5 Local deviations from the Hubble flow

The Hubble relation (radial velocity - distance) for 156 nearby galaxies is shown in Fig. 4.

Figure 4: Radial velocity - distance relation for 156 Local Volume galaxies. The galaxies with accurate distance estimates ("Cep'', "RGB'', "SBF'', and "mem'') are shown as filled circles, and galaxies with less reliable distance estimates ("BS'' and "TF'') are indicated as crosses. The members of M 81 and Cen A groups with distances in the range of 3.4-4.0 Mpc are shown by open circles and open squares, respectively. The regression line corresponds to the Hubble relation with H0 = 73 km s-1 Mpc-1, curved at small distances assuming a decelerating gravitational action of the Local Group with a total mass of $1.3 \times 10^{12}~ M_{\odot}$.

Here galaxies with accurate distance estimates ("Cep'', "RGB'', "SBF'', and "mem'') are represented by filled circles, and galaxies with less reliable distance estimates ("BS'' and "TF'') by crosses. In the considered volume there are two massive groups of galaxies around M 81 and Cen A, whose average distances of $3.73\pm0.04$ Mpc (Karachentsev et al. 2002a), and $3.63\pm0.07$ Mpc (Karachentsev et al. 2002b) are very similar. Members of these two groups are shown in Fig. 4 as open circles and open squares, respectively. The solid line corresponds to the Hubble relation with H₀ = 73 km s^-1 Mpc^-1, curved at small distances because of the decelerating gravitational action of the Local Group (Sandage 1986) assuming a total mass of $1.3 \times 10^{12}~ M_{\odot}$(Karachentsev et al. 2002c). The Hubble diagram for the LV galaxies reveals some important properties.

1.

The largest deviations from the Hubble regression are seen in the range of distances between 3.5 and 3.8 Mpc. Their evident reason are the virial motions of galaxies inside the M 81 and Cen A groups. Other nearby groups, in particular those of M 83 and IC 342/Maffei, also contribute to the observed dispersion of radial velocities.

2.

The galaxies situated at the near end of the M 81 and Cen groups, in the distance range 2.5-3.4  Mpc, have radial velocities that are on the average $\sim$60 km s^-1 larger than the expected Hubble velocities. In contrast, radial velocities of galaxies within the distance range of 4.0-4.6 Mpc tend to have velocities systematically below the Hubble regression line. Such a kind of "S''- shaped deviation of radial velocities is typical of the vicinity of a massive attractor (see, for example, Fig. 1 in Tonry et al. 2000), when galaxies at the front and at the back of the attractor fall towards its center. In particular, because of this the galaxies UGC 6456 and NGC 4236 behind the M 81 group lie in Fig. 4 much lower than the Hubble regression line.

As was shown by Karachentsev & Makarov (1996), the local Hubble flow on a scale of $\sim$5 Mpc is significantly anisotropic. Based on rough estimates of distances to 145 galaxies obtained from the luminosity of their brightest stars, Karachentsev & Makarov (2001) derived that the local field of peculiar motions can be described as a tensor of the local Hubble parameter, H_ij, which has the main values of ($81\pm3$):($62\pm3$):$(48\pm5)$ in km s^-1. The minor axis of the corresponding ellipsoid is directed towards the polar axis of the Local Supercluster, and the major axis has an angle of $(29\pm5)\hbox{$^\circ$ }$ with respect to the direction towards the center of the Virgo cluster. Broadly speaking, the observed anisotropy of velocities corresponds to a Virgo-centric flow, however, a spherically symmetric Virgo-centric flow does not fit well the observed peculiar velocity field.

Our new, more accurate data on galaxy distances given in Table 2 confirm the presence of an anisotropy of the Hubble flow in the Local Volume. In particular, Fig. 4 shows that isolated galaxies situated at high supergalactic latitudes (UGC 3755, UGC 3974, UGC 4115, and KK 65) have radial velocities that are about twice lower than expected with H₀ = 73 km s^-1 Mpc^-1.

Figure 5 presents the all-sky distribution of 156 galaxies from Table 2 in Supergalactic coordinates.

Figure 5: Full-sky distribution of 156 galaxies from Table 2 in supergalactic coordinates. The galaxies with positive and with negative peculiar velocities with respect to the isotropic Hubble flow (H0 = 73 km s-1 Mpc-1) are shown as open and filled circles, respectively. The observed peculiar velocities of galaxies were smoothed with a 2D-Gaussian filter with a parameter $\sigma = 25\hbox {$^\circ $ }$, and then were plotted as contour map with intervals of 20 km s-1.

The galaxies with positive and negative peculiar velocities with regard to the isotropic Hubble flow (H₀ = 73 km s^-1 Mpc^-1) are represented by open and filled circles, respectively. The position of the supergiant elliptical galaxy M 87 at the center of the Virgo cluster (SGL = 102.9 $\hbox{$^\circ$ }$, SGB = $-2.3\hbox{$^\circ$ }$) is indicated with an asterisk. The observed peculiar velocities of galaxies were smoothed with a spatial 2D-Gaussian filter with dispersion $\sigma = 25\hbox {$^\circ $ }$, and then were plotted in Fig. 5 as a contour map with intervals of 20 km s^-1. As can be seen, the local peculiar velocity field is quite symmetric about to the Local Supercluster equator. The most slowly expanding region of the local Hubble flow with an amplitude of -80 km s^-1 occupies the southern Supergalactic polar cap (Monoceros constellation). Another negative peculiar velocity area with a lower amplitude, -20 km s^-1, corresponds to the northern Supergalactic cap, also pointing towards the Local Void (Draco constellation). Two regions of outflow peculiar velocity within the +20 km s^-1 contours lie just on the Supergalactic equator in the Centaurus and Pisces constellations. However, they are located far from the Virgo/anti-Virgo directions, as would be expected in a spherical Virgo-centric flow.

The same map of the local field of peculiar velocities is shown in Fig. 6 in galactic coordinates.

Figure 6: The same map of the local field of peculiar velocities as shown in Fig. 5, but in galactic coordinates.

Figure 6 is useful for comparison with the all-sky contour map of the predicted peculiar velocity field (see Fig. 1 in Mendez et al. 2002). That map derived from the IRAS galaxy distribution represents deviations from the pure Hubble flow on the shell corresponding to velocity $V_{\rm LG} = 500$ km s^-1. In general, the observed peculiar velocity map fits the predicted one, but has a 4-6 times lower amplitude and significantly different positions of the regions of outflow peculiar velocity.