Monday, February 27, 2012

Galactic Structure Caused By Black Hole Barf

The behavior of stars in the central bulge of a galaxy is closely related to the size of its central black hole, despite the fact that gravity alone is too weak by six orders of magnitude to explain the range of the effect. Why is this the case?

It looks increasingly plausible that the link between central black hole size and the behavior of stars in the central bulge of those galaxies may be due to flows of matter away from the central black holes called "ultra-fast outflows" (consider the acronym) distinct from the X-rays that shoot out in both directions in narrow jets from the axis of a central black hole, that are more aptly described as black hole barf, in amounts roughly comparable to the amount of new matter that they gobble up each year.

It turns out the the black holes at the center of galaxies often seem to have bulimia, and are not simply gluttons.

Near the inner edge of the disk, a fraction of the matter orbiting a black hole often is redirected into an outward particle jet. Although these jets can hurl matter at half the speed of light, computer simulations show that they remain narrow and deposit most of their energy far beyond the galaxy's star-forming regions. . . .

At the centers of some active galaxies, X-ray observations at wavelengths corresponding to those of fluorescent iron show that this radiation is being absorbed. This means that clouds of cooler gas must lie in front of the X-ray source. What's more, these absorbed spectral lines are displaced from their normal positions to shorter wavelengths -- that is, blueshifted, which indicates that the clouds are moving toward us. . . .

The outflows turned up in 40 percent of the sample, which suggests that they're common features of black-hole-powered galaxies. On average, the distance between the clouds and the central black hole is less than one-tenth of a light-year. Their average velocity is about 14 percent the speed of light, or about 94 million mph, and the team estimates that the amount of matter required to sustain the outflow is close to one solar mass per year -- comparable to the accretion rate of these black holes. . . .

By removing mass that would otherwise fall into a supermassive black hole, ultra-fast outflows may put the brakes on its growth. At the same time, UFOs may strip gas from star-forming regions in the galaxy's bulge, slowing or even shutting down star formation there by sweeping away the gas clouds that represent the raw material for new stars. Such a scenario would naturally explain the observed connection between an active galaxy's black hole and its bulge stars.

Also, any time we understand more about what is causing the structure we observe in galaxies, it follows that before long our understanding will also reduce the need for black box alternatives, like exotic dark matter, to explain it. It is possible that a meaningful share of dark matter halo effects may actually consist of a halo of ordinary matter barfed out by central black holes in galaxies at unexpectedly high speeds in unexpectedly high volumes, something that is rare at the galactic fringe and not accounted for in previous matter census efforts unaware that ultra-fast outflows existed. This would also explain why this potential contributor to dark matter has been so hard to directly observe on Earth.

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