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Chapter I. Introduction

1. Groups of galaxies

It seems to be a recognized fact that a considerable number of the galaxies in the general field (in this paper the term "general field" will be used to designate the space outside the big clusters) are not isolated objects but rather members of groups. Even a superficial examination reveals that some of the nearby giant spiral systems are surrounded by clouds of smaller and larger satellites. A detailed study of such physical groups would be of great importance, since it could furnish valuable information about a galaxian population in a given volume of space, especially as regards the statistical distributions of absolute characteristics, such as luminosities, diameters, and masses. If the group members form a representative sample of all general field galaxies, the results would be of universal significance. It may not be necessary to point out that information of this type is very hard, if not impossible, to obtain for low-luminosity galaxies by other methods, on account of the difficulty in determining individual distances. In the case of a physical group the central dominant galaxy serves as a distance indicator.

In a previous investigation the writer (1950) has presented some results referring to groups of galaxies. The distribution curves derived for absolute luminosities and diameters were of a very preliminary nature, since they referred only to members of the Local Group, the M81 group, and the M101 group.

The present paper gives the results of a study of physical companions belonging to 174, more or less nearby, prominent spiral galaxies. The investigation is based on plates previously taken with the Mount Wilson 60-inch and 100-inch telescopes (cf. Holmberg 1958, 1964), and on the prints of the National Geographic Society-Palomar Observatory Sky Atlas, the latter being evaluated down to the practical limit, as regards galaxies. From an examination of over 3000 galaxies in circular survey areas around the spiral systems and in nearby comparison areas, a total of 274 physical companions has been picked out. A summary of the material is presented in Tables 2 and 7. As will be shown in the following sections, the analysis leads to a series of interesting results, for instance, concerning the number of satellites, as related to different properties of the central spiral system, and as regards the luminosity functions of different types of galaxies.

As regards the important question whether the group members form a representive sample of all general field galaxies, it may be stated here that the results obtained seem to give an affirmative answer. In the present material there are 53 spiral systems above gal. lat. +30° out to an absolute distance modulus of 30.0, which have altogether 82 physical companions with absolute pg magnitudes brighter than M = - 15.0 (the observed number out to a separation of 50 kpc has been multiplied by 3.3; cf. the next section). On the other hand, the smoothed-out space density of galaxies brighter than M = - 15.0 is about 0.17 per Mpc3 (cf. sect. 14 - 15), which in the volume considered leads to a total number of approximately 180. Thus, over 70% of all galaxies can be referred to the groups. Since the present investigation does not claim to be complete, it seems likely that the great majority of galaxies in the general field are members of physical groups of the type studied here. The conclusion is that the present material approximates to a random sample.

At this point some data will be presented for the three nearest and best-known groups of galaxies: the Milky Way group, the M31 group, and the M81 group. These physical groups, which in all respects appear to be comparable to the groups studied in the present paper, will later on be referred to for comparison. There does not seem to be any doubt that the Local Group is really two associations that happen to be located rather close together: one group around each of the two giant spiral galaxies (there may be some uncertainty as to the status of the two Ir I systems IC 1613 and Wolf-Lundmark-Melotte, which are exceptionally distant both from the Milky Way and from M31). The distances listed in Table 1 have been taken from the very detailed study of the Local Group by van den Bergh (1968), which is also the source for the absolute magnitudes and masses of SMC and LMC; the Capricornus system, not listed by van den Bergh, has been assumed to be at a distance < 250 kpc on account of the high resolution. The remaining data are from my previous works (1958, 1964); the log. masses in brackets have been derived from absolute luminosities and integrated color indices by a relation given in the last-mentioned paper. The M81 group has been studied previously (Holmberg 1950). A new addition is Ho IX, a highly resolved Ir I system very close to M81 at alpha = 9h53m.5, delta = + 69°16'(1950); the total pg magnitude has been measured on a Mount Wilson 60-inch plate.

Table 1 probably includes all group members outside the galactic absorption belt that are brighter than M = - 13.5; those objects for which information on M is not listed (app. magnitude not accurately known) are all fainter than this limit. It may be noted that down to an absolute magnitude of M = - 9 the three groups probably have more than 100 members (cf. the luminosity function derived in sect. 12). In spite of the incompleteness, the material available permits certain conclusions. We find that seven out of 22 members (IC 1613 and WLM not included have separations from the central galaxy of less than 50 kpc; for the Milky Way group the separation is assumed to be equal to the distance multiplied by pi / 4, which is the average projection factor for a random orientation of the distance vectors The arithmetical mean of the separations (21 members) amounts to 111 kpc, whereas the maximum separation is about 400 kpc.

Table 1 Data for members of the Local Group (= MW group + M31 group) and the M81 group.

Columns 2-6 give the number according to de Vaucouleurs et al. (1964), the adopted distance, the separation from the central galaxy, the abs. pg magnitude, and the log. mass (solar units).

Designation Ref. Cat. Distance Separation M logM

Milky Way group
Milky Way       - 11.1
NGC 6822   465 kpc - -15.3 9.1
SMC A 51 60 - -16.2 9.2
Sculptor A 58 84 - - -
Fornax A 237 188 - - -
LMC A 524 50 - -17.6 9.8
Leo I A 1006 220 - -10.8 -
Leo II A 1111 220 - - 9.1 -
Draco A 1719 67 - - -
Capricornus A 2144 < 250 - - -
Ursa Minor - 67 - - -
M31 group
NGC 224   690 kpc   -20.4 11.5
NGC 147   690 89 kpc -14.2 (9.0)
NGC 185   690 85 -14.5 (9.2)
NGC 205   690 7 -15.9 (9.4)
NGC 221   690 5 -15.7 9.6
NGC 598   690 180 -18.4 10.6
IC 1613   740 > 500 -14.5 8.6
WLM A 2359 760 > 500 -13.5 (7.6)
M81 group
NGC 3031   2900 kpc   -19.8 11.2
NGC 2976   2900 71 kpc -17.0 (9.7)
NGC 3034   2900 32 -18.5 (10.7)
NGC 3077   2900 40 -17.1 (9.9)
IC 2574   2900 155 -16.8 (8.9)
Ho I A 936 2900 125 -14.4 (7.9)
Ho II A 814 2900 425 -16.6 (8.9)
Ho IX - 2900 9 -13.5 (7.6)

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