Sky and Telescope - October 2015 - 12

News Notes

Late last year, astronomers reported a
weird ﬁnd: 47 "dark" galaxies mingling
with the other denizens of the Coma
Cluster (S&T: Mar. 2015, p. 12). Now
another team has discovered a whopping
854 of these faint, ﬂuﬀ y, and hard-toexplain objects in the same cluster.
Even ordinary galaxies contain a lot of
dark matter, which makes up about 83%
of the universe's mass. But the dark galaxies discovered within the Coma Cluster
contain even more of the exotic and invisible stuﬀ, up to 98% of their total mass.
The galaxies are incredibly dim, making them diﬃcult to pick out even with
advanced instruments. Yet despite their

lack of stars, most still span roughly the
size of the Milky Way. Only dark matter
could hold this diﬀ use collection of stars
together in the collision-prone environment of a crowded galaxy cluster.
Jin Koda (Stony Brook University)
and colleagues found the dark galaxies
by poring over archival images from the
8.2-meter Subaru Telescope. Astronomers
haven't seen so many of these objects
elsewhere, suggesting that the crowded
cluster environment strips star-forming
gas out of these galaxies, leaving only
dark matter behind, the team reports in
the July 1st Astrophysical Journal Letters.
■ MONICA YOUNG

. . . and Mystery of Dust-Poor Early Galaxies
New submillimeter observations reveal
low levels of dust in nine early galaxies.
The result is in line with previous predictions, but it does highlight a problem with
some observers' calculations.
Peter Capak (Caltech) and colleagues
used the ALMA array, with other optical
and infrared data, to look at nine galaxies
shining at us from about a billion years
after the Big Bang. The astronomers
detected dust emission from only four of
the nine galaxies, but detected a form of
ionized carbon known as [CII] in all nine.
As the authors explain in the June
25th Nature, the presence of all this
ionized carbon suggests a low level of
dust. Carbon likes linking up with other
elements to form molecules, so it doesn't
hang around by itself for long. But with
few heavy elements to bond with, and
with minimal dust around to protect the
carbon atoms from the ionizing inﬂuence of ultraviolet radiation - which is
pouring out from the young stars in these
nascent galaxies - the normally rare
[CII] has become fairly concentrated.
The implication is that these galaxies have amounts of dust similar to that
in the Small Magellanic Cloud (SMC).
That's unsurprising: dwarf galaxies likely
undergo star formation in on-and-oﬀ ﬁts,
so they'll take longer to build up dust.
12

October 2015 sky & telescope

NAOJ

GALAXIES I Dark Galaxies Suffuse Coma . . .

This color composite of Subaru images shows
some of the newly discovered "dark" galaxies,
circled in yellow. Blue circles highlight two galaxies discovered late last year. The image above
covers less than 1% of the studied area.

EXOPLANETS I
Planet with a Tail

Interestingly, two of the team's galaxies have similar amounts of dust to
A1689-zD1, the remarkably dusty galaxy
that raised eyebrows earlier this year
(S&T: June 2015, p. 14). But A1689-zD1
existed 300 million years before these
galaxies. What counts for a "moderate"
amount of dust a billion years after
the Big Bang is "challenging" only 700
million years after the Big Bang, says
Veronique Buat (Astrophysics Laboratory
of Marseille, France). Perhaps in general
dust buildup was slow, but some galaxies
jumped the gun and got dusty fast.
Dust warmed by starlight is usually
the dominant source of a galaxy's infrared emission, a fact astronomers exploit
to estimate the rate of starbirth in distant
galaxies. If there's less dust in early
galaxies, then the particular correlation
used to calculate the starbirth rate in an
individual galaxy will give an estimated
rate that's too high. But the assumption probably has much less of an eﬀect
on estimates of star formation across
cosmic time, because most stars form in
less luminous galaxies that we already
assume have very little dust. And studies
that assume a galaxy's dust properties are
similar to the SMC's (such as S&T: Feb.
2014, p. 10) appear okay as well.

Astronomers have conﬁrmed that
the exoplanet Gliese 436b is trailing a
gigantic, coma-like cloud behind itself
in its orbit. Gliese 436b has about the
mass and size of Neptune but orbits
its red dwarf star in just 2.6 days. In
2014, astronomers watched Gliese 436b
pass in front of its host star in ultraviolet (UV) light and found that the star
stayed dim long after the planet had
ostensibly ﬁnished transiting.
David Ehrenreich (Geneva Observatory, Switzerland) and colleagues
have now conﬁrmed that observation.
The team studied the planet's transit
in a speciﬁc UV spectral line called
Lyman alpha, which is associated
with hydrogen. The astronomers thus
found that the planet blocks more
than 50% of the host star's UV light,
whereas it blocks less than 1% of its
visible light. The result suggests that
the planet is trailing a gigantic, cometlike tail of hydrogen behind itself
that continues to block starlight after
the main transit, the team writes in
the June 25th Nature. Read more and
watch a video of the system at http://
is.gd/436btail.

■ CAMILLE M. CARLISLE

■ JOHN BOCHANSKI

http://www.is.gd/436btail http://www.is.gd/436btail

Sky and Telescope - October 2015

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