This episode's coordinates point to a location in the southern constellation Hydrus. At first, I thought that Hydrus was an alternate name for the constellation Hydra, which represents the multi-headed snake that Hercules fought in Greek mythology, but as it turns out, Hydrus is not that mythological beast but instead the "lesser water snake". It's a constellation that is supposed to represent a sea snake [1]. This constellation and several others located in the southern part of the sky were created by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman in the 1500s when they charted the southern skies [1], and since they seemed to like naming constellations after tropical animals, this one got named after a sea snake. It's possible to draw a squiggly line with one extra-large squiggle through all of the stars in this constellation, so it technically does look like a sea snake, but it could have also been named after an earthworm or a sea cucumber.
Anyhow, the specific object that this episode's coordinates point to is the galaxy Hydrus I, which was named after the constellation that it's found in. Hydrus I was discovered by the Dark Energy Survey [2], a survey covering 5000 square degrees of the southern sky using the Blanco 4-m telescope at Cerro Telolo Inter-American Observatory [3, 4]. This survey, which started in 2013, has been mapping hundreds of millions of distant galaxies and will be attempting to measure the large scale structure of the universe, to measure very slight gravitational distortions on the light from these distant galaxies, and to detect new supernovae, with the ultimate goal of measuring the effects of dark energy [3]. Dark energy, which has one of the coolest names in physics, is basically a type of exotic form of energy associated with voids in space that is causing the universe's expansion to accelerate over time, and trying to understand dark energy has been one of the major challenges in astrophysics during this century.
However, Hydrus I has almost nothing to do with the study of dark energy. This object is not a very distant galaxy that can be used in the analysis of the effects of dark energy but is instead a dwarf galaxy very close to the Milky Way that was discovered by accident (or, to use the fancy term, serendipitously) by the Dark Energy Survey when it pointed at the part of the sky covering the constellation Hydrus [2]. A few astronomers have specialized in studying things that appear in these types of cosmological surveys that that the cosmologists are not interested in because they are way too close and therefore way too easy to understand. Sergey E. Koposov is one of those people. He likes to specialize in studying star clusters and streams of stars created when the Milky Way tidally tears apart dwarf galaxies, and he was the lead author on the paper published in 2018 announcing the discovery of Hydrus I [2].
So, first of all, one of the important things to say about Hydrus I is that it is an ultrafaint dwarf galaxy, which has made it very difficult to find. While many dwarf galaxies orbiting the Milky Way were discovered either by people looking through telescopes in the nineteenth century or using photographic plates in the twentieth century, these dwarf galaxies have been relatively bright objects. Very faint satellite galaxies like Hydrus I have only been detected in modern digital surveys, which means that, in recent years, astronomers have been spending a lot of time working to try to understand these newly-discovered galaxies and how they fit into the grand scheme of things.
Additionally, in terms of galaxies orbiting the Milky Way, Hydrus I is very close to Earth. The distance to the galaxy is about 94.6 million light years (28 kpc) from the Solar System [2]. It also appears to orbit the Large Magellanic Cloud, a relatively large dwarf galaxy orbiting our own [2, 5]. This means that Hydrus I is actually a satellite galaxy of another satellite galaxy. It might also somehow be associated with a stream of matter connecting the Large Magellanic Cloud with the Small Magellanic Cloud, another dwarf galaxy orbiting our own [2].
Hydrus I is also very small, or else it would not be in orbit around a dwarf galaxy like the Large Magellanic Cloud. The galaxy has a mass that is 220 thousand times the mass of the Sun [2]. If you listened to episode 129 in my podcast series, you would have heard me talk about how it could be difficult to differentiate between a relatively large, distant globular cluster and a relatively small, nearby dwarf galaxy, and this issue is relevant to Hydrus I, which has a mass that is similar to that of many globular clusters. However, dwarf galaxies are expected to contain dark matter, while globular clusters should not [6, 7]. (Just for reference, dark matter is matter that cannot be seen in any part of the electromagnetic spectrum but appears to exert gravitational forces on visible matter, and dark matter should not be confused with dark energy.) A comparison of the orbital velocities of the stars in Hydrus I to the brightness of the entire system indicates that it does indeed contains a lot of dark matter, which means that it definitely should be classified as a dwarf galaxy [2].
Hydrus I and dwarf galaxies like it are important to study for a few reasons. The primary reason why identifying these dwarf galaxies is important to get a complete picture of the Milky Way's neighborhood, sort of like people should (hypothetically) get to know their neighbors in real life, although I would admit that I hardly know who my neighbors are. However, these galaxies are often weird because they contain relatively large amounts of dark matter, which indicates that they formed in relatively unusual ways. Additionally, these galaxies usually contain very old stars; the stars in Hydrus I have an age of around 12 billion years. Hence, if astronomers want to understand what very old stars are like, they might want to look at Hydrus I. In any case, Hydrus I definitely seems like something that a lot of astronomers would be interested in studying with the exception of those cosmology people who aren't really interested in nearby galaxies and just want to look at the really far away and difficult-to-see things behind Hydrus I.