The spectral-type/luminosity and the spectral-type/satellite-density relations in the 2dFGRS

¹ Dipartimento di Astronomia, Università di Bologna, V. Ranzani 1, 40127 Bologna, Italy e-mail: birgit.kelm@unibo.it
² INAF – Osservatorio Astronomico di Capodimonte, V. Moiariello 16, 80131 Napoli, Italy

Received: 10 June 2004
Accepted: 19 June 2005

Abstract

We examine the relative fractions of passive (type 1), quiet-SF (type 2) and active-SF (type 3+4) galaxies as a function of luminosity and number of neighbours in several volume-limited samples selected from the 2dFGRS. Neighbours are counted within 1  Mpc projected distance and ±1000 km s^-1 depth. We apply a maximum magnitude difference criterion and require neighbours to be fainter than the galaxy itself. We show that, whatever the environment, passive galaxies dominate in bright samples and active-SF galaxies in faint samples, whereas quiet-SF galaxies never dominate. We further show that in bright samples (M_B – 5 h₇₅ ≤) the fraction of passive galaxies grows steadily with fainter neighbour density, whereas in faint samples a threshold-like dependence is observed. This suggests that the spectral-type/density (≈morphology/density) relation extends to the intermediate dense environment, but only in the surroundings of luminous galaxies and that it reflects an enhancement of the number of satellites rather than stronger clustering among galaxies themselves. Our analysis indicates that, in general, luminosity is a good tracer of galaxy halo mass and that it dominates over environment (satellite density) in setting the spectral type mix of a population. However, minority populations exist, such as luminous SF galaxies and faint passive galaxies, whose luminosity is an inaccurate tracer of halo mass.