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The integrated H I line flux, the characteristic widths of 21 cm line profile W50and W20 (for a Hanning smoothing of 10.6 km s-1), and the derived H I
mass
for HS 0822+3542 are presented in Table 1.
The small H I mass 2.4
of this galaxy
is consistent with its low optical luminosity.
The very narrow H I profile is indicative of its very low amplitude of rotational
velocity, which does not exceed 30 km s-1. It is difficult to assess the
inclination angle correction, since the optical morphology can be
unrelated to global properties of the associated H I cloud, as exemplified by the
case of SBS 0335-052 (Pustilnik et al. [2000]).
The role of chaotic gas motions is, in general, more important in very low mass
galaxies, where the amplitude of random velocities reaching ten km s-1 or more
can be commensurate with the rotational velocity.
The measured H I mass and the profile width are in the range characteristic of very
low mass galaxies. The mass-to-light parameter M(H I)/LB =
1.40 /
,
is comparable to those of I Zw 18 and SBS 0335-052E.
It is not as high as in some gas-rich dwarfs from van Zee et
al. ([1997]), but these galaxies, although they
have a few H II regions, are in relatively quiescent state.
The extremely metal-poor BCGs discussed here experience significant
luminosity enhancement due to very intense current and recent
SF activity; this results in a significant decrease of their mass-to-light
ratios relative to their non-active state.
A rough estimation can be made also on the dynamical mass of HS 0822+3542.
From the width of H I profile at the 20% level,
a maximum rotation velocity of 30 km s-1 can be assumed.
The extent of the H I cloud associated with a BCG is normally many times
larger than its optical size.
The optical radius R25 is the radius
of a disc galaxy at the isophotal level
= 25.0 mag arcsec-2.
A conservative lower limit to the ratio of H I-to-optical radii
is 4 (see, e.g. Taylor et al. [1995];
Chengalur et al. [1995]; Salzer et al. [1991];
van Zee et al. [1998]; Pustilnik et al. [2000]).
By equating gravitation and centrifugal force at the edge of H I disc
an estimate of the dynamical mass inside 1.5 kpc is obtained; this is
3.4
108
,
one order of magnitude larger than the total visible mass of the
galaxy
+
+
.
Even rather unprobable case of H I radius equal to the optical one
results in the total dynamical mass 3 times higher than the visible mass.
Hence, HS 0822+3542 like other extremely metal-deficient dwarf
galaxies, is dynamically dominated by a dark matter halo, supporting the
modern view of primeval galaxy formation process (e.g. Rees [1988]).
In turn the mass of its DM halo is one of the smallest for galaxies.