Sky & Telescope - October 2023 - 23

ESA / HUBBLE & NASA / W. KEEL (2)
analogs to the ones Webb sees. Thanks
to carefully selected galaxies and Hubble
filters, Leonardo Clarke (now University
of California, Los Angeles) and others
looked past the bright emission from the
Peas' gas at the galaxies themselves. The
young blue stars, they found, sit within a
diffuse, redder region. If that color comes
from older stars, then their presence
could mark a key difference between
Green Peas and early galaxies: Early galaxies
are too young to have such a mature
stellar generation. It's crucial to understand
that star-formation history, and the
role of older stars in galactic processes, to know what Green
Peas can and can't tell us about the early universe.
Unusual galaxies can also help us understand the universe
today, Keel says. He points to the Zoo Gems category
of overlapping galaxy pairs. What's unusual there, he says, is
not the galaxies themselves, but the fact that one sits neatly
behind the other in telescopic images. It's like putting the
foreground galaxy on a light table, he explains: With that
backlighting, astronomers have another way to study the
closer galaxy's dust.
Or, he says, consider these rarities: spiral galaxies with
double radio lobes created by powerful jets, which launch
during the voracious accretion activity of a central black hole.
Most such double-lobed jets come from huge elliptical galaxies.
Keel says the rare spiral hosts might reveal details about
how these jets are produced.
To that end, Zihao Wu (Peking University, China) and
others analyzed Zoo Gems images and collected 18 that they
YOU'RE BLOCKING MY VIEW The spiral
LEDA 2073461 lies in front of another, more edgeon
spiral called SDSS J115331.86+360024.2. It's
unclear how close the two galaxies are in reality,
and there's no obvious sign of interaction. But the
foreground galaxy has an intriguing double dust
lane seen in other backlit Zoo Gems - is that
normal?
believe show true spiral hosts of these
radio lobes. The galaxies tend to have
high stellar masses for spirals, which
implies they also have especially massive
black holes. If so, then the black hole's
mass might play a key role in determining
whether it launches jets and how powerful those jets can be.
" They've done a really systematic job " of finding promising
candidates for jet-launching spiral galaxies, says Clive
Tadhunter (University of Sheffield, UK), who was not part
of the research. But although this study reinforces past
associations between mass and power, he cautions that the
link between stellar mass and black hole mass isn't perfect.
There's a lot more work to be done in untangling the factors
that drive jet formation.
Ultimately, by inspecting galactic outliers, " we have a better
idea of what's even possible, " Brunker says. " If you're going
to paint a whole picture, you can't leave out the weird things. "
¢ MADISON GOLDBERG is a science journalist based in New
York City. She has written about topics ranging from sleep to
sewer systems.
Join the Zooite ranks: https://is.gd/galaxy_zoo.
MULTI-ARM MERGER The Zoo Gem CGCG 396-2 lies about 520 million light-years away in Orion.
sk yand tele scope .o r g * OCTOBER 2023 23
https://www.is.gd/galaxy_zoo https://skyandtelescope.org/
Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023
