Surface Brightness Fluctuations Research Articles

The Next Generation Virgo Cluster Survey (NGVS). III. A Catalog of Surface Brightness Fluctuation Distances and the Three-dimensional Distribution of Galaxies in the Virgo Cluster

The surface brightness fluctuation (SBF) method is a robust and efficient way of measuring distances to galaxies containing evolved stellar populations. Although many recent applications of the method have used space-based imaging, SBF remains a powerful technique for ground-based telescopes. Deep, wide-field imaging surveys with subarcsecond seeing enable SBF measurements for numerous nearby galaxies. Using a preliminary calibration, Cantiello et al. presented SBF distances for 89 bright, mainly early-type galaxies observed in the Next Generation Virgo Cluster Survey. Here we present a refined calibration and SBF distances for 278 galaxies extending several magnitudes fainter than in previous work. The derived distances have uncertainties of 5%–12% depending on the properties of the individual galaxies, and our sample is more than 3 times larger than any previous SBF study of this region. Virgo has a famously complex structure with numerous subclusters, clouds, and groups; we associate individual galaxies with the various substructures and map their three-dimensional spatial distribution. Curiously, subcluster A, centered around M87, appears to have two peaks in distance: the main peak at ∼16.5 Mpc, and a smaller one at ∼19.4 Mpc. Subclusters B and C have distances of ∼15.8 Mpc. The W and W ′ groups form a filament-like structure, extending more than 15 Mpc behind the cluster with a commensurate velocity increase of ∼1000 km s−1 along its length. These measurements are a valuable resource for future studies of the relationship between galaxy properties and local environment within a dynamic and evolving region.

Underluminous 1991bg-like Type Ia supernovae are standardizable candles

Abstract It is widely accepted that the width-luminosity relation used to standardize normal Type Ia supernovae (SNe Ia) breaks down in underluminous, 1991bg-like SNe Ia. This breakdown may be due to the choice of parameter used as a stand-in for the width of the SN Ia light curve. Using the colour stretch parameter sBV instead of older parameters resolves this issue. Here, I assemble a sample of 14 nearby 1991bg-like SNe Ia from the literature, all of which have independent host-galaxy distance moduli and little to no reddening. I use Gaussian process regression to fit the light curves of these SNe in U/u, B, V, g, R/r, I/i, and H, and measure their peak absolute magnitudes. I find statistically significant (>5σ confidence level in the optical and >4σ in the near-infrared) correlations between the peak absolute magnitudes of the 1991bg-like SNe Ia and their sBV values in the range 0.2 < sBV < 0.6. These correlations are broadly consistent with fits to sBV < 0.7 SNe Ia with preliminary B- and V-band peak absolute magnitudes from the Carnegie Supernova Project and significantly inconsistent with similar fits to normal and transitional SNe Ia (with 0.7 < sBV < 1.1). The underluminous width-luminosity relation shown here needs to be properly calibrated with a homogeneous sample of 1991bg-like SNe Ia, after which it could be used as a rung on a new cosmological distance ladder. With surface-brightness fluctuations (or another non-Cepheid method) used to calibrate distances to nearby 1991bg-like SNe Ia, such a ladder could produce an independent measurement of the Hubble-Lemaître Constant, H0.

Toward Precision Cosmology with Improved Planetary Nebula Luminosity Function Distances Using VLT-MUSE. II. A Test Sample from Archival Data

Thanks to the MUSE integral field spectrograph on board the Very Large Telescope (VLT), extragalactic distance measurements with the [O iii] λ5007 planetary nebula luminosity function (PNLF) are now possible out to ∼40 Mpc. Here we analyze the VLT/MUSE data for 20 galaxies from the ESO public archive to identify the systems’ planetary nebulae (PNe) and determine their PNLF distances. Three of the galaxies do not contain enough PNe for a robust measure of the PNLF, and the results for one other system are compromised of the galaxy’s internal extinction. However, we obtain robust PNLF distances for the remaining 16 galaxies, two of which are isolated and beyond 30 Mpc in a relatively unperturbed Hubble flow. From these data, we derive a Hubble constant of 74.2 ± 7.2 (stat) ±3.7 (sys) km s−1 Mpc−1, a value that is very similar to that found from other quality indicators (e.g., Cepheids, the tip of the red giant branch, and surface brightness fluctuations). At present, the uncertainty is dominated by the small number of suitable galaxies in the ESO archive and their less-than-ideal observing conditions and calibrations. Based on our experience with these systems, we identify the observational requirements necessary for the PNLF to yield a competitive value for H 0 that is independent of the Type Ia supernova distance scale.

Modelling of surface brightness fluctuation measurements. Methodology, uncertainty, and recommendations

The goal of this work is to scrutinise the surface brightness fluctuation (SBF) calculation methodology. We analysed the SBF derivation procedure, measured the accuracy of the fitted SBF under controlled conditions, retrieved the uncertainty associated with the variability of a system that is inherently stochastic, and studied the SBF reliability under a wide range of conditions. Additionally, we address the possibility of an SBF gradient detection. We also examine the problems related with biased measurements of the SBF and low luminosity sources. All of this information allows us to put forward guidelines to ensure a valid SBF retrieval. To perform all the experiments described above, we carried out Monte Carlo simulations of mock galaxies as an ideal laboratory. Knowing its underlying properties, we attempted to retrieve SBFs under different conditions. The uncertainty was evaluated through the accuracy, the precision, and the standard deviation of the fitting. We demonstrate how the usual mathematical approximations taken in the SBF theoretical derivation have a negligible impact on the results and how modelling the instrumental noise reduces the uncertainty. We conducted various studies where we varied the size of the mask applied over the image, the surface and fluctuation brightness of the galaxy, its size and profile, its point spread function (PSF), and the sky background. It is worth highlighting that we find a strong correlation between having a high number of pixels within the studied mask and retrieving a low uncertainty result. We address how the standard deviation of the fitting underestimates the actual uncertainty of the measurement. Lastly, we find that, when studying SBF gradients, the result is a pixel-weighted average of all the SBFs present within the studied region. Retrieving an SBF gradient requires high-quality data and a sufficient difference in the fluctuation value through the different radii. We show how the SBF uncertainty can be obtained and we present a collection of qualitative recommendations for a safe SBF retrieval. Our main findings are as follows. It is important to model the instrumental noise, rather than fitting it. The target galaxies must be observed under appropriate observational conditions. In a traditional SBF derivation, one should avoid pixels with fluxes lower than ten times the SBF estimate to prevent biased results. The uncertainty associated with the intrinsic variability of the system can be obtained using sets of Monte Carlo mock galaxy simulations. We offer our computational implementation in the form of a simple code designed to estimate the uncertainty of the SBF measurement. This code can be used to predict the quality of future observations or to evaluate the reliability of those already conducted.

Radio surface fluctuations in radio relics

Recent observations have revealed detailed structures of radio relics across a wide range of frequencies. In this work, we performed three-dimensional magnetohydrodynamical (3D MHD) simulations of merger shocks propagating through a turbulent magnetized intracluster medium. We then employed on-the-fly Lagrangian particles to explore the physical processes behind the origination of radio substructures and their appearance in high and low-frequency observations. We employed two cosmic-ray (CR) electron acceleration models, with a fresh injection of electrons from the thermal pool and the re-acceleration of mildly relativistic electrons. We used the relative surface brightness fluctuations, $ nu $, to define a ``degree of patchiness.'' First, we found that patchiness is produced if the shock's surface has a distribution of Mach numbers, rather than a single Mach number. Second, radio relics appear patchier if the Mach number distribution consists of a large percentage of low Mach numbers ($ M Furthermore, as the frequency increases, the patchiness also becomes larger. Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz), they necessarily will also be patchy at low frequencies (e.g., 150 MHz). Then, to produce noticeable differences in the patchiness at low and high frequencies, the shock front should have a Mach number spread of $ M Finally, the extent of the patchiness depends on the Mach number distribution as well as the CR acceleration model. We propose $ nu $ as a potential tool for extracting merger shock properties and information on particle acceleration processes at shocks in radio observations.

An effective description of Laniakea: impact on cosmology and the local determination of the Hubble constant

We propose an effective model to describe the bias induced on cosmological observables by Laniakea, the gravitational supercluster hosting the Milky Way, which was defined using peculiar velocity data from Cosmicflows-4 (CF4). The structure is well described by an ellipsoidal shape exhibiting triaxial expansion, reasonably approximated by a constant expansion rate along the principal axes. Our best fits suggest that the ellipsoid, after subtracting the background expansion, contracts along the two smaller axes and expands along the longest one, predicting an average expansion of ∼ -1.1 km/s/Mpc. The different expansion rates within the region, relative to the mean cosmological expansion, induce line-of-sight-dependent corrections in the computation of luminosity distances. We apply these corrections to two low-redshift datasets: the Pantheon+ catalog of type Ia Supernovae (SN Ia), and 63 measurements of Surface Brightness Fluctuations (SBF) of early-type massive galaxies from the MASSIVE survey. We find corrections on the distances of order ∼ 2-3%, resulting in a shift in the inferred best-fit values of the Hubble constant H 0 of order ΔH 0 SN Ia ≈ 0.5 km/s/Mpc and ΔH 0 SBF ≈ 1.1 km/s/Mpc, seemingly worsening the Hubble tension.

Power spectrum estimation methods on intracluster medium surface brightness fluctuations

Accurate estimation of galaxy cluster masses is a central problem in cosmology. Turbulence is believed to introduce significant deviations from the hydrostatic mass estimates. Estimation of turbulence properties is complicated by projection of the 3D cluster onto the 2D plane of the sky, and is commonly done in the form of indirect probes from fluctuations in the X-ray surface brightness and Sunyaev-Zeldovich effect maps. In this paper, we address this problem using simulations. We examine different methods for estimating the power spectrum on 2D projected fluctuation data, emulating data projected onto a 2D plane of the sky, and comparing them to the original, expected 3D power spectrum. Noise can contaminate the power spectrum of ICM observations, so we also briefly compare a few methods of reducing noise in the images for better spectral estimation.

Measuring distances to galaxies with globular cluster velocity dispersions

ABSTRACT Accurate distances are key to obtaining intrinsic properties of astronomical objects such as luminosity or size. Globular clusters (GCs) follow a well-defined relation between their absolute magnitudes and internal stellar velocity dispersions (σ), offering an independent way to measure distances to their host galaxies via high-resolution spectroscopy. This is reminiscent of the ‘Faber–Jackson’ for elliptical galaxies. However, unlike galaxies, GCs have a very narrow range of mass-to-light ratios and simple star formation histories. Here, we show that the GC MV−log10(σ) relation is linear, whose slope is identical for the Milky Way and M31 GC systems. Based on this, we use 94 Milky Way GCs which have distances from GAIA parallaxes, or proper-motion dispersion profiles to derive a ‘GC velocity dispersion’ distance (GCVD) to M31, obtaining (m − M)0 = 24.51 ± 0.08 (d = 798 ± 28 kpc), in excellent agreement with independent measurements. Combining data for these two galaxies to create a fiducial relation using 296 GCs with high-quality measurements, we obtain a zero-point uncertainty (±0.06 mag) corresponding to a distance uncertainty of $\sim 3~{{\ \rm per\ cent}}$. We then use GCVD to obtain a distance to the giant elliptical galaxy NGC 5128 (Centaurus A), finding (m − M)0 = 27.95 ± 0.09 (d = 3.89 ± 0.16 Mpc). This is in excellent agreement with, and in some cases more precise than, literature estimates from the tip of the red giant branch or surface brightness fluctuations. We apply GCVD to Local Group galaxies with appropriate data and find good agreement with literature values even in cases with only one GC velocity dispersion measurement.

Testing the surface brightness fluctuation method on dwarf galaxies in the COSMOS field

ABSTRACT Dwarf galaxies are important tracers of small-scale cosmological structure, yet much of our knowledge about these systems comes from the limited sample of dwarf galaxies within the Local Group. To make a comprehensive inventory of dwarf populations in the local Universe, we require effective methods for deriving distance estimates for large numbers of faint, low surface brightness objects. Here we test the surface brightness fluctuation (SBF) method, traditionally applied to brighter early-type galaxies, on a sample of 20 nearby dwarf galaxies detected in the Cosmological Evolution Survey (COSMOS) field. These objects are partially resolved in the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and have confirmed redshift distances in the range 17–130 Mpc. We discuss the many model choices required in applying the SBF method and explore how these affect the final distance estimates. Amongst other variations on the method, when applying the SBF method, we alter the standard equation to include a term accounting for the power spectrum of the background, greatly improving our results. For the most robust modelling choices, we find a roughly Gaussian SBF signal that correlates linearly with distance out to distances of 50–100 Mpc, but with only a fraction of the power expected. At larger distances, there is excess power relative to that predicted, probably from undetected point sources. Overall, obtaining accurate SBF distances to faint, irregular galaxies remains challenging, but may yet prove possible with the inclusion of more information about galaxy properties and point source populations, and the use of more advanced techniques.

Connecting Infrared Surface Brightness Fluctuation Distances to Type Ia Supernova Hosts: Testing the Top Rung of the Distance Ladder

We compare infrared surface brightness fluctuation (IR SBF) distances measured in galaxies that have hosted type Ia supernovae (SNe Ia) to distances estimated from SNe Ia light-curve fits. We show that the properties of the SNe Ia found in IR SBF hosts are very different from those exploding in Cepheid calibrators, therefore this is a direct test of systematic uncertainties on the estimation of the Hubble constant (H 0) using SNe. The IR SBF results from Jensen et al. provide a large and uniformly measured sample of IR SBF distances which we directly compare with the distances to 25 SN Ia host galaxies. We divide the Hubble flow SNe Ia into subsamples that best match the divergent SN properties seen in the IR SBF hosts and Cepheid hosts. We further divide the SNe Ia into a sample with light-curve widths and host masses that are congruent to those found in the SBF-calibrated hosts. We refit the light-curve stretch and color correlations with luminosity, and use these revised parameters to calibrate a sample of “Hubble flow” SNe Ia with IR SBF calibrators. Relative to the Hubble flow sample, the average calibrator distance moduli vary by 0.03 mag depending on the SN Ia subsample examined and this adds a 1.8% systematic uncertainty to our Hubble constant estimate. Based on the IRSBF calibrators, H 0 = 74.6 ± 0.9(stat) ± 2.7(syst) km s−1 Mpc−1, which is consistent with the Hubble constant derived from SNe Ia calibrated from Cepheid variables. We conclude that IR SBFs provide reliable calibration of SNe Ia with a precision comparable to Cepheid calibrators, and with a significant saving in telescope time.

Inferences from Surface Brightness Fluctuations of Zwicky 3146 via the Sunyaev–Zel’dovich Effect and X-Ray Observations

The galaxy cluster Zwicky 3146 is a sloshing cool-core cluster at z = 0.291 that in Sunyaev–Zel’dovich (SZ) imaging does not appear to exhibit significant pressure substructure in the intracluster medium. We perform a surface brightness fluctuation analysis via Fourier amplitude spectra on SZ (MUSTANG-2) and X-ray (XMM-Newton) images of this cluster. These surface brightness fluctuations can be deprojected to infer pressure and density fluctuations from the SZ and X-ray data, respectively. In the central region (Ring 1, r < 100″ = 440 kpc, in our analysis), we find fluctuation spectra that suggest injection scales around 200 kpc (∼140 kpc from pressure fluctuations and ∼250 kpc from density fluctuations). When comparing the pressure and density fluctuations in the central region, we observe a change in the effective thermodynamic state from large to small scales, from isobaric (likely due to the slow sloshing) to adiabatic (due to more vigorous motions). By leveraging scalings from hydrodynamical simulations, we find an average 3D Mach number ≈0.5. We further compare our results to other studies of Zwicky 3146 and, more broadly, to other studies of fluctuations in other clusters.

Investigating the turbulent hot gas in X-COP galaxy clusters

Context. Turbulent processes at work in the intracluster medium perturb this environments, impacting its properties, displacing gas, and creating local density fluctuations that can be quantified via X-ray surface brightness fluctuation analyses. Improved knowledge of these phenomena would allow for a more accurate determination of the mass of galaxy clusters, as well as a better understanding of their dynamic assembly. Aims. In this work, we aim to set constraints on the structure of turbulence using X-ray surface brightness fluctuations. We seek to consider the stochastic nature of this observable and to constrain the structure of the underlying power spectrum. Methods. We propose a new Bayesian approach, relying on simulation-based inference to account for the whole error budget. We used the X-COP cluster sample to individually constrain the power spectrum in four regions and within R500. We spread the analysis on the entire set of 12 systems to alleviate the sample variance. We then interpreted the density fluctuations as the result of either gas clumping or turbulence. Results. For each cluster considered individually, the normalisation of density fluctuations correlate positively with the Zernike moment and centroid shift, but negatively with the concentration and the Gini coefficient. The spectral index within R500 and evaluated over all clusters is consistent with a Kolmogorov cascade. The normalisation of density fluctuations, when interpreted in terms of clumping, is consistent within 0.5R500 with the literature results and numerical simulations; however, it is higher between 0.5 and 1R500. Conversely, when interpreted on the basis of turbulence, we deduce a non-thermal pressure profile that is lower than the predictions of the simulations within 0.5 R500, but still in agreement in the outer regions. We explain these results by the presence of central structural residues that are remnants of the dynamical assembly of the clusters.

A deep dive: Chandra observations of the NGC 4839 group falling into the Coma cluster

ABSTRACT Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium acts to strip the gas from the infalling groups, forming large tails of stripped gas, which eventually become part of the main cluster. In this work, we present a detailed analysis of our new deep Chandra observations of the NGC 4839 group falling into the nearby Coma cluster, providing a unique opportunity to explore the way galaxy clusters in the local Universe continue to grow. Our analysis reveals a cold front feature at the leading head of the group, preceded by a bow shock of hot gas in front with a Mach number of ∼1.5. The power spectrum of surface brightness fluctuations in the tail shows that the slope gets less steep as the distance from the leading head increases, changing from $-2.35_{-0.06}^{+0.07}$ at the inner part of the tail to $-1.37_{-0.07}^{+0.09}$ at the outermost part of the tail. These values are shallower than the slope of the Kolmogorov 2D power spectrum, indicating that thermal conduction is being suppressed throughout the tail, enabling long-lived small-scale turbulence, which would typically be washed out if thermal conduction was not inhibited. The characteristic amplitude of surface brightness fluctuations in the tail suggests a mild level of turbulence with a Mach number in the range of 0.1–0.5, agreeing with that found for the infalling group in Abell 2142.

ThePlanckclusters in the LOFAR sky

Context.The footprint of the recent second data release of the LOFAR Two-metre Sky Survey (LoTSS-DR2) covers 309PlanckSunyaev-Zeldovich (SZ) selected galaxy clusters, 83 of which host a radio halo and 26 host a radio relic(s). It provides an excellent opportunity to statistically study the properties of extended cluster radio sources, especially their connection with merging activities.Aims.We quantify cluster dynamic states to investigate their relation with the occurrence of extended radio sources. We also search for connections between intracluster medium (ICM) turbulence and nonthermal characteristics of radio halos in the LoTSS-DR2.Methods.We analyzedXMM-NewtonandChandraarchival X-ray data of allPlanckSZ clusters in the footprint of LoTSS-DR2. We computed concentration parameters and centroid shifts that indicate the dynamic states of the clusters. We also performed a power spectral analysis of the X-ray surface brightness fluctuations to investigate large-scale density perturbations and estimate the turbulent velocity dispersion. Furthermore, we searched for the relation between radio halo power and the turbulent dissipation flux channeled to particle acceleration.Results.The concentration parameters measured by the two telescopes agree well, but the centroid shift has a larger scatter. The surface brightness power spectral analysis results in a large scatter of the surface brightness and density fluctuation amplitudes. We therefore only found a marginal anticorrelation between density fluctuations and cluster relaxation state, and we did not find a correlation between density fluctuations and radio halo power. Nevertheless, the injected power for particle acceleration calculated from turbulent dissipation is correlated with the radio halo power, where the best-fit unity slope supports the turbulent (re)acceleration scenario. Two different acceleration models, transit-time damping and adiabatic stochastic acceleration, cannot be distinguished due to the large scatter of the estimated turbulent Mach number. We introduced a new quantity [kBT ⋅ YX]rRH, which is proportional to the turbulent acceleration power assuming a constant Mach number. This quantity is strongly correlated with radio halo power, where the slope is also unity.

Discovery of a red backsplash galaxy candidate near M81

ABSTRACT Understanding quenching mechanisms in low-mass galaxies is essential for understanding galaxy evolution overall. In particular, isolated galaxies are important tools to help disentangle the complex internal and external processes that impact star formation. Comparisons between quenched field and satellite galaxies in the low-mass regime offer a substantial opportunity for discovery, although very few quenched galaxies with masses below $M_{\star }\, \sim \, 10^{9} {\rm M}_{\odot }$ are known outside the virial radius, Rvir, of any host halo. Importantly, simulations and observations suggest that an in-between population of backsplash galaxies also exists that may complement interpretations of environmental quenching. Backsplash galaxies – like field galaxies – reside outside the virial radius of a host halo, but their star formation can be deeply impacted by previous interactions with more massive systems. In this paper, we report the concurrent discovery of a low-mass ($M_{\star }\, \sim \, 10^{7} {\rm M}_{\odot }$) quenched galaxy approximately 1Rvir in projection from the M81 group. We use surface brightness fluctuations (SBF) to investigate the possibility that the new galaxy, dw0910+7326 (nicknamed Blobby), is a backsplash galaxy or a more distant field galaxy. The measured SBF distance of $3.21\substack{+0.15 +0.41 \\-0.15 -0.36}$ Mpc indicates that Blobby likely lies in the range 1.0 &amp;lt; R/Rvir &amp;lt; 2.7 outside the combined M81–M82 system. Given its distance and quiescence, Blobby is a good candidate for a backsplash galaxy and could provide hints about the formation and evolution of these interesting objects.

Cosmicflows-4

With Cosmicflows-4, distances are compiled for 55,877 galaxies gathered into 38,065 groups. Eight methodologies are employed, with the largest numbers coming from the correlations between the photometric and kinematic properties of spiral galaxies (TF) and elliptical galaxies (FP). Supernovae that arise from degenerate progenitors (type Ia SNe) are an important overlapping component. Smaller contributions come from distance estimates from the surface brightness fluctuations of elliptical galaxies and the luminosities and expansion rates of core-collapse supernovae (SNe II). Cepheid period–luminosity relation and tip of the red giant branch observations founded on local stellar parallax measurements along with the geometric maser distance to NGC 4258 provide the absolute scaling of distances. The assembly of galaxies into groups is an important feature of the study in facilitating overlaps between methodologies. Merging between multiple contributions within a methodology and between methodologies is carried out with Bayesian Markov chain Monte Carlo procedures. The final assembly of distances is compatible with a value of the Hubble constant of H 0 = 74.6 km s−1 Mpc−1 with the small statistical error of ±0.8 km s−1 Mpc−1 but a large potential systematic error of ∼3 km s−1 Mpc−1. Peculiar velocities can be inferred from the measured distances. The interpretation of the field of peculiar velocities is complex because of large errors on individual components and invites analyses beyond the scope of this study.

Etherington duality breaking: gravitational lensing in non-metric space–times versus intrinsic alignments

ABSTRACT The Etherington distance duality relation is well-established for metric theories of gravity, and confirms the duality between the luminosity distance and the angular diameter distance through the conservation of surface brightness. A violation of the Etherington distance duality due to lensing in a non-metric space–time would lead to fluctuations in surface brightness of galaxies. Likewise, fluctuations of the surface brightness can arise in classical astrophysics as a consequence of intrinsic tidal interaction of galaxies with their environment. Therefore, we study these in two cases in detail: First, for intrinsic size fluctuations and the resulting changes in surface brightness, and secondly, for an area-metric space–time as an example of a non-metric space–time, where the distance duality relation itself acquires modifications. The aim of this work is to quantify whether a surface brightness fluctuation effect due to area-metric gravity would be resolvable compared to the similar effect caused by intrinsic alignment. We thus compare the auto- and cross-correlations of the angular spectra in these two cases and show that the fluctuations in intrinsic brightness can potentially be measured with a cumulative signal-to-noise ratio Σ(ℓ) ≥ 3 in a Euclid-like survey. The measurement in area-metric space–times, however, depends on the specific parameter choices, which also determine the shape and amplitude of the spectra. While lensing surveys do have sensitivity to lensing-induced surface brightness fluctuations in area-metric space–times, the measurement does not seem to be possible for natural values of the Etherington-breaking parameters.

Chandra measurements of gas homogeneity and turbulence at intermediate radii in the Perseus Cluster

ABSTRACT We present a Chandra study of surface brightness fluctuations in the diffuse intracluster medium of the Perseus Cluster. Our study utilizes deep, archival imaging of the cluster core as well as a new mosaic of 29 short $\, 5\,$-ks observations extending in eight different directions out to radii of $r_{500} \sim 2.2\, r_{2500}$. Under the assumption that the distribution of densities at a given radius is log-normally distributed, two important quantities can be derived from the width of the log-normal density distribution on a given spatial scale: the density bias, that is equal to the square root of the clumping factor C; and the one-component turbulent velocity, vk,1D. We forward-model all contributions to the measured surface brightness, including astrophysical and particle background components, and account for the Poisson nature of the measured signal. Measuring the distribution of surface brightness fluctuations in 1 arcmin2 regions, spanning the radial range $0.3\rm {-}2.2\, r_{2500}$ ($7.8\rm {-}57.3\,$ arcmin), we find a small to moderate average density bias of around $3{{\ \rm per\ cent}}$ at radii below $1.6\, r_{2500}$. We also infer an average turbulent velocity at these radii of V1D &amp;lt; 400 km s−1. Direct confirmation of our results on turbulent velocities inferred from surface brightness fluctuations should be possible using the X-ray calorimeter spectrometers to be flown aboard the XRISM and Athena observatories.

The Missing Satellite Problem outside of the Local Group. II. Statistical Properties of Satellites of Milky Way–like Galaxies

We present a new observation of satellite galaxies around seven Milky Way (MW)–like galaxies located outside of the Local Group (LG) using Subaru/Hyper Suprime-Cam imaging data to statistically address the missing satellite problem. We select satellite galaxy candidates using magnitude, surface brightness, Sérsic index, axial ratio, FWHM, and surface brightness fluctuation cuts, followed by visual screening of false positives such as optical ghosts of bright stars. We identify 51 secure dwarf satellite galaxies within the virial radius of nine host galaxies, two of which are drawn from the pilot observation presented in Paper I. We find that the average luminosity function of the satellite galaxies is consistent with that of the MW satellites, although the luminosity function of each host galaxy varies significantly. We observe an indication that more massive hosts tend to have a larger number of satellites. Physical properties of the satellites such as the size–luminosity relation are also consistent with the MW satellites. However, the spatial distribution is different; we find that the satellite galaxies outside of the LG show no sign of concentration or alignment, while that of the MW satellites is more concentrated around the host and exhibits a significant alignment. As we focus on relatively massive satellites with M V < −10, we do not expect that the observational incompleteness can be responsible here. This trend might represent a peculiarity of the MW satellites, and further work is needed to understand its origin.

Commotion in their Motions: Proper Motion Anomalies of Nearby AGB Stars

Proper motion anomalies (PMa) are often useful for identifying companions, but using this technique for large and luminous stars like Asymptotic Giant Branch (AGB) stars is problematic. We studied nearby AGB stars and their potential companions in a PMa catalog derived from Hipparcos and Gaia DR2 proper motion measurements. We found that companion configurations based on PMa for AGB stars are often implausible, such that the radius of the primary star is larger than the orbit of the putative companion. This may suggest an alternative origin of PMa in AGB stars, possibly surface brightness fluctuations due to their large convective surfaces.