The study of galaxies in the LV, conditionally limited by the D=10 Mpc radius, has an obvious advantage, since a lot of dwarf galaxies, otherwise inaccessible for observations at large distances were discovered in it. These "test particles" with measured radial velocities and distances are tracing the Hubble flow with an unprecedentedly high detail. For a comparison note that in the most extensive Sloan Digital Sky Survey (Abazajian et al. 2009) the average distance between galaxies with known radial velocities is about 8 Mpc, whereas in the LV (D < 10 Mpc), the number of galaxies with measured velocities is greater than 630.

Until the late 1990s development of observational cosmology in the Local universe was hampered by the scarcity of data on distances of even the most nearby galaxies, located just outside the Local Group boundaries. Deployment of unique capabilities of the Hubble Space Telescope (HST), combined with a new method for determining distances to galaxies by the luminosity of the tip of their red giant branch (TRGB) (Lee et al. 1993) made it possible to carry out mass distance measurements to more than 250 nearby galaxies with an accuracy of 5%–10%. The summary of data on distances, radial velocities and other parameters of galaxies in the LV (D ≤ 10 Mpc) was presented in the Catalog of Neighboring Galaxies (CNG; Karachentsev et al. 2004). This volume contains dwarf galaxies with luminosities 10⁴ times lower than that of the Milky Way, and it includes more than a dozen groups, similar to our Local Group in size and population. A detailed pattern of motions of galaxies in these groups and around them has for the first time revealed some unexpected features in the Hubble flow at 1–3 Mpc scales. New evidence appeared that the Hubble velocity–distance diagrams around the Local Group and other neighboring groups are characterized by a small dispersion of peculiar velocities ~30 km/s (Karachentsev et al. 2009). The Local Group overdensity decelerates surrounding galaxies that leads to curving the local Hubble flow. This effect can be observed because of small chaotic motions as well as minor distance measurement errors of nearby galaxies. The achieved distance accuracy allows to determine the total mass of nearby groups with a relative error of ~30% by the value of radius of the "zero-velocity sphere" R₀, which separates the group volume from remaining expanding neighborhood (Karachentsev et al. 2009, Karachentsev 2005).

It should be emphasized that the "R₀" method gives an estimate of the group mass, independent of the virial theorem, and this total mass estimate refers to a scale 3.5–4.0 times larger than the virial radius of the group. It is noteworthy that the agreement of mass estimates of nearby groups based on external and internal (virial) motions of galaxies is achieved only in the presence of the cosmological parameter Ω_λ ≈ 0.7. This means that the observed properties of the Local Hubble flow give a direct and independent evidence of the presence in the Universe of a specific medium, the dark energy, discovered from observations of distant Supernovae.

As shown by Dalcanton et al. (2009), Weisz et al. (2011) and other authors, the deep color–magnitude diagrams obtained at the Hubble Space Telescope for stellar population of nearby galaxies provides an opportunity to reconstruct the history of star formation in them with a resolution of ~(0.1-1) Gyr. This approach is an important observational tool for modeling the evolution of galaxies in different environments.

Creation of a representative sample of galaxies in the LV originates from a list of 179 galaxies by Kraan-Korteweg&Tammann (1979), which contains the galaxies with radial velocities V_LG < 500 km/s relative to the Local Group centroid, except for members of the nearby Virgo cluster. Later, Karachentsev (1994) and Karachentsev et al. (1999) have increased the number of galaxies in the LV to 226 and 303 objects, respectively. In 1998–2001 Karachentseva and her colleagues undertook a systematic search for new nearby dwarf galaxies using the POSS-II/ESO/SERC photographic sky survey. These efforts (Karachentseva&Karachentsev 1998, Karachentseva et al., 1999, Karachentseva&Karachentsev 2000, Karachentsev et al. 2000) along with the subsequent survey of new objects in the HI line of neutral hydrogen (Huchtmeier et al. 2000, 2001, 2003) have considerably enriched the sample of galaxies in the LV. A significant number of new irregular dwarf galaxies with radial velocities V_LG < 500 km/s were observed within the "blind" HI survey of the southern sky performed at the Parks radio telescope (Staveley-Smith et al. 1998, Kilborn et al. 2002, Zwaan et al. 2003, Koribalski et al. 2004, Meyer et al. 2004). The summary of these data, increasing the number of galaxies in the LV up to N = 450 is reflected in the CNG catalog (Karachentsev et al. 2004).

In subsequent years, the growth of the sample of the LV occurred via detecting new dwarf galaxies within the SDSS optical sky survey (Abazajian et al. 2009), the HIPASS (Wong et al. 2006), ALFALFA (Giovanelli et al. 2005, Haynes et al. 2011), and Westerbork (Kovaĉ et al. 2009) HI surveys of the northern sky, and as a result of systematic search for dwarf satellites of extremely low luminosity, resolved into stars, around the Milky Way (Willman et al. 2005, Belokurov et al. 2006), M31 (Ibata et al. 2007, Martin et al. 2009) and M81 (Chiboucas et al., 2009). By now the number of candidate members in the LV with distances D ≤ 10 Mpc has reached 720. It is clear that massive optical sky surveys like the Pan-STARRS (Tonry et al. 2012) and deeper "blind" HI surveys of the northern and southern sky will increase this number up to 1000 and over.

The Local Volume sample criterion


Selection of galaxies in the local spherical volume of 10 Mpc radius by the condition V_LG < 500 km/s, used by Kraan-Korteweg&Tammann (1979), assumed the Hubble parameter value of H₀=50 km s^-1 Mpc^-1. At the current value of H₀=73 km s^-1 Mpc^-1 (Spergel et al. 2007), the limit set for radial velocities should be raised to V_LG < 730 km/s. However, the radial velocity of galaxy is only an approximate indication of its distance. In addition to the virial component of velocity inherent in the nearby group members, the local velocity field is also affected by the presence of a nearby rich Virgo cluster at a distance of 16.5 Mpc with the velocity dispersion of σ_v ≅ 650 km/s, and an extensive Local Void (Tully 1988), which occupies about a quarter of the celestial sphere. According to Tully et al. (2008), the presence of these two main elements of the local large-scale structure generates two velocity components of the Local Group and surrounding galaxies: ~180 km/s towards the Virgo cluster center (12^h30^m + 12^°), and ~260 km/s in the direction away from the Local Void center, located in the region ~(19^h00^m + 3^°). An almost complete absence of galaxies in the region of the Local Void and their relative excess in the opposite direction creates a specific selection effect: most of the galaxies at a distance of D=10 Mpc generally have radial velocities much lower than the expected value of ~730 km/s.

There are recent indications that filaments and walls of the large-scale structure may have collective motions with an amplitude of ~500 km/s. Perhaps the closest example of such great non virial motions is the Coma~I region, where a "flock" of galaxies around NGC4150 at a distance of D ~ 15 Mpc is moving towards us with an average peculiar velocity of -800 km/s (Karachentsev et al. 2011). To our regret, we have to state that the local field of peculiar velocities of galaxies is poorly studied by now, and the proposed radial velocity correction schemes for coherent non-Hubble motions such as the model of pure Virgo-centric flow (Kraan-Korteweg 1986, Masters 2005) turn out to be too simplified. Therefore, a low radial velocity of the assumed nearby galaxy is not yet a reliable indicator of its proximity.

An ideal solution would be a direct measurement of distances to all nearby galaxy candidates using the Hubble Space Telescope. As it was shown by Rizzi et al. (2007), the tip of the red giant branch method (TRGB) gives the distance accuracy of ~5% regardless of the galaxy morphological type. During the exposure time with Advanced Camera for Survey at HST, corresponding to 1–2 orbits, the TRGB method allows to measure accurate distances up to 7–10 Mpc, i.e., to completely solve the problem of creating a fair sample of the LV. A cost of the issue is, however, one to two thousand orbits of the HST.

Other methods of distance measurement may be either used for a small number of objects (the Supernova method, the Cepheid method), or be applicable only to galaxies of fixed morphology (method of surface brightness fluctuations, Tully-Fisher and Faber-Jackson methods), or have an accuracy not better than 25% (method of brightest stars).

Given all these circumstances, we have included in the LV sample the galaxies having radial velocities with respect to centroid of the Local Group

V_LG < 600 km/s,     (1)

or the galaxies with distance estimates

D < 11.0 Mpc.     (2)

A simultaneous fulfillment of both conditions (1) and (2) is not required. Here, we took into account the fact that some galaxies at distances ~(7-10) Mpc may have orbital/virial velocities that place them in the region of V_LG > 600 km/s on the Hubble diagram, while other galaxies, projected onto the Virgo cluster, are expected to have an additional positive velocity component due to their infall toward the cluster.

The velocity–distance diagram for galaxies of the LV is presented in Figure 1. Beyond the upper edge of the figure, there are 16 galaxies with D = (7-11) Mpc and V_LG > 1100 km/s, almost all of them are located near the line of sight, directed to the Virgo cluster center. Beyond the right edge of the figure, 72 galaxies with V_LG < 600 km/s are located, but with the distance estimates over 11 Mpc. We did not exclude such objects from the sample for two reasons: a) their distance estimates can not be quite reliable, b) the distribution of such galaxies on the sky may outline coherent motions in nearby diffuse filaments. In addition, the list of objects in the LV contains 108 galaxies with individual distance estimates of D < 10 Mpc, which still have their radial velocities unmeasured.

Therefore, our total list of galaxies in the LV consists of more than 850 objects. As can be seen from Figure 1, about one third of galaxies located in its upper right corner could be considered to belong to the D < 10 Mpc volume only conditionally, since their typical distance measurement error is ~2 Mpc. It also follows from these data that limiting the sample by the condition (1) only would introduce a strong selective effect, distorting the kinematic pattern of the LV.

Distribution of the LV galaxies on the sky is shown in Figure 2 in the equatorial (bottom panel) and galactic (top panel) coordinates. The galaxies with distances of D = (1-11) Mpc are shown by circles, whose size reflects the luminosity of galaxy, and color — its distance. Here the members of the Local Group with D < 1 Mpc were excluded. The shaggy gray lane in the panels corresponds to the region of strong extinction in our Galaxy. The distribution of galaxies shows their concentration in the region of well-known nearby groups around M81, Centaurus A, M83, IC342, NGC253, M101, NGC6946, Leo I, etc. The figure exhibits an extensive area in the Hercules-Aquila, almost completely devoid of galaxies, i.e. the Local Void (Tully 1988). This void extends far beyond the LV boundary.