ORCID Profile
0000-0002-9112-0184
Current Organisation
Istituto nazionale di astrofisica
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Publisher: EDP Sciences
Date: 07-2017
Publisher: Oxford University Press (OUP)
Date: 22-08-2020
Abstract: Collisions between galaxy clusters dissipate enormous amounts of energy in the intra-cluster medium (ICM) through turbulence and shocks. In the process, Mpc-scale diffuse synchrotron emission in form of radio halos and relics can form. However, little is known about the very early phase of the collision. We used deep radio observations from 53 MHz to 1.5 GHz to study the pre-merging galaxy clusters A1758N and A1758S that are ∼2 Mpc apart. We confirm the presence of a giant bridge of radio emission connecting the two systems that was reported only tentatively in our earlier work. This is the second large-scale radio bridge observed to date in a cluster pair. The bridge is clearly visible in the LOFAR image at 144 MHz and tentatively detected at 53 MHz. Its mean radio emissivity is more than one order of magnitude lower than that of the radio halos in A1758N and A1758S. Interestingly, the radio and X-ray emissions of the bridge are correlated. Our results indicate that non-thermal phenomena in the ICM can be generated also in the region of compressed gas in-between infalling systems.
Publisher: EDP Sciences
Date: 07-2017
Publisher: Oxford University Press (OUP)
Date: 04-02-2022
Abstract: 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 (∼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 $S_{\\rm 144\\, MHz}=(1.52\\pm 0.31)$ Jy. The spectral index distribution of the source is patchy with an average spectral index α ∼ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index $\\alpha \\, \\sim \\, 2.1$ 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.
Publisher: EDP Sciences
Date: 10-2014
Publisher: EDP Sciences
Date: 07-2022
DOI: 10.1051/0004-6361/202243833
Abstract: Context. Radio observations of galaxy clusters reveal a plethora of diffuse, steep-spectrum sources related to the re-acceleration of cosmic-ray electrons, such as halos, relics, and phoenices. In this context, the LOw Frequency ARray Low-Band Antenna (LOFAR-LBA) Sky Survey (LoLSS) provides the most sensitive images of the sky at 54 MHz to date, allowing us to investigate re-acceleration processes in a poorly explored frequency regime. Aims. We study diffuse radio emission in the galaxy cluster Abell 1550, with the aim of constraining particle re-acceleration in the intra-cluster medium. Methods. We exploited observations at four different radio frequencies: 54, 144, 400, and 1400 MHz. To complement our analysis, we made use of archival Chandra X-ray data. Results. At all frequencies we detect an ultra-steep spectrum radio halo ( S ν ∝ ν −1.6 ) with an extent of ∼1.2 Mpc at 54 MHz. Its morphology follows the distribution of the thermal intra-cluster medium inferred from the Chandra observation. West of the centrally located head-tail radio galaxy, we detect a radio relic with a projected extent of ∼500 kpc. From the relic, a ∼600 kpc long bridge departs and connects with the halo. Between the relic and the radio galaxy, we observe what is most likely a radio phoenix, given its curved spectrum. The phoenix is connected to the tail of the radio galaxy through two arms, which show a nearly constant spectral index for ∼300 kpc. Conclusions. The halo could be produced by turbulence induced by a major merger, with the merger axis lying in the NE-SW direction. This is supported by the position of the relic, whose origin could be attributed to a shock propagating along the merger axis. It is possible that the same shock has also produced the phoenix through adiabatic compression, while we propose that the bridge could be generated by electrons which were pre-accelerated by the shock, and then re-accelerated by turbulence. Finally, we detect hints of gentle re-energisation in the two arms that depart from the tail of the radio galaxy.
Publisher: Oxford University Press (OUP)
Date: 07-07-2022
Abstract: Large-scale magnetic fields reveal themselves through diffuse synchrotron sources observed in galaxy clusters such as radio halos. Total intensity filaments of these sources have been observed in polarization as well, but only in three radio halos out of about 100 currently known. In this paper we analyse new polarimetric Very Large Array data of the diffuse emission in the galaxy cluster Abell 523 in the frequency range 1–2 GHz. We find for the first time evidence of polarized emission on scales of ∼2.5 Mpc. The total intensity emission is observed only in the central part of the source, likely due to observational limitations. To look for total intensity emission beyond the central region, we combine these data with single-dish observations from the Sardinia Radio Telescope and we compare them with multifrequency total intensity observations obtained with different instruments, including the LOw Frequency ARray and the Murchison Widefield Array. By analysing the rotation measure properties of the system and utilizing numerical simulations, we infer that this polarized emission is associated with filaments of the radio halo located in the outskirts of the system, in the peripheral region closest to the observer.
Publisher: Oxford University Press (OUP)
Date: 29-08-2017
Publisher: Oxford University Press (OUP)
Date: 07-08-2020
Abstract: Observations of radio relics at very high frequency (& GHz) can help to understand how particles age and are (re-)accelerated in galaxy cluster outskirts and how magnetic fields are lified in these environments. In this work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25 GHz Effelsberg observations of the well-known northern radio relic of CIZA J2242.8+5301. We detected the relic which shows a length of ∼1.8 Mpc and a flux density equal to $S_{14.25\\, \\mathrm{ GHz}}=(9.5\\pm 3.9)\\, \\mathrm{ mJy}$ and $S_{18.6\\, \\mathrm{ GHz}}=(7.67\\pm 0.90)\\, \\mathrm{ mJy}$ at 14.25 and 18.6 GHz, respectively. The resulting best-fitting model of the relic spectrum from 145 MHz to 18.6 GHz is a power-law spectrum with spectral index α = 1.12 ± 0.03: no evidence of steepening has been found in the new data presented in this work. For the first time, polarization properties have been derived at 18.6 GHz, revealing an averaged polarization fraction of $\\sim\\! 40{{\\ \\rm per\\ cent}}$ and a magnetic field aligned with the ’filaments’ or ’sheets’ of the relic.
Publisher: EDP Sciences
Date: 20-11-2018
DOI: 10.1051/0004-6361/201732507
Abstract: Context. Superclusters form from the largest enhancements in the primordial density perturbation field and extend for tens of Mpc, tracing the large-scale structure of the Universe. X-ray detections and systematic characterisations of superclusters and the properties of their galaxies have only been possible in the last few years. Aims. We characterise XLSSsC N01, a rich supercluster at z ~ 0.3 detected in the XXL Survey, composed of X-ray clusters of different virial masses and X-ray luminosities. As one of the first studies on this topic, we investigate the stellar populations of galaxies in different environments in the supercluster region. Methods. We study a magnitude-limited ( r ≤ 20) and a mass-limited s le (log( M * ∕ M ⊙ ) ≥ 10.8) of galaxies in the virialised region and in the outskirts of 11 XLSSsC N01 clusters, in high-density field regions, and in the low-density field. We compute the stellar population properties of galaxies using spectral energy distribution (SED) and spectral fitting techniques, and study the dependence of star formation rates (SFR), colours, and stellar ages on environment. Results. For r ≤ 20, the fraction of star-forming/blue galaxies, computed either from the specific-SFR (sSFR) or rest-frame colour, shows depletion within the cluster virial radii, where the number of galaxies with log (sSFR/ yr −1 ) −12 and with ( g − r ) restframe 0.6 is lower than in the field. For log( M * ∕ M ⊙ ) ≥ 10.8, no trends with environment emerge, as massive galaxies are mostly already passive in all environments. No differences among low- and high-density field members and cluster members emerge in the sSFR-mass relation in the mass-complete regime. Finally, the luminosity-weighted age–mass relation of the passive populations within cluster virial radii show signatures of recent environmental quenching. Conclusions. The study of luminous and massive galaxies in this supercluster shows that while environment has a prominent role in determining the fractions of star-forming/blue galaxies, its effects on the star formation activity in star-forming galaxies are negligible.
Publisher: EDP Sciences
Date: 20-11-2018
DOI: 10.1051/0004-6361/201731606
Abstract: Context. In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster s les is critical. Aims. The XXL Survey has been specifically designed to provide a well characterised s le of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues of more specific objects such as super-clusters and fossil groups. Methods. Following from the publication of the hundred brightest XXL clusters, we now release a s le containing 365 clusters in total, down to a flux of a few 10 −15 erg s −1 cm −2 in the [0.5–2] keV band and in a 1′ aperture. This release contains the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, spectroscopically confirmed. In this paper, we give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster s le, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5–2] keV flux within 1 arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80% of the clusters having a flux larger than 9 × 10 −15 erg s −1 cm −2 . Results. Our cluster s le extends from z ~ 0 to z ~ 1.2, with one cluster at z ~ 2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. The largest number of confirmed spectroscopic members in a cluster is 41. Our updated luminosity function and luminosity–temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a s le of 18 possible fossil groups. Conclusions. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50 deg 2 area out to z ~ 1.
Publisher: EDP Sciences
Date: 20-11-2018
DOI: 10.1051/0004-6361/201832974
Abstract: Context. We present the results of a study of the active galactic nucleus (AGN) density in a homogeneous and well-studied s le of 167 bona fide X-ray galaxy clusters (0.1 z 0.5) from the XXL Survey, from the cluster core to the outskirts (up to 6 r 500 ). The results can provide evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters, testing the efficiency of ram pressure gas stripping and galaxy merging in dense environments. Aims. The XXL cluster s le mostly comprises poor and moderately rich structures ( M = 10 13 –4 × 10 14 M ⊙ ), a poorly studied population that bridges the gap between optically selected groups and massive X-ray selected clusters. Our aim is to statistically study the demographics of cluster AGNs as a function of cluster mass and host galaxy position. Methods. To investigate the effect of the environment on AGN activity, we computed the fraction of spectroscopically confirmed X-ray AGNs ( L X [0.5-10 keV] 10 42 erg cm −1 ) in bright cluster galaxies with M i * − 2 M M i * + 1, up to 6 r 500 radius. The corresponding field fraction was computed from 200 mock cluster catalogues with reshuffled positions within the XXL fields. To study the mass dependence and the evolution of the AGN population, we further ided the s le into low- and high-mass clusters (below and above 10 14 M ⊙ , respectively) and two redshift bins (0.1–0.28 and 0.28–0.5). Results. We detect a significant excess of X-ray AGNs, at the 95% confidence level, in low-mass clusters between 0.5 r 500 and 2 r 500 , which drops to the field value within the cluster cores ( r 0.5 r 500 ). In contrast, high-mass clusters present a decreasing AGN fraction towards the cluster centres, in agreement with previous studies. The high AGN fraction in the outskirts is caused by low-luminosity AGNs, up to L X [0.5-10 keV] = 10 43 erg cm −1 . It can be explained by a higher galaxy merging rate in low-mass clusters, where velocity dispersions are not high enough to prevent galaxy interactions and merging. Ram pressure stripping is possible in the cores of all our clusters, but probably stronger in deeper gravitational potentials. Compared with previous studies of massive or high-redshift clusters, we conclude that the AGN fraction in cluster galaxies anti-correlates strongly with cluster mass. The AGN fraction also increases with redshift, but at the same rate with the respective fraction in field galaxies.
Publisher: EDP Sciences
Date: 20-11-2018
DOI: 10.1051/0004-6361/201833238
Abstract: Context. Galaxy clusters trace the highest density peaks in the large-scale structure of the Universe. Their clustering provides a powerful probe that can be exploited in combination with cluster mass measurements to strengthen the cosmological constraints provided by cluster number counts. Aims. We investigate the spatial properties of a homogeneous s le of X-ray selected galaxy clusters from the XXL survey, the largest programme carried out by the XMM-Newton satellite. The measurements are compared to Λ-cold dark matter predictions, and used in combination with self-calibrated mass scaling relations to constrain the effective bias of the s le, b eff , and the matter density contrast, Ω M . Methods. We measured the angle-averaged two-point correlation function of the XXL cluster s le. The analysed catalogue consists of 182 X-ray selected clusters from the XXL second data release, with median redshift ⟨ z ⟩ = 0.317 and median mass ⟨ M 500 ⟩≃ 1.3 × 10 14 M ⊙ . A Markov chain Monte Carlo analysis is performed to extract cosmological constraints using a likelihood function constructed to be independent of the cluster selection function. Results. Modelling the redshift-space clustering in the scale range 10 r [ h −1 Mpc] 40, we obtain Ω M = 0.27 −0.04 +0.06 and b eff = 2.73 −0.20 +0.18 . This is the first time the two-point correlation function of an X-ray selected cluster catalogue at such relatively high redshifts and low masses has been measured. The XXL cluster clustering appears fully consistent with standard cosmological predictions. The analysis presented in this work demonstrates the feasibility of a cosmological exploitation of the XXL cluster clustering, paving the way for a combined analysis of XXL cluster number counts and clustering.
Publisher: Oxford University Press (OUP)
Date: 02-02-2022
Abstract: The XXL Survey is the largest homogeneous survey carried out with XMM-Newton. Covering an area of 50 deg2, the survey contains several hundred galaxy clusters out to a redshift of ≈2, above an X-ray flux limit of ∼6 × 10−15 er g cm−2 s−1. The GAMA spectroscopic survey of ∼300 000 galaxies covers ≈286 deg2, down to an r-band magnitude of r & 19.8 mag. The region of overlap of these two surveys (covering 14.6 deg2) represents an ideal opportunity to study clusters selected via two independent selection criteria. Generating two independently selected s les of clusters, one drawn from XXL (spanning a redshift range 0.05 ≤ z ≤ 0.3) and another from GAMA (0.05 ≤ z ≤ 0.2), both spanning 0.2 ≲ M500 ≲ 5 × 1014 M⊙, we investigate the relationship between X-ray luminosity and velocity dispersion (LX − σv relation). Comparing the LX − σv relation between the X-ray selected and optically selected s les, when not accounting for the X-ray selection, we find that the scatter of the X-ray selected s le is 2.7 times higher than the optically selected s le (at the 3.7σ level). Accounting for the X-ray selection to model the LX − σv relation, we find that the difference in the scatter increases (with the X-ray selected s le having a scatter 3.4 times larger than the optically selected s le). Although the scatter of the optically selected s le is lower, we find 13 optically selected GAMA groups undetected in X-rays. Inspection of the difference in magnitude between the first and second brightest galaxies in the cluster, and a stacked X-ray image of these 13 groups, suggests that these are young systems still in the process of forming.
Publisher: EDP Sciences
Date: 09-2017
Publisher: EDP Sciences
Date: 02-2019
DOI: 10.1051/0004-6361/201833861
Abstract: Context. A number of merging galaxy clusters show the presence of large-scale radio emission associated with the intra-cluster medium (ICM). These synchrotron sources are generally classified as radio haloes and radio relics. Aims. Whilst it is commonly accepted that mergers play a crucial role in the formation of radio haloes and relics, not all the merging clusters show the presence of giant diffuse radio sources and this provides important information concerning current models. The Abell 781 complex is a spectacular system composed of an apparent chain of clusters on the sky. Its main component is undergoing a merger and hosts peripheral emission that is classified as a candidate radio relic and a disputed radio halo. Methods. We used new LOw Frequency ARay (LOFAR) observations at 143 MHz and archival Giant Metrewave Radio Telescope (GMRT) observations at 325 and 610 MHz to study radio emission from non-thermal components in the ICM of Abell 781. Complementary information came from XMM-Newton data, which allowed us to investigate the connection with the thermal emission and its complex morphology. Results. The origin of the peripheral emission is still uncertain. We speculate that it is related to the interaction between a head tail radio galaxy and shock. However, the current data allow us only to set an upper limit of ℳ 1.4 on the Mach number of this putative shock. Instead, we successfully characterise the surface brightness and temperature jumps of a shock and two cold fronts in the main cluster component of Abell 781. Their positions suggest that the merger is involving three substructures. We do not find any evidence for a radio halo either at the centre of this system or in the other clusters of the chain. We place an upper limit to the diffuse radio emission in the main cluster of Abell 781 that is a factor of 2 below the current radio power-mass relation for giant radio haloes.
Publisher: EDP Sciences
Date: 15-06-2016
Publisher: EDP Sciences
Date: 20-11-2018
DOI: 10.1051/0004-6361/201834022
Abstract: Context. We present an estimation of cosmological parameters with clusters of galaxies. Aims. We constrain the Ω m , σ 8 , and w parameters from a stand-alone s le of X-ray clusters detected in the 50 deg 2 XMM-XXL survey with a well-defined selection function. Methods. We analyse the redshift distribution of a s le comprising 178 high signal-to-noise ratio clusters out to a redshift of unity. The cluster s le scaling relations are determined in a self-consistent manner. Results. In a lambda cold dark matter (ΛCDM) model, the cosmology favoured by the XXL clusters compares well with results derived from the Planck Sunyaev-Zel’dovich clusters for a totally different s le (mass/redshift range, selection biases, and scaling relations). However, with this preliminary s le and current mass calibration uncertainty, we find no inconsistency with the Planck CMB cosmology. If we relax the w parameter, the Planck CMB uncertainties increase by a factor of ~10 and become comparable with those from XXL clusters. Combining the two probes allows us to put constraints on Ω m = 0.316 ± 0.060, σ 8 = 0.814 ± 0.054, and w = −1.02 ± 0.20. Conclusions. This first self-consistent cosmological analysis of a s le of serendipitous XMM clusters already provides interesting insights into the constraining power of the XXL survey. Subsequent analysis will use a larger s le extending to lower confidence detections and include additional observable information, potentially improving posterior uncertainties by roughly a factor of 3.
Publisher: American Astronomical Society
Date: 07-2022
Abstract: We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ∼30′ from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio–X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.
Publisher: Oxford University Press (OUP)
Date: 04-05-2018
No related grants have been discovered for Fabio Gastaldello.