Galaxies in the Fields of z~1.5 Radio-Loud Quasars

Hall, P.B., Sawicki, M., Martini, P., Finn, R.A., Pritchet, C.J., Osmer, P.S., McCarthy, D.W., Evans, A.S., Lin, H., Hartwick, F.D.A. 2001, AJ, 121, 1840

We have previously identified an excess population of predominantly red galaxies around a sample of 31 radio-loud quasars (RLQs) at 1 < z < 2. Here we show that these fields have a surface density of extremely red objects (EROs, with R-K > 6) 2.7 times higher than the general field. Assuming these EROs are passively evolved galaxies at the quasar redshifts, they have characteristic luminosities of only ~ L_*. Only one of four RLQ fields has an excess of J-K selected EROs with J-K > 2.5; thus, those objects are probably unrelated to the quasars.

We also present new multiwavelength data and analyses on the fields of four of these quasars at z ~ 1.54, obtained to build more detailed pictures of the environments of these quasars and the galaxies within them. First, wide-field J and K_s data shows that the galaxy excess around Q0835+580 is of Abell richness 2 +/- 1 and extends to 140" at 2.35 sigma, and that the galaxy excess around Q1126+101 extends to only 50" even though the overall counts in the field are higher than the literature average. Second, in three fields we present the deepest narrow-band redshifted Halpha observations yet published. We detect five candidate galaxies at the quasar redshifts, a surface density ~ 30 times higher than shallower surveys. However, photometric spectral energy distribution (SED) fitting of one candidate suggests it is an [OIII] detection background to the quasar. Third, submillimeter observations of three fields with SCUBA detect two of the quasars and two galaxies with SEDs best fit as highly reddened galaxies at the quasar redshifts. Fourth, H-band adaptive optics (AO) imaging is used to estimate redshifts for two moderately red bulge-dominated galaxies in the Q0835+580 field using the Kormendy relation between central surface brightness and half-light radius. Both have structural redshifts consistent with early-type galaxies foreground to the quasar at z ~ 0.2 or 1 < z < 1.35. Photometric redshifts do not confirm these structural redshifts, however, possibly because our optical photometry for these objects is corrupted by scattered light from the nearby bright AO guidestar. Finally, quantitative SED fits and photometric redshifts z_ph are presented for numerous galaxies of interest in two fields. Most galaxies in the spatially compact group around Q0835+580 are consistent with being at the quasar redshift z_q. One of these is a candidate very old, non-star-forming galaxy, while the others appear to have ongoing star formation. Many very and extremely red objects across both fields have z_ph ~ z_q, and significant dust is required to fit most of them, including about half of the objects whose fits require relatively old stellar populations. Large reddenings of E(B-V) ~ 0.6 +/- 0.3 are also required to fit four J-K selected EROs in the Q1126+101 field, but all but one of them have best-fit redshifts z_ph > z_q. These objects may represent a population of dusty high-redshift galaxies underrepresented in optically selected samples.

Taken together, these observations reinforce the claim that radio-loud quasars at z > 1 can be found in galaxy overdensities. Ongoing star formation with moderate amounts of dust seems to be common among all but the very reddest galaxies in these overdensities.