Monster Black Hole Found Hiding Behind Dust in Center of Dwarf Galaxies
Washington DC—January 16, 2023. In a paper published in the Astrophysical Journal, scientists describe the discovery of an intermediate black hole buried in gas and dust in a dwarf galaxy. The discovery of the dust-obscured black hole of a previously undiscovered size is exciting since it may help provide clues about how such black holes were seeded in the very early universe.
Typically, galaxies have supermassive black holes at their centers, and astronomers can observe them as the surrounding gas and dust fall into them.
Using NASA's NuSTAR observatory to study a black hole containing 160,000 times the mass of the Sun in the center of the dwarf galaxy J144013+024744, Universities Space Research Association’s Dr. Chien-Ting Chen, who is the Principal Investigator, and Shrey Ansh, the lead author and graduate student at University of Alabama in Huntsville, found the black hole to be deeply buried by the gas and dust in the galaxy.
The black hole is a ``tiny monster'', as it is much more massive than typical stellar mass black holes that are only a few times the mass of the sun, but it is still smaller than the million to billion solar mass supermassive black holes found in typical galaxies. The authors showed the black hole to be enshrouded by a thick column of dust, similar to what we typically see in many actively accreting supermassive black holes.
The host galaxy J144013+024744 has a stellar mass of about 3 billion times the mass of the sun, which is only about 10% of that of the Milky Way.
There are many dwarf galaxies out there in the local Universe (for example, the Milky Way has a few dozens of satellite dwarf galaxies including the Magellanic Clouds), and the fraction of them have a massive black hole (mBH) in the center and it is “key to understand how the supermassive black holes were seeded in the early Universe, because these dwarf galaxies are thought to have undergone fewer galaxy mergers, therefore they are the "fossil records" of the very early Universe that could tell us about how supermassive black holes came to existence in the first place,” says Dr. Chen.
“Observationally, finding smaller black holes are harder because they are fainter, and it is even harder to find them when you add a substantial amount of intervening dust,” says Dr. Chen.
“We suspected this dwarf galaxy to hide a massive black hole because observations from NASA's Spitzer Observatory have showed the target to have strong mid infrared (IR) emission lines that could only be powered by very energetic photons coming from accreting massive black holes” stated Dr. Chen. Mid-IR emission lines originate from regions that are further away from the black hole, and their wavelengths are longer than typical dust particles, making it possible to see them despite of the presence of dust.
“We thought this dwarf galaxy could have a dust-obscured black hole based on its multi-wavelength properties,” Ansh says. “The amount of obscuring column density that we found could imply that a torus-like geometry found in supermassive black holes could extend into intermediate black holes as well.” It was unclear whether the smaller black holes in dwarf galaxies would behave similarly to the supermassive black holes in typical galaxies.
“By targeting J144013+024744 with NASA's NuSTAR telescope that can detect high energy X-ray photons with more penetrating power, we can directly measure the amount of dust in our line of sight,’ noted Dr. Chen.
Finding J144013+024744’s monster black hole to be actually buried behind a screen of dust is quite exciting, since the census of the prevalence of the monster black hole in dwarf galaxies is one of the few actually measurable constrains of how all these supermassive black holes were seeded in the very early Universe.
With the confirmation of the dust obscured monster in J144013+024744, scientists should have a better idea on how and where to look for more of these objects -- by combining hard X-ray and mid-IR emission line observations. Although Spitzer is no longer operating, it's got an even more powerful successor JWST. Combining JWST, NuSTAR and NASA's future hard X-ray missions, astronomers should be able to find more of these tiny monsters lurking in our cosmic neighborhood, and hence getting closer to have a complete census of the black hole population in dwarf galaxies.
Additional Resources: https://arxiv.org/abs/2209.09913
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