arXiv subjects

S. Giacintucci

Publications and source records attributed to S. Giacintucci.

At least 55 records · Page 3Linked to original sources

arXiv2015

The Extended GMRT Radio Halo Survey II: Further results and analysis of the full sample

The intra-cluster medium contains cosmic rays and magnetic fields that are manifested through the large scale synchrotron sources, termed as radio halos, relics and mini-halos. The Extended Giant Metrewave Radio Telescope (GMRT) Radio Halo Survey (EGRHS) is an extension of the GMRT Radio Halo Survey (GRHS) designed to search for radio halos using GMRT 610/235 MHz observations. The GRHS+EGRHS consists of 64 clusters in the redshift range 0.2 -- 0.4 that have an X-ray luminosity larger than 5x10^44 erg/s in the 0.1 -- 2.4 keV band and with declinations > -31 deg in the REFLEX and eBCS X-ray cluster catalogues. In this second paper in the series, GMRT 610/235 MHz data on the last batch of 11 galaxy clusters and the statistical analysis of the full sample are presented. A new mini-halo in RXJ2129.6+0005 and candidate diffuse sources in Z5247, A2552 and Z1953 are discovered. A unique feature of this survey are the upper limits on the detections of 1 Mpc sized radio halos; 4 new are presented here making a total of 31 in the survey. Of the sample, 58 clusters that have adequately sensitive radio information were used to obtain the most accurate occurrence fractions so far. The occurrence of radio halos in our X-ray selected sample is ~22%, that of mini-halos is 13% and that of relics is ~5%. The radio power - X-ray luminosity diagrams for the radio halos and mini-halos with the detections and upper limits are presented. The morphological estimators namely, centroid shift (w), concentration parameter (c) and power ratios (P_3/P_0) derived from the Chandra X-ray images are used as proxies for the dynamical states of the GRHS+EGRHS clusters. The clusters with radio halos and mini-halos occupy distinct quadrants in the c-w, c-P_3/P_0 and w - P_3/P_0 planes, corresponding to the more and less morphological disturbance, respectively. The non-detections span both the quadrants.

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arXiv2014

Cluster Radio Halos at the crossroads between astrophysics and cosmology in the SKA era

Giant Radio Halos (RH) are diffuse, Mpc-sized, synchrotron radio sources observed in a fraction of merging galaxy clusters. The current scenario for the origin of RHs assumes that turbulence generated during cluster mergers re-accelerates pre-existing fossil and/or secondary electrons in the intra-cluster-medium (ICM) to the energies necessary to produce the observed radio emission. Moreover, more relaxed clusters could host diffuse "off state" halos produced by secondary electrons. In this Chapter we use Monte Carlo simulations, that combine turbulent-acceleration physics and the generation of secondaries in the ICM, to calculate the occurrence of RHs in the Universe, their spectral properties and connection with properties of the hosting clusters. Predictions for SKA1 surveys are presented at low (100-300 MHz) and mid (1-2 GHz) frequencies assuming the expected sensitivities and spatial resolutions of SKA1. SKA1 will step into an unexplored territory allowing us to study the formation and evolution of RHs in a totally new range of cluster masses and redshift, allowing firm tests of the current theoretical hypothesis. In particular, the combination of SKA1-LOW and SUR will allow the discovery of ~1000 ultrasteep- spectrum halos and to detect for the very first time "off state" RHs. We expect that at least ~2500 giant RHs will be discovered by SKA1-LOW surveys up to z~0.6. Remarkably these surveys will be sensitive to RHs in a cluster mass range (down to ~10^14 solar masses) and redshifts (up to ~1) that are unexplored by current observations. SKA1 surveys will be highly competitive with present and future SZ-surveys in the detection of high-redshift massive objects.

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arXiv2014

Deep Chandra Observations of HCG 16 - I. Active Nuclei, Star formation and Galactic Winds

We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope 610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we use to examine nuclear activity, star formation and the high luminosity X-ray binary populations in the major galaxies. We confirm the presence of obscured active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized nuclear source in NGC 838. All three nuclei are variable on timescales of months to years, and for NGC 833 and NGC 835 this is most likely caused by changes in accretion rate. The deep Chandra observations allow us to detect for the first time an Fe-K$α$ emission line in the spectrum of the Seyfert 2 nucleus of NGC 835. We find that NGC 838 and NGC 839 are both starburst-dominated systems, with only weak nuclear activity, in agreement with previous optical studies. We estimate the star formation rates in the two galaxies from their X-ray and radio emission, and compare these results with estimates from the infra-red and ultra-violet bands to confirm that star formation in both galaxies is probably declining after galaxy-wide starbursts were triggered ~400-500 Myr ago. We examine the physical properties of their galactic superwinds, and find that both have temperatures of ~0.8 keV. We also examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in the group, and show that it is dominated by emission from its starburst.

astro-ph.GA↗

arXiv2014

Deep Chandra Observations of HCG 16 - II. The Development of the Intra-group Medium in a Spiral-Rich Group

We use a combination of deep Chandra X-ray observations and radio continuum imaging to investigate the origin and current state of the intra-group medium in the spiral-rich compact group HCG 16. We confirm the presence of a faint ($L_{X,{\rm bolo}}$=1.87$^{+1.03}_{-0.66}$$\times$10$^{41}$ erg/s), low temperature (0.30$^{+0.07}_{-0.05}$ keV) intra-group medium (IGM) extending throughout the ACIS-S3 field of view, with a ridge linking the four original group members and extending to the southeast, as suggested by previous Rosat and XMM-Newton observations. This ridge contains 6.6$^{+3.9}_{-3.3}$$\times$10$^9$ solar masses of hot gas and is at least partly coincident with a large-scale HI tidal filament, indicating that the IGM in the inner part of the group is highly multi-phase. We present evidence that the group is not yet virialised, and show that gas has probably been transported from the starburst winds of NGC 838 and NGC 839 into the surrounding IGM. Considering the possible origin of the IGM, we argue that material ejected by galactic winds may have played a significant role, contributing 20-40% of the observed hot gas in the system.

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arXiv2014

Discovery of large-scale diffuse radio emission and of a new galaxy cluster in the surroundings of MACSJ0520.7-1328

We report the discovery of large-scale diffuse radio emission South-East of the galaxy cluster MACS J0520.7-1328, detected through high sensitivity Giant Metrewave Radio Telescope 323 MHz observations. This emission is dominated by an elongated diffuse radio source and surrounded by other features of lower surface brightness. Patches of these faint sources are marginally detected in a 1.4 GHz image obtained through a re-analysis of archival NVSS data. Interestingly, the elongated radio source coincides with a previously unclassified extended X-ray source. We perform a multi-wavelength analysis based on archival infrared, optical and X-ray Chandra data. We find that this source is a low-temperature (~3.6 keV) cluster of galaxies, with indications of a disturbed dynamical state, located at a redshift that is consistent with the one of the main galaxy cluster MACS J0520.7-132 (z=0.336). We suggest that the diffuse radio emission is associated with the non-thermal components in the intracluster and intergalactic medium in and around the newly detected cluster. We are planning deeper multi-wavelength and multi-frequency radio observations to accurately investigate the dynamical scenario of the two clusters and to address more precisely the nature of the complex radio emission.

astro-ph.CO↗

arXiv2013

Revisiting scaling relations for giant radio halos in galaxy clusters

Many galaxy clusters host Megaparsec-scale radio halos, generated by ultrarelativistic electrons in the magnetized intracluster medium. Correlations between the power of radio halos and the thermal properties of the hosting clusters were established in the last decade, including the connection between the presence of a halo and cluster mergers. The X-ray luminosity and redshift limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for statistical studies of the halos. We uniformly analyze the radio and X-ray data for the GMRT cluster sample, and use the new Planck SZ catalog, to revisit the correlations between the power of halos and the thermal properties of galaxy clusters. We find that the radio power of halos at 1.4 GHz scales with the cluster X-ray (0.1--2.4 keV) luminosity computed within R_500 as P_1.4 L_500^2.0. Our bigger and more homogenous sample confirms that the X-ray luminous (L_500 > 5x10^44 erg/s) clusters branch into two populations --- radio halos lie on the correlation, while clusters with upper limits to radio-halo emission are well below that correlation. This bimodality remains if we excise cool cores from the X-ray luminosities. Correlating with Planck data, we find that P_1.4 scales with the cluster integrated SZ signal within R_500 as P_1.4 Y_500^2.1, in line with previous findings. However, contrary to previous studies that were limited by incompleteness and small sample size, we find that the "SZ-luminous" Y_500 > 6x10^-5 Mpc^2 clusters show a bimodal behavior similar to that in the radio-X-ray diagram. Bimodality of both correlations can be traced to clusters dynamics, with radio halos found exclusively in merging clusters. These results confirm the key role of mergers for the origin of giant radio halos, suggesting that they trigger the relativistic particle acceleration.

astro-ph.CO↗

arXiv2013

Radio emission at the centre of the galaxy cluster Abell 3560: evidence for core sloshing?

Previous radio observations of the galaxy cluster A3560 in the Shapley Concentration showed complex radio emission associated with the brightest cluster member.To understand its origin we observed it with the GMRT, the VLA and ATCA at 240 and 610 MHz, 1.28,1.4, 2.3,4.8 and 8.4 GHz, and performed a detailed morphological and spectral study of the radio emission associated with the BCG. We also observed the cluster with XMM-Newton and Chandra to derive the properties of the ICM. The radio emission of the N-E nucleus of the dumb-bell BCG shows an active radio galaxy, plus aged diffuse emission, which is not refurbished at present. Our Chandra data show that the radio active nucleus of the BCG has extended X-ray emission, which we classify as a low-luminosity corona. A residual image of the XMM-Newton brightness shows the presence of a spiral-like feature, which we interpret as the signature of gas sloshing. The presence of a subgroup is clear in the surface brightness residual map, and in the XMM-Newton temperature analysis. The optical 2D analysis shows substructure in A3560. A galaxy clump was found at the location of the X-ray subgroup, and another group is present south of the cluster core, close to the spiral-like feature. The aged part of the radio emission closely follows the spiral pattern of the X-ray residual brightness distribution, while the two active radio lobes are bent in a completely different direction. We conclude that the complex radio emission associated with the cluster BCG is the result of a minor merger event in A3560. The aged diffuse emission is strongly affected by the sloshing motion in the ICM. On the other hand, the bent jets and lobes of the current radio AGN activity may reflect a complex gas velocity field in the innermost cluster regions and/or sloshing-induced oscillations in the motion of the cD galaxy.

astro-ph.CO↗

arXiv2013

A multi-wavelength view of cooling vs. AGN heating in the X-ray luminous cool-core of Abell 3581

We report the results of a multi-wavelength study of the nearby galaxy group, Abell 3581 (z=0.0218). This system hosts the most luminous cool core of any nearby group and exhibits active radio mode feedback from the super-massive black hole in its brightest group galaxy, IC 4374. The brightest galaxy has suffered multiple active galactic nucleus outbursts, blowing bubbles into the surrounding hot gas, which have resulted in the uplift of cool ionised gas into the surrounding hot intragroup medium. High velocities, indicative of an outflow, are observed close to the nucleus and coincident with the radio jet. Thin dusty filaments accompany the uplifted, ionised gas. No extended star formation is observed, however, a young cluster is detected just north of the nucleus. The direction of rise of the bubbles has changed between outbursts. This directional change is likely due to sloshing motions of the intragroup medium. These sloshing motions also appear to be actively stripping the X-ray cool core, as indicated by a spiraling cold front of high metallicity, low temperature, low entropy gas.

astro-ph.CO↗

arXiv2013

Chandra View of the Ultra-Steep Spectrum Radio Source in Abell 2443: Merger Shock-Induced Compression of Fossil Radio Plasma?

We present a new Chandra X-ray observation of the intracluster medium in the galaxy cluster Abell 2443, hosting an ultra-steep spectrum radio source. The data reveal that the intracluster medium is highly disturbed. The thermal gas in the core is elongated along a northwest to southeast axis and there is a cool tail to the north. We also detect two X-ray surface brightness edges near the cluster core. The edges appear to be consistent with an inner cold front to the northeast of the core and an outer shock front to the southeast of the core. The southeastern edge is coincident with the location of the radio relic as expected for shock (re)acceleration or adiabatic compression of fossil relativistic electrons.

astro-ph.CO↗

arXiv2013

The Extended GMRT Radio Halo Survey I: New upper limits on radio halos and mini-halos

A fraction of galaxy clusters host diffuse radio sources called radio halos, radio relics and mini-halos. We present the sample and first results from the Extended GMRT Radio Halo Survey (EGRHS)- an extension of the GMRT Radio Halo Survey (GRHS, Venturi et al. 2007, 2008). It is a systematic radio survey of galaxy clusters selected from the REFLEX and eBCS X-ray catalogs . Analysis of GMRT data at 610/ 235/ 325 MHz on 12 galaxy clusters are presented. We report the detection of a newly discovered mini-halo in the cluster RXJ1532.9+3021 at 610 MHz. A small scale relic (~200 kpc) is suspected in the cluster Z348. We do not detect cluster-scale diffuse emission in 11 clusters. Robust upper limits on the detection of radio halo of size of 1 Mpc are determined. We also present upper limits on the detections of mini-halos in a sub-sample of cool-core clusters. The upper limits for radio halos and mini-halos are plotted in the radio power- X-ray luminosity plane and the correlations are discussed. Diffuse extended emission, not related to the target clusters, but detected as by-products in the sensitive images of two of the cluster fields (A689 and RXJ0439.0+0715) are reported. Based on the information about the presence of radio halos (or upper limits), available on 48 clusters out of the total sample of 67 clusters (EGRHS+GRHS), we find that ~23% of the clusters host radio halos. The radio halo fraction rises to ~31%, when only the clusters with X-ray luminosities >8x10^44 erg/s are considered. Mini-halos are found in ~50 % of cool-core clusters. A qualitative examination of the X-ray images of the clusters with no diffuse radio emission indicates that a majority of these clusters do not show extreme dynamical disturbances and supports the idea that mergers play an important role in the generation of radio halos/relics.

astro-ph.CO↗

arXiv2013

Sloshing cold fronts in galaxy groups and their perturbing disk galaxies: an X-ray, Optical and Radio Case Study

We present a combined X-ray, optical, and radio analysis of the galaxy group IC 1860 using the currently available Chandra and XMM data, literature multi-object spectroscopy data and GMRT data. The Chandra and XMM imaging and spectroscopy reveal two surface brightness discontinuities at 45 and 76 kpc shown to be consistent with a pair of cold fronts. These features are interpreted as due to sloshing of the central gas induced by an off-axis minor merger with a perturber. This scenario is further supported by the presence of a peculiar velocity of the central galaxy IC 1860 and the identification of a possible perturber in the optically disturbed spiral galaxy IC 1859. The identification of the perturber is consistent with the comparison with numerical simulations of sloshing. The GMRT observation at 325 MHz shows faint, extended radio emission contained within the inner cold front, as seen in some galaxy clusters hosting diffuse radio mini-halos. However, unlike mini-halos, no particle reacceleration is needed to explain the extended radio emission, which is consistent with aged radio plasma redistributed by the sloshing. There is strong analogy of the X-ray and optical phenomenology of the IC 1860 group with two other groups, NGC 5044 and NGC 5846, showing cold fronts. The evidence presented in this paper is among the strongest supporting the currently favored model of cold-front formation in relaxed objects and establishes the group scale as a chief environment to study this phenomenon.

astro-ph.CO↗

arXiv2013

153 MHz GMRT follow-up of steep-spectrum diffuse emission in galaxy clusters

In this paper we present new high sensitivity 153 MHz Giant Meterwave Radio Telescope follow-up observations of the diffuse steep spectrum cluster radio sources in the galaxy clusters Abell 521, Abell 697, Abell 1682. Abell 521 hosts a relic, and together with Abell 697 it also hosts a giant very steep spectrum radio halo. Abell 1682 is a more complex system with candidate steep spectrum diffuse emission. We imaged the diffuse radio emission in these clusters at 153 MHz, and provided flux density measurements of all the sources at this frequency. Our new flux density measurements, coupled with the existing data at higher frequencies, allow us to study the total spectrum of the halos and relic over at least one order of magnitude in frequency. Our images confirm the presence of a very steep "diffuse component" in Abell 1682. We found that the spectrum of the relic in Abell 521 can be fitted by a single power-law with $α=1.45\pm0.02$ from 153 MHz to 5 GHz. Moreover, we confirm that the halos in Abell 521 and Abell 697 have a very steep spectrum, with $α=1.8-1.9$ and $α=1.52\pm0.05$ respectively. Even with the inclusion of the 153 MHz flux density information it is impossible to discriminate between power-law and curved spectra, as derived from homogeneous turbulent re-acceleration. The latter are favored on the basis of simple energetic arguments, and we expect that LOFAR will finally unveil the shape of the spectra of radio halos below 100 MHz, thus providing clues on their origin.

astro-ph.CO↗

arXiv2011

A hidden radio halo in A1682?

High sensitivity observations of radio halos in galaxy clusters at frequencies lower than 330 MHz are still relatively rare, and very little is known compared to the classical 1.4 GHz images. The few radio halos imaged down to 150-240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A1682, imaging is particularly challenging due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that the radio galaxies are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here.

astro-ph.CO↗

arXiv2011

An elusive radio halo in the merging cluster Abell 781?

Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610 MHz observations and to archival VLA 1.4 GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and on the basis of simple spectral considerations we do not support their claim of a radio halo with flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source.

astro-ph.CO↗

arXiv2011

GMRT 150 MHz follow up of diffuse steep spectrum radio emission in galaxy clusters

It has been recently found that a few galaxy clusters host diffuse radio halo emission with very steep synchrotron spectra ($α$ > 1.6), which may be classified as Ultra Steep Spectrum Radio Halos (USSRHs). USSRHs are expected in the turbulence re-acceleration model for the origin of cluster radio halos, and are best discovered and studied at low frequencies. We performed GMRT follow up observations of three galaxy clusters at 150MHz, selected from the GMRT radio halo survey, which are known to host an USSRH or candidate very steep spectrum diffuse emission. This project is aimed to characterize the low frequency spectrum of USSRHs for a detailed study of their origin and connection with cluster mergers. We present preliminary results at 150 MHz of the cluster A697.

astro-ph.CO↗

arXiv2011

ICM Signatures from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback

We present results from Chandra observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. We focus on three main results. 1) The diffuse gas shows clear signatures from three distinct outbursts of the central AGN, with three pairs of roughly collinear cavities. The inner two cavity pairs are associated with unambiguous elliptical shock fronts, with Mach numbers M~1.7 and M~1.5 for the inner and outer shocks, respectively. 2) The mean power of the two most recent outbursts differs by a factor of six, indicating that the mean jet power varies over long (~10^7 yr) time scales. 3) The heating from the shocks alone is sufficient to balance radiative cooling of the gas within at least the central 10 kpc, allowing feedback to operate isotropically at small radii.

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arXiv2010

A radio minihalo in the extreme cool-core galaxy cluster RXCJ1504.1-0248

Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a massive galaxy cluster that has an extremely luminous cool core. To date, only 9 radio minihalos are known, thus the discovery of a new one, in one of the most luminous cool-core clusters, provides important information on this peculiar class of sources and sheds light on their origin. Methods. The diffuse radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA data at 1.46 GHz. The minihalo has a radius of $\sim$140 kpc. A Chandra gas temperature map shows that the minihalo emission fills the cluster cool core and has some morphological similarities to it, as has been previously observed for other minihalos. Results. The Chandra data reveal two subtle cold fronts in the cool core, likely created by sloshing of the core gas, as observed in most cool-core clusters. Following previous work, we speculate that the origin of the minihalo is related to sloshing. Sloshing may result in particle acceleration by generating turbulence and/or amplifying the magnetic field in the cool core, leading to the formation of a minihalo.

astro-ph.HE↗

arXiv2010

Dying Radio Sources in Clusters

We present the study of five `dying' nearby radio galaxies belonging to the WENSS minisurvey and to the B2 bright catalogs: WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. These sources have been selected on the basis of their extremely steep broad-band radio spectra. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed by two distinct dying galaxies, which appear blend together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is detected also in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources of our sample are located (at least in projection) at the center of an X-ray emitting cluster. Our results suggest that the duration of the dying phase for a radio source in cluster can be significantly higher with respect to that of a radio galaxy in the field. The simplest interpretation is that the low-frequency emission from the fading radio lobes last longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. Radio sources in dense environment, like e.g. the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger samples of objects.

astro-ph.CO↗