E. O’Sullivan

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 (AGNs) 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 co-exist with current activity is found in six groups of the sample.

On the Anomalous Temperature Distribution of the Intergalactic Medium in the NGC 3411 Group of Galaxies

We present XMM-Newton, Chandra, and VLA observations of the USGC S152 group and its central elliptical NGC 3411. Imaging of the group X-ray halo suggests that it is relaxed with little apparent structure. We investigate the temperature and metal abundance structure of the group halo and find that while the abundance distribution is fairly typical, the temperature profile is highly unusual, showing a hot inner core surrounded by a cool shell of gas with a radius of ~20-40 kpc, at the center of the larger group halo. Spectral mapping confirms an irregular ring of gas ~0.15 keV cooler than its surroundings. We estimate the total mass, entropy, and cooling time profiles within ~200 kpc, and find that the cool shell contains ~9 × 109 M☉ of gas. VLA observations at 1.4, 5, and 8 GHz reveal a relatively weak nuclear radio source, with a core radio luminosity LR = 2.7 × 1038 ergs s-1 and a diffuse component extended on scales of a few arcseconds (or more). A lack of evidence for activity at optical or X-ray wavelengths supports the conclusion that the central black hole is currently in a quiescent state. We discuss possible mechanisms for the formation of temperature features observed in the halo, including a previous period of AGN activity, and settling of material stripped from the halo of one of the other group member galaxies.

A deep Chandra observation of the poor cluster AWM 4 – II. The role of the radio jets in enriching the intracluster medium

We use a Chandra observation of the poor cluster AWM 4 to map the temperature and abundance of the intracluster medium, so as to examine the influence of the central radio galaxy on its environment. While the cluster core is generally enriched to near-solar abundances, we find evidence of supersolar abundances correlated with the radio jets, extending ∼35 kpc from the core of the central dominant galaxy NGC 6051 along its minor-axis. We conclude that the enriched gas has been transported out of the central galaxy through the action of the radio source. We estimate the excess mass of iron in the entrained gas to be ∼1.4 x 10 6 M ⊙ and find that this can be produced in the core of NGC 6051 within the time-scale of the active galactic nucleus (AGN) outburst. The energy required to transport this gas to its current location is ∼4.5 × 10 57 erg, a significant fraction of the estimated total mechanical energy output of the AGN, though this estimate is dependent on the degree of enrichment of the uplifted gas. The larger near-solar abundance region is also compatible with enrichment by metals mixed outwards from NGC 6051 over a much longer time-scale.

Cold gas in group-dominant elliptical galaxies ,

We present IRAM 30 m telescope observations of the CO(1–0) and (2–1) lines in a sample of 11 group-dominant elliptical galaxies selected from the CLoGS nearby groups sample. Our observations confirm the presence of molecular gas in 4 of the 11 galaxies at >4σ significance, and combining these with data from the literature we find a detection rate of 43 ± 14%, comparable to the detection rate for nearby radio galaxies, suggesting that group-dominant ellipticals may be more likely to contain molecular gas than

RECURRENT RADIO OUTBURSTS AT THE CENTER OF THE NGC 1407 GALAXY GROUP

We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at 240, 330, and 610 MHz of the complex radio source at the center of the NGC 1407 galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse emission enclosing the central twin-jet radio galaxy. This has been interpreted as an indication of two possible radio outbursts occurring at different times. Both the inner double and diffuse components are detected in the new GMRT images at high levels of significance. Combining the GMRT observations with archival Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both components. The inner double has a spectral index α = 0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with α = 1.8 between 240 MHz and 1.4 GHz. The radiative age of the large-scale component is very long, ~300 Myr, compared to ~30 Myr estimated for the central double, confirming that the diffuse component was generated during a former cycle of activity of the central galaxy. The current activity have so far released an energy which is nearly one order of magnitude lower than that associated with the former outburst. The group x-ray emission in the Chandra and XMM -Newton images and extended radio emission show a similar swept-back morphology. We speculate that the two structures are both affected by the motion of the group core, perhaps due to the core sloshing in response to a recent encounter with the nearby elliptical galaxy NGC 1400.

A Giant Metrewave Radio Telescope 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, 327, 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 ~ 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 ~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 center. 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.

The Presence of Thermally Unstable X-Ray Filaments and the Production of Cold Gas in the NGC 5044 Group

We present the results of a deep Chandra observation of the X-ray bright, moderate cooling flow group NGC 5044 along with the observed correlations between the ionized, atomic, and molecular gas in this system. The Chandra observation shows that the central AGN has undergone two outbursts in the past 100 Myrs, based on the presence of two pairs of nearly bipolar X-ray cavities. The molecular gas and dust within the central 2kpc is aligned with the orientation of the inner pair of bipolar X-ray cavities, suggesting that the most recent AGN outburst had a dynamical impact on the molecular gas. NGC 5044 also hosts many X-ray filaments within the central 8kpc, but there are no obvious connections between the X-ray and H

AGN Feedback in Galaxy Groups: A Joint GMRT/X‐ray Study

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Disturbed Fossil Group Galaxy NGC 1132

filaments and the more extended X-ray cavities that were inflated during the prior AGN outburst. Using the linewidth of the blended Fe-L line complex as a diagnostic for multiphase gas, we find that the majority of the multiphase, thermally unstable gas in NGC 5044 is confined within the X-ray filaments. While the cooling time and entropy of the gas within the X-ray filaments are very similar, not all filaments show evidence of gas cooling or an association with Ha emission. We suggest that the various observed properties of the X-ray filaments are suggestive of an evolutionary sequence where thermally unstable gas begins to cool, becomes multiphased, develops Ha emitting plasma, and finally produces cold gas.

AGN Feedback in the Compact Group of Galaxies HCG 62—as Revealed by Chandra, XMM and GMRT Data

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.