NGC 1172 is a lenticular galaxy located at a distance of 69.1 million light years (21.2 Mpc) [1] in the constellation Eridanus, which represents a river and more or less looks like a river. This is only the second time that the random number generator has selected an object in Eridanus despite the fact that it is the sixth largest constellation in the sky. On the other hand, the constelaltion Cetus is only about 10% larger, and the random number generator has so far selected 12 objects from that constellation. Go figure.
Anyway, as a lenticular galaxy, NGC 1172 has a morphology halfway between that of a spiral galaxy and an elliptical galaxy. It has a relatively smooth disk with a really large bulge, but it's hard to tell the two apart because we are looking at the galaxy face-on from Earth, and if you look up images of it online, you will see that it looks like a big ball of stars with an extended haze around it (although you will, for some reason, find pictures of rare historical coins for sale such as a medieval coin associated with Richard the Lionheart's reign over France [2] and a nineteenth century Indian one quarter rupee coin [3]). This is kind of indicative of the fact that not many people have devoted that much time looking at the galaxy NGC 1172. It's also worth keeping in mind that NGC can stand for not only "New General Catalogue" [4], which is a catalog of nearby star clusters, nebulae, and galaxies, but also "Numismatic Guaranty Company" [5], which is a company that assesses the authenticity and grade of antique coins. If the galaxy NGC 1172 was frequently studied scientifically, a Google search would return more results for scientific papers, and if it was a more popular amateur astronomy object, a Google search would produce more astrophotography results, but instead, a Google search turns up a combination of some amateur astronomy webpages that look like they were automatically computer generated, some randon numismatic results, and a load of links to one very specific paper with Ana Inés Ennis as the first author [6], and despite the lack of other research work on NGC 1172, I personally found that one paper sufficiently interesting that I thought I could devote an entire podcast episode to it (albeit a short one).
The paper is entitled "Wide-field study of the peculiar globular cluster system hosted by the field lenticular NGC 1172" [6]. While most amateur astronomers, when they hear the phrase "globular cluster" will typically think of the globular clusters orbiting within the Milky Way's halo, other galaxies also have their own collections of globular clusters orbiting within their halos. Ennis's group decided to focus on studying the globular clusters in NGC 1172 specifically because it was a relatively isolated lenticular galaxy [6], which meant that it was relatively straightforward to identify all of the globular clusters that belonged to that galaxy alone rather than clusters that might actually be orbiting different galaxies instead.
What they found was that the globular clusters belonged to two separate groups. One group was redder in color than the other, and this was interpreted as indicating that one group of globular clusters contained fewer elements heavier than hydrogen and helium in their outer atmospheres compared to the other group of clusters [6]. In turn, this implied that the stars in globular cluster with fewer heavy elements formed out of interstellar gas with fewer heavy elements to begin with. These heavy elements typically end up in the interstellar medium when either very massive stars explode as supernovae or stars about the size of the Sun die and shed their outer gas layers to form planetary nebulae. The stars that form out of that gas then end up with those heavy elements that were formed by that previous generation of stars.
The conclusion that Ennis and her collaborators reached was that a smaller galaxy collided with NGC 1172 a long time ago, and that smaller merging galaxy brought along its own globular clusters [6]. Before the merger, that smaller galaxy would have had fewer stars that could die and dump heavy elements into the galaxy's interstellar medium, so any globular clusters from the smaller galaxy would have also had fewer heavy elements in comparison to the clusters already in NGC 1172 before the merger event. After the two galaxies completely merged, the globular clusters from the smaller infalling galaxy would not have been destroyed but would just end up orbiting NGC 1172 along with the globular clusters that NGC 1172 had before the merger.
The reason why I find this specific analysis on NGC 1172 very interesting is that I have now talked about quite a few different globular clusters in our own galaxy where, for various reasons, astronomers have been able to determine that they originated from other smaller galaxies (typically dwarf galaxies) that merged with our own. This even includes the Messier object M56, which I discussed back in episode 33. I suppose I just found it interesting that people have now expanded this focus on the origins of globular clusters to other galaxies and had used the globular clusters to show that other galaxies had undergone merger events in their past. However, I suspect that this type of analysis has also been done on the globular clusters in other galaxies, and I think I want to spend more time hunting around in the scientific literature for those science results. Also, I now want to spend more time looking up pictures of ancient and medieval coins, but that's a separate thing.