arXiv subjects

Yongmin Yoon

Publications and source records attributed to Yongmin Yoon.

At least 19 recordsLinked to original sources

arXiv2026

Constructing a Mock Galaxy Catalog for the All-sky SPECtroscopic Survey of Nearby Galaxies (A-SPEC) Using the Machine-assisted Semi-Simulation Model

We present a methodology for constructing a mock galaxy catalog for the All-sky SPECtroscopic survey of nearby galaxies (A-SPEC) using the Machine-assisted Semi-Simulation Model. The model is trained on the cosmological magnetohydrodynamical simulation IllustrisTNG to predict baryonic properties of subhalos from dark-matter-only features and is applied to our own N-body simulation tailored to satisfy the requirements of A-SPEC. We have improved the model's accuracy by introducing additional features such as subhalo anisotropy parameters and modified definitions of the subhalo environment, which result in the coefficient of determination R^2=0.96, 0.90, 0.70, 0.79 for stellar mass, gas mass, star formation rate, and gas metallicity, respectively. The resulting mock galaxies reproduce the luminosity-dependent clustering of the target galaxies when tuned to match the number density. We discuss avenues for further improvement, including the role of environment in the predictions. We release the mock galaxy catalog with the baryonic properties predicted from the model.

astro-ph.CO↗

arXiv2026

Only obscured yet luminous active galactic nuclei are closely associated with galaxy mergers: Direct observational evidence from type 2 active galactic nuclei

To establish a more comprehensive understanding of the connection between galaxy mergers and active galactic nuclei (AGNs), it is essential to disentangle the contributions of intrinsic AGN luminosity and dust extinction to the merger-AGN connection. Since tidal features identified in deep images serve as direct evidence of recent mergers, we studied the fraction of AGN hosts with tidal features ($f_T$) for a large sample of 748 type 2 AGNs at $z<0.063$. Specifically, we examined $f_T$ as a function of $E(B-V)$, derived from the Balmer decrement, and the internal-extinction-corrected luminosity of the [O III] $\lambda$5007 emission line ($L_{\text{[O III]}}$), which is a proxy for bolometric AGN luminosity. Our main finding is that $f_T$ is only significantly higher for AGNs that are simultaneously luminous and heavily dust-obscured. Specifically, AGNs with $\log L_{\text{[O III]}}\gtrsim41.5$ and $E(B-V)\gtrsim0.7$ exhibit a high $f_T$ of $\sim0.7$. In contrast, AGNs with either low luminosity ($\log L_{\text{[O III]}}\lesssim41.0$) or low dust obscuration ($E(B-V)\lesssim0.3$) show a low $f_T$ of $\lesssim0.2$. This trend suggests that galaxy mergers preferentially trigger AGNs that are simultaneously luminous and dust-obscured, whereas AGNs that are either luminous but unobscured or dust-obscured but less luminous are not strongly associated with merger-driven triggering. Based on several assumptions, our result can also be interpreted, despite certain caveats, within the framework of a merger-initiated evolution model for AGNs, suggesting that AGNs that are both obscured and luminous are temporally closer to merger events than those with lower luminosities and less dust obscuration.

astro-ph.GA↗

arXiv2026

Black Hole Properties of Type-1 Active Galactic Nuclei in the North Ecliptic Pole Wide Field: I. Mid-infrared Sources with Optical Counterparts

We present measurements of black hole (BH) properties of 861 Type-1 active galactic nuclei (AGNs) in the North Ecliptic Pole (NEP)-Wide field. These AGNs are detected in both optical and mid-infrared (MIR) surveys and are identified as Type-1 AGNs in optical spectroscopic surveys. By performing spectral energy distribution (SED) and line fitting, we obtained their MIR continuum luminosities ($L_{\rm MIR}$) as well as full width at half maximum (FWHM) values for the \ion{C}{4}, \ion{Mg}{2}, H$β$, and H$α$ lines. Using these measurements, we derived bolometric luminosities ($10^{43.20}$--$10^{47.27}~{\rm erg~s^{-1}}$), BH masses ($10^{7.29}$--$10^{9.67}$\,$M_{\odot}$), and Eddington ratios ($10^{-2.74}$--$10^{-0.08}$) for $\sim$450 objects over a wide redshift range ($z=0.09$--$4.71$). The use of $L_{\rm MIR}$ and FWHM values effectively alleviates the effects of dust extinction, enabling reliable estimates of BH properties even for dust-obscured AGNs. Moreover, we find that 34\,\% of the Type-1 AGNs in the NEP-Wide field are dust-obscured, and that their bolometric luminosities can be significantly underestimated without proper dust extinction correction. Our relatively extinction-free BH property estimates can (i) be combined with multi-wavelength data in the NEP-Wide field to facilitate diverse studies of AGN environments, number densities, host galaxies, and related topics, and (ii) serve as fiducial estimates for SPHEREx and other upcoming infrared (IR) spectroscopic missions covering the NEP-Wide field.

astro-ph.GA↗

arXiv2026

K-DRIFT Science Theme: New Theoretical Framework Using the Galaxy Replacement Technique for LSB studies

Low-surface-brightness (LSB) structures provide critical insights into the hierarchical formation of galaxies and galaxy clusters. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is designed to detect such diffuse features through deep, wide-field optical imaging with a surface brightness reaching $\sim$$30~\rm{mag}~\rm{arcsec}^{-2}$. To interpret the observation data expected from K-DRIFT, we have developed the Galaxy Replacement Technique (GRT), an $N$-body simulation framework optimized for tracing the gravitational evolution of stellar components. The GRT works by inserting high-resolution galaxy models, including a dark matter (DM) halo and stellar disk, in place of multiple low-resolution DM halos in the base $N$-body cosmological simulation. It allows us to achieve very high mass ($m_{star}=5.4\times10^4\msun\ h^{-1}$) and spatial resolution (10~$\rm{pc}~h^{-1}$) with shorter computation time compared to full hydrodynamic cosmological simulations. Therefore, this technique is particularly well-suited for studying LSB structures, with a surface brightness reaching $\sim$$31~\rm{mag}~\rm{arcsec}^{-2}$. In this paper, we present the motivation and methodology of the GRT, summarize key results from previous studies, and highlight its synergy with K-DRIFT observations. We further discuss planned science cases using the GRT, aiming to build a theoretical basis for interpreting LSB features in various environments.

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arXiv2026

K-DRIFT Science Theme: Galaxies in the Faint Universe

Low-surface-brightness (LSB) structures serve as evidence of the intricate mass assembly of galaxies, and dedicatedly studying them promises to give us profound insights into the evolutionary history of galaxies. Furthermore, delving into the properties of star formation (SF) in the LSB regime can broaden our understanding of SF activity in regions characterized by low surface gas density, thereby shedding light on fundamental cosmic processes. However, systematic uncertainties may hamper the exploration of the LSB universe by limiting detectable SB levels. Indeed, despite dedicated advancements in telescope and observing techniques over decades, achieving ultra-deep photometric depths in optical wavelengths remains a formidable challenge. To overcome this challenge and explore the LSB universe that we have yet to see, we have been developing a novel telescope called K-DRIFT. This paper outlines the telescope's specification and describes various LSB features we aim for, explicitly focusing on nearby individual galaxies. To further advance the capabilities of the K-DRIFT survey, focused on LSB detection, we present several feasible research topics that utilize other survey data together and discuss the role of LSB observation in understanding the evolution of galaxies.

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arXiv2026

A New Collision Avoidance Fiber Assignment Algorithm for Robotic Fiber Positioners in Multi-Object Spectroscopy

We present a new fiber assignment algorithm for a robotic fiber positioner system in multi-object spectroscopy. Modern fiber positioner systems typically have overlapping patrol regions, resulting in the number of observable targets being highly dependent on the fiber assignment scheme. To maximize observable targets without fiber collisions, the algorithm proceeds in three steps. First, it assigns the maximum number of targets for a given field of view without considering any collisions between fiber positioners. Then, the fibers in collision are grouped, and the algorithm finds the optimal solution resolving the collision problem within each group. We compare the results from this new algorithm with those from a simple algorithm that assigns targets in descending order of their rank by considering collisions. As a result, we could increase the overall completeness of target assignments by 10% with this new algorithm in comparison with the case using the simple algorithm in a field with 150 fibers. Our new algorithm is designed for the All-sky SPECtroscopic survey of nearby galaxies (A-SPEC) based on the K-SPEC spectrograph system, but can also be applied to similar fiber-based systems with heavily overlapping fiber positioners.

astro-ph.IM↗

arXiv2025

K-DRIFT Science Theme: Illuminating the Next Era of Galaxy Cluster Science

The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is a pioneering instrument designed to explore low-surface-brightness (LSB) phenomena. This white paper presents a compelling array of science cases that showcase K-DRIFT's unique capabilities in unraveling the mysteries of intracluster light (ICL) and other LSB components within galaxy clusters. Exploring the origin of ICL in galaxy clusters and comparing the spatial distributions of ICL and dark matter will offer new insights into galaxy cluster dynamics. Moreover, investigating LSB objects in galaxy clusters, such as LSB structures in the brightest cluster galaxy, ultra-diffuse galaxies, and tidal features, will enhance our understanding of galaxy evolution within the cluster environment. We present our strategies for addressing scientific queries, encompassing LSB observation and analysis techniques, specialized simulations, and machine-learning approaches. Additionally, we examine the potential synergies between K-DRIFT and other ongoing or forthcoming multi-wavelength surveys. This white paper advocates for the recognition and support of K-DRIFT as a dedicated tool for advancing our understanding of the universe's subtlest phenomena.

astro-ph.GA↗

arXiv2025

Direct observational evidence that higher-luminosity type 1 active galactic nuclei are most commonly triggered by galaxy mergers

We examine the connection between galaxy mergers and the triggering of active galactic nuclei (AGNs) using a sample of 614 type 1 AGNs at $z<0.07$, along with a control sample of inactive galaxies matched to the AGNs for comparison. We used tidal features, detected in deep images from the DESI Legacy Imaging Survey, as direct evidence of recent mergers. We find that the fraction of type 1 AGN hosts with tidal features ($f_T$) is higher for AGNs with higher luminosities and (to a lesser extent) more massive black holes. Specifically, $f_T$ rapidly increases from $0.05\pm0.03$ to $0.75\pm0.13$ as the luminosity of the $[\mathrm{O\,{\scriptstyle III}}]$ $λ$5007 emission line ($L_{[\mathrm{O\,{\scriptstyle III}}]}$), an indicator for bolometric AGN luminosity, increases in the range $10^{39.5}\lesssim L_{[\mathrm{O\,{\scriptstyle III}}]}/(\mathrm{erg\,s}^{-1}) \lesssim10^{42.5}$. In addition, $f_T$ increases from $0.13\pm0.03$ to $0.43\pm0.09$ as black hole mass ($M_\mathrm{BH}$) increases in the range $10^{6.0}\lesssim M_\mathrm{BH}/M_{\odot}\lesssim10^{8.5}$. The fraction $f_T$ also increases with the Eddington ratio, although the trend is less significant compared to that with $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and $M_\mathrm{BH}$. The excess of $f_T$, defined as the ratio of $f_T$ for AGNs to that of their matched inactive counterparts, exhibits similar trends, primarily increasing with $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and weakly with $M_\mathrm{BH}$. Our results indicate that, in the local Universe, galaxy mergers are the predominant triggering mechanism for high-luminosity AGNs, whereas they play a lesser role in triggering lower-luminosity AGNs. Additionally, strong events, such as galaxy mergers, may be more necessary to activate massive black holes in more massive galaxies due to their lower gas fractions.

astro-ph.GA↗

arXiv2025

K-DRIFT: Unveiling New Imagery of the Hidden Universe

Low-surface-brightness (LSB) structures play a crucial role in understanding galaxy evolution by providing significant insights into galaxy interactions, the histories of mass assembly, and the distribution of dark matter. Nevertheless, their inherently faint nature, coupled with observational difficulties such as stray light interference and variations in the sky background, has significantly impeded comprehensive studies of LSB features. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) project aims to address these observational challenges by developing off-axis freeform three-mirror telescopes and observational strategies specifically designed for LSB imaging surveys. The first generation of the K-DRIFT (K-DRIFT G1) has been successfully completed, and the forthcoming survey, scheduled to commence shortly, is expected to yield novel insights into the LSB universe. This paper outlines the scientific motivations of the project, discusses the technical challenges encountered, highlights the innovative solutions devised, and describes the future trajectory of the K-DRIFT.

astro-ph.GA↗

arXiv2025

Diverse Rotation Curves of Galaxies in a Simulated Universe: the Observed Dependence on Stellar Mass and Morphology Reproduced

We use the IllustrisTNG cosmological hydrodynamical simulation to study the rotation curves of galaxies in the local universe. To do that, we first select the galaxies with 9.4 $<$ $\log{(M_\mathrm{star}/M_\odot)}$ $<$ 11.5 to make a sample comparable to that of SDSS/MaNGA observations. We then construct the two-dimensional line-of-sight velocity map and conduct the fit to determine the rotational velocity and the slope of the rotation curve in the outer region ($R_\mathrm{t}<r<3\times r_\mathrm{half,*}$). The outer slopes of the simulated galaxies show diverse patterns that are dependent on morphology and stellar mass. The outer slope increases as galaxies are more disky, and decreases as galaxies are more massive, except for the very massive early-type galaxies. The outer slope of the rotation curves shows a correlation with the dark matter fraction, slightly better than for the gas mass fraction. Our study demonstrates that the observed dependence of galaxy rotation curves on morphology and stellar mass can be successfully reproduced in cosmological simulations, and provides a hint that dark matter plays an important role in shaping the rotation curve. The sample of simulated galaxies in this study could serve as an important testbed for the subsequent study tracing galaxies back in time, enabling a deeper understanding of the physical origin behind the diverse rotation curves.

astro-ph.GA↗

arXiv2024

The Most Massive Early-type Galaxies Exhibit Tidal Features More Frequently in Lower-density Environments

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.

astro-ph.GA↗

arXiv2024

Shell-type Tidal Features Are More Frequently Detected in Slowly Rotating Early-type Galaxies than Stream- and Tail-type Features

To enhance our understanding of the impact of galaxy mergers on the kinematics of early-type galaxies (ETGs), we examine differences in specific stellar angular momentum within the half-light radius ($λ_{R_e}$) among ETGs with different types of tidal features and those without such features. This is accomplished by categorizing tidal features, which serve as direct evidence of recent mergers, into shells, streams, and tails, through deep images from the DESI Legacy Survey, and by using MaNGA data for the analysis of the kinematics of 1244 ETGs at $z<0.055$. We find that ETGs with tidal features typically have reduced $λ_{R_e}$ values that are lower by 0.12 dex than ETGs without tidal features. ETGs with shells contribute most to the reduction in $λ_{R_e}$. Consequently, nearly half of ETGs with shells are classified as slow rotators, a fraction that is more than twice as high as that of ETGs with tails or streams, and over three times higher than that of ETGs without tidal features. These trends generally remain valid even when ETGs are divided into several mass bins. Our findings support the idea that radial mergers, which are more effective at reducing $λ_{R_e}$ than circular mergers, are more closely associated with the formation of shells rather than streams or tails. The detection of shells in slightly more massive ETGs compared to streams and tails may be attributed to the fact that massive satellite galaxies are more likely to be accreted through radial orbits, due to the nature of dynamical friction.

astro-ph.GA↗

arXiv2023

Low-mass Quiescent Galaxies Are Small in Isolated Environments: Environmental Dependence of the Mass-Size Relation of Low-mass Quiescent Galaxies

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 $0.01<z<0.04$ from the Sloan Digital Sky Survey. We find that the mass-size relation of low-mass quiescent galaxies (LQGs) with $\log(M_\mathrm{star}/M_{\odot})\lesssim10.0$ depends on their environment, with LQGs in the highest-density environments exhibiting an average size $\sim70\%$ larger than those in isolated environments. Moreover, the slope of the mass-size relation for LQGs in high-density environments is significantly shallower than that of their counterparts in isolated environments. This is in contrast with high-mass quiescent galaxies with $\log(M_\mathrm{star}/M_{\odot})\gtrsim10.5$ that show a nearly identical mass-size relation across all environments. Combined with additional discoveries that the mass-size relation slopes of LQGs and star-forming galaxies are similar to each other in high-density environments, and that LQGs in higher-density environments exhibit more disk-like structures, our results support the idea that LQGs in high-density environments have evolved from star-forming galaxies through environmental effects, which are capable of causing their quenching and transformation into quiescent galaxies. With the aid of an analysis of merger rates for simulated galaxies from a cosmological galaxy formation simulation, we suggest that the steep slope and low normalization of the mass-size relation of LQGs in the lowest-density environments may originate from recent gas-rich mergers, which occur over 10-30 times more frequently in the progenitors of LQGs in the lowest-density environments than in their counterparts in high-density environments at low redshifts.

astro-ph.GA↗

arXiv2023

Cosmological Parameter Constraints from the SDSS Density and Momentum Power Spectra

We extract the galaxy density and momentum power spectra from a subset of early-type galaxies in the SDSS DR7 main galaxy catalog. Using galaxy distance information inferred from the improved fundamental plane described in \citet{Yoon_2020}, we reconstruct the peculiar velocities of the galaxies and generate number density and density-weighted velocity fields, from which we extract the galaxy density and momentum power spectra. We compare the measured values to the theoretical expectation of the same statistics, assuming an input $Λ$CDM model and using a third-order perturbative expansion. After validating our analysis pipeline with a series of mock data sets, we apply our methodology to the SDSS data and arrive at constraints $fσ_{8} = 0.471_{-0.080}^{+0.077}$ and $b_{1}σ_{8} = 0.920_{-0.070}^{+0.070}$ at a mean redshift $\bar{z} = 0.04$. Our result is consistent with the Planck cosmological best fit parameters for the $Λ$CDM model. The momentum power spectrum is found to be strongly contaminated by small scale velocity dispersion, which suppresses power by $\sim {\cal O}(30\%)$ on intermediate scales $k \sim 0.05 \, h \, {\rm Mpc}^{-1}$.

astro-ph.CO↗

arXiv2023

Dependence of galactic bars on the tidal density field in the SDSS

As a key driver of the secular evolution of disc galaxies, bar formation is potentially linked to the surrounding tidal field. We systematically investigate the dependence of bars on both the small (${<}2\,\mathrm{Mpc}/h$) and large-scale (${>}5\,\mathrm{Mpc}/h$) tidal fields using galaxies observed between $0.01{<}z{<}0.11$ by the Sloan Digital Sky Survey (SDSS). We characterise bar strength using the ellipticity of the isophote that corresponds to the bar, $e_{\mathrm{bar}}$, derived from the galaxy image after subtracting the 2D disc component. We demonstrate the efficacy of our bar detection method by performing an extensive comparison with the visual identifications from SDSS and the DESI Legacy Surveys. Using the Yang et al. SDSS group catalogue, we confirm the results from a recent study that the average $e_{\mathrm{bar}}$ of galaxies within interacting clusters is higher than that within isolated ones at $0.01{<}z{<}0.06$, but this small-scale tidal enhancement of bars disappears after we increase the cluster sample by a factor of five to $z{=}0.11$. On large scales, we explore the dependence of $e_{\mathrm{bar}}$ on $α_{5}$, the tidal anisotropy of the density field defined over $5\,\mathrm{Mpc}/h$. We do not detect any such dependence for $98\%$ of the galaxies with $α_{5}{<}10$. Intriguingly, among the $2\%$ with $α_{5}{\ge}10$, we detect some hint of a boost in bar strength in the underdense regions and a suppression in the overdense regions. Combining our results on both scales, we conclude that there is little evidence for the tidal dependence of bar formation in the local Universe, except for the extremely anisotropic environments.

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arXiv2023

Impact of Galaxy Mergers on Stellar Population Profiles of Early-type Galaxies

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.

astro-ph.GA↗

arXiv2022

The Fundamental Plane Is Not a Plane: Warped Nature of the Fundamental Plane of Early-type Galaxies and Its Implication for Galaxy Formation

Based on $16,283$ early-type galaxies (ETGs) in $0.025\le z_\mathrm{spec}<0.055$ from Sloan Digital Sky Survey data, we show that the fundamental plane (FP) of ETGs is not a plane in the strict sense but is a curved surface with a twisted shape whose orthogonal direction to the surface is shifted as the central velocity dispersion ($σ_0$) or mean surface brightness within the half-light radius ($μ_e$) changes. When ETGs are divided into subsamples according to $σ_0$, the coefficient of $μ_e$ of the FP increases, whereas the zero-point of the FP decreases at higher $σ_0$. Taking the $z$ band as an example, the coefficient of $μ_e$ rises from $0.28$ to $0.36$ as $σ_0$ increases from $\sim100$ to $\sim300$ km s$^{-1}$. At the same time, the zero-point of the FP falls from $-7.5$ to $-9.0$ in the same $σ_0$ range. The consistent picture on the curved nature of the FP is also reached by inspecting changes in the FP coefficients for ETG subsamples with different $μ_e$. By examining scaling relations that are projections of the FP, we suggest that the warped nature of the FP may originate from dry merger effects that are imprinted more prominently in ETGs with higher masses.

astro-ph.GA↗

arXiv2022

The Infrared Medium-deep Survey. IX. Discovery of Two New $z\sim6$ Quasars and Space Density down to $M_{1450}\sim-23.5$ mag

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.

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