Australian Sky & Telescope - June 2018

6 AUSTRALIAN SKY & TELESCOPE July 2018

NEWS NOTES

NGC 1052-DF2: P. VAN DOKKUM / R. ABRAHAM / STSCI; ‘OUMUAMUA: © WILLIAM K. HARTM

ASTRONOMERS HAVE LONGthought ANN

youcouldn’thavegalaxieswithoutdark

matter,justasyoucan’thaveacupof

coffee without the cup. But the fluffy

galaxyNGC1052-DF2contradictsthis

picture. Combining ground- and space-

based observations, Pieter van Dokkum

(YaleUniversity)andcolleagueshave

foundthatthisdiffusegalaxyhas

400 times less dark matter compared to

other systems of similar mass. In fact, it

mightnothaveanydarkmatteratall.

Thepeculiarobjectispartofagroup

of galaxies 65 million light-years away

that’sdominatedbythebeefyelliptical

NGC 1052. Astronomers already

knew NGC 1052-DF2 existed, but its

appearance in images from the team’s

Dragonfly Telephoto Array puzzled the

researchers. Follow-up revealed the

galaxy is so sparse that it’s see-through.

Ten strangely big and bright

globular clusters surround the galaxy.

Bymeasuringhowquicklytheyorbit

DF2,theastronomersestimatedthe

galaxy’smass,whichtheyfoundtobe

surprisinglylow—roughlyequaltothe

massvisibleasstars,withessentiallyno

dark matter. No other objects in DF2’s

class of so-calledultra-diffuse galaxies

show this paucity of dark matter.

It’sunclearhowthegalaxycame

to exist. One possibility is that the

ellipticalhadstrippedDF2ofitsdark

matter when it passed too close, causing

theremainingstarsto‘puffup’.But

the Dragonfly team doesn’t see signs of

disturbance around the galaxy, which

it thinks disfavours this scenario. The

researchers suggest a few other possible

origins for DF2, but nothing fits

perfectly.

Thatthecoffeecanexistwithout

thecupindicatesthatthecupand

coffeearebothrealentities,theteam

concludes. If the presence of dark

matter were only an illusion, arising

because we’re using the wrong theory

of gravity, then we’d always see signs

ofitingalaxies.Butifthedarkmatter

canbesometimesthereandsometimes

absent, then dark matter must exist.

This result therefore undermines

alternative approaches, including

modified Newtonian dynamics(MOND),

atheoryofgravitythatdoeswithout

dark matter by suggesting gravity works

slightlydifferentlythaninEinstein’s

framework, the team says.

MOND expert Stacy McGaugh (Case

Western Reserve University) isn’t so

sure.HeagreesthatMONDpredicts

that DF2’s globulars should move faster

thantheydo.Buthe’ssqueamishabout

tweakingthefractionofdarkmatterin

any given galaxy just to make sense of

itsstars’motions—that’saninference,

notapredictionthatcanbeprovedor

disproved, he says.

The Dragonfly team continues

tolookformoregalaxieslikeDF2,

andthey’vefoundthreepotentially

similarobjectsforfurtherstudy.With

morethanonesuchgalaxyinhand,

astronomers might be able to say more

abouthowthesegalaxiesformandwhat

theymeanfordarkmatter.

■CAMILLE M. CARLISLE

A galaxy without dark matter?

A new, pancake-shaped

artist’s concept of ‘Oumuamua.

A NEW ANALYSIS of 818 telescopic

observations suggests that our first

known interstellar visitor could have

the shape of a flattened disk.

Small-body specialist Michael Belton

(Belton Space Exploration Initiatives) and

colleagues pooled brightness estimates

from nearly a dozen major instruments

— including the Hubble Space Telescope

— in order to unravel the details of

1I/‘Oumuamua’s shape and spin.

Belton and co-author Karen Meech

(University of Hawai‘i) say that

‘Oumuamua is tumbling, spinning

every 54.48 hours with a pronounced

wobble that takes 8.67 hours to com-

plete. However, it’s unclear if the object

is rotating mainly end-over-end or if

the primary spin axis is close to its long

axis, like a wobbling, badly thrown

football. How it spins determines our

understanding of its shape, which Bel-

Is ‘Oumuamua

pancake-shaped?

This Hubble Space Telescope

image of NGC 1052-DF2 shows

the blob-shaped galaxy’s diffuse

nature — it has so few stars and

is so sparse that background

galaxies are visible through it.