Australian Sky & Telescope — July 2017

10 AUSTRALIAN SKY & TELESCOPE July 2017

SPIRAL GALAXY:

ESO / L. CALÇADA

NASA narrows Mars

2020 landing sites

Three candidates are now in the running

for where NASA’s Mars 2020 rover will land

— Columbia Hills, the crater Jezero, and

northeastern Syrtis Major. All three are near the

equator. The latter two are close to each other,

on the edge of Isidis Planitia; Columbia Hills

was explored by NASA’s Spirit rover, which

discovered evidence for ancient hot springs

there. The sites selected include terrains

where water might once have flowed and, just

possibly, sustained microbial Martian life. Read

more athttps://is.gd/mars2020threesites.

■DAVID DICKINSON

Meteorites date

solar nebula’s demise

A study of ancient meteorites has refined the

date for the dissolution of the solar nebula, the

cloud of dust and gas from which the planets

formed. From snapshots of infant exoplanet

systems, astronomers had crudely estimated

that the nebula lasted 1 to 10 million years,

until the newborn Sun’s radiation cleared it

away. But by looking at some of the oldest

ANEWSTUDYof six young, star-

forminggalaxiessuggeststhey’reless

influencedbydarkmatterthanexpected.

Buttheresultsperhapssaymoreabout

galaxyevolutionthanaboutdarkmatter.

Oneofthemainargumentsforthe

existenceofdarkmatteristhatgalaxies’

outer reaches rotate more quickly

than expected based on the matter we

observe. Now, in March 16th’sNature,

Reinhard Genzel (Max Planck Institute

forExtraterrestrialPhysics,Germany)

andcolleaguesreportthattheoutskirts

of six distant galaxies do the opposite:

They rotate more slowly than the

inclusionofdarkmatterwouldpredict.

Averaged data from 97 other (fainter)

distantgalaxiesshowthesameresult.

That’snottosaythatdarkmatter

isabsent—there’sjustnotasmuchas

astronomers expected. The dark matter

cushionsthesegalaxieslieinappearto

be rather threadbare.

Onepossibility,saysMarkSwinbank

(Durham University, UK), who

authored an accompanying article in

Nature,isthatthedarkmatterhalos

ofthesegalaxiesarestillgrowing.But

that would fundamentally change how

we view galaxy evolution — the halos

shouldbelargelyinplacebeforethegas

and stars come together.

Another possibility is that we’re

RUNAWAY STARS IN ORION

ARUNAWAYPROTOSTARin the Orion

Nebulasuggestsatussleoccurred

betweenfourstars540yearsago.

The protostar, labeled Source X, lies

inthevicinityoftheKleinmann-Low

Nebula, the most active part of the

star-forming Orion Nebula complex.

BycomparingHubbleSpaceTelescope

images from 1998 and 2015, Kevin

Luhman (Penn State University) and

colleaguesfoundthatSourceXismoving

atmorethan55km/s,awayfromthe

sameoriginpointastwootherhigh-

velocitystars—calledBNandSourceI,

which are zooming in opposite directions

at26km/sand10km/s,respectively.

Thethreeobjectsmightoncehave

beenpartofafour-starsystem.When

twoofthestarscametooclosetogether,

theyformedabinaryormerged

(SourceI),sendingBNandSourceX

flying. The new measurement of Source

X’smotionaccountsforthesystem’s

missing kinetic energy and seals the deal.

■MONICA YOUNG

simply viewing these galaxies during

a crucial era. Genzel’s team chose

to observe massive, star-forming

disk galaxies during cosmic noon, the

universe’s peak in star formation, about

2 to 8 billion years after the Big Bang.

Recent computer simulations by Adi

Zolotov (Ohio State University) and

colleagues show that virtually all such

massive galaxies take a fast track toward

evolution, their burst of star formation

instigated by a single event. As a result,

massive, star-forming galaxies will look

a lot more compact during this cosmic

era than they actually are.

So measuring their rotations

won’t reveal the influence of the full

dark matter halo around them, says

simulation coauthor Joel Primack

(University of California, Santa Cruz).

The dark matter simulations actually

predict the Genzel team’s result.

■ MONICA YOUNG

Artistic galaxy, with arrow lengths

corresponding to rotation speeds

meteorites on Earth, known as angrites,

Huapei Wang (MIT) and colleagues put the

solar nebula’s lifetime at 3 to 4 million years.

The magnetism ‘frozen’ into meteorites of

various ages dropped more than tenfold in

strength between 2 and 3.8 million years

after the Solar System’s formation, the team

reports in the February 10th Science. The

magnetic field would have been associated

with the solar nebula.

■ DAVID DICKINSON

Black hole gnaws on star

The black hole in the galaxy SDSS

J1500+0154 has been eating the same

star for at least 11 years — a feast that

normally takes one to two years. X-ray

observations suggest the object has been

gorging itself on the shredded star’s remains

at what should be an unsustainable rate,

called super-Eddington accretion, Dacheng

Lin (University of New Hampshire) and

colleagues report February 6th in Nature

Astronomy. The detection supports the idea

that the universe’s first big black holes could

grow this way.

■ MONICA YOUNG

IN BRIEF

Less dark matter in young galaxies?

NEWS NOTES