Sky and Telescope - August 2015 - 14

News Notes

IN BRIEF
Ancient Galaxies Seen Dying InsideOut. In the April 17th Science, Sandro
Tacchella (ETH Zurich, Switzerland) and
colleagues report their analysis of 22 galaxies forming stars 10 to 11 billion years
ago (average redshift: 2.2). The team
tracked starbirth inside the galaxies and
found that, in the most massive ones, star
formation is shutting down, beginning in
the galaxies' centers and moving out from
there. These big galaxies rival the Milky
Way in number of stars, and even with
death approaching they should be able to
churn out stars for another 2 or 3 billion
years. The team suggests such galaxies
will form enough stars to grow into some
of today's large elliptical galaxies. But the
most massive ellipticals probably grew by
merging with smaller galaxies.
■ CAMILLE M. CARLISLE

Black Holes More Certain. A team of
astronomers has found indirect evidence
of the event horizon of M87's supermassive black hole, providing further proof
that these wacky objects actually exist.
Avery Broderick (Perimeter Institute for
Theoretical Physics, Canada) and colleagues peered into the heart of the elliptical galaxy using three radio telescopes
in the still-developing Event Horizon Telescope network (S&T: Feb. 2012, p. 20). As
material streams onto any massive object,
it heats up and emits torrents of highenergy photons. If there's a surface, then
the infalling stuﬀ will splash onto the surface, glowing hotter and brighter than it
would if there wasn't one. The team found
that, given the amount of material falling
onto the black hole in M87, if there were
a surface the gas should glow at least 10
times brighter than it does. Hence, the
gas seems to be falling through an event
horizon, the team concludes April 5th
in a paper posted to arXiv.org. Previous
studies have come to similar conclusions
looking at stellar-mass black holes and
the Milky Way's central beast, Sgr A*, but
the new study expands the list of missing
surfaces beyond our galaxy.
■ SHANNON HALL

August 2015 sky & telescope

SUN I Do Explosive Bursts Heat the Corona?
New evidence suggests that nanoﬂares,
small but potent bursts of energy, might
heat the Sun's outer atmosphere. But the
evidence is still up for debate.
For decades, scientists have struggled
to understand why the Sun's outer
atmosphere, known as the corona, is a
blistering-hot few million degrees when
its visible "surface," the photosphere,
bubbles away far below at only thousands
of degrees. Theories range from waves
rippling along magnetic ﬁeld lines to ﬁeld
lines twisted so tightly they snap, releasing tremendous amounts of energy.
There's also another question to
answer: is the heating process steady, or
does it occur in explosive bursts?
These bursts would be small stuﬀ -
they release only a billionth the energy
of regular ﬂares, and they're too small,
quick, and faint for telescopes to capture
individually. But even a nanoﬂare would
be equivalent to a 50-megaton hydrogen
bomb, and millions going oﬀ per second
could feasibly heat the corona.
Now astronomers have found indirect
evidence that these small explosions heat
the solar corona. On April 28th Adrian
Daw (NASA Goddard) presented results

from the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding
rocket mission at the ﬁrst Triennial EarthSun Summit in Indianapolis. The EUNIS
results point to short-lived pockets of
superhot plasma that are about 10 million
Kelvin, ten times hotter than the sizzling
corona around them. The researchers
argue that these temperatures could only
come from the combined echo of many
tiny ﬂashes in the solar atmosphere.
But if nanoﬂares behave like mini versions of larger, well-observed ﬂares (and
we don't know whether that's true), then
the energy they unleash ought to accelerate particles, which will emit a particular
pattern of X-rays. Observations with
NASA's NuSTAR spacecraft reveal X-ray
emission from the super-hot gas, but
the telescope sees no sign of the particle
signature needed to cinch the case.
That might be because the Sun is
still too active, overwhelming NuSTAR's
bandwidth. As solar activity decreases,
NuSTAR will have a better shot at making out fainter, non-thermal emission -
and perhaps providing deﬁnitive evidence
for nanoﬂares.
■ MONICA YOUNG

GALAXIES I Runaway Compact Galaxies?
Astronomers have discovered
195 compact elliptical galaxies, upping
the known number of these weird objects
sixfold. These dwarf galaxies are puny
balls of old stars, each having maybe
a few billion solar masses. Most lie in
clusters, and observers have found debris
around some, suggesting that they were
once larger but had their outer edges
ripped from them by other galaxies. But
a couple sulk oﬀ by themselves, making
the tidal disruption theory problematic: if
there's nothing around to strip material
oﬀ these galaxies, did they really shrink?
In the April 24th Science, Igor Chilingarian (Smithsonian Astrophysical
Observatory) and Ivan Zolotukhin (Moscow State University, Russia) report their
discovery of 195 of these objects, with 56
in clusters, 128 in galaxy groups, and 11 in

isolation. Given the galaxies' motions and
sizes, the authors suggest that interactions
with two or more other galaxies could
sling these diminutive ellipticals into the
cosmic outback. That would explain how
galaxies created via tidal disruption sit
out in the sticks, where there aren't many
galaxies to interact with.
But astronomers need to check this
theory with rigorous computer simulations - it's by no means conclusive. Other
researchers have suggested that isolated
compact ellipticals might arise when
dwarf galaxies merge. With no additional
stuﬀ nearby to feed on, the compact ellipticals would simply stop growing and never
reach full size. This new, larger population
will thus help astronomers explore these
galaxies' origin. ✦
■ CAMILLE M. CARLISLE

http://www.arXiv.org

Sky and Telescope - August 2015

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