Shock waves and non-thermal phenomena in merging galaxy clusters

In this Thesis, we used state of the art radio and X-ray datasets and techniques to derive constraints on the formation mechanisms of diffuse radio emission in merging galaxy clusters. In particular, turbulence is believed to be responsible for the formation of the central and likely spherical sources called radio halos, while shocks are the origin of the elongated and polarized emissions found in cluster outskirts known as radio relics. Although this scenario seems supported by current observations, the processes that originate these synchrotron sources are still poorly constrained. An important goal achieved during the Thesis is a progress on the relic-shock connection and on the origin of radio relics. This was obtained thanks to the detection in the X-rays of new shocks in merging galaxy clusters. In combination with the analysis of radio observations, this allowed us to derive efficient constraints on the mechanisms of particle (re)acceleration and on the magnetic fields in relics. Notably, we demonstrated for the first time, in an homogeneous way, that merger shocks can not reproduce the luminosity of radio relics if particles are accelerated from the thermal pool. This strongly support that other mechanisms, such as shock re-acceleration, are involved in the formation of this kind of sources. LOFAR is a new generation interferometer that is providing a revolutionary view of clusters at low frequencies. For this reason, the exploitation of LOFAR observations represented a central task of the Thesis. We used LOFAR observations in combination with X-ray and radio data coming from other facilities to study non-thermal phenomena in two dynamically complex cluster mergers providing also first hints of a radio bridge of emission connecting two clusters in a pre-merging phase. Our results proved the extraordinary potential of LOFAR in galaxy cluster science

A New Galaxy Cluster Merger Capable of Probing Dark Matter: Abell 56

We report the discovery of a binary galaxy cluster merger via a search of the redMaPPer optical cluster catalog, with a projected separation of 535 kpc between the BCGs. Archival XMM-Newton spectro-imaging reveals a gas peak between the BCGs, suggesting a recent pericenter passage. We conduct a galaxy redshift survey to quantify the line-of-sight velocity difference ($153\pm281$ km/s) between the two subclusters. We present weak lensing mass maps from archival HST/ACS imaging, revealing masses of $M_{200}=4.5\pm0.8\times10^{14}$ and $2.8\pm0.7\times10^{14}$ M$_\odot$ associated with the southern and northern galaxy subclusters respectively. We also present deep GMRT 650 MHz data revealing extended emission, 420 kpc long, which may be an AGN tail but is potentially also a candidate radio relic. We draw from cosmological n-body simulations to find analog systems, which imply that this system is observed fairly soon (60-271 Myr) after pericenter, and that the subcluster separation vector is within 22$^\circ$ of the plane of the sky, making it suitable for an estimate of the dark matter scattering cross section. We find $\sigma_{\rm DM}=1.1\pm0.6$ cm$^2$/g, suggesting that further study of this system could support interestingly tight constraints.Comment: accepted to Ap

Surface brightness discontinuities in radio halos. Insights from the MeerKAT Galaxy Cluster Legacy Survey

Dynamical motions in the ICM can imprint distinctive features on the X-ray images that map the thermal emission from clusters, such as sharp surface brightness discontinuities due to shocks and cold fronts. The gas dynamics during cluster mergers may also drive large-scale turbulence in the ICM which in turn generates extended synchrontron sources known as radio halos. The presence of surface brightness edges in the thermal gas of clusters has been established by a number of X-ray observations. In contrast, edges in radio halos have been observed only in a handful of cases. Our goal is to search for new radio surface brightness discontinuities in the ICM. We inspected the images of the Bullet Cluster and the other 25 radio halos reported in the MeerKAT Galaxy Cluster Legacy Survey. To aid the identification of surface brightness discontinuities, we applied a gradient filtering edge detection method to the radio images. We found that the adopted filtering technique is helpful to identify surface brightness edges in radio images, allowing us to identify at least one gradient in half of the radio halos studied. For the Bullet Cluster, we found excellent agreement between the locations of the 4 radio discontinuities detected and X-ray edges. This similarity informs us that there is substantial interplay between thermal and non-thermal components in galaxy clusters. This interplay is likely due to the forzen-in ICM magnetic field which mediates the advection of cosmic rays while being dragged by thermal gas flows. We conclude that radio halos are shaped by dynamical motions in the ICM and that they often display surface brightness discontinuities apparently co-located with edges in the thermal gas emission. Our results demonstrate that new and future generations of radio telescopes will provide a complementary approach to X-rays to efficiently detect shocks and cold fronts in the ICM.Comment: 10 pages, 5 figures, 1 table (excluding Appendixes). Abstract abridged to meet arXiv requirements. Submitted to A&

Walk on the Low Side: LOFAR explores the low-frequency radio emission of GASP jellyfish galaxies

Jellyfish galaxies, characterized by long filaments of stripped interstellar medium extending from their disks, are the prime laboratories to study the outcomes of ram pressure stripping. At radio wavelengths, they often show unilateral emission extending beyond the stellar disk, and an excess of radio luminosity with respect to that expected from their current star formation rate. We present new 144 MHz images provided by the LOFAR Two-metre Sky Survey for a sample of six galaxies from the GASP survey. These galaxies are characterized by a high global luminosity at 144 MHz ($6-27\times10^{22}$ W Hz$^{-1}$), in excess compared to their ongoing star formation rate. The comparison of radio and H$\alpha$ images smoothed with a Gaussian beam corresponding to $\sim$10 kpc reveals a sub-linear spatial correlation between the two emissions with an average slope $k=0.50$. In their stellar disk we measure $k=0.77$, which is close to the radio-to-star formation linear relation. We speculate that, as a consequence of the ram pressure, in these jellyfish galaxies the cosmic rays transport is more efficient than in normal galaxies. Radio tails typically have higher radio-to-H$\alpha$ ratios than the disks, thus we suggest that the radio emission is boosted by the electrons stripped from the disks. In all galaxies, the star formation rate has decreased by a factor $\leq10$ within the last $\sim10^8$ yr. The observed radio emission is consistent with the past star formation, so we propose that this recent decline may be the cause of their radio luminosity-to-star formation rate excess.Comment: 22 pages, 7 figures. Accepted for publication on ApJ on 24/08/202

On the encounter between the GASP galaxy JO36 and the radio plume of GIN 049

We report on the serendipitous discovery of an unprecedented interaction between the radio lobe of a radio galaxy and a spiral galaxy. The discovery was made thanks to LOFAR observations at 144 MHz of the galaxy cluster Abell 160 ($z=0.04317$) provided by the LOFAR Two-metre Sky Survey. The new low-frequency observations revealed that one of the radio plumes of the central galaxy GIN 049 overlaps with the spiral galaxy JO36. Previous studies carried out with MUSE revealed that the warm ionized gas in the disk of JO36, traced by the H$\alpha$ emission, is severely truncated with respect to the stellar disk. We further explore this unique system by including new uGMRT observations at 675 MHz to map the spectral index. The emerging scenario is that JO36 has interacted with the radio plume in the past 200-500 Myr. The encounter resulted in a positive feedback event for JO36 in the form of a star formation rate burst of $\sim14$ $M_\odot$ yr$^{-1}$. In turn, the galaxy passage left a trace in the radio-old plasma by re-shaping the old relativistic plasma via magnetic draping.Comment: 20 pages, 11 figures. Accepted for publication on ApJ on September 4th, 202

The discovery of a radio galaxy of at least 5 Mpc

We discover what is in projection the largest known structure of galactic origin: a giant radio galaxy with a projected proper length of $4.99 \pm 0.04\ \mathrm{Mpc}$. The source, named Alcyoneus, was first identified in low-resolution LOFAR Two-metre Sky Survey images from which angularly compact sources had been removed. Being an extreme example in its class, Alcyoneus could shed light on the main mechanisms that drive radio galaxy growth. We find that - beyond geometry - Alcyoneus and its host galaxy appear suspiciously ordinary: the total low-frequency luminosity density, stellar mass and supermassive black hole mass are all lower than, though similar to, those of the medial giant radio galaxy (percentiles$45 \pm 3\%$,$25 \pm 9 \%$and$23 \pm 11 \%$, respectively). The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. At$5 \cdot 10^{-16}\ \mathrm{Pa}$, the pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents one of the most promising radio galaxies yet to probe the warm-hot intergalactic medium.Comment: 18 pages, 14 figures, 3 tables, accepted for publication in Astronomy & Astrophysic

Fast magnetic field amplification in distant galaxyclusters

In the present-day Universe, magnetic fields pervade galaxy clusters, with strengths of a few microGauss obtained from Faraday Rotation. Evidence for cluster magnetic fields is also provided by Megaparsec-scale radio emission, namely radio halos and relics. These are commonly found in merging systems and are characterized by a steep radio spectrum. It is widely believed that magneto-hydrodynamical turbulence and shock-waves (re-)accelerate cosmic rays, producing halos and relics. The origin and the amplification of magnetic fields in clusters is not well understood. It has been proposed that turbulence drives a small-scaledynamo that amplifies seed magnetic fields (primordial and/or injected by galactic outflows, as active galactic nuclei, starbursts, or winds). At high redshift, radio halos are expected to be faint, due to the Inverse Compton losses and dimming effect with distance. Moreover, Faraday Rotation measurements are difficult to obtain. If detected, distant radio halosprovide an alternative tool to investigate magnetic field amplification. Here, we report LOFAR observations which reveal diffuse radio emission in massive clusters when the Universe was only half of its present age, with a sample occurrence fraction of about 50%. The high radio luminosities indicate that these clusters have similar magnetic field strengths to those in nearby clusters, and suggest that magnetic field amplification is fast during the first phases ofcluster formation.Comment: Published in Nature Astronomy on 2 November 2020. The published version is available at this URL https://www.nature.com/articles/s41550-020-01244-5#citea

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multiwavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (similar to 1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120-168 MHz and the Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size of about 1.8 Mpc and a flux density S-144 MHz = (1.52 +/- 0.31) Jy. The spectral index distribution of the source is patchy with an average spectral index alpha similar to 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index alpha similar to 2.1 has been obtained between 1.410 and 1.782 GHz. A previously unseen patch of steep spectrum emission is clearly detected at 144 MHz in the south of the cluster. Overall, our findings suggest that we are observing an overlapping of different structures, powered by the turbulence associated with the primary and a possible secondary merger.Peer reviewe