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The Local Group and the Local Neighbourhood

  The AVP has in literature mainly been applied to a limited number of galaxies, mainly due to computational and observational limitations. A natural choice of system would then be the Local Group (LG hereafter) and the Local Neighbourhood (LN hereafter), where we have extensive and relatively accurate observations to base and compare the model to. This section will be devoted to our ``home'' and ``neighbourhood'' in the Universe.

The LG is composed of 29 likely members and some 20 possible members Bergh (including the recently discovered Antlia galaxy by Whit,Fouq). Among the probable members, there are two dominant galaxies, the MW and Andromeda (M31 hereafter), and a small spiral galaxy M33. The rest are small elliptical and irregular dwarf galaxies, most of them probably being satellites of the two large galaxies. The mass of the MW and M31 are clearly dominating the mass of the LG--the other galaxies being from one to six orders of magnitude smaller. The closest galaxy of comparable size to the MW and M31 is the NGC 55 galaxy at a distance of 2 Mpc, which is in the LN. Thus, the MW, M31, and their companions are relatively isolated. The LG is therefore often approximated by an isolated double-galaxy system.

The separation between the MW and M31 is about 750 kpc, and the rate of change of the separation is (measured relative to the centre of the galaxies) about -123 km s tex2html_wrap_inline5219 . Consequently, M31 is approaching the MW, which is not what one would expect from the general expansion. KW explained that this motion was due to mutual gravitational attraction of the masses of the two galaxies. The alternative would be that the MW and M31 are accidentally passing by, but, if the galaxies had moved at constant speed, this alternative does not seem promising [page 483]P-93. The Kahn-Woltjer dynamical picture (often referred to as the ``timing argument'') together with the spherical model [description given by][]P-80, allows us to estimate certain parameters describing a system of two galaxies. If we take the assumption of LG being a isolated two-body system to be valid, the relative orbit of the MW and M31 should be on the following form Gunn,GT:




where r is the distance between the Galaxy and M31, tex2html_wrap_inline5397 the collapse time of the LG, and tex2html_wrap_inline5399 is a parameter. The age of the universe when the LG reaches its point of maximum expansion tex2html_wrap_inline5401 is tex2html_wrap_inline5403 . Mass estimates of this two-body systems ranges from about tex2html_wrap_inline5405 to twice that. P-93 arrived at the latter, in addition to an estimate of the mass-to-light ratio, tex2html_wrap_inline5407 .gif In comparison, analysis of the rotation curves of disc galaxies indicate tex2html_wrap_inline5413 Pad. R-LB showed that taking masses from the LN into account do not alter the mass estimate of the LG, which indicates that the approximation of treating the LG as a isolated double-galaxy system is valid. Results for tex2html_wrap_inline5397 seems to support the assumption that the MW and M31 have crossing times comparable to the Hubble time, indicating that the two galaxies are approaching each other for the first time.

The LN is dominated by groups of galaxies not too different from our own LG: the Maffei, Sculptor, M81, Centaurus, and Canes Venatici I (CV I) groups. They all have masses comparable to M31. Details on these groups and the galaxies in the LG considered in this thesis are given in Table 5.4. The data are taken from various sources--more on this in Subsection 5.4.1.

On larger scales, the two dominant structures are the Virgo cluster and the Coma cluster. The Virgo cluster, located about 15 Mpc away, has a diameter of about 3 Mpc and contains several thousand galaxies. The Coma cluster has also an overall size of 3 Mpc and is located about 80 Mpc from us. Studies of the distribution of the galaxies in our neighbourhood has shown that most of the galaxies lie in a plane, called the supergalactic plane, which is approximately perpendicular to the plane of our galaxy. The dense set of galaxies in this plane is called the Local Supercluster, and the Virgo cluster is situated close to the center of this cluster. There are also indications of even larger structures: the LG seems to have a net velocity of about 600 km s tex2html_wrap_inline5219 in the direction of the Centaurus group. This hypothetical large structure is often referred to as the Great Attractor.

In this thesis only the LG and the LN will be considered.

next up previous contents
Next: The Dynamics of Galaxies Up: Formation of Large-scale Structures Previous: Peculiar Velocity Fields

Trond Hjorteland
Mon Jul 5 02:59:28 MET DST 1999