Colour-magnitude diagrams (CMDs) were constructed for our photometric data to reveal the brightest blue (B-V<0.4) and red (B-V>1.6) stars. All of CMDs show a prominent blue bump with only a few or absent red stars. The photometric limit of our data (about in V) as well as the actually low number of red supergiants in dwarf irregulars can be the reasons for this situation. Therefore, the very few cases of distance estimation via the brightest red stars are presented just for reference. Our CMDs should not be too heavily contaminated by background stars because of the high galactic latitude of the objects in the Canes Venatici cloud.
Following Sandage & Tamman (1974a-d) and de Vaucouleurs (1978a,b) we measured the galaxy distance moduli using the mean magnitude of three brightest blue stars, <B(3B)>. Taking into account its dependence on the total magnitude of the parent galaxy, , we have:
where AB is the galactic extinction. The coefficients were calibrated using the galaxy distances derived from cepheids (Karachentsev & Tikhonov 1994).
The results of our measurement and some basic parameters of the dwarf galaxies under consideration are presented in Table 20. We used the PGC-ROM catalog (Paturel et al. 1992) as a source of global parameters of the galaxies. We also obtained the total B magnitude and the colour index from our frames, where it was possible. These data are marked with "c'' in Table 20. The Table contains the following columns: (1) - the galaxy name; (2) -the standard diameter in arcmin; (3) - the total B magnitude; (4) - the total colour index (B-V) measured within the standard diameter; (5) - the value of the galactic extinction; (6) -the heliocentric radial velocity (km/s); (7) - the radial velocity with respect to the centroid of the Local Group according to Karachentsev & Makarov (1996); (8), (9) -the mean magnitude and the mean colour for the three brightest blue stars in the galaxy; (10) - the distance modulus of the galaxy; (11) - the corresponding distance in Mpc.
|UGC 7131||1.5||15.8:c||0.7:c||0.05||+253||+226||>23||-||> 30.8||> 14|
Below we note some properties of the galaxies observed.
UGC 6782 = DDO 97. This irregular galaxy from van den Bergh's (1966) list is hardly resolved into stars. Within its apparent boundary we can distinguish about a dozen faint bluish stars. Using the mean magnitude of the three brightest blue stars (Nos. 13, 9 and 15) and we derive a distance modulus , which is typical of the Virgo cluster galaxies. Probably, the galaxy actually belongs to the Virgo periphery, and its low radial velocity is caused by the considerable peculiar motion with respect to the Virgo center.
UGC 7131. Unlike other galaxies in our sample, this one was observed in the V and I - bands only. It is entirely unresolved into stars. The Table 3 data refer to foreground stars and, probably, distant compact galaxies. The apparent absence of blue stars brighter than yields the galaxy distance: D>14 Mpc. As in the previous case, UGC 7131 may also belong to the Virgo cluster periphery. Note that a very blue star 10 with V-I=-0.80 is projected onto the galaxy northern side.
UGC 7356. The galaxy is unresolved on our frames. Taking into account its regular shape and yellow colour, we may consider UGC 7356 as a distant dwarf system of dE/dSph type. Its radial velocity was measured by Thuan & Seiter (1979). However, UGC 7356 lies only 5 arcmin away from the southern spiral arm of NGC 4258, which has a radial velocity = +450 km/s and a HI line width of about 420 km/s. That is why the radial velocity estimate for UGC 7356 should be considered as fictitious, because of confusion with the HI flux of the neighbouring spiral.
UGC 7559 = DDO 126. This irregular dwarf system has been resolved into stars by Hopp & Shulte-Ladbeck (1995) and Georgiev et al. (1997). On our CCD frames we detected and measured 192 stars. The colour-magnitude diagram for them is presented in Fig. 20. From the three brightest blue stars with we obtained the distance modulus . Note that some rather red stars are present in the main body of the galaxy. If the brightest one (No. 57) is assumed to be a red supergiant, then using the relation
Figure 20: V vs. (B-V) diagram for UGC 7559. Here and after open squares correspond to stars measured in the main body of the galaxy and crosses indicate foreground stars
we derive the "red'' distance modulus , in agreement with the "blue'' one. Expression (2) reflects a slight correlation between the luminosity of the brightest red supergiant and of its parent galaxy (Karachentsev et al. 1994). According to Hopp & Shulte-Ladbeck (1995) and Georgiev et al. (1997), in UGC 7559 the brightest blue stars are slightly brighter ( and , respectively) giving a smaller distance modulus: and , respectively.
UGC 7599 = DDO 127. The galaxies DDO 126 and DDO 127 are separated by 15 arcmin, and their radial velocities differ by 60 km/s only. Nevertheless, their spatial distances turn out to be different. Unlike its neighbour, DDO 127 has a regular shape with a considerable brightness gradient towards the center. We found the distance modulus of UGC 7599 to be equal to 29.19 from the brightest blue stars (see CMD for the galaxy in Fig. 21). However, the distance estimate via the brightest red star (No. 160) yields a modulus of 29.84, which differs largely from the blue one.
Figure 21: V vs. (B-V) diagram for UGC 7599
UGC 7605. The faint outlying part of the galaxy has a more regular shape than its patchy core. According to our aperture photometry the integrated colour of the galaxy increases smoothly from B-V=+0.19 in the center to +0.38 within the faintest isophotes. This colour gradient indicates clearly the existence of a young blue stellar population in the center of the galaxy as well as an older population dominated at the edge. The galaxy distance estimate via the brightest blue stars () is in agreement with the red one ().
UGC 7639. Similarly to the previous object this galaxy shows a regular outlying shape. The galaxy integrated colour grows smoothly from B-V=+0.30 to +0.60, suggesting the presence of different stellar populations. Based on the luminosity of the brightest blue stars (Nos. 12, 17, 27), UGC 7639 is situated at a distance of 8.0 Mpc. The bright blue star No. 33 in the outer part of the galaxy is probably a foreground object.
UGC 7698 = DDO 133. This is a low surface brightness galaxy with
a standard angular diameter of 5.0
arcmin. Because of its large size, the outer parts of the galaxy are left from our CCD frame. The distance modulus from the three brightest blue stars () does not correspond to the distance modulus from the brightest red star (). Since other blue and red stars may occur outside the CCD frame, our distance estimate should be considered uncertain.
UGCA 290 = Arp 211 = VV 42. This patchy dwarf galaxy looks like an interacting binary system (Vorontsov-Velyaminov 1959). In the "Atlas of peculiar galaxies'' (Arp 1966) it looks well resolved into stars. Recently we observed this galaxy with the 6-m telescope in bands and derived a distance modulus of 27.58 mag from the brightest blue stars. Our present estimate (from stars Nos. 5, 11, 13) differs slightly from the previous one. We did not consider the stars No. 12 and 14 in the distance estimation due to their location in the extremely crowded region which may lead to overestimation of the star brightness.
UGCA 292 = PGC 42275 = CVn dwA. The very low surface brightness and the presence of some blue stellar complexes characterize the galaxy structure. According to our photometry, the system has an extremely blue total colour: B-V = 0.08. Probably, this is the case of a very young galaxy undergoing active star formation process. The VLA observations of Lo et al. (1993) revealed a larger size for the galaxy in neutral hydrogen than in the optical light. The HI mass-to-light ratio for UGCA 292 is 6.1 in solar units, which is one of the highest value among the known galaxies. With a distance of 3.1 Mpc derived by us, the galaxy HI mass-to-total mass ratio reaches about 70%. The brightest blue object No. 23 lies at the edge of the galaxy may be a foreground star.
UGC 7866 = IC 3687 = DDO 141. Extending over the entire CCD field of view, the galaxy image shows many blue stars and associations as well as a few red stars. Our estimate of UGC 7866 distance based on the three brightest blue stars is 27.37. However, the red modulus, , differs significantly from the blue one.
UGC 7949 = DDO 147 = K 200. The galaxy has a low surface brightness without prominent clumps. All stars except for one in the galaxy are fainter than which leads to a distance of 10.4 Mpc. However, if the galaxy is a moderatly young system without massive blue stars, its distance may be overestimated. Thus this estimate is an upper limit.
UGC 8024 = DDO 154. The galaxy has a very asymmetric shape. Its standard optical diameter is 2.6 arcmin. According to Hoffman et al. (1993), in HI, the galaxy extends to 26 arcmin, i.e. 10 times the optical diameter. Photometry of the stellar population of the galaxy was made by Carignan & Beaulieu (1989) and Hopp & Shulte-Ladbeck (1995). The former authors estimated the galaxy distance to be Mpc, via the brightest blue stellar objects. It became clear from a comparison of their data with ours that, the authors treated some multiple stars as single ones. According to our photometry of the brightest blue stars the galaxy distance is 4.3 Mpc.
K 215 = F. This very low-surface-brightness galaxy was discovered first by Karachentseva (1968), and then studied by Schombert et al. (1992). Within its apparent boundary we detected only a dozen faint bluish stars. Based on the three brightest stars we obtained a galaxy distance D = 19 Mpc which suggests that the galaxy may be a member of the Virgo cluster. However, the galaxy distance may be overestimated because of the lack of young massive stars at the present stage of the galaxy evolution.
UGC 8638 = VV 133. Some bright blue stellar complexes are clearly identified
against the regular amorphous body of the galaxy. The integrated colour of the galaxy
increases smoothly from +0.27 to +0.48 outwards. Except for the
blue object No. 7 (which is a contact binary on the V-frame) the three
brightest blue stars
(Nos. 18, 20, 23) lie too much above the other stars (see CMD in Fig. 22 (click here)). Perhaps, they are compact stellar clusters rather than stars. In this case, the derived distance of 2.3 Mpc, should be considered as a lower limit. To refine the galaxy distance value, a much higher resolution, which is achievable with the Hubble space telescope, is needed.
Figure 22: V vs. (B-V) diagram for UGC 8638
UGC 8651 = DDO 181. The galaxy has a curved bow-like shape very similar to DDO 165 (Karachentsev et al. 1991). Some crowded, semi-resolved stellar complexes are seen in its body. The three brightest blue stars have the mean apparent magnitude which yields a distance D = 3.4 Mpc. If the brightest one (No. 12) is not a single star but a multiple one, the galaxy distance increases up to 5.4 Mpc.
UGC 8760 = DDO 183. The object has an overall symmetric shape. It was resolved into stars by Hopp & Shulte-Ladbeck (1995). The distance modulus, which is derived from the brightest blue stars (28.54) agrees well with the red one ().
UGC 8833. The galaxy core looks much more irregular than its outer parts. Our photometry reveals a slight increase in the colour (from +0.25 to +0.32) along the galaxy radius, which may be caused by the presence of different stellar generations. In the central part of the system, we distinguish about two dozen blue stars brighter than V = 24 mag. The three brightest ones yield a galaxy distance D = 3.2 Mpc.