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Register a new userTunable-filter imaging of quasar fields at z ~ 1. II. The star-forming galaxy environments of radio-loud quasars
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We have scanned the fields of six radio-loud quasars using the Taurus Tunable Filter to detect redshifted [OII] 3727 line-emitting galaxies at redshifts 0.8 < z < 1.3. Forty-seven new emission-line galaxy (ELG) candidates are found. This number corresponds to an average space density about 100 times that found locally and, at L([OII]) < 10^{42} erg s^{-1} cm^{-2}, is 2 - 5 times greater than the field ELG density at similar redshifts, implying that radio-loud quasars inhabit sites of above-average star formation activity. The implied star-formation rates are consistent with surveys of field galaxies at z ~ 1. However, the variation in candidate density between fields is large and indicative of a range of environments, from the field to rich clusters. The ELG candidates also cluster -- both spatially and in terms of velocity -- about the radio sources. In fields known to contain rich galaxy clusters, the ELGs lie at the edges and outside the concentrated cores of red, evolved galaxies, consistent with the morphology-density relation seen in low-redshift clusters. This work, combined with other studies, suggests that the ELG environments of powerful AGN look very much the same from moderate to high redshifts, i.e. 0.8 < z < 4.
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We have carried out multicolour imaging of a complete sample of radio-loud quasars at 0.6 < z < 1.1 and find groups or clusters of galaxies in the fields of at least 8 and possibly 13 of the 21 sources. There is no evidence for an evolution in the richness of the environments of radio-loud quasars from other low-redshift studies to z >~ 0.9. The quasars associated with groups and clusters in our sample do not necessarily reside in the centre of the galaxy distribution which rarely displays a spherical geometry. Clustering is preferentially associated with small or asymmetric steep-spectrum radio sources. The quasars with the largest projected angular size are, in nearly all cases, found in non-clustered environments. Radio-based selection (including source size) of high-redshift groups and clusters can be a very efficient method of detecting rich environments at these redshifts. We find that in optical searches for galaxy overdensities above z ~ 0.6 multiple filters must be used. If the single-filter counting statistics used by groups at lower redshift are applied to our data, uncertainties are too large to make accurate quantifications of cluster richness. This means that genuine clustering of galaxies about quasars will be missed and, in ~10% of cases, putative clusters turn out to be false detections. The statistics are further diluted by the fact that galaxy overdensities are generally not centred on the quasar.
We have used the Taurus Tunable Filter to search for Lyman alpha emitters in the fields of three high redshift quasar fields: two at z~2.2 (MRC B1256-243 and MRC B2158-206) and one at z~4.5 (BR B0019-1522). Our observations had a field of view of around 35 square arcminutes, and reached AB magnitudes of magnitudes of ~21 (MRC B1256-243), ~22 (MRC B2158-206), and ~22.6 (BR B0019-1522), dependent on wavelength. We have identified candidate emission line galaxies in all three of the fields, with the higher redshift field being by far the richest. By combining our observations with simulations of the instrumental response, we estimate the total density of emission line galaxies in each field. Seventeen candidate emission line galaxies were found in within 1.5 Mpc of BR0019-1522, a number density of 4.9 +/- 1.2 x 10^-3 Mpc^-3, suggesting a significant galaxy overdensity at z~4.5.
We have obtained multi-colour imaging of a representative, statistically complete sample of low-frequency selected (S_408MHz > 0.95Jy) radio loud quasars at intermediate (0.6 < z < 1.1) redshifts. These sources are found in a variety of environments, from the field through to rich clusters. We show that statistical measures of environmental richness, based upon single-band observations are inadequate at these redshifts for a variety of reasons. Environmental richness seems correlated with the size and morphology of the radio source, as expected if the energy density in the radio lobes is approximately the equipartition value and the lobes are in pressure equilbrium with a surrounding intragroup/cluster medium. Selecting on radio size therefore efficiently selects dense galactic sytems at these redshifts.
We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6<z<1.1, in order to determine the characteristics of their environments. The largest radio sources are located in the field, and smaller steep-spectrum sources are more likely to be found in richer environments, from compact groups through to clusters. This radio-based selection (including source size) of high redshift groups and clusters is a highly efficient method of detecting rich environments at these redshifts. Although our single filter clustering measures agree with those of other workers, we show that these statistics cannot be used reliably on fields individually, colour information is required for this.
Radio-loud Active Galactic Nuclei at z~2-4 are typically located in dense environments and their host galaxies are among the most massive systems at those redshifts, providing key insights for galaxy evolution. Finding radio-loud quasars at the highest accessible redshifts (z~6) is important to study their properties and environments at even earlier cosmic time. They would also serve as background sources for radio surveys intended to study the intergalactic medium beyond the epoch of reionization in HI 21 cm absorption. Currently, only five radio-loud ($R=f_{ u,5{rm GHz}}/f_{ u,4400AA}>10$) quasars are known at z~6. In this paper we search for 5.5 < z < 7.2 quasars by cross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radio information allows identification of quasars missed by typical color-based selections. While we find no good 6.4 < z <7.2 quasar candidates at the sensitivities of these surveys, we discover two new radio-loud quasars at z~6. Furthermore, we identify two additional z~6 radio-loud quasars which were not previously known to be radio-loud, nearly doubling the current z~6 sample. We show the importance of having infrared photometry for z>5.5 quasars to robustly classify them as radio-quiet or radio-loud. Based on this, we reclassify the quasar J0203+0012 (z=5.72), previously considered radio-loud, to be radio-quiet. Using the available data in the literature, we constrain the radio-loud fraction of quasars at z~6, using the Kaplan--Meier estimator, to be $8.1^{+5.0}_{-3.2}%$. This result is consistent with there being no evolution of the radio-loud fraction with redshift, in contrast to what has been suggested by some studies at lower redshifts.
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