Galactic bimodality

Galaxies can also be divided into the two most common types: elliptical and spiral (see Fig. 1.8). According to many estimates1, these types of galaxies account for more than 70% of the total number of galaxies in the Universe. If we take into account that SO-type galaxies, which are a kind of intermediate class between spiral and elliptical galaxies, are the third most common, the other classes of galaxies account for no more than 15% of the total number of galaxies. What distinguishes the two main types of galaxies in the most general terms?

SHAPE AND STRUCTURE. Elliptical galaxies have a very simple shape and internal structure.

Elliptical galaxies, unlike the others, are structureless. Regarding the structure of elliptical galaxies, we can speak only in terms of the distribution of densities in them along the radius2

Spiral galaxies have a complex form of logarithmic spiral arms, disks, nucleus, core, and many other features of the structure. Moreover, every spiral galaxy has a halo, which is an elliptical formation of globular clusters of stars. Virtually every spiral galaxy has an inset, an elliptical inside it "galaxy" (see Fig. 1.52).

AGE. The age of most galaxies is estimated by astrophysicists to be about the same. They all formed at the very beginning of the expansion of the Universe, at the moment when it crossed the threshold of \(10^{27}\) cm.

At the same time in elliptical galaxies, star formation has practically stopped, they have no material — gas and dust — and they consist of old population II stars.

In spiral galaxies, old stars form a spherical "insert" — the halo — while dust and new stars are mainly distributed in the disk and spiral branches. In spiral galaxies, the process of star birth continues.

That is why we can say that elliptical galaxies (here we include elliptical components of spiral galaxies) are old systems of the first generation, and spiral galaxies (in terms of their specific structure) are young systems of the second epoch of star formation.

SIZES AND MASSES. The largest in size are spiral galaxies, the largest of them exceeding a diameter of \(10^{23}\) cm. The author was unable to find in the literature any mention of spiral galaxies with sizes smaller than \(10^{22}\) cm. On the other hand, all dwarf galaxies are elliptical, and for them the lower threshold is \(10^{20}\) cm.

In addition, according to statistical data 3, the mass range of spiral galaxies lies within \(10^9{-}10^{12}\) solar masses. For elliptical galaxies, however, this range is strongly shifted towards low masses: \(10^5{-}10^{12}\) solar masses. This indicates that spiral galaxies are on average larger and more massive, while elliptical galaxies are on average smaller and less massive.

ENTRY INTO CLUSTERS. Galaxy clusters include about 76% of elliptical galaxies and only less than 50% of spiral galaxies 4.

So, we see that the Universe is populated mainly by two types of galaxies. Elliptical galaxies are older, on average smaller in size, simpler in shape and structure, and they are part of the metagalactic cellular skeleton of clusters. Spiral galaxies are younger, the largest and most massive, and they are free from the structural skeleton of the Metagalaxy.

It is very important to note that there is a clear distinction between these two types in a multitude of parameters, which indicates their fundamental difference.

Based on these qualitative data, we can PROPOSE that elliptical galaxies belong to the first mode of the assumed bimodal size distribution of galaxies, while spiral galaxies belong to the second mode. And by analogy with atoms and stars, we can PROPOSE that there is a gap in the statistical distribution of galaxies by size between these modes.

  1. Vorontsov-Vel'yaminov B.A. Essays on the Universe. Moscow: Nauka, 1969. ↩

  2. Vorontsov-Vel'yaminov B.A. Essays on the Universe. Moscow: Nauka, 1969. С. 130. ↩

  3. Vorontsov-Vel'yaminov B.A. Essays on the Universe. Moscow: Nauka, 1969. С. 114. ↩

  4. Vorontsov-Vel'yaminov B.A. Essays on the Universe. Moscow: Nauka, 1969. С. 125. ↩