Sky & Telescope - April 2022 - 37

NUMBER OF SATELLITES OBSERVABLE VS. DISTANCE: YAO-YUAN MAO
(RUTGERS); SAGA THUMBNAILS: Y.-Y. MAO (RUTGERS), WITH IMAGES FROM
LEGACY SURVEYS / D. LANG (PERIMETER INSTITUTE)
Number of satellites/magnitude limit
In 2017, they finally published the results on eight systems.
In February 2021, the tally rose to 127 satellites around 36
Milky Way-like galaxies. Each system of satellites is named
after an iconic literary work or its main characters, including
Gilgamesh, Catch-22, and Harry Potter. The Lord of the Rings,
so far, has secured two spots with systems named after Frodo
and Bilbo.
Since the 2017 study, the work has mostly been smooth
sailing - except, of course, for the global pandemic. In the
early days of COVID-19, most professional telescopes shut
down. And even when astronomers were able to return to
work by observing from the comfort of their living rooms,
they faced unique hurdles.
Geha, for example, has spent 25 nights on the AAT since
the start of the pandemic. To do so, she had four computer
screens and two laptops set up in her living room on cobbledtogether
furniture - all while her five-year-old sat on the
couch in remote kindergarten.
Her days would begin at 3 a.m. and run until dinner. " It's
a mental fatigue, it's a marathon, " Geha says. After a week,
she wouldn't let herself drive. " You have to make sure not to
operate heavy machinery outside of the telescope itself - just
because it's so exhausting and so tiring. "
Local group
Local volume
<80 million light-years
The SAGA Survey
80-130 million light-years
Main galaxy sample
of the Sloan Digital Sky Survey
Up to 650 million light-years
But now, the team is finally close to its goal of 100 systems,
with more than 300 satellites among them. " I'm just
over the moon, " Wechsler says.
Counting Galaxies
At first take, these systems do resemble our own, at least in
the number of satellites per big galaxy. And that's reassuring.
Astronomers have long mused over the number of satellites
- or more specifically, the drastically low number of
satellites - that swirl around the Milky Way. More than two
decades ago, computer simulations predicted that the Milky
Way's neighborhood should host hundreds or thousands of
tiny companions. And yet at the time, we had only discovered
a dozen. (We've since pushed that number up to roughly 60.)
It was a worrying conundrum that might have meant we
were missing an important piece to the puzzle. Perhaps we
didn't understand how galaxies evolve. Or maybe dark matter
was even more exotic than we imagined.
Luckily, astronomers have since found the missing puzzle
piece: The smallest satellites simply can't hold onto the gas
that forms stars (S&T: Oct. 2016, p. 12). The Milky Way is
embedded within a cloud of dark matter that extends well
beyond the iconic spiral disk. Within this halo are mini-halos,
Distance (light-years)
VISIBILITY Finding Milky Way analogs requires imaging galaxies to large distances. But as distance increases, astronomers can't see as many
dwarfs for a given brightness limit. SAGA's aim is to image satellites around 100 Milky Way siblings between 80 million and 130 million light-years
away. The thumbnails at bottom show a selection of SAGA satellites. (Each box is 1 arcminute on a side; host galaxies are not shown.)
sk yand tele scope .o r g * APRIL 2022 37

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