arXiv subjects

N. Lyskova

Publications and source records attributed to N. Lyskova.

At least 19 recordsLinked to original sources

arXiv2026

Average soft X-ray surface brightness profile of massive galaxy clusters in Magneticum simulations

The self-similar growth of massive galaxy clusters suggests that radial profiles of their key thermodynamic properties should have identical shapes after proper mass- and redshift-dependent re-scaling. This property, tested within the virial radius on samples of well-studied individual objects, together with clear and robust observational characteristics such as sensitivity and background accounting, enables the possibility of stacking observations that can be confronted with identically-derived population-averaged predictions from theory or numerical simulations at large radii. Such a comparison not only eliminates effects of inevitable stochasticity in properties of individual objects, but also allows one to reach higher sensitivity for the faintest regions on the outskirts of the clusters. In this study, we conduct a one-to-one comparison of the observed and simulated average soft X-ray surface brightness profiles of several dozen massive galaxy clusters at low redshift. We find a very good out-of-the-box agreement between the 0.3 - 2.3 keV surface brightness profile of stacked galaxy clusters recently measured by SRG/eROSITA and the corresponding predictions from the Magneticum cosmological hydrodynamical simulations, which are known to reproduce other scaling relations observed for massive galaxy clusters. A significant difference between the observed and simulated profiles is present in the very central region, where effective implementation of the AGN feedback likely results in excessive gas redistribution within the core. The simulations predict a very noisy surface brightness profile beyond several times the virial radius of the cluster, with the mean signal being orders of magnitude lower than the local radially-flat but strongly fluctuating emission background, meaning that a proper detection of this component would be very challenging even with larger samples in the future.

astro-ph.CO↗

arXiv2026

The Treble Clef radio phoenix and its old nonthermal filaments

By inspecting data from the LOFAR Two-meter Sky Survey (LoTSS), we noticed a peculiar bright and filamentary radio source at low-galactic latitude ($b \approx 0.5 °$). This source, detected also in previous radio observations, was originally believed to be a pulsar until Green et al. (2004) suggested that it is located in a heavily obscured galaxy cluster behind the Galactic plane. In this paper, we characterize for the first time the main properties of the host cluster (redshift, mass, temperature, X-ray luminosity, and dynamical status) by using X-ray observations performed with Chandra and SRG/eROSITA. In addition, by combining new uGMRT follow-up data with observations from the e LOFAR LBA Sky Survey (LoLSS), we perform a multifrequency, spatially resolved spectral analysis of the filamentary radio source (VLSS J0318.9+5755, nicknamed here the "Treble Clef" due to its morphology). We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the Zone of Avoidance. We speculate that its complex morphology is shaped by gas motions generated in the intracluster medium during the ongoing merger, which are also likely responsible for the generation of the candidate radio halo tentatively observed in the cluster center. Owing to its highly filamentary morphology, brightness at $\lesssim$1 GHz, and extremely steep spectrum, reaching values of $α> 4$ between 400 and 650 MHz, this source represents an ideal target for high-resolution, very-low-frequency follow-up observations with LOFAR2.0.

astro-ph.CO↗

arXiv2026

Dark and Luminous Matter in the Coma Cluster: Probing Galaxy Cluster Assembly Through Filaments with Weak Lensing and Multiwavelength Observations

The Coma cluster (Abell 1656; $z=0.023$) is a nearby rich galaxy cluster and a key laboratory for studying cluster assembly in the Cosmic Web. We characterize its projected dark matter distribution and connection to galaxies, the intracluster medium, and reported intracluster filaments (ICFs) with wide-field ($\sim$12-deg$^2$) Subaru/Hyper Suprime-Cam weak-lensing (WL) analysis. We reconstruct the two-dimensional mass distribution, fit Navarro-Frenk-White (NFW) models, derive an aperture mass densitometry profile, and compare the WL signal with optical spectroscopy, eROSITA X-ray observations, radio data, and gas fraction diagnostics. A single-halo NFW fit yields $M_{200\mathrm{c}}=8.2\pm0.7\times10^{14}~M_{\odot}$. The aperture mass profile agrees with the best-fit NFW model and the X-ray hydrostatic mass at $R\gtrsim20'$ ($\sim$560 kpc), suggesting little merger-induced bias in the global WL mass, while the inner region shows substantial hydrostatic bias. A two-halo NFW fit centered on NGC 4874 and NGC 4839 gives masses of $7.8\pm0.6$ and $0.9\pm0.2\times10^{14}~M_{\odot}$, implying a $\sim$1:8 minor merger. The gas mass fraction suggests that the system is returning from first apocenter. We find a positive spatial correlation between the WL signal and X-ray surface brightness, strongest along the ICF directions ($110^{\circ}$ and $340^{\circ}$), where shear-selected subhalos are predominantly detected. The Coma $r$-band mass-to-light ratio is radially constant with $\langle M/L_r\rangle\simeq250\pm66~M_{\odot}/L_{\odot}$ within $R_{200\mathrm{c}}$, whereas the northern and western ICFs show higher values of $\sim1000~M_{\odot}/L_{\odot}$, suggesting stronger dark matter dominance. These results show that joint WL and multiwavelength analyses can effectively probe cluster assembly and the dark matter content of ICFs.

astro-ph.GA↗

arXiv2026

Spectroscopic galaxy redshifts in the Peanut cluster -- a massive nearly head-on cluster merger shortly after pericenter passage

The Peanut cluster (SRGe J023820.8+200556, SRGe CL0238.3+2005, $z_{spec}$ = 0.42) has recently emerged as a candidate for a rare, massive merger, potentially analogous to the Bullet cluster. We present the results of optical identification and spectroscopic redshift measurements for 31 galaxies in the Peanut cluster, including 26 new redshifts obtained with the 6-m telescope BTA (Big Telescope Alt-azimuthal) at SAO RAS between October 2024 and January 2025. The derived distribution of line-of-sight velocities reveals the possible presence of two subclusters with a line-of-sight velocity difference of ~2000 km/s. However, statistical tests and the Dressler-Schectman test show that the hypothesis that the observed velocity distribution can be described by a normal distribution for a single cluster cannot be ruled out, and the evidence for the existence of two gravitationally bound substructures remains ambiguous. Assuming a single cluster with the normal velocity distribution, the estimated galaxy velocity dispersion is $σ_{los} = 1455 \pm 83$ km/s, corresponding to the total cluster mass of $M_{200} = 2 \times 10^{15} M_\odot$ based on the mass-velocity dispersion scaling relation. In either scenario -- a single extremely massive cluster or an ongoing merger -- the Peanut cluster appears to be a very rare and peculiar object, comparable to such extreme systems as the Bullet cluster (1E 0657-56) or El Gordo (ACT-CL J0102-4915).

astro-ph.GA↗

arXiv2026

ComPACT: Mass-Redshift Properties of the galaxy cluster catalogue

Machine-learning methods are increasingly applied to astronomical surveys, providing powerful tools for detecting and studying galaxy clusters. We investigate the mass-redshift properties and completeness of the ComPACT galaxy cluster catalogue, constructed using a convolutional neural network applied to publicly available combined ACT+Planck maps. The ComPACT catalogue contains 2,962 SZ-selected galaxy cluster candidates. We confirm clusters by estimating redshifts using literature information and photometric techniques based on DESI Legacy Imaging Surveys data. Cluster masses are derived from ACT+Planck and Planck Compton-y maps via SZ scaling relations. The completeness is assessed using simulated cluster injections into real microwave maps. We confirm approximately $\sim$60 % of the ComPACT candidates as galaxy clusters. The redshifts span the range $0.007 < z < 1.7$, including approximately 116 new measurements. Masses are obtained for 56 % of the sample, covering the range $(0.25 - 13.1) \times 10^{14} ~M_\odot$ and including 158 new mass determinations. We identify five previously unreported massive clusters ($M_{500c} > 6 \times 10^{14}~M_\odot$) at $z > 0.7$, increasing the known population of such systems by approximately 10 %. The ComPACT catalogue expands the SZ-selected Planck-like cluster population, especially at high redshift and high mass, demonstrating the effectiveness of deep-learning approaches for cluster detection in microwave data.

astro-ph.CO↗

arXiv2025

Non-thermal filaments and AGN recurrent activity in the galaxy group Nest200047: a LOFAR, uGMRT, MeerKAT, VLA radio spectral analysis

Nest200047 is a clear example of multiple radio bubbles from an Active Galactic Nucleus (AGN) in a galaxy group, featuring non-thermal filaments likely shaped by buoyancy, gas motions, and stabilized by magnetic fields. This study presents high-quality data obtained from uGMRT, MeerKAT, and VLA, alongside existing LOFAR data, to analyze the system's morphology and spectrum over a broad frequency range (53-1518 MHz). Our findings reveal new filamentary emission in the inner 60 kpc, surrounding and extending from the inner bubbles and jets, suggesting complex dynamical evolution of the non-thermal plasma in the group core. The filaments have widths of a few kpc and lengths from tens to hundreds of kpc, with a steep and curved radio spectrum ($\rm α=1\sim2$). They exhibit a constant spectral index profile along their length, implying particles are either (re-)accelerated together or move at super-Alfvenic speeds. Spectral aging analysis yields jet active times between 50 and 100 Myr with short inactive phases, suggesting continuous energy injection typical of AGN feedback in galaxy groups. This study highlights the potential of combining high-quality radio data to understand recurrent jet activity and feedback, with implications for future research with the SKA observatory.

astro-ph.GA↗

arXiv2025

X-ray investigation of the remarkable galaxy group Nest200047

Galaxy groups are more susceptible to feedback from the central active galactic nuclei (AGN) due to their lower gravitational binding energy compared to clusters. This makes them ideal laboratories to study feedback effects on the overall energy and baryonic mass budget. We study the LOFAR-detected galaxy group Nest200047, where there is clear evidence of multiple generations of radio lobes from the AGN. Using 140 ks Chandra and 25 ks XMM-Newton data, we investigate thermodynamic properties of the the intragroup medium including any excess energy due to the central AGN. We also investigate X-ray properties of the central black hole and constrain the $2-10$ keV X-ray flux. We used spectral analysis techniques to measure various thermodynamic profiles across the whole field of view. We also used both imaging and spectral analysis to detect and estimate the energy deposited by potential shocks and cavities. Due to the faint emission from the object beyond the core, various background effects were considered. Nest200047 has significant excess entropy, and the AGN likely contributes to a part of it. There is an excess energy of $(5-6.5) \times 10^{60}$ erg within 400 kpc, exceeding the binding energy. The pressure profile indicates that gas is likely being ejected from the system, resulting in a baryon fraction of $\sim4\%$ inside $r_{500}$. From scaling relations, we estimate a black hole mass of $(1-4)\times 10^9 M_{\odot}$. An upper limit of $2.1 \times 10^{40}$ erg s$^{-1}$ was derived on the black hole bolometric luminosity, which is $\sim$2.5% of the Bondi accretion power. Nest200047 is likely part of a class of over-heated galaxy groups like ESO 3060170, AWM 4 and AWM 5. Such excessive heating may lead to high quenching of star formation. Moreover, the faint X-ray nuclear emission in Nest is likely due to the accretion energy being converted into jets rather than radiation.

astro-ph.CO↗

arXiv2025

Average energy of the X-ray spectrum as a model-independent proxy for the mass of galaxy clusters

Temperature of the hot gas in galaxy clusters is known to be a reliable proxy for their total gravitating mass, allowing one to use spectroscopic X-ray observations for halo mass function measurements. Data of shallow wide area surveys, however, often precludes direct fitting of the X-ray spectra, given possible biases arising due to unresolved (multi-temperature) inner structure of the intracluster medium (ICM), projection effects and necessity of certain model assumptions to be made to allow for robust spectral fitting. We consider using a simple observable value - the average energy of the observed cluster X-ray spectrum - as a model-independent proxy for the ICM temperature, and consequently cluster's mass. We calibrate relation of this proxy to the cluster parameters using mock observations for a sample of 84 massive galaxy clusters extracted from the \textit{Magneticum} cosmological hydro simulations. We consider observational parameters corresponding to the all-sky survey observations by \textit{SRG/eROSITA}. Taking into account contributions of various background and foreground signals, average energy of the simulated X-ray spectra in the $0.4-7.0$ keV band is shown to be a stable indicator of the ICM temperature with $\sim10\%$ scatter and cluster's mass $M_{500}$ with a $\sim 20\%$ scatter. A database containing simulated X-ray images and their spectra (subtracted in several concentric rings) is publicly available.

astro-ph.CO↗

arXiv2025

Limits and challenges of the detection of cluster-scale diffuse radio emission at high redshift: The Massive and Distant Clusters of WISE Survey (MaDCoWS) in LoTSS-DR2

Diffuse radio emission in galaxy clusters is a tracer of ultra-relativistic particles and $μ$G-level magnetic fields, and is thought to be triggered by cluster merger events. In the distant Universe (i.e. $z>0.6$), such sources have been observed only in a handful of systems, and their study is important to understand the evolution of large-scale magnetic fields over the cosmic time. Previous studies of nine {\it Planck} clusters up to $z\sim0.9$ suggest a fast amplification of cluster-scale magnetic fields, at least up to half of the current Universe's age, and steep spectrum cluster scale emission, in line with particle re-acceleration due to turbulence. In this paper, we investigate the presence of diffuse radio emission in a larger sample of galaxy clusters reaching even higher redshifts (i.e. $z\gtrsim1$). We selected clusters from the Massive and Distant Clusters of {\it WISE} Survey (MaDCoWS) with richness $λ_{15}>40$ covering the area of the second data release of the LOFAR Two-Meter Sky Survey (LoTSS-DR2) at 144 MHz. These selected clusters are in the redshift range $0.78-1.53$ (with a median value of 1.05). We detect the possible presence of diffuse radio emission, with the largest linear sizes of $350-500$ kpc, in 5 out of the 56 clusters in our sample. If this diffuse radio emission is due to a radio halo, these radio sources lie on or above the scatter of the $P_ν-M_{500}$ radio halo correlations (at 150 MHz and 1.4 GHz) found at $z<0.6$, depending on the mass assumed. We also find that these radio sources are at the limit of the detection by LoTSS, and therefore deeper observations will be important for future studies.

astro-ph.CO↗

arXiv2024

Merger of massive galaxy cluster CL0238.3+2005 at z~0.4: just after pericenter passage?

Massive clusters of galaxies are very rare in the observable Universe. Even rarer are mergers of such clusters observed close to pericenter passage. Here, we report on one such case: a massive (~ $10^{15}\,M_\odot$) and hot (kT ~ 10 keV) cluster CL0238.3+2005 at $z\approx 0.42$. For this cluster, we combine X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with the projected separation of subhalos of $\sim 200$ kpc. The line-of-sight velocity of galaxies tentatively associated with the two merging halos differs by 2000-3000 km/s. We conclude that, most plausibly, the merger axis is neither close to the line of sight nor to the sky plane. We compare CL0238 with two well-known clusters MACS0416 and Bullet, and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster has recently (only $\lesssim 0.1$ Gyr ago) experienced an almost head-on merger. We argue that this "just after" system is a very rare case and an excellent target for lensing, Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in intracluster medium that are associated with shock waves, e.g., electron-ion equilibration efficiency and relativistic particle acceleration.

astro-ph.CO↗

arXiv2024

ComPACT: ACT+Planck galaxy cluster catalogue

Galaxy clusters are the most massive gravitationally bound systems consisting of dark matter, hot baryonic gas and stars. They play an important role in observational cosmology and galaxy evolution studies. We develop a deep learning model for segmentation of Sunyaev-Zeldovich (SZ) signal on ACT+Planck intensity maps and construct a pipeline for microwave cluster detection in the ACT footprint. The proposed model allows us to identify previously unknown galaxy clusters, i.e. it is capable of detecting SZ sources below the detection threshold adopted in the published galaxy clusters catalogues (such as ACT DR5 and PSZ2). In this paper, we use the derived SZ signal map to considerably improve a cluster purity in the extended catalogue of Sunyaev-Zeldovich objects from Planck data (SZcat) in the ACT footprint. From SZcat, we create a new microwave galaxy cluster catalogue (ComPACT), which includes 2,962 SZ objects with cluster purity conservatively estimated as $\gtrsim74-84$\%. We categorise objects in the catalogue into 3 categories, based on their cluster reliability. Within the ComPACT catalogue, there are $\gtrsim{}977$ new clusters with respect to the ACT DR5 and PSZ2 catalogues.

astro-ph.GA↗

arXiv2023

Radio-continuum spectra of ram pressure stripped galaxies in the Coma Cluster

$Aims:$ We used the nearby Coma Cluster as a laboratory in order to probe the impact of ram pressure on star formation as well as to constrain the characteristic timescales and velocities for the stripping of the non-thermal ISM. $Methods:$ We used high-resolution ($6.5'' \approx 3\,\mathrm{kpc}$), multi-frequency ($144\,\mathrm{MHz} - 1.5\,\mathrm{GHz}$) radio continuum imaging of the Coma Cluster to resolve the low-frequency radio spectrum across the discs and tails of 25 ram pressure stripped galaxies. With resolved spectral index maps across these galaxy discs, we constrained the impact of ram pressure perturbations on galaxy star formation. We measured multi-frequency flux-density profiles along each of the ram pressure stripped tails in our sample. We then fit the resulting radio continuum spectra with a simple synchrotron aging model. $Results:$ We showed that ram pressure stripped tails in Coma have steep ($-2 \lesssim α\lesssim -1$) spectral indices. The discs of galaxies undergoing ram pressure stripping have integrated spectral indices within the expected range for shock acceleration from supernovae ($-0.8 \lesssim α\lesssim -0.5$), though there is a tail towards flatter values. In a resolved sense, there are gradients in spectral index across the discs of ram pressure stripped galaxies in Coma. These gradients are aligned with the direction of the observed radio tails, with the flattest spectral indices being found on the `leading half'. From best-fit break frequencies we estimated the projected plasma velocities along the tail to be on the order of hundreds of kilometers per second, with the precise magnitude depending on the assumed magnetic field strength.

astro-ph.GA↗

arXiv2023

X-ray surface brightness and gas density profiles of galaxy clusters up to 3*R500c with SRG/eROSITA

Using the data of the SRG/eROSITA all-sky survey, we stacked a sample of ~40 galaxy cluster images in the 0.3--2.3 keV band, covering the radial range up to $10\times R_{\rm 500c}$. The excess emission on top of the galactic and extragalactic X-ray backgrounds and foregrounds is detected up to $\sim 3\times R_{\rm 500c}$. At these distances, the surface brightness of the stacked image drops below $\sim 1$% of the background. The density profile reconstructed from the X-ray surface brightness profile agrees well (within $\sim30$%) with the mean gas profile found in numerical simulations, which predict the local gas overdensity of $\sim$ 20--30 at $3\times R_{\rm 500c}$ and the gas fraction close to the universal value of $\frac{Ω_b}{Ω_m}\approx 0.15$ in the standard $Λ$CDM model. Taking at face value, this agreement suggests that up to $\sim 3\times R_{\rm 500c}$ the X-ray signal is not strongly boosted by the gas clumpiness, although a scenario with a moderately inhomogeneous gas cannot be excluded. A comparison of the derived gas density profile with the electron pressure profile based on the SZ measurements suggests that by $r\sim 3\times R_{\rm 500c}$ the gas temperature drops by a factor of $\sim$ 4--5 below the characteristic temperature of a typical cluster in the sample within $R_{\rm 500c}$, while the entropy keeps growing up to this distance. Better constraints on the gas properties just beyond $3\times R_{\rm 500c}$ should be possible with a sample larger than used for this pilot study.

astro-ph.CO↗

arXiv2023

Prospects of detecting soft X-ray emission from typical WHIM filaments around massive clusters and the Coma cluster soft excess

While hot ICM in galaxy clusters makes these objects powerful X-ray sources, the cluster's outskirts and overdense gaseous filaments might give rise to much fainter sub-keV emission. Cosmological simulations show a prominent "focusing" effect of rich clusters on the space density of the Warm-Hot Intergalactic Medium (WHIM) filaments up to a distance of $\sim 10\,{\rm Mpc}$ ($\sim$ turnaround radius, $r_{ta}$) and beyond. Here, we use Magneticum simulations to characterize their properties in terms of integrated emission measure for a given temperature and overdensity cut and the level of contamination by the more dense gas. We suggest that the annuli $(\sim 0.5-1)\times \,r_{ta}$ around massive clusters might be the most promising sites for the search of the gas with overdensity $\lesssim 50$. We model spectral signatures of the WHIM in the X-ray band and identify two distinct regimes for the gas at temperatures below and above $\sim 10^6\,{\rm K}$. Using this model, we estimate the sensitivity of X-ray telescopes to the WHIM spectral signatures. We found that the WHIM structures are within reach of future high spectral resolution missions, provided that the low-density gas is not extremely metal-poor. We then consider the Coma cluster observed by SRG/eROSITA during the CalPV phase as an example of a nearby massive object. We found that beyond the central $r\sim 40'$ ($\sim 1100\,{\rm kpc}$) circle, where calibration uncertainties preclude clean separation of the extremely bright cluster emission from a possible softer component, the conservative upper limits are about an order of magnitude larger than the levels expected from simulations.

astro-ph.HE↗

arXiv2022

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA. II. Shock & Relic

This is the second paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained during the calibration and performance verification phase of the mission. Here, we focus on the region adjacent to the radio source 1253+275 (radio relic, RR, hereafter). We show that the X-ray surface brightness exhibits its steepest gradient at $\sim 79'$ ($\sim 2.2\,{\rm Mpc}\approx R_{200c}$), which is almost co-spatial to the outer edge of the RR. As in the case of several other relics, the Mach number of the shock derived from the X-ray surface brightness profile ($M_X\approx 1.9$) appears to be lower than needed to explain the slope of the integrated radio spectrum in the diffusive shock acceleration (DSA) model ($M_R\approx 3.5$) if the magnetic field is uniform and the radiative losses are fast. However, the shock geometry is plausibly much more complicated than a spherical wedge centered on the cluster, given the non-trivial correlation between radio, X-ray, and SZ images. While the complicated shock geometry alone might cause a negative bias in $M_X$, we speculate on a few other possibilities that may affect the $M_X$-$M_R$ relation, including the shock substructure that might be modified by the presence of non-thermal filaments stretching across the shock and the propagation of relativistic electrons along the non-thermal filaments with a strong magnetic field. We also discuss the "history" of the radio galaxy NGC4789, which is located ahead of the relic in the context of the Coma-NGC4839 merger scenario.

astro-ph.CO↗

arXiv2022

High-resolution, High-sensitivity, Low-frequency uGMRT View of Coma Cluster of Galaxies

We present high-resolution, high-sensitivity upgraded Giant Metrewave Radio Telescope observations of the Coma cluster (A1656) at 250-500 MHz and 550-850 MHz. At 250-500 MHz, 135 sources have extensions $>$ 0.45 arcmin (with peak-to-local-noise ratio $> 4$). Of these, 24 sources are associated with Coma-member galaxies. In addition, we supplement this sample of 24 galaxies with 20 ram pressure stripped galaxies from Chen et al. (2020, eight are included in the original extended radio source sample) and an additional five are detected and extended. We present radio morphologies, radio spectra, spectral index maps, and equipartition properties for these two samples. In general, we find the equipartition properties lie within a narrow range (e.g., $P_{\rm min}$ = 1-3 dynes cm$^{-2}$). Only NGC 4874, one of the two brightest central Coma cluster galaxies, has a central energy density and pressure about five times higher and a radio source age about 50 % lower than that of the other Coma galaxies. We find a diffuse tail of radio emission trailing the dominant galaxy of the merging NGC 4839 group that coincides with the "slingshot" tail, seen in X-rays. The southwestern radio relic, B1253$+$275, has a large extent $\approx$ 32$^\prime$ $\times$ 10$^\prime$ ($\simeq$ 1.08 $\times$ 0.34 Mpc$^2$). For NGC 4789, whose long radio tails merge into the relic and may be a source of its relativistic seed electrons, and we find a transverse radio spectral gradient, a steepening from southwest to northeast across the width of the radio source. Finally, radio morphologies of the extended and RPS samples suggest that these galaxies are on their first infall into Coma on (predominantly) radial orbits.

astro-ph.GA↗

arXiv2021

A snapshot of the oldest AGN feedback phases

Active Galactic Nuclei (AGN) inject large amounts of energy into their host galaxies and surrounding environment, shaping their properties and evolution. In particular, AGN jets inflate cosmic-ray lobes, which can rise buoyantly as light `bubbles' in the surrounding medium, displacing and heating the encountered thermal gas and thus halting its spontaneous cooling. These bubbles have been identified in a wide range of systems. However, due to the short synchrotron lifetime of electrons, the most advanced phases of their evolution have remained observationally unconstrained, preventing us to fully understand their coupling with the external medium, and thus AGN feedback. Simple subsonic hydrodynamic models predict that the pressure gradients, naturally present around the buoyantly rising bubbles, transform them into toroidal structures, resembling mushroom clouds in a stratified atmosphere. The way and timescales on which these tori will eventually disrupt depend on various factors including magnetic fields and plasma viscosity. Here we report LOFAR observations below 200 MHz, sensitive to the oldest radio-emitting particles, showing the late evolution of multiple generations of cosmic-ray AGN bubbles in a galaxy group with unprecedented level of detail. The bubbles' buoyancy power can efficiently offset the radiative cooling of the intragroup medium. However, the bubbles have still not thoroughly mixed with the thermal gas, after hundreds of million years, likely under the action of magnetic fields.

astro-ph.HE↗

arXiv2021

Tempestuous life beyond R_500: X-ray view on the Coma cluster with SRG/eROSITA. I. X-ray morphology, recent merger, and radio halo connection

This is the first paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained in course of the Calibration and Performance Verification observations. The data cover $\sim3^\circ\times 3^\circ$ area around the cluster with a typical exposure time of more than 20 ks. The stability of the instrumental background and operation of the SRG Observatory in the scanning mode provided us with an excellent data set for studies of the diffuse emission up to a distance of $\sim 1.5R_{200}$ from the Coma center. In this study, we discuss the rich morphology revealed by the X-ray observations (also in combination with the SZ data) and argue that the most salient features can be naturally explained by a recent (on-going) merger with the NGC 4839 group. In particular, we identify a faint X-ray bridge connecting the group with the cluster, which is convincing proof that NGC 4839 has already crossed the main cluster. The gas in the Coma core went through two shocks, first through the shock driven by NGC 4839 during its first passage through the cluster some Gyr ago, and, more recently, through the "mini-accretion shock" associated with the gas settling back to quasi-hydrostatic equilibrium in the core. After passing through the primary shock, the gas should spend much of the time in a rarefaction region, where radiative losses of electrons are small, until the gas is compressed again by the mini-accretion shock. Unlike "runway" merger shocks, the mini-accretion shock does not feature a rarefaction region downstream and, therefore, the radio emission can survive longer. Such a two-stage process might explain the formation of the radio halo in the Coma cluster.

astro-ph.HE↗