Lenticular galaxies look like spirals that have run out of fuel
Picture a spiral galaxy with its arms erased and its star-making gas gone. What remains is a lenticular galaxy: a smooth disc around a fat central bulge, filled with stars all thought to be older than about a billion years. Astronomers still argue over whether these are faded spirals, merger products, or something else entirely.
On the classic sequence of galaxy shapes, lenticulars, labelled S0, sit between the round, featureless ellipticals and the spirals. Like spirals they have a disc, often with a central bar and sometimes lanes of dust; like ellipticals they are dominated by a bulge and by old, reddish stars. With little or no cold molecular gas, they barely form new stars. Because their discs are featureless, astronomers subdivide them not by arm shape but by how much dust dims the disc, or, for barred ones, by how prominent the bar is.
Telling one from an elliptical can be tricky. The trick is motion. Stars in a bulge buzz around randomly, the way air molecules hold up a balloon, while a disc is held together by orderly rotation. Comparing rotation speed with the random scatter of stellar velocities separates them: ellipticals rotate relatively little. Measuring lenticulars is harder than measuring spirals, though, because they lack the glowing gas that normally reveals rotation, forcing astronomers to rely on fainter absorption lines in starlight.
The strongest clue to their past comes from the Tully-Fisher relation, which links a disc galaxy's rotation to its brightness. Lenticulars follow the same slope as spirals but sit offset by about 1.5 magnitudes, just what you would expect if spirals had stopped forming bright young stars and aged into redder populations. Gas-poor, anaemic spirals and gravitational harassment from neighbours in crowded regions could help drive that transformation.
It cannot be the whole story. Some lenticulars outshine spirals, suggesting mergers or gradual accretion built them instead. Very isolated examples such as LEDA 2108986 undercut the idea that crowding strips them. A scheme that originated with Sidney van den Bergh, a Canadian, arranges lenticulars in a sequence that mirrors the spirals, hinting at a parallel family.
Source: Lenticular galaxy
