Abstract

I find a significant excess of K>19 galaxies in the fields of 31 z=1--2 RLQs, on two spatial scales. One component is at <40'' from the quasars and is significant compared to the galaxy surface density at >40'' in the same fields. The other component appears roughly constant across the fields to ~100'' from the quasars and is significant compared to the galaxy surface density seen in random-field surveys in the literature. The <40'' component may be produced by as few as ~25% of the fields, but the large-scale component is present in >~50% of them.

The r-K color distributions of the excess galaxy populations are indistinguishable from each other and are significantly redder than the color distribution of the field population, consistent with the excess being predominantly at z>1. There is no evidence that the excess is associated with intervening MgII absorption systems. However, there is a deficit of blue (r-K<~3.5) galaxies at K=20--21 which is difficult to understand as either real or spurious. Additional control-field data would be useful in verifying the reality of this deficit. The magnitudes and colors of the excess galaxies are thus consistent with a population of predominantly early-type galaxies at the quasar redshifts, such as would be found in quasar host clusters or groups.

Spectroscopic surveys in these fields are needed to confirm the association of the excess galaxies with the quasars, to robustly determine the strength of the overdensities at the quasar redshifts, and to constrain the spectral properties of the excess galaxy population. Assuming that the excess galaxies are located at the quasar redshifts, the average excess within 0.5h_75^-1 Mpc (~65'') of the quasars corresponds to Abell richness class ~0+-1 compared to the galaxy surface density at >0.5h_75^-1 Mpc from the quasars, and to Abell richness class ~1.5+-1.5 compared to the galaxy surface density from the literature. This suggests that on a large scale (>~0.75h_75^-1 Mpc) RLQs at z=1--2 are located within clusters and/or large scale galaxy structures of Abell richness ~1, and that on a smaller scale (<~0.5h_75^-1 Mpc) within those structures, RLQs can be located in unremarkable ``field'' environments or in groups or clusters up to Abell richness ~0.

By assuming the excess galaxies are at the quasar redshifts and fitting their K-band luminosity function, I find --0.65 (-0.55,+0.41) magnitudes of luminosity evolution in M_K^* to =1.67. This is in contrast to the trends seen at z>1 by Cowie et al. (1996) and Arag'on-Salamanca et al. (1994), but plausibly in agreement with the work of Schade et al. (1996).

For four fields with data in at least rJK_s, I find that the SEDs of most of the excess galaxies are consistent with them being 2--3 Gyr old early-type galaxies at the quasar redshifts of z~1.5, but that there are galaxies whose SEDs cannot be fit by such simple models. Several objects have SEDs consistent with being 4--5 Gyr old at z~1.5 and a number of others are consistent with old but dust-reddened galaxies at the quasar redshifts. These potentially different galaxy types suggest that there is considerable dispersion in the properties of early-type cluster galaxies at z~1.5. Spectroscopic followup will be needed to confirm this suggestion. In particular, age determinations from deep spectra of the candidate 4--5 Gyr old galaxies offer the possibility of constraining the cosmological model by requiring a relatively old universe at large lookback times.

There are also several dozen galaxies in the four fields with good J data (and particularly in the Q1126+101 field) whose SEDs are best explained if they are background galaxies at z>~2.5. Many of these galaxies seem to be dusty or to have composite stellar populations, or both, and some may be already >~2 Gyr old at z>~2.5. These latter galaxies may also offer the possibility of constraining the cosmological model.

Papers

Data

"Scaled" color images

Q0835+580 (3C205) Field rJKs Scaled Color Image

Color image of the field of Q0835+580 (z=1.534) using r, J, and Ks images to drive the blue, green and red color guns, respectively. Saturation occurs at r=24, Ks=20, and J=21 mag/arcsec^2. North is up and East is left; the area in black is approximately 3.25' by 3.25'.

Q1126+101 Field rJKs Scaled Color Image

Color image of the field of Q1126+101 (z=1.516). See key to color picture of Q0835+580 for details. The area in black is approximately 3.3' by 3.3'.

Q2345+061 Field rJKs Scaled Color Image

Color image of the field of Q2345+061 (z=1.540). See key to color picture of Q0835+580 for details. The area in black is approximately 3.8' by 3.2'. The J image of this field has 2.68" seeing and does not reach particularly deep, accounting for the different appearance of the colors in this picture compared to the previous two Figures.

"Faint" color images

Q0835+580 (3C205) Field rJKs Faint Color Image

Color image of the field of Q0835+580 (z=1.534) using r, J, and Ks images to drive the blue, green and red color guns, respectively. The saturation level in each filter is 8 times the iterative image RMS so as to emphasize the faintest galaxies. North is up and East is left; the area in black is approximately 3.25' by 3.25'.

Q1126+101 Field rJKs Faint Color Image

Color image of the field of Q1126+101 (z=1.516). See key to color picture of Q0835+580 for details. The area in black is approximately 3.3' by 3.3'.

Q2345+061 Field rJKs Faint Color Image

Color image of the field of Q2345+061 (z=1.540). See key to color picture of Q0835+580 for details. The area in black is approximately 3.8' by 3.2'. The J image of this field has 2.68" seeing and does not reach particularly deep, accounting for the different appearance of the colors in this picture compared to the previous two Figures.