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Myriam Gitti

Publications and source records attributed to Myriam Gitti.

At least 73 records · Page 4Linked to original sources
arXiv2011

Evidence for AGN Feedback in Galaxy Clusters and Groups

The current generation of flagship X-ray missions, Chandra and XMM-Newton, has changed our understanding of the so-called "cool core" galaxy clusters and groups. Instead of the initial idea that the thermal gas is cooling and flowing toward the center, the new picture envisages a complex dynamical evolution of the intra-cluster medium (ICM) regulated by the radiative cooling and the nongravitational heating from the active galactic nucleus (AGN). Understanding the physics of the hot gas and its interplay with the relativistic plasma ejected by the AGN is key for understanding the growth and evolution of galaxies and their central black holes, the history of star formation, and the formation of large-scale structures. It has thus become clear that the feedback from the central black hole must be taken into account in any model of galaxy evolution. In this paper, we draw a qualitative picture of the current knowledge of the effects of the AGN feedback on the ICM by summarizing the recent results in this field.

astro-ph.CO↗
arXiv2011

Heating the hot atmospheres of galaxy groups and clusters with cavities: the relationship between jet power and low-frequency radio emission

We present scaling relations between jet power and radio power measured using the Giant Metrewave Radio Telescope (GMRT), Chandra and XMM-Newton, for a sample of 9 galaxy groups combined with the Birzan et al. sample of clusters. Cavity power is used as a proxy for mechanical jet power. Radio power is measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz-10 GHz radio luminosity is estimated from the GMRT 610-235 MHz spectral index. The use of consistently analysed, high resolution low-frequency radio data from a single observatory makes the radio powers for the groups more reliable than those used by previous studies, and the combined sample covers 6-7 decades in radio power and 5 decades in cavity power. We find a relation of the form Pjet proportional to Lradio^~0.7 for integrated radio luminosity, with a total scatter of sigma_Lrad=0.63 and an intrinsic scatter of sigma_i,Lrad=0.59. A similar relation is found for 235 MHz power, but a slightly flatter relation with greater scatter is found for 1.4 GHz power, suggesting that low-frequency or broad band radio measurements are superior jet power indicators. We find our low-frequency relations to be in good agreement with previous observational results. Comparison with jet models shows reasonable agreement, which may be improved if radio sources have a significant low-energy electron population. We consider possible factors which could bias our results or render them more uncertain, and find that correcting for such factors in those groups we are able to study in detail leads to a flattening of the Pjet:Lradio relation.

astro-ph.CO↗
arXiv2011

A combined low-radio frequency/X-ray study of galaxy groups I. Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz

We present new Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz of 18 X-ray bright galaxy groups. These observations are part of an extended project, presented here and in future papers, which combines low-frequency radio and X-ray data to investigate the interaction between central active galactic nuclei (AGN) and the intra-group medium (IGM). The radio images show a very diverse population of group-central radio sources, varying widely in size, power, morphology and spectral index. Comparison of the radio images with Chandra and XMM-Newton X-ray images shows that groups with significant substructure in the X-ray band and marginal radio emission at >= 1 GHz host low-frequency radio structures that correlate with substructures in IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction in our sample, are found in half of the systems, and are typically associated with small, low- or mid-power double radio sources. Two systems, NGC5044 and NGC4636, possess multiple cavities, which are isotropically distributed around the group center, possibly due to group weather. In other systems the radio/X-ray correlations are less evident. However, the AGN/IGM interaction can manifest itself through the effects of the high-pressure medium on the morphology, spectral properties and evolution of the radio-emitting plasma. In particular, the IGM can confine fading radio lobes in old/dying radio galaxies and prevent them from dissipating quickly. Evidence for radio emission produced by former outbursts that coexist with current activity is found in six groups of the sample.

astro-ph.CO↗
arXiv2011

A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst

We present the results of a Chandra study of the Hydra A galaxy cluster, where a powerful AGN outburst created a large-scale cocoon shock. We investigated possible azimuthal variations in shock strength and shape, finding indications for a weak shock with a Mach number in the range ~1.2-1.3. We measured the temperature change across the shock front. However, the detection of a temperature rise in the regions immediately inside of the front is complicated by the underlying temperature profile of the cluster atmosphere. We measured the global temperature profile of the cluster up to 700 kpc, which represents the farthest measurement obtained with Chandra for this cluster. A "plateau" in the temperature profile in the range ~70-150 kpc indicates the presence of cool gas, which is likely the result of uplift of material by the AGN outburst. After masking the cool filaments visible in the hardness ratio map, the plateau disappears and the temperature profile recovers a typical shape with a peak around 190 kpc, just inside the shock front. However, it is unlikely that such a temperature feature is produced by the shock as it is consistent with the general shape of the temperature profiles observed for relaxed galaxy clusters. We studied the spectral properties of the cool filaments finding evidence that ~10^11 M_sun of low-entropy material has been dredged up by the rising lobes from the central 30 kpc to the observed current position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x 10^60 erg, which is comparable to the work required to inflate the cavities and is ~25% of the total energy of the large-scale shock. Our results show that the AGN feedback in Hydra A is acting not only by directly heating the gas, but also by removing a substantial amount of potential fuel for the SMBH.

astro-ph.CO↗
arXiv2010

A decline and fall in the future of Italian Astronomy?

On May 27th 2010, the Italian astronomical community learned with concern that the National Institute for Astrophysics (INAF) was going to be suppressed, and that its employees were going to be transferred to the National Research Council (CNR). It was not clear if this applied to all employees (i.e. also to researchers hired on short-term contracts), and how this was going to happen in practice. In this letter, we give a brief historical overview of INAF and present a short chronicle of the few eventful days that followed. Starting from this example, we then comment on the current situation and prospects of astronomical research in Italy.

astro-ph.IM↗
arXiv2010

Cavities and shocks in the galaxy group HCG 62 as revealed by Chandra, XMM and GMRT data

We report on the results of an analysis of Chandra, XMM-Newton and new GMRT data of the X-ray bright compact group of galaxies HCG 62, which is one of the few groups known to possess clear, small X-ray cavities in the inner regions. This is part of an ongoing X-ray/low-frequency radio study of 18 groups, initially chosen for the availability of good-quality X-ray data and evidence for AGN/hot gas interaction. At higher frequency (1.4 GHz), the HCG 62 cavity system shows minimal if any radio emission, but the new GMRT observations at 235 MHz and 610 MHz clearly detect extended low-frequency emission from radio lobes corresponding to the cavities. By means of the synergy of X-ray and low-frequency radio observations, we compare and discuss the morphology, luminosity and pressure of the gas and of the radio source. We find that the radio source is radiatively inefficient, with a ratio of radio luminosity to mechanical cavity power of $\sim 10^{-4}$, and that the radio pressure of the lobes is about one order of magnitude lower than the X-ray pressure of the surrounding thermal gas. Thanks to the high spatial resolution of the Chandra surface brightness and temperature profiles, we also identify a shock front located at 36 kpc to the south-west of the group center, close to the southern radio lobe, with a Mach number $\sim 1.5$ and a total power which is about one order of magnitude higher than the cavity power. Such a shock may have heated the gas in the southern region, as indicated by the temperature map. The shock may also explain the arc-like region of enriched gas seen in the iron abundance map, as this may be produced by a non-Maxwellian electron distribution near its front.

astro-ph.CO↗
arXiv2009

The inverse iron-bias in action in Abell 2028

Recent work based on a global measurement of the ICM properties find evidence for an increase of the iron abundance in galaxy clusters with temperature around 2-4 keV. We have undertaken a study of the metal distribution in nearby clusters in this temperature range, aiming at resolving spatially the metal content of the ICM. The XMM observation of the first object of the sample, the cluster Abell 2028, reveals a complex structure of the cluster over scale of ~ 300 kpc, showing an interaction between two sub-clusters in a ``cometary'' configuration. We show that a naive one-component fit for the core of Abell 2028 returns a biased high metallicity. This is due to the inverse iron-bias, which is not related to the presence in the spectrum of both Fe-L and Fe-K emission lines but to the behavior of the fitting code in shaping the Fe-L complex of a one temperature component to adjust to the multi-temperature structure of the projected spectrum.

astro-ph.CO↗
arXiv2007

Mass distribution in the most X-ray-luminous galaxy cluster RX J1347.5-1145 studied with XMM-Newton

We report on the analysis of XMM-Newton observations of RX J1347.5-1145 (z=0.451), the most X-ray-luminous galaxy cluster. We present a detailed total and gas mass determination up to large distances (~1.7 Mpc), study the scaling properties of the cluster, and explore the role of AGN heating in the cluster cool core. By means of spatially resolved spectroscopy we derive density, temperature, entropy, and cooling time profiles of the intra-cluster medium. We compute the total mass profile of the cluster in the assumption of hydrostatic equilibrium. If the disturbed south-east region of the cluster is excluded from the analysis, our results on shape, normalization, scaling properties of density, temperature, entropy, and cooling time profiles are fully consistent with those of relaxed, cool core clusters. We compare our total and gas mass estimates with previous X-ray, lensing, dynamical, and SZ studies. We find good agreement with other X-ray results, dynamical mass measurements, weak lensing masses and SZ results. We confirm a discrepancy of a factor ~2 between strong lensing and X-ray mass determinations and find a gross mismatch between our total mass estimate and the mass reconstructed through the combination of both strong and weak lensing. We explore the effervescent heating scenario in the core of RX J1347.5-1145 and find support to the picture that AGN outflows and heat conduction are able to quenching radiative cooling.

astro-ph↗
arXiv2007

Discovery of diffuse radio emission at the center of the most X-ray-luminous cluster RX J1347.5-1145

We report on new VLA radio observations of the distant cluster RX J1347.5-1145, which is the most luminous in X-rays. We aim at investigating the possible presence of diffuse and extended radio emission in this very peculiar system which shows both a massive cooling flow and merging signatures. New low resolution (~18 arcsec) VLA radio observations of this cluster are combined with higher resolution (~2 arcsec) data available in the VLA archive. We discover the presence of a diffuse and extended (~500 kpc) radio source centered on the cluster, unrelated to the radio emission of the central AGN. The properties of the radio source, in particular a) its occurrence at the center of a massive cooling flow cluster, b) its total size comparable to that of the cooling region, c) its agreement with the observational trend between radio luminosity and cooling flow power, indicate that RX J1347.5-1145 hosts a radio mini-halo. We suggest that the radio emission of this mini-halo, which is the most distant object of its class discovered up to now, is due to electron re-acceleration triggered by the central cooling flow. However, we also note that the morphology of the diffuse radio emission shows an elongation coincident with the position of a hot subclump detected in X-rays, thus suggesting that additional energy for the electron re-acceleration might be provided by the submerger event.

astro-ph↗
arXiv2007

Cosmological Effects of Powerful AGN Outbursts in Galaxy Clusters: Insights from an XMM-Newton Observation of MS0735+7421

We report on the results of an analysis of XMM-Newton observations of MS0735+7421, the galaxy cluster which hosts the most energetic AGN outburst currently known. The previous Chandra image shows twin giant X-ray cavities (~200 kpc diameter) filled with radio emission and surrounded by a weak shock front. XMM data are consistent with these findings. The total energy in cavities and shock (~6 \times 10^{61} erg) is enough to quench the cooling flow and, since most of the energy is deposited outside the cooling region (~100 kpc), to heat the gas within 1 Mpc by ~1/4 keV per particle. The cluster exhibits an upward departure (factor ~2) from the mean L-T relation. The boost in emissivity produced by the ICM compression in the bright shells due to the cavity expansion may contribute to explain the high luminosity and high central gas mass fraction that we measure. The scaled temperature and metallicity profiles are in general agreement with those observed in relaxed clusters. Also, the quantities we measure are consistent with the observed M-T relation. We conclude that violent outbursts such as the one in MS0735+7421 do not cause dramatic instantaneous departures from cluster scaling relations (other than the L-T relation). However, if they are relatively common they may play a role in creating the global cluster properties.

astro-ph↗
arXiv2005

Multifrequency VLA radio observations of the X-ray cavity cluster of galaxies RBS797: evidence of differently oriented jets

We report on the peculiar activity of the radio source located at the center of the cooling flow cluster RBS797 (z=0.35), the first distant cluster in which two pronounced X-ray cavities have been discovered. New multifrequency (1.4, 4.8, and 8.4 GHz) observations obtained with the Very Large Array clearly reveal the presence of radio emission on three different scales showing orientation in different directions, all of which indicates that RBS797 represents a very peculiar case. The lowest resolution images show large-scale radio emission characterized by amorphous morphology and a steep spectrum, extended on a scale of hundreds of kpc. On a scale of tens of kpc, there is evidence of 1.4 GHz radio emission elongated in the northeast-southwest direction exactly towards the holes detected in X-rays. The highest resolution image shows the details of the innermost 4.8 GHz radio jets on a kpc scale; they are remarkably oriented in a direction that is perpendicular to that of the extended structure detected at a lower resolution. We therefore find evidence of a strong interaction between the central radio source and the intra-cluster medium in RBS797. We suggest a scenario in which the 1.4 GHz emission filling the X-ray cavities consists of buoyant bubbles of radio emitting plasma that are created by twin jets in the past and whose expansion has displaced the thermal gas that was formerly in the X-ray holes, whereas the two jets visible at 4.8 GHz are related to the present nuclear activity that has restarted at a different position angle from the original outburst that created the outer radio lobes. The total radio luminosity is ~ 10^42 erg/s, corresponding to a factor of a few thousand times less than the estimated cooling luminosity.

astro-ph↗
arXiv2005

Modeling the interaction between ICM and relativistic plasma in cooling flows: the case of the Perseus Cluster

We argue that the presence of diffuse synchrotron emission forming the so-called radio mini-halos in some cooling flow clusters can be explained by reacceleration of relativistic electrons, the necessary energetics being supplied by the cooling flows themselves. In particular, the reacceleration due to MHD turbulence has the correct radial dependence on the parameters to naturally balance the radiative losses. As an application we show that the main properties of the radio mini-halo in the Perseus cluster (brightness profile, total radio spectrum and radial spectral steepening) can be accounted for by the synchrotron radiation from relic relativistic electrons in the cluster, which are efficiently reaccelerated by MHD turbulence via Fermi-like processes due to the compression of the cluster magnetic field in the cooling flow region. Since the presence of an observable radio mini-halo in a cooling flow region critically depends on the combination of several physical parameters, we suggest that the rarity of radio mini-halos found in cooling flow clusters is due to the fact that the physical conditions of the ICM are intermediate between those which lead to the formation of extended radio halos and those holding in cooling flows without radio halos. The basic results of our model remain unchanged even if the cooling flow is stopped somewhere in the innermost region of the cluster.

astro-ph↗
arXiv2004

A first XMM-Newton look at the most X-ray-luminous galaxy cluster RX J1347.5-1145

We present the first results from an XMM-Newton observation of RX J1347.5-1145 (z=0.451), the most luminous X-ray cluster of galaxies currently known, with a luminosity L_X = 6.0 \pm 0.1 \times 10^{45} erg/s in the [2-10] keV energy band. The cluster has an overall temperature of kT=10.0 \pm 0.3 keV and is not isothermal: the temperature profile shows a decline in the outer regions and a drop in the centre, indicating the presence of a cooling core. The spectral analysis identifies a hot region at radii 50-200 kpc to south-east of the main X-ray peak, at a position consistent with the subclump seen in the X-ray image. Excluding the data of the south-east quadrant, the cluster appears relatively relaxed and we estimate a total mass within 1 Mpc of 1.0 \pm 0.2 \times 10^{15} M_sun.

astro-ph↗
arXiv2004

XMM-Newton observation of the most X-ray-luminous galaxy cluster RX J1347.5-1145

We report on an XMM-Newton observation of RX J1347.5-1145 (z=0.451), the most luminous X-ray cluster of galaxies currently known, with a luminosity L_X = 6.0 \pm 0.1 \times 10^45 erg/s in the [2-10] keV energy band. We present the first temperature map of this cluster, which shows a complex structure. It identifies the cool core and a hot region at radii 50-200 kpc to south-east of the main X-ray peak, at a position consistent with the subclump seen in the X-ray image. This structure is probably an indication of a submerger event. Excluding the data of the south-east quadrant, the cluster appears relatively relaxed and we estimate a total mass within 1.7 Mpc of 2.0 \pm 0.4 \times 10^15 M_sun. We find that the overall temperature of the cluster is kT=10.0 \pm 0.3 keV. The temperature profile shows a decline in the outer regios and a drop in the centre, indicating the presence of a cooling core which can be modelled by a cooling flow model with a minimum temperature ~2 keV and a very high mass accretion rate, \dot{M} ~ 1900 M_sun/yr. We compare our results with previous observations from ROSAT, ASCA and Chandra.

astro-ph↗
arXiv2004

Feedback from Intra-Cluster Supernovae on the ICM in Cooling Flow Galaxy Clusters

We study the effect of heating and metal enrichment from supernovae (SNe) residing between galaxies on the Intra-Cluster Medium (ICM). Recent observations indicate that a considerable fraction (~20 %) of the SN Ia parent stellar population in galaxy clusters is intergalactic. By considering their effect on the relaxed progenitors of cooling flow clusters we propose that intra-cluster SNe can act as a distributed heating source which may influence the initial stages of the formation of cooling flows. We investigate the increase in cooling time as a function of the energy input supplied by SNe and their assumed spatial distribution, and conclude that intra-cluster SNe represent a heating source which in some clusters can cause a delay of the formation of cooling flows. This would imply that some cooling flows are younger than previously thought. We also discuss the impact that a large population of intra-cluster SNe could have on the chemical evolution of the ICM in cooling flow clusters.

astro-ph↗
arXiv2004

The cooling flow cluster Abell 2626 and the associated radio emission

We present VLA data at 330 MHz and 1.5 GHz of the radio emission observed in the cooling flow cluster A2626. By producing images at different resolutions we found that the radio source consists of different components: an unresolved core plus a jet-like feature, two elongated parallel features, and an extended diffuse emission (radio mini-halo). Low resolution images allow us to derive morphological and spectral information of the diffuse emission: the radio mini-halo is extended on a scale comparable to that of the cooling flow region and is characterized by amorphous morphology, lack of polarized flux and very steep spectrum which steepens with distance from the center. We then applied to this new mini-halo source a model for particle re-acceleration in cooling flows (Gitti, Brunetti & Setti 2002). In particular, we found that its main radio properties (brightness profile, integrated radio spectrum and radial spectral steepening) can be accounted for by the synchrotron radiation from relic relativistic electrons in the cluster, which are efficiently re-accelerated by MHD turbulence amplified by the compression of the cluster magnetic field in the cooling flow region.

astro-ph↗
arXiv2003

Particle acceleration in cooling flow clusters of galaxies: the case of Abell 2626

It has recently been proposed a theoretical model which accounts for the origin of radio mini-halos observed in some cooling flow clusters as related to electron re-acceleration by MHD turbulence (Gitti, Brunetti & Setti 2002). The MHD turbulence is assumed to be frozen into the flow of the thermal ICM and thus amplified in the cooling flow region. Here we present the application of this model to a new mini-halo candidate, the cluster A2626, and compare the results with those obtained for the mini-halo in the Perseus cluster. We present VLA data at 330 MHz and 1.5 GHz of the diffuse radio emission observed in A2626, and we show that its main properties can be explained by the model. We find that the power necessary for the re-acceleration of the relic electron population is only a factor ~ 0.7% of the maximum power that can be extracted by the cooling flow (as estimated on the basis of the standard model). We also discuss the observational properties of known mini-halos in connection with those of host clusters, showing that the radio power of mini--halos increases with the maximum power of cooling flows. This trend is expected in the framework of the model. Possible effects of new Chandra and XMM-Newton estimates of $\dot{M}$ on this trend are considered: we conclude that even if earlier derived cooling rates were overestimated, cooling flow powers are still well above the radio powers emitted by mini-halos.

astro-ph↗