7 Concluding remarks

A general view of the Local Volume within a radius of 5.5 Mpc is presented in Fig. 7.

Figure 7: Panorama of the Local Volume within a radius of 5.5 Mpc. The upper panel shows the galaxy distribution projected onto the Supergalactic plane, and the lower panel corresponds to the edge-on view. The galaxies with known radial velocities are shown as filled circles, the 35 galaxies of dSph, dE types without radial velocities are indicated as open circles. The brightest members of nearest groups are shown as asterisks.

Figure 7: Continued.

Figure 7: Continued.

Its upper panel shows the galaxy distribution projected onto the Supergalactic plane, and the lower panel corresponds to an edge-on view. Apart from 156 galaxies with radial velocities known so far (shown with filled circles), we also plot in Fig. 7 35 galaxies without radial velocity estimates (open circles). All of them are dwarf galaxies of the dSph and dwarf elliptical (dE) morphological types. In the considered volume there are six known groups, besides the LG, whose brightest members: M 81, NGC 5128 (=Cen A), M 83, IC 342, NGC 4736, and NGC 253 are shown with asterisks. Altogether, 121 galaxies, or 63% of their total number inside the shell of 1.0 < D < 5.5 Mpc, belong to these compact or loose groups.

Apart from the well-known groups, where 1 or 2 giant galaxies dominate over other members, there are also some groups consisting entirely of dwarf galaxies. Tully et al. (2002) found four groups of this kind, the principal members of which are NGC 3109, UGC 8760, UGC 3974, and NGC 784, respectively. In the Local Volume we found six more such groups. Their complete list is given in Table 3.

 

 

Table 3: Properties of nearby groups of dwarf galaxies.

Group	N	< D >	$< R_{\rm p}>$	$\sigma_V$	M1	LB	$M_{\rm vir}/L_B$	$M_{\rm orb}/L_B$	$T_{\rm cross}$
		Mpc	kpc	km s-1	mag	$10^8L_{\odot}$	$M_{\odot}/L_{\odot}$	$M_{\odot}/L_{\odot}$	Gyr
N3109, SexB,	4	1.36	414	18	-15.57	3.58	214	201	23
Antlia, SexA
U8760, U8651,	3	3.20	162	7	-13.23	0.59	398	430	23
U8833
U8320, U8215,	4	4.20	84	37	-15.46	2.58	869	948	2.3
U8308, U8331
N4395, N4244,	5	4.43	320	54	-17.69	35.9	625	452	5.9
U7559, U7605,
IC 3687
N784, U1281,	4	4.96	184	16	-16.58	8.52	45	84	12
KK 16, KK 17
U3974, U3755,	4	5.10	412	19	-14.97	3.43	222	1945	22
KK 65, U4115
Orion, KK 49,	3	5.95	300	41	-16.33	6.94	2045	2999	7.3
U3817
U3966,	2	6.25	142	1	-14.80	1.94	-	7	142
U3860
U5272, KK 78,	4	7.10	114	14	-14.91	1.91	33	859	8.1
KKH 54, U5186
N2337, U3698,	3	7.90	174	6	-16.77	9.09	7	3	27
U3817
Median	4	5.0	179	18	-15.52	3.5	218	441	23

The table columns contain: (1) group member names, where the brightest galaxy ranks first, (2) number of galaxies in the group, (3) mean distance to the group, (4) mean projected linear radius of the group, (5) radial velocity dispersion, (6) absolute B magnitude of the brightest member, (7) integrated luminosity of the group, (8,9) virial and orbital (Karachentsev et al. 2002a) mass estimate normalized to the luminosity unit, (10) crossing time.

It follows from the presented data that a typical group of dwarf galaxies (N = 4 members) is characterized by a median projected radius of $\sim$180 kpc, a median velocity dispersion of only 18 km s^-1, a median absolute magnitude of the brightest member of -15.5 mag, and a median virial/orbital mass-to-luminosity ratio of (220-440)  $M_{\odot}/L_{\odot}$. Tully et al. (2002) suggest that these galaxy groups contain a large amount of dark matter as low mass halos, as expected in a $\Lambda$ CDM cosmology, which have never hosted significant star formation. The high virial mass-to-luminosity ratios favour this idea. However, the typical crossing time for these groups, 23 Gyr, exceeds largely the age of the Universe, which means that virial/orbital mass estimates are fictitious. Altogether, about 13% of the Local Volume galaxies belong to these loose associations of dwarf galaxies.

Together with the usual groups and groups of dwarf galaxies, the Local Volume contains small empty regions of different sizes, which are completely devoid of any galaxy. The biggest one is known as the Local Void (Tully 1988). In this respect, a study of the topology of the Local Volume would be of interest for cosmology (Gottlober et al. 2002).

Acknowledgements

We thank the referee, J. Lequeux, for his very useful comments. Support for this work was provided by NASA through grant GO-08601.01-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This work was partially supported by RFBR grant 01-02-16001 and DFG-RFBR grant 02-02-04012. D.G. gratefully acknowledges support from the Chile Centro de Astrofísica FONDAP No. 15010003.

The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on the Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with permission of these institutions.

This project made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.