Sky & Telescope - June 2022 - 15

MASS COMPARISONS: GREGG DINDERMAN / S&T, SOURCE: LIGO-G2000963;
TALLY: LIGO-VIRGO / AARON GELLER / NORTHWESTERN UNIVERSITY
Solar masses
Researchers use advanced statis " There
were no theoretical reatical
techniques to take a step back
from these individual finds and
study the big picture. This population
analysis tells astrophysicists
what they would detect if their
instruments were perfect, explains
LIGO's Zoheyr Doctor (Northwestern).
" What we're after is
what's going on out in the universe,
not just what our detector is
seeing, " he says.
This extrapolation has turned
GW150914
31
solar
masses
= 0.86
36
solar
masses
GW190814
up something unexpected: Black
holes tend to cluster at 10 and 35
solar masses. " We've totally baffled
a lot of the theorists, because our
mass distribution looks nothing
like any mass distribution drawn
before, " says LIGO's Maya Fishbach
(also Northwestern).
Below roughly 10 solar masses, the number of black holes
2.6 solar masses
= 0.11
23 solar masses
Most mergers
involve black
holes of similar
masses,
including the
first detection,
GW150914.
But some,
such as
GW190814,
pair one object
with a second
that's much
larger than it.
sons, " says Feryal Özel (University
of Arizona), who helped bring the
paucity to astronomers' attention.
Small stars are more common than
big ones, she explains, " and if the
evolution does nothing special and
the explosion mechanism does
nothing special, then there should
have been far more low-mass black
holes in the 2- to 5-solar-mass
range than, for example, the 5- to
10-solar-mass range. "
Gravitational waves have turned
up a few objects in the valley,
notably the smaller member of
GW190814. This event involved a
23-solar-mass black hole swallowing
a mystery object of 2.6 solar
masses. Many astrophysicists think
the mystery object was a tiny black
plummets, creating a valley in the plot. This dearth of small
black holes has puzzled astronomers long before the advent
of gravitational-wave science. Neutron stars can only survive
up to maybe 2½ solar masses before they collapse under their
own weight; black holes should appear just above that limit.
Yet observations of compact objects in stellar binaries have
found almost no black holes between 2 and 5 solar masses.
200
LIGO-Virgo-KAGRA black holes
100
50
?
20
10
5
EM black holes
?
2
EM neutron stars
1
THE TALLY SO FAR Scientists have now detected 90 gravitational-wave events (blue and orange), expanding considerably on the compact objects
detected at various wavelengths of light (electromagnetic, or " EM " ). The dots indicate the two masses of the objects that merged and of the object
they created. Almost all of the new discoveries appear to be black holes. Objects that lie just above 2 solar masses are of uncertain nature.
sk yand tele scope .o r g * JUNE 2022 15
LIGO-Virgo-KAGRA
neutron stars
?
hole, although a rapidly spinning neutron star might be able
to withstand collapse at that mass.
Other observations also suggest the low-mass valley isn't
empty. Careful monitoring of tiny flashes created as unseen
objects pass between us and our galaxy's stars, bending and
magnifying the stars' light, have turned up eight objects
that might lie in the valley, Łukasz Wyrzykowski (Warsaw
(continued on page 18)

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Sky & Telescope - June 2022

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