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3. HI line observations of M 81 group dwarfs

Of the 40 objects listed in Table 1 (click here), only 13 have previously been detected in the 21cm HI line (see also Table 3 (click here)). Four galaxies (Kar 1N, A952+69, Kar 61, and Garland), as well as 8 of the peculiar low-surface brightness objects first noted by Börngen et al. (1984) have never before been observed in the 21cm line. These 4 galaxies, as well as the four brightest objects of unknown nature, i.e. Kar 8N, and Anon 5, 6 and 7, were selected for our deep HI search.

Four objects (HolmbergI, Kar 52 (= M 81 dwarf A), Kar 73 and Kar 3N) were mapped in HI with the VLA (Westpfahl & Prugniel 1994). The first two look like rather face-on incomplete rings with evidence for slow rotation, while the latter two resemble discs with a central hole seen edge-on and complex or confused velocity fields without clear signs of systematic rotation in the dwarf systems. No HI profile parameters are given in this reference - note, that neither Kar 73 nor Kar 3N has been reported as detected in any single-dish study, including ours.

Fifteen of these galaxies have previously been searched for with the 90 m Green Bank (Schneider et al. 1992) and the 100 m Effelsberg dish (Huchtmeier & Skillman 1994, 1997), with an rms noise varying from 7 to 15 mJy, and 4 with an rms noise of 18 mJy only for earlier work by Fisher & Tully (1981) at Green Bank and Effelsberg. We reobserved these galaxies at Nançay with considerably higher sensitivity (tex2html_wrap_inline1618 rms).

The total sample of 23 dwarf galaxy candidates selected for observation in HI at Nançay are marked in the last column of Table 1 (click here). The searches for 21cm HI line emission were made with the Nançay decimetric radio telescope in the period December 1995 - September 1996. The telescope is a meridian transit-type instrument, which permits the tracking of such high-declination galaxies for about 3 hours per day. The telescope has the equivalent collecting surface of a 94 m diameter round dish, but its HPBW is about 4' tex2html_wrap_inline1622 22' (tex2html_wrap_inline1626), due to its elongated geometry. Most objects were observed for about 4 hours each, while the 3 low-surface brightness Anonymous objects were observed for about 8 hours each. It should be noted that the telescope is somewhat less sensitive at high declinations, like those observed for the present survey (tex2html_wrap_inline1628), then at lower declinations, due to its geometry.

The 1024 channel correlator set-up used permitted an HI line search in both H and V polarisation in the radial velocity range of -529 to 1826 tex2html_wrap_inline1496, with a 190 tex2html_wrap_inline1496 overlap between the filter banks, from 548 to 747 tex2html_wrap_inline1496, at a resolution of about 6 tex2html_wrap_inline1496.

After averaging the individual spectra, the data was smoothed to a velocity resolution of 12.7 tex2html_wrap_inline1496, and third-order baselines were fit. For the conversion of antenna temperature to flux density in mJy we used the standard declination-dependent calibration relation established by the Nançay staff through regular monitoring of strong continuum sources, and our observations of a sample strong late-type galaxies (Matthews et al. 1997) observed earlier at Nançay as well as with other telescopes.


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