Since there is a correlation between the gas content of a galaxy and its spiral type, it is natural to ask whether there are any spiral galaxies with either no gas at all, or at any rate less than the few per cent contained in Sa types, or else with more gas than the Sd galaxies. Such systems do indeed exist; they are the LENTICULAR and IRREGULAR GALAXIES respectively.

Lenticular galaxies strongly resemble the Sa types, but they lack the extreme Population I component (i.e. gas and dust). Usually they have a large nuclear bulge, and where seen edge-on they show a characteristic outward cusp due to the stars in the plane of the galaxy. Many appear to contain no interstellar matter, although a small proportion shows absorption lanes when seen edge-on. These inconspicuous dust lanes appear to be practically circular when a lenticular galaxy is seen face-on . Because of their similarity to Sa types, the lenticular galaxies are held to belong to the spiral sequence, and therefore are often called .SO galaxies. It is less certain whether the lenticulars form a true intermediate state between spiral and elliptical galaxies. On the one hand, the large nuclear bulge of many lenticulars is reminiscent of an E7 type, but on the other hand the difference between a
spheroidal star cloud and a system with a fundamental plane is very profound, in shape as well as dynamically . Also, the idea of an E— S transition is in conflict with the finding that among the flattest stellar systems known there are many lenticular galaxies . Lastly, there is the curious fact that the masses of spiral galaxies are much more sharply defined than are those of ellipticals. Whereas E types range from 106 to almost 1013 solar masses, the S types are typically found between 1010 and a few times 1011 M0. Perhaps this effect is connected with the pos- sible existence of large haloes around galaxies or perhaps it is a selection effect, but its cause is not yet known. If it is real, this phenomenon speaks against lenticulars forming an E-S transition.

Irregular galaxies are more or less chaotic stellar systems. There exist two kinds of irregulars, which are usually easily distinguishable by their colour. The so-called TYPE i IRREGULARS are very blue, with a B-V index around 0.35; the TYPE n IRREGULARS have the ordinary yellowish colour of about 0.8. The Irr I are held to be intrinsically irregular, whereas the Irr II are made irregular by some disturbance, for example an explosion in the nucleus, as in M82 or a galaxy collision. Only the intrinsically irregular galaxies are considered here. They have the largest gas content of all galaxies . New stars formed from this gas give the systems their blue colour, while generating regions of ionized hydrogen that completely dominate the optical picture of the galaxy . Although some irregulars look vaguely like Sd galaxies with a very open spiral-like pattern, it is not certain whether the irregulars belong to the spiral sequence. The mass of an irregular galaxy is typically lower (1010 M0) than that of an average spiral. Also, irregulars less frequently have nuclei; the composition of their interstellar matter is sometimes highly atypi¬cal, as in the case of the Small Magellanic Cloud which has one third of its mass as gas, but practically no dust.

Many spiral galaxies have a pattern that winds inwards until, at a radius of five to 10 kpc. it breaks off, being replaced by what looks like a bar-like structure centred on the galaxy’s nucleus. Such systems are called BARRED SPIRAL GALAXIES. Apart from their inner shape, they resemble ordinary spirals hi every respect. They show a steady progression in the tightness of winding of their spiral parts ? Accordingly, their shapes are indicated as SBa through SBd. Much less is known about their gross properties than is known about ordinary spirals, partly because there are no nearby ones visible from the northern hemisphere, where mos large observatories are located. Bars also exist hi other types of galaxy. For example, the equivalent of the lenticulars are the THETA GALAXIES, named after their similarity to the Greek letter . Barred irregular galaxies also appear to exist; for example the
Large Magellanic Cloud contains a distinct elongated cloud of stars.

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