4 Status of the measured distances in the Local Volume

Apart from 35 members of the Local Group with distances D < 1.0 Mpc, there are so far 191 galaxies with distance estimates D < 5.5 Mpc. Among them 35 galaxies have no measured radial velocities. The present sample of data on radial velocities and distances of nearby galaxies is presented in Table 2. Its columns give: (1) galaxy name, (2) apparent integrated blue magnitude from the NED or some recent sources (Makarova 1999; Parodi et al. 2002), (4) Galactic extinction from Schlegel et al. (1998), (5) heliocentric radial velocity in km s^-1 from the NED or recent measurements by Huchtmeier et al. (2003), (6) radial velocity in the frame of the Local Group, (7) galaxy distance with indication of the used method: "Cep'' - Cepheids, "RGB'' - tip of red giant branch stars, "SBF'' - surface brightness fluctuations, "mem'' - membership of known nearby groups, "BS'' - luminosity of the brightest stars, and "TF'' - Tully-Fisher relation. The last column gives the reference for the distance.

Figure 3 shows the distribution of the LV galaxies according to their distances determined using various distance indicators.

Figure 3: Distribution of 223 Local Volume galaxies according to their distances derived by different methods: "Cep'' - from cepheids, "RGB'' - from the tip of red giant branch stars, "SBF'' - from surface brightness fluctuations, "mem'' - from the galaxy membership in the known nearby groups, "BS'' - from the luminosity of the brightest stars, and "TF'' - from the Tully-Fisher relation. Two upper panels present distance distributions for 35 galaxies without radial velocities, as well as for 30 galaxies with distance estimates from the Hubble relation $D = V_{\rm LG}/H_0$, where H0 = 73 km s-1 Mpc-1 is adopted.

The three lower panels correspond to the most reliable methods giving distances with an accuracy of $\sim$5-15%. The same error is probably similar for the members of some nearby groups (around M81, Cen A, and M83) with well determined average distances. A characteristic error on distances estimated via brightest stars or via TF- relation might be $\sim$(20-30)%. The two upper panels present distance distributions for 35 galaxies without radial velocities, and also for 32 galaxies with distance estimates from the Hubble relation $D = V_{\rm LG}/H_0$, for which H₀ = 73 km s^-1 Mpc^-1 is adopted. As seen from the histograms, the TRGB method is, in practice, the most efficient method to measure distances within $\sim$5 Mpc. Besides, 99% of the TRGB distances have been obtained during the last three years taking advantage of the superior angular resolution of HST. It should be noted, however, that so far the relative number of the LV galaxies with radial velocities and accurate distance estimates is 111/223 or only 50%. The remaining 112 galaxies might be suitable targets for the next snapshot survey with the Advanced Camera at HST.