Sky and Telescope - November 2017 - 10

NEWS NOTES
GALAXIES

SUN

Milky Way May Be Made with Swapped Gas

Solar Waves
Reveal Core's Spin

HIGH-POWERED SIMULATIONS suggest that half of the material in the
Milky Way could come from other galaxies. Although galaxies might look like
pristine "island universes," exchanging
stars and gas only when they crash into
each other, stellar winds and supernovae regularly blow huge amounts
of gas out of their host galaxies. This
gas might rain back down, or it might
escape into intergalactic space, where
other galaxies can sweep it up.
Using the FIRE simulations (S&T:
May 2017, p. 34), Daniel Anglés-Alcázar
(Northwestern University) and col-

■ CAMILLE M. CARLISLE

STARS

ALMA Sees New Star Launch Wind
º Observations from the Atacama Large Millimeter/submillimeter Array

(ALMA), published in the July 2017 Astronomy & Astrophysics, reveal a
still-forming star as it launches a wind from the edge of the disk that feeds
it. The color gradient, corresponding to the intensity of 1.3-mm radiation,
represents heat emitted from the dusty disk that surrounds the protostar
BHB07-11 out to 80 a.u. A sparser envelope (outer blue regions) feeds the
dense central disk. Other wavelengths tracing gas motions show a lopsided wind (white contours) blowing off the disk's edge, between 90 and 130
a.u. from the star. Astronomers knew that protostars must produce winds
in order to carry away the infalling gas's native spin and enable accretion.
But these observations are the first to identify the launch location as the
region where infalling gas meets the disk's edge.
■ MONICA YOUNG

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

* p-modes
convective *
zone
core *
* g-modes
* radiative
zone

p Cutaway diagram of the Sun

SOLAR ASTRONOMERS might

ﬁnally have detected the effect of
gravity waves in our star's core,
revealing that the Sun's central
region rotates about four times faster
than its outer layers.
Astronomers have studied solar
oscillations since the 1960s, but so far
they've only detected acoustic waves,
not gravity waves (called g modes).
Gravity waves are a sloshing motion
spurred by the turbulent convection
that happens in the Sun's outermost
layer. They don't survive long there,
though - instead, they become
trapped below, in the radiative zone
and the core, where material moves in
a way that doesn't erase them.

p The barred spiral NGC 4911, surrounded by
galactic friends in the Coma Cluster.

leagues took a closer look at these processes. The team watched the development of multiple systems, each with a
different kind of galaxy in the middle.
The simulations tracked gas particles as
the gas ﬂowed here and there, moving
out of and back into individual galaxies,
as well as between the central member
and the ones surrounding it.
Previous work had suggested maybe
10-20% of the gas in a Milky Way-mass
galaxy would come from other galaxies.
Reporting in the October 1st Monthly
Notices of the Royal Astronomical Society,
the FIRE team's numbers are much
higher: Somewhere between 20% and
60% of the gas in a modern Milky Way
is "intergalactic" - stuff blown out of
one galaxy and nabbed by another.
In today's universe, the team ﬁnds,
the exchange of gas between galaxies via
galactic-scale winds can even dominate
how a big stellar metropolis acquires
material. Once a galaxy's early years
of formation have passed, gas swaps
outstrip in importance the accretion of
fresh, unprocessed gas and the recycling of the galaxy's own outﬂows. The
simulations also suggest that a Milky
Way-like system permanently chucks out
a ton of gas, equivalent to the mass of all
our galaxy's stars - that's in the range of
tens of billions of Suns.

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017