Publication Search Results

ID: 156917
Type: article
Authors: Schellenberger, Gerrit; David, Laurence P.; Vrtilek, Jan; O'Sullivan, Ewan; Lim, Jeremy; Forman, William; Sun, Ming; Combes, Francoise; Salome, Philippe; Jones, Christine; Giacintucci, Simona; Edge, Alastair; Gastaldello, Fabio; Temi, Pasquale; Brighenti, Fabrizio; Bardelli, Sandro
Abstract: The fate of cooling gas in the centers of galaxy clusters and groups is still not well understood, as is also the case for the complex processes of triggering star formation in central dominant galaxies, reheating of cooled gas by active galactic nuclei (AGN), and the triggering or "feeding" of supermassive black hole outbursts. We present CO observations of the early-type galaxy NGC 5044, which resides at the center of an X-ray bright group with a moderate cooling flow. For our analysis we combine CO(2-1) data from the 7 m antennae of the Atacama Compact Array (ACA) and the ACA total power array (TP). We demonstrate, using the 7 m array data, that we can recover the total flux inferred from IRAM 30 m single-dish observations, which corresponds to a total molecular mass of about 4 × 10⁷M_☉. Most of the recovered flux is blueshifted with respect to the galaxy rest frame and is extended on kiloparsec-scales, suggesting low filling factor dispersed clouds. We find eight concentrations of molecular gas out to a radius of 10″ (1.5 kpc), which we identify with giant molecular clouds. The total molecular gas mass is more centrally concentrated than the X-ray emitting gas, but is extended in the northeast-southwest direction beyond the IRAM 30 m beam. We also compare the spatial extent of the molecular gas to the Hα emission: The CO emission coincides with the very bright Hα region in the center. We do not detect CO emission in the fainter Hα regions. Furthermore, we find two CO absorption features spatially located at the center of the galaxy, within 5 pc projected distance of the AGN, infalling at 255 and 265 km s^-1 relative to the AGN. This indicates that the two giant molecular clouds seen in absorption are most likely within the sphere of influence of the supermassive black hole.

ID: 154705
Type: article
Authors: Nagai, H.; Onishi, K.; Kawakatu, N.; Fujita, Y.; Kino, M.; Fukazawa, Y.; Lim, J.; Forman, William; Vrtilek, Jan; Nakanishi, K.; Noda, H.; Asada, K.; Wajima, K.; Ohyama, Y.; David, Laurence; Daikuhara, K.
Abstract: We present observations using the Atacama Large Millimeter/submillimeter Array of the CO(2-1), HCN(3-2), and HCO⁺(3-2) lines in the nearby radio galaxy/brightest cluster galaxy (BCG) NGC 1275 with a spatial resolution of ̃20 pc. In previous observations, the CO(2-1) emission was detected as radial filaments lying in the east-west direction on a kiloparsec scale. We resolved the inner filament and found that it cannot be represented by a simple infalling stream on a sub-kiloparsec scale. The observed complex nature of the filament resembles the cold gas structure predicted by numerical simulations of cold chaotic accretion. Within the central 100 pc, we detected a rotational disk of molecular gas whose mass is ̃10⁸ M _☉. This is the first evidence of the presence of a massive cold gas disk on this spatial scale for BCGs. A crude estimate suggests that the accretion rate of the cold gas can be higher than that of hot gas. The disk rotation axis is approximately consistent with the radio- jet axis. This probably suggests that the cold gas disk is physically connected to the innermost accretion disk, which is responsible for jet launching. We also detected absorption features in the HCN(3-2) and HCO⁺(3-2) spectra against the radio continuum emission mostly radiated by a jet of size ̃1.2 pc. The absorption features are blueshifted from the systemic velocity by ̃300-600 km s^-1, suggesting the presence of outflowing gas from the active galactic nucleus (AGN). We discuss the relation of the AGN feeding with cold accretion, the origin of blueshifted absorption, and an estimate of the black hole mass using molecular gas dynamics.

ID: 154444
Type: article
Authors: O'Sullivan, Ewan; Schellenberger, Gerrit; Burke, Douglas J.; Sun, Ming; Vrtilek, Jan M.; David, Laurence P.; Sarazin, Craig
Abstract: We present deep Chandra, XMM-Newton, Giant Metrewave Radio Telescope, and Hα observations of the group-group merger NGC 6338. X-ray imaging and spectral mapping show that as well as trailing tails of cool, enriched gas, the two cool cores are embedded in an extensive region of shock-heated gas with temperatures rising to ̃5 keV. The velocity distribution of the member galaxies show that the merger is occurring primarily along the line of sight, and we estimate that the collision has produced shocks of Mach number M = 2.3 or greater, making this one of the most violent mergers yet observed between galaxy groups. Both cool cores host potential AGN cavities and Hα nebulae, indicating rapid radiative cooling. In the southern cool core around NGC 6338, we find that the X-ray filaments associated with the Hα nebula have low entropies (²) and short cooling times (̃200-300 Myr). In the northern core, we identify an Hα cloud associated with a bar of dense, cool X-ray gas offset from the dominant galaxy. We find no evidence of current jet activity in either core. We estimate the total mass of the system and find that the product of this group-group merger will likely be a galaxy cluster.

ID: 154403
Type: article
Authors: Schellenberger, Gerrit; David, Laurence; O'Sullivan, Ewan; Vrtilek, Jan M.; Haines, C. P.
Abstract: The system A1758 is made up of two galaxy clusters, a more massive, northern cluster and a southern cluster. Both parts are undergoing major merger events at different stages. Although the mass of the merger constituents provides enough energy to produce visible shock fronts in the X-ray, none have been found to date. We present detailed temperature and abundance maps based on Chandra ACIS data and identify several candidates for shocks and cold fronts from a smoothed gradient map of the surface brightness. One candidate can be confirmed as the missing shock front in the northern cluster through X-ray spectroscopy. Nonthermal radio emission observed with the GMRT confirms the presence of radio halos in the northern and southern clusters and shows evidence for a relic in the periphery of the southern cluster. We do not find evidence for shocked gas between A1758 N and A1758 S.

ID: 149411
Type: article
Authors: O'Sullivan, Ewan J.; Combes, F.; Salomé, P.; David, L. P.; Babul, A.; Vrtilek, Jan M.; Lim, J.; Olivares, V.; Raychaudhury, S.; Schellenberger, Gerrit
Abstract: We present IRAM 30 m and APEX telescope observations of CO(1-0) and CO(2-1) lines in 36 group-dominant early-type galaxies, completing our molecular gas survey of dominant galaxies in the Complete Local-volume Groups Sample. We detect CO emission in 12 of the galaxies at > 4σ significance, with molecular gas masses in the range ˜ 0.01 - 6 × 10⁸ M_⊙, as well as CO in absorption in the non-dominant group member galaxy NGC 5354. In total 21 of the 53 CLoGS dominant galaxies are detected in CO and we confirm our previous findings that they have low star formation rates (0.01-1 M_⊙yr^-1) but short depletion times (>1 Gyr) implying rapid replenishment of their gas reservoirs. Comparing molecular gas mass with radio luminosity, we find that a much higher fraction of our group-dominant galaxies (60 ± 16%) are AGN-dominated than is the case for the general population of ellipticals, but that there is no clear connection between radio luminosity and the molecular gas mass. Using data from the literature, we find that at least 27 of the 53 CLoGS dominant galaxies contain H I, comparable to the fraction of nearby non-cluster early type galaxies detected in H I and significantly higher that the fraction in the Virgo cluster. We see no correlation between the presence of an X-ray detected intra-group medium and molecular gas in the dominant galaxy, but find that the H I-richest galaxies are located in X-ray faint groups. Morphological data from the literature suggests the cold gas component most commonly takes the form of a disk, but many systems show evidence of galaxy-galaxy interactions, indicating that they may have acquired their gas through stripping or mergers. We provide improved molecular gas mass estimates for two galaxies previously identified as being in the centres of cooling flows, NGC 4636 and NGC 5846, and find that they are relatively molecular gas poor compared to our other detected systems. Based on observations carried out with the IRAM 30m telescope and the Atacama Pathfinder Experiment (APEX). IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.The reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A126

ID: 145789
Type: article
Authors: O'Sullivan, Ewan; Kolokythas, Konstantinos; Kantharia, Nimisha G.; Raychaudhury, Somak; David, Laurence P.; Vrtilek, Jan M.
Abstract: The NGC 5903 galaxy group is a nearby (∼30 Mpc) system of ∼30 members, dominated by the giant ellipticals NGC 5903 and NGC 5898. The group contains two unusual structures: a ∼110 kpc long H I filament crossing NGC 5903 and a ∼75 kpc wide diffuse, steep-spectrum radio source of unknown origin that overlaps NGC 5903 and appears to be partly enclosed by the H I filament. Using a combination of Chandra, XMM-Newton, Giant Meterwave Radio Telescope (GMRT) and Very Large Array (VLA) observations, we detect a previously unknown ∼0.65 keV intra-group medium filling the volume within 145 kpc of NGC 5903 and find a loop of enhanced X-ray emission extending ∼35 kpc south-west from the galaxy, enclosing the brightest part of the radio source. The northern and eastern parts of this X-ray structure are also strongly correlated with the southern parts of the H I filament. We determine the spectral index of the bright radio emission to be α _{150}^{612} = 1.03 ± 0.08, indicating a radiative age >360 Myr. We discuss the origin of the correlated radio, X-ray and H I structures, either through an interaction-triggered active galactic nucleus (AGN) outburst with enthalpy 1.8 × 10⁵⁷ erg, or via a high-velocity collision between a galaxy and the H I filament. While neither scenario provides a complete explanation, we find that an AGN outburst is the most likely source of the principal X-ray and radio structures. However, it is clear that galaxy interactions continue to play an important role in the development of this relatively highly evolved galaxy group. We also resolve the question of whether the group member galaxy ESO 514-3 hosts a double-lobed radio source, confirming that the source is a superposed background AGN.

ID: 143363
Type: article
Authors: David, Laurence P.; Vrtilek, Jan; O'Sullivan, Ewan; Jones, Christine; Forman, William; Sun, Ming
Abstract: 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 10⁸ years, based on the presence of two pairs of nearly bipolar X-ray cavities. The molecular gas and dust within the central 2 kpc 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 8 kpc, but there are no obvious connections between the X-ray and Hα filaments and the more extended X-ray cavities that were inflated during the prior AGN outburst. Using the line width 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 Hα 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 Hα emitting plasma, and finally produces cold gas.

ID: 144806
Type: article
Authors: Lim, Jeremy; Dinh-V-Trung; Vrtilek, Jan; David, Laurence P.; Forman, William
Abstract: We present observations in CO (3-2) that, combined with previous observations in CO (2-1), constrain the physical properties of the filamentary molecular gas in the central ~6.5 kpc of NGC 1275, the central giant elliptical galaxy of the Perseus Cluster. We find this molecular gas to have a temperature ? 20 K and a density ~ {10}²{--}{10}⁴ {{cm}}^-3, typically warmer and denser than the bulk of Giant Molecular Clouds (GMCs) in the Galaxy. Bathed in the harsh radiation and particle field of the surrounding intracluster X-ray gas, the molecular gas likely has a much higher ionization fraction than that of GMCs. For an ionization fraction of ~10^-4, similar to that of Galactic diffuse (? 250 {{cm}}^-3) partially molecular clouds that emit in HCN (1-0) and HCO⁺ (1-0), we show that the same gas traced in CO can produce the previously reported emissions in HCN (3-2), HCO⁺ (3-2), and CN (2-1) from NGC 1275; the dominant source of excitation for all the latter molecules is collisions with electrons. For the molecular filaments to not collapse, as evidenced by their lack of star formation, they must consist of thin strands that have cross-sectional radii ?0.2-2 pc if supported solely by thermal gas pressure; larger radii are permissible if turbulence or poloidal magnetic fields provide additional pressure support. We point out that the conditions required to relate CO luminosities to molecular gas masses in our Galaxy are unlikely to apply in cluster central elliptical galaxies. Rather than being virialized structures analogous to GMCs, we propose that the molecular gas in NGC 1275 comprises pressure-confined structures created by turbulent flows.

ID: 145637
Type: article
Authors: O'Sullivan, Ewan; Ponman, Trevor J.; Kolokythas, Konstantinos; Raychaudhury, Somak; Babul, Arif; Vrtilek, Jan M.; David, Laurence P.; Giacintucci, Simona; Gitti, Myriam; Haines, Chris P.
Abstract: 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 four optically bright (log L_B ≥ 10.2 L_B⊙) 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 65 kpc and with luminosity >10⁴¹ erg s^-1, while a further three groups host smaller galaxy-scale gas haloes. The X-ray bright groups have masses in the range M₅₀₀ ≃ 0.5-5 × 10¹³ M_⊙, based on system temperatures of 0.4-1.4 keV, and X-ray luminosities in the range 2-200 × 10⁴¹ erg s^-1. We find that ∼53-65 per cent of the X-ray bright groups have cool cores, a somewhat lower fraction than found by previous archival surveys. Approximately 30 per cent of the X-ray bright groups show evidence of recent dynamical interactions (mergers or sloshing), and ∼35 per cent of their dominant early-type galaxies host active galactic nuclei 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} ⁴² erg s^-1) with no concentrated cool core, or highly disturbed. This leads us to suggest that ∼20 per cent of X-ray bright groups in the local universe may still be unidentified.

ID: 143858
Type: article
Authors: Schellenberger, G.; Vrtilek, J. M.; David, L.; O'Sullivan, E.; Giacintucci, S.; Johnston-Hollitt, M.; Duchesne, S. W.; Raychaudhury, S.
Abstract: Low-mass galaxy cluster systems and groups will play an essential role in upcoming cosmological studies, such as those to be carried out with eROSITA. Though the effects of active galactic nuclei (AGNs) and merging processes are of special importance to quantify biases like selection effects or deviations from hydrostatic equilibrium, they are poorly understood on the galaxy-group scale. We present an analysis of recent deep Chandra and XMM-Newton integrations of NGC 741 that provides an excellent example of a group with multiple concurrent phenomena: both an old central radio galaxy and a spectacular infalling head-tail source, strongly bent jets, a 100-kpc radio trail, intriguing narrow X-ray filaments, and gas-sloshing features. Supported principally by X-ray and radio continuum data, we address the merging history of the group, the nature of the X-ray filaments, the extent of gas-stripping from NGC 742, the character of cavities in the group, and the roles of the central AGN and infalling galaxy in heating the intra-group medium.

ID: 136402
Type: article
Authors: Peris, Charith S.; Vrtilek, Saeqa D.; Steiner, James F.; Vrtilek, Jan M.; Wu, Jianfeng; McClintock, Jeffrey E.; Longa-Peña, Penelope; Steeghs, Danny; Callanan, Paul; Ho, Luis C.; Orosz, Jerome A.; Reynolds, Mark T.
Abstract: We present a spectroscopic analysis of the black hole binary Nova Muscae 1991 in quiescence using data obtained in 2009 with Magellan Echelette on the Magellan Clay telescope and in 2010 with IMACS on the Magellan Baade telescope at the Las Campanas Observatory. Emission from the disc is observed in Ha, Hß and Ca II (?8662). A prominent hotspot is observed in the Doppler maps of all three emission lines. The existence of this spot establishes ongoing mass transfer from the donor star in 2009-2010 and, given its absence in the 1993-1995 observations, demonstrates the presence of a variable hotspot in the system. We find the radial distance to the hotspot from the black hole to be consistent with the circularization radius. Our tomograms are suggestive of stream-disc overflow in the system. We also detect possible Ca II (?8662) absorption from the donor star.

ID: 136418
Type: article
Authors: Zhang, B.; David, L. P.; Jones, C.; Andrade-Santos, F.; O'Sullivan, E.; Dahle, H.; Nulsen, P. E. J.; Clarke, T. E.; Pointecouteau, E.; Pratt, G. W.; Arnaud, M.; Vrtilek, J. M.; Ji, L.; van Weeren, R. J.; Kraft, R. P.; Kong, X.
Abstract: We present Chandra and XMM-Newton observations of PLCK G036.7+14.9 from the Chandra–Planck Legacy Program. The high resolution X-ray observations reveal two close (∼72″ = 193 kpc in projection) subclusters, G036N and G036S, which were not resolved by previous ROSAT, optical, or recent Planck observations. We perform detailed imaging and spectral analyses and use a simplified model to study the kinematics of this system. The basic picture is that PLCK G036.7+14.9 is undergoing a major merger (mass ratio close to unity) between the two massive subclusters, with the merger largely along the line of sight (∼80° between the merger axis and the plane of the sky from the simplified model) and probably at an early stage (less than ∼0.4–0.7 Gyr since the merger began). G036N hosts a small (∼27 kpc), moderate cool core (cooling time {{t}_cool}∼ 2.6–4.7 Gyr), while G036S has at most a very weak cool core ({{t}_cool}∼ 5.7–10.3 Gyr) in the central ∼40 kpc region. The difference in core cooling times is unlikely to be caused by the ongoing merger disrupting a pre-existing cool core in G036S. G036N also hosts an unresolved radio source in the center, which may be heating the gas if the radio source is extended. The total mass of the whole cluster determined from XMM-Newton is ∼ (5.9-8.0)× {{10}¹⁴} {{M}_ȯ }, and is ∼ (6.7-9.9)× {{10}¹⁴} {{M}_ȯ } from Chandra. The Planck derived mass, ∼ (5.1-6.0)× {{10}¹⁴} {{M}_ȯ }, is higher than the X-ray measured mass of either subcluster, but is lower than the X-ray measured mass of the whole cluster, due to the fact that Planck does not resolve PLCK G036.7+14.9 into subclusters and interprets it as a single cluster. This mass discrepancy could induce significant bias to the mass function if such previously unresolved systems are common in the Planck cluster sample. High resolution X-ray observations are necessary to identify the fraction of such systems and correct such a bias for the purpose of precision cosmological studies.

ID: 130140
Type: article
Authors: David, Laurence P.; Lim, Jeremy; Forman, William; Vrtilek, Jan; Combes, Francoise; Salome, Philippe; Edge, Alastair; Hamer, Stephen; Jones, Christine; Sun, Ming; O'Sullivan, Ewan J.; Gastaldello, Fabio; Bardelli, Sandro; Temi, Pasquale; Schmitt, Henrique; Ohyama, Youichi; Mathews, William; Brighenti, Fabrizio; Giacintucci, Simona; Trung, Dinh-V
Abstract: An ALMA observation of the early-type galaxy NGC 5044, which resides at the center of an X-ray bright group with a moderate cooling flow, detected 24 molecular structures within the central 2.5 kpc. The masses of the molecular structures vary from 3 × 10⁵ M _? to 10⁷ M _? and the CO(2-1) linewidths vary from 15 to 65 km s^-1. Given the large CO(2-1) linewidths, the observed structures are likely giant molecular associations (GMAs) and not individual giant molecular clouds (GMCs). Only a few of the GMAs are spatially resolved and the average density of these GMAs yields a GMC volume filling factor of about 15%. The masses of the resolved GMAs are insufficient for them to be gravitationally bound, however, the most massive GMA does contain a less massive component with a linewidth of 5.5 km s^-1 (typical of an individual virialized GMC). We also show that the GMAs cannot be pressure confined by the hot gas. Given the CO(2-1) linewidths of the GMAs (i.e., the velocity dispersion of the embedded GMCs) they should disperse on a timescale of about 12 Myr. No disk-like molecular structures are detected and all indications suggest that the molecular gas follows ballistic trajectories after condensing out of the thermally unstable hot gas. The 230 GHz luminosity of the central continuum source is 500 times greater than its low frequency radio luminosity and probably reflects a recent accretion event. The spectrum of the central continuum source also exhibits an absorption feature with a linewidth typical of an individual GMC and an infalling velocity of 250 km s^-1.

ID: 118823
Type: article
Authors: O'Sullivan, Ewan J.; David, Laurence P.; Vrtilek, Jan M.
Abstract: We present temperature and abundance maps of the central 125 kpc of the NGC 5044 galaxy group, based on a deep XMM-Newton observation. The abundance map reveals an asymmetrical abundance structure, with the centroid of the highest abundance gas offset ~22 kpc north-west of the galaxy centre, and moderate abundances extending almost twice as far to the south-east than in any other direction. The abundance distribution is closely correlated with two previously identified cold fronts and an arc-shaped region of surface brightness excess, and it appears that sloshing, induced by a previous tidal encounter, has produced both the abundance and surface brightness features. Sloshing dominates the uplift of heavy elements from the group core on large scales, and we estimate that the south-east extension (the tail of the sloshing spiral) contains at least 1.2 × 10⁵ M_? more iron than would be expected of gas at its radius. Placing limits on the age of the encounter we find that if, as previously suggested, the disturbed spiral galaxy NGC 5054 was the perturber, it must have been moving supersonically when it transited the group core. We also examine the spectral properties of emission from the old, detached radio lobe south-east of NGC 5044, and find that they are consistent with a purely thermal origin, ruling out this structure as a significant source of spectrally hard inverse-Compton emission.

ID: 131120
Type: article
Authors: O'Sullivan, Ewan J.; Vrtilek, J. M.; David, L. P.; Giacintucci, S.; Zezas, Andreas; Ponman, T. J.; Mamon, G. A.; Nulsen, P.; Raychaudhury, S.
Abstract: 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 (IGM) in the spiral-rich compact group HCG 16. We confirm the presence of a faint (L _{X, bolo} = 1.87^{+1.03}_{-0.66}\times10⁴¹ erg s^-1), low-temperature (0.30^{+0.07}_{-0.05} keV) 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 10⁹ M _⊙ of hot gas and is at least partly coincident with a large-scale {H}\,\scriptsize {I} 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 virialized, 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.

ID: 131121
Type: article
Authors: O'Sullivan, Ewan J.; Zezas, Andreas; Vrtilek, J. M.; Giacintucci, S.; Trevisan, M.; David, L. P.; Ponman, T. J.; Mamon, G. A.; Raychaudhury, S.
Abstract: 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 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 Kα 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 infrared and ultraviolet 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.

ID: 112709
Type: article
Authors: Giacintucci, Simona; O'Sullivan, Ewan J.; Clarke, Tracy E.; Murgia, Matteo; Vrtilek, Jan M.; Venturi, Tiziana; David, Laurence P.; Raychaudhury, Somak; Athreya, Ramana M.
Abstract: 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 ? = 0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with ? = 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.

ID: 99077
Type: article
Authors: David, Laurence P.; O'Sullivan, Ewan J.; Jones, Christine; Giacintucci, Simona; Vrtilek, Jan; Raychaudhury, Somak; Nulsen, Paul E. J.; Forman, William R.; Sun, Ming; Donahue, Megan

ID: 101189
Type: article
Authors: Giacintucci, Simona; O'Sullivan, Ewan J.; Vrtilek, Jan; David, Laurence P.; Raychaudhury, Somak; Venturi, Tiziana; Athreya, Ramana M.; Clarke, Tracy E.; Murgia, Matteo; Mazzotta, Pasquale; Gitti, Myriam; Ponman, Trevor; Ishwara-Chandra, C. H.; Jones, Christine; Forman, William R.
Abstract: 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 gsim1 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.

ID: 101651
Type: article
Authors: O'Sullivan, Ewan J.; Giacintucci, S.; David, L. P.; Gitti, M.; Vrtilek, J. M.; Raychaudhury, S.; Ponman, T. J.
Abstract: 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 nine galaxy groups combined with the Bîrzan 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 analyzed, 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 P _jetvprop L ^~0.7 _radio for integrated radio luminosity, with a total scatter of ? _Lrad = 0.63 and an intrinsic scatter of ? _{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 broadband 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 that 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 P _jet:L _radio relation.