Tracy Ellen Clarke

THE COMPLEX COOLING CORE OF A2029: RADIO AND X-RAY INTERACTIONS

We present an analysis of Chandra observations of the central regions of the cooling flow cluster A2029. We find a number of X-ray filaments in the central 40 kpc, some of which appear to be associated with the currently active central radio galaxy. The outer southern lobe of the steep-spectrum radio source appears to be surrounded by a region of cool gas and is at least partially surrounded by a bright X-ray rim similar to that seen around radio sources in the cores of other cooling flow clusters. Spectroscopic fits show that the overall cluster emission is best fitted by either a two-temperature gas (kThigh = 7.47 keV and kTlow = 0.11 keV) or a cooling flow model with gas cooling over the same temperature range. This large range of temperatures (over a factor of 50) is relatively unique to A2029 and may suggest that this system is a very young cooling flow in which the gas has only recently started cooling to low temperatures. The cooling flow model gives a mass deposition rate of = 56 M☉ yr-1. In general, the cluster emission is elongated along a position angle of 22° with an ellipticity of 0.26. The distribution of the X-ray emission in the central region of the cluster is asymmetric, however, with excess emission to the northeast and southeast compared with that to the southwest and northwest, respectively. Fitting and subtracting a smooth elliptical model from the X-ray data reveals a dipolar spiral excess extending in a clockwise direction from the cluster core to radii of ~150 kpc. We estimate a total mass of Mspr ~ 6 × 1012 M☉ in the spiral excess. The most likely origin of the excess is either stripping of gas from a galaxy group or from bare dark matter potential that has fallen into the cluster or sloshing motions in the cluster core induced by a past merger.

Chandra Observation of the Central Region of the Cooling Flow Cluster A262: A Radio Source That Is a Shadow of Its Former Self?

We present a Chandra observation of the cooling flow cluster A262. Spectral fits show that the intracluster medium (ICM) in A262 cools by a factor of 3, from 2.7 to 0.9 keV, at the cluster center. A mass deposition rate of = 19 M☉ yr-1 is measured. Complex structure is found in the very inner regions of the cluster, including knots of emission and a clear deficit of emission to the east of the cluster center. The bright X-ray structures are located in the same regions as optical line emission, indicating that cooling to low temperatures has occurred in these regions. The X-ray deficit is spatially coincident with the eastern radio lobe associated with the active galactic nucleus hosted by the central cD galaxy. The region surrounding the X-ray hole is cool and shows no evidence that it has been strongly shocked. This joins the ranks of other cooling flow clusters with Chandra-detected bubbles blown by central radio sources. This source is different from the other well-known cases, in that the radio source is orders of magnitude less luminous and has produced a much smaller bubble. Comparing the energy output of the radio source with the luminosity of the cooling gas shows that energy transferred to the ICM from the radio source is insufficient to offset the cooling flow unless the radio source is currently experiencing a less powerful than average outburst and was more powerful in the past.

Low-Frequency Radio Observations of X-Ray Ghost Bubbles in A2597: A History of Radio Activity in the Core

A previous analysis of the Chandra X-ray image of the center of the cooling core cluster A2597 showed two ghost holes in the X-ray emission to the west and northeast of the central radio galaxy PKS 2322-123. Previous radio observations did not detect any radio emission coming from the interior of the X-ray holes. We present new low-frequency radio observations of A2597. At 330 MHz, radio emission extends into the interior of the western ghost bubble, but not the northeast one. Our reanalysis of the archival Chandra data shows evidence for an X-ray tunnel (elongated region of reduced X-ray emission) extending from near the center of the cD galaxy out to the west ghost bubble. We also detect a smaller X-ray hole to the northeast of the center of the cD and closer than the outer ghost bubbles. Radio observations at 1.3 GHz show extensions to the west along the X-ray tunnel toward the west ghost bubble, to the northeast into the new X-ray hole, and to the northwest. All of these structures are much larger than the two inner radio lobes seen previously at 8 GHz. The X-ray tunnel suggests that the west ghost bubble is part of a continuous flow of radio plasma out from the active galactic nucleus, rather than a detached buoyant old radio lobe, and thus it may be an intermediate case between an active radio galaxy and a buoyant lobe.

FARADAY ROTATION OBSERVATIONS OF MAGNETIC FIELDS IN GALAXY CLUSTERS

The presence of magnetic fields in the intracluster medium in clusters of galaxies has been revealed through several different observational techniques. These fields may be dynamically important in clusters as they will provide additional pressure support to the intracluster medium as well as inhibit transport mechanisms such as thermal conduction. Here, we review the current observational state of Faraday rotation measure studies of the cluster fields. The fields are generally found to be a few to 10 G in non-cooling core clusters and ordered on scales of 10 - 20 kpc. Studies of sources at large impact parameters show that the magnetic fields extend from cluster cores to radii of at least 500 kpc. In central regions of cooling core systems the field strengths are often somewhat higher (10 - 40 G) and appear to be ordered on smaller scales of a few to 10 kpc. We also review some of the recent work on interpreting Faraday rotation measure observations through theory and numerical simulations. These techniques allow us to build up a much more detailed view of the strength and topology of the fields.

HYDRA A AT LOW RADIO FREQUENCIES

We present new, low-frequency images of the powerful FR I radio galaxy Hydra A (3C 218). Images were made with the Very Large Array at frequencies of 1415, 330, and 74 MHz, with resolutions on the order of 20. The morphology of the source is seen to be more complex and even larger than previously known and extends nearly 8 (530 kpc) in a north-south direction. The southern lobe is bent to the east and extends in that direction for nearly 3 (200 kpc). In addition, we find that the northern lobe has a flatter spectral slope than the southern lobe, consistent with the appearance of greater confinement to the south. We measure overall spectral indices α = -0.83 and α = -0.89.

Are the Faraday rotating magnetic fields local to intracluster radio galaxies

We investigate the origin of the high Faraday rotation measures (RMs) found for polarized radio galaxies in clusters. The two most likely origins are magnetic fields local to the source or magnetic fields located in the foreground intracluster medium. The latter is identified as the null hypothesis. Rudnick & Blundell have recently suggested that the presence of magnetic fields local to the source may be revealed in correlations of the position angles (PAs) of the source-intrinsic linear polarization and the RMs. We investigate the claim of Rudnick & Blundell that they have found a relationship between the intrinsic PA0 of the radio source PKS 1246-410 and its RM by testing the clustering strength of the PA0-RM scatter plot. We show that the claimed relationship is an artifact of an improperly performed null experiment. We describe a gradient alignment statistic aimed at finding correlations between PA0 and RM maps. This statistic does not require any null experiment since it gives a unique (zero) result in the case of uncorrelated maps. We apply it to a number of extended radio sources in galaxy clusters (PKS 1246-410, Cygnus A, Hydra A, and 3C 465). In no case is a significant large-scale alignment of PA0 and RM maps detected. We find significant small-scale co-alignment in all cases, but we are able to fully identify this with map-making artifacts through a suitable statistical test. We conclude that there is presently no existing evidence for Faraday rotation local to radio lobes. Given the existing independent pieces of evidence, we favor the null hypothesis that the observed Faraday screens are produced by intracluster magnetic fields.

Active galactic nucleus feedback in clusters of galaxies

Observations made during the last ten years with the Chandra X-ray Observatory have shed much light on the cooling gas in the centers of clusters of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of the hot intracluster medium in cluster centers can feed the supermassive black holes found in the nuclei of the dominant cluster galaxies leading to AGN outbursts which can reheat the gas, suppressing cooling and large amounts of star formation. AGN heating can come in the form of shocks, buoyantly rising bubbles that have been inflated by radio lobes, and the dissipation of sound waves.

Monitoring the Sky with the Prototype All-Sky Imager on the LWA1

We present a description of the Prototype All-Sky Imager (PASI), a backend correlator and imager of the first station of the Long Wavelength Array (LWA1). PASI cross-correlates a live stream of 260 dual-polarization dipole antennas of the LWA1, creates all-sky images, and uploads them to the LWA-TV website in near real time. PASI has recorded over 13,000hr of all-sky images at frequencies between 10 and 88MHz creating opportunities for new research and discoveries. We also report rate density and pulse energy density limits on transients at 38, 52, and 74MHz, for pulse widths of 5s. We limit transients at those frequencies with pulse energy densities of >2.7×10−23, >1.1×10−23, and >2.8×10−23Jm−2Hz−1 to have rate densities <1.2×10−4, <5.6×10−4, and <7.2×10−4 year−1deg−2.

The Distance to the Draco Intermediate-Velocity Cloud

The understanding of the nature of intermediate- and high-velocity gas in the Milky Way is hampered by a paucity of distance estimates to individual clouds. A project has been started at the David Dunlap Observatory to address this lack of distance measures by observing early-type stars along the line of sight toward these clouds and searching for sodium doublet absorption at the clouds systemic velocities. Distances to foreground stars (no absorption) and background stars (with absorption) are estimated from spectroscopic parallax, and thus the distance to the bracketed cloud is estimated. In this Letter, we present the first result from this ongoing project: a measurement of the distance to the Draco Cloud, which is the most studied of the intermediate-velocity clouds. The result presented here is the first distance bracket that tightly constrains the position of the Draco Cloud. We briefly describe our target selection and observing methodology and then demonstrate absorption at the velocity of the Draco Cloud for one star (TYC 4194 2188) and a lack of absorption for several other stars. We derive a distance bracket to the Draco Cloud of 463+ 192−136 to 618+ 243−174 pc.

Soft X-Ray Absorption Due to a Foreground Edge-on Spiral Galaxy toward the Core of A2029

We have detected an X-ray absorption feature against the core of the galaxy cluster Abell 2029 (z=0.0767) which we identify with the foreground galaxy UZC J151054.6+054313 (z=0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87 pm 3 degrees. HI observations give a rotation velocity of 108 kms and an atomic hydrogen mass of M_{HI} = 3.1 X 10^9 d_{90}^2 msun, where d_{90} is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 pm 0.4) X 10^{21} cm^{-2} assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is M_H = (6.2 pm 1.2) X 10^8 d_{90}^2 msun. Since the absorbing gas in the galaxy is probably concentrated to the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.We have detected an X-ray absorption feature against the core of the galaxy cluster A2029 (z = 0.0767) that we identify with the foreground galaxy UZC J151054.6+054313 (z = 0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87° ± 3°. H I observations give a rotation velocity of 108 km s-1 and an atomic hydrogen mass of M = 3.1 × 109d M☉, where d90 is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 ± 0.4) × 1021 cm-2, assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is MH = (6.2 ± 1.2) × 108d M☉. Since the absorbing gas in the galaxy is probably concentrated toward the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.