arXiv subjects

Myriam Gitti

Publications and source records attributed to Myriam Gitti.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Complete Local Volume Groups Sample -- III. Characteristics of group central radio galaxies in the Local Universe

Using new 610 MHz and 235 MHz observations from the Giant Metrewave Radio Telescope (GMRT) in combination with archival GMRT and Very Large Array (VLA) survey data we present the radio properties of the dominant early-type galaxies in the low$-$richness sub-sample of the Complete Local-volume Groups Sample (CLoGS; 27 galaxy groups) and provide results for the radio properties of the full CLoGS sample for the first time. We find a high radio detection rate in the dominant galaxies of the low-richness sub-sample of 82% (22/27); for the full CLoGS sample, the detection rate is 87% (46/53). The group-dominant galaxies exhibit a wide range of radio power, 10$^{20}$ $-$ 10$^{25}$ W Hz$^{-1}$ in the 235 and 610 MHz bands, with the majority (53%) presenting point-like radio emission, 19% hosting currently active radio jets, 6% having remnant jets, 9% being diffuse and 13% having no detected radio emission. The mean spectral index of the detected radio sources in the 235$-$610 MHz frequency range is found to be $α_{235}^{610}\sim$0.68, and $α_{235}^{1400}\sim$0.59 in the 235$-$1400 MHz one. In agreement with earlier studies, we find that the fraction of ultra-steep spectrum sources ($α>$1.3) is $\sim$4%, mostly dependent on the detection limit at 235 MHz. The majority of point-like systems are found to reside in dynamically young groups, whereas jet systems show no preference between spiral-rich and spiral-poor group environments. The mechanical power of the jet sources in the low$-$richness sample groups is estimated to be $\sim$10$^{42}$ $-$ 10$^{44}$ erg s$^{-1}$ with their black hole masses ranging between 2$\times$10$^{8}$ $-$ 5$\times$10$^{9}$ M$_{\odot}$. We confirm previous findings that, while radio jet sources tend to be associated with more massive black holes, black hole mass is not the decisive factor in determining jet activity or power.

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arXiv2018

The Complete Local Volume Groups Sample - II. A study of the Central Radio Galaxies in the High$-$Richness Sub-sample

We present a study of the radio properties of the dominant early-type galaxies in 26 galaxy groups, the high-richness sub-sample of the Complete Local-volume Groups Sample (CLoGS). Combining new 610 MHz and 235 MHz observations of 21 groups from the Giant Metrewave Radio Telescope (GMRT) with archival GMRT and Very Large Array (VLA) survey data, we find a high detection rate, with 92% of the dominant galaxies hosting radio sources. The sources have a wide range of luminosities, 10$^{20}$ $-$ 10$^{24}$ W/Hz in the 235 and 610 MHz bands. The majority (54%) are point-like, but 23% have radio jets, and another 15% are diffuse radio sources with no clear jet/lobe morphology. The spectral index of the detected radio sources ranges from very flat values of ~0.2 to typical radio synchrotron spectra of ~0.9 with only two presenting steep radio spectra with $α_{235}^{610}$ > 1. We find that jet sources are more common in X-ray bright groups, with radio non-detections found only in X-ray faint systems. Radio point sources appear in all group environments irrespective of their X-ray properties or spiral fraction. We estimate the mechanical power (Pcav) of the jet sources in the X-ray bright groups to be 10$^{41}$ $-$ 10$^{43}$ erg/s, with the two large-scale jet systems (NGC 193 and NGC 4261) showing jet powers two orders of magnitude greater than the radiative losses from the cool cores of their groups. This suggests that central AGN are not always in balance with cooling, but may instead produce powerful periodical bursts of feedback heating.

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arXiv2018

Thermal and non-thermal connection in radio mini-halos

Several cool-core clusters are known to host a radio mini-halo, a diffuse, steep-spectrum radio source located in their cores, thus probing the presence of non-thermal components as magnetic field and relativistic particles on scales not directly influenced by the central AGN. The nature of the mechanism that produces a population of radio-emitting relativistic particles on the scale of hundreds of kiloparsecs is still unclear. At the same time, it is still debated if the central AGN may play a role in the formation of mini-halos by providing the seed of the relativistic particles. We aim to investigate these open issues by studying the connection between thermal and non-thermal components of the intra-cluster medium. We performed a point-to-point analysis of the radio and the X-ray surface brightness of a compilation of mini-halos. We find that mini-halos have super-linear scalings between radio and X-rays, with radio brightness declining more steeply than the X-ray brightness. This trend is opposite to that generally observed in giant radio halos, thus marking a possible difference in the physics of the two radio sources. Finally, using the scalings between radio and X-rays and assuming a hadronic origin of mini-halos we derive constraints on the magnetic field in the core of the hosting clusters.

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arXiv2018

Radio-continuum surveys with SKA and LOFAR: a first look at the perspectives for radio mini-halos

Diffuse synchrotron radio emission has been observed in a number of cool-core clusters on scales comparable to that of the cooling region. These radio sources are called `mini-halos'. In order to understand their origin, which is still unclear, joint radio and X-ray statistical studies of large cluster samples are necessary to investigate the radio mini-halo properties and their connection with the cluster thermodynamics. We here extend our previous explorative study and investigate the perspectives offered by surveys in the radio continuum with LOFAR and SKA, in particular examining the effect of the intra-cluster magnetic field in the mini-halo region for the first time. By considering the minimum flux detectable in radio surveys and exploiting the $P_{radio}-L_X$ correlation observed for known mini-halos, we estimate the detection limits achievable by future radio observational follow-up of X-ray cluster samples, such as HIFLUGCS and eROSITA. This allows us to estimate the maximum number of radio mini-halos that can potentially be discovered in future surveys as a function of redshift and magnetic field strength. We show that future radio surveys with LOFAR and SKA1 (at 140 MHz and 1.4 GHz) have the potential to discover ~1,000-10,000 radio mini-halo candidates up to redshift z=1. We further note that future SKA1 radio surveys at redshift z>0.6 will allow us to distinguish between different magnetic fields in the mini-halo region, because higher magnetic fields are expected to produce more powerful mini-halos, thus implying a larger number of mini-halo detected at high redshift. For example, the non-detection with SKA1 of mini-halos at z>0.6 will suggest a low magnetic field (B < few $μ$G). The synergy of these radio surveys with future X-ray observations and theoretical studies is essential in establishing the radio mini-halo physical nature. [abridged]

astro-ph.CO↗

arXiv2018

ALMA observations of molecular clouds in three group centered elliptical galaxies: NGC 5846, NGC 4636, and NGC 5044

We present new ALMA CO(2--1) observations of two well studied group-centered elliptical galaxies: NGC~4636 and NGC~5846. In addition, we include a revised analysis of Cycle 0 ALMA observations of the central galaxy in the NGC~5044 group that has been previously published. We find evidence that molecular gas, in the form of off-center orbiting clouds, is a common presence in bright group-centered galaxies (BGG). CO line widths are $\gtrsim 10$ times broader than Galactic molecular clouds, and using the reference Milky Way $X_{CO}$, the total molecular mass ranges from as low as $2.6\times 10^5 M_\odot$ in NGC~4636 to $6.1\times 10^7 M_\odot$ in NGC~5044. With these parameters the virial parameters of the molecular structures is $\gg 1$. Complementary observations of NGC~5846 and NGC~4636 using the ALMA Compact Array (ACA) do not exhibit any detection of a CO diffuse component at the sensitivity level achieved by current exposures. The origin of the detected molecular features is still uncertain, but these ALMA observations suggest that they are the end product of the hot gas cooling process and not the result of merger events. Some of the molecular clouds are associated with dust features as revealed by HST dust extinction maps suggesting that these clouds formed from dust-enhanced cooling. The global nonlinear condensation may be triggered via the chaotic turbulent field or buoyant uplift. The large virial parameter of the molecular structures and correlation with the warm ($10^3 - 10^5 K$)/hot ($\ge10^6$) phase velocity dispersion provide evidence that they are unbound giant molecular associations drifting in the turbulent field, consistently with numerical predictions of the chaotic cold accretion process. Alternatively, the observed large CO line widths may be generated by molecular gas flowing out from cloud surfaces due to heating by the local hot gas atmosphere.

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arXiv2017

The Complete Local Volume Groups Sample - I. Sample Selection and X-ray Properties of the High-Richness Subsample

We present the Complete Local-Volume Groups Sample (CLoGS), a statistically complete optically-selected sample of 53 groups within 80 Mpc. Our goal is to combine X-ray, radio and optical data to investigate the relationship between member galaxies, their active nuclei, and the hot intra-group medium (IGM). We describe sample selection, define a 26-group high-richness subsample of groups containing at least 4 optically bright (log L_B>=10.2 LBsol) galaxies, and report the results of XMM-Newton and Chandra observations of these systems. We find that 14 of the 26 groups are X-ray bright, possessing a group-scale IGM extending at least 65kpc and with luminosity >10^41 erg/s, while a further 3 groups host smaller galaxy-scale gas halos. The X-ray bright groups have masses in the range M_500=0.5-5x10^13 Msol, based on system temperatures of 0.4-1.4 keV, and X-ray luminosities in the range 2-200x10^41 erg/s. We find that ~53-65% of the X-ray bright groups have cool cores, a somewhat lower fraction than found by previous archival surveys. Approximately 30% of the X-ray bright groups show evidence of recent dynamical interactions (mergers or sloshing), and ~35% of their dominant early-type galaxies host AGN with radio jets. We find no groups with unusually high central entropies, as predicted by some simulations, and confirm that CLoGS is in principle capable of detecting such systems. We identify three previously unrecognized groups, and find that they are either faint (L_X,R500<10^42 erg/s) with no concentrated cool core, or highly disturbed. This leads us to suggest that ~20% of X-ray bright groups in the local universe may still be unidentified.

astro-ph.GA↗

arXiv2017

New JVLA observations at 3 GHz and 5.5 GHz of the `Kite' radio source in Abell 2626

We report on new JVLA observations performed at 3 GHz and 5.5 GHz of Abell 2626. The cluster has been the object of several studies in the recent years due to its peculiar radio emission, which shows a complex system of symmetric radio arcs characterized by a steep spectrum. The origin of these radio sources is still unclear. Due to their mirror symmetry toward the center, it has been proposed that they may be created by pairs of precessing jets powered by the inner AGN. The new JVLA observations were requested with the specific aim of detecting extended emission on frequencies higher than 1.4 GHz, in order to constrain the jet-precession model by analyzing the spectral index and radiative age patterns alongs the arcs. We performed a standard data reduction of the JVLA datasets with the software CASA. By combining the new 3 GHz data with the archival 1.4 GHz VLA dataset we produced a spectral index maps of the extended emission, and then we estimated the radiative age of the arcs by assuming that the plasma was accelerated in moving hot-spots tracing the arcs. Thanks to the high sensitivity of the JVLA, we achieve the detection of the arcs at 3 GHz and extended emission at 5.5 GHz. We measure a mean spectral index <-2.5 for the arcs up to 3 GHz. No clear spectral index, or radiative age, trend is detected across the arcs which may challenge the interpretation based on precession or put strong constraints on the jet-precession period. In particular, by analyzing the radiative age distribution along the arcs, we were able to provide for the first time a time-scale < 26 Myr of the jet-precession period.

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arXiv2016

Discovery of a fourth arc in Abell 2626 at 610 MHz with the GMRT: Spectral properties and possibilities for the origin

We report the discovery of a fourth eastern arc (Arc E) towards the cool-core cluster Abell 2626 using 610 MHz Giant Metrewave Radio Telescope observations. Three arcs towards north, west and south were known from earlier works at 1400 MHz and proposed to have originated in precessing radio jets of the central active galactic nucleus. The 610 - 1400 MHz integrated spectral indices of the arcs are in the range 3.2 - 3.6 and the spectral index map shows uniform distribution along the lengths of the arcs. If associated with A2626, the arcs have linear extents in the range 79 - 152 kpc. The detection of Arc E favours the scenario in which a pair of bipolar precessing jets were active and halted to produce the arc system. Based on the morphological symmetry and spectral similarity, we indicate a possible role of gravitational lensing. Further high resolution low frequency observations and measurements of the mass of the system are needed to disentangle the mystery of this source.

astro-ph.GA↗

arXiv2016

On the connection between radio mini-halos and gas heating in cool core clusters

In this work, we present a study of the central regions of cool-core clusters hosting radio mini-halos, which are di use synchrotron sources extended on cluster-scales surrounding the radio-loud brightest galaxy. We aim to investigate the interplay between the thermal and non-thermal components in the intracluster medium in order to get more insights into these radio sources, whose nature is still unclear. It has recently been proposed that turbulence plays a role for heating the gas in cool cores. A correlation between the radio luminosity of mini-halos, $νP_ν$, and the cooling flow power, $P_{\rm CF}$, is expected in the case that this turbulence also plays a role for the acceleration of the relativistic particles. We carried out a homogeneous re-analysis of X-ray Chandra data of the largest sample of cool-core clusters hosting radio mini-halos currently available ($\sim20$ objects), finding a quasi-linear correlation, $νP_ν \propto P_{\rm CF}^{0.8}$. We show that the scenario of a common origin of radio mini-halos and gas heating in cool-core clusters is energetically viable, provided that mini-halos trace regions where the magnetic field strength is $B \gg 0.5 μG$.

astro-ph.CO↗

arXiv2016

Radio mini-halos and AGN heating in cool core clusters of galaxies

The brightest cluster galaxy (BCG) in the majority of relaxed, cool core galaxy clusters is radio loud, showing non-thermal radio jets and lobes ejected by the central active galactic nucleus (AGN). Such relativistic plasma has been unambiguously shown to interact with the surrounding thermal intra-cluster medium (ICM) thanks to spectacular images where the lobe radio emission is observed to fill the cavities in the X-ray-emitting gas. This `radio-mode AGN feedback' phenomenon, which is thought to quench cooling flows, is widespread and is critical to understand the physics of the inner regions of galaxy clusters and the properties of the central BCG. At the same time, mechanically-powerful AGN are likely to drive turbulence in the central ICM which may contribute to gas heating and also play a role for the origin of non-thermal emission on cluster-scales. Diffuse non-thermal emission has been observed in a number of cool core clusters in the form of a radio mini-halo surrounding the radio-loud BCG on scales comparable to that of the cooling region. This contribution outlines the main points covered by the talk on these topics. In particular, after summarizing the cooling flow regulation by AGN heating and the non-thermal emission from cool core clusters, we present a recent study of the largest collection of known mini-halo clusters (~ 20 objects) which investigated the scenario of a common origin of radio mini-halos and gas heating. We further discuss the prospects offered by future radio surveys with the Square Kilometre Array (SKA) for building large (>> 100 objects), unbiased mini-halo samples while probing at the same time the presence of radio-AGN feedback in the host clusters.

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arXiv2015

Do radio mini-halos and gas heating in cool-core clusters have a common origin?

In this letter we present a study of the central regions of cool-core clusters hosting radio mini-halos, which are diffuse synchrotron sources extended on cluster-scales surrounding the radio-loud brightest cluster galaxy. We aim to investigate the interplay between the thermal and non-thermal components in the intra-cluster medium in order to get more insights into these radio sources, whose nature is still unclear. It has recently been proposed that turbulence plays a role for heating the gas in cool cores. By assuming that mini-halos are powered by the same turbulence, we expect that the integrated radio luminosity of mini-halos, $νP_ν$, depends on the cooling flow power, $P_{\rm CF}$, which in turn constrains the energy available for the non-thermal components and emission in the cool-core region. We carried out a homogeneous re-analysis of X-ray Chandra data of the largest sample of cool-core clusters hosting radio mini-halos currently available ($\sim$ 20 objects), finding a quasi-linear correlation, $νP_ν \propto P_{\rm CF}^{0.8}$. We show that the scenario of a common origin of radio mini-halos and gas heating in cool-core clusters is energetically viable, provided that mini-halos trace regions where the magnetic field strength is $B \gg 0.5\, μ$G .

astro-ph.CO↗

arXiv2015

Twin SMBH candidates in the BCG of RBS 797

The radio-loud BCG at the center of the cool core cluster RBS 797 is known to exhibit a misalignment of its 5 GHz radio emission observed at different VLA resolutions, with the innermost kpc-scale jets being almost orthogonal to the radio lobes which extends for tens of kpc filling the X-ray cavities seen by Chandra. The different radio directions may be caused by rapid jet reorientation due to interaction with a secondary supermassive black hole (SMBH), or to the presence of two AGN, probably in a merging phase, which are emitting radio jets in different directions. We present the results of new 5 GHz observations performed with the EVN in May 2013. In particular, we detected two compact radio components, with a projected separation of 77 pc. We discuss two possible scenarios for the origin and nature of the EVN double source, showing that both interpretations are consistent with the presence of a SMBH binary system in the BCG of RBS 797.

astro-ph.GA↗

arXiv2015

Mega-parsec scale magnetic fields in low density regions in the SKA era: filaments connecting galaxy clusters and groups

The presence of magnetic fields in galaxy clusters has been well established in recent years, and their importance for the understanding of the physical processes at work in the Intra Cluster Medium has been recognized. Halo and relic sources have been detected in several tens clusters. A strong correlation is present between the halo and relic radio power and the X-ray luminosity. Since cluster X-Ray luminosity and mass are related, the correlation between the radio power and X-ray luminosity could derive from a physical dependence of the radio power on the cluster mass, therefore the cluster mass could be a crucial parameter in the formation of these sources. The goal of this project is to investigate the existence of non-thermal structures beyond the Mpc scale, and associated with lower density regions with respect to clusters of galaxies: galaxy filaments connecting rich clusters. We present a piece of evidence of diffuse radio emission in intergalactic filaments. Moreover, we present and discuss the detection of radio emission in galaxy groups and in faint X-Ray clusters, to analyze non-thermal properties in low density regions with physical conditions similar to galaxy filaments. We discuss how SKA1 observations will allow the investigation of this topic and the study of the presence of diffuse radio sources in low density regions. This will be a fundamental step to understand the origin and properties of cosmological magnetic fields.

astro-ph.GA↗

arXiv2014

Non-thermal emission from galaxy clusters: feasibility study with SKA1

Galaxy clusters are known to host a variety of extended radio sources: tailed radio galaxies whose shape is modelled by the interaction with the intra-cluster medium (ICM); radio bubbles filling cavities in the ICM distribution and rising buoyantly through the thermal gas; diffuse giant radio sources ("halos" and "relics") revealing the presence of relativistic electrons and magnetic fields in the intra-cluster volume. It is currently the subject of an active debate how the non-thermal components that we observe at radio wavelengths affect the physical properties of the ICM and depend on the dynamical state of galaxy clusters. In this work we start our SKA1 feasibility study of the "radio cluster zoo" through simulations of a typical radio-loud cluster, hosting several bright tailed radio galaxies and a diffuse radio halo. Realistic simulations of SKA1 observations are obtained through the MeqTrees software. A new deconvolution algorithm, based on sparse representations and optimised for the detection of faint diffuse astronomical sources, is tested and compared to the classical CLEAN method.

astro-ph.CO↗

arXiv2014

The SKA view of cool-core clusters: evolution of radio mini-halos and AGN feedback

In about 70% of the population of relaxed, cool-core galaxy clusters, the brightest cluster galaxy (BCG) is radio loud, showing non-thermal radio jets and lobes ejected by the central active galactic nucleus (AGN). In recent years such relativistic plasma has been unambiguously shown to interact with the surrounding thermal intra-cluster medium (ICM) thanks to spectacular images where the lobe radio emission is observed to fill the cavities in the X-ray-emitting gas. This `radio feedback' phenomenon is widespread and is critical to understand the physics of the inner regions of galaxy clusters and the properties of the central BCG. At the same time, mechanically-powerful AGN are likely to drive turbulence in the central ICM which may also play a role for the origin of non-thermal emission on cluster-scales. Diffuse non-thermal emission has been observed in a number of cool-core clusters in the form of a radio mini-halo surrounding the radio-loud BCG on scales comparable to that of the cooling region. Large mini-halo samples are necessary to establish their origin and connection with the cluster thermal properties and dynamics, especially in light of future X-ray characterization of the cluster cores as it is expected by Athena-XIFU. We show that All-Sky reference survey at Band 2 with SKA1 at confusion limit (rms ~2 μJy per beam) has the potential to detect up to ~620 mini-halos at redshift z<0.6, whereas Deep Tier reference surveys at Band 1/2 with SKA1 at sub-arcsec resolution (rms ~0.2 μJy per beam) will allow a complete census of the radio-loud BCGs at any redshift down to a 1.4 GHz power of 10^{22} W/Hz. We further anticipate that SKA2 might detect up to ~1900 new mini-halos at redshift z<0.6 and characterize the radio-mode AGN feedback in every cluster and group up to redshift z ~1.7 (the highest-z where virialized clusters are currently detected).

astro-ph.CO↗

arXiv2013

The puzzling radio source in the cool core cluster A 2626

We report on new VLA radio observations performed at 1.4 GHz and 4.8 GHz with unprecedented sensitivity and angular resolution (~1 arcsec) of the cool core cluster A 2626, which is known to possess a radio mini-halo at its center. The most unusual features of A 2626 are two elongated radio features detected in previous observations to the north and south, having morphologies not common to the typical jet-lobe structures in cool cores. In our new sensitive images the two elongated features appears clearly as bright radio arcs, and we discover the presence of a new arc to the west. These radio arcs are not detected at 4.8 GHz, implying a steep (alpha >1) spectrum, and their origin is puzzling. After subtracting the flux density contributed by these discrete features from the total flux measured at low resolution, we estimate a residual 18.0 +/- 1.8 mJy flux density of diffuse radio emission at 1.4 GHz. We therefore confirm the detection of diffuse radio emission, which appears distinct from the discrete radio arcs embedded in it. Although its radio power is lower (1.4x10^23 W/Hz) than previously known, the diffuse emission may still be classified as a radio mini-halo.

astro-ph.CO↗

arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

astro-ph.HE↗

arXiv2012

A Chandra - VLA Investigation of the X-ray Cavity System and Radio Mini-Halo in the Galaxy Cluster RBS 797

We present a study of the cavity system in the galaxy cluster RBS 797 based on Chandra and VLA data. RBS 797 (z = 0.35), is one of the most distant galaxy clusters in which two pronounced X-ray cavities have been discovered. The Chandra data confirm the presence of a cool core and indicate an higher metallicity along the cavity directions. This is likely due to the AGN outburst, which lifts cool metal-rich gas from the center along the cavities, as seen in other systems. We find indications that the cavities are hotter than the surrounding gas. Moreover, the new Chandra images show bright rims contrasting with the deep, X-ray deficient cavities. The likely cause is that the expanding 1.4 GHz radio lobes have displaced the gas, compressing it into a shell that appears as bright cool arms. Finally we show that the large-scale radio emission detected with our VLA observations may be classified as a radio mini-halo, powered by the cooling flow (CF), as it nicely follows the trend P_radio vs. P_CF predicted by the re-acceleration model.

astro-ph.CO↗