A Dark Matter Dwarf Galaxy Peeks Through the Milky Way's Murk and Dust

Some ​400,000 light-years away and just below the horizon line of the Milky Way's equatorial plane lie four stars. Known properly as Cepheid variables, they pulsate at regular intervals (days, usually) and act as intergalactic sounding instruments, helping astronomers determine distances between our galaxy's center and other nearby structures. These four offer something more, however: a dark matter-bound clue into one of the Milky Way's more profound mysteries: the "missing satellite" problem.

The Milky Way's immediate neighborhood is a lonelier place than it should be. Our simulations, based on the wobbles and perturbances at the edges of the galaxy, indicate the presence of a lot of stuff just beyond the Milky Way's boundaries. There should be entire galaxies out there, but we haven't observed nearly as many as simulations predict. This is frustrating.

Part of the problem is the galactic disk itself, which is basically a huge frisbee of dust, gas, and young stars radiating outward across a two-dimensional plane from the center of a galaxy. (Which introduces another problem: the rotational velocity of the Milky Way's disk doesn't match the amount of galactic matter that we're able to observe.) This disc blocks our view of any satellite galaxies or clusters that might be orbiting around the disk's outer perimeter. Even bright spiral galaxies can be obscured.

According to astronomers at Rochester Institute of Technology, these four Cepheid variables may likely represent a previously unidentified dwarf galaxy. The team analyzed near-infrared observations collected by the European Southern Observatory's VISTA sky survey—a database of tens of millions of stars—hoping to confirm an earlier prediction made by RIT's Sukanya Chakrabarti based on gravitational ripples found within the Milky Way's outer disk.

"The infrared tool offers an alternative view that's able to penetrate the disk's dust shield, at least somewhat," Chakrabarti and her group write ​in a new paper. "These are the most distant Cepheid variables close to the plane of our Galaxy discovered to date. The fact that the Cepheids that we detect are at an average distance of 90 [kiloparsecs], highly clustered in angle and in distance, is difficult to explain without invoking the hypothesis of these stars being associated with a dwarf galaxy."

As a cluster of dim lights, it seems likely that these four Cepheids are members of a dark matter-dominated galaxy, which would align well enough with most predictions. Dark matter is a solution to the above-mentioned missing satellites problem, as it offers a way to collect large amounts of material without collecting large amounts of light, or ​baryonic matter in general. In 2007, ​a survey of ultra-faint Milky Way satellites found dwarf galaxies composed of, in most cases, 99 percent dark matter.

"The discovery of the Cepheid variables shows that our method of finding the location of dark-matter dominated dwarf galaxies works," Chakrabarti offered in a statement. "It may help us ultimately understand what dark matter is made up of. It also shows that Newton's theory of gravity can be used out to the farthest reaches of a galaxy."