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The galaxy cluster PSZ2 G181.06+48.47 ($z=0.234$) is a post-merging system that exhibits symmetric double radio relics separated by ~2.7 Mpc. We present the first weak-lensing analysis of PSZ2 G181.06+48.47 and propose possible merging scenarios using numerical simulations. Our analysis with Subaru Hyper Suprime-Cam imaging identifies a binary dark matter structure consisting of northern and southern components, separated by ~500 kpc. Assuming Navarro-Frenk-White (NFW) halos, the masses for the northern and southern subclusters are $M_{200c}^{N} = 0.88_{-0.30}^{+0.35} \times 10^{14} M_{\odot}$ and $M_{200c}^{S} = 2.71_{-0.48}^{+0.51} \times 10^{14} M_{\odot}$, respectively. By superposing the two NFW halos, we determine the total mass of the cluster to be $M_{200c} = 4.22_{-1.00}^{+1.10} \times 10^{14} M_{\odot}$ ($M_{500c} = 2.90_{-0.69}^{+0.75} \times 10^{14} M_{\odot}$). Our mass estimate suggests that the two relics are located around the cluster $R_{200c}$, where the density of the intracluster medium is very low. Our idealized simulations find that an off-axis collision of a 3:1 major merger can simultaneously reproduce the observed relic and dark matter halo separations. From these findings, we suggest that the system is observed ~0.9 Gyr after the first pericenter passage and is returning from the first apocenter., Comment: Submitted to ApJ on Jan 12, 2025. 21 pages, 14 figures

We report upgraded Giant Metrewave radio telescope and Karl J. Jansky Very Large Array radio observations of a low-mass merging galaxy cluster PSZ2 G181.06+48.47. This exceptional galaxy cluster hosts two megaparsec-scale diffuse sources, symmetrically located with respect to the cluster center and separated by about 2.6 Mpc. We detect these low surface brightness sources in our new high-frequency observations (0.3-2 GHz) and classify them as radio relics associated with merger-driven shock fronts. The southwest relic exhibits an inverted morphology and shows evidence of spectral steepening in the post-shock region, potentially tracing a high Mach number shock ($\sim 4$) under the framework of diffusive shock acceleration. The northeast relic is found to be highly polarized with a 22% average polarization fraction at 1.5 GHz and aligned magnetic field vectors. Its spectral and polarization properties, along with the presence of a nearby tailed galaxy, support re-acceleration scenarios. The merger axis defined by the two relics is tilted by $\sim 45$ degree with respect to the plane of the sky, which implies an unprecedented projected separation of $\sim 3.5$ Mpc. We also detect a possible faint radio halo, suggesting weak turbulence in the central cluster region. We conclude that the faint double relics can be best explained by two outward moving shock waves in which particles are (re-)accelerated and that the cluster is in an evolved merger state. PSZ2 G181.06+48.47 presents a unique opportunity to investigate particle acceleration in low mass systems characterized by large relic separations., Comment: 23 pages, 11 figures, Submitted to ApJ

Relics are diffuse, highly-polarized radio sources that trace merger-driven shocks at the periphery of merging galaxy clusters. The LOFAR survey recently discovered a rare example of double relics in the low-mass cluster PSZ2 G181.06+48.47. Through a detailed exploration of new Chandra and XMM-Newton observations, we reveal that PSZ2 G181.06+48.47 has a lower mass ($M_{500,X}=2.57^{+0.37}_{-0.38}\times10^{14}$ M$_{\odot}$) than previously thought. Despite its cool global temperature of $kT_{500}=3.86^{+0.27}_{-0.29}$ keV, PSZ2 G181.06+48.47 is one of the most disturbed clusters in the Planck sample, with a complex morphological and thermodynamic structure. We discover a set of three discontinuities within <500 kpc of the cluster center, and, from a surface brightness analysis, place $5\sigma$ upper limits of $M_{NE}<1.43$ and $M_{SW}<1.57$ for any shock associated with the relic locations. We also revise established scaling relations for double radio-relics by adding 12 new systems not included in previous work. The PSZ2 G181.06+48.47 relics have the widest separation (scaled for $r_{500}$) of all known double-relic systems. The exceptional distance from the cluster center ($>r_{200}$), indicates the relics may be associated with shocks in the ``run-away" phase. We propose that this late-stage, post-apocenter merger is captured as the two subclusters with a mass ratio of 1.2-1.4 fall back into each other. The outer relic shocks were likely produced at the first core passage, while the inner discontinuities are associated with the second infall., Comment: Submitted to ApJ on Jan 12, 2025. 29 pages, 11 figures. Companion papers discussing the radio properties and the weak-lensing reconstruction will appear on ArXiv in the next days

Type Ia supernovae (SNe Ia) serve as the most crucial standardizable candles in cosmology, providing direct measurements of the universe's expansion history. However, it is well-known that the post-standardization brightness of SNe Ia is influenced by the properties of their host galaxies, such as mass and star formation rate, both of which are closely related to progenitor age. In this study, we reaffirm the ubiquitous and robust correlation between SN Ia luminosity and host age, showing that this host property dependence arises primarily from stellar population age of the host galaxy. This analysis was conducted using an expanded sample of over 300 hosts across a broad redshift range up to $z \sim 0.4$, ensuring sufficient statistical significance of the result. To quantify the relationship between host age and Hubble residual (HR), we employed two linear regression techniques: LINMIX, which assumes a Gaussian error, and Bayesian hierarchical linear regression, which utilizes a full posterior for the age error. Both models demonstrate a robust correlation between host age and HR, with high statistical significance approaching $5.5 \sigma$. While our new regression analyses yield the slopes that are similar or slightly shallower compared to our previous results, the significance of these slopes has notably increased. These findings robustly validate our previous suggestions that post-standardization SN Ia luminosity varies with progenitor age, which is currently not properly accounted for in SN cosmology., Comment: 13 pages, 10 figures, 2 tables. Submitted to ApJ

We present a multiwavelength analysis of 29 merging galaxy clusters that exhibit radio relics. For each merging system, we perform a weak-lensing analysis on Subaru optical imaging. We generate high-resolution mass maps of the dark matter distributions, which are critical for discerning the merging constituents. Combining the weak-lensing detections with X-ray emission, radio emission, and galaxy redshifts, we discuss the formation of radio relics from the past collision. For each subcluster, we obtain mass estimates by fitting a multi-component NFW model with and without a concentration-mass relation. Comparing the two mass estimate techniques, we find that the concentration-mass relation underestimates (overestimates) the mass relative to fitting both parameters for high- (low-) mass subclusters. We compare the mass estimates of each subcluster to their velocity dispersion measurements and find that they preferentially lie below the expected velocity dispersion scaling relation, especially at the low-mass end (~$10^{14}\ M_\odot$). We show that the majority of the clusters that exhibit radio relics are in major mergers with a mass ratio below 1:4. We investigate the position of the mass peak relative to the galaxy luminosity peak, number density peak, and BCG locations and find that the BCG tends to better trace the mass peak position. Finally, we update a golden sample of 8 galaxy clusters that have the simplest geometries and can provide the cleanest picture of the past merger, which we recommend for further investigation to constrain the nature of dark matter and the acceleration process that leads to radio relics., Comment: 55 pages, 36 figures, submitted to ApJS

The galaxy cluster Abell 746 (A746; $z$=0.214), featuring a double radio relic system, two isolated radio relics, a possible radio halo, disturbed V-shaped X-ray emission, and intricate galaxy distributions, is a unique and complex merging system. We present a weak-lensing analysis of A746 based on wide-field imaging data from Subaru/Hyper Suprime-Cam observations. The mass distribution is characterized by a main peak which coincides with the center of the X-ray emission. At this main peak, we detect two extensions toward the north and west, tracing the cluster galaxy and X-ray distributions. Despite the ongoing merger, our estimate of the A746 global mass $M_{500}=4.4\pm1.0\times10^{14}~M_{\odot}$ is consistent with the previous results from SZ and X-ray observations. We conclude that reconciling the distributions of mass, galaxies, and intracluster medium with the double radio relic system and other radio features remains challenging., Comment: Published in ApJ

It is well established that the Hubble residuals of type Ia supernovae (SNe Ia) show the luminosity step with respect to their host galaxy stellar masses. This `mass-step' is taken as an additional correction factor for the SN Ia luminosity standardization. Here we investigate the root cause of the mass-step and propose that the bimodal nature of the host $age$ distribution is responsible for the step. In particular, by using the empirical $nonlinear$ mass-to-age relation of local galaxies, we convert the mass function of SN Ia hosts to their age distribution. We find that the age distribution shows clear bimodality: a younger ($<$ 6 Gyr) group with lower mass ($\sim 10^{9.5}{\rm M}_{\rm sun}$) and an older ($>$ 6 Gyr) group with higher mass ($\sim 10^{10.5}{\rm M}_{\rm sun}$). On the Hubble residual versus host mass plane, the two groups create the mass-step at $\sim 10^{10}{\rm M}_{\rm sun}$. This leads us to conclude that the host galaxy mass-step can be attributed to the bimodal age distribution in relation to a nonlinear relation between galaxy mass and age. We suggest that the mass-step is another manifestation of the old `red sequence' and the young `blue cloud' observed in the galactic color--magnitude diagram., Comment: Accepted for publication in ApJ, 10 pages, 5 figures, 1 table

Our concordance cosmological model predicts that galaxy clusters grow at the intersection of filaments structuring the cosmic web stretching tens of Mega parsecs. Although this hypothesis has been supported by the baryonic components, no observational study has detected the dark matter component of the intracluster filaments (ICFs), the terminal segment of the large-scale cosmic filaments at their conjunction with individual clusters. We report weak-lensing detection of ICFs in the Coma cluster field from the ~12 sq. deg Hyper Suprime-Cam imaging data. The detection is based on two methods: matched-filter technique and shear-peak statistic. The matched-filter technique (shear-peak statistic) yields detection significances of 6.6- (3.1) $\sigma$ and 3.6- (2.8) $\sigma$ for the northern and western ICFs at 110$^{\circ}$ and 340$^{\circ}$, respectively. Both ICFs are highly correlated with the overdensities in the WL mass reconstruction and are well-aligned with the known large-scale ($>10$ Mpc) cosmic filaments comprising the Coma supercluster., Comment: Accepted for publication by Nature Astronomy (2024). The current submission will differ from the published one

A dissociative merger is formed by the interplay of ram pressure and gravitational forces, which can lead to a spatial displacement of the dark matter and baryonic components of the recently collided subclusters. CIZA J0107.7+5408 is a nearby (z=0.105) dissociative merger that hosts two X-ray brightness peaks and a bimodal galaxy distribution. Analyzing MMT/Hectospec observations, we investigate the line-of-sight and spatial distribution of cluster galaxies. Utilizing deep, high-resolution Hubble Space Telescope Advanced Camera for Surveys imaging and large field-of-view Subaru Hyper-Suprime-Cam observations, we perform a weak-lensing analysis of CIZA J0107.7+5408. Our weak-lensing analysis detects a bimodal mass distribution that is spatially consistent with the cluster galaxies but significantly offset from the X-ray brightness peaks. Fitting two NFW halos to the lensing signal, we find an equal-mass merger with subcluster masses of $M_{200,NE}=2.8^{+1.1}_{-1.1}\times10^{14}$ M$_\odot$ and $M_{200,SW}=3.1^{+1.2}_{-1.2}\times10^{14}$ M$_\odot$. Moreover, the mass-to-light ratios of the subclusters, $(M/L)_{NE}=571^{+89}_{-91}$ $M_\odot/L_{\odot,B}$ and $(M/L)_{SW}=564^{+87}_{-89}$ $M_\odot/L_{\odot,B}$, are found to be consistent with each other and within the range of mass-to-light ratios found for galaxy clusters., Comment: 15 pages, 10 figures, accepted to ApJ

We present a multiwavelength study of the double radio relic cluster A1240 at z=0.195. Our Subaru-based weak lensing analysis detects three mass clumps forming a ~4 Mpc filamentary structure elongated in the north-south orientation. The northern ($M_{200}=2.61_{-0.60}^{+0.51}\times10^{14} M_{\odot}$) and middle ($M_{200}=1.09_{-0.43}^{+0.34}\times10^{14} M_{\odot}$) mass clumps separated by ~1.3 Mpc are associated with A1240 and co-located with the X-ray peaks and cluster galaxy overdensities revealed by Chandra and MMT/Hectospec observations, respectively. The southern mass clump ($M_{200}=1.78_{-0.55}^{+0.44}\times10^{14} M_{\odot}$), ~1.5 Mpc to the south of the middle clump, coincides with the galaxy overdensity in A1237, the A1240 companion cluster at z=0.194. Considering the positions, orientations, and polarization fractions of the double radio relics measured by the LOFAR study, we suggest that A1240 is a post-merger binary system in the returning phase with the time-since-collision ~1.7 Gyr. With the SDSS DR16 data analysis, we also find that A1240 is embedded in the much larger-scale (~80 Mpc) filamentary structure whose orientation is in remarkable agreement with the hypothesized merger axis of A1240., Comment: Replaced with version accepted for publication in ApJ, including minor revisions after reviewer's report. 23 pages, 14 figures, 2 tables

We present an evolutionary population synthesis model of the surface brightness fluctuation (SBF) for normal and He-enriched simple stellar populations (SSPs). While our SBF model for the normal-He population agrees with other existing models, the He-rich population, containing hotter horizontal-branch stars and brighter red-clump stars than the normal-He population, entails a substantial change in the SBF of SSPs. We show that the SBF magnitudes are affected by He-rich populations at least $\sim$0.3~mag even in $I$- and near-IR bands at given colors, from which the SBF-based distances are often derived. Due to uncertainties both in observations and models, however, the SBFs of Galactic globular clusters and early-type galaxies do not allow verifying the He-enriched model. We propose that when combined with independent metallicity and age indicators such as ${\rm Mg}_2$ and ${\rm H}\beta$, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems at a distance out to $\gtrsim 20$\,Mpc. A full set of the spectro-photometric and SBF data for SSPs from the Yonsei Evolutionary Population Synthesis (YEPS) model is available for download at http://cosmic.yonsei.ac.kr/YEPS.htm., Comment: 24 pages, 14 figures, 4 tables, Accepted for publication in ApJS

We present new HST optical and near-infrared (NIR) photometry of the rich globular cluster (GC) system of NGC 4874, the cD galaxy in the core of the Coma cluster (Abell 1656). NGC 4874 was observed with the HST Advanced Camera for Surveys in the F475W (g) and F814W (I) passbands and the Wide Field Camera 3 IR Channel in F160W (H). The GCs in this field exhibit a bimodal optical color distribution with more than half of the GCs falling on the red side at g-I > 1. Bimodality is also present, though less conspicuously, in the optical-NIR I-H color. Consistent with past work, we find evidence for nonlinearity in the g-I versus I-H color-color relation. Our results thus underscore the need for understanding the detailed form of the color-metallicity relations in interpreting observational data on GC bimodality. We also find a very strong color-magnitude trend, or "blue tilt," for the blue component of the optical color distribution of the NGC 4874 GC system. A similarly strong trend is present for the overall mean I-H color as a function of magnitude; for M_814 < -10 mag, these trends imply a steep mass-metallicity scaling with $Z\propto M_{\rm GC}^{1.4\pm0.4}$, but the scaling is not a simple power law and becomes much weaker at lower masses. As in other similar systems, the spatial distribution of the blue GCs is more extended than that of the red GCs, partly because of blue GCs associated with surrounding cluster galaxies. In addition, the center of the GC system is displaced by 4+/-1 kpc towards the southwest from the luminosity center of NGC 4874, in the direction of NGC 4872. Finally, we remark on a dwarf elliptical galaxy with a noticeably asymmetrical GC distribution. Interestingly, this dwarf has a velocity of nearly -3000 km/s with respect to NGC 4874; we suggest it is on its first infall into the cluster core and is undergoing stripping of its GC system by the cluster potential., Comment: 24 pages, 20 figures, accepted for publication in ApJ

We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance method for the F110W (J) and F160W (H) bandpasses of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope. The calibrations are based on data for 16 early-type galaxies in the Virgo and Fornax clusters observed with WFC3/IR and are provided as functions of both the optical (g-z) and near-infrared (J-H) colors. The scatter about the linear calibration relations for the luminous red galaxies in the sample is approximately 0.10 mag, corresponding to a statistical error of 5% in distance. Our results imply that the distance to any suitably bright elliptical galaxy can be measured with this precision out to about 80 Mpc in a single-orbit observation with WFC3/IR, making this a remarkably powerful instrument for extragalactic distances. The calibration sample also includes much bluer and lower-luminosity galaxies than previously used for IR SBF studies, revealing interesting population differences that cause the calibration scatter to increase for dwarf galaxies., Comment: 19 pages, 11 figures; version accepted for publication in the Astrophysical Journal; revised version contains a number of clarifications and additional references

We combine new Wide Field Camera~3 IR Channel (WFC3/IR) F160W (H) imaging data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W (g), F606W (V), F814W (I), and F850LP (z) optical data from the Advanced Camera for Surveys (ACS). The purely optical g-I, V-I, and g-z colors of NGC1399's rich globular cluster (GC) system exhibit clear bimodality, at least for magnitudes $I_814 > 21.5$. The optical-IR I-H color distribution appears unimodal, and this impression is confirmed by mixture modeling analysis. The V-H colors show marginal evidence for bimodality, consistent with bimodality in V-I and unimodality in I-H. If bimodality is imposed for I-H with a double Gaussian model, the preferred blue/red split differs from that for optical colors; these "differing bimodalities" mean that the optical and optical-IR colors cannot both be linearly proportional to metallicity. Consistent with the differing color distributions, the dependence of I-H on g-I for the matched GC sample is significantly nonlinear, with an inflection point near the trough in the g-I color distribution; the result is similar for the I-H dependence on g-z colors taken from the ACS Fornax Cluster Survey. These g-z colors have been calibrated empirically against metallicity; applying this calibration yields a continuous, skewed, but single-peaked metallicity distribution. Taken together, these results indicate that nonlinear color-metallicity relations play an important role in shaping the observed bimodal distributions of optical colors in extragalactic GC systems., Comment: 15 pages, 12 figures, accepted for publication in the Astrophysical Journal