerical simulations suggest that z57OD and z66OD are protoclusters that are progenitors of the present-day clusters with halo masses of $\sim10^{14}\ \mathrm{M_\odot}$.

astro-ph.GA↗

arXiv2019

CHORUS. III. Photometric and Spectroscopic Properties of Ly$α$ Blobs at $z=4.9-7.0$

We report the Subaru Hyper Suprime-Cam (HSC) discovery of two Ly$α$ blobs (LABs), dubbed z70-1 and z49-1 at $z=6.965$ and $z=4.888$ respectively, that are Ly$α$ emitters with a bright ($\log L_{\rm Lyα}/{\rm [erg\ s^{-1}]}>43.4$) and spatially-extended Ly$α$ emission, and present the photometric and spectroscopic properties of a total of seven LABs; the two new LABs and five previously-known LABs at $z=5.7-6.6$. The z70-1 LAB shows the extended Ly$α$ emission with a scale length of $1.4\pm 0.2$ kpc, about three times larger than the UV continuum emission, making z70-1 the most distant LAB identified to date. All of the 7 LABs, except z49-1, exhibit no AGN signatures such as X-ray emission, {\sc Nv}$λ$1240 emission, or Ly$α$ line broadening, while z49-1 has a strong {\sc Civ}$λ$1548 emission line indicating an AGN on the basis of the UV-line ratio diagnostics. We carefully model the point-spread functions of the HSC images, and conduct two-component exponential profile fitting to the extended Ly$α$ emission of the LABs. The Ly$α$ scale lengths of the core (star-forming region) and the halo components are $r_{\rm c}=0.6-1.2$ kpc and $r_{\rm h}=2.0-13.8$ kpc, respectively. The average $r_{\rm h}$ of the LABs falls on the extrapolation of the $r_{\rm h}$-Ly$α$ luminosity relation of the Ly$α$ halos around VLT/MUSE star-forming galaxies at the similar redshifts, suggesting that typical LABs at $z\gtrsim5$ are not special objects, but star-forming galaxies at the bright end.

astro-ph.GA↗

arXiv2019

A spectroscopic study of a rich cluster at z=1.52 with Subaru $\&$ LBT: the environmental impacts on the mass-metallicity relation

We present the results of our near-infrared (NIR) spectroscopic observations of a rich cluster candidate around a radio galaxy at $z=1.52$ (4C65.22) with Subaru/MOIRCS and LBT/LUCI. We observed 71 galaxies mostly on the star-forming main sequence selected by our previous broad-band (photo-$z$) and narrow-band H$α$ imaging observation in this cluster environment. We successfully confirmed the redshifts of 39 galaxies, and conclude that this is a gravitationally bound, real cluster at $z=1.517$. Our spectroscopic data also suggest a hint of large-scale filaments or sheet-like three-dimensional structures crossing at the highest-density cluster core. By stacking the spectra to derive their average interstellar medium (ISM) gas-phase metallicity based on the [NII]/H$α$ emission line flux ratio, we find that the mass-metallicity relation (MZR) in the 4C65.22 cluster environment is consistent with that of H$α$-selected field galaxies at similar redshifts. Our results suggest that the environmental impacts on the MZR is small at high redshifts, but a larger sample of high-$z$ clusters and their member galaxies is still required to fully address the effect of environment as well as its cluster-cluster variation.

astro-ph.GA↗

arXiv2019

"Big Three Dragons": a z = 7.15 Lyman BreakGalaxy Detected in [OIII] 88 $μ$m, [CII] 158 $μ$m, and Dust Continuum with ALMA

We present new ALMA observations and physical properties of a Lyman Break Galaxy at z=7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, $M_{\rm UV}\approx-22.4$, and has been spectroscopically identified in Ly$α$ with a small rest-frame equivalent width of $\approx4$ Å. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [OIII] 88 $μ$m, [CII] 158 $μ$m, and their underlying dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII] lines have consistent redshifts of $7.1520\pm0.0003$, and the [OIII] luminosity, $(34.4\pm4.1)\times10^{8}L_{\rm \odot}$, is about three times higher than the [CII] luminosity, $(11.0\pm1.4)\times10^{8}L_{\rm \odot}$. With our two continuum flux densities, the dust temperature is constrained to be $T_{\rm d}\approx50-60$ K under the assumption of the dust emissivity index of $β_{\rm d}=2.0-1.5$, leading to a large total infrared luminosity of $L_{\rm TIR}\approx1\times10^{12}L_{\rm \odot}$. Owing to our high spatial resolution data, we show that the [OIII] and [CII] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by $\approx200$ km s$^{-1}$. We also find these two clumps have comparable UV, infrared, [OIII], and [CII] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR$=260^{+119}_{-57}$ Gyr$^{-1}$, inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [OIII]-to-[CII] luminosity ratio.

astro-ph.GA↗

arXiv2019

The brightest UV-selected galaxies in protoclusters at $z\sim4$: Ancestors of Brightest Cluster Galaxies?

We present the results of a survey of the brightest UV-selected galaxies in protoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates are drawn from 179 overdense regions of $g$-dropout galaxies at $z\sim4$ from the Hyper Suprime-Cam Subaru Strategic Program identified previously as good protocluster candidates. This study is the first to extend the systematic study of the progenitors of BCGs from $z\sim2$ to $z\sim4$. We carefully remove possible contaminants from foreground galaxies and, for each structure, we select the brightest galaxy that is at least 1 mag brighter than the fifth brightest galaxy. We select 63 proto-BCG candidates and compare their properties with those of galaxies in the field and those of other galaxies in overdense structures. The proto-BCG candidates and their surrounding galaxies have different rest-UV color $(i - z)$ distributions to field galaxies and other galaxies in protoclusters that do not host proto-BCGs. In addition, galaxies surrounding proto-BCGs are brighter than those in protoclusters without proto-BCGs. The image stacking analysis reveals that the average effective radius of proto-BCGs is $\sim28\%$ larger than that of field galaxies. The $i-z$ color differences suggest that proto-BCGs and their surrounding galaxies are dustier than other galaxies at $z\sim4$. These results suggest that specific environmental effects or assembly biasses have already emerged in some protoclusters as early as $z \sim 4$, and we suggest that proto-BCGs have different star formation histories than other galaxies in the same epoch.

astro-ph.GA↗

arXiv2019

Tracing the formation history of galaxy clusters into the epoch of reionization

The large-scale overdensities of galaxies at z~2-7 known as protoclusters are believed to be the sites of cluster formation, and deep, wide survey projects such as the Large Synoptic Survey Telescope (LSST) and the Wide Field Infrared Survey Telescope (WFIRST) will deliver significant numbers of these interesting structures. Spectroscopic confirmation and interpretation of these targets, however, is still challenging, and will require wide-field multi-plexed spectroscopy on >20 m-class telescopes in the optical and near-infrared. In the coming decade, detailed studies of protoclusters will enable us, for the first time, to systematically connect these cluster progenitors in the early universe to their virialized counterparts at lower redshifts. This will allow us to address observationally the formation of brightest cluster galaxies and other cluster galaxy populations, the buildup of the intra-cluster light, the chemical enrichment history of the intra-cluster medium, and the formation and triggering of supermassive black holes in dense environments, all of which are currently almost exclusively approached either through the fossil record in clusters or through numerical simulations. Furthermore, at the highest redshifts (z~5-10), these large extended overdensities of star-forming galaxies are believed to have played an important role in the reionization of the universe, which needs to be tested by upcoming experiments. Theory and recent simulations also suggest important links between these overdensities and the formation of supermassive black holes, but observational evidence is still lacking. In this white paper we review our current understanding of this important phase of galaxy cluster history that will be explored by the next generation of large aperture ground-based telescopes GMT and TMT.

astro-ph.CO↗

arXiv2019

Discovery of the First Low-Luminosity Quasar at z > 7

We report the discovery of a quasar at z = 7.07, which was selected from the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. This quasar, HSC J124353.93+010038.5, has an order of magnitude lower luminosity than do the other known quasars at z > 7. The rest-frame ultraviolet absolute magnitude is M1450 = -24.13 +/- 0.08 mag and the bolometric luminosity is Lbol = (1.4 +/- 0.1) x 10^{46} erg/s. Its spectrum in the optical to near-infrared shows strong emission lines, and shows evidence for a fast gas outflow, as the C IV line is blueshifted and there is indication of broad absorption lines. The Mg II-based black hole mass is Mbh = (3.3 +/- 2.0) x 10^8 Msun, thus indicating a moderate mass accretion rate with an Eddington ratio 0.34 +/- 0.20. It is the first z > 7 quasar with sub-Eddington accretion, besides being the third most distant quasar, known to date. The luminosity and black hole mass are comparable to, or even lower than, those measured for the majority of low-z quasars discovered by the Sloan Digital Sky Survey, and thus this quasar likely represents a z > 7 counterpart to quasars commonly observed in the low-z universe.

astro-ph.GA↗