Andromedaanditsplaceinthelocalgroup
YlvaPihlström,UNM
M31-TheAndromedaGalaxy
• Atadistanceof2.5millionlightyears,ourclosestlargeneighbor
• Canseewithnakedeye
• Spiralgalaxy
• Mass1.2x1012M⊙M32
M110
N
Messier&Hubblemadeitknown
From Messier’s work in 1700’s.
• Messiercataloguedit
• Hubblein1920’susedCepheidvariablesinAndromedatodeterminedistance– “Islanduniverse”
• Significantlychangedtheviewoftheuniverseasweknewit
MulO-wavelengthview
X-ray
MulO-wavelengthview
UV
MulO-wavelengthview
Optical
MulO-wavelengthview
Infrared
MulO-wavelengthview
Radio
MulO-wavelengthview
Gascomponents
Atomic Hydrogen CO gas
StructuresintheUniverse
• Hierarchyofstructures:– Galaxy
– Group/cluster
– Supercluster
• Agrouporclusterofgalaxiescontain10-1000sofmembers
• M31andMilkyWaybelongtotheLocalGroup– About50members
The Antennae
NumericalsimulaDons–whatgravitydoes
Walker et al. 1996
StandardevoluDonarymergerscenario
• TheToomresequence(fromsimulaOons),belowinimages.Asequenceofobservedgalaxiesthatarethoughttobeindifferentstagesofmajormergerbetweentwospirals.
• MergerrateasafuncOonoflook-backOmeinformusabouthowstructuresformandevolve
Mergers
• AffectthestarformaOonrate– Minorversusmajormergers– Smallonescontributesignificantly
(Kavirajetal.2016)
• Metallicity
• Shapeofcentralbulges
• Growthofcentralblackholes
=>Extremelyimportantforgalaxyevolu3onscenarios
NGC7714
EvoluDonwithintheLocalGroup
WhatistheevoluOonaryscenariofortheLocalGroup?
• DominatedbyM31andtheMilkyWay(MW),movingtowardeachotherwithapproximately120km/s
• AssumingtheLocalGroupisgravitaOonallybound=>mergerbetweentheMWandM31
Cox & Loeb 2008
DistancebetweenMWandM31asafuncOonofOme
Cox & Loeb 2008
DistancebetweenMWandM31asafuncOonofOme,mulOplemodels
Cox & Loeb 2008
> 3.5 Gyr
2 Gyr
NASA; ESA; A. Feild and R. van der Marel, STScI
• ThemodeledhistoryandevoluOonoftheM31-MWsystemneedsconstraints
• DependsstronglyonthetransversevelocityofM31– OnlyhaveradialvelociOes,need3D
vy =120 km/s
MW
M31
y
x
• ThemodeledhistoryandevoluOonoftheM31-MWsystemneedsconstraints
• DependsstronglyonthetransversevelocityofM31– OnlyhaveradialvelociOes,need3D
MW
M31
v = f(vx, vy) = ?
y
x
vy =120 km/s
vx = ?
Wecanmeasuretheradialvelocity,vy,viaDopplershid.
• Forfullvelocityvectorweneedthetransversevelocity,vx
• Getfromthepropermo3onintheplaneofthesky:theapparentangularmoOonofanobjectacrossthesky
TimebetweenobservaOonsknown,andthedistance=>transversevelocity
t t + dt
x
𝛼d
vx
However,verysmallangularchangesandM31ishuge!
Plan:tofindsomethingmuchsmaller,verycompact,withinthegalaxyandfollowthatobjectwithhighangularresoluOonobservaOons.
Expect motion of 15-50𝜇as/yr, only a small fraction of the galaxy extent.📍
Itmustbecompact,bright,andpreferablynon-variable.
ConOnuumsourcesinM31:
• M31*:acompactradiosourceinthecenterofthegalaxy,thedynamicalcenter
• X-RayBinaries
Willnotwork:InM31theseonesaretooweak.
📍Whatcanweuseasapin?Intensity
Wavelength
X-ray binary, artist’s impression
AnotheropOonislineemission,frommolecules.
We’remainlyinterestedinmasers:
BrightamplifiedemissionfromexcitedmoleculartransiOons
Ex:H2Oat22GHz,OHat1.6GHz,CH3OHat6.7and12GHz,SiOat43GHz
Intensity
Wavelength
• MicrowaveAmplificaOonbySOmulatedEmissionofRadiaOon
absorption spontaneous emission
n1< n2
amplification
E
E
n1
n2
n2
n1
AlsoneedhighangularresoluOon,tobeabletomapthemaser‘spots’asafuncOonofOme.
• VeryLongBaselineInterferometry
(VLBI)measuresposiOonswithafew10𝜇asaccuracy.
Examples:theVeryLongBaselineArray(VLBA),theEuropeanVLBINetwork
ResoluOondependsontelescopedistance–thefurtherapart,thehigherangularresoluOon.
M33&IC10H2OmaserssuccessfullyobservedinM33andIC10,givingpropermoOons(Brunthaleretal.2005,2007)
Brunthaler et al. 2007
Task:FindmaseremissioninM31
H2Oisaprimechoice:
• ProvideshighposiOonalaccuracy
• FoundinregionsofstarformaOon,frequentlyobservedinMWandalsoinM33andIC10
SizeofM31isaproblem,asthefield-of-viewissmallforthewavelengthsofinterest
• TakesverylongOmetodoablindsearchofM31
• TargetedsearchesforH2Oat22GHzchallenging
– Mainbeamissmall:Hardtocoverfullextentofgalaxy
• 1.6GHzOH– LargermainbeambutworseposiOonalaccuracy=>toolongOmebaseline!
– Also,noOHmasersinM31…
• Methanolat6.7/12.2GHz
– Intermediatesizedbeam
Darling et al. 2016
Methanolat6.7GHz
• WithVLAupgradein2009,webeganablindsurveysearchingfor6.7GHzmethanolmasers
N
• Severalweakmasersdetected,sofarmostinthemid-galaxy
Dec
linat
ion
(J20
00)
Right Ascension (J2000)00 45 44 43 42 41
42 00
41 50
40
30
20
10
00
40 50
40
X
Y
Flux
(mJy
/bea
m)
Heliocentric velocity (km/s)
-340 -320 -300 -280 -260 -240
12
10
8
6
4
2
0
-2
-4
Flux
(mJy
/bea
m)
Heliocentric velocity (km/s)
-270 -260 -250 -240 -230 -220 -210
8
7
6
5
4
3
2
1
0
-1
-2
September 2009
VLAfollow-up
• Widthabout5km/s,peakabout8mJy/beam.
• High-endtailofthe6.7GHzmethanolmaserdistribuOonintheMilkyWay– W3(OH)3700Jy=>M31maserat
samedistance1300Jy
• Noassociated22GHzwatermaser
• Notclose(<50pc)ofastarformingregion,H-alphaemissionlineregionoracompactHIIregion.
Sjouwerman et al. 2010
July 2010
Methanolmaserisapossiblepin
• VLAtelescopesaretooclosetogetsufficientposiOonaccuracy
• GlobalobservaOonsincluding44telescopesacrosstheworld
– 4x6hrs
• Currentlyworkingonthisdata
– Maybetoo‘diffuse’,spreadoversizescalesthetelescopearrangementisnotsensiOvetoo
Atthispoint,wesOlldonotknowthetransversevelocityofM31,butthereareseveralgroupsworkingonit.
Whatmayhappentothesolarsystem?
Oncewehavethetransversevelocity,wewillbeabletobeverconstraintheLocalGroupfuture.
Meanwhile,speculate:Whathappenstothesolarsystem?
ExcludinginteracOon:Sunwilldevelopintoredgiant:
WithinteracOon:TaggingparOcles
Summaryremarks:
TheAndromeda–MilkyWaysystemisofgreatinterest:Cluestothepast,andtotheevoluOonofourLocalGroup
NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
