2 Sample and imaging

The basis of the chosen sample is the catalogue of nearby galaxies by Schmidt & Boller (1992), which lists all galaxies with radial velocities $\leq$ 500kms^-1 referred to the center of the LG, corresponding roughly to a distance limit of 10 Mpc. We selected 19 field dwarf candidates in the northern sky, i.e galaxies which could not be assigned to a group or a cloud of galaxies, and for which no reliable photometry was available (at least at the time of selection). A list of the objects selected and observed along with some basic data is given in Table 1. The columns of Table 1 are as follows:

Columns (2) and (3): identification of the observed galaxy;

Columns (4) and (5): their 2000.0 epoch coordinates (from NED);

Column (6): morphological type in the classification system of Sandage & Binggeli (1984);

Column (7): total B band magnitude, corrected for galactic extinction (from this paper);

Columns (8) and (9): heliocentric radial velocity in kms^-1(from the NED) and distance in Mpc (from Karachentsev et al. 1999);

Column (10): absolute B band magnitude, based on the data given in Cols. (7) and (9).

The observed dwarfs are spread out over almost half of the northern sky. As the table shows, velocities and distance estimates are available for most of the selected objects. The distances from Karachentsev et al. (1999), based either on the Hubble flow velocity or the luminosity of the brightest resolved blue stars, have to be regarded as estimates only. DDO97 with an estimated distance of 14 Mpc would formally fall off the sample limiting distance of 10 Mpc, but because of the basic distance uncertainty just mentioned we have chosen to keep this galaxy in our sample. Otherwise the distances are in the range 5.3 < d < 7.9, which is beyond the well-known groups of M 81 and M 101. Accordingly, our sample dwarfs are relatively bright, with a median absolute magnitude M_B = -14.9. Kar49 and Kar50, though without known distance, are morphologically (by their very low surface brightness) very likely nearby dwarfs. One galaxy in the sample, UGC 2689, is probably not nearby but is likely a S0 member galaxy of the Perseus cluster of galaxies (Abell 426) - despite a velocity of 277 kms^-1 listed in the NED, which we deem wrong (see Sect. 5, UGC 2689). The photometric data of this galaxy are presented here nonetheless.

With the possible exception of Kar49 (which might be a dwarf elliptical), and disregarding UGC 2689, the sample galaxies are all late-type dwarfs, which is of course expected for the low-density field. Mostly, they are typical irregulars classified Im, but there are also some intriguing objects included, like NGC 2537 (classified as Sm/BCD), that seem to undergo a burst of star formation. A CCD gallery of our sample galaxies is shown in Fig. 1.

The CCD imaging was carried out between November 7, 1999 and January 8, 2000 on the 1.2-m telescope of the Observatoire de Haute Provence (OHP). We used the No. 2 Tektronix $1024 \times 1024$ CCD camera. For 12 (7) galaxies the frames are 40 (10) minutes B and 20 (5) minutes V and R exposures. The field of view is $11 \hbox{$.\mkern-4mu^\prime$ }8 \times 11 \hbox{$.\mkern-4mu^\prime$ }8$ with a resolution of $0 \hbox{$.\!\!^{\prime\prime}$ }69$ per pixel. The gain was set to 3.5e^- per ADU, and the CCD was read out in the fast mode, with a readout noise of 8.5e^-. Seeing was between $1 \hbox{$.\!\!^{\prime\prime}$ }5$ and $3 \hbox{$.\!\!^{\prime\prime}$ }3$.

Figure 1: B-band CCD images. Image size: $5\hbox {$.\mkern -4mu^\prime $ }9 \times 5\hbox {$.\mkern -4mu^\prime $ }9$. North is up and east to the left.

Figure 1: continued.