arXiv subjects

Minhee Hyun

Publications and source records attributed to Minhee Hyun.

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

Modeling of the diffuse background produced by the Vera C. Rubin Observatory M2 baffle scattered light

The Vera C. Rubin Observatory, with its unprecedented field of view and fast focal ratio, will survey the entire sky every 3.5 nights. This unique capacity requires dealing with off axis light that can produce stray light artefacts on the images. The secondary mirror (M2) baffle restricts the light that reaches the LSSTCam detector and it contributes to shaping the inner edge of the telescope optical pupil. This work studies the contribution to the background from the light scattered by the M2 baffle itself. The evanescence of this feature, together with the challenge of isolating it from the sky background, led to the necessity of performing in dome tests using a Collimated Beam Projector (CBP), normally used for calibration purposes. To complete the analysis, in addition to the in dome tests, an on sky observational campaign was conducted. This campaign employed both stellar targets and the Moon as illumination sources in order to determine the actual energy associated with the feature. The test data have been retro fitted thanks to the combination of ray tracing simulation, CBP and on sky data to infer the intensity and spatial distribution of the background scattered light within the different LSSTCam filters. We quantified the on sky impact of scattered light from the M2 baffle, both for light coming from bright and red stars and from the Moon. We also developed an approximate relation to transform the in dome measurements into predictions of on sky behavior. This transformation was achieved by comparing the illumination footprint produced by an off axis star with that generated by the CBP and by mapping the stellar Spectral Energy Distribution (SED) onto the CBP's set of discrete wavelengths. Finally, we extrapolated the scattered light behavior of the Moon to stellar sources, in order to build a compplete description of the M2 baffle contribution over the full range of magnitudes.

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arXiv2026

An overview of stray light findings and interpretation during on-sky commissioning of LSSTCam

Wide-field telescopes are intrinsically difficult to shield from unwanted stray and scattered light, while the search to identify sources of contaminating light is frequently a challenging task. The Vera C.~Rubin Observatory, which achieved its first photon with the LSST Camera (LSSTCam) on April 15, 2025, will initiate a revolutionary era for the study of dark matter, dark energy, the transient sky, the Solar System, and the Milky Way. LSSTCam will provide near seeing-limited images of the sky in six bands ($u,g,r,i,z,y$) over a $3.^\circ 5$-diameter field of view, and over the course of a decade, it will execute the Legacy Survey of Space and Time (LSST). This work provides an overview of the dedicated stray and scattered light test campaign that has been undertaken since the start of Rubin commissioning. In particular, we highlight the processes used to characterize, model, and mitigate stray light present in LSSTCam images. The Rubin commissioning team created a series of testing and analysis tools to track stray light artifacts from their initial discovery through reproduction with timely observations, simulation using ray tracing to identify opto-mechanical origins, and finally devising corrective actions. The complex stray light features encountered by Rubin provide a wealth of experience for the future wide-field and extremely wide-field observatories. This work covers the many stages of a long journey that started with conceiving an innovative and challenging optical design, followed by the engineering and system engineering efforts to build it, to finally delivering an optimized and revolutionary cutting-edge facility.

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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.

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arXiv2024

Multi-Object Spectroscopy of Galaxy Clusters at $z \sim 0.95$ in Ultra Deep Survey Field with Different Star-formation Properties and Large-scale Environments

While galaxy clusters are dominated by quiescent galaxies at local, they show a wide range in quiescent galaxy fraction (QF) at higher redshifts. Here, we present the discovery of two galaxy clusters at $z \sim 0.95$ with contrasting QFs despite having similar masses (log ($M_{200}/M_{\odot}$)$ \sim 14$) and spectra and redshifts of 29 galaxies in these clusters and 76 galaxies in the surrounding area. The clusters are found in the Ultra Deep Survey (UDS) field and confirmed through multi-object spectroscopic (MOS) observation using the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Magellan telescope. The two clusters exhibit QFs of $0.094^{+0.11}_{-0.032}$ and $0.38^{+0.14}_{-0.11}$, respectively. Analysis of large-scale structures (LSSs) surrounding these clusters finds that properties of these clusters are consistent with the anti-correlation trend between the QF and the extent of surrounding LSS, found in Lee et al. (2019), which can be interpreted as a result from the replenishment of young, star-forming galaxies keeps the QF low when galaxy clusters are accompanied by rich surrounding environments.

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arXiv2023

PEARLS: Near Infrared Photometry in the JWST North Ecliptic Pole Time Domain Field

We present Near-Infrared (NIR) ground-based Y, J, H, and K imaging obtained in the James Webb Space Telescope North Ecliptic Pole Time Domain Field (TDF) using the MMT-Magellan Infrared Imager and Spectrometer (MMIRS) on the MMT.These new observations cover a field of approximately 230 arcmin^2 in Y, H, and K and 313 arcmin^2 in J. Using Monte Carlo simulations we estimate a 1 sigma depth relative to the background sky of (Y, J, H, K}) = (23.80, 23.53, 23.13, 23.28) in AB magnitudes for point sources at a 95% completeness level. These observations are part of the ground-based effort to characterize this region of the sky, supplementing space-based data obtained with Chandra, NuSTAR, XMM, AstroSat, HST, and JWST. This paper describes the observations and reduction of the NIR imaging and combines these NIR data with archival imaging in the visible, obtained with the Subaru Hyper-Suprime-Cam, to produce a merged catalog of 57,501 sources. The new observations reported here, plus the corresponding multi-wavelength catalog, will provide a baseline for time-domain studies of bright sources in the TDF.

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arXiv2023

The JCMT SCUBA-2 Survey of the James Webb Space Telescope North Ecliptic Pole Time-Domain Field

The James Webb Space Telescope Time-Domain Field (JWST-TDF) is an $\sim$14$'$ diameter field near the North Ecliptic Pole that will be targeted by one of the JWST Guaranteed Time Observations programs. Here, we describe our James Clerk Maxwell Telescope SCUBA-2 850 $\mu$m imaging of the JWST-TDF and present the submillimeter source catalog and properties. We also present a catalog of radio sources from Karl J. Jansky Very Large Array 3 GHz observations of the field. These observations were obtained to aid JWST's study of the dust-obscured galaxies that contribute significantly to the cosmic star formation at high redshifts. Our deep 850 $\mu$m map covers the JWST TDF at a noise level of $\sigma_{850}$ = 1.0 mJy beam$^{-1}$, detecting 83/31 sources in the main/supplementary signal-to-noise ratio (S/N $>$ 4 / S/N = 3.5 - 4) sample respectively. The 3 GHz observations cover a 24$'$ diameter field with a 1 $\sigma$ noise of 1$\mu$Jy beam$^{-1}$ at a 0$.\!\!^{\prime\prime}$7 FWHM. We identified eighty-five 3 GHz counterparts to sixty-six 850 $\mu$m sources and then matched these with multiwavelength data from the optical to the mid-infrared wave bands. We performed spectral energy distribution fitting for 61 submillimeter galaxies (SMGs) matched with optical/near-infrared data, and found that SMGs at S/N $>$ 4 have a median value of $z_{phot} = $2.22 $\pm$ 0.12, star formation rates of 300 $\pm$ 40 M$_{\odot}\,{\rm yr^{-1}}$ (Chabrier initial mass function), and typical cold dust masses of 5.9 $\pm$ 0.7 $ \times$ 10$^{8} $M$_{\odot}$, in line with bright SMGs from other surveys. The large cold dust masses indicate correspondingly large cool gas masses, which we suggest are a key factor necessary to drive the high star formation rates seen in this population

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arXiv2022

JWST's PEARLS: Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results

We give an overview and describe the rationale, methods, and first results from NIRCam images of the JWST "Prime Extragalactic Areas for Reionization and Lensing Science" ("PEARLS") project. PEARLS uses up to eight NIRCam filters to survey several prime extragalactic survey areas: two fields at the North Ecliptic Pole (NEP); seven gravitationally lensing clusters; two high redshift proto-clusters; and the iconic backlit VV 191 galaxy system to map its dust attenuation. PEARLS also includes NIRISS spectra for one of the NEP fields and NIRSpec spectra of two high-redshift quasars. The main goal of PEARLS is to study the epoch of galaxy assembly, AGN growth, and First Light. Five fields, the JWST NEP Time-Domain Field (TDF), IRAC Dark Field (IDF), and three lensing clusters, will be observed in up to four epochs over a year. The cadence and sensitivity of the imaging data are ideally suited to find faint variable objects such as weak AGN, high-redshift supernovae, and cluster caustic transits. Both NEP fields have sightlines through our Galaxy, providing significant numbers of very faint brown dwarfs whose proper motions can be studied. Observations from the first spoke in the NEP TDF are public. This paper presents our first PEARLS observations, their NIRCam data reduction and analysis, our first object catalogs, the 0.9-4.5 $\mu$m galaxy counts and Integrated Galaxy Light. We assess the JWST sky brightness in 13 NIRCam filters, yielding our first constraints to diffuse light at 0.9-4.5 {\mu}m. PEARLS is designed to be of lasting benefit to the community.

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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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arXiv2022

High-$z$ Universe probed via Lensing by QSOs (HULQ) II. Deep GMOS spectroscopy of a QSO lens candidate

Galaxies and their central supermassive black holes are known to coevolve, but the physical background for this is unknown as of yet. The High-$z$ Universe probed via Lensing by QSOs (HULQ) project aims to investigate this coevolution by using quasi-stellar object (QSO) host galaxies acting as gravitational lenses (QSO lenses). We present the results of the spectroscopic observation of the first QSO lens candidate from the HULQ project, HULQ J0002+0239, which consists of a QSO host galaxy at $z_{\rm d} = 1.455$ and four seemingly lensed objects in a cross-like configuration. Deep optical spectra of two of the possibly lensed objects with $z \sim 24.5$ mag were obtained with the Gemini Multi-Object Spectrograph on the Gemini North Telescope. Their spectra reveal that the objects are newly discovered galaxies at $z=0.29$ and $z=1.11$, and we conclude that HULQ J0002+0239 is not a QSO lens. Our QSO lens search results are so far in agreement with the predicted number of QSO lenses, and we discuss how the future investigation of additional QSO lens candidates could tell us more about the evolution of the black hole mass and host galaxy scaling relations.

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arXiv2020

The Infrared Medium-deep Survey. VIII. Quasar Luminosity Function at $z\sim5$

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 ($α=-1.2^{+1.4}_{-0.6}$), 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 $ε_{912}=(3.7$--$7.1)\times10^{23}$ 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 $z\sim5$.

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arXiv2020

The Infrared Medium-deep Survey. \Romannum{7}. Faint Quasars at $z \sim 5$ in the ELAIS-N1 Field

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 $z\sim5$

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arXiv2019

More connected, more active: galaxy clusters and groups at z~1 and the connection between their quiescent galaxy fractions and large-scale environments

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.

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arXiv2019

Intensive Monitoring Survey of Nearby Galaxies (IMSNG)

Intensive Monitoring Survey of Nearby Galaxies (IMSNG) is a high cadence observation program monitoring nearby galaxies with high probabilities of hosting supernovae (SNe). IMSNG aims to constrain the SN explosion mechanism by inferring sizes of SN progenitor systems through the detection of the shock-heated emission that lasts less than a few days after the SN explosion. To catch the signal, IMSNG utilizes a network of 0.5-m to 1-m class telescopes around the world and monitors the images of 60 nearby galaxies at distances D < 50 Mpc to a cadence as short as a few hours. The target galaxies are bright in near-ultraviolet (NUV) with M_NUV < -18.4 AB mag and have high probabilities of hosting SNe (0.06 SN/yr per galaxy). With this strategy, we expect to detect the early light curves of 3.4 SNe per year to a depth of R ~ 19.5 mag, enabling us to detect the shock-heated emission from a progenitor star with a radius as small as 0.1 R_sun. The accumulated data will be also useful for studying faint features around the target galaxies and other science projects. So far, 18 SNe have occurred in our target fields (16 in IMSNG galaxies) over 5 years, confirming our SN rate estimate of 0.06 SN/yr per galaxy.

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arXiv2019

Extremely Massive Quasars are not Good Proxies for Dense Environments Compared to Massive Galaxies: Environments of Extremely Massive Quasars and Galaxies

Black hole mass scaling relations suggest that extremely massive black holes (EMBHs) with $M_\mathrm{BH}\ge10^{9.4}\,M_{\odot}$ are found in the most massive galaxies with $M_\mathrm{star}\ge10^{11.6}\,M_{\odot}$, which are commonly found in dense environments, like galaxy clusters. Therefore, one can expect that there is a close connection between active EMBHs and dense environments. Here, we study the environments of 9461 galaxies and 2943 quasars at $0.24 \le z \le 0.40$, among which 52 are extremely massive quasars with $\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, using Sloan Digital Sky Survey and MMT Hectospec data. We find that, on average, both massive quasars and massive galaxies reside in environments more than $\sim2$ times as dense as those of their less massive counterparts with $\log(M_\mathrm{BH}/M_{\odot}) \le 9.0$. However, massive quasars reside in environments about half as dense as inactive galaxies with $\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, and only about one third of massive quasars are found in galaxy clusters, while about two thirds of massive galaxies reside in such clusters. This indicates that massive galaxies are a much better signpost for galaxy clusters than massive quasars. The prevalence of massive quasars in moderate to low density environments is puzzling, considering that several simulation results show that these quasars appear to prefer dense environments. Several possible reasons for this discrepancy are discussed, although further investigation is needed to obtain a definite explanation.

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arXiv2018

The Infrared Medium-deep Survey. VI. Discovery of Faint Quasars at $z\sim5$ with a Medium-band-based Approach

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 $-25<M_{1450}<-23$ at $z\sim5$, among which three have been missed in a previous survey using the same optical data over the same area, implying the necessity for improvements in high redshift faint quasars selection. We derived photometric redshifts from the medium-band data, and find that they have high accuracies of $\langle|Δz|/(1+z)\rangle=0.016$. The medium-band-based approach allows us to rule out many of the interlopers that contaminate $\gtrsim20~\%$ of the broad-band-selected quasar candidates. These results suggest that the medium-band-based approach is a powerful way to identify $z\sim5$ quasars and measure their redshifts at high accuracy (1-2 %). It is also a cost-effective way to understand the contribution of quasars to the cosmic re-ionization history.

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arXiv2018

The Infrared Medium-deep Survey. IV. Low Eddington Ratio of A Faint Quasar at $z\sim6$: Not Every Supermassive Black Hole is Growing Fast in the Early Universe

To date, most of the luminous quasars known at $z\sim6$ have been found to be in maximal accretion with the Eddington ratios, $λ_{\rm{Edd}}\sim1$, 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 $λ_{\rm{Edd}}$. 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 $λ1549$ emission line in the NIR spectrum, we find that IMS J2204+0112 harbors a SMBH with about a billion solar mass and $λ_{\rm{Edd}} \sim 0.1$, 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 $λ_{\rm{Edd}}$ 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 $z \sim 6$.

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arXiv2017

The Infrared Medium-deep Survey. III. Survey of Luminous Quasars at 4.7 $\leq$ z $\leq$ 5.4

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.

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