A combined JVLA and Chandra study of the Abell 2626 galaxy cluster

In this thesis work we carried out an accurate analysis of new observations of the galaxy cluster Abell 2626 in the radio and X-ray bands. Its radio emission is characterized by the presence of a system of peculiar radio sources, the arcs , whose properties differs from the properties of the common cluster diffuse radio sources. In the past years, several models have been proposed to explain their origin. Our work aims at investigating the origin of the arcs. We analyzed new JVLA observations of the cluster at 3.0 GHz and 5.5 GHz. During the radio analysis we detected the arcs at 3.0 GHz and possible evidences for extended emission inside them at 5.5 GHz. By combining archival maps at 1.4 GHz and the new ones at 3.0 GHz, we estimated that the arcs have a spectral index α ∼ -3. The analysis of a new CHANDRA observation of the cluster produced surprising results. We highlighted the a spatial connection between the radio arcs and a cold region inside the cluster core and, moreover, we discovered a cold front which coincides with the south-west junction of the radio arcs. It is well known that the turbulence inside the cold fronts is able to accelerate the relativistic particles, triggering radio emission in form of mini-halos. Under certain conditions, this emission results in form of collimated steep-spectrum arcs that traces the dynamics of the cold fronts. So we proposed a scenario, in which the arcs are not related to the precession of AGN jets, but instead are produced by non-thermal processes (particle acceleration and magnetic fled amplification) that are related to the formation and the evolution of the cold fronts in the core of Abell 2626. In this case the peculiar morphology of the arcs is the result of the dynamics of the cold fronts, that will be studied with future MHD simulations

Introducing PT-REX, the point-to-point TRend EXtractor

Investigating the spatial correlation between different emissions in an extended astrophysical source can provide crucial insights into their physical connection, hence it can be the key to understand the nature of the system. The point-to-point analysis of surface brightness is a reliable method to do such an analysis. In this work we present PT-REX, a software to carry out these studies between radio and X-ray emission in extended sources. We discuss how to reliably carry out this analysis and its limitation and we introduce the Monte Carlo point-to-point analysis, which allows to extend this approach to poorly-resolved sources. Finally we present and discuss the application of our tool to study the diffuse radio emission in a galaxy cluster...

New JVLA observations at 3 GHz and 5.5 GHz of the `Kite' radio source in Abell 2626

We report on new JVLA observations performed at 3 GHz and 5.5 GHz of Abell 2626. The cluster has been the object of several studies in the recent years due to its peculiar radio emission, which shows a complex system of symmetric radio arcs characterized by a steep spectrum. The origin of these radio sources is still unclear. Due to their mirror symmetry toward the center, it has been proposed that they may be created by pairs of precessing jets powered by the inner AGN. The new JVLA observations were requested with the specific aim of detecting extended emission on frequencies higher than 1.4 GHz, in order to constrain the jet-precession model by analyzing the spectral index and radiative age patterns alongs the arcs. We performed a standard data reduction of the JVLA datasets with the software CASA. By combining the new 3 GHz data with the archival 1.4 GHz VLA dataset we produced a spectral index maps of the extended emission, and then we estimated the radiative age of the arcs by assuming that the plasma was accelerated in moving hot-spots tracing the arcs. Thanks to the high sensitivity of the JVLA, we achieve the detection of the arcs at 3 GHz and extended emission at 5.5 GHz. We measure a mean spectral index <-2.5 for the arcs up to 3 GHz. No clear spectral index, or radiative age, trend is detected across the arcs which may challenge the interpretation based on precession or put strong constraints on the jet-precession period. In particular, by analyzing the radiative age distribution along the arcs, we were able to provide for the first time a time-scale < 26 Myr of the jet-precession period.Comment: 8 pages, 5 figures. Accepted for publication in A&

Interplay between relativistic and thermal plasma in relaxed galaxy clusters

Galaxy clusters are complex ecosystems, where galaxies with powerful active galactic nuclei, thermal plasma, magnetic fields, and cosmic rays can interact and flourish in spectacular astrophysical phenomena. Decades of studies have investigated these systems, and as the boundaries of our knowledge were pushed forward, new questions emerged awaiting to be answered. The main focus of this Thesis is the interplay between the relativistic and thermal plasma of the intra-cluster medium. The study of the intra-cluster medium (ICM) allows to understand the way energy is injected from large scale dynamics and dissipated in different channels. These channels include viscous heating and the generation of non-thermal components that, eventually, generate diffuse radio emission. Furthermore, studying the ICM provides complementary insights into the physics of cluster galaxy evolution. We mainly focus on the case of relaxed galaxy clusters, where the connection between the central, massive, radio-loud galaxies and the diffuse radio emission is still unclear. In this context, we investigate also the interactions between the ICM and the cluster galaxies. The cornerstone of our works is a multi-wavelength analysis based on radio and X-ray observations, which features brand-new Low-Frequency Array (LOFAR) observations and new approaches to the study of the spatial correlation between these emissions

Radio and X-ray connection in radio mini-halos: implications for hadronic models

The radio mini halos (MH) observed in relaxed clusters probe the presence of relativistic particles on scales of hundreds of kpc, beyond the scales directly influenced by the central AGN, but the nature of the mechanism that produces the relativistic electrons is still debated. In this work we explore the connection between thermal and non-thermal components of the ICM in a sample of MH and we study its implications for hadronic models for the origin of the relativistic electrons. We studied the thermal and non-thermal connection by carrying out a point-to-point comparison of the radio and the X-ray surface brightness. We extended the method generally applied to giant radio halos by considering the effects of a grid randomly generated through a Monte Carlo chain. Contrary to what is generally observed for giant radio halos, we find that the mini-halos in our sample have super-linear scaling between radio and X-rays, which suggests a peaked distribution of relativistic electrons and magnetic field. We used the radio and X-ray correlation to constrain the physical parameters of a hadronic model and we compared the model predictions with current observations. Specifically, we focus on a model where cosmic rays are injected by the central AGN and they generate secondaries in the ICM, and we assume that the role of turbulent re-acceleration is negligible. This model allows us to constrain the AGN cosmic ray luminosity in the range $\sim10^{44-46}$ erg s$^{-1}$ and the central magnetic field in the range 10-40 $\mu$G. The resulting $\gamma$-ray fluxes calculated assuming these model parameters do not violate the upper limits on $\gamma$-ray diffuse emission set by the Fermi-LAT telescope. Further studies are now required to explore the consistency of these large magnetic fields with Faraday rotation studies and to study the interplay between the secondary electrons and the ICM turbulence.Comment: 17 pages, 13 figures. Accepted for publication on A&

Ram pressure stripping in the EAGLE simulation

Ram pressure stripping of satellite galaxies is thought to be a ubiquitous process in galaxy clusters, and a growing number of observations reveal satellites at different stages of stripping. However, in order to determine the fate of any individual galaxy, we turn to predictions from either simulations or analytic models. It is not well-determined whether simulations and analytic models agree in their predictions, nor the causes of disagreement. Here we investigate ram pressure stripping in the reference EAGLE hydrodynamical cosmological simulation, and compare the results to predictions from analytic models. We track the evolution of galaxies with stellar mass $M_{*} > 10^{9} \, \mathrm{M_{\odot}}$ and initial bound gas mass $M_{\mathrm{gas}} > 10^{9} \, \mathrm{M_{\odot}}$ that fall into galaxy clusters ($M_{\mathrm{200c}} > 10^{14} \, \mathrm{M_{\odot}}$) between $z = 0.27$ and $z = 0$. We divide each galaxy into its neutral gas disk and hot ionized gas halo and compare the evolution of the stripped gas fraction in the simulation to that predicted by analytic formulations for the two gas phases, as well as to a toy model that computes the motions of gas particles under the combined effects of gravity and a spatially uniform ram pressure. We find that the analytic models generally underpredict the stripping rate of neutral gas and overpredict that of ionized gas, with significant scatter between the model and simulation stripping timescales. This is due to opposing physical effects: the enhancement of ram pressure stripping by stellar feedback, and the suppression of stripping by the compaction of galactic gas.Comment: 28 pages, 12 figures, plus 1 appendix. Version accepted to Ap

Unveiling the Interplay between the GASP Jellyfish Galaxy JO194 and Its Environment with Chandra

X-ray studies of jellyfish galaxies opened a window into the physics of the interplay between the intracluster medium (ICM) and interstellar medium (ISM). In this paper, we present the study of an archival Chandra observation of the GASP jellyfish galaxy JO194. We observe X-ray emission extending from the stellar disk to the unwinding spiral arms with an average temperature of kT = 0.79 ± 0.03 keV. To investigate the origin of the X-ray emission, we compare the observed X-ray luminosities with those expected from the star formation rates (SFRs) obtained from Hα emission. We estimate an X-ray luminosity excess of a factor ∼2–4 with respect to the SF; therefore, we conclude that SF is not the main event responsible for the extended X-ray emission of JO194. The metallicity in the spiral arms (Z = 0.24+0.19 Z ) is consistent with that of the ICM around JO194 -0.12 (Z = 0.35 ± 0.07); thus, we suggest that ICM radiative cooling dominates the X-ray emission of the arms. We speculate that the X-ray plasma results from the ISM–ICM interplay, although the nature of this interplay is still mostly unknown. Finally, we observe that the X-ray properties of JO194 are consistent with those of two other GASP galaxies with different stellar mass, phase-space conditions in their hosting clusters, and local ICM conditions. We suggest that the conditions required to induce extended X-ray emission in jellyfish galaxies are established at the beginning of the stripping, and they can persist on long timescales so that galaxies in different clusters and evolutionary stages can present a similar extended X-ray emission

Ram-pressure stripped radio tail and two ULXs in the spiral galaxy HCG 97b

We report LOFAR and VLA detections of extended radio emission in the spiral galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio emission detected at 144 MHz, 1.4 GHz and 4.8 GHz is elongated along the optical disk and has a tail that extends 27 kpc in projection towards the centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data show two off-nuclear ultra-luminous X-ray sources (ULXs) with the more distant one being a suitable candidate for an accreting intermediate-mass black hole (IMBH) embedded in an environment with an increased density of molecular gas. Given the observed morphology in optical, CO, and radio continuum, we propose that the galaxy is undergoing ram-pressure stripping and the relativistic plasma accelerated in star-forming regions is transported from the galactic disc by galaxy-intragroup medium interaction. Finally, we also demonstrate that the formation of the radio tail could, in principle, be the result of putative IMBH-induced activity, which could facilitate the stripping or inject the radio plasma via jets.Comment: 15 pages, 11 figures, submitted to MNRAS, comments are welcom

GASP XXIII: a jellyfish galaxy as an astrophysical laboratory of the baryonic cycle

© 2019. The American Astronomical Society. All rights reserved. With MUSE, Chandra, VLA, ALMA, and UVIT data from the GASP program, we study the multiphase baryonic components in a jellyfish galaxy (JW100) with a stellar mass 3.2 × 1011 M o hosting an active galactic nucleus (AGN). We present its spectacular extraplanar tails of ionized and molecular gas, UV stellar light, and X-ray and radio continuum emission. This galaxy represents an excellent laboratory to study the interplay between different gas phases and star formation and the influence of gas stripping, gas heating, and AGNs. We analyze the physical origin of the emission at different wavelengths in the tail, in particular in situ star formation (related to Hα, CO, and UV emission), synchrotron emission from relativistic electrons (producing the radio continuum), and heating of the stripped interstellar medium (ISM; responsible for the X-ray emission). We show the similarities and differences of the spatial distributions of ionized gas, molecular gas, and UV light and argue that the mismatch on small scales (1 kpc) is due to different stages of the star formation process. We present the relation Hα-X-ray surface brightness, which is steeper for star-forming regions than for diffuse ionized gas regions with a high [O i]/Hα ratio. We propose that ISM heating due to interaction with the intracluster medium (either for mixing, thermal conduction, or shocks) is responsible for the X-ray tail, observed [O i] excess, and lack of star formation in the northern part of the tail. We also report the tentative discovery in the tail of the most distant (and among the brightest) currently known ULX, a pointlike ultraluminous X-ray source commonly originating in a binary stellar system powered by either an intermediate-mass black hole or a magnetized neutron star