Observational Support for Massive Black Hole Formation Driven by Runaway
  Stellar Collisions in Galactic Nuclei
  We explore here an scenario for massive black hole formation driven by
stellar collisions in galactic nuclei, proposing a new formation regime of
global instability in nuclear stellar clusters triggered by runaway stellar
collisions. Using order of magnitude estimations, we show that observed nuclear
stellar clusters avoid the regime where stellar collisions are dynamically
relevant over the whole system, while resolved detections of massive black
holes are well into such collision-dominated regime. We interpret this result
in terms of massive black holes and nuclear stellar clusters being different
evolutionary paths of a common formation mechanism, unified under the standard
terminology of being both central massive objects. We propose a formation
scenario where central massive objects more massive than $\rm \sim 10^8 \,
Msun$, which also have relaxation times longer that their collision times, will
be too dense (in virial equilibrium) to be globally stable against stellar
collisions and most of its mass will collapse towards the formation of a
massive black hole. Contrarily, this will only be the case at the core of less
dense central massive objects leading to the formation of black holes with much
lower black hole efficiencies $\rm \epsilon_{BH} = \frac{M_{BH}}{M_{CMO}}$,
with these efficiencies $\rm \epsilon_{BH}$ drastically growing for central
massive objects more massive than $\rm \sim 10^7 \, Msun$, approaching unity
around $\rm M_{CMO} \sim 10^8 \, Msun$. We show that the proposed scenario
successfully explains the relative trends observed in the masses, efficiencies,
and scaling relations between massive black holes and nuclear stellar clusters.
