This episode's object, which is a dwarf galaxy that looks like a slightly overdense elliptical region of stars in the constellation Pisces, goes by a couple of different names. When it is mentioned in any popular astronomy references, it is called the Pisces Galaxy or Pisces Dwarf Galaxy or something like that, but professional astronomers generally call it LGS 3. The letters LGS stand for "Local Group Suspected", which indicates that this was initially identified as only possibly located within the Local Group, the group of galaxies containing the Milky Way and the Andromeda Galaxy. However, LGS 3 seemed to have been confirmed as part of the Local Group a few decades ago [1], so the name LGS 3 seems to be a bit outdated, although people are still using that name for some reason. By the way, I also found information for objects named LGS 1, 2, 4, and 5. LGS 1 and 4 are definitely located outside the Local Group [2], LGS 2 is just a weird reflection nebula in our galaxy and not a nearby dwarf galaxy [3] (and while it is technically in the Local Group, it doesn't really count if it's in our galaxy), and no one seems to have tried looking at LGS 5 since the 1970s, so, in all seriousness, I'm not even certain that LGS 5 exists.
Anyway, let's get back to LGS 3. Valentina E. Karachentseva has been credited with discovering this galaxy in 1976 [4], but the paper describing the discovery is not online, so I can't say anything about how it was discovered. What I can say that this was an era where people were identifying nearby dwarf galaxies by either making photographic plates of the sky or by staring for hours at the photographic plates provided by the Palomar Sky Survey. The first technique woiuld require luck, while the second would require huge amounts of commitment and eyedrops. The galaxy is located at a distance of about 2.1 million light years (650 kpc) [5] and seems to be close to both the Andromeda Galaxy and Messier 33, which means that it could be orbiting either of them (but most likely not both).
One of the interesting things about LGS 3 is that it is one of the very few galaxies in the sky that is actually moving towards the Milky Way rather than moving away because of the expansion of the universe [6]. This is because LGS 3 is within the gravitationally bound Local Group, and because the Local Group is gravitationally bound, the galaxies within it are not going to move away from each other as the universe expands. Instead, it is likely that some of the galaxies, or at least the Milky Way and the Andromeda Galaxy, will eventually merge together, although that will take billions of years. Smaller galaxies like LGS 3 tend to fall into larger galaxies like the Milky Way, the Andromeda Galaxy, and Messier 33, so it could merge into one of those larger galaxies at some point, but then again, this will take billions of years.
Since LGS 3 is close enough that it is possible to identify many of the individual stars in the galaxy and since dwarf galaxies like LGS 3 contain stars that are a little different from what we see in the Milky Way, people have spent a lot of time making detailed observations of the stars in this galaxy. Among other things, people have studies the current stellar population to infer what has happened in the past in terms of star formation. I found a 2011 paper by Sebastian L. Hidalgo et al. that gave such a good description of this [7] that I’m just going to summarize that paper’s results right now.
So, to begin with, LGS 3 would have formed shortly after the Big Bang out of some slightly extra dense region of dark matter and gas. That gas and dark matter would have gravitationally pulled in other gas and dark matter from the nearby area, and the infalling gas specifically would provide the material for forming new stars. In fact, it looks like the majority of stars in LGS 3 formed during a time period staring 11.7 billion years ago that ran for about 1.4 billion years [7]. After that, the rate at which stars formed dropped dramatically, although it didn’t stop entirely. This is in contrast to spiral galaxies like the Milky Way, where stars have been forming at a relatively steady state for billions of years.
The slowdown in star formation in LGS 3 could have occurred in part because a lot of the newly formed stars were the types of very large stars that have short lifespans and that explode as supernovae, and those supernovae could have driven all a lot of the gas needed to create stars out of the dwarf galaxy. However, the energy expected from these supernovae would be inadequate for stopping stars from forming in LGS 3, so Hidalgo et al. proposed that the gas within LGS 3 may have also been partly ionized by the relatively intense ultraviolet light from both the stars forming within the galaxy itself but also from other nearby galaxies where lots of stars were forming at the same time [7], and when interstellar gas gets ionized, it also stops gravitationally collapsing to form stars. This result may explain why not only LGS 3 but also many other similar dwarf galaxies did not continue to form stars at the same rapid rates at which they formed stars when the universe was much younger. In contrast, the gas in much larger protogalaxies in the in the early universe, like spiral galaxies, may have been more resistant to being fully ionized, and so the gas in those galaxies could continue to be used to form stars, and that would explain why spiral galaxies like the Milky Way look different from dwarf galaxies like LGS 3.
As a final note, it’s worth pointing out that LGS 3 might be gravitationally interacting with a blob of intergalactic hydrogen gas and dark matter moving at a relatively high velocity compared to the dwarf [8]. This blob could provide a lot of extra material for forming new stars if it fell into LGS 3, but it looks more like the blob is just going to continue speeding past the dwarf galaxy instead of falling into it.