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10 AUSTRALIAN SKY & TELESCOPE JULY 2016

Giant cloud’s gas came from our galaxy

A massive cloud falling toward our
galaxy’s disk is likely from the Milky Way
itself, not a visitor, a new study shows.
This object, called the Smith Cloud, lies
40,000 light-years away and contains
2 million Suns’ worth of gas, primarily
hydrogen. It’s falling toward the galactic
disk at nearly 102,000 km/h.
The Smith Cloud is one of the best
characterisedhigh-velocity clouds, speedy
wispsthatraindownasalightdrizzle
onto our galaxy. This shower helps fuel
star formation. But despite these clouds’
importance,astronomersknowapaltry
amount about what they’re made of and
wheretheycomefrom.
Andrew Fox (Space Telescope Science
Institute) and colleagues used the Hubble
Space Telescope and the 91-metre Green
Bank Telescope to better understand the
Smith Cloud’s makeup. The astronomers
took advantage of three distantactive

Shedding light on the ‘diamond planet’
New observations are helping characterise
the atmosphere of an exotic exoplanet.
Angelos Tsiaras (University College
London) and his team took an infrared
spectrum of the super-Earth 55 Cancri e
(now called Janssen). It’s the first
successful measurement of a super-Earth’s
atmospheric composition. The team found
tantalising hints of hydrogen cyanide
(HCN). This molecule would only dominate
in a carbon-rich environment, instead of one
based on oxygen-rich silicates as on Earth.
That supports researchers’ suspicion that
this dense world, nicknamed the ‘diamond
planet,’ contains crystallised carbon in its
interior. The team also found no trace of
water vapour, which would form easily if
oxygen were widespread on the planet, the
astronomers report in an upcoming issue of
the Astrophysical Journal.
■ MONICA YOUNG

galactic nuclei(AGN)thatliebehindthe

cloud from our perspective. With Hubble,

the team observed the AGN through the

cloud to see how well the gas absorbed

certainwavelengthsfromtheaccreting

black holes’ light. Which wavelengths

the gas absorbs, and how deeply, reveals

how tainted the cloud is by elements

heavierthanhelium,knownasmetalsin

astrospeak. The researchers found that

the Smith Cloud contains about half the

concentrationofmetalsthattheSunhas.

But metals come from stars — they’re

synthesised in stellar cores or produced

when stars die — and astronomers

don’t know of a single star within the

SmithCloud.Soit’shighlyunlikelythat

this object is a chewed-up dwarf galaxy

or a pristine, first-time visitor from

intergalactic space.

However, the heavy-element level is

aclosematchforthegasintheMilky

Way’s own outer disk. The team argues

in the journalAstrophysical Journal Letters

that the cloud’s gas was either spewed

from or torn out of our galaxy. And given

the cloud’s orbit, it either passed through

or came from the outer disk about 70

million years ago. So the Milky Way

might be feeding on itself.

■CAMILLE M. CARLISLE

This artist’s conception shows the comet-

like Smith Cloud (red) and its current trajec-

tory with respect to the Milky Way’s disk. If

visible, the cloud would span 30 full Moons.

NASA / ESA

WFIRST:

Next decade’s

space telescope

A

fter several years of design studies,

NASA has announced it is commencing

work on the Wide-Field Infrared Survey

Telescope (WFIRST), the successor to the

Hubble and the yet-to-be-launched James

Webb space telescopes.

WFIRST will incorporate one of the two

2.4-m mirrors donated to NASA in 2011 by

the US National Reconnaissance Office. The

mirror is comparable in size to Hubble’s

primary but, thanks to its faster focal ratio

(f/7.8 compared with Hubble’s f/24), will

have a field of view 100 times larger in area.

However, its resolution will be less sharp

than Webb’s, and Webb will peer deeper

into the universe.

Although the ready-made ‘spy’ mirror

speeds development, a significant amount

of time and money will still go into

instrumentation. The mission is set to

Learn more about the

mission and watch a recent

Google Hangout with some

of WFIRST’s key players:

http://is.gd/wfirstago

launch in the mid-2020s.

Originally conceived as the Joint Dark

Energy Mission, WFIRST was the No. 1

mission on the astronomy community’s

decadal survey wish list. The two main

themes it will address are dark energy

and exoplanets. Its wide-field imager

will search for Type Ia supernovae and

gravitationally lensed galaxies in an effort

to better understand cosmic expansion,

complementing the upcoming European

Space Agency’s Euclid mission.

On the other hand, the telescope’s

coronagraphic imager will home in on

large, close-orbiting exoplanets to reveal

their atmospheres and compositions,

complementing space- and ground-based

direct-imaging projects.

■ DAVID DICKINSON

NASA

News Notes

IN BRIEF