The properties of poor groups of galaxies. III. The galaxy luminosity function

The form of the galaxy luminosity function (GLF) in poor groups - regions of intermediate galaxy density that are common environments for galaxies - is not well understood. Multiobject spectroscopy and wide-field CCD imaging now allow us to measure the GLF of bound group members directly (i.e., without statistical background subtraction) and to compare the group GLF with the GLFs of the field and of rich clusters. We use R-band images in 1.5 × 1.5 degree² mosaics to obtain photometry for galaxies in the fields of six nearby (2800 < cz < 7700 km s^-1) poor groups for which we have extensive spectroscopic data, including 328 new galaxy velocities. For the five groups with luminous X-ray halos, the composite group GLF for group members with -23 + 5 log h < M_R < -16 + 5 log h and within projected radii of ≲0.4-0.6 h^-1 Mpc from the group center is fit adequately by a Schechter function with M^*_R = -21.6 ± 0.4 + 5log h and α = -1.3 ± 0.1. We also find that (1) the ratio of dwarfs (-17 + 5 log h ≥ M_R > -19 + 5 log h) to giants (M_R ≤ -19 + 5 log h) is significantly larger for the five groups with luminous X-ray halos than for the one marginally X-ray-detected group; (2) the composite GLF for the luminous X-ray groups is consistent in shape with two measures of the composite R-band GLF for rich clusters (Trentham; Driver et al.) and flatter at the faint end than another (α ≈ -1.5; Smith et al.); (3) the composite group GLF rises more steeply at the faint end than the R-band GLF of the Las Campanas Redshift Survey (LCRS; α = -0.7 from Lin et al.), a large volume survey dominated by galaxies in environments more rarefied than luminous X-ray groups; (4) the shape difference between the LCRS field and composite group GLFs results mostly from the population of non-emission line galaxies (EW [O II] < 5 Å), whose dwarf-to-giant ratio is larger in the denser group environment than in the field (cf. Ferguson & Sandage; Bromley et al.); and (5) the non-emission line dwarfs are more concentrated about the group center than the non-emission line giants, except for the central, brightest (M_R < M^*_R) group elliptical (BGG). This last result indicates that the dwarfs, giants, and BGGs occupy different orbits (i.e., have not mixed completely) and suggests that at least one of these populations formed at a different time. Our results show that the shape of the GLF varies with environment and that this variation is due primarily to an increase in the dwarf-to-giant ratio of quiescent galaxies in higher density regions, at least up to the densities characteristic of X-ray luminous poor groups. This behavior suggests that, in some environments, dwarfs are more biased than giants with respect to dark matter. This trend conflicts with the prediction of standard biased galaxy formation models.