# Medium sized Black Holes

For many years the existence of medium sized Black Holes (IMBH$$^1$$) have eluded scientists. BH of several times the mass of our sun have been found, as well as SMBH with millions of sun masses. SMBH's and small ones mass grows with time, as matter gets transferred to the accretion disc and later gets absorbed or by merging. Quoting a paper from 2018:

Although many IMBH candidates have been identified, none are accepted as definitive; thus, their very existence is still debated.

My question is: If we know BH grow, where are the medium sized Black Holes? How come small sized ones are easier to spot than medium ones? Why is their mere existence being debated on?

$$^1$$ For the purposes of this question we could define an IMBH to be one of mass between about $$10^2$$ and $$10^5$$ solar masses (https://en.wikipedia.org/wiki/Intermediate-mass_black_hole).

• IMBH are uncommon because they either form via collision of two or more lesser black holes, or form via an extremely massive star via direct collapse. SMBH form through a IMBH "eating" a whole nebula in the early universe, or other unknown factors such as swallowing a star. Quasars - well, we don't know much about how they form, but they are indeed very massive SMBH. Mar 3 at 16:50
• Is there any evidence supporting those claims about IMBH? Would you please point a paper out. And why are they so elusive compared to small ones? Mar 3 at 16:54
• See this Wiki article about it. Mar 3 at 16:59
• Ok so there has been no confirmed IMBH. The Wiki article only talks about "potential discoveries" and "IMBH candidates" Mar 3 at 17:03
• different but related: What is the “hypothesized lower mass gap” between 2.5 and 5 solar masses?
– uhoh
Mar 4 at 0:26

This review detailing the state of the art of searching for intermediate BH candidates, e.g. $$m \sim 10^2 - 10^5$$ M$$_{\odot}$$, from electromagnetic observations "for the few hundreds of nearby IMBH candidates found in dwarf galaxies, globular clusters, and ultra-luminous X-ray sources, as well as the possible discovery of a few seed BHs at high redshift."
In fact, there is a binary black hole merger event detected by LIGO/Virgo, dubbed GW190521, where the resultant post-merger BH had a mass of ~150 M$$_{\odot}$$, which is the first direct observation of an intermediate mass BH, e.g., m is within $$10^2 - 10^5$$ M$$_{\odot}$$ to high confidence, and thus the first conclusive experimental evidence that the intermediate mass range can be populated by mergers of high stellar-mass BHs. How the high stellar-mass BBH system originated is speculative currently: it could have originated from hierarchical mergers of stellar mass progenitor BBHs in a dense stellar cluster, or as a high-mass isolated stellar binary or a stellar binary in an accretion disk, or more exotic possibilities...