Detecting black holes at the center of dwarf galaxies has proven difficult. This is partly because they tend to “wander” and are not located at the center of the galaxy. There are many galaxies that could contain such a black hole, but so far we don’t have enough data to confirm their existence. A new paper by Megan Sturm of Montana State University and her colleagues analyzed additional data from Chandra and Hubble on a set of 12 potential galaxy candidates for active galactic nuclei (AGN). They were only able to confirm three, highlighting the difficulty of isolating these giant wanderers.
Why is it important to find black holes at the center of galaxies? Early black holes may have formed the “seeds” of galaxies. However, mergers have occurred repeatedly in large galaxies like our Milky Way, obscuring the evolutionary history of the supermassive black hole at their center. Dwarf galaxies, on the other hand, have not undergone as many changes, so their black holes more closely resemble what they looked like in the early days of the universe, allowing astronomers to place greater constraints on the formation of these galactic seeds.
The process through which they “migrate” is also interesting. Some simulations of dwarf galaxies predict that up to 50% of their central black holes could be offset from their center. This could be caused either by a merger (which happens even in some smaller dwarf galaxies), in which the black hole is “thrown” from the center by gravity, or possibly by their own formation process. They could have formed in gas clouds that weren’t at the center of the galaxy at all, and the gas and dust around them either didn’t have time to adjust to their gravity, or they’re trapped in an unstable gravitational dance in which the black hole will never actually be at the center of the galaxy.
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To try to find these elusive giants, the authors examined data from Chandra and Hubble for 12 dwarf galaxies found using the Very Large Array. They were “radioselected” from a list of 111 dwarf galaxies because they had radio signals typical of accreting black holes, but which could still have formed through normal star formation. The authors wanted to find out the cause of these signals and either confirm or refute the existence of these black holes.
Of the 12, they were only able to fully confirm three, using “multi-wavelength evidence” of strong signals in the radio (VLA), X-ray (Chandra), and optical (Hubble) wavelengths, although even after this confirmation, not all were particularly bright in all three wavelengths. One (known as “ID 26” because of its categorization in the larger list of AGN candidates) was the only one confirmed to be “bright” in all three wavelengths. Another (ID 82) was only visible in X-rays, meaning its optical light is likely obscured by gas and dust, although other studies had found “coronal” lines that suggested it was an accreting black hole. ID 83, on the other hand, was very bright in the X-ray range and had optical wavelengths consistent with those of a black hole.
There were two “imposters” in the dataset that originally looked like they could also be AGNs, but the authors found other causes for their luminosity. ID 64 had a very bright optical source offset from the center of its galaxy. However, after examining the redshift of the galaxy compared to the optical source using data from the Palomar Observatory, the authors determined that the source of the optical source was actually a background galaxy, consistent with the dwarf galaxy in the foreground. The background galaxy’s AGN essentially made it look like it was “walking” in the foreground galaxy, even though it is billions of years older and therefore further away.
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Another false alarm was ID 92. Hubble data showed that the radio signal coming from this galaxy was aimed at a very active star-forming region. Further analysis of the data led the authors to conclude that the radio source likely came from a “super cluster” rather than an AGN.
This left seven more radio sources that had not been detected in either the X-ray or optical range and for which there was no clear explanation. However, a lack of scientific confirmation sometimes leads to new theories, and that is the case here. The authors believe that three of the “ghost” candidates are actually background sources, in large part because they are extremely distant from their galactic centers. A particular ghost (ID 65) may actually be the source of a Fast Radio Burst (FRB), the origins of which are still debated.
These theories will remain unresolved unless time for a more powerful telescope such as the James Webb Space Telescope becomes available. He will soon make decisions about where his observation time will go in the fifth year, and it is unclear whether the Montana State team has submitted a proposal to track down these ghostly wanderers. Even if they didn’t, the new paper is at least a step toward catching up with at least some of these interesting connections to the early universe.
Learn more:
MR Sturm et al. – Chandra and HST observations of radioselected (wandering) massive black hole candidates in dwarf galaxies
UT – This wandering black hole has left its galactic center
UT – A nearby dwarf galaxy has a surprisingly massive black hole at its heart
UT – Dwarf galaxies could be the answer to the dark matter debate
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