4 Conclusions

Taking advantage of the high quality of the Nordic telescope optics, we carried out a detailed two-colour imaging of 20 irregular galaxies. Most of them were resolved into stars for the first time. We made photometry of the brightest stellar population and also measured the integral magnitudes and colour indices of the galaxies. Based on the magnitudes of the brightest blue stars we estimated distances to the galaxies, which lie in the interval of [5.1 - 9.4] Mpc.

Besides UGC 3698 and UGC 5086, the studied galaxies are well isolated systems, whose crossing time with respect to their nearest massive neighbours is of the order of the cosmological time, $\sim\!10^{10}$ years. Evolution of such isolated dwarf galaxies should not be affected by tidal disturbances. Nevertheless, these objects show impressively diversified morphologies, which are likely to result from their starburst activity.

The absolute magnitudes of the galaxies lie in the interval [-12.5, -16.9] with a median of -14.6, and their integral colours, $(B-V)\rm _T^0$, are concentrated within [0.30, 0.70] with a median of 0.47. Outside this colour index interval there is only one galaxy, UGC 5340, with $(B-V)\rm _T^0 = 0.17$. It has also a high M(HI)/L_B ratio, being probably a young system.

Note that among these 20 nearby irregular dwarfs only 4 objects are IRAS sources. If we rank them according to their apparent magnitude, they occupy positions #3-6 in our sample, whereas their classification by the mean surface brightness give them positions #2-5. Therefore, the IR-flux from dwarf galaxies seems to exhibit a correlation both with their optical flux and with their surface brightness. Besides, the dwarf galaxies detected in the IR have a slightly bluer integral colour.

Because the radial velocities of the considered galaxies can not be much affected by neighbours, one can use them to estimate the local value of the Hubble parameter, H = V₀/D. According to the data of Table 18, individual estimates of H have a median of 65 km s^-1/Mpc. This value is in reasonable agreement with other estimates of the Hubble parameter.

Acknowledgements

We are indebted to Leo Takalo and Pekka Heinamaki, who took part in the observations, and Serafim Kajsin, who made preliminary processing of the data. This work is partly supported by INTAS-RFBR grant No. 95-1390.