Radio Emitting Galaxies in Clusters

NASA ADS Astronomy Abstract Service

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Title:
An Unexpectedly High Fraction of Active Galactic Nuclei in Red Cluster Galaxies
Authors:
Martini, Paul; Kelson, Daniel D.; Mulchaey, John S.; Trager, Scott C.
Affiliation:
AA(Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292 martini@ociw.edu, kelson@ociw.edu, mulchaey@ociw.edu, sctrager@ociw.edu), AB(Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292 martini@ociw.edu, kelson@ociw.edu, mulchaey@ociw.edu, sctrager@ociw.edu), AC(Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292 martini@ociw.edu, kelson@ociw.edu, mulchaey@ociw.edu, sctrager@ociw.edu), AD(Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292 martini@ociw.edu, kelson@ociw.edu, mulchaey@ociw.edu, sctrager@ociw.edu)
Journal:
The Astrophysical Journal, Volume 576, Issue 2, pp. L109-L112. (ApJ Homepage)
Publication Date:
09/2002
Origin:
UCP
ApJ Keywords:
Galaxies: Active, Galaxies: Clusters: Individual: Alphanumeric: A2104, X-Rays: Galaxies: Clusters, X-Rays: General
Abstract Copyright:
(c) 2002: The American Astronomical Society
Bibliographic Code:
2002ApJ...576L.109M

Abstract

As part of a program to study the evolution of active galactic nuclei (AGNs) in clusters of galaxies, we present our results for A2104. A deep Chandra observation of this massive, z=0.154 cluster reveals a significant X-ray point-source excess over the expectations of blank fields, including eight X-ray counterparts with R<20 mag. Our spectroscopy shows that all six X-ray sources associated with red counterparts are cluster members and their X-ray properties are consistent with all of them being AGNs. Only one of the six has the emission lines characteristic of optically selected AGNs; the remaining five would not have been classified as AGNs based on their optical spectra. This suggests the existence of a large population of obscured, or at least optically unremarkable, AGNs in clusters of galaxies. These six sources correspond to a lower limit of ~5% of the AGN fraction in cluster galaxies with R<20 mag (rest-frame MV=-19.5 mag) and are comparable to the blue galaxy fraction in the cluster. Such an obscured AGN population in clusters of galaxies has many implications for cluster galaxy evolution, the hidden growth of their central, supermassive black holes, estimates of the star formation rate at infrared and radio wavelengths, and the observed variance in the hard X-ray background.

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Title:
Far infrared and radio emission in dusty starburst galaxies
Authors:
Bressan, A.; Silva, L.; Granato, G. L.
Affiliation:
AA(INAF, Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy; SISSA, Strada Costiera, 34131 Trieste, Italy), AB(INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy), AC(INAF, Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy; SISSA, Strada Costiera, 34131 Trieste, Italy)
Journal:
Astronomy and Astrophysics, v.392, p.377-391 (2002) (A&A Homepage)
Publication Date:
09/2002
Origin:
A&A
A&A Keywords:
ISM: dust, extinction, galaxies: stellar content, infrared: galaxies, radio continuum: galaxies
Abstract Copyright:
(c) 2002: Astronomy & Astrophysics
Bibliographic Code:
2002A&A...392..377B

Abstract

We revisit the nature of the far infrared (FIR)/radio correlation by means of the most recent models of star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. \cite{Sil98}). For the radio emission, we revisit the simple model of Condon & Yin (\cite{Con90}). We find that a tight FIR/radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electron cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non-thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest a strategy to complement the analysis of the deviations from the FIR/radio correlation with the radio slope (q-radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q-radio slope diagram to assess the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.

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Title:
A Radio Study of Abell 2255: Star Formation and AGN in a Cluster Merger
Authors:
Miller, N. A.; Owen, F. N.
Affiliation:
AA(NASA's GSFC), AB(NRAO)
Journal:
American Astronomical Society Meeting 200, #45.02
Publication Date:
05/2002
Origin:
AAS
Abstract Copyright:
(c) 2002: American Astronomical Society
Bibliographic Code:
2002AAS...200.4502M

Abstract

Deep 1.4 GHz radio continuum imaging of Abell 2255 is presented. This cluster is among the better nearby candidates for rich cluster- cluster merger systems, with evidence including an elongated X-ray morphology, the presence of a radio halo, and substructure present in its galaxy distribution. Our radio observations reach an rms sensitivity of 40 microJansky per beam, enabling us to detect (at 5-sigma) star formation rates as low as 1.4 solar masses per year from the center of the cluster out to a radial distance of 3h75-1 Mpc. The radio data are complemented by optical imaging and a large spectroscopic database, allowing us to separate all galaxies brighter than MR=-20 into cluster members and foreground/background galaxies. The spectra are also used to associate the galaxies' radio emission with either star formation or AGN. By comparing Abell 2255 to other nearby clusters, we demonstrate an unusual amount of activity in this cluster. It appears that the merger strongly effects the evolution of cluster members, including both the powerful AGN and the optically-faint star-forming galaxies.

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Title:
μJy radio sources in the cluster MS1054-03
Authors:
Best, P. N.; van Dokkum, P. G.; Franx, M.; Röttgering, H. J. A.
Affiliation:
AA(Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ), AB(California Institute of Technology, Mail Stop 105-24, 1200 E. California Blvd, Pasadena, CA 91125, USA), AC(Sterrewacht Leiden, Postbus 9513, 2300RA Leiden, the Netherlands), AD(Sterrewacht Leiden, Postbus 9513, 2300RA Leiden, the Netherlands)
Journal:
Monthly Notices of the Royal Astronomical Society, Volume 330, Issue 1, pp. 17-34. (MNRAS Homepage)
Publication Date:
02/2002
Origin:
MNRAS
MNRAS Keywords:
GALAXIES: CLUSTERS: INDIVIDUAL: MS1054-03, GALAXIES: EVOLUTION, GALAXIES: STARBURST, RADIO CONTINUUM: GALAXIES
Abstract Copyright:
(c) 2002 The Royal Astronomical Society
Bibliographic Code:
2002MNRAS.330...17B

Abstract

An extremely deep 5-GHz radio observation is presented of the rich cluster MS1054-03 at redshift 34 radio sources are detected down to a 6σ level of 32μJy, compared with about 25 expected from previous blank-field radio source count determinations; the sources giving rise to these excess counts lie predominantly within 2arcmin (~700kpc) of the cluster centre. Existing imaging and spectroscopic observations have provided optical identifications for 21 of the radio sources and redshifts for 11, of which eight are confirmed cluster members. Four of these eight confirmed cluster sources are associated with close galaxy pairs projected offset) of similar magnitude, implying that the radio source may be triggered by an interaction. However, although MS1054-03 has a very high fraction (17 per cent) of ongoing mergers (separations <<10kpc), no radio emission is detected towards any of these merger events, setting a mean upper limit of 10Msolar yr-1 for any star-formation associated with these mergers. This supports the hypothesis that low-luminosity radio sources may be onset by initial weak interactions rather than direct mergers. The host galaxies of the other four confirmed cluster radio sources are all isolated, and show a range of morphologies from early type to Sc. A comparison between the emission-line and radio luminosities suggests that two of these four radio sources are low-luminosity active galactic nuclei (AGNs), whilst for at least one of the other two the radio emission is associated with ongoing star formation. All the radio sources associated with the galaxy pairs appear more likely to be AGN than starburst in origin. The overall proportion of radio sources associated with AGNs in this cluster (>~75 per cent) is higher than that detected at these flux-density levels in the field per cent).

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Title:
A radio perspective on galaxy evolution in the cluster environment
Authors:
Miller, Neal Andrew
Affiliation:
NEW MEXICO STATE UNIVERSITY
Journal:
Thesis (PhD). NEW MEXICO STATE UNIVERSITY, Source DAI-B 62/02, p. 898, Aug 2001, 419 pages.
Publication Date:
08/2001
Category:
Physics: Astronomy and Astrophysics
Origin:
UMI
Abstract Copyright:
(c) 2001: UMI Company
Comment:
Publication Number: 3004857; Advisor: Anderson, Kurt S.
Bibliographic Code:
2001PhDT........16M

Abstract

Using the recently-completed NRAO VLA Sky Survey (Condon et al. 1998) and new radio continuum observations, a comprehensive catalog of active galaxies in twenty nearby Abell clusters has been created. Basing the catalog on radio emission insures that both active galactic nuclei (AGN) as well as galaxies forming stars at rates comparable to the Milky Way are identified. The catalog also canvasses large physical areas within each cluster, from their cores out to radii where the local galaxy density is consistent with the field. Velocities determined from a rigorous program of follow-up optical spectroscopy and from public sources were used to finalize lists of cluster and non-cluster members. In total, 401 cluster members were confirmed, 159 radio galaxies were identified as foreground/background objects, and only 23 radio galaxies lack velocities. Consequently, a statistical database of active galaxies in a broad range of cluster environment has been established. This database was used to explore galaxy evolution in clusters, including studies within the identified sample and comparisons with clusters of galaxies at higher redshift. In summary, a picture develops wherein large-scale influences govern the evolution of galaxies in clusters. Outlying groups are accreted by clusters, and over relatively short timescales star formation in the group members has been quenched. In dramatic cases, entire clusters may merge and drive numerous outlying groups and field galaxies into the cluster environment during the process. Such structure formation processes were more common in the past, explaining the changes seen in clusters from high redshift up until the present. (Abstract shortened by UMI.)

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Title:
The Radio Galaxy Populations of Nearby Northern Abell Clusters
Authors:
Miller, Neal A.; Owen, Frazer N.
Affiliation:
AA(National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801; nmiller@aoc.nrao.edu, fowen@aoc.nrao.edu; Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.; Also Department of Astronomy, New Mexico State University, P.O. Box 30001, Department 4500, Las Cruces, NM 88003.), AB(National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801; nmiller@aoc.nrao.edu, fowen@aoc.nrao.edu)
Journal:
The Astrophysical Journal Supplement Series, Volume 134, Issue 2, pp. 355-383. (ApJS Homepage)
Publication Date:
06/2001
Origin:
UCP
ApJ Keywords:
Catalogs, Galaxies: Clusters: General, Galaxies: Distances and Redshifts, Radio Continuum: Galaxies
Abstract Copyright:
(c) 2001: The American Astronomical Society
Bibliographic Code:
2001ApJS..134..355M

Abstract

We report on the use of the NRAO VLA Sky Survey (NVSS) to identify radio galaxies in 18 nearby Abell clusters. The listings extend from the cores of the clusters out to radii of 3 h-175 Mpc, which corresponds to 1.5 Abell radii and approximately 4 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 that are in actuality cluster members. The resulting catalog consists of 329 cluster radio galaxies plus 138 galaxies deemed foreground or 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 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 (AGNs) at higher luminosities. The flux limit of the NVSS allows us to identify AGNs and star-forming galaxies down to star formation rates less than 1 Msolar yr-1. 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. Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium (ARC).

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Title:
The Far-Infrared-Radio Correlation in Nearby Abell Clusters
Authors:
Miller, Neal A.; Owen, Frazer N.
Affiliation:
AA(National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801-0387 nmiller@aoc.nrao.edu, fowen@aoc.nrao.edu; Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.; Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.; Also Department of Astronomy, Box 30001/Dept. 4500, New Mexico State University, Las Cruces, NM 88003.), AB(National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801-0387 nmiller@aoc.nrao.edu, fowen@aoc.nrao.edu)
Journal:
The Astronomical Journal, Volume 121, Issue 4, pp. 1903-1914. (AJ Homepage)
Publication Date:
04/2001
Origin:
UCP
AJ Keywords:
Galaxies: Clusters: General, Galaxies: Statistics, Infrared Radiation, Radio Continuum
Abstract Copyright:
(c) 2001: The American Astronomical Society
Bibliographic Code:
2001AJ....121.1903M

Abstract

A comprehensive study of the effect of the cluster environment on the far-infrared (FIR)-radio correlation in nearby Abell clusters is presented. Using a cluster radio galaxy database, optical spectroscopy, and high-resolution radio images to remove active galactic nuclei (AGNs), 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 AGNs are also well correlated. In the case of AGNs, 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 2 or 3. The FIR-radio correlation for cluster star-forming galaxies is also tested against the 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 magnetic field strengths of the cluster galaxies through compression, the velocity data do not confirm this model. Although a contribution from ram pressure cannot 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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Title:
Circumnuclear Star Formation in Active Galaxies
Authors:
Colina, Luis; Alberdi, A.; González-Delgado, R.; Torrelles, J. M.; Leitherer, C.; Panagia, N.; Wilson, A. S.
Affiliation:
AA(Instituto de Estructura de la Materia (CSIC), Serrano 121, 28006 Madrid, Spain), AB(Instituto de Astrofísica de Andalucia (CSIC), Granada, Spain), AC(Instituto de Astrofísica de Andalucia -- CSIC, Granada, Spain), AD(Instituto de Estudios Espaciales -- CSIC, Barcelona, Spain), AE(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA), AF(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA), AG(Department of Astronomy, University of Maryland, College Park, USA)
Journal:
The Central Kiloparsec of Starbursts and AGN: The La Palma Connection, ASP Conference Proceedings Vol. 249. Edited by J. H. Knapen, J. E. Beckman, I. Shlosman, and T. J. Mahoney. ISBN: 1-58381-089-7. San Francisco: Astronomical Society of the Pacific, 2001, p. 119.
Publication Date:
00/2001
Origin:
ADS
Bibliographic Code:
2001cksa.conf..119C

Abstract

Studies of ongoing star formation in the central kpc of active galaxies require the use of techniques that are natural detectors of young massive stars and provide the high angular resolution needed to disentangle AGN from luminous massive stellar clusters. HST ultraviolet studies of the Seyfert 2/LINER galaxy NGC 4303 and VLA radio studies of the infrared luminous Seyfert 1 galaxy NGC 7469 are presented.

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Title:
Poststarburst Models of LINERS
Authors:
Taniguchi, Yoshiaki; Shioya, Yasuhiro; Murayama, Takashi
Affiliation:
AC(Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan; tani@astr.tohoku.ac.jp, shioya@astr.tohoku.ac.jp, murayama@astr.tohoku.ac.jp)
Journal:
The Astronomical Journal, Volume 120, Issue 3, pp. 1265-1272. (AJ Homepage)
Publication Date:
09/2000
Origin:
UCP
AJ Keywords:
Galaxies: Active, Galaxies: Evolution, Galaxies: Nuclei, Galaxies: Starburst
Abstract Copyright:
(c) 2000: The American Astronomical Society
Bibliographic Code:
2000AJ....120.1265T

Abstract

Since the discovery of low-ionization nuclear emission-line regions (LINERs) in many galaxies, it has been recognized that they constitute a class of active galactic nuclei (AGNs) that are thought to be powered by gas accretion onto a central, supermassive black hole. LINERs are observed in approximately one-third of galaxies in the local universe, and it has been often thought that they harbor an AGN-like central engine with moderate activity. However, some LINERs show no direct evidence for AGNs, such as broad emission lines, radio jets, hard X-ray emission, spectral energy distributions that are inconsistent with those of starlight, and so on. For such LINERs (a subset of type 2 LINERs), we present new poststarburst models that explain some of their most important optical narrow emission line ratios. In these models, the ionization sources are planetary nebula nuclei (PNNs) with temperature of ~105 K that appear in the late-phase evolution of intermediate-mass stars with mass between ~3 and ~6 Msolar. Such PNNs left in a typical starburst nucleus can produce an Hα luminosity of L(Hα)~1038 ergs s-1 for typical poststarburst LINERs and ~1039 ergs s-1 only in exceptionally bright cases. The PNN phase lasts until the death of the lowest-mass stars formed in the starburst, which is ~5×108 yr for an assumed lower limit of the initial mass function of 3 Msolar. This long duration appears consistent with the observed higher frequency of occurrence of LINERs if every galaxy could experience the starburst activity several times in its life. We therefore propose that some LINERs that show no direct evidence for AGNs may be poststarburst nuclei powered by a cluster of PNNs.

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Title:
Microjansky Radio Sources in DC 0107-46 (Abell 2877)
Authors:
Hopkins, A.; Georgakakis, A.; Cram, L.; Afonso, J.; Mobasher, B.
Affiliation:
AA(Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, PA 15260), AB(School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK), AC(School of Physics, University of Sydney, NSW 2006, Australia), AD(Astrophysics Group, The Blackett Laboratory, Prince Consort Road, London SW7 2BZ, UK), AE(Astrophysics Group, The Blackett Laboratory, Prince Consort Road, London SW7 2BZ, UK)
Journal:
The Astrophysical Journal Supplement Series, Volume 128, Issue 2, pp. 469-478. (ApJS Homepage)
Publication Date:
06/2000
Origin:
UCP
ApJ Keywords:
Galaxies: Clusters: Individual: Name: Abell 2877, Galaxies: Evolution, Galaxies: Starburst, Radio Continuum: Galaxies
Abstract Copyright:
(c) 2000: American Astronomical Society
Bibliographic Code:
2000ApJS..128..469H

Abstract

The cluster DC 0107-46 (Abell 2877) lies within the Phoenix Deep Survey, made at 1.4 GHz with the Australia Telescope Compact Array. Of 89 known optical cluster members, 70 lie within the radio survey area. Of these 70 galaxies, 15 (21%) are detected, with luminosities as faint as 1.0×1020 W Hz-1. Spectroscopic observations are available for 14/15 of the radio-detected cluster galaxies. Six galaxies show only absorption features and are typical low-luminosity active galactic nucleus (AGN) radio sources. One galaxy hosts a Seyfert 2 nucleus, two are star-forming galaxies, and the remaining five may be star-forming galaxies, AGNs, or both.

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Title:
Cluster Mergers as Triggers of Star Formation and Radio Emission: A Comparative Study of the Rich Clusters Abell 2125 and 2645
Authors:
Owen, Frazer N.; Ledlow, Michael J.; Keel, William C.; Morrison, Glenn E.
Affiliation:
AA(National Radio Astronomy Observatory, P.O. Box O, Socorro, New Mexico 87801), AB(Department of Physics & Astronomy, University of New Mexico, Albuquerque, NM 87131), AC(Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487-0324), AD(Department of Physics & Astronomy, University of New Mexico, Albuquerque, NM 87131; National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801)
Journal:
The Astronomical Journal, Volume 118, Issue 2, pp. 633-644. (AJ Homepage)
Publication Date:
08/1999
Origin:
AJ
AJ Keywords:
GALAXIES: ACTIVE, GALAXIES: CLUSTERS: INDIVIDUAL (ABELL 2125, ABELL 2645), GALAXIES: DISTANCES AND REDSHIFTS, GALAXIES: EVOLUTION, GALAXIES: KINEMATICS AND DYNAMICS, GALAXIES: PHOTOMETRY, GALAXIES: STARBURST
Abstract Copyright:
(c) 1999: The American Astronomical Society
Bibliographic Code:
1999AJ....118..633O

Abstract

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 H_0=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 10^23 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.

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Title:
Radio Emission and the Butcher-Oemler Effect in Abell 2125
Authors:
Ledlow, M. J.; Owen, F. N.; Dwarakanath, K. S.; Keel, W. C.; Morrison, G. E.
Affiliation:
AA(Institute for Astrophysics, Dept. of Physics & Astronomy, University of New Mexico, Albuquerque, NM 87131), AB(National Radio Astronomy Observatory, Socorro, NM 87801), AC(Raman Research Institute, Bangalore, India 560 080), AD(Dept of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487), AE(National Radio Astronomy Observatory, Socorro, NM 87801, and Dept of Physics & Astronomy, University of New Mexico, Albuquerque, NM 87131)
Journal:
Observational Cosmology: The Development of Galaxy Systems, Proceedings of the International Workshop held at Sesto Pusteria, Bolzano, Italy, 30 June - 3 July, 1998, Eds.: G. Giuricin, M. Mezzetti, and P. Salucci, Astronomical Society of the Pacific, Vol. 176, p. 83
Publication Date:
06/1999
Origin:
AUTHOR
Bibliographic Code:
1999obco.conf...83L

Abstract

We present radio, optical, and X-ray observations of the Butcher-Oemler cluster Abell 2125. We find an interesting correlation between an excess of radio galaxies, X-ray substructure and an apparent ongoing cluster merger. Interestingly, the excess radio galaxy population appears to be nearly evenly divided between blue, star-forming disk galaxies and red cluster ellipticals. It appears that whatever mechanisms are responsible for the Butcher-Oemler effect in this cluster also stimulate weak AGN emission in these cluster ellipticals. Observations suggest that we may have caught this cluster in the act, in that the signatures of the enhanced activity may have occurred fairly recently (within the last few × 10^8 years), and detailed observations may provide important clues as to the cause of the excess activity. We review several possible mechanisms and compare to the observed properties of this cluster.

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Title:
Mid-Infrared Emission from E+A Galaxies in the Coma Cluster
Authors:
Quillen, A. C.; Rieke, G. H.; Rieke, M. J.; Caldwell, N.; Engelbracht, C. W.
Journal:
The Astrophysical Journal, Volume 518, Issue 2, pp. 632-640. (ApJ Homepage)
Publication Date:
06/1999
Origin:
APJ
ApJ Keywords:
GALAXIES: CLUSTERS: INDIVIDUAL: NAME: COMA, GALAXIES: STELLAR CONTENT, INFRARED: GALAXIES
Abstract Copyright:
(c) 1999: The American Astronomical Society
Bibliographic Code:
1999ApJ...518..632Q

Abstract

We have used ISO to observe at 12 μm seven E+A galaxies plus an additional emission-line galaxy, all in the Coma Cluster. E+A galaxies lacking narrow emission lines have 2.2-12 μm flux density ratios or limits similar to old stellar populations (typical of early-type galaxies). Only galaxies with emission lines have enhanced 12 μm flux density. Excess 12 μm emission is therefore correlated with the presence of ongoing star formation or an active galactic nucleus (AGN). From the mid- and far-infrared colors of the brightest galaxy in our sample, which was detected at longer wavelengths with IRAS, we estimate the far-infrared luminosity of these galaxies. By comparing the current star formation rates with previous rates estimated from the Balmer absorption features, we divide the galaxies into two groups: those for which star formation has declined significantly following a dramatic peak ~1 Gyr ago; and those with a significant level of ongoing star formation or/and an AGN. There is no strong difference in the spatial distribution on the sky between these two groups. However, the first group has systemic velocities above the mean cluster value and the second group has systemic velocities below that value. This suggests that the two groups differ kinematically. Based on surveys of the Coma Cluster in the radio, the IRAS sources, and galaxies detected in Halpha emission, we sum the far-infrared luminosity function of galaxies in the cluster. We find that star formation in late-type galaxies is probably the dominant component of the Coma Cluster far-infrared luminosity. The presence of significant emission from intracluster dust is not yet firmly established. The member galaxies also account for most of the far-infrared output from nearby rich clusters in general. We update estimates of the far-infrared luminosities of nearby, rich clusters and show that such clusters are likely to undergo luminosity evolution from z=0.4 at a rate similar to, or faster than, field galaxies.

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