Jubee Sohn

We identify compact groups (CGs) of galaxies in an IllustrisTNG300 simulation using a friends-of-friends algorithm. Our approach is designed to be comparable to systematic CG searches based on spectroscopic surveys, while avoiding the conventional Hickson selection criteria, which can bias samples toward relatively low-density environments. We construct two CG catalogs: one based on a three-dimensional distance linking length of 73 kpc (i.e., 50 h ⁻¹ kpc), and another based on projected and radial linking lengths of 73 kpc and 1000 km s ⁻¹ . We refer to these as the position–position–position (PPP) and position–position–velocity (PPV) CG catalogs, respectively. The PPV catalog provides a direct analog to observed CG samples. At z = 0 in TNG300, we identify 383 PPP CGs and 1666 PPV CGs. A large fraction (∼80%) of PPV CGs are not physically compact systems but are contaminated by line-of-sight interlopers. We demonstrate that the scaling relation between total group stellar mass and velocity dispersion is an effective diagnostic for identifying false positives with line-of-sight interlopers. We further examine the large-scale environments of CGs and show that they reside in a wide range of densities, including the central regions of galaxy clusters. These CG catalogs provide a robust foundation for studying the formation and evolution of CGs in cosmological simulations.

We measure galaxy stellar mass functions (SMFs) for nine of the most massive galaxy clusters in the local Universe (0.07 < z < 0.11) using deep and complete spectroscopy from the MAssive Cluster Survey with Hectospec (MACH). We construct the cluster SMFs down to log ( M * / M ⊙ ) ≳ 8.5 . For comparison, we measure the SMF for field galaxies, complete to log ( M * / M ⊙ ) ≳ 10.5 , based on Sloan Digital Sky Survey spectroscopy over the same redshift range. The mean MACH SMF shows a shape similar to that of the field SMF but with a significantly higher amplitude at log ( M * / M ⊙ ) < 11.4 . At log ( M * / M ⊙ ) > 11.4 , the MACH SMF shows a clear excess, indicating the contribution of massive galaxies, including brightest cluster galaxies (BCGs). Based on homogeneous MACH spectroscopy, we compare SMF shapes for quiescent and star-forming members as a function of the cluster-centric distance. The quiescent SMFs display a curved shape with a peak at log ( M * / M ⊙ ) ≈ 10.5 ; the star-forming SMFs decline monotonically with increasing stellar mass. We further compare the mean MACH SMF with SMFs derived from similarly massive clusters in the IllustrisTNG-300 simulations. The shape of the observed and simulated SMFs agrees well overall. However, the MACH clusters contain roughly a factor of 2 more galaxies at 9.0 < log ( M * / M ⊙ ) < 10.5 . These results demonstrate that constructing cluster SMFs from complete spectroscopic samples can test simulations and provide powerful constraints on galaxy formation and evolution in dense environments.

The recently initiated SPHEREx and 7DS surveys will deliver low-resolution spectra ( R ∼ 20 − 130) for hundreds of millions of galaxies over the optical to near-infrared range (0.4 − 5.0 μm), covering a wide sky area without sample selection. These unique datasets will improve redshift estimation and provide a rich redshift catalog for the community. In this study, we forecast the performance of photometric redshift estimations using simulated SPHEREx and 7DS data. Four widely used template-fitting approaches and two machine-learning (ML) methods are used to derive photometric redshifts from low-resolution spectrophotometric data. We measured redshifts using mock catalogs based on the GAMA and COSMOS galaxy samples and achieved high precision for bright (13 < i < 18) galaxies, with σ _NMAD ≲ 0.005, bias ≲0.005, and a catastrophic failure rate ≲0.005 for all methods employed. We find that the combined SPHEREx + 7DS dataset significantly improves redshift estimation compared to using either the SPHEREx or 7DS datasets alone, highlighting the synergy between the two surveys. Moreover, we compare the redshift estimation performance across magnitude ranges for the different methods and examine the probability distribution functions (PDFs) produced by the template-fitting approaches. As a result, we identify some factors that can affect the redshift measurements, for example, treatments on dust extinction or inclusion of flux uncertainty in the ML model. We also show that the PDFs are relatively well calibrated, although the confidence intervals are generally underestimated, particularly for bright galaxies in the template-fitting methods. This study demonstrates the strong potential of SPHEREx and 7DS to deliver improved redshift measurements from low-resolution spectrophotometric data, underscoring the scientific value of jointly utilizing both datasets.

We investigated the stellar velocity dispersion functions (VDFs) of pressure-supported galaxies in the EAGLE cosmological simulations. The central stellar velocity dispersion is one of the fundamental dynamical tracers of the total mass of galaxy subhalos, alongside luminosity and stellar mass. Because it reflects the gravitational potential, the stellar velocity dispersion is expected to be relatively insensitive to feedback from active galactic nuclei (AGNs), a critical process that regulates the connection between other galaxy observables and subhalo masses. To examine the impact of AGN feedback, we analyzed the VDFs from five EAGLE simulation runs, each adopting a different AGN feedback model: one “standard”, two “enhanced”, one “reduced”, and one with no AGN feedback. We computed the stellar velocity dispersions of pressure-supported galaxies using member stellar particles, mimicking fiber spectroscopy. The VDFs from the standard and enhanced AGN feedback models show little difference. However, contrary to our initial expectation that the VDF shape would be largely insensitive to AGN feedback, the simulations with reduced and no AGN feedback show a significant excess of high velocity dispersion galaxies ( σ _* > 200 km s ⁻¹ ) and a deficit of low velocity dispersion galaxies (100 < σ _* (km s ⁻¹ ) < 200), compared to those with standard or enhanced AGN feedback. The presence of high velocity dispersion galaxies in the no-AGN model arises from enhanced central star formation, due to the absence of AGN-driven gas heating or expulsion. Our results demonstrate that the shape of the theoretical VDF is sensitive to the strength of AGN feedback. These predictions offer a theoretical benchmark for future observational studies of the galaxy VDF using large-scale spectroscopic surveys.

We derived structural parameters for a mass-limited sample ( M _★ > 10 ¹⁰ M _⊙ ) of 27 000 quiescent galaxies with 0.2 < z < 0.6 using grizy photometry from Subaru/Hyper Suprime-Cam and dense spectroscopy from the HectoMAP survey. Based on Sérsic profile fits in all five bands, we modeled the wavelength dependence of the circularized half-light radius ( R _e, c , a proxy for size) and Sérsic index ( n , a proxy for central concentration). We estimated the structural parameters in two rest frames: UV (3500 Å, tracing young and metal-poor stellar populations) and red (7000 Å, tracing the stellar mass distribution). Combined with the stellar mass, redshift, and D _n 4000, the estimates of R _e, c and n enable us to explore the evolution in the correlations between structural properties and stellar mass for quiescent galaxies with different stellar population ages. At intermediate redshift, quiescent galaxies at all stellar masses exhibit a systematic decline in size and a rise in central concentration with the rest-frame wavelength. Over the full redshift range of the sample, variations in the Sérsic index are larger for galaxies that recently joined the quiescent population (newcomers, with 1.5 < D _n 4000 < 1.6) than for descendants of galaxies that are already quiescent at the survey limit, z ∼ 0.6 (aging, or resident, population). Variations in size with the rest-frame wavelength are greater for newcomers than for the resident population, with z < 0.3. The combined evidence supports inside-out quenching as the dominant mechanism halting star formation during this epoch. The normalization of the size-stellar mass relation (the typical size of a M _★ ∼ 10 ¹¹ M _⊙ quiescent galaxy) increases by ∼30% between z ∼ 0.6 and z ∼ 0.2 in the red rest frame and remains constant in the rest-frame UV. Size growth is age-independent, and newcomers are found to be ∼20% larger than the aging population across redshifts and rest frames. Based on the Sérsic index-stellar mass relation in the UV, we find that M _★ ∼ 10 ¹¹ M _⊙ quiescent galaxies maintain a constant central concentration ( n ∼ 4 for the aging population and n ∼ 2 for newcomers). In the red rest frame, both subpopulations exhibit de Vaucouleurs profiles after accounting for selection effects. The larger sizes and disk-like UV profiles of newcomers link them to their direct progenitors in the star-forming population. For the aging population, the steady red rest-frame growth and elevated central concentration in both red and UV rest frames suggest minor mergers with progressively redder satellites at lower redshifts as the primary driver of galaxy evolution in the quiescent phase. Large samples of low-surface-brightness galaxies available in forthcoming sensitive large-area imaging surveys are ideal testbeds for this prediction.

We present the second catalog of bright quasars from the All-sky BRIght, Complete Quasar Survey (AllBRICQS), focusing on spectroscopically observed quasars in the Northern Hemisphere with Galactic latitude ∣b∣ > 10°. This catalog includes their spectral data, redshifts, and luminosities. AllBRICQS aims to identify the last remaining optically bright quasars using data from the Wide-field Infrared Survey Explorer and Gaia all-sky survey Data Release 3 (DR3). AllBRICQS searches for quasars that are brighter than B _P = 16.5 or R _P = 16 mag in Gaia DR3, based on simple selection criteria. Here, we report 62 new AllBRICQS quasars spanning various types, which include typical broad emission line quasars and the most luminous iron low-ionization broad absorption line quasars discovered to date. Spectroscopic observations were conducted using the Long-Slit Spectrograph on the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory, the Yunnan Faint Object Spectrograph and Camera on the 2.4 m telescope at Lijiang Observatory, and the Beijing Faint Object Spectrograph and Camera on the 2.16 m telescope at Xinglong Observatory. We applied flux calibration using Zwicky Transient Facility broadband photometry to correct for attenuation due to intermittent thin clouds during the observations. Redshifts were determined using inverse-variance weighted cross-correlation methods. Our targets span the bolometric luminosity range of 44.9 < log L bol / erg s − 1 < 48.0 at redshifts between 0.09 and 2.48. These confirmed AllBRICQS quasars provide a valuable resource for future research into quasar evolution, black holes, their environments, and their host galaxies across multiple wavelengths.

We introduce RVSNUpy, a new Python package designed to measure spectroscopic redshifts. Based on inverse-variance-weighted cross correlation, RVSNUpy determines redshifts by comparing observed spectra with various rest-frame template spectra. We test the performance of RVSNUpy based on ∼6000 objects in the HectoMAP redshift survey observed with both the Sloan Digital Sky Survey (SDSS) and MMT/Hectospec. We demonstrate that a slight redshift offset (∼40 km s⁻¹) between the SDSS and MMT/Hectospec measurements reported from previous studies results from the small offsets in the redshift template spectra used for SDSS and Hectospec reductions. We construct a universal set of template spectra, including empirical SDSS template spectra, carefully calibrated to the rest frame. Our test for the HectoMAP objects with duplicated observations shows that RVSNUpy with the universal template spectra yields homogeneous redshifts for spectra obtained with different spectrographs. We highlight that RVSNUpy is a powerful redshift measurement tool for current and future large-scale spectroscopy surveys, including A-SPEC, DESI, 4MOST, and the Subaru Prime Focus Spectrograph.

We introduce RVSNUpy, a new Python package designed to measure spectroscopic redshifts. Based on inverse-variance weighted cross-correlation, RVSNUpy determines the redshifts by comparing observed spectra with various rest-frame template spectra. We test the performance of RVSNUpy based on ~ 6000 objects in the HectoMAP redshift survey observed with both SDSS and MMT/Hectospec. We demonstrate that a slight redshift offset (~ 40 km/s) between SDSS and MMT/Hectospec measurements reported from previous studies results from the small offsets in the redshift template spectra used for SDSS and Hectospec reductions. We construct the universal set of template spectra, including empirical SDSS template spectra, carefully calibrated to the rest frame. Our test for the HectoMAP objects with duplicated observations shows that RVSNUpy with the universal template spectra yields the homogeneous redshift from the spectra obtained with different spectrographs. We highlight that RVSNUpy is a powerful redshift measurement tool for current and future large-scale spectroscopy surveys, including A-SPEC, DESI, 4MOST, and Subaru/PFS.

The HectoMAP spectroscopic survey provides a unique mass-limited sample of more than 35,000 quiescent galaxies (D _n 4000 > 1.5) covering the redshift range 0.2 < z < 0.6. We segregate galaxies in bins of properties based on stellar mass, D _n 4000, and redshift to construct a set of high signal-to-noise spectra representing a massive (M _* > 10¹⁰ M _⊙) quiescent population at intermediate redshift. These high-quality summed spectra enable full spectrum fitting and the related extraction of the average stellar population age and metallicity. The average galaxy age increases with the central D _n 4000 as expected. The correlation is essentially invariant with stellar mass; thus, D _n 4000 is a robust proxy for quiescent galaxy stellar population age. HectoMAP provides the first quiescent sample at intermediate redshift comparable with z ∼ 0 mass-complete data sets. Scaling relations derived from the HectoMAP summed spectra connect stellar age and metallicity with quiescent galaxy stellar mass up to z ∼ 0.5. Anticorrelation between the equivalent width (EW) of the [O ii] emission line and stellar age, together with the mild increase in stellar age with stellar mass, supports a broad range of timescales for the mass assembly of intermediate-redshift quiescent systems. On average, the most massive galaxies (M _* > 10¹¹ M _⊙) assemble the bulk of their stars at earlier epochs. A strong increase in the average stellar metallicity with stellar mass, along with the correlation between the [O ii] EW and metallicity at 0.2 < z < 0.4, suggests that lower mass galaxies are more likely to have experienced recent star formation episodes; related feedback from massive stars affects the chemical enrichment of these galaxies.

We perform an MMT/Hectospec redshift survey of the North Ecliptic Pole Wide (NEPW) field covering 5.4 deg² and use it to estimate the photometric redshifts for the sources without spectroscopic redshifts. By combining 2572 newly measured redshifts from our survey with existing data from the literature, we create a large sample of 4421 galaxies with spectroscopic redshifts in the NEPW field. Using this sample, we estimate photometric redshifts of 77,755 sources in the band-merged catalog of the NEPW field with a random forest model. The estimated photometric redshifts are generally consistent with the spectroscopic redshifts, with a dispersion of 0.028, an outlier fraction of 7.3%, and a bias of −0.01. We find that the standard deviation of the prediction from each decision tree in the random forest model can be used to infer the fraction of catastrophic outliers and the measurement uncertainties. We test various combinations of input observables, including colors and magnitude uncertainties, and find that the details of these various combinations do not change the prediction accuracy much. As a result, we provide a catalog of 77,755 sources in the NEPW field, which includes both spectroscopic and photometric redshifts up to z ∼ 2. This data set has significant legacy value for studies in the NEPW region, especially with upcoming space missions such as JWST, Euclid, and SPHEREx.