The Hubble Space Telescope is tracking down a suspected black hole that shredded a wayward star that came too close for comfort.
The famed space observatory made the find while hunting down the source of a powerful burst of X-rays caught in 2006 by two other cosmic telescopes: NASA's Chandra X-ray Observatory and the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton).
At the time, astronomers weren't sure if the X-rays had come from inside or outside of the Milky Way galaxy, but new high-resolution photography by Hubble shows that the X-ray source (known as 3XMM J215022.4−055108) is located in a star cluster at the edge of another galaxy. That's exactly where an intermediate-sized black hole (IMBH) may lurk — at least, according to theory.
The team's work further suggests the star cluster may have been the core of a small dwarf galaxy disrupted long ago, when the dwarf galaxy strayed too close to the larger galaxy that currently hosts the star cluster. Gravitational interactions with the larger galaxy may have ripped the dwarf galaxy apart, leaving only a small cluster of stars in its wake. In much the same way, astronomers believe that the gravitational pull of the black hole inside this cluster shredded a star that got too close, thereby producing the X-ray flare detected in 2006.
If the X-rays indeed came from a black hole, astronomers suspect the object is about 50,000 times the mass of our sun. That's a featherweight compared with the supermassive black hole that lies in the center of our Milky Way galaxy, which has a mass four million times greater than that of the sun.
"IMBHs have been particularly difficult to find because they are smaller and less active than supermassive black holes; they do not have readily available sources of fuel, nor as strong a gravitational pull to draw stars and other cosmic material which would produce telltale X-ray glows," NASA officials said in a statement. "Astronomers essentially have to catch an IMBH red-handed in the act of gobbling up a star."
IMBHs are believed to be a "missing link" in our understanding of how black holes evolve. Astronomers have seen many examples of small black holes that are similar in size to a star, and other examples of much larger black holes that typically reside in the centers of galaxies.
But IMBHs remain difficult to confirm as astronomers struggle to understand how supermassive black holes got so darn big, compared to stellar-sized black holes. Hubble and other observatories have found other IMBH candidates in the past, but this newest observation is considered the strongest evidence yet for this class of black hole, according to NASA.
"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," Dacheng Lin, a research assistant professor at the University of New Hampshire and principal investigator of the new study, said in the statement.
One other theory for the X-rays was that a neutron star — the dense remnants of an exploded supernova star — may have been responsible. Based on the X-ray glow of the chewed-up star, however, astronomers calculated that the black hole's mass was 50,000 solar masses, which is far more massive than a neutron star (typically about the size of a small city). The mass was calculated using both the X-ray luminosity — the inherent brightness of the X-rays — and the shape of the spectrum.
Hubble also helped track down another possible IMBH in 2009. The object, called HLX-1, was spotted on the edge of a galaxy known as ESO 243-49 — and also resides in a star cluster that could have been a dwarf galaxy in the ancient past. The X-rays spotted in the case of HLX-1, however, likely came from the accretion disk surrounding the black hole. The accretion disk is considered the point of no return for light or other objects to escape a black hole's gravity.
"The main difference is that our object is tearing a star apart, providing strong evidence that it is a massive black hole, instead of a stellar-mass black hole as people often worry about for previous candidates including HLX-1," Lin said.
A paper based on the study was published March 31 in Astrophysical Journal Letters.
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