Massive Clusters Research Articles

Distinct distributions of elliptical and disk galaxies across the Local Supercluster as a ΛCDM prediction

Galaxies of different types are not equally distributed in the Local Universe. In particular, the supergalactic plane is prominent among the brightest ellipticals, but inconspicuous among the brightest disk galaxies. This striking difference provides a unique test for our understanding of galaxy and structure formation. Here we use the SIBELIUS DARK constrained simulation to confront the predictions of the standard Lambda Cold Dark Matter (ΛCDM) model and standard galaxy formation theory with these observations. We find that SIBELIUS DARK reproduces the spatial distributions of disks and ellipticals and, in particular, the observed excess of massive ellipticals near the supergalactic equator. We show that this follows directly from the local large-scale structure and from the standard galaxy formation paradigm, wherein disk galaxies evolve mostly in isolation, while giant ellipticals congregate in the massive clusters that define the supergalactic plane. Rather than being anomalous as earlier works have suggested, the distributions of giant ellipticals and disks in the Local Universe and in relation to the supergalactic plane are key predictions of the ΛCDM model.

Prospects for detecting the circum- and intergalactic medium in X-ray absorption using the extended intracluster medium as a backlight

ABSTRACT The warm-hot plasma in cosmic web filaments is thought to comprise a large fraction of the gas in the local Universe. So far, the search for this gas has focused on mapping its emission, or detecting its absorption signatures against bright, point-like sources. Future, non-dispersive, high-spectral resolution X-ray detectors will, for the first time, enable absorption studies against extended objects. Here, we use the Hydrangea cosmological hydrodynamical simulations to predict the expected properties of intergalactic gas in and around massive galaxy clusters, and investigate the prospects of detecting it in absorption against the bright cores of nearby, massive, relaxed galaxy clusters. We probed a total of 138 projections from the simulation volumes, finding 16 directions with a total column density $N_{{\rm O\, {\small VII}}} > 10^{14.5}$ cm−2. The strongest absorbers are typically shifted by ±1000 km s−1 with respect to the rest frame of the cluster they are nearest to. Realistic mock observations with future micro-calorimeters, such as the Athena X-ray Integral Field Unit or the proposed Line Emission Mapper (LEM) X-ray probe, show that the detection of cosmic web filaments in ${\rm O\, {\small VII}}$ and ${\rm O\, {\small VIII}}$ absorption against galaxy cluster cores will be feasible. An ${\rm O\, {\small VII}}$ detection with a 5σ significance can be achieved in 10–250 ks with Athena for most of the galaxy clusters considered. The ${\rm O\, {\small VIII}}$ detection becomes feasible only with a spectral resolution of around 1 eV, comparable to that envisioned for LEM.

The cluster mass function and the σ8 tension

ABSTRACT We use a large set of halo mass function (HMF) models in order to investigate their ability to represent the observational cluster mass function (CMF), derived from the $\tt {GalWCat19}$ cluster catalogue, within the Lambda cold dark matter (ΛCDM) cosmology. We apply the χ2 minimization procedure to constrain the free parameters of the models, namely Ωm and σ8. We find that all HMF models fit well the observational CMF, while the Bocquet et al. model provides the best fit, with the lowest χ2 value. Utilizing the index of inconsistency (IOI) measure, we further test the possible inconsistency of the models with respect to a variety of Planck 2018 ΛCDM cosmologies, resulting from the combination of different probes (CMB–BAO or CMB–DES). We find that the HMF models that fitted well the observed CMF provide consistent cosmological parameters with those of the Planck CMB analysis, except for the Press and Schechter, Yahagi et al., and Despali et al. models that return large IOI values. The inverse $\chi _{\rm min}^2$-weighted average values of Ωm and σ8, overall 23 theoretical HMF models are ${\bar{\Omega }_{m,0}}=0.313\pm 0.022$ and ${\bar{\sigma }_8}=0.798\pm 0.040$, which are clearly consistent with the results of Planck–CMB, providing S8 = σ8(Ωm/0.3)1/2 = 0.815 ± 0.05. Within the ΛCDM paradigm and independently of the selected HMF model in the analysis, we find that the current CMF shows no σ8 tension with the corresponding Planck–CMB results.

The Hydrostatic Mass of A478: Discrepant Results from Chandra, NuSTAR, and XMM-Newton

Galaxy clusters are the most recently formed and most massive, gravitationally bound structures in the Universe. The number of galaxy clusters formed is highly dependent on cosmological parameters, such as the dark matter density, σ 8, and Ω m . The number density is a function of the cluster mass, which can be estimated from the density and temperature profiles of the intracluster medium under the assumption of hydrostatic equilibrium. The temperature of the plasma, hence its mass, is calculated from the X-ray spectra. However, effective-area calibration uncertainties in the soft band result in significantly different temperature measurements from various space-based X-ray telescopes. NuSTAR is potentially less susceptible to these issues than Chandra and XMM-Newton, having larger effective area, particularly at higher energies, enabling high-precision temperature measurements. In this work, we present analyses of Chandra, NuSTAR, and XMM-Newton data of A478 to investigate the nature of this calibration discrepancy. We find that NuSTAR temperatures are on average ∼11% lower than those of Chandra, and XMM-Newton temperatures are on average ∼5% lower than those of NuSTAR. This results in a NuSTAR mass at r 2500,Chandra of M ⊙, which is ∼10% lower than that of M 2500,Chandra and ∼4% higher than M 2500,XMM−Newton.

The Path of Integrating Curriculum Civics into Advertising Design Teaching in Higher Vocational Colleges and Universities Based on Big Data Analysis

Abstract In order to cultivate advertising design applied talents, this paper firstly combs through the status quo of the current teaching system of advertising design and designs the model of advertising design teaching system under the concept of curriculum ideology and political education. Secondly, on the basis of the classical division clustering algorithm, by adding a fuzzy set, fuzzy clustering and fuzzy center of mass and explaining the importance of distance, a weighted fuzzy clustering teaching quality evaluation model based on big data analysis is designed. In the end, the teaching model was employed to evaluate the clustering of teaching quality by accumulating educational data. The percentage of excellent teaching quality in the clustering results is about 0.35, the smallest average percentage is failing and average, which are 0.12 and 0.11, respectively, and the highest average weight is the teacher’s professional knowledge and the student’s learning achievement, which are 0.12 and 0.125, respectively.

Characterizing cool, neutral gas, and ionized metals in the outskirts of low-z galaxy clusters

ABSTRACT We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z ≈ 0.2 background quasar-foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalogue with optically- and SZ-selected cluster catalogues. The median cluster mass of our sample is ≈1014.2 $\rm {\rm M}_{\odot }$ with a median impact parameter (ρcl) of ≈5 Mpc. We detect significant Lyα, marginal C iv, but no O vi absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096 ± 0.011, 0.032 ± 0.015, and <0.009 Å (3σ) for our sample. The Lyα REW shows a declining trend with increasing ρcl (ρcl/R500), which is well explained by a power-law with a slope of −0.79 (−0.70). The covering fractions (CFs) measured for Lyα (21 per cent), C iv (10 per cent), and O vi (10 per cent) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centres or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lyα along with the non-detection of Lyα signal when the strong absorbers ($N(\rm{H\,{\small I}}) > 10^{13}$ cm−2) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.

Observational bias and young massive cluster characterization − II. Can Gaia accurately observe young clusters and associations?

ABSTRACT Observations of clusters suffer from issues such as completeness, projection effects, resolving individual stars, and extinction. As such, how accurate measurements and conclusions are likely to be? Here, we take cluster simulations (Westerlund2- and Orion-type), synthetically observe them to obtain luminosities, accounting for extinction, and the inherent limits of Gaia, then place them within the real Gaia DR3 catalogue. We then attempt to rediscover the clusters at distances of between 500 and 4300 pc. We show the spatial and kinematic criteria that are best able to pick out the simulated clusters, maximizing completeness, and minimizing contamination. We then compare the properties of the ‘observed’ clusters with the original simulations. We looked at the degree of clustering, the identification of clusters and subclusters within the data sets, and whether the clusters are expanding or contracting. Even with a high level of incompleteness (e.g. $\lt 2~{{\ \rm per\ cent}}$ stellar members identified), similar qualitative conclusions tend to be reached compared to the original data set, but most quantitative conclusions are likely to be inaccurate. Accurate determination of the number, stellar membership, and kinematic properties of subclusters are the most problematic to correctly determine, particularly at larger distances due to the disappearance of cluster substructure as the data become more incomplete, but also at smaller distances where the misidentification of asterisms as true structure can be problematic. Unsurprisingly, we tend to obtain better quantitative agreement of properties for our more massive Westerlund2-type cluster. We also make optical style images of the clusters over our range of distances.

Core-collapse supernova inside the core of a young massive star cluster: 3D MHD simulations

ABSTRACT Young massive stars in compact stellar clusters could end their evolution as core-collapse supernovae a few million years after the cluster was built. The blast wave of a supernova propagates through the inner cluster region with multiple stellar winds of young luminous stars. We present the results of 3D magnetohydrodynamic simulations of the plasma flows produced by a supernova event inside a cluster with a population of massive stars similar to that in Westerlund 1. We followed its evolution over a few thousand years (i.e. a few shock crossing times). The plasma temperature, density, and magnetic field, which are highly disturbed by supernova event, relax to values close to the initial over the studied period. The relaxation time of a cluster is a few thousand years, which is a sizeable fraction of the period between the successive supernova events for a massive cluster of a few million years age. The spectra of the cluster diffuse X-ray emission simulated here should be representative for the galactic and extragalactic young massive clusters. The resultant magnetic fields are highly intermittent, so we derived the volume filling factors for a set of magnetic field ranges. Highly amplified magnetic fields of magnitude well above 100 µG fill in a few per cent of the cluster volume, but still dominate the magnetic energy. The structure of the magnetic fields and high-velocity plasma flows with shocks in the system are favourable for both proton and electron acceleration to energies well above TeV.

Catalogue of model star clusters in the Milky Way and M31 galaxies

ABSTRACT Detailed understanding of the formation and evolution of globular clusters (GCs) has been recently advanced through a combination of numerical simulations and analytical models. We employ a state-of-the-art model to create a comprehensive catalogue of simulated clusters in three Milky Way (MW) and three Andromeda (M31) analogue galaxies. Our catalogue aims to connect the chemical and kinematic properties of GCs to the assembly histories of their host galaxies. We apply the model to a selected sample of simulated galaxies that closely match the virial mass, circular velocity profile, and defining assembly events of the MW and M31. The resulting catalogue has been calibrated to successfully reproduce key characteristics of the observed GC systems, including total cluster mass, mass function, metallicity distribution, radial profile, and velocity dispersion. We find that clusters in M31 span a wider range of age and metallicity, relative to the MW, possibly due to M31’s recent major merger. Such a merger also heated up the in-situ GC population to higher orbital energy and introduced a large number of ex-situ clusters at large radii. Understanding the impacts of galaxy mergers and accretion on the GC populations is crucial for uncovering the galaxy assembly histories.

Exploring the low-mass regime of galaxy-scale strong lensing: Insights into the mass structure of cluster galaxies

Context. Several recent studies have highlighted a discrepancy between the strong lensing (SL) properties of observed cluster galaxies and the predictions of Λ cold dark matter (CDM) cosmological hydrodynamical simulations. This discrepancy can be interpreted as the result of observed cluster members being more compact than their simulated counterparts. Aims. In this work, we aim at a direct measurement of the compactness of a few selected galaxy-scale lenses in massive clusters, testing the accuracy of the scaling laws adopted to describe the members in SL models of galaxy clusters. Methods. We selected the multiply imaged sources MACS J0416.1−2403 ID14 (z = 3.221), MACS J0416.1−2403 ID16 (z = 2.095), and MACS J1206.2−0847 ID14 (z = 3.753). Eight multiple images were observed for the first SL system, and six for the latter two. We focused on the main deflector of each galaxy-scale SL system (identified as members 8971, 8785, and 3910, respectively), and modelled its total mass distribution with a truncated isothermal sphere. To account for the lensing effects of the remaining components of the cluster, we took the most accurate SL model of its mass distribution available. To include the uncertainty and the systematics affecting the cluster-scale mass models, we explored the posterior probability distribution of its parameters and extracted 100 cluster mass distributions. For each of them, we optimised the mass parameters of the galaxy-scale lens: the bootstrapping procedure allowed us to obtain a realistic estimate of the uncertainty on their values. Results. We measured a truncation radius value of 6.1−1.1+2.3 kpc, 4.0−0.4+0.6 kpc, and 5.2−1.1+1.3 kpc for members 8971, 8785, and 3910, corresponding to total mass values of M = 1.2−0.1+0.3 × 1011 M⊙, M = 1.0−0.1+0.2 × 1010 M⊙, and M = 6.3−1.1+1.0 × 1010 M⊙, respectively. Alternative non-truncated models with a higher number of free parameters do not lead to an improved description of the SL system and show some parametric degeneracies. We measured the stellar-to-total mass fraction within the effective radius for the three cluster members, finding 0.51 ± 0.21, 1.0 ± 0.4, and 0.39 ± 0.16, respectively. Conclusions. We find that a parameterisation of the physical properties of cluster galaxies in SL models based on power-law scaling relations with respect to the observed total luminosity cannot accurately describe the compactness of the members over their full total mass range. Our results, instead, agree with recent modelling of the cluster members based on the Fundamental Plane relation. Finally, we report good agreement between our predicted values of the stellar-to-total mass fraction within the effective radius and those of early-type galaxies from the Sloan Lens ACS Survey. Our work significantly extends the regimes of the current samples of lens galaxies, towards the mass range that will be probed by the Euclid, Rubin, and James Webb Telescopes.

Optical Identification and Spectroscopic Redshift Measurements of 216 Galaxy Clusters from the SRF/eROSITA All-Sky Survey

We present the results of the optical identification and spectroscopic redshift measurements of216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observationswere performed in 2020–2023 with the 6-m BTA telescope at the Special Astrophysical Observatory ofthe Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of theSternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope atthe Sayan Solar Observatory of the Institute of Solar–Terrestrial Physics of the Siberian Branch of theRussian Academy of Sciences, and the 1.5-m Russian–Turkish telescope (RTT-150) at the TU¨ BI˙ TAKObservatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements havebeen obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in theSRG/eROSITA survey and 22 galaxy clusters are at redshifts zspec 0.7, including three at zspec 1.Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters atzspec 0.7. For these observations we chose the most massive clusters and, therefore, most of the galaxyclusters presented here with the spectroscopic redshifts measured by us will most likely enter in future intothe cosmological samples of galaxy clusters from the SRG/eROSITA survey.

Impact of Galaxy Clusters on the Propagation of Ultrahigh-energy Cosmic Rays

Galaxy clusters are the largest objects in the Universe kept together by gravity. Most of their baryonic content is made of a magnetized diffuse plasma. We investigate the impact of such a magnetized environment on the propagation of ultrahigh-energy cosmic rays (UHECRs). The intracluster medium (ICM) is described according to the self-similar assumption, in which gas density and pressure profiles are fully determined by the cluster mass and redshift. The magnetic field is scaled to the thermal components of the ICM under different assumptions. We model the propagation of UHECRs in the ICM using a modified version of the Monte Carlo code SimProp, where hadronic processes and diffusion in the turbulent magnetic field are implemented. We provide a universal parameterization that approximates the UHECR fluxes escaping from the environment as a function of the most relevant quantities, such as the mass of the cluster, the position of the source with respect to the center of the cluster, and the nature of the accelerated particles. We show that galaxy clusters are an opaque environment, especially for UHECR nuclei. The role of the most massive nearby clusters in the context of the emerging UHECR astronomy is finally discussed.

Composite Bulges. III. A Study of Nuclear Star Clusters in Nearby Spiral Galaxies

We present photometric and morphological analyses of nuclear star clusters (NSCs)—very dense, massive star clusters present in the central regions of most galaxies—in a sample of 33 massive disk galaxies within 20 Mpc, part of the “Composite Bulges Survey.” We use data from the Hubble Space Telescope including optical (F475W and F814W) and near-IR (F160W) images from the Wide Field Camera 3. We fit the images in 2D to take into account the full complexity of the inner regions of these galaxies (including the contributions of nuclear disks and bars), isolating the NSC and bulge components. We derive NSC radii and magnitudes in all three bands, which we then use to estimate NSC masses. Our sample significantly expands the sample of massive late-type galaxies with measured NSC properties. We clearly identify NSCs in nearly 80% of our galaxies, putting a lower limit on the nucleation fraction in these galaxies that is higher than previous estimates. We find that the NSCs in our massive disk galaxies are consistent with previous NSC mass–NSC radius and galaxy mass–NSC mass relations. However, we also find a large spread in NSC masses, with a handful of galaxies hosting very low-mass, compact clusters. Our NSCs are aligned in PA with their host galaxy disks but are less flattened. They show no correlations with bar or bulge properties. Finally, we find the ratio of NSC to BH mass in our massive disk galaxy sample spans a factor of ∼300.

OMEGACat. I. MUSE Spectroscopy of 300,000 Stars within the Half-light Radius of ω Centauri

Omega Centauri (ω Cen) is the most massive globular cluster of the Milky Way and has been the focus of many studies that reveal the complexity of its stellar populations and kinematics. However, most previous studies have used photometric and spectroscopic data sets with limited spatial or magnitude coverage, while we aim to investigate it having full spatial coverage out to its half-light radius and stars ranging from the main sequence to the tip of the red giant branch. This is the first paper in a new survey of ω Cen that combines uniform imaging and spectroscopic data out to its half-light radius to study its stellar populations, kinematics, and formation history. In this paper, we present an unprecedented MUSE spectroscopic data set combining 87 new MUSE pointings with previous observations collected from guaranteed time observations. We extract spectra of more than 300,000 stars reaching more than 2 magnitudes below the main-sequence turnoff. We use these spectra to derive metallicity and line-of-sight velocity measurements and determine robust uncertainties on these quantities using repeat measurements. Applying quality cuts we achieve signal-to-noise ratios (S/Ns) of 16.47/73.51 and mean metallicity errors of 0.174/0.031 dex for the main-sequence stars (18 mag <mag F625W < 22 mag) and red giant branch stars (16 mag <mag F625W < 10 mag), respectively. We correct the metallicities for atomic diffusion and identify foreground stars. This massive spectroscopic data set will enable future studies that will transform our understanding of ω Cen, allowing us to investigate the stellar populations, ages, and kinematics in great detail.

A Formation Mechanism for “Wrong Way” Radio Relics

Radio relics are typically found to be arc-like regions of synchrotron emission in the outskirts of merging galaxy clusters, bowing out from the cluster center. In most cases they show synchrotron spectra that steepen toward the cluster center, indicating that they are caused by relativistic electrons being accelerated at outward traveling merger shocks. A number of radio relics break with this ideal picture and show morphologies that are bent the opposite way and show spectral index distributions that do not follow expectations from the ideal picture. We propose that these “wrong way” relics can form when an outward traveling shock wave is bent inward by an infalling galaxy cluster or group. We test this in an ultra-high-resolution zoom-in simulation of a massive galaxy cluster with an on-the-fly spectral cosmic-ray model. This allows us to study not only the synchrotron emission at colliding shocks, but also their synchrotron spectra to address the open question of relics with strongly varying spectral indices over the relic surface.

R-process Abundance Patterns in the Globular Cluster M92

Whereas light-element abundance variations are a hallmark of globular clusters, there is little evidence for variations in neutron-capture elements. A significant exception is M15, which shows a star-to-star dispersion in neutron-capture abundances of at least one order of magnitude. The literature contains evidence both for and against a neutron-capture dispersion in M92. We conducted an analysis of archival Keck/HIRES spectra of 35 stars in M92, 29 of which are giants, which we use exclusively for our conclusions. M92 conforms to the abundance variations typical of massive clusters. Like other globular clusters, its neutron-capture abundances were generated by the r-process. We confirm a star-to-star dispersion in r-process abundances. Unlike M15, the dispersion is limited to “first-generation” (low-Na, high-Mg) stars, and the dispersion is smaller for Sr, Y, and Zr than for Ba and the lanthanides. This is the first detection of a relation between light-element and neutron-capture abundances in a globular cluster. We propose that a source of the main r-process polluted the cluster shortly before or concurrently with the first generation of star formation. The heavier r-process abundances were inhomogeneously distributed while the first-generation stars were forming. The second-generation stars formed after several crossing times (∼0.8 Myr); hence, the second generation shows no r-process dispersion. This scenario imposes a minimum temporal separation of 0.8 Myr between the first and second generations.

Hidden Giants in JWST's PEARLS: An Ultramassive z = 4.26 Submillimeter Galaxy that Is Invisible to HST

We present a multiwavelength analysis using the Submillimeter Array (SMA), James Clerk Maxwell Telescope, NOEMA, JWST, the Hubble Space Telescope (HST), and the Spitzer Space Telescope of two dusty strongly star-forming galaxies, 850.1 and 850.2, seen through the massive cluster lens A 1489. These SMA-located sources both lie at z = 4.26 and have bright dust continuum emission, but 850.2 is a UV-detected Lyman-break galaxy, while 850.1 is undetected at ≲ 2 μm, even with deep JWST/NIRCam observations. We investigate their stellar, interstellar medium, and dynamical properties, including a pixel-level spectral energy distribution analysis to derive subkiloparsec-resolution stellar-mass and A V maps. We find that 850.1 is one of the most massive and highly obscured, A V ∼ 5, galaxies known at z > 4 with M * ∼1011.8 M ⊙ (likely forming at z > 6), and 850.2 is one of the least massive and least obscured, A V ∼ 1, members of the z > 4 dusty star-forming population. The diversity of these two dust-mass-selected galaxies illustrates the incompleteness of galaxy surveys at z ≳ 3–4 based on imaging at ≲ 2 μm, the longest wavelengths feasible from HST or the ground. The resolved mass map of 850.1 shows a compact stellar-mass distribution, ∼1 kpc, but its expected evolution means that it matches both the properties of massive, quiescent galaxies at z ∼ 1.5 and ultramassive early-type galaxies at z ∼ 0. We suggest that 850.1 is the central galaxy of a group in which 850.2 is a satellite that will likely merge in the near future. The stellar morphology of 850.1 shows arms and a linear bar feature that we link to the active dynamical environment it resides within.

Tracking the evolution of satellite galaxies: mass stripping and dark-matter deficient galaxies

ABSTRACT Satellite galaxies undergo a variety of physical processes when they are accreted by groups and clusters, often resulting in the loss of baryonic and dark matter (DM) mass. In this work, we evaluate the predictions from the IllustrisTNG hydrodynamical simulation regarding the evolution of the matter content of satellites, focusing on a population that are accreted at z &amp;gt; 1 and retain their identity as satellites down to z = 0. At fixed host halo mass, the amount of DM and stellar mass stripped depends mostly on the pericentric distance, dperi, here normalized by host halo virial radius. The closest encounters result in significant loss of DM, with subhaloes retaining between 20 and a few per cent of their z = 1 mass. At fixed dperi, DM mass stripping seems more severe in lower mass haloes. Conversely, the average satellite in higher mass haloes has its stellar mass growth halted earlier, having lost a higher fraction of stellar mass by z = 0. We also show that mass stripping has a strong impact on the quenched fractions. The IllustrisTNG boxes are qualitatively consistent in these predictions, with quantitative differences mostly originating from the distinct subhalo mass ranges covered by the boxes. Finally, we have identified DM-deficient systems in all TNG boxes. These objects are preferentially found in massive clusters (Mhost ≳ 1013 M⊙), had very close encounters with their central galaxies ($d_{\rm peri}\simeq 0.05\, R_{\rm vir}$), and were accreted at high redshift (zinfall ≳ 1.4), reinforcing the notion that tidal stripping is responsible for their remarkable lack of DM.

Observing gravitational redshift with X-ray emission in galaxy clusters withAthenaX-IFU

Context.The Doppler shift predicted by general relativity for light escaping a gravitational potential has been observed on Earth as well as in the direction of various stars and galaxy clusters at optical wavelengths.Aims.Observing the gravitational redshift in the X-ray band within galaxy clusters could provide information on their properties and, in particular, their gravitational potential. We present a feasibility study of such a measurement, using the capabilities of the next-generation European X-ray observatoryAthena.Methods.We used a simple generalized Navarro–Frenk–White potential model along with aβ-model for the density of baryonic matter, which sets the emission to provide an estimation of the observed redshift in the simplest of cases. We generated mock observations with theAthenaX-ray Integral Field Unit (X-IFU) for a nearby massive cluster, while seeking to recover the gravitational redshift along with other properties of the toy model cluster.Results.We investigated the observability of the gravitational redshift in an idealized test case of a nearby massive cluster with theAthenaX-IFU instrument, as well as its use in probing the properties of the potential well. We were also able to constrain the mass to a ∼20% level of precision and the cosmological redshift to less than ∼1%, within a simplified and idealized observational framework. More refined simulations accounting for further effects such as the internal gas motions and the actual shape of the potential well are required to fully investigate the feasibility of measuring the gravitational redshift for a single target or statistically over a sample of galaxy clusters.

Observational signatures of forming young massive clusters: continuum emission from dense H ii regions

ABSTRACT Young massive clusters (YMCs) are the most massive star clusters forming in nearby galaxies and are thought to be a young analogue to the globular clusters. Understanding the formation process of YMCs leads to looking into very efficient star formation in high-redshift galaxies suggested by recent JWST observations. We investigate possible observational signatures of their formation stage, particularly when the mass of a cluster is increasing via accretion from a natal molecular cloud. To this end, we study the broad-band continuum emission from ionized gas and dust enshrouding YMCs, whose formation is followed by recent radiation hydrodynamics simulations. We perform post-process radiative transfer calculations using simulation snapshots and find characteristic spectral features at radio and far-infrared frequencies. We show that a striking feature is long-lasting, strong free–free emission from a ∼10-pc-scale H ii region with a large emission measure of ≳107 cm−6 pc, corresponding to the mean electron density of ≳103 cm−3. There is a turnover feature below ∼10 GHz, a signature of the optically thick free–free emission, often found in Galactic ultracompact H ii regions. These features come from the peculiar YMC formation process, where the cluster’s gravity effectively traps photoionized gas for a long duration and enables continuous star formation within the cluster. Such large and dense H ii regions show distinct distribution on the density–size diagram, apart from the standard sequence of Galactic H ii regions. This is consistent with the observational trend inferred for extragalactic H ii regions associated with YMCs.