A 'Galaxy X' Found Orbiting the Milky Way

 

 

The search for a mystery Galaxy X orbiting the Milky Way may have been solved. Many large galaxies are thought to have dark matter satellite galaxies too dim to be detected. Theoretical astronomer Sukanya Chakrabarti explains how she has found a way to locate "dark" satellite galaxies by analyzing the ripples in the hydrogen gas distributed in spiral galaxies such as the Milky Way. Chakrabarti predicted the mass of Galaxy X to be one-hundredth that of the Milky Way, in a parabolic orbit around our galaxy, at a distance of about 300,000 light years from the galactic center.

The Milky Way is surrounded by some 80 known or suspected dwarf galaxies that are called satellite galaxies, even though some of them may just be passing through, not captured into orbits around the galaxy. The Large and Small Magellanic Clouds are two such satellites, both of them irregular dwarf galaxies.

Theoretical models of rotating spiral galaxies, however, predict that there should be many more satellite galaxies, perhaps thousands, with small ones even more prevalent than large ones. Dwarf galaxies, however, are faint, and some of the galaxies may be primarily invisible dark matter.
"The method is like inferring the size and speed of a ship by looking at its wake," said Chakrabarti's colleague Leo Blitz, a UC Berkeley professor of astronomy . "You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner."

Earlier this year, Chakrabarti used her mathematical method to predict that a dwarf galaxy sits on the opposite side of the Milky Way, but is not able to be seen due to gas and dust in the galaxy's disk.

Chakrabarti and Blitz also calculated that the predicted galaxy is in a parabolic orbit around the Milky Way, now at a distance of about 300,000 light years from the galactic center. The galactic radius is about 50,000 light years.

"Our paper is a proof of principle, but we need to look at a much larger sample of spiral galaxies with optically visible galactic companions to determine the incidence of false positives," and thus the method's reliability, Chakrabarti said.

Earlier this year, Chakrabarti used her mathematical method to predict that a dwarf galaxy sits on the opposite side of the Milky Way from Earth, and that it has been unseen to date because it is obscured by the intervening gas and dust in the galaxy's disk. One astronomer has already applied for time on the Spitzer Space Telescope to look in infrared wavelengths for this hypothetical Galaxy X.

"My hope is that this method can serve as a probe of mass distribution and of dark matter in galaxies, in the way that gravitational lensing today has become a probe for distant galaxies," Chakrabarti said.

Since her prediction for the Milky Way, Chakrabarti has gained confidence in her method after successfully testing it on two galaxies with known, faint satellites," a method that has broad implications for many fields of physics and astronomy – for the indirect detection of dark matter as well as dark-matter dominated dwarf galaxies, planetary dynamics, and for galaxy evolution driven by satellite impacts," she said.

Chakrabarti's colleague Leo Blitz, a UC Berkeley professor of astronomy, said that the method could also help test an alternative to dark matter theory, which proposes a modification to the law of gravity to explain the missing mass in galaxies."The matter density in the outer reaches of spiral galaxies is hard to explain in the context of modified gravity, so if this tidal analysis continues to work, and we can find other dark galaxies in distant halos, it may allow us to rule out modified gravity."

Chakrabarti and Blitz realized that dwarf galaxies would create disturbances in the distribution of cold atomic hydrogen gas (H I) within the disk of a galaxy, and that these perturbations could reveal not only the mass, but the distance and location of the satellite. The cold hydrogen gas in spiral galaxies is gravitationally confined to the plane of the galactic disk and extends much farther out than the visible stars – sometimes up to five times the diameter of the visible spiral. The cold gas can be mapped by radio telescopes.

"The method is like inferring the size and speed of a ship by looking at its wake," said Blitz. "You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner."

The technique Chakrabarti developed involves a Fourier analysis of the gas distribution determined by high-resolution radio observations for satellite galaxies as small as one-thousandth the mass of the primary galaxy. Her initial predication of Galaxy X around the Milky Way was made possible by of data already available on the atomic hydrogen in the Milky Way.

To test her theory on other galaxies, she and her collaborators used recent data from a radio survey called The HI Nearby Galaxy Survey (THINGS), conducted by the Very Large Array, as well as its extension to the Southern Hemisphere, THINGS-SOUTH, a survey carried out by the Australia Telescope Compact Array.

"Our paper is a proof of principle, but we need to look at a much larger sample of spiral galaxies with optically visible galactic companions to determine the incidence of false positives," and thus the method's reliability, Chakrabarti said.

 

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