A recently discovered ultra-faint dwarf galaxy is sending astronomers clues about the makeup of dark matter in the neighborhood of the Milky Way. It is one more clue that a type of stellar object called massive compact halo objects (MACHOs) are probably not the dominant component of dark matter in the universe.
A star cluster in the Eridanus II ultra-faint dwarf galaxy is hinting to Timothy Brandt, a researcher at the Institute for Advanced Study at Princeton University, that dark matter cannot be made of MACHOs of more than about five times the mass of our sun.
MACHOs are any type of non-luminous, ultra-compact object that emits little radiation, such that they aren’t detected through normal means. It’s believed that a grouping of these objects could account for the missing mass of the universe. Other work has made it seem unlikely that MACHOs make up the bulk of dark matter in other mass ranges, so the future looks dim for MACHOs.
Eridanus II is one of nine satellite galaxies of the Milky Way discovered last year as part of the Dark Energy Survey (DES). Ultra-faint dwarf galaxies like Eridanus II were predicted by computer models but only in recent years observed. They may contain only 1,000 to 10,000 stars within a space of only 50 light-years across for the smallest of them, although the extent of their dark matter components may be much bigger.
Brandt likes to use the analogy of a mixture of helium atoms and air molecules to describe the mixture of stars and potential dark matter MACHOs in Eridanus II: “If you have a mixture of helium and air, the helium atoms will move faster because they are less massive than air molecules.” The two types of objects exchange kinetic energy as they bump into each other, but for the same kinetic energy, the smaller helium atoms move faster than the more massive air molecules.
“The same thing applies with stars and more massive MACHOs,” says Brandt. “If the stars are the light objects compared to, say, 30-solar-mass black holes, the stars will tend to go faster than the MACHOs, and the visible galaxy will be puffier. Over a long period of time, the MACHO part of the galaxy (which we cannot see) would become more compact, and the stars (which we can see) would become more extended.”
In his observations of a star cluster in the heart of Eridanus II, Brandt does not observe the expected puffiness, so he estimates that the dark matter in Eridanus II must not be MACHOs with masses of more than about five solar masses.
Brandt’s conclusions were published in the June 22, 2016, Astrophysical Journal Letters.
According to measurements of the individual stars in the Eridanus II star cluster, the galaxy has had time for the stars to spread out if they were going to. The stars in most ultra-faint dwarf galaxies like Eridanus II have been around for a very long time.
“In addition to being the least luminous galaxies, they’re also the least chemically enriched,” says Marla Geha, an astronomer who studies ultra-faint dwarf galaxies at Yale University. “That suggests that they formed very early on in the universe, before star formation really got going in creating all these various different elements.”
Eridanus II swings around the Milky Way at a current distance of about 1.2 million light-years away from Earth. Astronomers can pin down the distance of an ultra-faint dwarf galaxy relatively easily, because they can resolve individual stars, and each star acts as a “standard candle.”
“A population of stars at a particular distance has a very unique track in color and brightness,” says Geha, “so we can use features in that space to determine what their distances are. The distances of these objects are actually pretty well-constrained.”
Only 11 satellite galaxies of the Milky Way, relatively bright ones, were known before the Sloan Digital Sky Survey (SDSS) brought the total up to 24. Now the DES, starting up in 2014 and using the Cerro Tololo Inter-American Observatory, in the Chilean Andes, has bumped the total up to about 50.
It’s a good thing, too. The theoretical predictions for satellite galaxies were so strong that before SDSS and DES started finding more, astronomers were worried about “the missing satellite problem.” Simulations of cosmic evolution predict that the Milky Way could have a total of up to a thousand dwarf satellite galaxies.
Precise measurements of the velocity of individual stars in an ultra-faint dwarf galaxy reveal how much unseen matter lurks inside the tiny galaxy. The greater the orbital velocity of a star, the more dark matter hides inside its orbit. The ultra-faints turn out to have a higher ratio of dark matter to luminous matter than full-size galaxies like the Milky Way.
“I think this is a sensible argument,” says Sergey E. Koposov from the Institute of Astronomy, University of Cambridge, of Brandt’s paper. “In my opinion, MACHOs aren’t really the most likely dark matter candidate, but I think under some assumptions he makes a reasonable argument.”
Koposov led one of two teams that found the group of nine ultra-faint dwarfs in the DES data and announced it in the May 28, 2015, Astrophysical Journal.
Geha is a little more skeptical: “I am not a huge fan of the Brandt paper. It has a large number of assumptions which I am not completely comfortable with, although it is a nice idea.”
“The stars in many of these dwarf galaxies are very, very old, and have had a long time to share energy with any MACHOs that might be floating around,” says Brandt. “It turns out that for many compact ultra-faint dwarfs, and in particular for one star cluster in a dwarf galaxy [Eridanus II], there has been plenty of time since the galaxies formed to puff them up to larger sizes than we observe.”
Allen Zeyher is a freelance writer in the Chicago area.