It is widely know that the more massive the galaxy, the more massive its central supermassive black hole. What we do not know is how far this holds in the opposite direction, i.e., if less massive galaxies have so called "intermediate mass black holes" (IMBHs) or if they even have black holes at all. Whether IMBHs exist is also important from the standpoint of the question of SMBH formation. If IMBHs exist, then they would be a natural starting point for the formation of SMBHs. If not, the SMBHs would somehow have to grow from scratch much faster than the currently known scenarios would allow.
Finding eventual IMBHs is difficult even among the relatively nearby galaxies. The reason for that is that we can only detect a black hole while it is consuming gas, giving rise to the active galactic nucleus (AGN). And the problem with the AGNs is that they can be difficult to identify observationally, especially if they are powered by these less massive black holes.
A recent study led by our grad student Anika Goel and published in the Astrophysical Journal focused on comparing two AGN indicators: the mid-infrared emission and the optical emission lines, in particular trying to understand the cases when one indicator would say there is an AGN and another would not. The study found that for the typical, more massive galaxies (where we expect the run-of-the-mill SMBH rather than an IMBH) the emission-line method is 5x more powerful in detecting AGN than the infrared method. The study explains that this is because most AGNs do not have enough dust near their black hole to produce sufficient amount of mid-infrared light.
However, in galaxies of smaller mass like the ones where IMBH might exist, the situation reverses - emission lines never suggest an AGN, whereas the infrared method sometimes does. The paper explores several alternative possibilities for the source of this infrared signature but concludes that the AGN interpretation is most likely, thus strengthening the case for the existence of IMBHs. Furthermore, the study found something hitherto unnoticed - the divide between the AGNs that the emission lines pick up opposed to those they do not, appears to correlate with the mass of the galaxy's bulge rather than the galaxy's total mass. To the extent the infrared signature coupled with the lack of AGN optical emission lines might indicate the presence of IMBH, the study implies that IMBHs are not present only in low-mass galaxies, but also in more massive galaxies that happen to have small bulges.
In addition to providing a better understanding of the strengths and limitations of different AGN indicators, the findings strengthen the case for the existence of IMBHs. The study also expands the population of galaxies where IMBHs might be found, thus offering new candidates for follow studies.
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