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Дата изменения: Sat Mar 11 12:16:53 2000
Дата индексирования: Sat Sep 11 21:00:18 2010
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H I and dynamical mass

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 $M_{\rm HI}$ for HS 0822+3542 are presented in Table 1. The small H I mass 2.4 $\times10^{7}$$M_{\odot}$ 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)/L_B = 1.40 $M_{\odot}$/$L_{\odot}$, 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 $\mu_{B}$ = 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$\times$10⁸ $M_{\odot}$, one order of magnitude larger than the total visible mass of the galaxy $M_{\rm neutral}$ + $M_{\rm stars}$ + $M_{\rm HII~region}$.

Even rather unprobable case of H I radius equal to the optical one results in the total dynamical mass $\sim$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.