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4.1 Morphological Classification

The classification of galaxies according to their shape is a fundamental tool in extragalactic astronomy. It is through classification schemes that astronomers identify how different types of galaxies are interrelated and thus build a deeper understanding of how galaxies form and evolve ([van den Bergh 1998]). The most widely used galaxy classification scheme was first proposed by [Hubble 1926] and later variously refined by Hubble himself and others. In its definitive form, described by [Sandage 1961] and visually illustrated by the famous ``tuning fork'' diagram shown in Figure 4.1, Hubble's scheme consists of four main morphological classes: These four basic classes can be divided into several further subclasses, but these finer distinctions will not be considered here.

Figure 4.1: An early version of Hubble's galaxy classification scheme of regular galaxies, also known as the ``tuning fork'' diagram. Reproduced from [Hubble 1936].

As for the relative frequencies of the different morphological types, in recent years they have been extensively investigated via both visual and automated classification procedures. In particular, deep observation campaigns carried out with the HST, namely the Medium Deep Survey (MDS) and the Hubble Deep Field North (HDF-N), have proved that when such frequencies are calculated over magnitude-limited samples of different depths, then they strongly depend on the limiting magnitude, showing a sharp decrease in the number of spirals and an increase in the number of ellipticals and unclassifiable galaxies at faint magnitudes. Relative frequencies of morphological types in magnitude-limited samples of different limiting magnitudes are given in Table 4.1.

Table 4.1: Relative frequencies of DDO morphological types in magnitude-limited samples of different limiting magnitude. SAC stands for Shapley-Ames Catalog ([Shapley and Ames 1932]), MDS for Medium Deep Survey ([Ratnatunga et al. 1999]) and HDF-N for Hubble Deep Field North ([Williams et al. 1996]), from Table 2 in [van den Bergh et al. 1996]. Morphological classifications from [van den Bergh 1960], [Abraham et al. 1996a] and [Abraham et al. 1996b], respectively. Wider DDO classification bins (E, S0, S, Ir and Unclassified) are indicated by horizontal lines. Note that the SAC makes no distinction between E, E/S0 ans S0, and that for the MDS and HDF data, only galaxies with $ I$ less than $ 21$ and $ 24$, respectively, were considered, in order to obtain a truly magnitude-limited sample and thus more robust estimates.
E   16.6 23.9
E/S0 22.2 3.3 0.7
S0   6.9 4.3
S0/Sa 0.0 0.0 0.7
E/Sa 1.3 0.0 1.4
Sa 6.9 7.5 14.6
Sab 0.2 3.1 1.4
Sb 26.9 7.1 4.3
Sbc 0.3 4.0 0.0
Sc 22.9 12.8 1.4
S 10.0 14.6 13.2
Sc/Ir 0.2 0.9 0.0
Ir 2.0 6.4 2.5
Unclassified 7.0 16.8 31.4

In Section 4.4 regular galaxies are divided into two classes, namely elliptical galaxies (E) and disk galaxies (D), on the basis of their surface brightness distribution. Taken the MDS frequencies from Table 4.1 as representative, it can be concluded that the relative frequencies of these two classes are of about 20% and 80% for E and D galaxies, respectively.

next up previous contents
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Mattia Vaccari 2000-12-05