arXiv subjects

Neal A. Miller

Publications and source records attributed to Neal A. Miller.

28 records · Page 2Linked to original sources

arXiv2005

Star Formation and AGN in the Core of the Shapley Supercluster: A VLA Survey of A3556, A3558, SC1327-312, SC1329-313, and A3562

The core of the Shapley supercluster (A3556, A3558, SC1327-312, SC1329-313, and A3562) is an ideal region in which to study the effects of cluster mergers on the activity of individual galaxies. This paper presents the most comprehensive radio continuum investigation of the region, relying on a 63-pointing mosaic obtained with the Very Large Array yielding an areal coverage of nearly 7 square degrees. The mosaic provides a typical sensitivity of about 80 uJy at a resolution of 16", enabling detection of galaxies with star formation rates as low as 1 solar mass per year. The radio data are complemented by optical imaging in B and R, producing a catalog of 210 radio-detected galaxies with m_R <= 17.36 (M_R <= -19). At least 104 of these radio-detected galaxies are members of the supercluster on the basis of public velocity measurements. Across the entire core of the supercluster, there appears to be a significant deficit of radio galaxies at intermediate optical magnitudes (M_R between -21 and -22). This deficit is offset somewhat by an increase in the frequency with which brighter galaxies (M_R less than -22) host radio sources. More dramatic is the highly significant increase in the probability for fainter galaxies (M_R between -20 and -21) in the vicinity of A3562 and SC1329-313 to be associated with radio emission. The radio and optical data for these sources strongly suggest that these active galaxies are powered by star formation. In conjunction with recent X-ray analysis, this is interpreted as young starbursts related to the recent merger of SC1329-313 with A3562 and the rest of the supercluster.

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arXiv2005

Radio Emission Associated with the ULX in Holmberg II

We report the detection of radio emission coincident with the ultraluminous X-ray source (ULX) in Holmberg II. The radio emission is diffuse and resolved, covering an area ~60 by 40 pc in extent and well-matched to the recently discovered HeII nebula surrounding the X-ray source. Comparison of the radio and optical properties of this extended radio emission argue against its association with either an HII region or supernova remnant. This is additional evidence that this ULX is not powered by a stellar-mass object whose emission is relativistically beamed towards the observer, and thus is either a super-Eddington source or intermediate mass black hole as suggested by optical observations. Implications of this result to future and existing radio studies of ULXs are discussed.

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arXiv2004

Radio Detections of Ultraluminous X-ray Sources

We present some results from an archival VLA study of ultraluminous X-ray source s (ULXs). These unresolved non-nuclear X-ray sources have luminosities (L_X >= 1 0^39 ergs/sec) which may require somewhat exotic explanations, such as intermedi ate mass black holes or super-Eddington accretion. Radio emission is a powerful way to investigate such sources, through radio morphology and the implications m ade by source energy and lifetimes derived from the radio. The three galaxies we present here suggest that no single model explains all ULXs, yet there is growi ng evidence that some ULXs are powered by intermediate mass black holes.

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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.

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arXiv2003

Star Formation in Cluster-Cluster Mergers

Radio continuum emission at 1.4 GHz was used to identify active galaxies in a sample of 20 nearby Abell clusters. The radio emission indicates either an active galactic nucleus or current star formation, and optical spectroscopy was used to evaluate which of these dominates for each of the radio-selected galaxies. In an analysis which parallels the blue fractions of Butcher-Oemler studies, we calculated radio galaxy fractions for the clusters. One cluster in particular shows a dramatic increase in activity relative to the others: the cluster-cluster merger Abell 2255. We compare the results for Abell 2255 with those for Abell 2256 to assess the role of cluster-cluster mergers on the star formation activity of member galaxies. From these clusters, as well as several identified in other studies, a picture emerges in which substructure and cluster dynamical activity are of great importance in understanding evolutionary phenomenon such as the Butcher-Oemler Effect. Increased fractions of active galaxies (be they the blue galaxies of Butcher-Oemler studies or radio galaxies as discussed here) naturally result as clusters are built through mergers of smaller groups in hierarchical formation scenarios. The observed spread in fractions of active galaxies at any given redshift reflects the spread in cluster dynamical state.

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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.

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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.

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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.

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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.

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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.

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