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Register a new userExtragalactic Jets as Probes of Distant Clusters of Galaxies and the Clusters Occupied by Bent Radio AGN (COBRA) Survey
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Added by
Elizabeth L. Blanton
Publication date
2014
fields
Physics
and research's language is
English
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We are conducting a large survey of distant clusters of galaxies using radio sources with bent jets and lobes as tracers. These radio sources are driven by AGN and achieve their bent morphologies through interaction with the surrounding gas found in clusters of galaxies. Based on low-redshift studies, these types of sources can be used to identify clusters very efficiently. We present initial results from our survey of 653 bent-double radio sources with optical hosts too faint to appear in the SDSS. The sample was observed in the infrared with Spitzer, and it has revealed $sim$200 distant clusters or proto-clusters in the redshift range $zsim0.7 - 3.0$. The sample of bent-doubles contains both quasars and radio galaxies enabling us to study both radiative and kinetic mode feedback in cluster and group environments at a wide range of redshifts.
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The shape of bent, double-lobed radio sources requires a dense gaseous medium. Bent sources can therefore be used to identify galaxy clusters and characterize their evolutionary history. By combining radio observations from the Very Large Array Faint Images of the Radio Sky at Twenty centimeters (VLA FIRST) survey with optical and infrared imaging of 36 red sequence selected cluster candidates from the high-z Clusters Occupied by Bent Radio AGN (COBRA) survey (0.35 < z < 2.2), we find that radio sources with narrower opening angles reside in richer clusters, indicating that the cluster environment impacts radio morphology. Within these clusters, we determine 55.5% of our radio host galaxies are brightest cluster galaxies (BCGs) and that the remainder are associated with other luminous galaxies. The projected separations between the radio sources and cluster centers and the sizes of the opening angles of bent sources follow similar distributions for BCG and non-BCG host populations, suggesting that COBRA host galaxies are either BCGs or galaxies that may evolve into BCGs. By measuring the orientation of the radio sources relative to the cluster centers, we find between 30% and 42% of COBRA bent sources are outgoing and have passed through the cluster center, while between 8% and 58% of COBRA bent sources are infalling. Although these sources typically do not follow directly radial paths, the large population of outgoing sources contrasts what is observed in low-z samples of bent sources and may indicate that the intracluster medium is less dense in these high-z clusters.
We present 190 galaxy cluster candidates (most at high redshift) based on galaxy overdensity measurements in the spitzer/IRAC imaging of the fields surrounding 646 bent, double-lobed radio sources drawn from the Clusters Occupied by Bent Radio AGN (COBRA) Survey. The COBRA sources were chosen as objects in the VLA FIRST survey that lack optical counterparts in the Sloan Digital Sky Survey (SDSS) to a limit of $m_r=22$, making them likely to lie at high redshift. This is confirmed by our observations: the redshift distribution of COBRA sources with estimated redshifts peaks near $z=1$, and extends out to $zapprox3$. Cluster candidates were identified by comparing our target fields to a background field and searching for statistically significant ($ge2sigma$) excesses in the galaxy number counts surrounding the radio sources; 190 fields satisfy the $ge2sigma$ limit. We find that 530 fields (82.0%) have a net positive excess of galaxies surrounding the radio source. Many of the fields with positive excesses but below the $2sigma$ cutoff are likely to be galaxy groups. Forty-one COBRA sources are quasars with known spectroscopic redshifts, which may be tracers of some of the most distant clusters known.
Here we present new red sequence overdensity measurements for 77 fields in the high-$z$ Clusters Occupied by Bent Radio AGN (COBRA) survey, based on $r$- and $i$-band imaging taken with Lowell Observatorys Discovery Channel Telescope. We observe 38 COBRA fields in $r$-band and 90 COBRA fields in $i$-band. By combining the $r$- and $i$-band photometry with our 3.6$mu$m and 4.5$mu$m $Spitzer$ IRAC observations, we identify 39 red sequence cluster candidates that host a strong overdensity of galaxies when measuring the excess of red sequence galaxies relative to a background field. We initially treat the radio host as the cluster center and then determine a new cluster center based on the surface density of red sequence sources. Using our color selection, we identify which COBRA cluster candidates have strong red sequence populations. By removing foreground and background contaminants, we more securely determine which fields include cluster candidates with a higher significance than our single-band observations. Additionally, of the 77 fields we analyze with a redshift estimate, 26 include newly estimated photometric redshifts.
We present the results from a study with NSFs Karl G. Jansky Very Large Array (VLA) to determine the radio morphologies of extended radio sources and the properties of their host galaxies in 50 massive galaxy clusters at z~1. We find a majority of the radio morphologies to be Fanaroff-Riley (FR) type IIs. By analyzing the infrared counterparts of the radio sources, we find that ~40% of the host galaxies are the candidate brightest cluster galaxy (BCG) and ~83% are consistent with being one of the top six most massive galaxies in the cluster. We investigate the role of environmental factors on the radio-loud AGN population by examining correlations between environmental and radio-galaxy properties. We find that the highest stellar mass hosts ($M_{*} gtrsim$ 4$times 10^{11} M_{odot}$) are confined to the cluster center and host compact jets. There is evidence for an increase in the size of the jets with cluster-centric radius, which may be attributed to the decreased ICM pressure confinement with increasing radius. Besides this correlation, there are no other significant correlations between the properties of the radio-AGN (luminosity, morphology, or size) and environmental properties (cluster richness and location within the cluster). The fact that there are more AGN in the cluster environment than the field at this epoch, combined with the lack of strong correlation between galaxy and environmental properties, argues that the cluster environment fosters radio activity but does not solely drive the evolution of these sources at this redshift.
We present a study of the central radio activity of galaxy clusters at high redshift. Using a large sample of galaxy clusters at $0.7<z<1.5$ from the Massive and Distant Clusters of {it WISE} Survey and the Faint Images of the Radio Sky at Twenty-Centimeters $1.4$~GHz catalog, we measure the fraction of clusters containing a radio source within the central $500$~kpc, which we term the cluster radio-active fraction, and the fraction of cluster galaxies within the central $500$~kpc exhibiting radio emission. We find tentative ($2.25sigma$) evidence that the cluster radio-active fraction increases with cluster richness, while the fraction of cluster galaxies that are radio-luminous ($L_{1.4~mathrm{GHz}}geq10^{25}$~W~Hz$^{-1}$) does not correlate with richness at a statistically significant level. Compared to that calculated at $0 < z < 0.6$, the cluster radio-active fraction at $0 < z < 1.5$ increases by a factor of $10$. This fraction is also dependent on the radio luminosity. Clusters at higher redshift are much more likely to host a radio source of luminosity $L_{1.4~mathrm{GHz}}gtrsim10^{26}$~W~Hz$^{-1}$ than are lower redshift clusters. We compare the fraction of radio-luminous cluster galaxies to the fraction measured in a field environment. For $0.7<z<1.5$, we find that both the cluster and field radio-luminous galaxy fraction increases with stellar mass, regardless of environment, though at fixed stellar mass, cluster galaxies are roughly $2$ times more likely to be radio-luminous than field galaxies.
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