A significant trend of the global star formation rate (SFR) of galaxies with the projected
clustercentric distance from rich clusters of galaxies is well documented in the local universe
(
0.05<z<0.1). The mean SFR, as traced by the equivalent width of the H
line (Kennicutt 1989),
is found to decrease with decreasing distance from rich clusters (Lewis et al. 2002).
This pattern is dominated by the "morphology segregation'' effect (Dressler 1980), i.e.
there are more
elliptical and spheroidal galaxies with little or no current star formation
than the star forming galaxies in the center of rich clusters. What physical mechanism
(nature vs. nurture) is responsible for the morphological transformation taking place in
the densest environments is however not yet fully understood. To shed light on the various
possibilities, i.e. galaxy harassment (Moore et al. 1996, 1998), tidal stirring (Mayer et al. 2001)
or ram pressure stripping (Gunn & Gott 1972), it is crucial to establish observationally if,
beside the morphology segregation,
galaxies of a given morphological type, namely the spirals, are affected by a systematic SFR decrease
toward the center of nearby clusters.
If on the one hand Kennicutt (1983) found that spirals in the Virgo cluster
have their mean SFR as much as a factor of two lower than isolated galaxies,
Gavazzi et al. (1998) did not confirm this evidence in the Coma and A1367 clusters.
Moreover Iglesias-Paramo et al. (2002) found that the shape of the H
luminosity function of these two clusters does not differ significantly from the one
of isolated galaxies.
The result of Kennicutt (1983) was based on only a dozen giant galaxies with
H
measurements from aperture photometry,
thus requiring a confirmation on a larger sample with modern imaging data.
With the aim of solving this riddle we undertook an H
imaging survey
of two optically complete samples of galaxies.
The first is composed of nearly isolated objects selected from the CGCG
(Zwicky et al. 1961-68) in the bridge between Coma and A1367,
which we observed down to the limit of 15.7 mag. This constitutes our reference sample of non-cluster objects.
The cluster sample is focused on A1367, the Coma and the Virgo clusters.
We took H
imaging observations of these regions (Gavazzi et al. 1998; Gavazzi et al.
2002a, Paper I of this series; Boselli & Gavazzi 2002, Paper II;
Boselli et al. 2002b, Paper III). Our own observations were complemented with data taken from
the literature (Kennicutt & Kent 1983; Romanishin 1990; Gavazzi et al. 1991; Young et al.
1996; Koopmann et al. 2001).
Furthermore we performed a NIR imaging survey of the same regions (Gavazzi et al. 2000b and references therein), providing information on the old stars.
In the present paper we combine H
with NIR measurements to study
the young and the old components of the stellar population integrated over the
whole galaxy and we analyze the
properties of the stars as a function of the clustercentric projected distance, of the luminosity and
of the global gas content.
We postpone to a forthcoming paper the morphological aspects of the analysis related to
the spatial distribution of the young/old stars.
The present paper is organized as follows:
in Sect. 2 we briefly present the new H
imaging observations of 13 galaxies.
The sample used in the present investigation is illustrated in Sect. 3.
After defining the "birth-rate'' parameter (Sect. 4.1) and the "gas-deficiency'' parameter (Sect. 4.2),
we analyze in Sect. 5.1 the clustercentric dependence of the current star formation rate.
In Sects. 5.2 and 5.3 we study
the current star formation properties of galaxies in 3 local clusters
as a function of their global luminosity and gaseous properties.
The conclusions are briefly discussed in Sect. 6 and summarized in Sect. 7.
Copyright ESO 2002