HI observations of southern LSB dwarf galaxies from the Karachentsev catalog

Walter K. Huchtmeier , Matthias Ehle , PASA, 16 (1), in press.

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Most of the observed HI-profiles (of the whole sample) are narrow (13.5 kms-1 for the narrowest line) which is typical for dwarf galaxies (slow rotation). From the distribution of radial velocities of the detected galaxies within our sample we derive that most of these galaxies are within the Local Supercluster, about 25% of the detected galaxies are within the Local Volume. There are only a few low surface brightness (LSB) background objects. Typical global values for the galaxies in the Local Volume are fainter or equal to an absolute B magnitude of -15, linear diameter of 1.5 kpc, average HI mass of 5

$10^{7} M_{\odot}$, and a total mass of 6

$10^{8} M_{\odot}$ (the typical corrected linewidth being of the order of 50 kms-1).

Figure 2: A digital sky survey image of the galaxy A0523-87 with contours of the HI distribution. Contour levels are in steps of 10% of the peak (which corresponds to a column density of 6 1020 cm-2 of HI atoms), the lowest contour corresponds to 30% of the peak value. This galaxy is typical for our sample as it shows a low surface brightness with the HI distribution definitely larger in extent than the optical counterpart. Often there are asymmetric features seen in lower contour levels of the HI distribution.
\begin{figure} \begin{center} \hspace*{0.5cm} \psfig{figure=huchtf2.ps,height=14cm} \end{center} \end{figure}

The correlation between linear optical diameter in kpc (at the D25 level, de Vaucouleurs et al. 1976) and the total HI mass for the whole sample is the same as observed for the Kraan-Korteweg-Tammann sample (Kraan-Korteweg and Tammann 1979, for the HI observations e.g. Huchtmeier and Richter 1988). The same is true for the correlation between HI mass and total mass.

In nearly all cases the HI distribution is centered on the optical position of the galaxy. However, lower contours of the HI distribution often show asymmetric shapes (see Fig.2) and two galaxies are definitely disturbed in the HI distribution and/or the velocity field. On average the HI distribution is larger than the optical extent (D25) by a factor of 3.

Figure 3: The MHI/LB ratio for the detected galaxies (filled circles) are plotted versus blue luminosity LB. Upper limits are shown for five undetected galaxies with known redshifts (triangles).
\begin{figure} \begin{center} \hspace*{0.5cm} \psfig{figure=huchtf3.ps,height=11cm} \end{center} \end{figure}

The ratio of the total HI mass to blue luminosity MHI/LB often is taken as a measure of the relative HI content. In Fig.3 the MHI/LB ratio is plotted versus optical blue luminosity LB. The full line represents the relation found for the sample of nearby galaxies (e.g. Huchtmeier and Richter 1988). Part of the scatter might be due to uncertainties in the observable quantities, in many cases blue magnitudes are precise to 0.5 magnitudes only. For the upper limits 1 Jy kms-1 was assumed in all cases. This does change with the noise and the assumed line width (i.e. rotational velocity) of the undetected galaxies; r.m.s. errors correspond to about five times the size of the symbols in Fig.3. In addition to this we may miss some flux with the interferometer (missing flux) as the observed HI emission extends over more than 2' per channel for over 60% of the galaxies. In general the deduced values of MHI/LB seem to be lower than expected from the comparison sample by a factor of 2. However, in view of the error discussion above more precise optical magnitudes are needed. Therefore we recently started an observational project to obtain multicolor photometry in order to improve the total magnitudes and to investigate the light distribution of these low surface brightness galaxies. In the work of Mathewson et al. (1995) on single-dish observations of late-type galaxies two of our objects are included, AM0637-404 and AM1013-394. For AM0637-404 they detected a narrow emission line at 272 kms-1, classified as a galactic HVC, and an emission line at 821 kms-1 in good agreement with our velocity measurement of 824 kms-1. Their HI flux for this galaxies is twice as high as our value, a possible hint that we might miss some of the HI flux.

The upper limits of MHI/LB in Fig.3 are more or less close to our sensitivity limit except one very low value which corresponds to AM1012-443. There is an HI detection for AM1013-394 at a radial velocity of 263 kms-1 (Mathewson et al. 1995) with a flux of 2 Jy kms-1 which we probably did not see due to our sensitivity limit. Their other HI line for this source has a radial velocity of 2982 kms-1 which happened to lie at the edge of the bandpass of our observations and was not further considered.

Since the first compilation of galaxies in the LV in 1979 by Kraan-Korteweg and Tammann the number of known galaxies increased from 179 to 303 galaxies (Karachentsev 1998, priv. comm.). This search for nearby dwarf galaxies - the first part of which is presented here - is expected to further increase this number.

Next Section: Acknowledgements
Title/Abstract Page: HI observations of southern
Previous Section: Observations
Contents Page: Volume 16, Number 1

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