Sky & Telescope - July 2024 - 16

Chandra: 25 Years
Silicon
Sulfur
Calcium
Iron
High-energy X-rays
ELEMENTS OF LIFE Different chemical elements emit different, narrow ranges of photon energies, enabling astronomers to map their locations.
These images show where silicon (red), sulfur (yellow), calcium (green), and iron (purple) are within the Cassiopeia A supernova remnant. The highestenergy
X-rays (blue) depict the supernova's expanding blast wave.
observations of a dwarf galaxy called Henize 2-10. This small
galaxy, which is forming stars at a prodigious rate, harbors
a black hole that already has about 1 million times the mass
of our Sun. Because Henize 2-10 is analogous to galaxies in
the early universe, this discovery suggests primordial galaxies
could host hefty supermassive black holes, already grown to
large sizes before the galaxies built up substantially.
While local dwarfs may be analogs of galaxies in the
infant universe, to truly understand galactic and black hole
formation, we must explore the early universe itself. The
detection of such galaxies became possible only recently with
the launch of the James Webb Space Telescope. But do any of
these early galaxies host a supermassive black hole? Observations
from Chandra indicate that yes, some of them do.
I was extremely fortunate to lead a Chandra program that
followed up on a sample of JWST-detected galaxies, as we
searched for growing black holes in the heart of these infant
galaxies. Thanks to nearly month-long observations with
Chandra, we found a supermassive black hole in a galaxy
called UHZ1, which exists merely 470 million years after the
Big Bang, when the universe was barely 3% of its current age.
To everyone's great surprise, the mass of this black hole
(tens of millions of solar masses) was already comparable to
the stellar mass of the galaxy itself (S&T: May 2024, p. 20).
For such a black hole to be so massive relative to its host
might suggest that it originated from the collapse of a massive
gas cloud rather than from the collapse of one of the first
stars (S&T: Jan. 2017, p. 25).
Chandra can do much more than see black holes: It can
actually hear them! Galaxy clusters are filled with gas so
intensely hot - between 10 and 100 million degrees - that
it emits X-rays. The Perseus Cluster is a prime example. As
the supermassive black hole in the central galaxy of the
cluster accretes matter, it launches jets of material into the
surrounding gas, creating pressure waves. Deep Chandra
observations have revealed these waves as ripples in the X-rayemitting
gas.
Sound waves are also pressure waves. If we were to convert
these ripples in the Perseus Cluster into a note, it would
be 57 octaves below middle C, which the human ear cannot
hear. However, by scaling the sound waves of Perseus to be
within the range of human hearing, Chandra's sonification
project (called Universe of Sound) made it possible for anyone
to now listen to the sound of the cluster - as well as that of
many other astrophysical objects (S&T: Mar. 2024, p. 26).
The vast amount of hot, X-ray-emitting gas that fills galaxy
clusters makes them prime targets for Chandra. A wide range
of studies explore cluster evolution and dynamics as well as
the interaction between supermassive black holes and largescale
gas. Perhaps the most intriguing application of an X-ray
observation of galaxy clusters, though, is that it can provide
evidence for the existence of dark matter.
There's no better example of this than the Bullet Cluster, a
merger of two galaxy clusters whose light has traveled 3.4 billion
years to reach Earth. When astronomers observed the
cluster both with Chandra and in visible light, they discovered
an offset between the hot, X-ray-emitting gas and the
location of most of the cluster mass. This mass, most of it
" dark, " is measured through the effect of gravitational lensing,
in which its gravity distorts the visible light of distant
background galaxies.
Why was there an offset? The X-ray-emitting gas from
5 arcsec
DWARFED The dwarf galaxy Henize 2-10 is bursting with new stars
and hosts an actively feeding black hole with the mass of 1 million Suns.
16 JULY 2024 * SKY & TELESCOPE
one cluster interacts with that of the other, slowing it down
and resulting in the characteristic bullet-like feature. However,
the non-interacting dark matter didn't slow down and
instead kept moving forward. Stars within the cluster's galaxELEMENTS:
NASA / CXC / SAO; HENIZE 2-10: X-RAY: NASA / CXC / VIRGINIA / A. REINES ET AL; RADIO: NRAO / AUI / NSF;
OPTICAL: NASA / STSCI; NGC 6240; X-RAY: NASA / CXC / SAO / E. NARDINI ET AL; OPTICAL: NASA / STSCI

Sky & Telescope - July 2024

Table of Contents for the Digital Edition of Sky & Telescope - July 2024