Michael J. Ledlow

20 CM VLA Survey of Abell Clusters of Galaxies. VI. Radio/Optical Luminosity Functions

From a statistically complete sample of 188 radio galaxies in Abell clusters, we examine the radio/optical correlations, the FR I/II division, and the univariate and bivariate luminosity functions. As suggested by Owen (1993), the FR I/II division is shown to be a strong function of the optical luminosity of the host galaxy (proportional to L_opt^2). This dependence is also seen in the bivariate luminosity function, which suggests that the evolutionary tracks of radio sources and/or the initial conditions in the source are governed by the host galaxy properties. The probability for detecting radio emission increases with optical luminosity. The optical dependence is clearly separated in the integral luminosity functions which can be used as a constraint to models of FR I radio power evolution. Additionally, the source counts from the integrated univariate radio luminosity function (RLF) are consistent with our suggestion in paper V that radio sources may be a transient phenomenon which occurs in all elliptical galaxies at some time (or several times) over their lifetime. We find no statistically significant differences in the luminosity functions between rich cluster samples and radio sources not selected to reside in clusters. These results suggest that all radio galaxies live in similar environments in that the optical luminosity and the properties of the host galaxy are the most important parameters which affect radio source formation and evolution.

The coma cluster after lunch: Has a galaxcy group passed through the cluster core?

We propose that the Coma cluster has recently undergone a collision with the NGC 4839 galaxy group. The ROSAT X-ray morphology, the Coma radio halo, the presence of poststarburst galaxies in the bridge between Coma and NGC 4839, the usually high velocity dispersion for the NGC 4839 group, and the position of a large-scale galaxy filament to the NE of Coma are all used to argue that the NGC 4839 group passed through the core of Coma approximately 2 Gyr ago. We present a new Hydro/N-body simulation of the merger between a galaxy group and a rich cluster that reproduces many of the observed X-ray and optical properties of Coma/NGC 4839.

A 20 Centimeter VLA Survey of Abell Clusters of Galaxies. VII. Detailed Radio Images

We present detailed radio images, and models, obtained with the Very Large Array at 20 cm for 199 radio galaxies in Abell clusters of galaxies.

A Catalog of Nearby Poor Clusters of Galaxies

A catalog of 732 optically selected, nearby poor clusters of galaxies covering the entire sky north of -3° declination is presented. The poor clusters, called WBL clusters, were identified as concentrations of three or more galaxies with photographic magnitudes brighter than 15.7, possessing a galaxy surface overdensity of 104/3. These criteria are consistent with those used in the identification of the original Yerkes poor clusters, and this new catalog substantially increases the sample size of such objects. These poor clusters cover the entire range of galaxy associations up to and including Abell clusters, systematically including poor and rich galaxy systems spanning over 3 orders of magnitude in the cluster mass function. As a result, this new catalog contains a greater diversity of richness and structures than other group catalogs, such as the Hickson and Yerkes catalogs. The information on individual galaxies includes redshifts and cross-references to other galaxy catalogs. The entries for the clusters include redshift (where available) and cross-references to other group and cluster catalogs.

The X-Ray Luminosity Function and Gas Mass Function for Optically Selected Poor and Rich Clusters of Galaxies

We present the first X-ray luminosity function (XLF) for an optically selected sample of 49 nearby poor clusters of galaxies and a sample of 67 Abell clusters with z ≤ 0.15. We have extended the measured cluster XLF by more than a factor of 10 in X-ray luminosity. Our poor cluster sample was drawn from an optical catalog of groups with 0.01 ≤ z ≤ 0.03 composed of Zwicky galaxies. The X-ray emission was measured from the ROSAT all-sky survey. About 45% of the poor clusters were detected, with 0.5-2.0 keV luminosities from 1.7 to 65 × 1041 h-2 ergs s-1. These are among the X-ray brightest, optically selected poor clusters in the northern hemisphere. For this sample, the poor cluster XLF was found to be a smooth extrapolation of the rich cluster XLF. A new Hydro/N-body simulation of a hot + cold dark matter model with Ωtotal = 1, Ων = 0.2, and a baryon fraction of 7.5% was used to model and understand our observational selection effects. We found that the observed cluster gas mass function was consistent with our model.

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 20 cm 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 with 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 σ significance within a box of 2 × 2 h Mpc centered on Abells position.

Cluster Mergers as Triggers of Star Formation and Radio Emission: A Comparative Study of the Rich Clusters Abell 2125 and 2645

We report a detailed optical study of the clusters Abell 2125 and 2645. These clusters are very similar in redshift (z ≈ 0.25) and richness (Abell class 4), yet contrast strongly in blue fraction and radio-galaxy populations. In 1984 Butcher & Oemler reported that A2125 and A2645 have blue-galaxy fractions of 0.19 and 0.03, respectively, while more recent radio observations with the VLA and subsequent optical identifications on the digital Palomar Sky Survey show an apparent excess of radio galaxies in A2125 relative to A2645 (Dwarakanath & Owen). Our spectroscopic observations confirm this difference. We find 27 radio galaxies to be members of A2125 and only four in A2645, based on (nearly) complete observations to the same limiting magnitude and radio flux density. The radio galaxies in A2125 extend over about 5 Mpc (assuming H0 = 75 km s-1 Mpc-1) along a band running from northeast to southwest of the cluster center. About half the radio galaxies are red and have optical spectra that resemble old stellar populations. The other half are blue with emission lines, most of which indicate an origin in star formation rather than AGN. Many of the blue galaxies are in a distinct clump located about 2 Mpc in projection from the cluster center. The excess population of radio galaxies in A2125 occurs entirely at radio luminosities less than 1023 W Hz-1, where one expects star formation to be primarily responsible for the radio emission. Most of these radio galaxies have optical properties most consistent with systems later than E/S0. However, the optical line luminosities are often weaker than one would expect for the star formation rates implied by the radio emission. Thus we suspect that dust obscuration, larger than is usually found locally, hides most of the star-forming regions optically. The existence of a cluster-cluster merger in progress in A2125 seems likely to play some role in these phenomena, although the details are obscure.

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 (~1024 h−275 WHz−1 at 20 cm) FR I radio source in a highly flattened disk-dominated galaxy. Half the radio flux from this source is concentrated within the host galaxy, with the remainder in a pair of nearly symmetrical lobes of total extent ~200 kpc nearly perpendicular to the disk. 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 20 cm image for this galaxy, 0313-192. The optical and near-infrared images clearly show a disk. We detect apparent spiral arms in a deep B-band exposure, and a dust lane from a higher resolution B-band image. The reddened nucleus is consistent with extinction by a similar dust lane. The optical spectrum suggests a central AGN and shows some evidence of a starburst, with both the AGN and central starlight appearing substantially reddened (perhaps by the optical dust lane). From analysis of the extended line emission in [O III] and Hα, we derive a rotation curve consistent with an early-type, dusty spiral seen edge-on. From the IRAS detection at 60 and 100 μm, we find that the ratio of far-infrared 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 that are present in jet-fed powerful radio sources, and that such powerful, extended sources can (albeit extremely rarely) occur in a disk-dominated host.

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 Fanaroff-Riley type 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.

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 h kpc), powerful double radio source whose host galaxy is clearly a disk and most likely a spiral. This FR I-like radio galaxy is located very near the center of the richness class 0 cluster A428. The existence of such an object violates a fundamental paradigm for radio-loud active galactic nuclei (AGNs). In our first paper, we showed that this object was most consistent with a spiral host classification with optical emission-line ratios and colors suggestive of an active nucleus. However, we were not able to confirm actual radio jet emission based on the maps available at that time. In this paper, we present new, higher resolution radio imaging, a radio/millimeter continuum spectrum for the nucleus, a detection of H I absorption against the bright radio core, an upper limit to CO emission and the gas mass, and 70 (68 new) optical redshifts measured in the direction of A428. We confirm the existence of a radio jet at 20 cm, extending 42 h into the southern lobe. At 3.6 cm, we also detect a nuclear jet similar in length to that in M87, although 10 times weaker. We believe that this is the first detection of a radio jet on these scales in a disk/spiral host galaxy. The nuclear radio spectrum is similar to many blazar- or quasar-like objects, suggesting that the galaxy harbors an imbedded and obscured AGN. We model a turnover in the spectrum at low frequencies as a result of free-free absorption. We detect very strong and narrow H I absorption, with nearly the entire 20 cm continuum flux of the core being absorbed, implying an unusually large optical depth (τ ≈ 1). The most consistent model is that we are viewing the nucleus through a disklike distribution of gas in the interstellar medium, possibly through a spiral arm or a warp to account for the above-average column density. From the radial velocity distribution, we find that A428 is in fact made up of at least two clumps of galaxies separated by ~3300 km s-1, which themselves appear to be imbedded in a nearly continuous distribution of galaxies over 13,000 km s-1 in velocity space. Thus, the environment around this unusual radio source is more like that of a poor galaxy group imbedded in a filament-like structure viewed end-on.