How to Form a Star Near a Black Hole
Black holes get a bad rap. We tend to think of them as destructive, all encompassing destroyers of stuff, consuming all in their way to add to their own bloated mass. Okay maybe I’m being a wee bit dramatic, but that’s the kind of pop culture picture of one, right? Scientists have also wondered if it’s even possible to create stars close to a supermassive black hole. Now, we have evidence that it is possible, and it’s happening in the center of our Galaxy.
Astronomers have a rough idea of how stars form, even though many of the details are still open questions. A region of interstellar gas gets dense enough through some process to begin forming a star. The material surrounding a black hole, however, suffers intense shearing forces from rotating around such a dense object. It is hard to imagine how a clump of gas could hold itself together in such close proximity to a gravitational behemoth.
Nevertheless, evidence of star formation has been detected within two light-years of the supermassive black hole at the center of the Milky Way Galaxy, affectionately known as Sagittarius A* (pronounced “A-star,” abbreviated Sgr A*). Using the fledgling Atacama Large Millimeter/Submillimeter Array (ALMA) when it was just 12 antennas and the Combined Array for Millimeter-Wave Astronomy (CARMA), a group of astronomers led by Farhad Yusef-Zadeh of Northwestern University detected emission of silicon monoxide (SiO) near the supermassive black hole.
Finding the molecular clouds of SiO itself isn’t a definite tracer of star formation. However, how the SiO is moving through space is important. The emission lines of the SiO show the clumps to be moving at high velocity in a way that indicates an outflow of material. Such outflows are commonly seen in star forming regions as not all the material falling into a protostar makes it in the end. Some of it is simply blown away in the energetic process. The brightness of the SiO emission is also consistent with star formation scenarios.
Once the team had this evidence to start with, they looked for where these baby stars might actually reside. Using the infrared capabilities of the Very Large Telescope in Chile, two candidate young stellar objects were noticed in the vicinity. Infrared light can penetrate the dust and gas that obscures the visible light coming from forming stars.
How could these stars have formed in such a turbulent environment so close to the several million solar-mass black hole? Two scenarios are put forth. In one, the radiation pressure from the intense ultraviolet coming from the hot stars in the region around Sgr A* compresses the gas to the point at which it can collapse to form a star. That’s right, the very pressure of the photons of light from other stars helps to make the gas form new stars. Another mechanism, amusingly dubbed “clump-clump collision,” involves denser regions of gas smacking in to each other, thus causing a high enough density for star formation.
Either way, this makes a great stride in explaining how stars may form so close to a supermassive black hole. We know that a population of stars does exist there, as the motions of these very stars were used to make accurate measurements of the black hole’s mass. However, it was not clear whether these all came from somewhere further away and worked their way closer due to gravitational interactions, or if it was possible to form these stars in place. It is clear, once again, that new telescopes such as ALMA will continue to push our knowledge of some of the most extreme environments of the Universe.
Image: The green indicates radio emission mapped by the Very Large Array, showing the hot gas around Sgr A*. The red and blue areas are the SiO emission as mapped by ALMA, with the blue areas having the highest velocities. Credit: Yusef-Zadeh et al., ALMA (ESO, NAOJ, NRAO), NRAO/AUI/NSF
This work has been accepted for publication in Astrophysical Journal Letters, and a preprint is available at arXiv.org.