Searcharxiv⌕ Search

arXiv subjects

Naomi Ota

Publications and source records attributed to Naomi Ota.

At least 19 recordsLinked to original sources
arXiv2026

Weak-lensing Shear-Selected Galaxy Clusters from the Hyper Suprime-Cam Subaru Strategic Program: III. A precision cosmological sample enabled by optical confirmation

We develop fCAMIRA (forced-mode CAMIRA), a tool for optical cluster confirmation, and apply it to a sample of 129 weak-lensing (WL) shear-selected galaxy clusters identified in aperture-mass maps obtained from the Hyper Suprime-Cam Subaru Strategic Program Three-Year (HSC-SSP Y3) weak-lensing data. fCAMIRA is built upon the CAMIRA cluster-finding algorithm and relies on a red-sequence (RS) galaxy model that is calibrated in a data-driven way. The RS model adopts the metallicity-luminosity relation measured in this work using X-ray-selected clusters up to redshift $z\approx1.3$, followed by the calibration of color offsets using large spectroscopic samples. With the RS model, we build two types of galaxy richness maps, one obtained with a spatial filter matched to a typical cluster size of $R=0.8\,h^{-1}\,\mathrm{Mpc}$ and the other obtained with a fixed angular-size filter identical to that used in constructing the WL aperture-mass maps. The fCAMIRA algorithm utilizes these two richness maps, identifies all optical counterpart candidates along the line of sight of each shear-selected cluster, and measures the cluster photometric redshift from the highest-ranked counterpart. The ranking is determined by the fractional lensing contribution $f_{\mathrm{lens}}$ of each candidate. Using available spectroscopic cluster redshifts, we quantify the mean bias and scatter in the cluster photometric redshifts at levels of approximately 0.005 and 0.008, respectively, demonstrating excellent photometric-redshift performance. We compare the fCAMIRA photometric redshifts with estimates from direct positional cross-matching and find that approximately 8% of the total sample exhibits redshift discrepancies greater than 0.15. This outlier fraction is primarily attributed to projection effects, leading to the misidentification of the optical counterparts. (abridged)

astro-ph.CO↗
arXiv2026

Exploring the dynamics of the Coma galaxy cluster by mapping its X-ray emission line profiles with XRISM

The intracluster medium (ICM) in merging galaxy clusters exhibits turbulence and bulk flows. Unraveling these components is crucial not only for elucidating the geometry of the cluster mergers, but also for understanding the physics of magnetic field amplification and relativistic particle acceleration. XRISM/Resolve data for two $3'\times3'$ fields in the core of the Coma cluster reveales that the ICM in the central field moves with $Δcz = -430$~km~s$^{-1}$ relative to the cluster galaxy average, while that in the southern field moves with $Δcz = -730$~km~s$^{-1}$ (see \cite{2025ApJ...985L..20X}, hereinafter ``Paper I''). In this paper, we perform a more detailed analysis of these data sets to search for non-Gaussian features in the Fe-K line complex profiles. In the spectra from the northwest (NW) quadrant of the central field, in addition to the main and redshifted ICM components ($Δcz = -40$ km s$^{-1}$) reported in Paper I, we find evidence of another, blueshifted component, moving with $Δcz = -1250$ km s$^{-1}$. For a systematic search for other significant velocity components, we perform a bias-free 3 eV step multi-component fit to the Resolve full-array spectra from the central and southern fields. This search uncovers another redshifted component in the southern field, moving with $Δcz \sim +1230$ km s$^{-1}$. We estimate the energy densities of the ICM turbulence and bulk motion to be similar to each other and several times greater than the energy density of the cluster's $B\sim 5~μ$G magnetic field.

astro-ph.HE↗
arXiv2026

Non-Thermal Pressure due to Gas Motions in the Intracluster Medium: Confronting XRISM/Resolve with TNG-Cluster Simulations

Intracluster medium (ICM) gas motions probe cluster assembly, feedback, and non-thermal pressure support, but recent XRISM observations reveal velocity dispersions and non-thermal pressure fractions systematically lower than simulations predict, with extreme systems such as Abell 2029 falling below nearly all simulated clusters. Using the TNG-Cluster simulations, we show that the non-thermal pressure fraction depends sensitively on cool-core state and formation history, and provide a two-scale fitting function capturing both the inner cool-core suppression and outer rise of the radial profile. By forward-modeling mock XRISM observations and comparing them with both projected and intrinsic three-dimensional quantities, we find that azimuthal variations and projection effects contribute to the deficit in the observed velocity dispersion and non-thermal pressure fraction. This bias increases with radius and partially offsets the intrinsic outward rise in the true three-dimensional non-thermal pressure fraction. However, these effects cannot explain the extremely low values of the non-thermal pressure fraction observed in Abell 2029, which fall below approximately the 0th-6th percentiles of the simulated cool-core cluster distribution at every measured radius under both the turbulence-only and turbulence-plus-bulk definitions. The remaining tension points to rare dynamical conditions or missing physics affecting the amplitude of gas motions in current ICM models.

astro-ph.CO↗
arXiv2026

Kinetic structure of the intracluster medium across nearby clusters observed with XRISM

XRISM/Resolve is building a sample of galaxy clusters with directly measured ICM gas motions, revealing diverse projected dynamical states. We compile 45 XRISM/Resolve measurements in 19 nearby galaxy clusters and place them on a common, emission-weighted effective line-of-sight scale, $\ell_{\rm eff}$. We compare the line-of-sight velocity dispersion $σ_v$, bulk velocity amplitude $|v_{\rm bulk}|$, their ratio $R_v \equiv |v_{\rm bulk}|/σ_v$, and non-thermal pressure proxies. Disturbed non-cool-core systems are not simply higher-dispersion counterparts of relaxed cool-core regions. Instead, differences among cool-core centers, cool-core outer regions, and non-cool-core systems are driven mainly by coherent line-of-sight motion relative to unresolved line broadening: $R_v$ tends to remain below unity in cool-core regions but often exceeds unity in non-cool-core systems, with the mean $R_v$ rising from 0.45 in cool-core centers to 1.6 in non-cool-core systems. These diagnostics help separate local central line broadening, likely associated with AGN feedback in some cool cores, from larger-scale coherent motions associated with sloshing, mergers, and halo assembly. Comparison with forward-modeled TNG-Cluster predictions suggests that many cool-core measurements occupy the lower part of the predicted non-thermal pressure range, consistent with small hydrostatic-mass corrections in relaxed systems and larger corrections in disturbed ones. XRISM is thus beginning to resolve the projected kinetic structure of the ICM across cluster environments, rather than tracing a single sequence of increasing turbulence.

astro-ph.CO↗
arXiv2026

XRISM reveals sloshing-driven gas motions in the core of Abell 2029

We investigate the velocity structure of the intracluster medium (ICM) in the core of the relaxed cool-core cluster Abell 2029 using XRISM Resolve spectroscopy. We analyze combined XRISM Resolve observations and divide the central region into several subregions. To account for photon mixing caused by the XRISM point spread function, we perform a spatial-spectral mixing analysis. We detect an ordered line-of-sight bulk-velocity gradient across the cluster core: the northern regions are blueshifted relative to the brightest cluster galaxy (BCG), while the southern regions are close to zero velocity or slightly redshifted. The maximum velocity difference is about $280~{\rm km\,s^{-1}}$. In contrast, the turbulent velocity dispersion is smaller, with measured values and upper limits of $\lesssim150~{\rm km\,s^{-1}}$, implying a non-thermal pressure fraction below $\sim2.5\%$. The velocity pattern is consistent with gas sloshing associated with the spiral structure seen in Chandra X-ray images. Averaged over all regions, the inferred turbulent heating rate is below the radiative cooling rate, indicating that turbulent dissipation alone is insufficient to offset cooling in the entire core. These results reveal that A2029 is not kinematically featureless: sloshing-induced bulk motions are present, while the observed line-of-sight velocity dispersion indicates only a limited contribution to pressure support and core heating.

astro-ph.HE↗
arXiv2026

Chemical enrichment of the Perseus cluster core seen by XRISM/Resolve

The intracluster medium (ICM) is rich in chemical elements, produced by core-collapse (SNcc) and Type Ia supernovae (SNIa) over the last $\sim$12 Gyr. Whereas cluster outskirts are uniformly enriched with Fe at $\sim$0.3 solar - strongly suggesting that the gas had been pre-enriched during or before the assembly of galaxies into clusters, the Fe abundance is known to centrally increase in the core of relaxed clusters. The origin of these central Fe peaks however, as well as the apparent presence of mysterious drops previously reported in the very centre of a number of systems, remain to be clarified. In this paper, we address these two questions by measuring the spatial distribution of Fe and its relative Si/Fe, S/Fe, Ar/Fe, Ca/Fe, Cr/Fe, Mn/Fe, and Ni/Fe ratios in the X-ray bright, nearby Perseus cluster. We take advantage of the unprecedented spectral resolution ($\sim$5 eV) offered by the Resolve microcalorimeter on board XRISM, which observed four distinct pointings of Perseus out to $\sim$250 kpc ($\sim$0.2$r_{500}$) during its Performance Verification phase. Although the presence of an X-ray bright AGN challenges a precise quantification of absolute abundances in the very core, our baseline analysis rules out a strong drop with $>$2$σ$ confidence, at variance with previous CCD measurements. In addition, we find a remarkable spatial uniformity of X/Fe ratios, supporting the idea of negligible late SNIa enrichment from the brightest cluster galaxy NGC 1275. We also compare the overall chemical composition of the Perseus ICM with SNcc and SNIa nucleosynthesis yield models, finding that the co-existence of two separate SNIa enrichment channels is not needed to reproduce the ICM ratios satisfactorily.

astro-ph.GA↗
arXiv2026

The Quiescent Sloshing Core of Abell 496 with XRISM

Gas motions provide insight into the dynamical history and physical processes within galaxy clusters. We investigate the kinematics of the ICM in the core of A496, a nearby, X-ray bright, strong cool-core cluster, using high-resolution data from the Resolve micro-calorimeter on board XRISM. We compared our measurement with other Resolve cluster core measurements and further compared our results with simulations and multiwavelength observations. From an optical redshift analysis, we found that the BCG is at rest with respect to the systemic velocity of the cluster. Despite multiple previously detected cold fronts and harboring a weak central radio source, Resolve observation shows that the core of A496 is dynamically quiescent. The ICM is moving with respect to the BCG with a LOS bulk velocity of $v_{\rm bulk}=-69_{-20}^{+25}\,\mathrm{km\,s}^{-1}$. We measured a turbulent velocity of $σ_{\rm v}=78_{-16}^{+18}\,\mathrm{km\,s}^{-1}$, the lowest value reported by the instrument on a cluster core to date. This value is in good agreement with the velocity dispersion of the H$α$ filament in the core, which may indicate condensation of ICM in the wake of the radio bubble. Assuming isotropic turbulence, the ICM turbulent velocity corresponds to a subsonic 3D Mach number of $0.15_{-0.03}^{+0.04}$ and a non-thermal pressure fraction of $1.2_{-0.5}^{+0.6}\,\%$. The mechanical AGN feedback from the recent activity of the central radio source is estimated to contribute about 7-9% to the ICM heating. The 1D LOS bulk velocity from the SLOW constrained Universe simulation is consistent with the measured value, suggesting that AGN feedback has a negligible contribution. The A496 SLOW turbulent velocity, as in other reported Resolve--simulation comparisons, is higher, but remains within $1.5σ$ uncertainty. A496 may represent one of the most quiescent sloshing cores observed so far.

astro-ph.CO↗
arXiv2026

XRISM Observations of Abell 1795: Evidence for Low Turbulence and Resonant Scattering

We present high-resolution X-ray spectroscopic observations of the cool-core galaxy cluster Abell~1795 obtained with XRISM/Resolve. The cluster was observed with two deep pointings: a 225 ks central exposure and a 113 ks northern exposure, extending to a projected radius of 320 kpc from the cluster center. Single-temperature fits reveal a clear radial gradient in the line-of-sight velocity dispersion, decreasing from 114 $\pm$ 11 km/s in the core to 68 $\pm$ 39 km/s at 320 kpc. The bulk velocities in the central regions are very low (22 $\pm$ 12 and 7 $\pm$ 21 km/s), indicating no significant relative motion between the brightest cluster galaxy (BCG) and the intracluster medium (ICM). Given that the central region includes the southward extending cool gas tail, this result disfavors the ``cooling-wake'' scenario and instead supports an AGN-uplift origin. We find that the nonthermal pressure fraction decreases with radius, from $P_{\rm NT}/P_{\rm T}\approx2\%$ in the core to $\sim0.6\%$ at 330 kpc, suggesting that the northern ICM of A1795 is largely quiescent. Two-temperature and split energy-band (2--4 keV and 6--7 keV) fits identify two gas phases within the central $<1.5'$ region, providing strong evidence for multiphase gas in the cluster core. We detect a $\sim14\%$ resonant suppression of the optically thick Fe XXV $w$ line in the center. Additionally, we observe a significant excess in the Fe XXV $y$ line-flux relative to models. Accounting for uncertainties in the atomic data reduces this discrepancy, suggesting that atomic data uncertainties may contribute to the observed residual flux.

astro-ph.CO↗
arXiv2026

Bulk and turbulent gas motions in the interacting galaxy cluster Abell 3395 South observed with XRISM

We investigate the gas motions in the core region of the Abell~3395 South subcluster (A3395S) using high-resolution X-ray spectroscopy with XRISM/Resolve. By analyzing the Fe~XXV He$α$ emission line, we directly measure the line-of-sight bulk and turbulent velocities of the intracluster medium. We find that the one-dimensional turbulent velocity is low, at the level of $124\pm21~{\rm km\,s^{-1}}$, while a significant line-of-sight bulk velocity of $263\pm23~{\rm km\,s^{-1}}$ is detected. The coexistence of low turbulence and finite bulk motion suggests that A3395S has not yet reached a dynamically relaxed state. These results are consistent with the non-detection of a radio halo in A3395S, implying that turbulent particle reacceleration is currently inefficient in the cluster core. This study demonstrates that high-resolution X-ray spectroscopy with XRISM provides a powerful means to directly constrain intracluster medium dynamics in merging galaxy clusters, and it provides a reference for future comparative studies of A3395N and A3391 within the same large-scale structure.

astro-ph.CO↗
arXiv2026

Probable Detection of a Cooler Gas Component in the Perseus Cluster with XRISM

We present an analysis of the temperature structure of the Perseus cluster atmosphere using XRISM Resolve observations. The average temperature rises from 3.3 keV near the nucleus of NGC 1275 to 8 keV at 10 arcmin (210 kpc), which is consistent with Chandra and XMM measurements. The velocity and velocity dispersion profiles are broadly consistent with those in arXiv:2509.04421. While the gas at altitudes beyond $\sim60$ kpc can be modeled as a single temperature plasma, we find evidence for more than one gas phase in the inner $\sim60$ kpc. The hotter gas component, traced primarily by the Fe He$α$ line, has a velocity dispersion of $\lesssim140$ km s$^{-1}$. We detect a cooler, $\sim1.87-2.43$ keV, gas component with a velocity dispersion of $\sim300-400$ km s$^{-1}$ and a bulk velocity of $\sim 21-213$ km s$^{-1}$ with respect to the central galaxy. These ranges reflect large systematic uncertainties associated with modeling spatial-spectral mixing and the bright central point source. Potential low energy gain variations may add additional uncertainties. The cooler component is identified by broad wings in prominent emission lines, most notably S Ly$α$ and Fe He$α$. This cooler component's Mach number $\sim0.73-0.96$ and non-thermal pressure fraction of $\sim22.9-33.7\%$ are much higher than found for the hotter gas. The cooler gas may be associated with merging halos along the line of sight which formed the cool, sloshing spiral and/or cooling gas being disturbed by the radio jets and lobes.

astro-ph.HE↗
arXiv2026

A Fast, Hot Wind from a Nuclear Starburst

Galaxies with intense star formation often host multiphase, galaxy-scale winds powered by supernovae and fast stellar winds. These are strong enough to disrupt the star-forming interstellar medium, and they chemically enrich the surrounding circumgalactic medium. However, their launching mechanism remains unknown. Here we show that thermal gas pressure is sufficient to drive the multiphase wind in the prototypical starburst galaxy M82. Using a high energy-resolution ($ΔE = 4.5$ eV) XRISM Resolve spectrum, including detections of FeXXV 6.7 keV, ArXVII 3.1 keV, and SXVI 2.6 keV, we measure the temperature ($T = 2.3^{+0.5}_{-0.2} \times 10^7$ K) and mass ($M \approx 6 \pm 2 \times 10^5$ M$_\odot$) of the hot gas in the starburst and provide the first direct measurement of its line-of-sight velocity dispersion ($σ= 595^{+464}_{-128}$ km s$^{-1}$). These values are consistent with a freely-expanding wind exceeding the galactic escape velocity. The size of the FeXXV-emitting region suggests a hot gas outflow rate of $\dot{M} \approx 4$ M$_\odot$ yr$^{-1}$, carrying a total energy of $\dot{E} \approx 2 \times 10^{42}$ erg s$^{-1}$. This is sufficient to drive the molecular, atomic, and ionized outflows while transporting up to $\approx 2$ M$_\odot$ yr$^{-1}$ of hot gas to the intergalactic medium. The estimated supernova rate implies that $\approx$ 60% of the supernova energy must be thermalized in hot gas. Our results suggest that additional driving mechanisms, such as cosmic-ray pressure, are not required to launch the wind.

astro-ph.GA↗
arXiv2026

XMM-Newton observations of ten high-redshift CAMIRA clusters of galaxies

We present results from XMM-Newton observations of ten high-redshift ($0.81 < z < 1.17$) galaxy clusters selected from the CAMIRA catalog based on high richness ($N > 40$). These massive clusters, identified in the Hyper Suprime-Cam Subaru Strategic Program field, provide an ideal sample for probing the dynamical state of the intracluster medium (ICM) in the early Universe. We performed uniform X-ray imaging and spectral analyses to measure the ICM temperature and bolometric luminosity, and investigated cluster morphology through offsets between the brightest cluster galaxy (BCG) and the X-ray peak. Extended X-ray emission was detected from all targets, but only one system was classified as dynamically relaxed, indicating a low relaxed fraction ($\sim 10\%$) at high redshift. By combining this high-$z$ sample with a lower-redshift CAMIRA cluster sample, we derived scaling relations among richness, temperature, luminosity, and mass. The results are broadly consistent with predictions from both the self-similar model and the baseline model incorporating the mass--concentration relation. We find no significant redshift evolution, strengthening the view that cluster scaling relations are largely established by $z \sim 1$. We also examined the AGN fraction among member galaxies and found significantly higher AGN activity in high-redshift clusters, particularly in the outskirts, suggesting enhanced AGN triggering during early cluster assembly and a possible connection to the thermodynamic state of dynamically young clusters. These findings provide new insights into the formation and evolution of massive clusters and the thermodynamic history of the ICM, and complement large-area X-ray surveys such as eROSITA.

astro-ph.CO↗
arXiv2026

High Spectral Resolution X-ray Observations of the Evolved Supermassive Stellar Binary System $η$ Carinae - Iron K$α$ Band Profile Revealed with XRISM

The supermassive binary system, $η$ Carinae, is experiencing enormous wind-driven mass loss at a rate unparalleled in the rest of the Galaxy. Their wind-wind collision (WWC) continuously produces shock heated, X-ray emitting plasmas. The XRISM X-ray observatory observed the system in 2023 and 2024 when the X-ray emission began to increase toward periastron passage in 2025. This manuscript reports unprecedentedly high-resolution X-ray spectra in the iron K$α$ band between 6.2 and 7.1 keV, obtained with the Resolve X-ray microcalorimeter. The hydrogen-like (Ly$α$) and helium-like (He$α$) lines reveal three velocity components. Two of them are broadened with maximum velocities of 2000-3000 km/s, likely originating from the post-shock companion wind. The other is relatively narrow, with a Gaussian broadening of only ~290 km/s in 1 sigma, which may originate from the post-shock companion wind at the WWC stagnation point or penetrating the primary wind. The iron fluorescent lines exhibit a moderate blueshift and broadening with velocities at 100-200 km/s, consistent with the primary wind's velocity field. The spectra also confirm a Compton shoulder of the He$α$ line complex for the first time. Both fluorescing and scattering spectral profiles indicate that the binary system is seen from the companion side during these observations. The flux ratio of the Compton scattering emission to the fluorescent line suggests substantial hydrogen depletion of the primary wind, expected from CNO-cycled hydrogen nuclear fusion gas.

astro-ph.HE↗
arXiv2026

XRISM-Subaru views of Abell 754: an off-axis, near-line-of-sight merging cluster

We report a weak-lensing (WL) mass measurement for the merging cluster Abell 754 and impose constraints on the merger trajectory. The trajectory analysis adopts a two-body model with a point-mass approximation and dynamical friction, refined using numerical simulations of major mergers and characterized by Euler angles. We first conduct WL analysis using the two-dimensional shear pattern from the Subaru HSC in combination with Suprime-Cam images to assist in color selection. The WL mass map shows a distinct double-peak structure located around the western and eastern brightest cluster galaxies as reported in the literature. The two-halo component analysis, which utilizes the 2D shear pattern over the cluster entire region and considers the lensing covariance matrix from uncorrelated large-scale structures, indicates mass values of $M_{200}^W=3.13_{-1.00}^{+1.53}\times10^{14}h_{70}^{-1}M_\odot$ and $M_{200}^E=6.41_{-1.97}^{+2.92}\times10^{14}h_{70}^{-1}M_\odot$. Thus, the eastern mass component associated with the X-ray tadpole-shaped gas is the main cluster. No substantial structural components are detected in the line-of-sight velocities of the member galaxies. Utilizing WL parameters, line-of-sight velocities, and X-ray information on morphology and kinematics, we determine an impact parameter of approximately 0.77 Mpc at an initial separation of 2 Mpc from the main cluster. The merger plane is inclined at about 20 degrees relative to the line-of-sight. Interestingly, this system is an off-axis, near-line-of-sight merger. This characteristic arises because the trajectory within the merger plane is altered during the pericenter passage, causing the apparent motion to transition from predominantly along the line-of-sight before the core passage to mainly within the plane of the sky afterward. This study will assist in conducting numerical simulations to understand the XRISM observations.

astro-ph.CO↗
arXiv2025

Catalogs of optically-selected clusters and photometric luminous red galaxies from the Hyper Suprime-Cam Subaru Strategic Program final year dataset

We construct samples of optically-selected clusters and photometric luminous red galaxies (LRGs) from the Hyper Suprime-Cam Subaru Strategic Program final year dataset covering $\sim 1200$~deg$^2$. The cluster catalogs extend out to the redshift of $1.38$ and contain more than 10000 clusters with richness larger than $15$, where the richness is defined to be a membership probability weighted number of galaxies above the stellar masses of approximately $10^{10.2}M_\odot$. The total number of probable red cluster member galaxies in these clusters are more than $6\times 10^5$. Photometric redshifts of the clusters are shown to be precise with the scatter better than $\sim 0.01$ for a wide redshift range. We detect stacked weak lensing signals of clusters out to the redshift of $1$, and use them to update constraints on the mass-richness relation. Our catalog of about 6 million photometric LRGs extend out to the redshift of $1.25$, and have the scatter of the photometric redshift better than $\sim 0.02$ for the redshift range between $0.4$ and $1.0$.

astro-ph.CO↗
arXiv2025

Disentangling Multiple Gas Kinematic Drivers in the Perseus Galaxy Cluster

Galaxy clusters, the Universe's largest halo structures, are filled with 10-100 million degree X-ray-emitting gas. Their evolution is shaped by energetic processes such as feedback from supermassive black holes (SMBHs) and mergers with other cosmic structures. The imprints of these processes on gas kinematic properties remain largely unknown, restricting our understanding of gas thermodynamics and energy conversion within clusters. High-resolution spectral mapping across a broad spatial-scale range provides a promising solution to this challenge, enabled by the recent launch of the XRISM X-ray Observatory. Here, we present the kinematic measurements of the X-ray-brightest Perseus cluster with XRISM, radially covering the extent of its cool core. We find direct evidence for the presence of at least two dominant drivers of gas motions operating on distinct physical scales: a small-scale driver in the inner ~60 kpc, likely associated with the SMBH feedback; and a large-scale driver in the outer core, powered by mergers. The inner driver sustains a heating rate at least an order of magnitude higher than the outer one. This finding suggests that, during the active phase, the SMBH feedback generates turbulence, which, if fully dissipated into heat, could play a significant role in offsetting radiative cooling losses in the Perseus core. Our study underscores the necessity of kinematic mapping observations of extended sources for robust conclusions on the properties of the velocity field and their role in the assembly and evolution of massive halos. It further offers a kinematic diagnostic for theoretical models of SMBH feedback.

astro-ph.HE↗
arXiv2025

XRISM Observations of The Prototypical Cold Front in Abell 3667

We present high-resolution X-ray spectroscopy of the merging galaxy cluster Abell 3667 with \textit{XRISM}/Resolve. Two observations, targeting the cluster X-ray core and the prototypical cold front, were performed with exposures of 105 ks and 276 ks, respectively. We find that the gas in the core is blueshifted by $v_z\sim-200$ km s$^{-1}$ relative to the brightest cluster galaxy, while the low-entropy gas inside the cold front is redshifted by $v_z\sim 200$ km s$^{-1}$. As one moves further off-center across the front, the line-of-sight (LoS) velocity changes significantly, by $Δv_z=535^{+167}_{-154}$ km s$^{-1}$, back to the value similar to that in the core. There are no significant LoS velocity gradients perpendicular to the cluster symmetry axis. These features suggest that the gas forming the cold front is flowing in the plane oriented along the LoS, supporting an offset merger scenario in which the main cluster has passed in front of the subcluster and induced rotation of the core gas in the plane perpendicular to the sky. The region just inside the front exhibits the largest LoS velocity dispersion seen across two pointings, $σ_z\sim420$ km s$^{-1}$, which can be interpreted as a developing turbulence or a projection of the LoS velocity shear within the front. The large LoS velocity jump across the cold front, combined with the lack of Kelvin-Helmholtz instability on the surface of the front, suggests some mechanism to suppress it. For example, a magnetic field with $B>5\,μ$G is required if the cold front is stabilized by magnetic draping.

astro-ph.GA↗
arXiv2025

XRISM-Subaru views of Abell 754: Energetic ICM Motions Revealed by XRISM/Resolve

We present high-resolution X-ray spectroscopy of the merging cluster Abell~754 using \textit{XRISM}/Resolve. In GO1 phase, \textit{XRISM}/Resolve observed Abell 754 in two deep pointings, targeting the eastern primary core (114~ks) and the middle of the X-ray filamentary structure (190~ks). Spectral fits to full field-of-view data reveal a line-of-sight velocity difference of $656 \pm 35$~km~s$^{-1}$ between the two pointing, corresponding to a bulk Mach number of 0.45$\pm$0.03. Velocity dispersions are measured to be $220^{+26}_{-29}$~km~s$^{-1}$ and $279^{+24}_{-23}$~km~s$^{-1}$ in the eastern and middle pointing, respectively. Within the eastern core, the velocity dispersion shows spatial variation, reaching $497^{+144}_{-117}$~km~s$^{-1}$ in the southern core with high temperature -- among the largest values yet reported in galaxy clusters to date. Narrow-band analysis of the Fe-K complex in this region reveals systematically higher temperatures derived from He-like and H-like Fe line ratio compared to those obtained via broadband fits, indicating multi-phase structures. Two-temperature modeling further separates a cooler core phase from a hotter, shock or turbulence-heated phase whose velocity is blueshifted, similar to that of the middle pointing. These results point to a mixing interface where post-shock gas from the south overlaps, in projection, with cooler core gas, inflating the observed line widths in this region. Weak-lensing analysis with Subaru/HSC and Suprime-cam confirms that the eastern component is about twice as massive as the western one, consistent with disruption and gas stripping of the latter. The curved morphology of the eastern X-ray core, together with the measured kinematics, is naturally explained by an off-axis, post--core-passage merger that imparts angular momentum and drives large-scale rotational and fallback flows.

astro-ph.GA↗