Sky and Telescope - September 2015 - 14

News Notes

IN BRIEF

SUPERNOVAE I Type Ia, Two Ways

Most Luminous Galaxy Discovered.
Researchers using data from NASA's WideField Infrared Survey Explorer (WISE) have
discovered the most luminous galaxy to
date. Dubbed WISE J224607-052635, this
extremely luminous infrared galaxy (ELIRG)
shines brighter in the infrared than 300 trillion Suns, likely powered by a supermassive
black hole at its core that's a thousand times
more massive than the one in the center of
our Milky Way. J2246 is one of 20 ELIRGs
reported by Chao-Wei Tsai (JPL) and colleagues in the June 1st Astrophysical Journal
and has the highest redshift of the group:
its light left it just 1.3 billion years after the
Big Bang. ELIRGs are the brightest examples
of hot dust-obscured galaxies, or hot DOGs
(S&T: Dec. 2012, p. 14). Astronomers still
debate how gargantuan black holes like the
one in J2246 could have formed so early in
the universe's history.

Two new studies conﬁrm that the
white dwarfs that explode as Type Ia
supernovae can approach death on two
diﬀerent paths.
When white dwarfs accrete too much
material, they burst through a strict
weight restriction called the Chandrasekhar limit and trigger a thermonuclear blast deep inside themselves,
self-annihilating in a Type Ia supernova.
For many years, astronomers have
debated just how the white dwarf maxes
out its mass. There are two scenarios:
either it siphons gas from a "living"
companion star until it just can't swallow any more (called the single-degenerate model), or it merges with another
dead star like itself (the double-degenerate model). The growing sense is that
white dwarfs probably die both ways.
Two papers in the May 21st Nature
support this idea. In the ﬁrst, Yi Cao
(Caltech) and colleagues detected an
ultraviolet pulse in light from the Type Ia
supernova iPTF14atg that lasted for four
days after the explosion. The authors
propose the UV pulse arose when the

■ ANNE MCGOVERN

M31's Giant Gas Halo. Astronomers have
detected a massive yet elusive nimbus of
hot gas surrounding the Andromeda Galaxy.
The gargantuan cushion extends out at least
a million light-years, almost halfway to the
Milky Way, Nicolas Lehner (University of
Notre Dame) and colleagues report in the
May 10th Astrophysical Journal. Astronomers have suspected that this kind of halo
must surround most galaxies: observations
only account for about 40% of the normal
matter expected in galaxies (that's after
accounting for dark matter), and simulations
suggest that hot gas, both inﬂowing and outﬂowing, ought to envelop them. By observing 18 distant quasars whose light streams
through the space where Andromeda's halo
ought to be, the team detected the spectroscopic shadows that the halo's ions left on
the quasar light. The measurements show
that Andromeda's halo contains 3 billion
Suns' worth of gas within 200,000 light-years
and probably 10 times that amount out to
a million light-years. In short, Lehner says,
this study and an accompanying one of more
distant galaxies have "essentially solved" the
missing matter problem.
■ MONICA YOUNG

September 2015 sky & telescope

exploding white dwarf's ejecta slammed
into its companion star. The signal would
then have disappeared once the ejecta
engulfed the companion enough to hide
the shocked gas created by the collision.
In the second study, Rob Olling (University of Maryland) and colleagues used
archival observations from the Kepler
mission to study three supernovae. Two
of these supernovae (KSN 2011b and
KSN 2012a) are clearly Type Ia; the third
(KSN 2011c) is probably one. The team
used the same theoretical predictions as
Cao's team did to analyze their data and
found no sign of ejecta slamming into
companion stars.
Although Kepler doesn't look in UV
(where Cao's team saw iPTF14atg's
ejecta shock), Olling says that the
telescope's sensitivity is so tremendous
that, had there been a signal from material colliding with a companion star, his
team would have seen it in visible light.
Thus, the astronomers favor a dual
white dwarf death as the origin for these
three (likely) Type Ia events.
■ CAMILLE M. CARLISLE

STELLAR I Migrating Stars in 47 Tucanae
Observations of white dwarfs in
a densely populated globular cluster
conﬁrm astronomers' expectations that
stars migrate to a cluster's outskirts
after losing mass.
Globular clusters are balls of very old
stars organized by mass, with heavier,
slower stars congregated near the
middle and lighter, faster ones at the
cluster's edge. Astronomers think that
clusters organize themselves in this way
via dynamical relaxation: when two stars
interact gravitationally, the less massive
star gets nudged farther out and is eventually expelled to the cluster's edges.
Using Hubble's Wide Field Camera 3, Jeremy Heyl (University of British
Columbia, Canada) and colleagues have
now caught this migration in action,
by studying white dwarfs in the rich
globular cluster 47 Tucanae. White
dwarfs begin as the cores of stars like the

Sun. When the stars hit the end of their
fusion-powered lives, they shrug oﬀ their
outer layers to reveal collapsed, planetsize cores, losing about 40% of their
initial mass in the form of stellar winds.
Because the stars lose so much mass,
subsequent interactions with other stars
should propel them outward from their
initial positions over time.
White dwarfs cool as they age, and
because hotter (younger) white dwarfs
are brighter in ultraviolet wavelengths
than cooler (older) ones, the researchers
were able to estimate the cluster stars'
ages and identify two major populations
of white dwarfs. They found that the
younger ones gathered near the center
of the cluster, while the older ones were
dispersed at the outer edges, exactly as
expected. The result appears in the May
1st Astrophysical Journal. ✦
■ ANNE MCGOVERN

Sky and Telescope - September 2015

Table of Contents for the Digital Edition of Sky and Telescope - September 2015