Considerations for Models

Major questions:

IMBH born or grown to $M\sim 10^{2-4}\,M_\odot$?

If grown, what is the environment?

If IMBH were born with $\approx$ their current mass:

Can't be recent single star ($M$ too high).

Maybe born from Population III stars.

If IMBH were born with $M_{\rm init}\ll M_{\rm now}$:

Insignificant growth from Bondi-Hoyle

$M/{\dot M}_{\rm BH}\approx 10^{13}M_{10}^{-1}\rho_{-24}^{-1} v_6^3\;\;{\rm yr}\; .$

Growth must be accretion from stars or merger with stars or compact objects.

Many stars or compact objects needed $\Rightarrow$ dense stellar cluster. Could be young or old.