arXiv subjects

Frazer N. Owen

Publications and source records attributed to Frazer N. Owen.

At least 37 records · Page 2Linked to original sources

arXiv2009

The Deep SWIRE Field II. 90cm Continuum Observations and 20cm-90cm Spectra

We present one of the deepest radio continuum surveys to date at a wavelength ~1 meter, at 324.5 MHz. The data reduction and analysis are described and an electronic catalog of the sources detected above 5 sigma is presented. We also discuss the observed angular size distribution for the sample. Using our deeper 20cm survey of the same field, we calculate spectral indices for sources detected in both surveys. The spectral indices for 90cm-selected sources, defined as S ~nu^(-alpha}, shows a peak near 0.7 and only a few sources with very steep spectra. Thus no large population of very steep spectrum microJy sources seems to exist down to the limit of our survey. For 20cm-selected sources, we find similar mean spectral indices for sources with S_20>1 mJy. For weaker sources, below the detection limit for individual sources at 90cm, we use stacking to study the radio spectra. We find that the spectral indices of small (<3") 20cm-selected sources with S_20< 10 mJy have mean and median alpha(90,20)~0.3-0.5. This is flatter than the spectral indices of the stronger source population. We report log N-log S counts at 90cm which show a flattening below 5 mJy. Given the median redshift of the population, z~1, the spectral flattening and the flattening of the log N-log S counts occurs at radio luminosities normally associated with AGN rather than with galaxies dominated by star-formation.

astro-ph.CO↗

arXiv2008

The Deep SWIRE Field I. 20cm Continuum Radio Observations: A Crowded Sky

We present results from deep radio observations taken with the VLA at a center frequency of 1400 MHz cover a region of the SWIRE Spitzer Legacy survey, centered at 10 46 00, 59 01 00 (J2000). The reduction and cataloging of the radio sources are described. The survey presented is the deepest so far in terms of the radio source density on the sky. Perhaps surprisingly, the sources down to the bottom of the catalog have median angular sizes greater than 1 arcsecond, like their cousins 10-100 times stronger. If the log N - log S normalization remains constant at the lowest flux densities, there are about 6 sources per square arcminute down to 15 microJy at 20cm. Given the finite source sizes this implies we may reach the natural confusion limit near 1 microJy.

astro-ph↗

arXiv2006

The Spiral Host Galaxy of the Double Radio Source 0313-192

We present new Hubble, Gemini-S, and Chandra observations of the radio galaxy 0313-192, which hosts a 350-kpc double source and jets, even though previous data have suggested that it is a spiral galaxy. We measure the bulge scale and luminosity, radial and vertical profiles of disk starlight, and consider the distributions of H II regions and absorbing dust. In each case, the HST data confirm its classification as an edge-on spiral galaxy, the only such system known to produce such an extended radio source of this kind. The Gemini near-IR images and Chandra spectral fit reveal a strongly obscured central AGN, seen through the entire ISM path length of the disk and showing X-ray evidence of additional absorption from warm or dense material close to the central object. We consider several possible mechanisms for producing such a rare combination of AGN and host properties, some combination of which may be at work. These include an unusually luminous bulge (suggesting a black hole of mass 0.5-0.9 billion solar masses), orientation of the jets near the pole of the gas-rich disk, and some evidence of a weak gravitational interaction which has warped the disk and could have enhanced fuelling of the central engine. An X-ray counterpart of the kiloparsec-scale radio jet emerges to the south; jet/counterjet limits in both radio and X-ray allow them to be symmetric if seen more than 15 degrees from the plane of the sky, still consistent with the jet axes being within ~30 degrees of the poles of the gas-rich galaxy disk. A linear or disklike emission-line structure is seen around thenucleus, inclined by ~20 degrees to the stellar disk but nearly perpendicular to the jets; this may represent the aftermath of a galaxy encounter, where gas is photoionized by the nuclear continuum.

astro-ph↗

arXiv2005

Quasars in the MAMBO blank field survey

Our MAMBO 1.2 mm blank field imaging survey of ~0.75 sqd has uncovered four unusually bright sources, with flux densities between 10 and 90 mJy, all located in the Abell 2125 field. The three brightest are flat spectrum radio sources with bright optical and X-ray counterparts. Their mm and radio flux densities are variable on timescales of months. Their X-ray luminosities classify them as quasars. The faintest of the four mm bright sources appears to be a bright, radio-quiet starburst at z~3, similar to the sources seen at lower flux densities in the MAMBO and SCUBA surveys. It may also host a mildly obscured AGN of quasar-like X-ray luminosity. The three non-thermal mm sources imply an areal density of flat spectrum radio sources higher by at least 7 compared with that expected from an extrapolation of the lower frequency radio number counts.

astro-ph↗

arXiv2004

The Dynamics of Abell 2125

We present 371 galaxy velocities in the field of the very rich cluster Abell 2125 (z~0.25). These were determined using optical spectroscopy collected over several years from both the WIYN 3.5m telescope and NOAO Mayall 4m telescope. Prior studies at a variety of wavelengths (radio, optical, and X-ray) have indicated that A2125 is a likely cluster-cluster merger, a scenario which we are able to test using our large velocity database. We identified 224 cluster galaxies, which were subjected to a broad range of statistical tests using both positional and velocity information to evaluate the cluster dynamics and substructure. The tests confirmed the presence of substructures within the Abell 2125 system at high significance, demonstrating that A2125 is a complex dynamical system. Comparison of the test results with existing simulations strengthens the merger hypothesis, and provides clues about the merger geometry and stage. The merger model for the system can reconcile A2125's low X-ray temperature and luminosity with its apparently high richness, and might also explain A2125's high fraction of active galaxies identified in prior radio and optical studies.

astro-ph↗

arXiv2003

The X-ray Properties of Nearby Abell Clusters from the ROSAT All-Sky Survey: The Sample and Correlations with Optical Properties

We present an analysis of the X-ray emission for a complete sample of 288 Abell clusters spanning the redshift range 0.016<= z <= 0.09 from the ROSAT All-Sky Survey. This sample is based on our 20cm VLA survey of nearby Abell clusters. We find an X-ray detection rate of 83%. We report cluster X-ray fluxes and luminosities and two different flux ratios indicative of the concentration and extent of the emission. We examine correlations between the X-ray luminosity, Abell Richness, and Bautz-Morgan and Rood-Sastry cluster morphologies. We find a strong correlation between Lx and cluster richness coupled to a dependence on the optical morphological type. These results are consistent with the observed scatter between X-ray luminosity and temperature and a large fraction of cooling flows. For each cluster field we also report the positions, peak X-ray fluxes, and flux-ratios of all X-ray peaks above 3-sigma significance within a box of 2x2 Mpc centered on Abell's position.

astro-ph↗

arXiv2002

Radio-selected Galaxies in Very Rich Clusters at z < 0.25: II. Radio Properties and Analysis

(Abridged) We report on the properties of radio-selected galaxies within 30 very-rich Abell clusters with z < 0.25. These radio data sample the ultra-faint (L(1.4) > 2E22 W/Hz) radio galaxy population with M_R < -21 using the well-known FIR/radio correlation to link the radio with ongoing star formation within individual cluster galaxies. These radio-selected galaxies reveal the `active' galaxy population (starburst and active galactic nuclei) within these rich cluster environments that can be identified regardless of their level of dust obscuration. For all clusters in our sample, we are sensitive to star formation rates (M > 5 M_sun) > 5 M_sun/yr. We have found that the excess number of low-luminosity `starburst' radio-selected galaxies (SBRG) found by Owen et. al. 1999 in Abell 2125 is not indicative of other rich clusters in our sample. The average fraction of SBRG is = 0.022+/-0.003. The A2125 fraction is f(SBRG) = 0.09+/-10.03 which is significantly different from the sample average at a >99.99% confidence level. The bimodal structure of both the x-ray brightness distribution and optical adaptively smoothed images of A1278 and A2125 suggests that ongoing cluster-cluster mergers may be enhancing this SBRG population. There is a significant spatial distribution difference between the low and high-luminosity (HLRG) radio-selected populations. The HLRGs seem to be a subclass of the cluster's massive red elliptical population, while the SBRGs have a projected radial distribution more like the blue spiral population. Our results indicate that most of the SBRGs are probably gas-rich disk galaxies undergoing > 5 M_sun/yr of star-formation.

astro-ph↗

arXiv2002

Hubble Space Telescope Imaging of Brightest Cluster Galaxies

We used the HST WFPC2 to obtain I-band images of the centers of 81 brightest cluster galaxies (BCGs), drawn from a volume-limited sample of nearby BCGs. The images show a rich variety of morphological features, including multiple or double nuclei, dust, stellar disks, point source nuclei, and central surface brightness depressions. High resolution surface brightness profiles could be inferred for 60 galaxies. Of those, 88% have well-resolved cores. Twelve percent of the BCG sample lacks a well-resolved core; all but one of these BCGs have ``power-law'' profiles. Some of these galaxies have higher luminosities than any power-law galaxy identified by Faber et al. (1997), and have physical upper limits on the break radius well below the values observed for core galaxies of the same luminosity. These results support the idea that the central structure of early-type galaxies is bimodal in its physical properties, but also suggest that there exist high luminosity galaxies with power-law profiles (or unusually small cores). The BCGs in the latter category tend to fall at the low end of the BCG luminosity function and tend to have low values of the quantity alpha (the logarithmic slope of the metric luminosity as a function of radius, at 10 kpc). Since theoretical calculations have shown that the luminosities and alpha values of BCGs grow with time as a result of accretion, this suggests a scenario in which elliptical galaxies evolve from power-law profiles to core profiles through accretion and merging. This is consistent with theoretical scenarios that invoke the formation of massive black hole binaries during merger events (Abridged).

astro-ph↗

arXiv2002

Evolution of Star Forming and Active Galaxies in Nearby Clusters

(Abridged) We have used optical spectroscopy to investigate the radio- selected active galaxy populations in a sample of 20 nearby Abell clusters. The spectra were used to characterize the galaxies as AGN or galaxies forming stars at rates comparable to or greater than the Milky Way. A significant population of star forming galaxies with large amounts of nuclear dust extinction were revealed. This extinction eliminates bluer emission lines such as [OII], meaning their star formation could easily be overlooked in studies which focus on such features. Around 20% of cluster star forming galaxies have such spectra, and their radial distribution suggests they are the consequence of some cluster environmental effect. We note that galaxies of this type may be identified using their 4000-Angstrom break strengths. We also find that discrepancies in reported radio luminosity functions for AGN are likely the result of classification differences. There exists a large population of cluster galaxies whose radio fluxes, FIR fluxes, and optical magnitudes suggest their radio emission may be powered by stars, yet their spectra lack emission lines. Understanding the nature of these galaxies is critical to assessing the importance of AGN in the radio luminosity function at low luminosities.

astro-ph↗

arXiv2002

Redshifts for a Sample of Radio-Selected Poor Clusters

Multifiber optical spectroscopy has been performed on galaxies in the vicinity of strong, nearby radio galaxies. These radio galaxies were selected from the 3CR and B2 catalogs based on their exclusion from the Abell catalog, which is puzzling given the hypothesis that an external medium is required to confine the radio plasma of such galaxies. Velocities derived from the spectra were used to confirm the existence of groups and poor clusters in the fields of most of the radio galaxies. We find that all radio galaxies with classical FR I morphologies prove to reside in clusters, whereas the other radio galaxies often appear to be recent galaxy-galaxy mergers in regions of low galaxy density. These findings confirm the earlier result that the existence of extended X-ray emission combined with a statistical excess of neighboring galaxies can be used to identify poor clusters associated with radio galaxies.

astro-ph↗

arXiv2001

Magnetic fields in cluster cores: Faraday rotation in A400 and A2634

We present Faraday rotation data for radio sources in the centers of the Abell clusters A400 and A2634. These clusters contain large (> 100 kpc), tailed radio sources, each attached to the central cD galaxy. These clusters do not have strong cooling cores. Our data extend previous work on rotation measure in cluster centers to larger scales and non-cooling clusters. The rotation measure, and thus the magnetic field, is ordered on scales 10-20 kpc in both clusters. The geometry of the rotation measure appears to be determined by the distribution of the X-ray emitting gas, rather than by the radio tails themselves. We combine our data with previously published X-ray and radio data in order to analyze the magnetic fields in all 12 clusters whose central radio sources have been imaged in rotation measure. We find that the fields are dynamically significant in most clusters. We argue that the Faraday data measure fields in the intracluster medium, rather than in a skin of the radio source. Finally, we consider the nature and maintenance of the magnetic fields in these clusters, and conclude that either the cluster-wide field exists at similar levels, or that a weaker cluster-wide field is amplified by effects in the core.

astro-ph↗

arXiv2001

Current Star Formation in Post-Starburst Galaxies?

Radio continuum observations are a probe of star formation in galaxies, and are unaffected by dust extinction. Observations of the distant rich cluster Cl 0939+4713 have detected radio galaxies classified as post-starburst (``k+a'') on the basis of their optical spectra, and presumably this situation arises from heavily dust-obscured star formation (Smail et al. 1999). We present the results of a radio continuum survey of post-starburst galaxies identified from the Las Campanas Redshift Survey by Zabludoff et al. (1996). This sample was selected using very stringent criteria, and therefore provides an estimate on the incidence of potential star formation in galaxies whose optical spectra exhibit the strongest post-starburst features. We detected two of fifteen such galaxies at radio luminosities consistent with moderate levels of star formation. This result underscores the potential importance of dust extinction when investigating star formation in galaxies.

astro-ph↗

arXiv2001

The Far-Infrared/Radio Correlation in Nearby Abell Clusters

A comprehensive study of the effect of the cluster environment on the far- infrared (FIR)/radio correlation in nearby Abell clusters is presented. Using the cluster radio galaxy database from Miller & Owen (2001) and optical spectroscopy and high resolution radio images to remove AGN, we assess the FIR/radio correlation of cluster galaxies from the centers of the clusters out well past the classical Abell radius. The FIR/radio correlation is shown to hold quite well for star forming galaxies, and the FIR and radio fluxes for cluster AGN are also well correlated. In the case of AGN, the relative radio- to-FIR fluxes are greater and the scatter in the correlation is larger than those seen for star forming galaxies. We also find that there is a rare but statistically significant excess of star forming galaxies with enhanced radio emission in the centers of the clusters, and that the degree of this enhancement is typically a factor of two or three. The FIR/radio correlation for cluster star forming galaxies is also tested against line-of-sight velocity relative to the cluster systemic velocities, but no significant correlation is found. While the radial dependence of the FIR/radio correlation is consistent with the model wherein ram pressure increases the cluster galaxies' magnetic field strengths through compression, the velocity data do not confirm this model. Although a contribution from ram pressure can not be ruled out, the thermal pressure due to the ICM alone is an equally viable alternative. The high resolution radio images largely reject the hypothesis that the increased radio emission arises from an AGN component, strengthening the claim that the change in the correlation is caused by a change in the environment of the galaxies.

astro-ph↗

arXiv2001

The Radio Galaxy Populations of Nearby Northern Abell Clusters

We report on the use of the NRAO VLA Sky Survey (NVSS) to identify radio galaxie s in eighteen nearby Abell clusters. The listings extend from the cores of the clusters out to radii of 3 Mpc, which corresponds to 1.5 Abell radii and approximately four orders of magnitude in galaxy density. To create a truly useful catalog, we have collected optical spectra for nearly all of the galaxies lacking public velocity measurements. Consequently, we are able to discriminate between those radio galaxies seen in projection on the cluster and those which are in actuality cluster members. The resulting catalog consists of 329 cluster radio galaxies plus 138 galaxies deemed foreground/background objects, and new velocity measurements are reported for 273 of these radio galaxies. The motivation for the catalog is the study of galaxy evolution in the cluster environment. The radio luminosity function (RLF) is a powerful tool in the identification of active galaxies, as it is dominated by star-forming galaxies at intermediate luminosities and active galactic nuclei (AGN) at higher luminosities. The flux limit of the NVSS allows us to identify AGN and star- forming galaxies down to star formation rates (SFR) less than 1 solar mass per year. This sensitivity, coupled with the all-sky nature of the NVSS, allows us to produce a catalog of considerable depth and breadth. In addition to these data, we report detected infrared fluxes and upper limits obtained from IRAS data. It is hoped that this database will prove useful in a number of potential studies of the effect of environment on galaxy evolution.

astro-ph↗

arXiv2000

A Large-Scale Jet and FR I Radio Source in a Spiral Galaxy: The Host Properties and External Environment

We have identified a large (~200 kpc), powerful double radio source whose host galaxy is clearly a disk and most likely a spiral. This FR I radio galaxy is located very near the center of the RC 0 cluster Abell 428. The existence of such an object violates a fundamental paradigm for radio loud AGN. In paper I, we showed that this object was most likely a spiral host with optical line ratios and colors consistent with an AGN. In this paper, we present new, higher resolution radio imaging, a radio/mm continuum spectrum for the nucleus, a detection of HI absorption against the bright radio core, an upper-limit to CO and the gas mass, and 70 optical redshifts. We confirm the existence of a radio jet at 20cm extending 42 kpc into the southern lobe. At 3.6cm, we also detect a nuclear jet similar in length to that in M87 but 10 times weaker. We believe that this is the first detection of a radio jet on these scales in a disk or spiral host galaxy. The nuclear radio spectrum is similar to many blazar or QSO like objects, suggesting that the galaxy harbors an imbedded and obscured AGN. We model a turnover in the spectrum at low frequencies as Free-Free absorption. We detect very strong and narrow HI absorption with nearly the entire 20 cm core continuum absorbed, implying an unusually large optical depth (tau~1). We suggest that the nucleus is seen through a disk-like distribution of ISM gas, possibly through a spiral arm or a warp to account for the high column density. From the radial velocities, we find that A428 is in fact made up of at least 2 clumps of galaxies separated by 3300 km/s, which are imbedded in a nearly continuous distribution of galaxies over 13000 km/s in velocity. Thus, the environment resembles a poor group within a filament viewed end-on.

astro-ph↗

arXiv1999

The X-ray Luminosity Function of Nearby Rich and Poor Clusters of Galaxies: A Cosmological Probe

In this letter, we present a new determination of the local (z<0.09) X-ray luminosity function (XLF) using a large, statistical sample of 294 Abell clusters and the ROSAT All-Sky-Survey. Given our large sample size, we have reduced errors by a factor of two for L(X)(0.5-2keV)>10^43 ergs/sec. We combine our data with previous work in order to explore possible constraints imposed by the shape of the XLF on cosmological models. A set of currently viable cosmologies is used to construct theoretical XLFs assuming Lx is proportional to M^p and a sigma_8-Omega_0 constraint (from Viana & Liddle 1996) based on the local X-ray temperature function. We fit these models to our observed XLF and verify that the simplest adiabatic, analytic scaling relation (e.g. Kaiser 1986) disagrees strongly with observations. If we assume that clusters can be described by the pre-heated, constant core-entropy models of Evrard & Henry (1991) then the observed XLF is consistent only with 0.1 < Omega_0 < 0.4 if the energy per unit mass in galaxies is roughly equal to the gas energy (ie if beta=1). (abridged)

astro-ph↗

arXiv1997

An Unusual Radio Galaxy in Abell 428: A Large, Powerful FR I Source in a Disk-Dominated Host

We report the discovery of a powerful (~10^{24} Watts/Hz) FR I radio source in a highly flattened disk-dominated galaxy. Half of the radio flux from this source is concentrated within the host galaxy, with the remainder in a pair of nearly symmetrical lobes with total extent ~200kpc nearly perpendicular to the disk. The traditional wisdom maintains that powerful, extended radio sources are found only in ellipticals or recent merger events. We report B,R,J, and K imaging, optical spectroscopy, a rotation curve, an IRAS detection, and a VLA 20cm image for this galaxy, 0313-192. The optical and NIR images clearly show a disk. We detect apparent spiral arms and a dust-lane from B band imaging. The reddened nucleus is consistent with extinction by a similar dust-lane. The optical spectrum suggests a central AGN and some evidence of a starburst, with both the AGN and central starlight appearing substantially reddened. From analysis of the extended line emission in [OIII] and H-alpha we derive a rotation curve consistent with an early- type, dusty spiral seen edge-on. From the IRAS detection at 60 and 100 microns, we find that the ratio of Far IR to radio flux places this object firmly as a radio galaxy (i.e. the radio emission is not powered by star formation). The radio structure suggests that the radio source in this galaxy is related to the same physical mechanisms present in jet-fed powerful radio sources, and that such powerful, extended sources can (albeit extremely rarely) occur in a disk-dominated host.

astro-ph↗

arXiv1997

The Cluster of Galaxies Surrounding Cygnus A

We report optical imaging and spectroscopy of 41 galaxies in a 22 arcmin square region surrounding Cygnus A. The results show that there is an extensive rich cluster associated with Cygnus A of Abell richness at least 1 and possibly as high as 4. The velocity histogram has two peaks, one centered on Cygnus A, and a more significant peak redshifted by about 2060 km/s from the velocity of Cygnus A. The dynamical centroid of the spatial distribution is also shifted somewhat to the NW. However, statistical tests show only weak evidence that there are two distinct clusters. The entire system has a velocity dispersion of 1581 km/s which is slightly larger than other, well studied, examples of rich clusters.

astro-ph↗