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Register a new userGalaxy Population in a Cluster of Galaxies around the Radio Galaxy 3C 324 at z=1.2
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Authors
Fumiaki Nakata
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We discuss the properties of galaxies around the radio galaxy 3C 324 at z=1.2 based on BVRIK multi-band imaging data. We have applied a photometric-redshift technique to objects in the 3C 324 field, and identified 35 objects as plausible cluster members. We have found that red and luminous members are concentrated in a small region enclosed by a circle of 40 radius (0.33 Mpc at z=1.2 for Omega_0=0.3, lambda_0=0.7, H_0=70km/s/Mpc cosmology) from the 3C 324 galaxy. The 3C 324 cluster is probably much more compact in size compared with the local clusters. We constructed a K-band luminosity function of the cluster members and fit a Schechter function, and found the characteristic magnitude to be K*_{AB}=20.2+-0.6. This value is consistent with the extrapolation of the pure passive evolution seen for z<1 clusters. We have identified eight bright galaxies which form a red color-magnitude sequence. The slope of the sequence is consistent with the passive evolution model down to K_{AB}<22; we also found that there is no clear age variation in these bright red galaxies. However, seven out of these eight galaxies exhibit a significant excess in the rest UV light with respect to the passive evolution model. This may suggest that the massive early-type galaxies in this high-redshift cluster are still forming stars to some extent. We have confirmed a truncation of the color-magnitude sequence at K_{AB}sim22; faint passively-evolving galaxies may not yet be present in this cluster at zsim1.2. The overall color distribution of the cluster members, selected by the photometric redshift technique, is found to be very broad. We derived the fraction of blue galaxies in this cluster following a
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We have investigated the optical and near-infrared colors of K-selected galaxies in clusters at z ~ 1.2 near to the radio galaxy 3C 324 using images obtained with the Subaru telescope and archival HST data. The distribution of colors of the galaxies in the cluster region is found to be fairly broad, and it may imply significant scatter in their star-formation histories, although the effect of contamination of field galaxies is uncertain. The red sequence of galaxies whose R-K colors are consistent with passive evolution models for old galaxies is found to be truncated at K ~ 20 mag, and there are few fainter galaxies with similar red colors in the cluster region. We find that the bulge-dominated galaxies selected by quantitative morphological classification form a broad sequence in the color-magnitude diagram, whose slope is much steeper than that expected from metallicity variations within a passively evolving coeval galaxy population. We argue that the observed color-magnitude sequence can be explained by metallicity and age variations, and the fainter galaxies with K > 20 mag may be 1-2 Gyr younger than the brighter galaxies. Some spatial segregation of the color and K-band luminosity is seen in the sky distribution; the redder and the brighter objects tend to be located near 3C 324.
We have recently discovered a forming cluster around the radio galaxy MRC 1138-262 at redshift 2.2. Besides the population of Ly alpha emitting galaxies that have been confirmed spectroscopically, we have detected many candidate H alpha emitters that seem to have a different spatial distribution from the other galaxies: they are more clustered towards the center of the cluster and seem to be distributed along the same direction as the radio source. We present here the characteristics of the Ly alpha and H alpha emitters and study the nature of these populations.
We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity are consistent with those of galaxies that lie within a narrow redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10^14 solar masses, and are dense enough to collapse into virizalised structures by the present day: these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the fore- and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies, and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U -V ~ 0.6), suggesting most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence implies that these cluster galaxies form the bulk of their stars at z < 3.
The z~1 radio galaxy 3C280 has a striking rest-frame UV morphology, with multiple line and continuum components precisely aligned with the radio structure, including an obvious semi-circular arc. We explore the nature of these various components by bringing together HST and ground-based imaging, ground-based spectroscopy, and radio mapping. From plausible decompositions of the spectra, we show that the continuum of the nuclear component is likely dominated by a combination of nebular thermal continuum, quasar light, and light from old stars. A component that falls directly on the probable path of the radio jet shows mostly nebular thermal continuum and includes contributions from a relatively young stellar population with an age around 100 Myr. The arc appears to be completely dominated by line emission and nebular thermal continuum, with no evidence for a significant stellar contribution. Though much of the aligned light is in UV components, the underlying old elliptical is also well-aligned with the radio axis. The elliptical is well-fit by a de Vaucouleurs profile, probably has a moderately old stellar population (~3 Gyr), and is a massive system with a velocity dispersion of sigma ~ 270 km/s that implies it contains a supermassive black hole. Although the arc and the extended emission surrounding the eastern lobe suggest that interactions between the radio lobe and jet must have been important in creating the UV morphology, the ionization and kinematic properties in these componentsare more consistent with photoionization than shock excitation. 3C280 may be a transition object between the compact steep-spectrum radio galaxies which seem to be shock-dominated, and the extended radio sources which may have evolved past this phase and rarely show shock signatures.
We present an analysis of the magnetic-field fluctuations in the magnetoionic medium in front of the radio galaxy 3C 31 derived from rotation-measure (RM) fits to high-resolution polarization images. We first show that the Faraday rotation must be due primarily to a foreground medium. We determine the RM structure functions for different parts of the source and infer that the simplest form for the power spectrum is a power law with a high-frequency cutoff. We also present three-dimensional simulations of RM produced by a tangled magnetic field in the hot plasma surrounding 3C 31, and show that the observed RM distribution is consistent with a spherical plasma distribution in which the radio source has produced a cavity.
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