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Register a new userDiscovery of an overdensity of faint red galaxies in the vicinity of the z=1.786 radio galaxy 3C 294
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S. Toft
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We report the discovery of an overdensity of faint red galaxies in the vicinity of the z=1.786 radio galaxy 3C 294. The overdensity, discovered in a 84 min Ks-band ISAAC/VLT image is significant at the 2.4 sigma level (compared to the local field density), and overlaps with the extended X-ray emission around 3C 294 detected with the Chandra X-ray Observatory. The near-infrared colours of the galaxies making up the overdensity show a large scatter and the galaxies do not follow a red sequence in the colour magnitude diagram. If the galaxies are in a cluster at z=1.786 they must be dominated by young stellar populations with different star-formation histories.
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We have obtained imaging in the K band (~I-band rest frame) of the z=1.786 radio galaxy 3C 294 with the 36-element curvature-sensing adaptive optics system Hokupa`a and the Canada-France-Hawaii Telescope. At a resolution of < ~0.15, the galaxy is seen as a group of small but resolved knots distributed over a roughly triangular region ~1.4 across. The interpretation of the structure depends on the location of the nucleus, as indicated by the compact radio core. Its position is uncertain by > ~0.5 (2-sigma) because of uncertainties in the optical astrometry, but our best estimate places it at or near the southern apex of the distribution. If this location is correct, the most likely interpretation is that of a hidden quasar nucleus illuminating dusty infalling dwarf-galaxy-like clumps having characteristic sizes of ~1.5 kpc.
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
We present seeing-limited and adaptive optics (AO) images of the z = 1.786 radio galaxy 3C294 in the H and K infrared bands obtained at Keck Observatory. The infrared emission of 3C294 is dominated by two distinct components separated by ~1 (9 kpc). The eastern knot contains an unresolved core that contributes ~4% of the K-band light; we identify this core with the active nucleus. The western component is about 2.5 times brighter. The most plausible interpretation of the near-infrared morphology is an ongoing merger event, with the active nucleus located in the less massive of the two galaxies.
A five square arcminute region around the luminous radio-loud quasar SDSS J0836+0054 (z=5.8) hosts a wealth of associated galaxies, characterized by very red (1.3 < i_775 - z_{850} < 2.0) color. The surface density of these z~5.8 candidates is approximately six times higher than the number expected from deep ACS fields. This is one of the highest galaxy overdensities at high redshifts, which may develop into a group or cluster. We also find evidence for a substructure associated with one of the candidates. It has two very faint companion objects within two arcseconds, which are likely to merge. The finding supports the results of a recent simulation that luminous quasars at high redshifts lie on the most prominent dark-matter filaments and are surrounded by many fainter galaxies. The quasar activity from these regions may signal the buildup of a massive system.
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.
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