6.1.4 H I Clouds
Because it was observationally difficult to confirm the recurrent burst
picture
against the idea that blue compact dwarf galaxies are young, several H I
surveys have provided independent clues. In their decisive early H I
survey,
Lo and Sargent (1979)
showed that the space density of protogalactic clouds required by the youth
hypothesis must be at least 8 Mpc-3, each cloud having a mass
of about 5 x 108
M which is 2 to
3 orders of magnitude higher than what they found.
H I-selected sample of clouds unavoidably turn out to have optical
counterparts. In all cases, possible local H I primeval
clouds have been found to be associated with stars
(Djorgovski 1990,
Impey et al. 1990,
McMahon et al. 1990,
Salzer et al. 1991,
Chengalur et al. 1995).
Since then, other surveys have been carried out to find
isolated H I clouds but without success
(Briggs 1997).
Some interesting examples of H I without coincident optical emission
were however provided by ``off-scans'' in 21cm line studies
(Schneider 1989,
Giovanelli and
Haynes 1989,
Chengalur et al. 1995,
Giovanelli et al. 1995)
but they are likely to be associated with (or bridge) nearby large
visible galaxies.
Tyson and Scalo (1988)
have suggested that the majority of
dwarf galaxies may be in a quiescent state, not forming young stars, and
that they might consequently be missed by optical selection methods. A
recent analysis of H I-selected galaxies
(Szomoru et al. 1994)
shows that H I searches do not yield a population of optically underluminous
galaxies. Some faint nearby galaxies with strong emission lines remaining
undetected in H I must have very little gas or store their gas in ionised or
molecular form
(Doublier et al. 1999).
Interestingly, the recent work by
Schneider et al. (1998)
shows that the H I mass
function may become steeper at faint masses, in analogy with an upturn
in the optical luminosity function at faint luminosities. However
detection limits for H I surveys remain quite high
(NHI ~ 1018 cm-2) hence
smaller pockets may still be hidden. On the other hand, isolated H I
clouds with masses and sizes comparable to
present dwarf galaxies would be easily seen with modern radio telescopes
hence they must be very rare. This conclusion is reinforced by the damped QSO
absorption lines that tend to occur in the haloes of bright galaxies and not in
smaller H I clouds
(Lanzetta et al. 1995;
however see discussion in
Sect. 8.2). It has been
noted that the diffuse ionising background could drive H I clouds under the
detection limits of current surveys (see
Corbelli and
Salpeter 1993a,
b).