Sky and Telescope - November 2015 - 12

News Notes

GALAXIES I Gigantic Protogalaxy in the Cosmic Web
Astronomers have found that a massive ﬁlament of gas in the early universe
is actually a humongous, galaxy-forming
disk of cold gas.
One way that galaxies grow - and
possibly the predominant way in the
early universe - is from cold gas funneled like a pipeline into wells of dark
matter (S&T: Sept. 2015, p. 16). These
dark matter wells are dense ﬁlaments
in the weblike cosmic structure, along
which galaxies form. Although computer
simulations suggest that many galaxies
could have gotten their start as cold-ﬂowgrown disks, observing this accretion
in action is diﬃcult because the gas is
diﬀ use and faint.
Last year, astronomers detected a large,
bright ﬁlament of gas called UM 287
shining at us from about 11 billion years
ago (S&T: May 2014, p. 14). It's ﬂuorescing
thanks to the intense ultraviolet radiation
of a nearby quasar. At the time, the team
estimated that the structure was about 10
times more massive than expected, given
simulation results.
But it turns out the ﬁlament isn't
too massive, because it's not merely a
ﬁlament. Christopher Martin (Caltech)
and colleagues took a second look with
the Palomar Cosmic Web Imager, a
high-tech spectrograph they built and
installed on the 5-m (200-inch) Hale
Telescope on Palomar Mountain in

California. The spectrograph homed in
on the wavelength Lyman-alpha, which
comes from cold neutral hydrogen that's
been irradiated by ultraviolet light. By
analyzing the ﬁlament's spectra, the team
discovered that one part of the ﬁ lament is
moving toward us, while another section
is moving away from us. In other words,
the structure is actually a fuel line feeding a gigantic disk.
The disk is about 400,000 light-years
across, or three to four times the size of
the Milky Way's spiral disk. As the team
reports August 5th in Nature, the object's
rotational velocity suggests it's sitting in
a halo of 10 trillion solar masses' worth of
dark matter, an order of magnitude larger
than the halo our galaxy inhabits. There's
even a hint of star formation in its center,
but the researchers aren't sure of that yet.
"Overall, it's hard to say with certainty
that they're deﬁnitely seeing a coldﬂow disk - as opposed to some other
phenomenon that just happens to look
like a cold-ﬂow disk," says Kyle Stewart
(California Baptist University), whose
team has simulated the growth of these
objects. "But when you look at all the
observable properties of cold-ﬂow disks
from the simulations to determine what
they should look like in the real universe,
in my opinion, it's amazingly similar to
what these authors have just observed."

November 2015 sky & telescope

Most Luminous Supernova Yet. Astronomers have discovered an exploding star that
tops the "superluminous" charts. Dubbed
ASASSN-15lh, the supernova was found
on June 16th at magnitude 17 as part of the
All-Sky Automated Survey for Supernovae
(ASAS-SN). Follow-up observations suggest
it shines with the luminosity of 572 billion
Suns, and spectra conﬁrm its bluish light has
been traveling toward Earth for the past 2.8
billion years (redshift of 0.2326). The debris
lacks hydrogen emission lines, meaning the
star lost its outer layers prior to exploding.
■ MONICA YOUNG

Astronomers Spot Five-Star System.
A rare four-star system turns out to have a
ﬁfth wheel, Marcus Lohr (The Open University, UK) and colleagues report in the
June Astronomy & Astrophysics. Although
multiple-star systems are common, systems
containing more than three stars are rare. In
2013 Lohr's team discovered the quadruple
system 1SWASP J093010.78+533859.5 in
Ursa Major. It contains two eclipsing binaries, the two pairs separated from each other
by 140 astronomical units (21 billion km). The
ﬁfth star revealed itself when the team took
spectra to study the stars in more detail. The
system is 9 to 10 billion years old.
■ ANNE MCGOVERN

and near distant galaxy clusters. They
discovered four of them were in the space
between galaxies. But they couldn't resolve
the images clearly enough to conﬁrm that
the white dwarfs truly ﬂoated in solitude.
Now, using Hubble images, Melissa
Graham (University of California, Berkeley) and colleagues have conﬁrmed the
solitary nature of three of the supernovae
- the fourth belongs to a dwarf galaxy.
Given these statistics, the team calculates
that roughly 11% of normal matter ﬂoats
in intergalactic space. The result appears
in the July 1st Astrophysical Journal.

Aurora on a Dwarf Star. Astronomers have
detected auroral emission on a star. Ultracool dwarfs (UCDs) are the runts of the stellar family: they include both the least massive
stars and brown dwarfs. Several UCDs emit
periodic, aurora-esque radio signals, and a
few even show signs in optical wavelengths.
Now, Gregg Hallinan (Caltech) and colleagues report in the June 30th Nature that
they've detected these telltale radio and optical variations simultaneously from a UCD
called LSR J1835+3259. This object is an M8.5
star, right at the transition point between
stars and brown dwarfs. The pulsations'
period matches the dwarf's 2.84-hour day,
suggesting the aurora is rotating in and out
of view. A magnetically controlled stream of
electrons, somehow dumped into the star's
lower atmosphere, likely excites atoms and
incites them to emit the auroral photons.

■ ANNE MCGOVERN

■ CAMILLE M. CARLISLE

SUPERNOVAE I Lonely Explosions Between Galaxies
Researchers have conﬁrmed that three
white dwarfs went supernova in intergalactic space.
The space between galaxies is not
entirely empty. Sometimes observers come
across stars that have been ejected from
their hosts and left to drift alone. Although
solitary, they're still important for understanding how much mass the universe
contains and where that mass is.
But it's diﬃcult to see individual stars
in the distant universe. So astronomers
turn to supernovae, which are easier to
spot. In 2011, researchers conducted a
survey of 23 exploding white dwarfs in

IN BRIEF

Sky and Telescope - November 2015

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