Our results for 34 rich clusters from 0.02 < z < 0.4, clearly show strong evolution in the low-luminosity radio galaxy population from the present epoch back to redshifts of 0.4. The RSG population fraction has increased 6-10x over ~5 Gyr, from 2% for nearby rich clusters to 12-20% at z~0.4 (VLA observations: 17-24 hr per z=0.4 cluster). All four z~0.4 clusters have significant substructure suggesting recent (~1-2 Gyr) accretion of smaller groups and subclumps (Oemler et. al. 1997). Many of these RSG at $z=0.4$ have disturbed morphology, indicating that galaxy mergers are probably important. This agrees with Dressler et. al. (1999) whose results suggest that the active and recently active galaxies are more likely to be classified as strongly asymmetric or distorted. However, dust obscuration may modify the morphological appearance of the RSG, as was reported in Smail, Morrison et. al. (1999), based on HST WFPC2/NICMOS color extinction maps.
At lower redshift, several clusters which have an elevated population of weak RSG appear to be undergoing a cluster-cluster merger. This evidence seems to tell us that large-scale-structure (LSS) mergers (\eg cluster-cluster, cluster-group mergers) may trigger significant SF in some of the embedded galaxian population. Numerical simulations by Bekki (1999) support this conclusion.
Our results further suggest that these weak RSG are more widely distributed with respect to the cluster center than galaxies with higher radio luminosities (Ledlow & Owen 1995). This is consistent with the radio population being related to the less concentrated, late-type disk galaxy population in these clusters. Many of these RSG have Halpha luminosities which are consistent with lower SFR than the SFR derived from the radio (Owen et al. 1999). This suggests either that the optical estimates are underestimating the SFR (due to dust extinction) or that these late-type galaxies have a significant AGN component. Either of these results is intriguing. In the former case, these results would call into question the optical estimates of the evolution of the cosmological SFR. This idea is further supported by the WFPC2/NICMOS results which show significant amounts of dust within radio-selected `E+A' (optically classified post-starburst galaxies) galaxies in Cl0939+4713 at z=0.4 (Smail et. al. 1999a), where the radio emission is powered by SF that is hidden by the dust. In the latter case, an increase in the AGN populations in late-type galaxies is implied, at least on the outskirts of rich clusters. New and ongoing spectroscopic work will help answer these questions soon. However, the major questions left unanswered are: what physical processes (environmentally stimulated?) are responsible for the RSG population, and why is this population so much larger at higher redshifts? Are we witnessing the consequence of LSS formation?
Current Research Projects
SWIRE is a wide-area, high galactic latitude, imaging survey to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and active galactic nuclei as a function of environment from redshifts, z ~ 3, when the Universe was about 2 billion years old, to the present time. SWIRE will survey approximately 70 square degrees with the MIPS far-infrared camera and the with the IRAC mid-infrared camera. The key scientific goals of SWIRE are to determine:
(1) The evolution of both actively star-forming and passively-evolving galaxies to determine the history of galaxy formation (including the global Star Formation History - SFH), in the context of cosmic structure formation and galaxy environment.
(2) The spatial distribution and clustering of evolved galaxies, starbursts, and AGN, and the evolution of their clustering in the key redshift range from 0.5 < z < 3 over which much of cosmic evolution has occurred. The evolutionary relationship between galaxies and AGN, and the contribution of AGN accretion energy to the cosmic backgrounds, relative to that from nucleosynthesis.
The survey will be dominated by: (1) more than 105 luminous infrared galaxies (log LFar-IR > 1011 L); up to 40,000 with z > 2. (2) 1 million early-type galaxies; up to 400,000 with z > 2. (3) 30,000 classical AGN, and as many as 250,000 dust-obscured QSO/AGN.
The satellite data will be complemented by an extensive program of ground-based optical, near-infrared and radio observations, providing identifications, photometric-redshifts, spectral energy distributions and other physical information. All of the survey data will be available to the Astronomical Community through the NASA/IPAC Infrared Science Archive.
The 3 microJy rms Radio Observation of a z=0.4 Rich Cluster: We have been awarded 108 hours of VLA time to image a z=0.4 rich cluster, Cl0939+47 at 20 cm. The goals of this project are to study the star-forming radio population both in the cluster and in the general field to levels below any previous radio survey for any field. With our existing radio data (total VLA time for one pointing: 120 hr), we expect to reach 3 microJy rms noise and to detect > 2000 sources. Deep photometry and spectroscopy will be required to determine the properties of the detected ultra-faint radio galaxy population. Our radio detection limits in Cl0939+47 will be a little higher than the Milky Way radio luminosity, thereby allowing us to study the truncation of SF in the normal galaxy population falling into the cluster environment. In the field, we expect to measure evolution of the SFR out to z = 1 - 3, with a sensitivity equivalent to ~10 Msun/yr at z = 1 and ~200 Msun/yr at z = 3. A 8-10 m class telescope is clearly needed for all deep and high-resolution follow-up optical and near-IR imaging and spectroscopy observations.
HST WFPC2 Imaging of Abell 2125: We were granted Cycle 7 time for a twin-band, three-pointings of WFPC2 imaging of A2125 (1) to look for color and morphological evidence of the dominant mechanism among the competing theories for the BO-effect, and (2) to understand the origin of the enhanced radio activity. Enhancement of AGNs in the substructure region would suggest that the environment is affecting the galaxy on very small nuclear scales. For star-forming galaxies, the color distribution can discriminate among several proposed mechanisms for production of a blue excess during cluster mergers.
HST WFPC2 Mosaicked Image of Cl0939+4713: In collaboration with the MORPHS group (Dressler, Oemler, Smail, et al.) and using our 15 hours (and 120 hr) of VLA radio data on Cl0939+4713 we are studying the morphology of RGS. An 8'x8' region centered on the cluster core was mapped by HST-WFPC2. This project is investigating the morphological changes in the RSG as a function environmental density, from the cluster core region to the near field, a density contrast of 10000:1. This study will offer clues to the triggering agent for the RSG that are dependent on local galaxy density, e.g., galaxy mergers/interactions.
Halpha Imaging and Photometric Redshift Study of Cl0939 at z=0.4: A Jan 2001 KPNO 4-meter and 2-meter scheduled project will obtain multi-band imaging of the fields of two distant clusters of galaxies in the optical and near-IR, to measure the spectral energy distributions (SED's) for the faint radio source population detected in deep VLA surveys over 40'x40', fields. Nine bands (U,B,V,R, 8010A, 9170A, J, H, K') will be observed. This will assist in the study of both ID's in the cluster and along the line-of-sight in front and behind the cluster. These data will be used to study the SFR both in the cluster and in the general field as a function of epoch, using the radio emission as a SF measure unaffected by dust obscuration. We will use the SED's to measure photometric redshifts, to estimate the dust obscuration, and to catalog extremely red objects (ERO's) for further study. We will also measure H_alpha+[NII] emission near the cluster redshifts, to enhance our understanding of the Butcher-Oemler effect and the evolution of SF in cluster environments.
Dynamical Studies: The goals of this project are to investigate the cluster dynamical activity, and to relate these processes to SF and AGN activity of the member galaxies. The results from Morrison (1999) suggests that the dynamical state of a cluster has a direct impact on the RSG population. To study this impact, we are conducting a optical(B,R)/radio/X-ray survey of the 35 richest Abell clusters (Abell et al. 1989) with z < 0.16. This complete sample will be used to answer many questions raised by our previous work: 1) Is there a correlation between cluster morphology (gas and galaxy distribution) and the SFR of the embedded galaxian population? 2) Do cluster-cluster mergers trigger SF within their galaxy members? We are going 2x deeper in the radio than our previous work, which will yield a factor of ~3 increase in the number of optical IDs, allowing robust determination of differences in radio activity between clusters. The survey will detect radio galaxies with L_1.4 > 1E22 W/Hz (SFR > 2 Msun/yr) equally from the cluster core to the vast low density periphery at 2.5 Mpc from the cluster center. Building on our previous work, this sample contains the most massive nearby galaxy clusters spanning a range of dynamical states, or stages of dynamical evolution. Such a sample is ideal for studying how the evolution of LSS drives the SFR of galaxies within them. 19 nights of Palomar 1.5-meter and CTIO 1.5-meter have already been allocated to this project.
SIRTF GTO MIPS & IRS Study: SIRTF will be used for imaging (24, 70, and 160 um) and spectroscopy on a sample of RSG within rich clusters. The project is to resolve the issues of whether the radio luminosity is indeed due to starburst activity and determine if the discordance between the optical and radio (several optical classified poststarburst galaxies, E+As, have radio emission) is due to extinction. MIPS observations will be used to check if the FIR-radio relation still hold for distant clusters. The IRS observations will use the NeV/NeII line ratio as a diagnostic to separate RSG into starburst and AGNs.
Spectroscopy of Low Redshift RSG: Spring 2001 observing time has been assigned on the Palomar 5-meter to complete the optical spectroscopy for a sample of low-luminosity radio galaxies on the outskirts of 18 very rich clusters of galaxies. The purpose is to determine the probability of radio emission as a function of distance from the cluster center in the red and blue galaxy populations, respectively. The incomplete spectroscopic results suggest that the weaker radio galaxies are much more widely distributed than the previously-studied, high luminosity samples. This may be related to the increase in merger activity, which would be expected in the lower-velocity-dispersion groups in the lower-density regions surrounding rich cluster cores, or simply to the less effective stripping in the lower-density cluster medium outside of the rich cluster core.
By measuring the fraction of red and blue galaxies which are radio emitters as a function of cluster radius, we will be able to distinguish between the predictions of different models. We also will use the emission-line strengths to estimate SFR to compare with the radio estimates of this quantity and to separate AGN's from the SF-driven radio population.
ISO-IRAS Faint Galaxy Survey: The goal of the survey is to produce a database of luminous far-infrared galaxies at 0.1 < z < 1 to investigate the AGN-SB relationship and to study the cosmological evolution of luminous infrared galaxies. The sample has been culled from the IRAS Faint Source Survey using criteria for faint, infrared-bright galaxies. The ISOCAM observations verify the IRAS detections, yield sensitive 12um fluxes, and provide positions to 10", accuracy, allowing for unambiguous optical identifications. Ground-based follow-up observations are providing optical magnitudes and accurate redshifts for identified sources.
References
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