arXiv subjects

S. Giacintucci

Publications and source records attributed to S. Giacintucci.

At least 73 records · Page 4Linked to original sources

arXiv2010

On the connection between giant radio halos and cluster mergers

The frequently observed association between giant radio halos and merging galaxy clusters has driven present theoretical models of non-thermal emission from galaxy clusters, which are based on the idea that the energy dissipated during cluster-cluster mergers could power the formation of radio halos. To quantitatively test the merger-halo connection we present the first statistical study based on deep radio data and X-ray observations of a complete X-ray selected sample of galaxy clusters with X-ray luminosity > 5x 10^44 erg/s and redshift 0.2<z< 0.32. Using several methods to characterize cluster substructures, namely the power ratios, centroid shift and X-ray brightness concentration parameter, we show that clusters with and without radio halo can be quantitatively differentiated in terms of their dynamical properties. In particular, we confirm that radio halos are associated to dynamically disturbed clusters and cluster without radio halo are more "relaxed", with only a couple of exceptions where a disturbed cluster does not exhibit a halo.

astro-ph.CO↗

arXiv2010

Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback

We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7 and M~1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from 1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~10^7 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the ICM and the central SMBH.

astro-ph.CO↗

arXiv2009

The unusual X-ray morphology of NGC4636 revealed by deep Chandra observations: cavities and shocks created by past AGN outbursts

We present Chandra ACIS-I and ACIS-S observations (~200ks in total) of the X-ray luminous elliptical galaxy NGC4636, located in the outskirts of the Virgo cluster. A soft band (0.5-2 keV) image shows the presence of a bright core in the center surrounded by an extended X-ray corona and two pronounced quasi-symmetric, 8 kpc long, arm-like features. Each of this features defines the rim of an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located ~2 kpc south of the north-eastern arm, is detected as well. We present surface brightness and temperature profiles across the rims of the bubbles, showing that their edges are sharp and characterized by temperature jumps of about 20-25%. Through a comparison of the observed profiles with theoretical shock models, we demonstrate that a scenario where the bubbles were produced by shocks, probably driven by energy deposited off-center by jets, is the most viable explanation to the X-ray morphology observed in the central part of NGC4636. As a confirmation to this scenario, radio jets extending towards the bubbles and a central weak X-ray and radio source are detected and are most likely the signs of AGN activity which was more intense in the past. A bright dense core of ~1 kpc radius is observed at the center of NGC4636. A sharp decline in surface brightness from the core to the ambient gas is observed and is not accompanied by a variation in the temperature and thus could not be in thermal pressure equilibrium. However the bright core could be a long lived feature if the radio jets are acting as a balancing factor to thermal pressure or if the bright core is produced by steep abundance gradients.

astro-ph.CO↗

arXiv2009

A giant radio halo in the massive and merging cluster Abell 1351

We report on the detection of diffuse radio emission in the X-ray luminous and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we classify the diffuse source as a giant radio halo. X-ray and weak lensing studies show A1351 to be a system undergoing a major merger. The halo is associated with the most massive substructure. The presence of this source is explained assuming that merger-driven turbulence may re-accelerate high-energy particles in the intracluster medium and generate diffuse radio emission on the cluster scale. The position of A1351 in the logP$_{1.4 GHz}$ - logL$_{X}$ plane is consistent with that of all other radio-halo clusters known to date, supporting a causal connection between the unrelaxed dynamical state of massive ($>10^{15} M_{\odot}$) clusters and the presence of giant radio halos.

astro-ph.CO↗

arXiv2009

AGN Feedback in the Compact Group of Galaxies HCG 62 - as revealed by Chandra, XMM and GMRT data

As a part of an ongoing study of a sample of galaxy groups showing evidence for AGN/hot gas interaction, we report on the preliminary results of an analysis of new XMM and GMRT data of the X-ray bright compact group HCG 62. This is one of the few groups known to possess very clear, small X-ray cavities in the inner region as shown by the existing Chandra image. At higher frequencies (>1.4 GHz) the cavities show minimal if any radio emission, but the radio appears clearly at lower frequencies (<610 MHz). We compare and discuss the morphology and spectral properties of the gas and of the radio source. We find that the cavities are close to pressure balance, and that the jets have a "light" hadronic content. By extracting X-ray surface brightness and temperature profiles, we also identify a shock front located around 35 kpc to the south-west of the group center.

astro-ph.CO↗

arXiv2009

The unusual X-ray morphology of NGC4636 revealed by deep Chandra observations: cavities and shocks created by past AGN outbursts

We present Chandra ACIS-I and ACIS-S observations ($\sim$200 ks in total) of the X-ray luminous elliptical galaxy NGC 4636, located in the outskirts of the Virgo cluster. A soft band (0.5-2 keV) image shows the presence of a bright core in the center surrounded by an extended X-ray corona and two pronounced quasi-symmetric, 8 kpc long, arm-like features. Each of this features defines the rimof an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located $\sim2$ kpc south of the north-eastern arm, is detected as well. We present surface brightness and temperature profiles across the rims of the bubbles, showing that their edges are sharp and characterized by temperature jumps of about 20-25%. Through a comparison of the observed profiles with theoretical shock models, we demonstrate that a scenario where the bubbles were produced by shocks, probably driven by energy deposited off-center by jets, is the most viable explanation to the X-ray morphology observed in the central part of NGC 4636.

astro-ph.CO↗

arXiv2009

AGN feedback in galaxy groups: a joint GMRT/X-ray study

We present an ongoing study of 18 nearby galaxy groups, chosen for the availability of Chandra and/or XMM-Newton data and evidence for AGN/hot intragroup gas interaction. We have obtained 235 and 610 MHz observations at the GMRT for all the groups, and 327 and 150 MHz for a few. We discuss two interesting cases - NGC 5044 and AWM 4 - which exhibit different kinds of AGN/hot gas interaction. With the help of these examples we show how joining low-frequency radio data (to track the history of AGN outbursts through emission from aged electron populations) with X-ray data (to determine the state of hot gas, its disturbances, heating and cooling) can provide a unique insight into the nature of the feedback mechanism in galaxy groups.

astro-ph.CO↗

arXiv2009

Isotropic AGN Heating with Small Radio Quiet Bubbles in the NGC 5044 Group

(abridged) A Chandra observation of the X-ray bright group NGC 5044 shows that the X-ray emitting gas has been strongly perturbed by recent outbursts from the central AGN and also by motion of the central dominant galaxy relative to the group gas. The NGC 5044 group hosts many small radio quiet cavities with a nearly isotropic distribution, cool filaments, a semi-circular cold front and a two-armed spiral shaped feature of cool gas. A GMRT observation of NGC 5044 at 610 MHz shows the presence of extended radio emission with a "torus-shaped" morphology. The largest X-ray filament appears to thread the radio torus, suggesting that the lower entropy gas within the filament is material being uplifted from the center of the group. The radio emission at 235 MHz is much more extended than the emission at 610 MHz, with little overlap between the two frequencies. One component of the 235 MHz emission passes through the largest X-ray cavity and is then deflected just behind the cold front. A second detached radio lobe is also detected at 235 MHz beyond the cold front. All of the smaller X-ray cavities in the center of NGC 5044 are undetected in the GMRT observations. Since the smaller bubbles are probably no longer momentum driven by the central AGN, their motion will be affected by the group "weather" as they buoyantly rise outward. Hence, most of the enthalpy within the smaller bubbles will likely be deposited near the group center and isotropized by the group weather. The total mechanical power of the smaller radio quiet cavities is $P_c = 9.2 \times 10^{41}$erg s$^{-1}$ which is sufficient to suppress about one-half of the total radiative cooling within the central 10 kpc. This is consistent with the presence of H$α$ emission within this region which shows that at least some of the gas is able to cool.

astro-ph.CO↗

arXiv2009

Tailed radio galaxies as tracers of galaxy clusters. Serendipitous discoveries with the GMRT

We report on the discovery of four new radio galaxies with tailed morphology. Tailed radio galaxies are generally found in rich environments, therefore their presence can be used as tracer of a cluster. The radio galaxies were found in the fields of Giant Metrewave Radio Telescope (GMRT) observations carried out at 610 MHz and 327 MHz devoted to other studies. We inspected the literature and archives in the optical and X-ray bands to search for galaxy clusters or groups hosting them. All the tailed radio galaxies serendipitously found in the GMRT fields are located in rich environments. Two of them belong to the candidate cluster NCS J090232+204358, located at z(phot)=0.0746; one belongs to the cluster MaxBCGJ223.97317+22.15620 at z(phot)=0.2619; finally we suggest that the fourth one is probing a galaxy cluster at z=0.1177, located behind A262, and so far undetected in any band. Our results strenghten the relevance of high sensitivity and high resolution radio data in the detection of galaxy clusters at intermediate redshift.

astro-ph.CO↗

arXiv2009

AGN Feedback in groups and clusters of galaxies

The lack of very cool gas at the cores of groups and clusters of galaxies, even where the cooling time is significantly shorter than the Hubble time, has been interpreted as evidence of sources that re-heat the intergalactic medium. Most studies of rich clusters adopt AGN feedback to be this source of heating. From ongoing GMRT projects involving clusters and groups, we demonstrate how low-frequency GMRT radio observations, together with Chandra/XMM-Newton X-ray data, present a unique insight into the nature of feedback, and of the energy transfer between the AGN and the IGM.

astro-ph.CO↗

arXiv2009

A Joint GMRT/X-ray study of galaxy groups

We present results from combined low-frequency radio and X-ray studies of nearby galaxy groups. We consider two main areas: firstly, the evolutionary process from spiral-dominated, HI-rich groups to elliptical-dominated systems with hot, X-ray emitting gas halos; secondly, the mechanism of AGN feedback which appears to balance radiative cooling of the hot halos of evolved groups. The combination of radio and X-ray observations provides a powerful tool for these studies, allowing examination of gas in both hot and cool phases, and of the effects of shock heating and AGN outbursts. Low-frequency radio data are effective in detecting older and less energetic electron populations and are therefore vital for the determination of the energetics and history of such events. We present results from our ongoing study of Stephan's Quintet, a spiral-rich group in which tidal interactions and shock heating appear to be transforming HI in the galaxies into a diffuse X-ray emitting halo, and show examples of AGN feedback from our sample of elliptical-dominated groups, where multi-band low-frequency radio data have proved particularly useful.

astro-ph.CO↗

arXiv2009

A Chandra X-ray view of Stephan's Quintet: Shocks and Star-formation

We use a deep Chandra observation to examine the structure of the hot intra-group medium of the compact group of galaxies Stephan's Quintet. The group is thought to be undergoing a strong dynamical interaction as an interloper, NGC 7318b, passes through the group core at ~850 km/s. A bright ridge of X-ray and radio continuum emission has been interpreted as the result of shock heating, with support from observations at other wavelengths. We find that gas in this ridge has a similar temperature (~0.6 keV) and abundance (~0.3 solar) to the surrounding diffuse emission, and that a hard emission component is consistent with that expected from high-mass X-ray binaries associated with star-formation in the ridge. The cooling rate of gas in the ridge is consistent with the current star formation rate, suggesting that radiative cooling is driving the observed star formation. The lack of a high-temperature gas component is used to place constraints on the nature of the interaction and shock, and we find that an oblique shock heating a pre-existing filament of HI may be the most likely explanation of the X-ray gas in the ridge. The mass of hot gas in the ridge is only ~2 per cent of the total mass of hot gas in the group, which is roughly equal to the deficit in observed HI mass compared to predictions. The hot gas component is too extended to have been heated by the current interaction, strongly suggesting that it must have been heated during previous dynamical encounters.

astro-ph↗

arXiv2009

Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323

We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and distant giant radio halos known to date, suggesting that it may be undergoing a strong merger process. The aim of our multiwavelength study is to investigate the radio-halo cluster merger scenario. We studied the radio properties of the giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4 GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio Telescope, in combination with previously published GMRT data at 610 MHz. The dynamical state of the cluster was investigated by means of X-ray Chandra observations and optical ESO--NTT observations. Our study confirms that RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy morphology of the radio halo could be due to a combination of the filamentary structure of the magnetic field and turbulence in the inital stage of a cluster merger.

astro-ph.CO↗

arXiv2009

Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.

astro-ph.CO↗

arXiv2009

The GMRT Radio Halo Survey and low frequency follow-up

The GMRT Radio Halo Survey, carried out at 610 MHz to investigate the statistical properties of cluster radio halos in a complete cluster sample selected in the redshift interval z=0.2-0.4, has significantly improved our understanding of the origin of cluster radio halos and relics. Here we briefly summarize the most relevant results of our investigation. A low frequency follow-up is in progress with the GMRT at 325 MHz and 240 MHz on the diffuse sources and candidated found at 610 MHz. We briefly report some preliminary results on these low frequency observations. Cluster radio halos with different radio spectral properties have been unexpectedly found.

astro-ph.CO↗

arXiv2008

A low-frequency radio halo associated with a cluster of galaxies

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

astro-ph↗

arXiv2008

A GMRT multifrequency radio study of the isothermal core of the poor galaxy cluster AWM 4

We present a detailed radio morphological study and spectral analysis of the wide-angle-tail radio source 4C +24.36 associated with the dominant galaxy in the relaxed galaxy cluster AWM 4. Our study is based on new high sensitivity GMRT observations at 235 MHz, 327 MHz and 610 MHz, and on literature and archival data at other frequencies. We find that the source major axis is likely oriented at a small angle with respect to the plane of the sky. The wide-angle-tail morphology can be reasonably explained by adopting a simple hydrodynamical model in which both ram pressure (driven by the motion of the host galaxy) and buoyancy forces contribute to bend the radio structure. The spectral index progressively steepens along the source major axis from $α\sim$0.3 in the region close to the radio nucleus to beyond 1.5 in the lobes. The results of the analysis of the spectral index image allow us to derive an estimate of the radiative age of the source of $\sim$ 160 Myr. The cluster X-ray emitting gas has a relaxed morphology and short cooling time, but its temperature profile is isothermal out to at least 160 kpc from the centre. Therefore we seek evidence of energy ejection from the central AGN to prevent catastrophic cooling. We find that the energy injected by 4C +24.36 in the form of synchrotron luminosity during its lifetime is far less than the energy required to maintain the high gas temperature in the core. We also find that it is not possible for the central source to eject the requisite energy in the intracluster gas in terms of the enthalpy of buoyant bubbles of relativistic fluid, without creating discernible large cavities in the existing X-ray XMM-Newton observations.

astro-ph↗

arXiv2008

Shock acceleration as origin of the radio relic in A521?

We present new high sensitivity observations of the radio relic in A521 carried out with the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327-610 MHz. To this aim we used the new GMRT observations and other proprietary as well as archival data. We also searched for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of the theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is further supported by our finding of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is likely a shock front.

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