In principle, much information about modes of star formation can be obtained from spectroscopy of the HII regions, particularly by evaluating their metallicities. The recent contribution by Lee (2000) stands out, because he compared the metallicities of HII regions in 12 VC dIs with those of a comparison sample of dIs in the field.
Lee (2000) derived a method of predicting the total HI content of a galaxy from its oxygen abundance. This allowed him to establish the HI deficiency of dIs in the VC, which he found to correlate with the local X-ray flux. According to Lee, this is a supporting finding to the assertion that HI is stripped from galaxies, and in this case dIs, by ram interaction with the hot intracluster medium. Lee also identified an isolated HI cloud near VCC 1249 containing a low luminosity dI; he proposes this to be the result of a tidal interaction between VCC 1249 and M49, which loosened the ISM in the dwarf galaxy, followed by ram pressure stripping, the formation of a separate entity HI cloud, and subsequent star formation (see also Lee et al. 2000).
The issue of ram pressure stripping was discussed extensively at this meeting, mainly in the context of spiral galaxies. The case of NGC 4338 may be another example of stripping for the case of a dwarf galaxy (Yoshida et al. 2002). Also, the strange features shown here by Manfred Stickel regarding M86 and its dwarf neighbor VCC 882 may also point to stripping, although some kind of tidal interaction cannot be ruled out. The external HI contours, the dust feature located between the two HI peaks (Elmegreen et al. 2000), and perhaps VCC 882 itself, may all originate from an interaction of NGC 4402 with M86. Obviously, detailed modeling that could account for all the ingredients (stars, dust, and HI gas) is required for this object.
Another piece of evidence supporting the claim of HI gas removal from
cluster dwarf galaxies can be found in the recent survey of HI in
compact galaxies located in voids
(Pustilnik et al. 2002).
It was shown there that the dependence of the total HI content on the
luminosity of a compact galaxy depends on the local galaxy density. This
is emphasized in the two panels of Figure 3.
The top panel shows the relation for the Virgo BCDs in the
Almoznino & Brosch (1998)
sample. The bottom panel shows it for a combined sample of compact
galaxies selected to be at large distances from their nearest neighbors,
and considered to represent a void population of compact galaxies. The
comparison is done through the
parameter,
defined from the relation M (HI) / LB
LB.
Fits to the distribution of points show that
= 0.07 for
Virgo BCDs while
= -0.40 for
void BCDs. The local neighborhood density of galaxies seems to influence
the ability of a galaxy to retain its HI not only in the dense cores of
clusters, but also in the typical regions of the Local Universe.
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Figure 3. The HI mass to blue light ratio vs. the absolute B magnitude for a sample of extremely isolated (void) compact galaxies (lower panel) and a sample of BCDs in Virgo. The solid line in each panel is the best-fit relation for the specific sample described by the panel. The dotted lines are best-fits for other samples considered by Pustilnik et al. (2002), two of which (not shown here) are of compact galaxies in the Local Supercluster and in the General Field. |