by Sarah Scoles

Where astronomers expected to find at most one black hole, they found two, which is a bigger deal than it sounds like.

Strader, Chomiuk, Maccarone, Miller-Jones, and Seth used the VLA to look at the center of globular cluster M22 to determine whether an intermediate-mass black hole lived at the center. While they did not detect a central object, they did see two bright radio sources. These two radio sources, the recent Nature paper posits, are stellar-mass black holes.

If these sources do turn out to be black holes, not only will they be unexpected, but they will also be the first black holes discovered in a globular cluster and the first stellar-mass black holes discovered using radio waves. Like, ever.

Why did astronomers expect to find a single black hole at the center?

Globular clusters are crowded and geriatric--essentially cosmic nursing homes. Because their stars are so old, the most massive ones have already imploded into black holes. Because these black holes are more massive than the other stars in the cluster (which are still living their regular lives, since the less mass a star has, the longer it lives), they "sink" toward the center.

The theory goes that after the black holes sink to the center, they all get in a big gravitational territory fight in which there can be at most one winner. The winner gets to stay in the nursing home, at the center (the cafeteria?). The other black holes are, theoretically, kicked out of the nursing home and back into the real world (the galaxy)

But that fight obviously doesn't take place exactly the way astronomers thought, given these two black hole tenants.

But how do we know they're really black holes?

Well, we don't "know," per se, but it is, according to the paper, "the most likely explanation" for two reasons:

They are in the core of the globular cluster, which means they must have sunk there, which means they must have a lot of mass. The ratio of their brightness in radio waves compared to their brightness in X-rays is similar to the radio/X-ray ratio seen in black holes inside the galaxy. These sources were easily detectable in radio waves but were not detected at all in X-rays, which does not mean they are not emitting X-rays, but means they must be below a certain brightness.

The red M22 represents the properties of the two new black holes. The connected arrow goes to the left because their X-ray luminosities could be 2x10^30 erg/s or lower). Black holes tend to lie along the black line, neutron stars along the blue lines, and white dwarfs not higher than the green line.

In the plot to the right, the x-axis is X-ray luminosity, and the y-axis is radio luminosity. This plot shows the relationship described in #2. If something is emitting lots of X-rays and lots of radio waves, it shows up in the top right. If something is not emitting very much of either sort of radiation, it shows up in the bottom left.

The red M22, representing both new sources, clearly shares a radio-X-ray relationship more similar to black holes than to neutron stars or white dwarfs--the next heaviest things in the universe. (Note: in the paper, the authors give several reasons that M22 is not on the black hole line; I won't go into them here, but the paper is very readable and the rationalizations come just after Figure 2).

Okay, so these objects are massive, and they emit radiation in the same way that black holes do. Wait, how are we seeing radiation from them if they're black holes? Isn't the point of black holes that they're dark?

As described in the first "So your friend asks..." post, we can only detect black holes if material is in the process of falling into them and/or if other stars are orbiting them.

As the material swirls into the black hole's mouth, like water and your hair down a bathtub drain (yum), it heats up, and this heat causes radiation. The infall also causes huge jets to shoot out of the "top" and "bottom" of the black hole.

For us to see these two globular black holes, they must be actively feeding.

To find out which stars may be the black holes' companions, astronomers compare the locations of the purported black holes to Hubble Space Telescope images of the area. Anything in those two circles could be the stellar companions. Luckily, that looks like only one source for VLA1, and only 1.23 sources for VLA2.

Presumably, on a binary companion star. So these hungry black holes are hanging out in the nursing home, tired from gravitational bouts that were not quite as predicted, and then they decide to eat their long-time partners. Not all at once, but little by little.

To find these unfortunately companions, the team looked at Hubble Space Telescope images to see if any visible stars were very close in location to the black holes' locations. One of the sources is near a red dwarf, and the other is near an orange dwarf (a star somewhere between a red dwarf and the sun. Neither of those is a definite companion, but both are candidates.

How is the research team explaining their result, since it didn't agree with the predictions?

For these two black holes to be here, it must mean that gravitational interactions between black holes that migrate to the center of globular clusters must not always lead to a winner-take-all scenario. Multiple black holes can, it appears, exist in the core of one globular cluster. Strader, et al., say this means that the black holes must not have received a large velocity 'kick' when they were born. It also means that these two are probably not the only ones: if we can only see the black holes that are eating their friends, it stands to reason that there are many more black holes that are not eating their friends, and thus that we can't see. After all, for every person who is eating their friends, there are many more who are not.

Strader J, Chomiuk L, Maccarone TJ, Miller-Jones JC, & Seth AC (2012). Two stellar-mass black holes in the globular cluster M22. Nature, 490 (7418), 71-3 PMID: 23038466