Эллипстік және спиральдық галактикалар арасындағы галактика түрі

Class of galaxy between an elliptical galaxy and a spiral galaxy

A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large scale disc but does not have large scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies). Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate scale discs.

In many respects the composition of lenticular galaxies is like that of ellipticals. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the Tully–Fisher relation (see below). In addition to these general stellar attributes, globular clusters are found more frequently in lenticular galaxies than in spiral galaxies of similar mass and luminosity. They also have little to no molecular gas (hence the lack of star formation) and no significant hydrogen α or 21 cm emission. Finally, unlike ellipticals, they may still possess significant dust.

Lenticular galaxies share kinematic properties with both spiral and elliptical galaxies. This is due to the significant bulge and disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion. This situation is analogous to a balloon, where the motions of the air particles (stars in a bulge's case) are dominated by random motions. However, the kinematics of lenticular galaxies are dominated by the rotationally supported disk. Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy. Thus, kinematics are often used to distinguish lenticular galaxies from elliptical galaxies. Determining the distinction between elliptical galaxies and lenticular galaxies often relies on the measurements of velocity dispersion (σ), rotational velocity (v), and ellipticity (ε). The kinematics of disk galaxies are usually determined by Hα or 21 cm emission lines, which are typically not present in lenticular galaxies due to their general lack of cool gas. These effects make kinematic measurements of lenticular galaxies considerably more difficult compared to normal disk galaxies.

The kinematic connection between spiral and lenticular galaxies is most clear when analyzing the Tully–Fisher relation for spiral and lenticular samples. If lenticular galaxies are an evolved stage of spiral galaxies then they should have a similar Tully–Fisher relation with spirals, but with an offset in the luminosity / absolute magnitude axis. This would result from brighter, redder stars dominating the stellar populations of lenticulars. An example of this effect can be seen in the adjacent plot. Alternatively, it has been proposed that they grew their disks via (gas and minor merger) accretion events. It had previously been suggested that the evolution of luminous lenticular galaxies may be closely linked to that of elliptical galaxies, whereas fainter lenticulars might be more closely associated with ram pressure stripped spiral galaxies, although this latter galaxy harassment scenario has since been queried due to the existence of extremely isolated, low luminosity lenticular galaxies such as LEDA 2108986.

The absence of gas, presence of dust, lack of recent star formation, and rotational support are all attributes one might expect of a spiral galaxy which had used up all of its gas in the formation of stars. Moore et al. also document that tidal harassment – the gravitational effects from other, near by galaxies – could aid this process in dense regions. The clearest support for this theory, however, is their adherence to slightly shifted version of Tully–Fisher relation, discussed above. A 2012 paper that suggests a new classification system, first proposed by the Canadian astronomer Sidney van den Bergh, for lenticular and dwarf spheroidal galaxies (S0a S0b S0c dSph) that parallels the Hubble sequence for spirals and irregulars (Sa Sb Sc Im) reinforces this idea showing how the spiral–irregular sequence is very similar to this new one for lenticulars and dwarf ellipticals.

The analyses of Burstein and Sandage showed that lenticular galaxies typically have surface brightness much greater than other spiral classes. It is also thought that lenticular galaxies exhibit a larger bulge to disk ratio than spiral galaxies and this may be inconsistent with simple fading from a spiral. If S0s were formed by mergers of other spirals these observations would be fitting and it would also account for the increased frequency of globular clusters. It should be mentioned, however, that advanced models of the central bulge which include both a general Sersic profile and bar indicate a smaller bulge, and thus a lessened inconsistency. Mergers are also unable to account for the offset from the Tully–Fisher relation without assuming that the merged galaxies were quite different from those we see today.

The creation of disks in, at least some, lenticular galaxies via the accretion of gas, and small galaxies, around a pre existing spheroidal structure was first suggested as an explanation to match the high redshift compact massive spheroidal shaped galaxies with the equally compact massive bulges seen in nearby massive lenticular galaxies. In a "downsizing" scenario, bigger lenticular galaxies may have been built first – in a younger universe when more gas was available – and the lower mass galaxies may have been slower to attract their disk building material, as in the case of the isolated early type galaxy LEDA 2108986. Within galaxy clusters, ram pressure stripping removes gas and prevents the accretion of new gas that might be capable of furthering the development of the disk.