2 Sample and imaging

 The M101 group, with $D \approx 6.5$ Mpc (Karachentsev 1996), is the most distant one in the 10 Mpc volume. It is also the poorest group of all, including the LG. It is completely dominated by M101 itself: the second-ranked group member, NGC 5585, is already 3 magnitudes fainter than M101. Only 13 members and possible members of the group are known to date, half of which are lying very close to M101 and can therefore be regarded as M101 satellites. With one exception (the dwarf elliptical UGC 8882) they are all late-type dwarfs (Sd, Sm, Im). One peculiar feature of the group is its luminosity function: the population of very faint and diffuse dawarfs (elliptical or irregular), which is so frequent elsewhere, is apparently simply missing here (the faintest member known is as bright as $M_{B}\sim -14$). We have therefore made an attempt to find new candidate members on deep POSSII Schmidt films, but found only one additional possible member (the BCD MGC9-23-21). A recent blind HI survey of the M101 area has also not resulted in a single new member of the group (Kraan-Korteweg et al. 1999, in preparation). It will take surveys of highly increased sensitivity to uncover the sought-for exponential rise of the luminosity function of the M101 group.

  

Figure 1: Sky map (conical equidistant projection) of the M101 group and surrounding field. The galaxies are coded according to group membership and types. Group members and possible members are indicated by triangles and inverted triangles, respectively, field galaxies by squares. Early types are represented by filled symbols and late types by open ones. M101 is plotted as a large cross. The galaxies M51 and M63 are shown with smaller crosses. UGCA 342 is shown as a cross along with M63 in the lower right of the map (see Sect. 6)

  

Table 1: M101 members (M), possible members (PM) and Field dwarfs (F) observed

In Table 1 we give a complete list of the 11 presently known members (M) and possible members (PM) of the M101 group as well as 10 field (F) dwarfs that were imaged during the same run. This list was prepared by B. Binggeli based on the catalogue of Schmidt & Boller (1992a). A map showing the distribution of these objects on the sky is shown in Fig. 1, where the galaxies are coded according to their type and group membership. A gallery of images is given in Fig. 2. It should be noted that the objects listed in Table 1 and the images displayed in Fig. 2 include all M101 group members known to date with the exception of the two giant members M101 and NGC5585 for which data and images are given in Sandage & Tammann (1974, 1987).

The photometry of the 21 objects listed in Table 1 is based on images taken during eight nights in March 1997 on the $1.2\:\mathrm{ m}$ telescope of the Observatoire de Haute-Provence (OHP). They are 40 minute Cousins B and 20 minute Cousins R exposures. The camera used was the No. 2 Tektronix $1024 \times 1024$ CCD. One pixel corresponds to $0\hbox{$.\!\!^{\prime\prime}$}69$, giving a frame size of $11\hbox{$.\mkern-4mu^\prime$}8 \times 11\hbox{$.\mkern-4mu^\prime$}8$. The gain was set to $3.5\,\mathrm {e^-}$ per ADU, and the CCD was read out in the fast mode, with a readout noise of $8.5 \,\mathrm {e^-}$. The seeing during the observing run varied between $2.5\hbox{$^{\prime\prime}$}$ and $4\hbox{$^{\prime\prime}$}$ (FWHM), which is relatively poor but sufficient for our purposes.

   Figure 2: B-band CCD images (except NGC 5204 for which only a R-band image was available) of the M101 group dwarf members and the dwarfs in the vicinity of M101, shown in the same order as listed in Table 1. The scale is the same for all pictures and is given by the size of one image side $=5\hbox{$.\mkern-4mu^\prime$}9$. North is up and east to the left

 

  Figure 2: continued

   Figure 2: continued

   Figure 2: continued