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 .
Multi-object 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 GLF ’ s of the field and of rich clusters .
We use R-band images in 1.5 \times 1.5 degree ^ { 2 } 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 ( Zabludoff & Mulchaey 1998 ) , including 328 new galaxy velocities ( this paper ) .
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 \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim$% } } } } 0.4 - 0.6 h ^ { -1 } Mpc from the group center is fit adequately by a Schechter function with { M } ^ { * } _ { R } = -21.6 \pm 0.4 + 5 log h and \alpha = -1.3 \pm 0.1 .

We also find that ( 1 ) the ratio of dwarfs ( -17 + 5 log h \geq M _ { R } > -19 + 5 log h ) to giants ( M _ { R } \leq - 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 ( \alpha \approx - 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 ; \alpha = -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 GLF ’ s results mostly from the population of non-emission line galaxies ( EW [ OII ] < 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 BGG occupy different orbits ( i.e . , have not mixed completely ) and suggests that some of the populations formed at a different times .

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 .

Subject headings : galaxies : luminosity function — galaxies : evolution — galaxies : clusters : general — cosmology : large-scale structure of Universe