Sky & Telescope - March 2023 - 19

MAGNETIC FIELDS: NASA / SOFIA, COMPOSITE IMAGE: NASA / JPLCALTECH
/ ROMA TRE UNIV.
gaps in the distribution of stars in the disk, rather than the
massive clumps of D'Onghia's simulation. Sellwood likens
these gaps to the destabilizing effect of a scratch in a piece of
glass. Stars and gas clump on either side of the gap, resulting
in extensive spiral arms. As the arms decay, they " scratch "
new gaps (and associated clumps) elsewhere, causing additional
spiral arms to form.
Both swing amplification and groove modes use gravitational
instabilities to explain spiral patterns in isolated galaxies,
but in different ways. " Probably the right way to think of
the swing amplification is as a transitory event that makes
the instabilities much stronger than you would otherwise
think, " Tremaine suggests. " The groove instability is a kind
of self-propagating cycle of instabilities. " The two result in
different spiral types: While swing amplification would only
generate ragged, multiple-armed spirals, groove modes could
reproduce the entire gamut of patterns.
Grooves and swing amplifications might also work in
league with other gravitational exchanges. Some have suggested
that bars - the elongated structures that skewer many
spiral galaxies' central bulges - can interact with the rest of
the disk to drive a spiral pattern. However, bars' gravitational
effect is weak, and observations have shown most spiral
patterns rotate at speeds different from the bar. So while
these structures might have some effect, they're probably not
directly responsible for most spirals.
Companion galaxies, though, do drive spiral structure.
When a dwarf satellite swings close by a disk galaxy, the two
galaxies pull on each other. Like tides caused by the Moon
on Earth, the pull of the smaller galaxy stretches both the
near- and farside of the larger one, driving the creation of two
grand arms.
Theorist Clare Dobbs (University of Exeter, UK) led a team
in simulating the evolution of the Whirlpool Galaxy as it
interacts with its companion NGC 5195, recreating it almost
exactly. " The simulation produced some quite detailed kinks
in the spiral arm, " Dobbs says. " I hadn't expected it to reproduce
the observations quite that well. "
Perhaps interacting with companions could explain all
grand-design spirals. After all, muses Karen Masters (Haverford
College), " is it really the case that there are any isolated
galaxies? " Cosmological simulations suggest there are still a
lot of smaller clumps in the universe, whether of stars or dark
matter, so no galaxy is an island entire of itself. " Maybe, "
Masters suggests, " you don't need a way to make granddesign
arms in an isolated galaxy. "
And maybe every one of these processes is at work. " I don't
necessarily think that there is one cause of spiral structure, "
Dobbs says. " Gravity is the underlying force, but there are
different ways that spiral structure can come about. "
What Spiral Arms Do
Spirals, whether flocculent, multi-armed, or grand-design,
are more than just pretty. The patterns contribute to lasting
change in their galaxies. But they're not always appreciated
INVISIBLE ALIGNMENT Using a far-infrared camera aboard the
Stratospheric Observatory for Infrared Astronomy, astronomers revealed
the magnetic field of the spiral galaxy M77. The gravitational forces that
created M77's shape are also compressing its magnetic field lines, so
that the streamlines follow the arms (underlying image).
sk yand tele scope .o r g * MARCH 2023 19
as such. " I think a lot of astronomers just don't even really
think about spiral arms very much, " Masters says. " They'll
write papers about spiral galaxies and not mention spiral
arms and the impact they might have. "
Spiral arms' most obvious impact is the stellar nurseries
that they foment: The density waves sloshing through the
disk compress gas and ignite these new stars. The waves also
compress, and thereby amplify, magnetic fields, which help
regulate star formation.
A more subtle effect is the rearrangement of the galaxy's
stars. As arms sap angular momentum from the inner
regions, transferring it to the outer parts of the galaxy, stars
can surf the spiral and move inward, Masters says. As a
result, the bulk of a galaxy's spinning material moves toward
its core over time, while a much smaller amount of material
in the outskirts expands.
But this transfer of energy can shuffle stellar orbits in
complex ways, Tremaine adds, causing stars to migrate back
and forth relative to the galactic center. The Sun, in fact,
most likely began its life closer to the center: The amount
of heavy elements in our galaxy's star-forming gas generally
increases inward, and the Sun's composition corresponds to a
birthplace as close as 16,000 light-years from the core. Today,
it's 10,000 light-years farther out.
" That shuffling is almost certainly caused by spiral arms, "
Tremaine says. " We're only just beginning to understand how
strong that process is and what effects it had on the galaxy. "
Sky & Telescope - March 2023

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