Sky & Telescope - November 2022 - 20

NASA GODDARD SPACE FLIGHT CENTER
The Dark Hunt
mass black holes may reveal themselves when they collide
and release gravitational waves. The event GW190521, for
example, most likely was the merger of black holes with 85
and 66 solar masses, respectively, creating a
behemoth among stellar-mass black holes.
Our stellar-evolution theories said that
these two objects shouldn't exist, because
we expected that stars couldn't create black
holes between 50 and 120 Suns. But gravitational-wave
observations have turned up
a handful of black holes in this " mass gap. "
They might have formed in the mergers of a
previous generation of smaller objects (S&T:
June 2022, p. 12).
In summary, we are confident that black
Our discoveries so far suggest that the seeds of today's
We are entering
an era of
unprecedented
access to black
holes across the
cosmos.
holes extend down to 100,000 solar masses,
and that such objects are common in small
galaxies. We are also just starting to find black holes with
masses of hundreds of Suns. But while at least one 20,000Sun
black hole has been found in HLX1, we have no knowledge
yet of black holes of thousands of Suns. What's more,
while it's now clear that black holes do inhabit the intermediate-mass
range, we don't yet understand the formation paths
of all the ones we've found.
Supermassive Origins
Being a black hole hunter means more than just tallying up
discoveries. We want to understand how black holes form and
when. We search for midweight objects in the universe today,
hoping that they will give us a sneak peek into the conditions
in the early universe. That's when, somehow or other, the
heavyweights of today were bulking up.
gigantic black holes formed at masses of less than 100,000
Suns - and probably lower than 20,000 Suns, given the existence
of HLX1. Seeds of roughly 100 solar
masses could have come from the collapsed
cores of the universe's first stars. These stars
were humongous by modern standards, and
only they would have been large enough to
make seeds of this heft. For slightly larger
seeds, under the right conditions a cascade
of collisions between smaller black holes in
the center of dense star clusters could have
made 1,000-solar-mass seeds. Both of these
kinds of seeds would need to grow fast to
become the black holes containing billions
of solar masses that we see only 650 million
years after the Big Bang.
A third option is that gas clouds could have collapsed
directly into more massive black holes of 10,000 to 1 million
Suns, without forming a star first. The process requires
goldilocks conditions, though; the gas has to be not too hot
and not too cold (S&T: Jan 2017, p. 24). Such heavy seeds,
forming only early in the universe, would likely be pretty rare.
If runaway collisions happen, then we should be able to
observe such mergers in the modern universe. At the end of
2022, gravitational-wave detectors will once again be scanning
the sky for black hole mergers, this time with even
greater sensitivity, and we may find more massive black holes
on the start of a runaway trip to supermassive status.
But gravitational-wave detections like GW190521 add an
interesting wrinkle: We don't know how these pairs of black
holes came together in the first place, or whether they inhabit
GRAVITATIONAL CHIRP Massive black holes reveal themselves when they merge and release gravitational waves in the process. While most such
mergers have involved stellar-mass black holes, a few have resulted in an object of more than 100 Suns, making the smallest of the intermediatemass
black holes.
20 NOVEMBER 2022 * SKY & TELESCOPE

Sky & Telescope - November 2022

Table of Contents for the Digital Edition of Sky & Telescope - November 2022