2019-07-01_Australian_Sky_&_Telescope

NEWS NOTES

Astronomershavebeensearchingfor

blackholesin theAndromedaGalaxy.

NEWOBSERVATIONSLIMITtherole

thatprimordialblackholescanplayin

explainingdarkmatter.

Evenbeforetherewerestars,

primordialblackholes(PBHs)could

havebeenbornwhenpatchesofthe

radiationandplasmathatfilledthe

earlyuniversebecamesufficiently

densetocollapseintoblackholes.

AstronomershavelookedforPBHs

withmasses between that of Earth

andSaturn’s moon Mimas, then

theastronomers calculate that they

shouldhave turned up roughly a

thousandmicrolensing events. They

foundone.

Thecurrent data can’t determine

whethereven this flash definitely

comesfrom a PBH, Niikura and

colleagueswrite in Nature Astronomy.

However,the results nevertheless

limitwhatPBHs in this mass range

mightcontribute to dark matter: At

mosttheymake up a hundredth of the

universe’sinvisible mass.

Microlensing searches can’t catch

allevents,as the appearance of such

blipsdepends on details such as

theblackhole’s distance. The team

continuesto explore the data on

Andromeda for additional candidates,

andtheresearchers may even rope

incitizenscientists to help them

investigate. ■ CAMILLE M. CARLISLE

Could dark matter be black holes?

NGC 1052-DF2: NASA / ESA / P. VAN DOKKUM (YALE); ANDROMEDA: NASA / ESA / DIGITIZED SKY SURVEY 2 (ACKNOWLEDGEMENT: DAVIDE DE MARTIN)

Two galaxies are missing their dark matter

ASTRONOMERS HAVE DISCOVERED a

second galaxy without dark matter.

This ultra-diffuse galaxy is one of

thousands of large, star-poor objects

discovered in recent years. Most of

these galaxies have more than the

usual amount of dark matter — it’s

all that keeps their sparse stars from

disbanding. But last year, Pieter

van Dokkum (Yale) and colleagues

discoveredonewithoutmuchdark

matterat all,dubbedNGC1052-DF2.

Nowtheteamhasdiscovereda second

oneinthesameregion,suggestingmore

mightbepresent.

VanDokkumandcolleaguesfound

NGC1052-DF4inimagestaken

withthelensesoftheDragonfly

TelephotoArray.Theyfollowedup

withHubbleSpaceTelescopeimaging

andspectroscopythroughtheKeckI

telescopeonMaunaKea,Hawai‘i.The

observationsappearintheMarch 20

usingmicrolensing:Whena blackhole

passesinfrontofa moredistantstar,

itsgravitybendsthestar’slighttoward

us,boostingitsbrightness.NoPBHs

havebeendefinitivelydetectedyet.

Aspartofongoingefforts,Hiroko

Niikura(UniversityofTokyo)and

colleaguesturnedthe8.2-metre

SubaruTelescope’sHyperSuprime-

Camonanestimated 100 million

starsintheAndromedaGalaxy.The

galaxyis farenoughawaythateach

camerapixelcontainedthelightfrom

severalsuns,sotheastronomers

couldn’tlookatindividualstars’

behaviour.Instead,theysearched

forpixelsthatflashed,presumably

becauseoneofthestarshadbriefly

brightened.Theastronomersthen

reducedthesecandidatestothose

thatonlyflashedonce,brightenedand

fadedintherightway,andweren’t

createdduringimageprocessing.

If darkmatteris madeofPBHs

stars’ diffuse light. The measurements

confirmed the galaxy’s low mass.

Michelle Collins (University of

Surrey,UK) had voiced doubts

about DF2 when it was

first discovered but said

the new study has

convinced her there’s

something to the low-

mass claims.

Nevertheless,

there’s work to be

done. Circumstantial

evidence suggests that

DF2 and DF4 belong

tothe cluster of galaxies

around NGC 1052, but their

distanceremains uncertain.

“If[DF2and DF4] were truly closer,

thegalaxies would appear more typical,

andwouldhave a typical amount of

darkmatter,” Collins explains. Hubble

observations in July or August should

finallydecide the distance question.

■ MONICAYOUNG

issue of Astrophysical Journal Letters.

For both DF2 and DF4,the

team measured the velocities

of unusually luminous

globular clusters,

finding that the

galaxies’ total masses

were equivalent to

their mass in stars —

there’s no need for

darkmatterineither

case.

Thediscoveryofthe

seconddarkmatter–less

galaxylendscredencetothefirst

find,inpartbecausevanDokkumand

colleaguesfollowedananonymous

referee’ssuggestiontomeasurethemass

ofDF4(andDF2)a secondway:by

gaugingthevelocityofthegalaxy’sstars.

Unliketheglobularclusters,thesestars

can’tberesolved,sotheastronomers

usedKecktoobtainthevelocityofthe

NGC 1052-DF2, DF4’s

doppelgänger

12 AUSTRALIAN SKY & TELESCOPE July 2019