Object 23: NGC 5102
Podcast release date: 15 June 2020
Right ascension: 13:21:57.6
Corresponding Earth location: Slightly less than 2000 km east from the eastern tip of New Zealand's North Island
NGC 5102 is a lenticular galaxy  in the constellation Centaurus at a distance of about 12.2 million light years (3.74 Mpc) . A lenticular galaxy, which may also be called an S0 galaxy, is a type of galaxy about halfway between a spiral galaxy and an elliptical galaxy. A lenticular galaxy has a disk of stars and a bulge of stars in the center like spiral galaxies, but the bulge is relatively large compared to the disk. Also, the disks of lenticular galaxies do not have any spiral patterns in them, and while spiral galaxies contain a lot of interstellar gas and dust, lenticular galaxies contain very little interstellar material.
NGC 5102 is embedded within a nearby group of galaxies called the Centaurus A / M83 Group . This group is named after the two largest galaxies in the group, and it's debatable as to whether both of those galaxies are at the centers of their own groups or if they are both part of one larger gravitationally-bound, dumbell-shaped group. Assuming that this is one group, NGC 5102 is the third brightest galaxy in the part of the group that contains Centaurus A.
NGC 5102 is sufficiently close to Earth that it is actually a fairly good amateur astronomy object, although you have to be in the tropics or the Southern Hemisphere to see it. I recommend first finding the star Iota Centauri, which is a magnitude 2.7 star located near the top of the constellation Centaurus (and I recommend getting a star chart to help identify which star is Iota Centauri). After this, point a telescope to a location northwest of this star at a distance of about 17 arcminutes, which is slightly larger than half the diameter of the Moon. The galaxy is fairly compact and has a brightness of magnitude 9 in the visible part of the spectrum , so even in small telescopes with a diameter of 7.5 cm or 3 inches, it's possible to see the oval structure of the galaxy . In telescopes twice as large, it's possible to distinguish the light from the centre of the bulge and from the disk .
Professional astronomers an interest in NGC 5102 because, over time, they have observed a series of peculiarities about the galaxy that make it different from other lenticular galaxies. The first thing that people noticed is that the center of NGC 5102 is rather blue for a lenticular galaxy . A number of follow-up observations demonstrated that the galaxy actually contains blue stars in both its nucleus and its bulge [8,9,10].
Most lenticular galaxies contain old stars, which usually look red. Blue stars have short lifespans in astronomical terms, lasting only between a few million and a few hundred million years before they use up all of the hydrogen in their centers, evolve into red giants, and then evolve further until they either explode as supernovae (if they are large enough) or form planetary nebulae.
So, it looks like star formation has been taking place in NGC 5102, although the star formation has been at relatively low levels. As I indicated before, lenticular galaxies do not normally contain much interstellar gas, but galaxies need interstellar gas to form stars. Moreover, the younger stars are found in the bulge, which is weird because young stars are usually found in the disks of galaxies.
This leads to questions about how star formation was triggered in NGC 5102. A couple of hypotheses have been put forward, but the most likely explanation is that the younger stars formed out of gas that came from outside the galaxy .
The main proof for this is an observation that some of the stars near the center of NGC 5102 are rotating around the center of the galaxy in the opposite direction from all of the other stars . This is called counterrotation, and I even wrote my first science paper on this phenomenon . It might sound very difficult to envision stars travelling in different directions past each other. Some people might compare this to cars driving the wrong way down a highway, which would lead to many horrific accidents as well as quite a few traffic tickets. However, the gaps between stars are so big that individual stars not only avoid colliding with each other but even avoid gravitationally interacting with each other. So, a more apt comparison to counterrotating stars may be two flocks of birds where the birds are spaced far enough apart that the flocks pass straight through each other without any birds colliding in mid-air and falling to the ground.
In any case, some of the stars in NGC 5102 are travelling the wrong direction . Within a galaxy, most stars and virtually all of the interstellar gas usually orbit around the galaxy's center in the same direction, so any new stars that form from the interstellar gas will still travel in the same direction as all of the older stars. However, if a galaxy acquires gas from somewhere else, so for example if a smaller galaxy falls into it, then that new gas could end up rotating in the opposite direction from the existing older stars, and when that gas forms stars, the new stars will move in the opposite direction from the older stars.
That's what seemed to happen with NGC 5102 . It's the only practical explanation for why this galaxy contains counterrotating stars. It also explains how a lenticular galaxy was able to form new stars when it would normally not have the gas to form stars in the first place. Moreover, because these types of merger events are chaotic, the stars that form from the infalling interstellar gas may not stay in the plane of the galaxy but end up outside the plane in the bulge, which is exactly what we see here.
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 Davidge, T. J., The Star-forming Histories of the Nucleus, Bulge, and Inner Disk of NGC 5102: Clues to the Evolution of a Nearby Lenticular Galaxy, 2015, Astrophysical Journal, 799, 97
 Mitzkus, Martin et al., Dominant dark matter and a counter-rotating disc: MUSE view of the low-luminosity S0 galaxy NGC 5102, 2017, Monthly Notices of the Royal Astronomical Society, 464, 4789
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