Low-mass black holes as the remnants of primordial black hole formation
  This article documents our ongoing search for the elusive "intermediate-mass"
black holes. These would bridge the gap between the approximately ten solar
mass "stellar-mass" black holes that are the end-product of the life of a
massive star, and the "supermassive" black holes with masses of millions to
billions of solar masses found at the centers of massive galaxies. The
discovery of black holes with intermediate mass is the key to understanding
whether supermassive black holes can grow from stellar-mass black holes, or
whether a more exotic process accelerated their growth only hundreds of
millions of years after the Big Bang. Here we focus on searches for black holes
with masses of 10^4-10^6 solar masses that are found at galaxy centers. We will
refer to black holes in this mass range as "low-mass" black holes, since they
are at the low-mass end of supermassive black holes. We review the searches for
low-mass black holes to date and show tentative evidence, from the number of
low-mass black holes that are discovered today in small galaxies, that the
progenitors of supermassive black holes were formed as ten thousand to
one-hundred thousand solar mass black holes via the direct collapse of gas.
