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A. Dwarf galaxies or galactic cirrus clouds?

  During their extensive Schmidt survey of the M 81 group, Börngen et al. (1984) discovered an unusual cluster of 11 low-surface brightness objects in the SE corner of the region. Some of these objects, notably UGCA 220 and DDO 87 had been known before. DDO 87 is clearly a dwarf irregular galaxy, although its relatively high velocity renders it a questionable member of the M 81 group. UGCA 220, on the other hand, has not (yet) been detected in the radio (see Van Driel et al. 1998, but is morphologically fairly convincing as a galaxy (see Fig. A3 (click here)).

However, the remaining nine objects, all of which appear with a question mark in Table 1 (click here), are indeed very strange, cloudy looking and we must doubt their extra-galactic nature altogether. In our second observing campaign (Paper II) we have obtained images for three of these objects. Deep R band exposures are shown in Fig. A1 (click here) (covering objects BK 8N and BKK 11) and Fig. A2 (click here) (with BK 7N). Also shown, in Fig. A3 (click here), is the environment of UGCA 220 which contains numerous other, non-catalogued clouds, most notably a spectacular, boomerang-shaped feature that is apparently associated with the background spiral galaxy UGC 5932 (Börngen et al. 1984). The nature of these fuzzy objects has been discussed by Börngen et al. (1984) who rightly point out that they might simply be galactic cirrus clouds. The region of M 81 is indeed known to be an area of great confusion between galactic and extra-galactic objects - in the optical (Sandage 1976) as well as in the radio (Van Driel et al. 1998, and references therein). Nevertheless, the galactic case is dismissed by Börngen et al. (1984) on the grounds that these clouds appear to be fairly isolated with respect to the prominent filamentary nebulosities around M 81/M 82 found on a deep Schmidt plate by Sandage (1976).

However, this argument is no longer valid, as an IRAS sky survey map of the region in the tex2html_wrap_inline2566 band, reproduced here as Fig. A4 (click here), clearly shows some large-scale cirrus structure around R.A. = 11tex2html_wrap_inline2568 and tex2html_wrap_inline2570 = 65tex2html_wrap_inline2572 - very close to our cloudy objects whose positions are superposed on the IRAS picture.

On the other hand, Börngen et al.'s (1984) interpretation of this cluster of low-surface brightness objects as the fragmentation of a protogalaxy into separate dwarf galaxies - while it cannot be ruled out completely - appears unlikely.

One might think that the integrated colour is a tool to discriminate between a galactic cloud and an extragalactic stellar system. Since dust clouds are seen by the scattered light of nearby stars, they should appear quite blue - bluer than a galaxy. Indeed, a preliminary measurement of the colours of BK 7N, BK 8N and of the boomerang region in Fig. A3 (click here) by Pierre Lesaffre (see Paper II) gives tex2html_wrap_inline2574, which is rather blue. However, independent measurements of the colours of reflection nebulae and cirrus clouds have reveiled surprisingly red colours (Witt & Schild 1985; Guhathakurta & Tyson 1989). In any case, cirrus clouds and galaxies are largely overlapping in colour, rendering the two indistinguishable in terms of colour alone.

Nor would the surface brightness profile or the colour gradient help us much to discriminate the two kinds of object - again because there is a huge overlap in these properties. This is no surprise, because otherwise they could also be distinguished on purely morphological grounds.

Hence, for the time being, the nature of these low-surface brightness objects in the SE corner of the M 81 group remains unsolved. Only very high-resolution imaging could reveal the presence or absence of stars and thus settle the case. Nevertheless, the IRAS map shown in Fig. A4 (click here) makes us favour the cirrus cloud hypothesis and we will exclude these objects (but not DDO 87 and UGCA 220) from our forthcoming discussion of the properties of dwarf galaxies in Paper III.

Figure A1: 40 min. R image of BK 8 (middle) and BK 11 (top). The field of the image is tex2html_wrap_inline2578

Figure A2: 40 min. R image of BK 7N. The image field is as in Fig. A1 (click here)

Figure A3: 40 min. R image of UGCA 220 (lower right). The image field is as in Fig. A1 (click here)

Figure A4: IRAS 100 micron image of the M 81 region. The positions of the objects UGCA 220, DDO 87, BKK 4, BK 7N, BKK 6, BKK 7, BKK 8, BKK 9, BK 8N and BKK 11 are shown as crosses. Note the apparent association of these objects with a large-scale IRAS cirrus feature slightly to the east (left). The positions of M 81, M 82, NGC 2976 as well as NGC 3077 are also indicated

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