Probing the evolution of early-type galaxies using multi-colour number counts and redshift distributions
Fumiaki Nakata, Kazuhiro Shimasaku, Mamoru Doi, Nobunari Kashikawa, Wataru Kawasaki, Yutaka Komiyama, Sadanori Okamura, Maki Sekiguchi, Masafumi Yagi, Naoki Yasuda
Abstract
We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e., no number density change or morphology transition), and examine whether these models are consistent with observed number counts in the B, I and K bands and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time scale
τ
SF
and the time
t
gw
when galactic wind blows in addition to several other conventional parameters. We find the single-burst model (
τ
SF
=0.1 Gyr and
t
gw
=0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with redshift distributions of early-type galaxies in the field environment due to overpredictions of galaxies at $z\gsim1.4$ even with strong extinction which is at work until
t
gw
. In order for dust extinction to be more effective, we change
τ
SF
and
t
gw
as free parameters, and find that models with $\tau_{SF}\gsim0.5$ Gyr and
t
gw
>1.0
Gyr can be made consistent with both the observed redshift distributions and number counts, if we introduce strong extinction
(E(B−V)≥1
as a peak value). These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.