arXiv subjects

Takuji Yamashita

Publications and source records attributed to Takuji Yamashita.

At least 19 recordsLinked to original sources

arXiv2025

Early-type Galaxies on the Star Formation Main Sequence: Internal Star Formation Geometry Revealed with MaNGA and Their Environmental Origin

Star-forming galaxies on the main sequence (MS) are often regarded as a uniform population characterized by similar global star formation properties. However, there exists a diversity in galaxy morphologies at fixed stellar mass and SFR. In this study, using spatially-resolved properties from the MaNGA final data release, we classify MS galaxies into late-type (MS-late) and early-type (MS-early). In addition, we further divide the MS-early galaxies into two distinct subgroups based on their internal star formation and stellar mass distributions within the galaxies. The first group -- ``MS-early\_SF'' -- shows centrally concentrated star formation without prominent stellar bulges and resides preferentially in dense environments, suggesting environmentally-driven evolution. The second group -- ``MS-early\_stellar'' -- exhibits significant stellar bulges with suppressed central star formation, maintains disk-like star formation patterns, and inhabits environments similar to those of late-type galaxies, indicating evolution through internal secular processes. Our findings demonstrate that spatially-resolved observations play critical roles in revealing the diverse evolutionary pathways hidden within galaxies that share similar global properties.

astro-ph.GA↗

arXiv2024

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). X. The Massive and Passive Nature of Radio Galaxies at $z \sim 4$

High-$z$ radio galaxies (HzRGs) are considered important objects for understanding the formation and evolution of massive galaxies in the early universe. However, till date, detailed studies of the stellar population of HzRGs such as the star-formation history have been scarce. Therefore, this study conducted a new survey to establish a less-biased sample of HzRGs and consequently investigate their properties. We utilized a sample of $g$-dropout Lyman break galaxies (LBGs) obtained from an optical wide and deep imaging survey made by Subaru Hyper Suprime-Cam (HSC). Based on the cross-matching of this LBG sample with the VLA FIRST radio survey data, we constructed a photometric sample of high-redshift radio galaxies (HzRGs) at $z \sim 4$ for $\sim$560 deg$^2$ survey field. Consequently, we identified 146 HzRG candidates. To analyze the characteristics of these candidates, we focus on objects exhibiting the near-infrared photometry of VIKING or UKIDSS and the mid-infrared photometry of unWISE (28 objects). The results indicate that 7 objects exhibit SEDs consistent with galaxies at $z \sim 4$. The HzRG candidates have very large stellar masses with $\sim 4.2 \times 10^{11} M_{\odot}$ on average. This stellar mass is similar to that of previously discovered USS HzRGs at $z \sim 4$, though our sample is affected by a sample selection bias that selects only HzRGs with $M_{\star} > 10^{11} M_{\odot}$. Further, the SEDs of those HzRG candidates suggest a past fast quenching with a rough timescale of $\sim$0.1 Gyr, as evidenced from the rest-frame UVJ diagram.

astro-ph.GA↗

arXiv2024

The New Horizons Extended Mission Target: Arrokoth Search and Discovery

Following the Pluto fly-by of the New Horizons spacecraft, the mission provided a unique opportunity to explore the Kuiper Belt in-situ. The possibility existed to fly-by a Kuiper Belt object (KBO) as well as to observe additional objects at distances closer than are feasible from earth-orbit facilities. However, at the time of launch no KBOs were known about that were accessible by the spacecraft. In this paper we present the results of 10 years of observations and three uniquely dedicated efforts -- two ground-based using the Subaru Suprime Camera, the Magellan MegaCam and IMACS Cameras, and one with the Hubble Space Telescope -- to find such KBOs for study. In this paper we overview the search criteria and strategies employed in our work and detail the analysis efforts to locate and track faint objects in the galactic plane. We also present a summary of all of the KBOs that were discovered as part of our efforts and how spacecraft targetability was assessed, including a detailed description of our astrometric analysis which included development of an extensive secondary calibration network. Overall, these efforts resulted in the discovery of 89 KBOs including 11 which became objects for distant observation by New Horizons and (486958) Arrokoth which became the first post-Pluto fly-by destination.

astro-ph.EP↗

arXiv2023

New technique to select recent fast-quenching galaxies at $z\sim2$ using the optical colors

Many massive quiescent galaxies have been discovered at $z>2$ thanks to multi-wavelength deep and wide surveys, however, substantial deep near-infrared spectroscopic observations are needed to constrain their star-formation histories statistically. Here, we present a new technique to select quiescent galaxies with a short quenching timescale ($\leq0.1$ Gyr) at $z\sim2$ photometrically. We focus on a spectral break at $\sim1600$ Å~that appears for such fast-quenching galaxies $\sim1$ Gyr after quenching when early A-type stars go out, but late A-type stars still live. This spectral break at $z\sim2$ is similar to a Lyman break at $z\sim4$. We construct a set of color criteria for $z\sim2$ fast-quenching galaxies on $g-r$ vs. $r-i$ and $i-J$ vs. $J-H$ or $\rm i-[3.6]$ vs. $\rm [3.6]-[4.5]$ color diagrams, which are available with the existing and/or future wide imaging surveys, by simulating various model galaxy spectra and test their robustnesses using the COSMOS2020 catalog. Galaxies with photometric and/or spectroscopic redshifts $z\sim2$ and low specific star formation rates are successfully selected using these colors. The number density of these fast-quenching galaxy candidates at $z\sim2$ suggests that massive galaxies not so far above the star-formation main sequence at $z=3-4$ should be their progenitors.

astro-ph.GA↗

arXiv2023

AGN number fraction in galaxy groups and clusters at z < 1.4 from the Subaru Hyper Suprime-Cam survey

One of the key questions on active galactic nuclei (AGN) in galaxy clusters is how AGN could affect the formation and evolution of member galaxies and galaxy clusters in the history of the Universe. To address this issue, we investigate the dependence of AGN number fraction ($f_{\rm AGN}$) on cluster redshift ($z_{\rm cl}$) and distance from the cluster center ($R/R_{\rm 200}$). We focus on more than 27,000 galaxy groups and clusters at $0.1 < z_{\rm cl} < 1.4$ with more than 1 million member galaxies selected from the Subaru Hyper Suprime-Cam. By combining various AGN selection methods based on infrared (IR), radio, and X-ray data, we identify 2,688 AGN. We find that (i) $f_{\rm AGN}$ increases with $z_{\rm cl}$ and (ii) $f_{\rm AGN}$ decreases with $R/R_{\rm 200}$. The main contributors to the rapid increase of $f_{\rm AGN}$ towards high-$z$ and cluster center are IR- and radio-selected AGN, respectively. Those results indicate that the emergence of the AGN population depends on the environment and redshift, and galaxy groups and clusters at high-$z$ play an important role in AGN evolution. We also find that cluster-cluster mergers may not drive AGN activity in at least the cluster center, while we have tentative evidence that cluster-cluster mergers would enhance AGN activity in the outskirts of (particularly massive) galaxy clusters.

astro-ph.GA↗

arXiv2023

GREX-PLUS Science Book

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA's strategic L-class mission to be launched in the 2030s. Its primary sciences are two-fold: galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2-8 $μ$m wavelength band and a high resolution spectrometer with a wavelength resolution of 30,000 in the 10-18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high resolution spectrometer aims to identify the location of the water ``snow line'' in proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on.

astro-ph.CO↗

arXiv2023

Quasar Luminosity Function at z = 7

We present the quasar luminosity function (LF) at $z = 7$, measured with 35 spectroscopically confirmed quasars at $6.55 < z < 7.15$. The sample of 22 quasars from the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) project, combined with 13 brighter quasars in the literature, covers an unprecedentedly wide range of rest-frame ultraviolet magnitudes over $-28 < M_{1450} < -23$. We found that the binned LF flattens significantly toward the faint end populated by the SHELLQs quasars. A maximum likelihood fit to a double power-law model has a break magnitude $M^*_{1450} = -25.60^{+0.40}_{-0.30}$, a characteristic density $Φ^* = 1.35^{+0.47}_{-0.30}$ Gpc$^{-3}$ mag$^{-1}$, and a bright-end slope $β= -3.34^{+0.49}_{-0.57}$, when the faint-end slope is fixed to $α= -1.2$ as observed at $z \le 6$. The overall LF shape remains remarkably similar from $z = 4$ to $7$, while the amplitude decreases substantially toward higher redshifts, with a clear indication of an accelerating decline at $z \ge 6$. The estimated ionizing photon density, $10^{48.2 \pm 0.1}$ s$^{-1}$ Mpc$^{-3}$, is less than 1 % of the critical rate to keep the intergalactic medium ionized at $z = 7$, and thus indicates that quasars are not a major contributor to cosmic reionization.

astro-ph.GA↗

arXiv2023

eROSITA Final Equatorial-Depth Survey (eFEDS): eFEDS X-ray view of WERGS Radio Galaxies selected by the Subaru/HSC and VLA/FIRST survey

We construct the eROSITA X-ray catalog of radio galaxies discovered by the WERGS survey that is made by the cross-matching of the wide-area Subaru/HSC optical survey and VLA/FIRST 1.4 GHz radio survey. We find 393 eROSITA detected radio galaxies in the 0.5--2 keV band in the eFEDS field covering 140~deg$^2$. Thanks to the wide and medium depth eFEDS X-ray survey, the sample contains the rare and most X-ray luminous radio galaxies above the knee of the X-ray luminosity function, spanning 44 22$. Those obscured and radio AGN reside in $0.4<z<3.2$, indicating that they are obscured counterparts of the radio-loud quasar, which are missed in the previous optical quasar surveys. By combining radio and X-ray luminosities, the jet production efficiency $η_\mathrm{jet}$ is investigated, and we find 14 sources with extremely high jet production efficiency at $η_\mathrm{jet}\approx1$. This high $η_\mathrm{jet}$ value might be a result of 1) the decreased radiation efficiency of $η_\mathrm{rad}<0.1$ due to the low accretion rate for those sources and/or 2) the boosting due to the decline of $L_\mathrm{bol}$ by a factor of 10--100 by keeping $P_\mathrm{jet}$ constant in the previous Myr, indicating the experience of the AGN feedback. Finally, inferring the BH masses from the stellar-mass, we find that X-ray luminous sources show the excess of the radio emission with respect to the value estimated from the fundamental plane. Such radio emission excess cannot be explained by the Doppler booming alone, and therefore disk-jet connection of X-ray luminous eFEDS-WERGS is fundamentally different from the conventional fundamental plane which mainly covers low accretion regime.

astro-ph.HE↗

arXiv2023

Massive molecular gas companions uncovered by VLA CO(1-0) observations of the $z$ = 5.2 radio galaxy TN J0924$-$2201

We present Karl G. Jansky Very Large Array (VLA) K-band (19 GHz) observations of the redshifted CO(1-0) line emission toward the radio galaxy TN J0924$-$2201 at $z=5.2$, which is one of the most distant CO-detected radio galaxies. With the angular resolution of $\sim2''$, the CO(1-0) line emission is resolved into three clumps, within $\pm500$ km\,s$^{-1}$ relative to its redshift, where is determined by Ly$α$. We find that they locate off-center and 12-33 kpc away from the center of the host galaxy, which has counterparts in $HST$ $i$-band, $Spitzer$/IRAC and ALMA Band-6 (230 GHz; 1.3 mm). With the ALMA detection, we estimate $L_{\rm IR}$ and SFR of the host galaxy to be $(9.3\pm1.7)\times10^{11} L_{\odot}$ and $110\pm20$ $M_{\odot}\,\rm yr^{-1}$, respectively. We also derive the $3σ$ upper limit of $M_{\rm H_{2}}<1.3\times10^{10}$ $M_{\odot}$ at the host galaxy. The detected CO(1-0) line luminosities of three clumps, $L'_{\rm CO(1-0)}$ = (3.2-4.7)$\times10^{10}$ $\rm\,K\,km\,s^{-1}pc^{2}$, indicate the presence of three massive molecular gas reservoirs with $M_{\rm H_{2}}$ = (2.5-3.7)$\times10^{10}$ $M_{\odot}$, by assuming the CO-to-H$_{2}$ conversion factor $α_{\rm CO} = 0.8$ $M_{\rm \odot}\rm\,(K\,km\,s^{-1}pc^{2})^{-1}$, although the star formation rate (SFR) is not elevated because of the non-detection of ALMA 1.3 mm continuum (SFR $<$ 40 $M_\odot$ yr$^{-1}$). From the host galaxy, the nearest molecular gas clump labeled as clump A, is apparently aligning with the radio jet axis, showing the radio-CO alignment. The possible origin of these three clumps around TN J0924$-$2201 can be interpreted as merger, jet-induced metal enrichment and outflow.

astro-ph.GA↗

arXiv2022

Extreme nature of four blue-excess dust-obscured galaxies revealed by optical spectroscopy

We report optical spectroscopic observations of four blue-excess dust-obscured galaxies (BluDOGs) identified by Subaru Hyper Suprime-Cam. BluDOGs are a sub-class of dust-obscured galaxies (DOGs, defined with the extremely red color $(i-[22])_{\rm AB} \geq 7.0$; Toba et al. 2015), showing a significant flux excess in the optical $g$- and $r$-bands over the power-law fits to the fluxes at the longer wavelengths. Noboriguchi et al. (2019) has suggested that BluDOGs may correspond to the blowing-out phase involved in a gas-rich major merger scenario. However the detailed properties of BluDOGs are not understood because of the lack of spectroscopic information. In this work, we carry out deep optical spectroscopic observations of four BluDOGs using Subaru/FOCAS and VLT/FORS2. The obtained spectra show broad emission lines with extremely large equivalent widths, and a blue wing in the CIV line profile. The redshifts are between 2.2 and 3.3. The averaged rest-frame equivalent widths of the CIV lines are $160\pm33$ $\mathrm{\mathring{A}}$, $\sim$7 times higher than the average of a typical type-1 quasar. The FWHMs of their velocity profiles are between 1990 and 4470 ${\rm km\ s^{-1}}$, and their asymmetric parameters are 0.05 and 0.25. Such strong CIV lines significantly affect the broad-band magnitudes, which is partly the origin of the blue excess seen in the spectral energy distribution of BluDOGs. Their estimated supermassive black hole masses are $1.1\times10^8 < M_{\rm BH}/M_\odot < 5.5 \times 10^8$. The inferred Eddington ratios of the BluDOGs are higher than 1 ($1.1< λ_{\rm Edd} < 3.8$), suggesting that the BluDOGs are in a rapidly evolving phase of supermassive black holes.

astro-ph.GA↗

arXiv2022

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). IX. The Most Overdense Region at z~5 Inhabited by a Massive Radio Galaxy

We report on the galaxy density environment around a high-z radio galaxy (HzRG) at z=4.72, HSC J083913.17+011308.1 (HSC J0839+0113), probed using an r-dropout Lyman break galaxy (LBG) sample from the Hyper Suprime-Cam Subaru Strategic Program data. We find that HSC J0839+0113 resides in the outskirt of an overdense region identified by the r-dropout galaxies at a 4.7 sigma significance level. The projected distance between HSC J0839+0113 and the peak position of the overdense region is 0.4 physical Mpc which is shorter than the typical protocluster radius in this epoch. According to the extended Press Schechter and the light cone models, the HSC J0839+0113-hosted overdense region is expected to evolve into a halo > 10^14 Msun at z=0 with a high probability of >80 %. These findings suggest that HSC J0839+0113 is associated with a protocluster. The HSC J0839+0113 rich-system is the most overdense region of LBGs among the known protoclusters with LBGs in the same cosmic epoch.

astro-ph.GA↗

arXiv2022

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). VII. Redshift Evolution of Radio Galaxy Environments at z=0.3-1.4

We examine the redshift evolution of density environments around 2,163 radio galaxies with the stellar masses of $\sim10^{9}-10^{12} M_\odot$ between redshifts of $z=0.3-1.4$, based on the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and Faint Images of the Radio Sky at Twenty-cm (FIRST). We use the $k$-nearest neighbor method to measure the local galaxy number density around our radio galaxy sample. We find that the overdensities of the radio galaxies are weakly but significantly anti-correlated with redshift. This is consistent with the known result that the relative abundance of less-massive radio galaxies increases with redshift, because less-massive radio galaxies reside in relatively low density regions. Massive radio galaxies with stellar mass of $M_* >10^{11}M_\odot$ are found in high density environments compared with the control sample galaxies with radio-non-detection and matched-stellar-mass. Less-massive radio galaxies with $M_* <10^{11}M_\odot$ reside in average density environments. The fraction of the radio galaxies associated with the neighbors within a typical major merger scale, $<70$ kpc, is higher than (comparable to) that of the control galaxies at $M_* >10^{11}M_\odot$ ($M_* <10^{11}M_\odot$). We also find that the local densities around the radio galaxies are anti-correlated with the radio luminosities and black hole mass accretion rates at fixed stellar mass. These findings suggest that massive radio galaxies have matured through galaxy mergers in the past, and have supermassive black holes whose mass accretion almost ceased at $z>1.4$, while less-massive radio galaxies undergo active accretion just at this epoch, as they have avoided such merger events.

astro-ph.GA↗

arXiv2022

Detection of extended millimeter emission in the host galaxy of 3C273 and its implications for QSO feedback via high dynamic range ALMA imaging

We estimate the amount of negative feedback energy injected into the ISM of the host galaxy of 3C273, a prototypical radio loud quasar. We obtained 93, 233 and 343 GHz continuum images with the Atacama Large Millimeter/Sub-millimeter Array (ALMA). After self calibration and point source subtraction, we reach an image dynamic range of $\sim 85000$ at 93\ GHz, $\sim 39000$ at 233\ GHz and $\sim 2500$ at 343\ GHz. These are currently the highest image dynamic range obtained using ALMA. We detect spatially extended millimeter emission associated with the host galaxy, cospatial with the Extended Emission Line Region (EELR) observed in the optical. The millimeter spectral energy distribution and comparison with centimeter data show that the extended emission cannot be explained by dust thermal emission, synchrotron or thermal bremsstrahlung arising from massive star formation. We interpret the extended millimeter emission as thermal bremsstrahlung from gas directly ionized by the central source. The extended flux indicates that at least $\sim 7\%$ of the bolometric flux of the nuclear source was used to ionize atomic hydrogen in the host galaxy. The ionized gas is estimated to be as massive as $10^{10}$ to $10^{11}\ \mathrm{M_\odot}$, but the molecular gas fraction with respect to the stellar mass is consistent with other ellipticals, suggesting that direct ionization ISM by the QSO may not be sufficient to suppress star formation, or we are witnessing a short timescale before negative feedback becomes observable. The discovery of a radio counterpart to EELRs provides a new pathway to studying the QSO-host ISM interaction.

astro-ph.GA↗

arXiv2022

Multi-wavelength and Multi-CO View of The Minor Merger Driven Star Formation in the Nearby LIRG NGC 3110

We present Atacama Large Millimeter/submillimeter Array observations of multiple CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) lines and 2.9 mm and 1.3 mm continuum emission toward the nearby interacting luminous infrared galaxy NGC 3110, supplemented with similar spatial resolution H$α$, 1.4GHz continuum, and $K$-band data. We estimate the typical CO-to-H$_2$ conversion factor of 1.7 $M_{\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ within the disk using LTE-based and dust-based H$_2$ column densities, and measure the 1-kpc scale surface densities of star formation rate ($Σ_{\rm SFR}$), super star clusters ($Σ_{\rm SSC}$), molecular gas mass, and star formation efficiency (SFE) toward the entire gas disk. These parameters show a peak at the southern part of the southern spiral arm (SFE $\sim$ 10$^{-8.2}$ yr$^{-1}$, $Σ_{\rm SFR}$ $\sim$ 10$^{-0.6}$ $M_{\odot}$ kpc$^{-2}$ yr$^{-1}$, $Σ_{\rm SSC}$ $\sim$ 6.0 kpc$^{-2}$), which is likely attributed to the on-going tidal interaction with the companion galaxy MCG-01-26-013, as well as toward the circumnuclear region. We also find that thermal free-free emission contributes to a significant fraction of the millimeter continuum emission at the southern peak position. Those measurements imply that the peak of the southern arm is an active and young star-forming region, whereas the central part of NGC 3110 is a site of long-continued star formation. We suggest that, during the early stage of the galaxy-galaxy interaction with large mass ratio that in NGC 3110, fragmentation along the main galaxy's arms is an important driver of merger-induced star formation and massive gas inflow results in dusty nuclear starbursts.

astro-ph.GA↗

arXiv2022

Multi-line assessment of narrow-line regions in $z \sim$ 3 radio galaxies

In this paper, we utilize high-quality rest-UV spectra of three radio galaxies at $z \sim$ 3 observed with VLT/FORS2 to measure the flux of several emission lines including relatively faint ones, such as NIV]$λ$1486, OIII]$λ$1663, and [NeIV]$λ$2424. Additionally, we collect fluxes of faint rest-UV emission lines in 12 $z \sim$ 3 radio galaxies from the literature. Previously, physical and chemical properties of narrow-line regions (NLRs) in high-$z$ active galactic nuclei (AGNs) have been investigated mostly by using only strong rest-UV emission-lines (e.g., NV$λ$1240, CIV$λ$1549, HeII$λ$1640, and CIII]$λ$1909). Such strong-line diagnostics are based on various assumptions due to the limitation in the number of available emission-line constraints. In this work, both physical and chemical properties of NLR clouds in each object are estimated by fitting detailed photoionization models to the measured emission-line fluxes. We confirm that the metallicity of NLRs in AGNs at $z \sim$ 3 is solar or super-solar, without assuming the gas density and ionization parameter thanks to the constrains from the faint emission lines. This result suggests that high-$z$ radio galaxies are already chemically matured at $z \sim$ 3.

astro-ph.GA↗

arXiv2021

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). VI. Distant Filamentary Structures Pointed by High-z Radio Galaxies at z~4

We present the environmental properties around high-$z$ radio galaxies (HzRGs) at $z\sim4$, which have been poorly investigated because of their rarity. We use the largest samples of HzRGs and $g$-dropout galaxy overdense regions at $z\sim4$, which were constructed from Hyper Suprime-Cam Subaru Strategic Program, to characterize the HzRG environments statistically. We measure the $g$-dropout galaxy overdensities around 21 HzRGs whose rest-frame 1.4 GHz radio luminosities ($L_{1.4\mathrm{GHz}}$) are $10^{26-27}$ W Hz$^{-1}$. We find that the overdensities around the faint HzRGs with $L_{1.4\mathrm{GHz}}\sim10^{26.0-26.5}$ W Hz$^{-1}$ tend to be higher than that of the $g$-dropout galaxies. On the other hand, no significant difference of density environments is found between the luminous HzRGs with $ L_{1.4\mathrm{GHz}}\sim10^{26.5-27.0} $ W Hz$^{-1}$ and the $g$-dropout galaxies. The HzRGs are found to occupy more massive halos than $g$-dropout galaxies through a cross-correlation between the HzRGs and $g$-dropout galaxies. This trend is more pronounced in the faint HzRGs. These results are consistent with a scenario where HzRGs get older and more massive as the radio-luminosity decreases. The HzRGs are expected to trace the progenitors of local cluster halos from their calculated halo mass. In addition, we find that surrounding galaxies tend to distribute along the radio-jet major axis of the HzRGs at angular distances less than $\lesssim500$ physical kpc. Our findings imply the onset of the filamentary structures around the HzRGs at $z\sim4$.

astro-ph.GA↗

arXiv2021

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XVI. 69 New Quasars at 5.8 < z < 7.0

We present the spectroscopic discovery of 69 quasars at 5.8 < z < 7.0, drawn from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) imaging survey data. This is the 16th publication from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, and completes identification of all but the faintest candidates (i.e., i-band dropouts with zAB < 24 and y-band detections, and z-band dropouts with yAB < 24) with Bayesian quasar probability Pq > 0.1 in the HSC-SSP third public data release (PDR3). The sample reported here also includes three quasars with Pq < 0.1 at z ~ 6.6, which we selected in an effort to completely cover the reddest point sources with simple color cuts. The number of high-z quasars discovered in SHELLQs has now grown to 162, including 23 type-II quasar candidates. This paper also presents identification of seven galaxies at 5.6 < z < 6.7, an [O III] emitter at z = 0.954, and 31 Galactic cool stars and brown dwarfs. High-z quasars and galaxies comprise 75 % and 16 % respectively of all the spectroscopic SHELLQs objects that pass our latest selection algorithm with the PDR3 photometry. That is, a total of 91 % of the objects lie at z > 5.6. This demonstrates that the algorithm has very high efficiency, even though we are probing an unprecedentedly low-luminosity population down to M1450 ~ -21 mag.

astro-ph.GA↗

arXiv2021

Cold Molecular Gas in Merger Remnants. II. The properties of dense molecular gas

We present the 3 mm wavelength spectra of 28 local galaxy merger remnants obtained with the Large Millimeter Telescope. Fifteen molecular lines from 13 different molecular species and isotopologues were identified, and 21 out of 28 sources were detected in one or more molecular lines. On average, the line ratios of the dense gas tracers, such as HCN (1-0) and HCO$^{+}$(1-0), to $^{13}$CO (1-0) are 3-4 times higher in ultra/luminous infrared galaxies (U/LIRGs) than in non-LIRGs in our sample. These high line ratios could be explained by the deficiency of $^{13}$CO and high dense gas fractions suggested by high HCN (1-0)/$^{12}$CO (1-0) ratios. We calculate the IR-to-HCN (1-0) luminosity ratio as a proxy of the dense gas star formation efficiency. There is no correlation between the IR/HCN ratio and the IR luminosity, while the IR/HCN ratio varies from source to source (1.1-6.5) $\times 10^{3}$ $L_{\odot}$/(K km s$^{-1}$ pc$^{2}$). Compared with the control sample, we find that the average IR/HCN ratio of the merger remnants is higher by a factor of 2-3 than those of the early/mid-stage mergers and non-merging LIRGs, and it is comparable to that of the late-stage mergers. The IR-to-$^{12}$CO (1-0) ratios show a similar trend to the IR/HCN ratios. These results suggest that star formation efficiency is enhanced by the merging process and maintained at high levels even after the final coalescence. The dynamical interactions and mergers could change the star formation mode and continue to impact the star formation properties of the gas in the post-merger phase.

astro-ph.GA↗