Hubble finds the best evidence of an elusive medium-sized black hole

Mar 31, 2020 10:30 PM ET

This illustration shows a cosmic murder in action. A headstrong star is torn apart by the intense attraction of a black hole that contains tens of thousands of solar masses. The remains of the star form an accretion disk around the black hole. X-ray flares from the overheated gas disk alerted the astronomers to the position of the black hole; otherwise it was unknown in the dark. The elusive object is classified as a medium-mass black hole (IMBH) because it is much less massive than the black holes of the monsters that live in the centers of galaxies. Therefore IMBHs are mostly calm because they do not take up so much material and are difficult to find. Hubble observations provide evidence that the IMBH lives in a dense cluster. The cluster itself can be the stripped-down nucleus of a dwarf galaxy. Photo credits: NASA, ESA and D. Player (STScI)

Astronomers have found the best evidence for the culprit of a cosmic murder: a black hole of an elusive class known as the “intermediate mass” that betrayed its existence by tearing apart a stubborn star that came too close.

The black hole weighs about 50,000 times the mass of our sun and is smaller than the supermassive black holes (with millions or billions of solar masses) lying on the nuclei of large galaxies, but larger than those of formed black holes with star mass the collapse of a massive star.

These so-called intermediate mass black holes (IMBHs) are a long-sought “missing link” in the development of black holes. Although there were several other IMBH candidates, researchers consider these new observations to be the strongest evidence of medium-sized black holes in the universe to date.

It took the combined power of two X-ray observatories and the keen vision of NASA’s Hubble Space Telescope to pin the cosmic animal down.

“Medium-mass black holes are very elusive objects, so it’s important to carefully consider and rule out alternative explanations for each candidate. This allowed Hubble to do that for our candidate,” said Dacheng Lin of the University of New Hampshire, principal researcher in the study. The results will be published on March 31, 2020 in The Astrophysical Journal Letters.

Astronomers have found the best evidence of a black hole in an elusive class known as the “intermediate mass”, which has revealed its existence by tearing apart a stubborn star that came too close. This exciting discovery opens the door to the possibility that much more will be lurking undetected in the dark and waiting to be given away by a star that is passing too close. Credit: NASA’s Goddard Space Flight Center

The story of the discovery reads like a Sherlock Holmes story, in which the meticulous, step-by-step creation of cases is required to catch the perpetrator.

Lin and his team used Hubble to track the evidence from NASA’s Chandra X-ray Observatory and ESA (European Space Agency) X-Ray Multi-Mirror Mission (XMM-Newton). In 2006, these satellites discovered a strong flare of X-rays, but could not determine whether they came from inside or outside of our galaxy. The researchers attributed it to a star that was torn apart after it came too close to a gravitationally compact object such as a black hole.

Surprisingly, the X-ray source was named 3XMM J215022.4? 055108 not in the center of a galaxy, where there were usually massive black holes. This raised hope that an IMBH was the culprit, but first another possible source of the X-ray beam had to be excluded: a neutron star in our own Milky Way galaxy that cools down after being heated to a very high temperature. Neutron stars are the crushed remains of an exploded star.

Hubble was pointed at the X-ray source to determine its exact position. Deep, high-resolution imaging provides strong evidence that the X-rays were not from an isolated source in our galaxy, but in a distant, dense cluster of stars on the edge of another galaxy – just the kind of place where astronomers should find IMBH. Previous Hubble studies have shown that the mass of a black hole in the center of a galaxy is proportional to the central bulge of this host galaxy. In other words, the more massive the galaxy is, the more massive its black hole is. Therefore, the star cluster in which 3XMM J215022.4? 055108, the stripped-down nucleus of a dwarf galaxy of lower mass, which due to its close interactions with its current larger galaxy host was disturbed by gravity and tides.

This Hubble space telescope image identified the location of a medium-mass black hole that weighs 50,000 times the mass of our Sun (making it much smaller than super-massive black holes in the centers of galaxies). The black hole labeled 3XMM J215022.4? 055108 is identified by the white circle. The elusive type of black hole was first identified in an outbreak of telltale X-rays emitted by hot gas from a star when it was captured and destroyed by the black hole. Hubble was needed to determine the position of the black hole in visible light. Hubble’s deep, high-resolution imaging shows that the black hole is in a dense cluster of stars that lies far above our Milky Way. The star cluster is located near the galaxy in the center of the image. Background galaxies that look much smaller appear scattered around the image, including a spiral just above the central foreground galaxy. This photo was taken with Hubble’s Advanced Camera for Surveys. Photo credits: NASA, ESA and D. Lin (University of New Hampshire)

IMBHs were particularly difficult to find because they are smaller and less active than supermassive black holes. They have neither readily available fuel sources nor the power to draw stars and other cosmic material that would produce telltale X-rays. Essentially, astronomers must catch an IMBH red-handed when they devour a star. Lin and his colleagues searched the XMM Newton data archive and searched hundreds of thousands of observations to find an IMBH candidate.

The X-ray light from the shredded star allowed astronomers to estimate the mass of the black hole at 50,000 solar masses. The mass of the IMBH was estimated both on the basis of the X-ray brightness and the spectral shape. “This is much more reliable than using X-ray brightness alone, as was previously the case for previous IMBH candidates,” Lin said. “The reason we can use the spectral adjustments to estimate the IMBH mass for our object is that its spectral evolution has shown that it is in the thermal spectral state, a state that is common in black hole accretion observed with star mass and well understood.

This artist’s impression shows a star torn apart by a medium-sized black hole (IMBH) surrounded by an accretion disk. This thin, rotating disk of material consists of the remains of a star that was torn apart by the tidal forces of the black hole. Photo credit: ESA / Hubble, M. Kornmesser

This object is not the first to be considered a likely candidate for a medium-mass black hole. In 2009, Hubble worked with NASA’s Swift Observatory and ESA’s XMM Newton to find out what is interpreted as the IMBH called HLX-1 and is on the edge of the ESO 243-49 galaxy. It is also at the center of a young, massive cluster of blue stars, which may be a stripped-down dwarf galaxy nucleus. The X-rays come from a hot accretion disk around the black hole. “The main difference is that our object tears apart a star, which provides strong evidence that it is a solid black hole, rather than a star-mass black hole, as people often do for previous candidates, including HLX -1, fear, “said Lin.

Finding this IMBH opens the door to the possibility that many more will be lurking undetected in the dark and waiting to be given away by a star that is passing too close. Lin plans to continue his meticulous detective work using the methods that his team has proven successful. Many questions still have to be answered. Is a supermassive black hole growing out of an IMBH? How do IMBHs form themselves? Are dense star clusters their preferred home?

According to study results, the Galaxy NGC 3319 can host an active black hole with medium mass.

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