Sky & Telescope - April 2022 - 40

GALAXY/DARK MATTER SIMULATION: Y.-Y. MAO & RISA WECHSLER,
VISUALIZATION: RALF KAEHLER; DWARF GALAXY THUMBNAILS: M. GEHA,
WITH IMAGES FROM LEGACY SURVEYS / D. LANG (PERIMETER INSTITUTE)
SAGA Satellites
spectra of the dwarfs they found. These data will help enable
researchers to determine how the star-formation rate might
have been impacted as the satellite fell toward the host
galaxy. Projected distances will also show whether starformation
rates change with distance from the host. This is a
question that Brooks is already addressing from a theoretical
standpoint by simulating the evolution of dozens of Milky
Way-like galaxies and their satellites.
And Geha would like to expand the survey to other kinds
of systems, too. " Now that we have this toolbox, we're going
to apply it everywhere, " she says. The team wants to look at
galaxies of similar sizes to the ones in SAGA but in any environment.
That includes not only galaxies in isolation but also
galaxies that reside in Local Group-like systems, which could
reveal whether any special features of the Milky Way's system
are related to its close contact with Andromeda.
The survey will surely help address open questions about
galaxy evolution, but the SAGA team is hopeful that the data
will also provide a new window on dark matter.
Astronomers have deduced that dark matter exists within
our own galaxy and others based on the excessive speeds of
galaxies' stars, which move far too fast to stay bound if the
gravity of visible matter is all that's holding a galaxy together.
The SAGA galaxies are too far away to measure the velocities
of their individual stars, but astronomers can measure
velocities for clusters of stars and large regions of hydrogen
gas within the galaxies. Then they use those measurements to
estimate how quickly a galaxy spins around.
" Having a new data set to apply that to is really enticing, "
Tollerud says. " And to ask whether the hundreds of papers
that have been written on [dark matter] in the Milky Way
also apply to the larger universe. "
But the findings won't simply point to the existence of
dark matter; they will also hint at dark matter's nature.
SEEING THE UNSEEN Astronomers compare SAGA observations
to predictions from cosmological simulations. In this visualization of one
such simulation, clumps and filaments of dark matter (black areas) serve
as the scaffolding for the formation of cosmic structures such as stars,
galaxies, and galaxy clusters (bright areas). The simulated Milky Way-like
halo is the large yellow-white object near the center.
Already, Wetzel is running simulations that vary dark matter's
characteristics to see if they create observable differences
in the number of dwarf galaxies or in their properties, such as
the orbital speeds of their stars.
" The SAGA observations are and will continue to be a
very useful data set to compare [to] any such model - be
it a vanilla, cold-dark-matter model or some more exotic
alternative, " Wetzel asserts. " It's quite a useful observational
benchmark. "
¢ SHANNON HALL is an award-winning freelance science
journalist. She'd like to nominate " Samwise " for a SAGA name.
STAR-FORMING VS. QUENCHED SAGA satellites are more likely to be star-forming (top row) than not (bottom row). The former tend to be bluer in
color due to light from newer, more massive stars. These representative examples all have roughly the same stellar mass (10 million Suns' worth).
40 APRIL 2022 * SKY & TELESCOPE

Sky & Telescope - April 2022

Table of Contents for the Digital Edition of Sky & Telescope - April 2022