Tracking down black holes at the center of dwarf galaxies has proven difficult.
In part that is because they have a tendency to “wander” and are not located at the galaxy’s center.
There are plenty of galaxies that might contain such a black hole, but so far we’ve had insufficient data to confirm their existence.
A new paper from Megan Sturm of Montana State University and her colleagues analyzed additional data from Chandra and Hubble on a set of 12 potential Active Galactic Nuclei (AGN) galaxy candidates.
They were only able to confirm three, which highlights the difficulty in isolating these massive wanderers.
Why is it important to find black holes at the center of galaxies? Early black holes could have formed the “seeds” of galaxies.
However, large galaxies, like our Milky Way, have undergone repeated mergers, obscuring the developmental history of the supermassive black hole at their center.
Dwarf galaxies, on the other hand, haven’t undergone as many changes, so their black holes are more akin to what they would have looked like in the early days of the universe, allowing astronomers to put more constraints on the formation of these galactic seeds.
The process by which they “wander” 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 for some smaller dwarf galaxies), whereby the black hole gets gravitationally “kicked” out of the center, but possibly by their own formation process.
They could have formed in gas clouds that were never at the center of the galaxy in the first place, and the gas and dust around them either hasn’t had time to adjust to their gravitational pull or are locked in an unstable gravitational dance where the black hole will never truly be at the center of the galaxy.
To try to find these elusive giants, the authors looked at data from Chandra and Hubble for 12 dwarf galaxies that were found using the Very Large Array.
They were “radio-selected” from a list of 111 dwarf galaxies because they had radio signals that were typical of accreting black holes, but that still might have been created by standard star formation. The authors wanted to sort 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 3 of them using “multi-wavelength evidence” of strong signals in the Radio (VLA), X-ray (Chandra), and Optical (Hubble) wavelengths, though even with these confirmed not all were particularly bright in all three wavelengths. One (known as “ID 26” for its categorization in the larger list of AGN candidates) was the only one confirmed as “bright” in all three wavelengths.
Another (ID 82) was only noticeable in X-rays, meaning its optical light is likely obscured by gas and dust, though other studies had found “coronal” lines that indicated it was an accreting black hole. ID 83, on the other hand, was very bright in X-rays and had optical wavelengths that were consistent with a black hole.
There were two “imposters” in the dataset, that, while they originally looked like they could also be AGNs, the authors found other causes for their luminosity.
ID 64 had a very bright optical source offset from the center of its galaxy, but after looking at the redshift of the galaxy compared to the optical source with data from the Palomar observatory, the authors realized the source of the optical source was actually a background galaxy that aligned with the foreground dwarf galaxy. Essentially the AGN of the background galaxy made it look like it was “wandering” in the foreground one, despite being billions of years older and therefore farther away.
Another false alarm was ID 92. Hubble data showed that the radio signal coming from this galaxy aligned with a very active star forming region. Further analyzing the data allowed the authors to conclude that the radio source was likely from a “super star cluster”, rather than an AGN.
That still left 7 other radio sources with no detection in either X-rays or Optical bands, and no clear explanation for them. Lack of confirmation in science sometimes leads to new theories, though, and that is the case here.
The authors believe three of the “ghost” candidates are indeed background sources, in large part because they are extremely offset from their galactic centers. One particular ghost (ID 65) might actually be a source of a Fast Radio Burst (FRB), the origins of which are still debated.
Those theories will remain unresolved, unless time on a more powerful telescope, like the James Webb Space Telescope, becomes available.
That one will be making decisions about where its observational time for its fifth year will be going soon, and it’s unclear whether the Montana State Team submitted a proposal to track down these ghostly wanderers.
Even if they didn’t at least the new paper itself is a step towards catching up with some of these interesting connections to the early universe at least.
Written by Andy Tomaswick/Universe Today.
