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The most massive early-type galaxies (ETGs) are known to form through numerous galaxy mergers. Thus, it is intriguing to study whether their formation in low-density environments, where nearby companions are almost absent, is associated with mergers, which are directly traced by tidal features. Using the 436 most massive ETGs with $M_\mathrm{star}>10^{11.2}\,M_{\odot}$ at $z<0.04$, we determine the variation in the fraction of massive ETGs with tidal features ($f_T$) across different environments and verify whether the most massive ETGs commonly have tidal features in very low density environments. Our main discovery is that the most massive ETGs exhibit tidal features more frequently in lower-density environments. In the highest-density environments, like galaxy clusters, $f_T$ is $0.21\pm0.06$, while in the lowest-density environments it triples to $0.62\pm0.06$. This trend is stronger for more extremely massive ETGs, with $f_T$ reaching $0.92\pm0.08$ in the lowest-density environments. One explanation for our finding is that the most massive ETGs in lower-density environments have genuinely experienced recent mergers more frequently than their counterparts in higher-density environments, suggesting that they possess extended formation histories that continue into the present. Another possibility is that tidal features last shorter in denser environments owing to external factors inherent in these environments. Our additional findings that massive ETGs with bluer $u-r$ colors are a more dominant driver of our main discovery and that dust lanes are more commonly observed in massive ETGs in low-density environments imply that gas-abundant mergers primarily contribute to the increased rate of recent mergers in low-density environments., Comment: 16 pages, 10 figures, published on October 18 in ApJ

We study the mass-size relation of quiescent galaxies across various environments, with a particular focus on its environmental dependence at the low-mass part of $\log(M_\mathrm{star}/M_{\odot})\lesssim10.0$. Our sample consists of 13,667 quiescent galaxies with $\log(M_\mathrm{star}/M_{\odot})\ge9.4$ and $0.01

We investigate the fraction of quenched satellite galaxies in host galaxy groups and clusters using TNG300 in the IllustrisTNG cosmological magnetohydrodynamical simulations. Simulations show that most satellites are quenched after they fall into their final hosts: post-processing is a more dominant mechanism of galaxy quenching than pre-processing. We find the fraction of quenched satellites at $z=0$ increases with host mass, which implies that more massive hosts have higher quenching efficiency because more massive hosts have more massive groups infalling. Furthermore, we find that hosts that have many early-infall satellites show a higher fraction of quenched satellites at $z=0$ than those having many late-infall satellites, which results in a scatter of the quenched fraction of satellites in a given mass range of hosts at $z=0$. Our results highlight the significance of the mass of hosts and the different infall times of satellites in understanding galaxy quenching., Comment: 14 pages, 12 figures. Accepted for publication in The Astrophysical Journal, July 11th, 2023

We measure the specific star formation rates of \textit{K}-band selected galaxies from the ELAIS-N1 by stacking GMRT data at 610 MHz. We identify a sample of SFGs, spanning $\rm{0.1\leq\,\textit{z}\,\leq\,1.5}$ and $\rm{10^{8.5}<\,{\textit{M}_{\star}}/{\textit{M}_{\odot}}<10^{12.4}}$, using a combination of multi-wavelength diagnostics obtained from the deep LoTSS multi-wavelength catalogue. We measure the flux densities in the radio map and estimate the radio SFR in order to probe the nature of the galaxies below the noise and confusion limits. The massive galaxies in our sample have the lowest sSFRs which is in agreement with previous studies. For the different populations, we show that the sSFR-mass relation steepens with redshift, with an average slope of $\rm{\langle \beta_{All} \rangle\,=\, -0.49\pm0.01}$ for the whole sample, and $\rm{\langle \beta_{SFG} \rangle\,=\, -0.42\pm0.02}$ for the SFGs. Our results indicate that galaxy populations undergo 'downsizing', whereby most massive galaxies form their stars earlier and more rapidly than low mass galaxies. Both populations show a strong decrease in their sSFR toward the present epoch. The sSFR evolution with redshift is best described by a power law $\rm{(1\,+\,\textit{z})^\textit{n}}$, where $\rm{\langle \textit{n}_{ALL}\rangle\sim4.94\pm0.53}$ for all galaxies, and $\rm{\langle \textit{n}_{SFG}\rangle \sim3.51\pm0.52}$ for SFGs. Comparing our measured sSFRs to results from literature, we find a general agreement in the \textit{sSFR-M$_{\star}$} plane., Comment: 17 pages, 13 figures, accepted for publication in MNRAS

We study the impact of galaxy mergers on stellar population profiles/gradients of early-type galaxies (ETGs) using ETGs at $z<0.055$ in the Stripe 82 region of the Sloan Digital Sky Survey and MaNGA integral field unit spectroscopic data. Tidal features around ETGs, which are detected from deep coadded images, are regarded as direct observational evidence for recent mergers. We find that ETGs with tidal features have less negative metallicity gradients and more positive age gradients than ETGs without tidal features at $M_\mathrm{star}\gtrsim10^{10.6}M_\odot$. Moreover, when integrating all the resolved stellar populations, ETGs with tidal features have lower metallicities by $\sim0.07$ dex and younger ages by $\sim1$ - $2$ Gyr than ETGs without tidal features. Analyzing star formation histories, we discover that the mass fraction of young stellar populations with age $<5$ Gyr is higher in the central regions of ETGs with tidal features than in the same regions of the counterparts without tidal features. Compared to normal ETGs, ETGs with tidal features have a slow metal-enrichment history in the early universe, but they have been accelerating the metal enrichment through recently formed stars over the last few billion years. Many of our results can be explained if the effects of recently occurred mergers are different from those in the early universe, which are more likely to be rich in gas., Comment: 22 pages, 14 figures, 1 table, accepted for publication in the ApJ

In a $\Lambda$CDM universe, most galaxies evolve by mergers and accretions, leaving faint and/or diffuse structures, such as tidal streams and stellar halos. Although these structures are a good indicator of galaxies' recent mass assembly history, they have the disadvantage of being difficult to observe due to their low surface brightness (LSB). To recover these LSB features by minimizing the photometric uncertainties introduced by the optical system, we developed a new optimized telescope named K-DRIFT pathfinder, adopting a linear astigmatism free-three mirror system. Thanks to the off-axis design, it is expected to avoid the loss and scattering of light on the optical path within the telescope. To assess the performance of this prototype telescope, we investigate the photometric depth and capability to identify LSB features. We find that the surface brightness limit reaches down to $\mu_{r,1\sigma}\sim28.5$ mag arcsec$^{-2}$ in $10^{\prime\prime}\times10^{\prime\prime}$ boxes, enabling us to identify a single stellar stream to the east of NGC 5907. We also examine the characteristics of the point spread function (PSF) and find that the PSF wing reaches a very low level. Still, however, some internal reflections appear within a radius of $\sim$6 arcmin from the center of sources. Despite a relatively small aperture (0.3 m) and short integration time (2 hr), this result demonstrates that our telescope is highly efficient in LSB detection., Comment: 15 pages, 8 figures, accepted for publication in PASP

We present the result of the Infrared Medium-deep Survey (IMS) $z\sim6$ quasar survey, using the combination of the IMS near-infrared images and the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) optical images. The traditional color-selection method results in 25 quasar candidates over $86$ deg$^{2}$. We introduce the corrected Akaike Information Criterion (AICc) with the high-redshift quasar and late-type star models to prioritize the candidates efficiently. Among the color-selected candidates, seven plausible candidates finally passed the AICc selection of which three are known quasars at $z\sim6$. The follow-up spectroscopic observations for the remaining four candidates were carried out, and we confirmed that two out of four are $z\sim6$ quasars. With this complete sample, we revisited the quasar space density at $z\sim6$ down to $M_{1450}\sim-23.5$ mag. Our result supports the low quasar space density at the luminosity where the quasar's ultraviolet ionizing emissivity peaks, favoring a minor contribution of quasars to the cosmic reionization., Comment: 21 pages, 13 figures, 4 tables. Accepted for publication in AJ

To understand how well galaxies, gas and intracluster stars trace dark matter in and around galaxy clusters, we use the IllustrisTNG cosmological hydrodynamical simulation and compare the spatial distribution of dark matter with those of baryonic components in clusters. To quantify the global morphology of the density distribution of each component in clusters, we fit an ellipse to the density contour of each component and derive shape parameters at different radii. We find that ellipticity of dark matter is better correlated with that of galaxy mass-weighted number density, rather than with that of galaxy number density or galaxy velocity dispersion. We thus use the galaxy mass-weighted number density map as a representative of the galaxy maps. Among three different density maps from galaxies, gas, and intracluster stars, the ellipticity of dark matter is best reproduced by that of the galaxy map over the entire radii. The 'virialized' galaxy clusters show a better correlation of spatial distribution between dark matter and other components than the 'unvirialized' clusters, suggesting that it requires some time for each component to follow the spatial distribution of dark matter after merging events. Our results demonstrate that galaxies are still good tracers of dark matter distribution even in the non-linear regime corresponding to the scales in and around galaxy clusters, being consistent with the case where galaxies trace well the matter distribution in cosmologically large scales., Comment: accepted for publication in ApJ

We present the circumnuclear multi-phase gas properties of the brightest cluster galaxy (BCG) in the center of Abell 1644-South. A1644-S is the main cluster in a merging system, which is well known for X-ray hot gas sloshing in its core. The sharply peaked X-ray profile of A1644-S implies the presence of a strongly cooling gas core. In this study, we analyze ALMA CO (1-0) data, JVLA HI data, and KaVA 22 GHz data for the central region of A1644-S to probe the potential origin of the cool gas and its role in (re)powering the central active galactic nucleus (AGN). We find CO clumps distributed in an arc shape along the X-ray gas sloshing, which is suggestive of a connection between the cold gas and the hot intracluster medium (ICM). HI and CN are detected in absorption against the AGN continuum emission. The absorption dip is observed at the systemic velocity of the BCG with an extended, redshifted tail. Based on the spatial and spectral configurations of the HI, CN and CO gases, it is inferred that cool gas spirals into the core of the BCG, which is then fed to the central AGN. Indeed, our KaVA observation reveals a parsec-scale bipolar jet, implying that this AGN could have been (re)powered quite recently. Combining this, we suggest that some cold gas in A1644-S could have been formed from the cooling of the ICM, triggering the activity of the central AGN in the early development of a cool-core cluster., Comment: 13 pages, 6 figures, 1 table, Accepted for publication in ApJ, Comments are welcome

Abell 2261 (A2661) is a well-studied fossil cluster, but previous studies give contradictory results on its dynamical states, such as its X-ray central entropy and magnitude gap. To improve our understanding of its dynamical state, we conduct multi-object spectroscopic observations with Hectospec on the MMT, covering an area out to 5 virial radii from the cluster center, and get improved completeness and membership. Using this new data, we calculate multiple dynamical indicators including gaussianity, distance offset, and velocity offset. These indicators suggest that A2261 is moderately relaxed. However, Dressler-Shectman test reveals a group candidate to the south, at a projected distance that is near the virial radius, and that overlaps with an X-ray tail-like feature. One of the galaxies associated with that group would be sufficiently bright to reduce the fossil magnitude gap. This raises the possibility that A2261 could have recently transit in fossil status, if the group had previously crossed the cluster and is only now found outside. In the cluster outskirts, we see an extended feature of galaxies located on the opposite side of the cluster to the group candidate. On even larger scales, we find this feature connects, both on the sky and in velocity space, with a long (~4.4Mpc) filamentary structure in the Sloan Digital Sky Survey data. This could support the idea that a group was fed into the cluster through the filament, temporarily breaking the fossil status, and resulting in a minor merger that weakly disturbed the intracluster medium of the cluster., Comment: 18 pages, 9 figures, Accepted in ApJ

Galaxy clusters contain a diffuse stellar component outside the cluster's galaxies, which is observed as faint intracluster light (ICL). Using Gemini/GMOS-N deep imaging and multi-object spectroscopy of a massive fossil cluster at a redshift of $z=0.47$, RX J105453.3+552102 (J1054), we improve the observational constraints on the formation mechanism of the ICL. We extract the ICL surface brightness and colour profiles out to 155 kpc from the brightest cluster galaxy (BCG) with a detection limit of 28.7 mag/arcsec$^2$ (1$\sigma$, 4.8" x 4.8"; i-band). The colour of the diffuse light is similar to that of the BCG and central bright galaxies out to $\sim$ 70 kpc, becoming slightly bluer toward the outside. We find that the ICL distribution shows better agreement with the spatial distribution of member galaxies than with the BCG-dominated cluster luminosity distribution. We report the ICL fraction of J1054 as $15.07 \pm 4.57 \%$ in the range of $60 \sim 155$ kpc from the BCG, which appears to be higher than the ICL fraction-redshift trend in previous studies. Our findings suggest that intracluster stars seems not to be explained by one dominant production mechanism. However, a significant fraction of the ICL of J1054 may have been generated from the outskirts of infalling/satellite galaxies more recently rather than by the BCG at the early stage of the cluster., Comment: 16 pages, 15 figures, accepted for publication in MNRAS

Variability of a galaxy's core radio source can be a significant consequence of AGN accretion. However, this variability has not been well studied, particularly at high radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. From our representative sample, we show that most vary significantly over timescales of approximately one year and longer. Our highest cadence observations are at 15 GHz and are from the Owens Valley Radio Observatory (OVRO). They have a median time interval of 7 days and mostly span between 8 and 13 years. We apply a range of variability detection techniques to the sources' lightcurves to analyse changes on week to decade long timescales. Most notably, at least half of the sources show 20 per cent peak to trough variability on 3 year timescales, while at least a third vary by 60 per cent on 6 year timescales. Significant variability, which is important to studies of the Sunyaev-Zel'dovich Effect in the radio/sub-mm, is therefore a common feature of these sources. We also show how the variability relates to spectral properties at frequencies of up to 353 GHz using data from the Korean VLBI network (KVN), the NIKA2 instrument of the IRAM 30m telescope, and the SCUBA-2 instrument of the James Clerk Maxwell Telescope., Comment: Accepted for publication in MNRAS

We study the stellar and gas kinematics of the brightest group galaxies (BGGs) in dynamically relaxed and unrelaxed galaxy groups for a sample of 154 galaxies in the SAMI galaxy survey. We characterize the dynamical state of the groups using the luminosity gap between the two most luminous galaxies and the BGG offset from the luminosity centroid of the group. We find that the misalignment between the rotation axis of gas and stellar components is more frequent in the BGGs in unrelaxed groups, although with quite low statistical significance. Meanwhile galaxies whose stellar dynamics would be classified as `regular rotators' based on their kinemetry are more common in relaxed groups. We confirm that this dependency on group dynamical state remains valid at fixed stellar mass and Sersic index. The observed trend could potentially originate from a differing BGG accretion history in virialised and evolving groups. Amongst the halo relaxation probes, the group BGG offset appears to play a stronger role than the luminosity gap on the stellar kinematic differences of the BGGs. However, both the group BGG offset and luminosity gap appear to roughly equally drive the misalignment between the gas and stellar component of the BGGs in one direction. This study offers the first evidence that the dynamical state of galaxy groups may influence the BGG's stellar and gas kinematics and calls for further studies using a larger sample with higher signal-to-noise., Comment: 16 pages, 12 figures. Accepted for publication in ApJ

Faint $z\sim5$ quasars with $M_{1450}\sim-23$ mag are known to be the potentially important contributors to the ultraviolet ionizing background in the post-reionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our $z\sim5$ quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg$^{2}$. From our spectroscopic observations with the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South 8 m Telescope, we discovered eight new quasars at $z\sim5$ with $-26.1\leq M_{1450} \leq -23.3$. Combining our IMS faint quasars ($M_{1450}>-27$ mag) with the brighter Sloan Digital Sky Survey (SDSS) quasars ($M_{1450}<-27$ mag), we derive the $z\sim5$ quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of $M_{1450}=-23$ mag. We find that the faint-end slope of the QLF is very flat ($\alpha=-1.2^{+1.4}_{-0.6}$), with a characteristic luminosity of $M^{*}_{1450}=-25.8^{+1.4}_{-1.1}$ mag. The number density of $z\sim5$ quasars from the QLF gives an ionizing emissivity at 912 $\unicode{x212B}$ of $\epsilon_{912}=(3.7$--$7.1)\times10^{23}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$ and an ionizing photon density of $\dot{n}_{\rm ion}=(3.0$--$5.7)\times10^{49}$ Mpc$^{-3}$ s$^{-1}$. These results imply that quasars are responsible for only 10-20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at $z\sim5$, disfavoring the idea that quasars alone could have ionized the IGM at $z\sim5$., Comment: 21 pages, 14 figures, 7 tables. Accepted for publication in ApJ

A521 has been a subject of extensive panchromatic studies from X-ray to radio. The cluster possesses a number of remarkable features including a bright radio relic with a steep spectrum, more than three distinct galaxy groups forming a filament, and two disturbed X-ray peaks at odds with the distant position and tilted orientation of the radio relic. These several lines of evidence indicate a complex merger. In this paper, we present a multi-wavelength study of A521 based on Subaru optical, Hubble Space Telescope infrared, Chandra X-ray, GMRT radio, and MMT optical spectroscopic observations. Our weak-lensing (WL) analysis with improved systematics control reveals that A521 is mainly composed of three substructures aligned in the northwest to southeast orientation. These WL mass substructures are remarkably well-aligned with the cluster optical luminosity distribution constructed from our new enhanced cluster member catalog. These individual substructure masses are determined by simultaneously fitting three NFW profiles. We find that the total mass of A521 modeled by the superposition of the three halos is $13.0_{-1.3}^{+1.0} \times 10^{14}M_{\odot}$, a factor of two higher than the previous WL measurement. With these WL mass constraints combined with X-ray and radio features, we consider two merging scenarios, carry out the corresponding numerical simulations, and discuss strengths and weaknesses of each case., Comment: 15 pages, 11 figures, Accepted to ApJ

The intergalactic medium (IGM) at $z\sim$ 5 to 6 is largely ionized, and yet the main source for the IGM ionization in the early universe is uncertain. Of the possible contributors are faint quasars with $-26 \lesssim M_{\rm 1450} \lesssim -23$, but their number density is poorly constrained at $z\sim5$. In this paper, we present our survey of faint quasars at $z\sim5$ in the European Large-Area {\it ISO} Survey-North 1 (ELAIS-N1) field over a survey area of 6.51 deg$^2$ and examine if such quasars can be the dominant source of the IGM ionization. We use the deep optical/near-infrared data of the ELAIS-N1 field as well as the additional medium-band observations to find $z \sim 5$ quasars through a two-step approach using the broadband color selection, and SED fitting with the medium-band information included. Adopting Bayesian information criterion, we identify ten promising quasar candidates. Spectra of three of the candidates are obtained, confirming all of them to be quasars at $z\sim5$ and supporting the reliability of the quasar selection. Using the promising candidates, we derive the $z\sim5$ quasar luminosity function at $-26 \lesssim M_{\rm 1450} \lesssim -23$. The number density of faint $z\sim5$ quasars in the ELAIS-N1 field is consistent with several previous results that quasars are not the main contributors to the IGM-ionizing photons at $z\sim5$, Comment: 16 pages, 9 figures, 4 tables. Accepted for publication in ApJ

High-redshift galaxy clusters, unlike local counterparts, show diverse star formation activities. However, it is still unclear what keeps some of the high-redshift clusters active in star formation. To address this issue, we performed a multi-object spectroscopic (MOS) observation of 226 high-redshift (0.8 < z < 1.3) galaxies in galaxy cluster candidates and the areas surrounding them. Our spectroscopic observation reveals six to eight clusters/groups at z ~ 0.9 and z ~ 1.3. The redshift measurements demonstrate the reliability of our photometric redshift measurements, which in turn gives credibility for using photometric redshift members for the analysis of large-scale structures (LSSs). Our investigation of the large-scale environment (~10 Mpc) surrounding each galaxy cluster reveals LSSs --- structures up to ~10 Mpc scale --- around many of, but not all, the confirmed overdensities and the cluster candidates. We investigate the correlation between quiescent galaxy fraction of galaxy overdensities and their surrounding LSSs, with a larger sample of ~ 20 overdensities including photometrically selected overdensities at 0.6 < z < 0.9. Interestingly, galaxy overdensities embedded within these extended LSSs show a lower fraction of quiescent galaxies (~ 20 %) than isolated ones at similar redshifts (with a quiescent galaxy fraction of ~ 50 %). Furthermore, we find a possible indication that clusters/groups with a high quiescent galaxy fraction are more centrally concentrated. Based on these results, we suggest that LSSs are the main reservoirs of gas and star-forming galaxies to keep galaxy clusters fresh and extended in size at z ~ 1., Comment: 56 pages, 23 figures, 4 tables, published in MNRAS

We present medium-resolution spectra ($R \sim 2000$--4000) at 0.4--1.0\,$\mu$m and 0.7--2.5\,$\mu$m of 16 active galactic nuclei (AGNs) selected with red color in the near-infrared (NIR) of $J - K > 2.0$ mag at $z \sim 0.3$. We fit the H$\beta$, H$\alpha$, P$\beta$, and P$\alpha$ lines from these spectra to obtain their luminosities and line widths. We derive the $E(B-V)$ color excess values of the NIR-red AGNs using two methods, one based on the line-luminosity ratios and another based on the continuum slopes. The two $E(B-V)$ values agree with each other at rms dispersion $\sim$ 0.249. About half of the NIR-red AGNs have $g'-K < 5$ magnitude, and we find that these NIR red, but blue in optical-NIR AGNs, have $E(B-V) \sim 0$, suggesting that a significant fraction of the NIR color-selected red AGNs are unobscured or only mildly obscured. After correcting for the dust extinction, we estimate the black hole (BH) masses and the bolometric luminosities of the NIR-red AGNs using the Paschen lines to calculate their Eddington ratios ($\lambda_{\rm Edd}$). The median Eddington ratios of nine NIR-red AGNs ($\log (\lambda_{\rm Edd}) \simeq -0.654 \pm 0.176$) are only mildly higher than those of unobscured type 1 AGNs ($\log (\lambda_{\rm Edd}) \simeq -0.961 \pm 0.008$). Moreover, we find that the $M_{\rm BH}$--$\sigma_{\ast}$ relation for three NIR-red AGNs is consistent with that of unobscured type 1 AGNs at similar redshift. These results suggest that the NIR red color selection alone is not effective at picking up dusty, intermediate-stage AGNs., Comment: 26 pages, 16 figures, 8 tables, Published in ApJS

The faint quasars with $M_{1450}>-24$ mag are known to hold the key to the determination of the ultraviolet emissivity for the cosmic re-ionization. But only a few have been identified so far because of the limitations on the survey data. Here, we present the first results of the $z\sim5$ faint quasar survey with the Infrared Medium-deep Survey (IMS), which covers $\sim100$ deg$^{2}$ areas in $J$-band to the depths of $J_{\rm AB}\sim23$ mag. To improve selection methods, the medium-band follow-up imaging has been carried out using the SED camera for QUasars in Early uNiverse (SQUEAN) on the Otto Struve 2.1 m Telescope. The optical spectra of the candidates were obtained with 8-m class telescopes. We newly discovered 10 quasars with $-25

To date, most of the luminous quasars known at $z\sim6$ have been found to be in maximal accretion with the Eddington ratios, $\lambda_{\rm{Edd}}\sim1$, suggesting enhanced nuclear activities in the early universe. However, this may not be the whole picture of supermassive black hole (SMBH) growth since previous studies have not reached on faint quasars that are more likely to harbor SMBHs with low $\lambda_{\rm{Edd}}$. To gain a better understanding on the accretion activities in quasars in the early universe, we obtained a deep near-infrared (NIR) spectrum of a quasar, IMS J220417.92+011144.8 (hereafter IMS J2204+0112), one of the faintest quasars that have been identified at $z\sim6$. From the redshifted C IV $\lambda 1549$ emission line in the NIR spectrum, we find that IMS J2204+0112 harbors a SMBH with about a billion solar mass and $\lambda_{\rm{Edd}} \sim 0.1$, but with a large uncertainty in both quantities (0.41 dex). IMS J2204+0112 has one of the lowest Eddington ratios among quasars at $z\sim6$, but a common value among quasars at $z\sim2$. Its low $\lambda_{\rm{Edd}}$ can be explained with two scenarios; the SMBH growth from a stellar mass black hole through short-duration super-Eddington accretion events or from a massive black hole seed ($\sim10^{5}\,M_{\odot}$) with Eddington-limited accretion. NIR spectra of more faint quasars are needed to better understand the accretion activities of SMBHs at $z \sim 6$., Comment: 10 pages, 4 figures, 3 tables. Accepted for publication in ApJ

We present our first results of the survey for high redshift quasars at $5 \lesssim {\rm z} \lesssim 5.7$. The search for quasars in this redshift range has been known to be challenging due to limitations of filter sets used in previous studies. We conducted a quasar survey for two specific redshift ranges, 4.60 $\leq$ z $\leq$ 5.40 and 5.50 $\leq$ z $\leq$ 6.05, using multi-wavelength data that include observations using custom-designed filters, $is$ and $iz$. Using these filters and a new selection technique, we were able to reduce the fraction of interlopers. Through optical spectroscopy, we confirmed seven quasars at 4.7 $\leq$ z $\leq$ 5.4 with $-27.4 < M_{1450} < -26.4$ which were discovered independently by another group recently. We estimated black hole masses and Eddington ratios of four of these quasars from optical and near-infrared spectra, and found that these quasars are undergoing nearly Eddington-limited accretion which is consistent with the rapid growth of supermassive black holes in luminous quasars at z $\sim$ 5., Comment: 16 pages, 12 figures, and 9 tables. Accepted for publication in ApJS

We present a study of the connection between brightest cluster galaxies (BCGs) and their host galaxy clusters. Using galaxy clusters at $0.1

Under the $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological models, massive galaxies are expected to be larger in denser environments through frequent hierarchical mergers with other galaxies. Yet, observational studies of low-redshift early-type galaxies have shown no such trend, standing as a puzzle to solve during the past decade. We analyzed 73,116 early-type galaxies at $0.1\leq z < 0.15$, adopting a robust nonparametric size measurement technique and extending the analysis to many massive galaxies. We find for the first time that local early-type galaxies heavier than $10^{11.2}M_{\odot}$ show a clear environmental dependence in mass-size relation, in such a way that galaxies are as much as 20-40% larger in densest environments than in underdense environments. Splitting the sample into the brightest cluster galaxies (BCGs) and non-BCGs does not affect the result. This result agrees with the $\Lambda$CDM cosmological simulations and suggests that mergers played a significant role in the growth of massive galaxies in dense environments as expected in theory., Comment: 27 pages, 29 figures, 4 tables, accepted for publication in the ApJ

Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars ($M_{1450}>-24$ mag) at $z \gtrsim6$, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z~6 in a 12.5 deg$^{2}$ region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at $\sim8443~\rm{\AA}$, with emission lines redshifted to $z=5.944 \pm 0.002$ and rest-frame ultraviolet continuum magnitude $M_{1450}=-23.59\pm0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z~6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars . This suggests that the number of $M_{1450}\sim-23$ mag quasars at z~6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS., Comment: 5 pages, 4 figures, 1 table. Published in ApJ Letters

The main progenitor candidate of Type Ia supernovae (SNe Ia) is white dwarfs in binary systems where the companion star is another white dwarf (double degenerate system) or a less evolved non-degenerate star with R* >~ 0.1 Rsun (single degenerate system), but no direct observational evidence exists that tells which progenitor system is more common. Recent studies suggest that the light curve of a supernova shortly after its explosion can be used to set a limit on the progenitor size, R*. Here, we report a high cadence monitoring observation of SN 2015F, a normal SN Ia, in the galaxy NGC 2442 starting about 84 days before the first light time. With our daily cadence data, we catch the emergence of the radioactively powered light curve, but more importantly detect with a > 97.4% confidence a possible dim precursor emission that appears at roughly 1.5 days before the rise of the radioactively powered emission. The signal is consistent with theoretical expectations for a progenitor system involving a companion star with R* = ~0.1 -- 1 Rsun or a prompt explosion of a double degenerate system, but inconsistent with a typically invoked size of white dwarf progenitor of R* ~ 0.01 Rsun. Upper limits on the precursor emission also constrain the progenitor size to be R* < 0.1 Rsun, and a companion star size of R* < ~1.0 Rsun, excluding a very large companion star in the progenitor system. Additionally, we find that the distance to SN 2015F is 23.9 +-0.4 Mpc., Comment: 26 pages, 5 figures, 1 table, accepted for publication in ApJS

Using a stellar mass limited sample of $\sim 46,600$ galaxies ($M_* > 10^{9.1}\,M_{\odot}$) at $0.5 < z < 2$, we show that the tellar mass, rather than the environment, is the main parameter controlling quenching of star formation in galaxies with $M_* > 10^{10}\,M_{\odot}$ out to $z=2$. On the other hand, the environmental quenching becomes efficient at $z < 1$ regardless of galaxy mass, and it serves as a main star formation quenching mechanism for lower mass galaxies. Our result is based on deep optical and near-infrared imaging data over 2800 arcmin$^2$, enabling us to negate cosmic variance and identify 46 galaxy cluster candidates with $M \sim 10^{14}\,M_{\odot}$. From $M_* \sim 10^{9.5}$ to $10^{10.5}\,M_{\odot}$, the fraction of quiescent galaxies increases by a factor of $\sim 10$ over the entire redshift range, but the difference between cluster and field environment is negligible. Rapid evolution in the quiescent fraction is seen from $z=2$ to $z=1.3$ for massive galaxies suggesting a build-up of massive quiescent galaxies at $z > 1.3$. For galaxies with $M_* < 10^{10}\,M_{\odot}$ at $z < 1.0$, the quiescent fraction is found to be as much as a factor of 2 larger in clusters than in field, showing the importance of environmental quenching in low mass galaxies at low redshift. Most high mass galaxies are already quenched at $z > 1$, therefore environmental quenching does not play a significant role for them, although the environmental quenching efficiency is nearly identical between high and low mass galaxies., Comment: 18 pages, 22 figures, 1 table, Accepted for publication in ApJ