Our Faint Galactic Companions
And Their Possible Dark Matter Implications
It's long been known that our Milky Way, as one of the largest galaxies in its neighborhood, can claim some natural satellites to its name. As the Earth has its moon, and the Sun has its orbiting planets and other bodies, our home galaxy's enormous mass induces over two dozen objects into orbit around it. Some of these have been known to humanity since before we wrote our history down; the Large and Small Magellanic Clouds, satellites of the Milky Way, have been visible as faint smears of light to southern hemisphere observers for many millennia. Newer, more powerful observational tools have added many entries to this list in the 20th and 21st centuries. These satellites are not moons or planets, but rather globular clusters of stars, or dwarf galaxies.
The Large Magellanic Cloud, a dwarf galaxy which orbits the Milky Way at a distance of 163,000 light years, imaged from Earth. It has the equivalent mass of about 10 billion of our Suns.
A new paper purports to add nine more members to this family, at least three of which are dwarf galaxies (small galaxies with "only" a few billion stars). The authors used publicly available data from the Dark Energy Survey (DES), a project that observes the sky in the optical and near-infrared wavelengths to study the expansion of the universe and characterize its highest-level structure. The data comes from a 570 megapixel camera (yowza) with a 90 second exposure, affixed to a Chilean telescope. Though it is incidental to the primary purpose of DES, the collected imagery naturally contains a lot of information about the density of stars in our immediate galactic neighborhood. It was this information that the authors used to identify what they call "ultra faint objects" that orbit the Milky Way.
All of the identified satellites are in the vicinity of the Large and Small Magellanic clouds, and their spatial distribution is such that it is likely at least some were once associated with them. This jibes with a long held hypothesis that the LMC and SMC play some part in seeding more Milky Way satellites.
Furthermore, one of the objects, which the authors have dubbed Reticulum 2 and suspect is a dwarf galaxy, shows an elongated profile that is consistent with the effect of tidal forces imparted by its massive galactic neighbor, our Milky Way. Tidal forces are something we expect to see in any orbital relationship. The force imparted on you by gravity grows weaker the further you get from the source. On the scale of celestial bodies, this matters a lot. The fact that the side of the Earth that is facing the Moon is substantially closer to the Moon than the opposite side means that the Moon-facing side experiences a greater gravitational tug; this pulls unevenly on the Earth's oceans and results in the high tide and low tide we're familiar with.
Similarly, the moons of Jupiter experience immense tidal forces due to Jupiter's strong gravitational pull, which create friction and drive tectonic processes that generate heat internally. This effect must scale up to the galactic theater, and indeed it is expected that tidal forces of the Milky Way are responsible for the smudgy, irregular shape of the Large Magellanic Cloud, as well as tails of gas emanating from the Small Magellanic Cloud. The stretched out morphology of Reticulum 2 may be a further manifestation of this effect.
The Tadpole Galaxy demonstrates tidal forces on a nearly incomprehensible galactic scale -- it sports a "tail" of millions of stars, 280,000 light years in length, extruded by tidal forces imparted by an encounter with another nearby galaxy.
A follow-up paper by the same authors highlights an even greater potential implication of Reticulum 2, but a bit of background first:
As early as the 1930s, observers of the cosmos were noticing something troubling: galaxies were not behaving in a way that was explainable via the amount of matter they were able to observe. In particular, stars in the outer regions of galaxies moved a lot more quickly than they ought to have, and the paths of light traveling around large clusters of galaxies deviated due to gravity much more than was predicted. It all pointed to a lot more mass than what we could see. The inescapable conclusion was that large quantities of matter existed to produce these effects, matter that could not be seen or detected because it did not emit or absorb light or other electromagnetic radiation. For obvious reasons, observers termed this dark matter. And we've been able to infer that it makes up quite a bit of our observable universe -- almost 27% in fact, whereas the matter we see and interact with accounts for only 5%. It is suspected that dark matter forms massive envelopes that surround visible galaxies in what are termed halos.
But as you might imagine, matter that doesn't emit or absorb any type of radiation, including light, is nearly impossible to study. Consequently, it remains one of the most puzzling features of our universe, and scientists are still struggling to characterize it. Some potential models for dark matter provide a possible method of indirect observation. If correct, they predict that particles of dark matter will sometimes annihilate each other, producing gamma rays. Observing such products of dark matter annihilation would go a long way towards furthering our understanding of what, exactly, dark matter is.
A great place to look for dark matter is in very small galaxies. The smaller the better, in fact. Ethan Siegel explains why: The star formation that occurs in any given galaxy generates radiation pressure that accelerates particles of normal matter, but does nothing to dark matter, since the latter doesn't interact with any kind of radiation. In smaller galaxies, where there is less mass and hence less gravitational pull, the effect is that normal matter gets blown out of the galaxy while the dark matter remains. So generally, the smaller the galaxy, the higher the ratio of dark matter to normal matter.
One of our newfound Milky Way satellites, Reticulum 2, is suspected to be a small dwarf galaxy with a relatively large dark matter halo, making it an appealing target for trying to observe these products of dark matter annihilation. And the follow-up paper reports the observation of gamma rays from Reticulum 2 that are consistent with what the annihilation of dark matter particles is predicted to look like. This comes with a huge note of caution. There are plenty of potential sources of gamma rays in the universe that have nothing whatsoever to do with dark matter annihilation, and this is far from a sure thing. As astrophysicist Matt Buckley stated rather bluntly on Twitter:
Summary (cont.): WE DID NOT FIND DARK MATTER TONIGHT. I cannot emphasize this enough. If you say we did, you are bad and you should feel bad
Still, it is a fascinating result that will spark plenty of discussion and further study in the arena of dark matter research. And it would be oddly comforting if our closest galactic neighbors could help shed a little light on one of the most inaccessible mysteries of the deepest cosmos.
Image Credits: Cover Image -- European Southern Observatory; Large Magellanic Cloud -- Primoz Ciqler; Tadpole Galaxy -- Hubble/NASA/ESA