arXiv subjects

E. O'Sullivan

Publications and source records attributed to E. O'Sullivan.

69 records · Page 4Linked to original sources

arXiv2009

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

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

astro-ph.CO↗

arXiv2009

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

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

astro-ph.CO↗

arXiv2009

AGN Feedback in groups and clusters of galaxies

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

astro-ph.CO↗

arXiv2009

A Joint GMRT/X-ray study of galaxy groups

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

astro-ph.CO↗

arXiv2009

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

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

astro-ph↗

arXiv2009

Merging Cold Fronts in the Galaxy Pair NGC 7619 and NGC 7626

We present results from {\it Chandra} observations of the galaxy pair NGC 7619 and NGC 7626, the two dominant members of the Pegasus group. The X-ray images show a brightness edge associated with each galaxy, which we identify as merger cold fronts. The edges are sharp, and the axes of symmetry of the edges are roughly anti-parallel, suggesting that these galaxies are falling towards one another in the plane of the sky. The detection of merger cold fronts implies a merging subgroup scenario, since the alternative is that the galaxies are falling into a pre-existing $\sim1$ keV halo without a dominant galaxy of its own, and such objects are not observed. We estimate the 3D velocities from the cold fronts and show that the velocity vectors are indeed most likely close to the plane of the sky, with a relative velocity of $\sim1190\kms$. The relative velocity is consistent with what is expected from the infall of two roughly equal mass subgroups whose total viral mass equals that of the Pegasus group. We conclude that the Pegasus cluster is currently forming from a major merger of two subgroups, dominated by NGC 7619 and NGC 7626. NGC 7626 contains a strong radio source, a core with two symmetric jets and radio lobes. Although we find no associated structure in the X-ray surface brightness map, the temperature map reveals a clump of cool gas just outside the southern lobe, presumably entrained by the lobe, and an extension of cooler gas into the lobe itself. The jet axis is parallel with the projected direction of motion of NGC 7626 (inferred from the symmetry axis of the merger cold front), and the southern leading jet is foreshortened as compared to the northern trailing one, possibly due to the additional ram pressure the forward jet encounters.

astro-ph↗

arXiv2008

The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe out to 7 effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1 Re. The velocity dispersion profile declines with radius, though not very steeply, down to ~70 km/s at the last data point. We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component LCDM-motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fit solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model. Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration halos, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.

astro-ph↗

arXiv2007

The dark haloes of early-type galaxies in low-density environments: XMM-Newton and Chandra observations of NGC 57, NGC 7796 and IC 1531

We present analysis of Chandra and XMM-Newton observations of three early-type galaxies, NGC 57, NGC 7796 and IC 1531. All three are found in very low density environments, and appear to have no neighbours of comparable size. NGC 57 has a halo of kT~0.9 keV, solar metallicity gas, while NGC 7796 and IC 1531 both have ~0.55 keV, 0.5-0.6 Zsol haloes. IC 1531 has a relatively compact halo, and we consider it likely that gas has been removed from the system by the effects of AGN heating. For NGC 57 and NGC 7796 we estimate mass, entropy and cooling time profiles and find that NGC 57 has a fairly massive dark halo with a mass-to-light ratio of 44.7 (4.0,-8.5) Msol/Lsol (1 sigma uncertainties) at 4.75 Re. This is very similar to the mass-to-light ratio found for NGC 4555 and confirms that isolated ellipticals can possess sizable dark matter haloes. We find a significantly lower mass-to-light ratio for NGC 7796, 10.6 (+2.5,-2.3) Msol/Lsol at 5 Re, and discuss the possibility that NGC 7796 hosts a galactic wind, causing us to underestimate its mass.

astro-ph↗

arXiv2006

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

We present XMM, Chandra and VLA observations of the USGC S152 group and its central elliptical NGC 3411. Imaging of the group X-ray halo suggests 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 ~9x10^9 Msun of gas. VLA observations at 1.4, 5 and 8 GHz reveal a relatively weak nuclear radio source, with a core radio luminosity L_R=2.7x10^38 erg/s, 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.

astro-ph↗

arXiv2006

X-ray observations of three young, early-type galaxies

Massive halos of hot plasma exist around some, but not all elliptical galaxies. There is evidence that this is related to the age of the galaxy. In this paper new X-ray observations are presented for three early-type galaxies that show evidence of youth, in order to investigate their X-ray components and properties. NGC 5363 and NGC 2865 were found to have X-ray emission dominated by purely discrete stellar sources. Limits are set on the mass distribution in one of the galaxies observed with XMM-Newton, NGC 4382, which contains significant hot gas. We detect the X-ray emission in NGC 4382 out to 4r$_e$. The mass-to-light ratio is consistent with a stellar origin in the inner regions but rises steadily to values indicative of some dark matter by 4r$_e$. These results are set in context with other data drawn from the literature, for galaxies with ages estimated from dynamical or spectroscopic indicators. Ages obtained from optical spectroscopy represent central luminosity weighted stellar ages. We examine the X-ray evolution with age, normalised by B and K band luminosities. Low values of Log(L$_X$/L$_B$) and Log(L$_X$/L$_K$) are found for all galaxies with ages between 1 and 4 Gyrs. Luminous X-ray emission only appears in older galaxies. This suggests that the interstellar medium is removed and then it takes several gigayears for hot gas halos to build up, following a merger. A possible mechanism for gas expulsion might be associated with feedback from an active nucleus triggered during a merger.

astro-ph↗

arXiv2005

AGN feedback and gas mixing in the core of NGC 4636

Chandra observations of NGC 4636 show disturbances in the galaxy X-ray halo, including arm-like high surface brightness features (tentatively identified as AGN driven shocks) and a possible cavity on the west side of the galaxy core. We present Chandra and XMM spectral maps of NGC 4636 which confirm the presence of the cavity and show it to be bounded by the arm features. The maps also reveal a ~15 kpc wide plume of low temperature, high abundance gas extending 25-30 kpc to the southwest of the galaxy. The cavity appears to be embedded in this plume, and we interpret the structure as being entrained gas drawn out of the galaxy core during previous episodes of AGN activity. The end of the plume is marked by a well defined edge, with significant falls in surface brightness, temperature and abundance, indicating a boundary between galaxy and group/cluster gas. This may be evidence that as well as preventing gas cooling through direct heating, AGN outbursts can produce significant gas mixing, disturbing the temperature structure of the halo and transporting metals out from the galaxy into the surrounding intra-group medium.

astro-ph↗

arXiv2005

AWM 4 - an isothermal cluster observed with XMM-Newton

We present analysis of an XMM observation of the poor cluster AWM 4. The cluster is relaxed and its X-ray halo is regular with no apparent substructure. Azimuthally averaged radial spectral profiles suggest that the cluster is isothermal to a radius of at least 160 kpc, with no evidence of a central cooling region. Spectral mapping shows some significant temperature and abundance substructure, but no evidence of strong cooling in the cluster core. Abundance increases in the core, but not to the extent expected, and we find some indication of gas mixing. Modeling the three dimensional properties of the system, we show that ongoing heating by an AGN in the dominant elliptical, NGC 6051, is likely to be responsible for the lack of cooling. We also compare AWM 4 to MKW 4, a cluster of similar mass observed recently with XMM. While the two systems have similar gravitational mass profiles, MKW 4 has a cool core and somewhat steeper gas density profile, which leads to a lower core entropy. AWM 4 has a considerably larger gas fraction at 0.1 R200, and we show that these differences result from the difference in mass between the two dominant galaxies and the activity cycles of their AGN. We estimate the energy required to raise the temperature profile of MKW 4 to match that of AWM 4 to be 9x10^58 erg, or 3x10^43 erg/s for 100 Myr, comparable to the likely power output of the AGN in AWM 4.

astro-ph↗

arXiv2004

The isolated elliptical NGC 4555 observed with Chandra

We present analysis of a Chandra observation of the elliptical galaxy NGC 4555. The galaxy lies in a very low density environment, either isolated from all galaxies of similar mass or on the outskirts of a group. Despite this, NGC 4555 has a large gaseous halo, extending to ~60 kpc. We find the mean gas temperature to be ~0.95 keV and the Iron abundance to be ~0.5 solar. We model the surface brightness, temperature and abundance distribution of the halo and use these results to estimate parameters such as the entropy and cooling time of the gas, and the total gravitational mass of the galaxy. In contrast to recent results showing that moderate luminosity ellipticals contain relatively small quantities of dark matter, our results show that NGC 4555 has a massive dark halo and large mass-to-light ratio (56.8 [+34.2,-35.8] solar at 50 kpc, 42.7 [+14.6,-21.2] solar at 5 effective radii, 1 sigma errors). We discuss this disparity and consider possible mechanisms by which galaxies might reduce their dark matter content.

astro-ph↗

arXiv2003

XMM-Newton and Chandra observations of three X-ray faint early-type galaxies

We present XMM-Newton observations of three X-ray under-luminous elliptical galaxies, NGC 3585, NGC 4494 and NGC 5322. All three galaxies have relatively large optical luminosities (log LB=10.35-10.67 solar) but have X-ray luminosities consistent with emission from discrete sources only. In conjunction with a Chandra observation of NGC 3585, we analyse the XMM data and show that the three galaxies are dominated by discrete source emission, but do possess some X-ray emitting gas. The gas is at relatively low temperatures, kT=0.25-0.44 keV. All three galaxies show evidence of recent dynamical disturbance and formation through mergers, including kinematically distinct cores, young stellar ages, and embedded stellar disks. This leads us to conclude that the galaxies formed relatively recently and have yet to build up large X-ray halos. They are likely to be in a developmental phase where the X-ray gas has a very low density, making it undetectable outside the galaxy core. However, if the gas is a product of stellar mass loss, as seems most probable, we would expect to observe supersolar metal abundances. While abundance is not well constrained by the data, we find best fit abundances <0.1 solar for single-temperature models, and it seems unlikely that we could underestimate the metallicity by such a large factor.

astro-ph↗

arXiv2003

An XMM-Newton observation of the galaxy group MKW 4

We present an X-ray study of the galaxy group or poor cluster MKW 4. Working with XMM data we examine the distribution and properties of the hot gas which makes up the group halo. The inner halo shows some signs of structure, with circular or elliptical beta models providing a poor fit to the surface brightness profile. This may be evidence of large scale motion in the inner halo, but we do not find evidence of sharp fronts or edges in the emission. The temperature of the halo declines in the core, with deprojected spectral fits showing a central temperature of ~1.3 keV compared to ~3 keV at 100 kpc. In the central ~30 kpc of the group multi-temperature spectral models are required to fit the data, but they indicate a lack of gas at low temperatures. Steady state cooling flow models provide poor fits to the inner regions of the group and the estimated cooling time of the gas is long except within the central dominant galaxy, NGC 4073. Abundance profiles show a sharp increase in the core of the group, with mean abundance rising by a factor of two in the centre of NGC 4073. Fitting individual elements shows the same trend, with high values of Fe, Si and S in the core. We estimate that ~50% of the Fe in the central 40 kpc was injected by SNIa, in agreement with previous ASCA studies. Using our best fitting surface brightness and temperature models, we calculate the mass, gas fraction, entropy and mass-to-light ratio of the group. At 100 kpc (~0.1 virial radii) the total mass and gas entropy of the system (~2x10^13 Msol and ~300 keV cm^2) are quite comparable to those of other systems of similar temperature, but the gas fraction is rather low (~1%). We conclude that MKW 4 is a fairly relaxed group, which has developed a strong central temperature gradient but not a large-scale cooling flow.

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