Sky & Telescope - April 2022 - 39

5′
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Field of view
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NGC 7541
sion between the Milky Way and Andromeda, which would
happen very differently if our galaxy is actually more massive
than we think.
Or maybe Andromeda is in on the secret. M31, like the
Milky Way, has a dearth of star-forming satellites. It's possible
that instead of being independent systems, the two
galaxies share one large dark matter halo - with a bunch of
hot gas deadly to tiny companions. The SAGA team members
specifically avoided close pairs of galaxies like that of the
Milky Way and Andromeda, so at the moment we have no
analog with which to test the idea. But perhaps if astronomers
can study similar duos in the future, they might find
only quenched satellite galaxies as well.
Then again, the discrepancy could have to do with our
galaxy's unique history. If our satellites fell into the Local
Group much earlier, then our hot, gaseous halo would have
had longer to act on them. Unfortunately, we don't know how
long ago most satellites were swept up by the Milky Way.
Lastly, it's possible our satellites are snuggled up so close
to the Milky Way that our halo can more efficiently remove
their gas. It's hard to tell if that's true in the SAGA systems,
where we don't have full three-dimensional pictures.
All these hypotheses assume that the answer lies in our
halo. But Andreea Font (Liverpool John Moores University,
UK), who was not involved in the survey, thinks another
mechanism might be the culprit. " There are many ways you
can kill a galaxy, " she says.
Ram-pressure stripping is favored in the case of the Milky
Way, but other physical mechanisms can also diminish a
dwarf galaxy's reservoir of star-forming gas. Tides from galaxy
clusters can strip it away. Other dwarfs can jostle a dwarf
about so that it loses its gas. And plenty of processes can ionize
gas, keeping it so warm that it can't form stars.
CATCH-22 To capture the satellites around NGC 7541, pictured here,
the survey included an area with a diameter of 1 degree around the host.
This large field of view encompasses most of the galaxy's dark matter
halo and the satellites within it. However, most of the galaxies pictured
here are distant background galaxies. (The galaxy and its satellites are
dubbed " Catch22 " in the SAGA catalog.)
There is no clear answer yet. And some theorists do wonder
if there is an observational bias. Since many galaxies that
are quenched tend to be fainter than star-forming galaxies
with the same mass, astronomers might have missed them.
But Geha says that SAGA can pick up faint satellites in this
mass range.
" I think it's really exciting, " says Alyson Brooks (Rutgers
University). Brooks is not involved in the SAGA project
directly, but she has already started to compare the current
generation of computer simulations to the newest results.
" For decades, we have had our own Milky Way and our nearest
neighbor Andromeda to compare our simulations to and
try to figure out if we understand galaxy evolution, and who
knows if they are biased. "
A Dark Future
These early results have prompted a never-ending list of
hypotheses, follow-up questions, and planned observations.
To start, the SAGA team would like to better understand
the influence that a Milky Way-like galaxy has on its satellites.
Dwarf galaxies as large as the Magellanic Clouds, for
example, should create new stars for a very long time, and yet
we see similar SAGA satellites that have ceased forming stars
if they are close to a massive galaxy like the Milky Way.
" The SAGA satellites are all influenced by their parent
Milky Way, " Geha says. " The question is, how much? " To
find the answer, the team plans to obtain high-resolution
sk yand tele scope .o r g * APRIL 2022 39
SATELLITES AROUND NGC-7541: Y.-Y. MAO (RUTGERS), WITH IMAGES FROM
LEGACY SURVEYS / D. LANG (PERIMETER INSTITUTE)
http://skyandtelescope.org
Sky & Telescope - April 2022

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