The Hubble Space Telescope has identified a new dwarf galaxy near our Local Group, about two megaparsecs (or about 6.9 million light-years) away from the Earth. Unlike our own galaxy, the dwarf—known as KKs 3—is a spheroid and has no spiral arms. While the object has been detected in previous sky surveys, it has now been conclusively identified as a spheroidal dwarf galaxy.

The discovery is important for cosmological models involving dwarfs. Dwarfs of any kind seem to be present in much lower numbers than expected. Thirty spheroidal dwarfs have been discovered near the Local Group in the past decade, but it’s not known how many there are in total—finding a definitive number is essential for creating more detailed models of galaxy formation and evolution.

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The galaxy’s stars are very ancient. It contains plenty of red giants—inflated stars close to the end of their lives—a population that our Sun will one day join. The galaxy is also no longer producing new stars—a star hasn’t been born in KKs 3 in the past billion years. That's because the galaxy itself is very old, estimated to be over twelve billion years old. Thus, it provides astronomers with a fascinating glimpse into the very early Universe.

This is typical for a spheroidal dwarf galaxy. These galaxies, shaped as their name implies, have very little of the gas that forms stars, unlike their cousins, irregular dwarf galaxies. Irregular dwarfs are rich in gas and have active star formation.

However, KKs 3 is not completely typical of a spheroidal dwarf. It’s also isolated, having not interacted with its neighbors in a very long time, which puts it in the very rare category of isolated spheroidal dwarfs. The fact that the galaxy is isolated puts some interesting constraints on our understandings of spheroidal dwarfs. The current thought is that most dwarf galaxies merged together early on in the Universe’s history, creating the larger galaxies like our own.

The dwarfs that survive this process would typically be gas-rich irregular galaxies. So any gas-poor spheroidal galaxies like KKs 3 are thought to have originally been irregulars but then interacted with a nearby object and had their gas stripped away by that object's gravity, turning them into the spheroids we see today. But since KKs 3 is isolated, it’s very unlikely that it formed this way, meaning that there’s likely another mechanism turning irregulars into spheroidal dwarfs.

A brief history of KKs 3

By plotting the galaxy’s stars on a color-magnitude diagram, the researchers were able to identify populations of stars and determine their respective ages. In that way, they gleaned information about the galaxy’s history. They identified three major episodes of star formation in KKs 3's history. The first occurred around the origin of the galaxy itself, 12-14 billion years ago (note that the Universe is only about 13.8 billion years old, so the galaxy must be younger than that). The second was about five billion years ago, and the third might have been around two billion years ago.

The latest of the three periods produced fewer stars, at a slower rate than the earlier episodes. Nevertheless, it made a significant contribution to the population of red giants, while the earlier two eras make weak contributions. Since red giants are old stars, this runs counter to our expectations.

The galaxy has a mass of about 23 million times the mass of the Sun. (Compare that to the Milky Way galaxy, which has about a trillion solar masses). Of KKs 3‘s mass, most of it (74 percent) was formed in the earliest of the three star formation episodes, as that period had the highest star formation rate. The later two episodes contribute about 14 and 12 percent, respectively, of the galaxy’s total stellar mass.

The earlier stars are poorer in metals, such as iron (for astrophysical purposes, "metals" refers to any element heavier than helium). As time went on, newly formed stars became more metallic, with the most recent star formation episode having the most present in the stars. This is to be expected, since metals are produced by nuclear fusion in the cores of stars, and in supernova explosions. So as time went on, more metals were produced, which could then be incorporated into newer stars.

Galactic neighborhood

Based on its location, it's relatively certain that KKs 3 hasn’t interacted with other galaxies in the past ten billion years, at least. It’s also not quite close enough to the Local Group to escape the effects of the expansion of the Universe, which the galaxies inside the group do because they’re gravitationally bound together. So if anything, it's getting further from contacting another galaxy.

A few other isolated spheroidal dwarfs have been discovered in the region near KKs 3 as well, and there are also some inside the Local Group, tied to it despite the Universe’s expansion. KKs 3‘s closest spheroidal dwarf neighbor, KKR 25, is also isolated, and it’s virtually certain that tbe two galaxies haven’t been interacting with each other. Now that there’s more than one example of an isolated spheroidal dwarf, it’s clearer that there’s another mechanism for the formation of spheroidal dwarfs besides gas being stripped from irregulars.

Galaxies like KKs 3 are extremely difficult to detect, largely because they’re so faint, lacking the gas to make new stars and thus produce more intense light. As a result, it’s possible that such galaxies are quite common, and only a few of them, the tip of the iceberg, have been discovered.

When the James Webb Space Telescope is launched in 2018, it should have the precision necessary to find more such objects—if many of them exist—and shed some light on the situation.

MNRAS Letters, 2014. DOI: 10.1093/mnrasl/slu181 (About DOIs)