Dong-Woo Kim

Quasars are extremely luminous objects that provide insights into the physics and evolution of supermassive black holes (SMBHs) and their accretion flows, galaxy evolution, and even cosmology. In this study, we present an exploratory study based on the ongoing fifth generation of the Sloan Digital Sky Survey (SDSS-V) and its unique dual-hemisphere, wide-field, and multiobject spectroscopic capabilities, with the aim of creating a comprehensive, all-sky quasar sample. The targets were selected through two novel methods, Gaia–unWISE AGN (GUA) and Skewt_QSO, that rely primarily on data from the Wide-field Infrared Survey Explorer (WISE) and Gaia, aiming to address gaps in previous large quasar samples. Our sample includes over 250,000 spectroscopically confirmed quasars reaching to z ∼ 5, with tens of thousands of newly identified quasars in the southern hemisphere. The selection methods are highly pure, with well over 80% of the spectra collected being genuine quasars; the main contaminants are M-type stars. The detailed spectral decomposition procedure we employed shows that the quasars in the sample span a wide range of luminosities ( L _bol ∼ 10 ⁴⁴ −10 ⁴⁸ erg s ⁻¹ ), SMBH masses ( M _BH ∼ 10 ⁶ −10 ¹⁰ M _⊙ ), and accretion rates ( L / L _Edd ∼ 0.01−1). The distributions of these properties are consistent with those of previous quasar catalogs, which are based on past generations of SDSS, once we account for potential selection biases related to the various survey depths. Our findings confirm that novel selection methods based on optical+infrared colors and/or astrometry can yield a large, high-purity quasar sample over wide sky areas, including in cases where more nuanced multiband photometry and/or multiwavelength data in the X-ray or radio are not available. This SDSS-V sample, which is expected to grow in the near future, establishes a robust reference for future southern (time-domain) surveys, while enhancing and complementing our understanding of quasar demographics and SMBH evolution.

We present a framework to crossmatch sources from the Chandra Source Catalog (CSC v2.1) with optical sources from Gaia Data Release 3. Unlike purely spatial approaches, we use source properties such as magnitudes, colors, and distances to identify true counterparts, detect chance coincidences, and resolve ambiguities when multiple plausible candidates exist. We define a training set of high-confidence matches using NWAY , a Bayesian crossmatching framework that accounts for positional errors and source densities. We train a gradient-boosted classifier ( LightGBM ) on a variety of features from both catalogs. Of the ≈254 k unique X-ray sources, we find counterparts for ≈113 k sources, of which plausible multiple counterparts are found for ≈7 k. We find no counterparts for ≈20 k sources for which separation-based crossmatching does find a match, and attribute half of these to chance coincidences. We validate the pipeline on the Chandra Orion Ultradeep Project (COUP), where the machine learning matches reproduce 95% of NWAY crossmatches without using any positional information. We release a catalog of ≈113 k Chandra–Gaia counterparts, together with ≈7 k alternative matches and ≈20 k ambiguous NWAY associations, supporting future population studies of sources detectable by both Chandra and Gaia. We discuss limitations and provide a generalization of the framework that is applicable in other crossmatching scenarios.

In most galaxies, the central black hole accounts for no more than a percent of the total mass in stars. Recently, however, extremely overmassive black holes (OMBHs) with ratios of 10% have been reported in dwarf galaxies at z < 1 and at cosmic dawn ( z > 5.5) by JWST. Both findings have been interpreted as signatures of the still mysterious origins of supermassive black holes. Accordingly, it is thought that most of the black hole mass was built at birth rather than through accretion. Here, we show that among evolved galaxies, OMBHs are also present, indicating that OMBHs are not a signature unique to black hole formation channels. The first large-area sky survey of the eROSITA X-ray telescope on board SRG identified 200 quasars by their luminous hard X-ray radiation. These quasars signpost rapidly growing black holes. Complementary optical spectroscopy from the Sloan Digital Sky Survey and archival UV to IR photometric data combined with galaxy-quasar decomposition techniques allowed us to make unbiased estimates of cosmological distances, black hole masses, and host galaxy stellar masses. We securely identified a sample of OMBHs with BH-to-host ratios of more than 5%, which may have undergone exponential accretion spurts lasting about a billion years. Our survey identified a high space density of at least 4/Gpc ³ of OMBHs near cosmic noon. This indicates an accretion channel disconnected from the stellar population that causes strong deviations from galaxy scaling relations. This channel is currently not part of galaxy evolution models. The identified channel, if applicable also for the first billion years of cosmic time, might explain JWST active galactic nuclei without requiring them to signify imprints of a black hole seeding mechanism.

Knowledge of the X-ray properties of the hot-gas halos of early-type galaxies (ETGs) has significantly advanced in the past years, for large and homogeneously investigated samples. We compare these results with the X-ray properties of an exploratory set of gas evolution models in realistic ETGs, produced with our high-resolution 2D hydrodynamical code MACER that includes active galactic nucleus feedback and accretion from a circumgalactic medium. The model X-ray emission and absorption are integrated along the line of sight, to obtain maps of the surface brightness Σ_X and temperature T _X. The X-ray diagnostics considered are the luminosity and average temperature for the whole galaxy (L _X and 〈T _X〉) and within five optical effective radii (L _X,5 and 〈T _X,5〉) and the circularized profiles Σ_X(R) and T _X(R). The values for L _X, L _X,5, 〈T _X〉, and 〈T _X,5〉 compare very well with those observed. The Σ_X(R) and T _X(R) also present qualitative similarities with those of the representative galaxy NGC 5129 and those of ETGs with the most commonly observed shape for T _X(R): Σ_X(R) matches the observed profile over many optical effective radii R _e, and T _X(R) reproduces the characteristic bump that peaks at R = (1 ÷ 3)R _e. Inside the peak position, T _X(R) declines toward the center, but the explored models are systematically hotter by ≃30%; possible explanations for this discrepancy are discussed. Interestingly, Σ_X(R) and T _X(R) as large as those observed outside R ≃ R _e are reproduced only with significant accretion from a circumgalactic medium, highlighting its importance.

We present one of the largest uniform optical spectroscopic surveys of X-ray selected sources to date that were observed as a pilot study for the Black Hole Mapper (BHM) survey. The BHM program of the Sloan Digital Sky Survey (SDSS)-V is designed to provide optical spectra for hundreds of thousands of X-ray selected sources from the SRG/eROSITA all-sky survey. This significantly improves our ability to classify and characterise the physical properties of large statistical populations of X-ray emitting objects. Our sample consists of 13 079 sources in the eROSITA eFEDS performance verification field, 12 011 of which provide reliable redshifts from 0 ≲ z ≤ 5.8. The vast majority of these objects were detected as point-like sources (X-ray flux limit F_{0.5 − 2 keV} ≳ 6.5 × 10⁻¹⁵ erg/s/cm²) and were observed for about 20 years with fibre-fed SDSS spectrographs. After including all available redshift information for the eFEDS sources from the dedicated SDSS-V plate programme and archival data, we visually inspected the SDSS optical spectra to verify the reliability of these redshift measurements and the performance of the SDSS pipeline. The visual inspection allowed us to recover reliable redshifts (for 99% of the spectra with a signal-to-noise ratio of > 2) and to assign classes to the sources, and we confirm that the vast majority of our sample consists of active galactic nuclei (AGNs). Only ∼3% of the eFEDS/SDSS sources are Galactic objects. We analysed the completeness and purity of the spectroscopic redshift catalogue, in which the spectroscopic completeness increases from 48% (full sample) to 81% for a cleaner, brighter (r_AB < 21.38) sample that we defined by considering a high X-ray detection likelihood, a reliable counterpart association, and an optimal sky coverage. We also show the diversity of the optical spectra of the X-ray selected AGNs and provide spectral stacks with a high signal-to-noise ratio in various sub-samples with different redshift and optical broad-band colours. Our AGN sample contains optical spectra of (broad-line) quasars, narrow-line galaxies, and optically passive galaxies. It is considerably diverse in its colours and in its levels of nuclear obscuration.

We explore the evolution of the ∼10⁷ K hot gas in normal galaxies out to redshift = 0.5 (lookback time = 5 Gyr), using X-ray luminosity functions (XLFs) built from a sample of 575 normal galaxies with z < 0.6 detected in five high-galactic-latitude Chandra wide-field surveys. After estimating the emission due to the hot gas component (reducing the sample to ∼400 galaxies), we compared the XLF in three redshift bins (z = 0.1, 0.3, and 0.5), finding increases in the number of galaxies per unit comoving volume from z = 0.1 to 0.3 and then from z = 0.3 to 0.5. These XLF changes suggest a significant (∼5σ) X-ray luminosity evolution of the hot gas, with L _X,GAS decreasing by a factor of 6–10 in the last 5 Gyr (from z = 0.5 to 0.1). The relative abundance of L _X,GAS ∼ 10⁴¹ erg s⁻¹ galaxies at higher z suggests that high-z, moderate-L _X,GAS galaxies may be the optimal target to solve the missing baryon problem. In early-type galaxies, this observational trend is qualitatively consistent with (but larger than) the expected time-dependent mass-loss rate in cooling flow models without active galactic nucleus feedback. In late-type galaxies, the observational trend is also qualitatively consistent with (but larger than) the effect of the z-dependent star formation rate.

X-ray bright optically normal galaxies (XBONGs) are galaxies with X-ray luminosities consistent with those of active galactic nuclei (AGNs) but no evidence of AGN optical emission lines. Crossmatching the Chandra Source Catalog version 2 with the Sloan Digital Sky Survey sample of spectroscopically classified galaxies, we have identified 817 XBONG candidates with L _X > 10⁴² erg s⁻¹ and X-ray to optical flux ratio F _XO > 0.1. Comparisons with Wide-field Infrared Survey Explorer colors and near-IR, optical, UV, and radio luminosities show that the loci of XBONGs are in-between those of control samples of normal galaxies and quasars and are consistent with low-luminosity quasars. We find that 43% of the XBONG sample have X-ray colors suggesting N _H > 10²² cm⁻², double the fraction in the QSO sample, suggesting that a large fraction of XBONG are highly obscured AGNs. However, ∼50% of the XBONGs are not obscured and have X-ray colors harder than those of normal galaxies. Some of these XBONGs have spatially extended X-ray emission. These characteristics suggest that they may be unidentified galaxy groups and clusters. Using the X-ray luminosity functions of QSOs, galaxies, groups and clusters, we estimate the approximate fraction of extended XBONGs to be L _X–L _r relation.