The catastrophic effect of mergers on the angular momentum and morphology of galaxies in EAGLE

1.5 MB·

by  Claudia del P. Lagos

Mon.Not.R.Astron.Soc.000,1–18(2016) Printed18January2017 (MNLATEXstylefilev2.2)
The catastrophic effect of mergers on the angular momentum and
morphology of galaxies in EAGLE
7 Claudia del P. Lagos1,2⋆, Adam R. H. Stevens3, Richard G. Bower4, Timothy A. Davis5,
1 Sergio Contreras6, Nelson D. Padilla6,7, Danail Obreschkow1,2, Darren Croton3, James
0
2 W. Trayford4, Charlotte Welker1,2, Tom Theuns4

n
1InternationalCentreforRadioAstronomyResearch(ICRAR),M468,UniversityofWesternAustralia,35StirlingHwy,Crawley,WA6009,Australia.
a
2AustralianResearchCouncilCentreofExcellenceforAll-skyAstrophysics(CAASTRO),44RosehillStreetRedfern,NSW2016,Australia.
J
3CentreforAstrophysics&Supercomputing,SwinburneUniversityofTechnology,Hawthorn,VIC3122,Australia.
6 4InstituteforComputationalCosmology,DepartmentofPhysics,UniversityofDurham,SouthRoad,Durham,DH13LE,UK.
1 5Astronomy,CardiffUniversity,QueensBuildings,TheParade,CardiffCF243AA,UnitedKingdom.
6InstitutodeAstrofs´ica,PontificiaUniversidadCatl´icadeChile,Avda.Vicun˜aMackenna4860,782-0436Macul,Santiago,Chile.
] 7CentrodeAstro-Ingenier´ıa,PontificiaUniversidadCato´licadeChile,Avda.Vicun˜aMackenna4860,782-0436Macul,Santiago,Chile.
A
G 18January2017
.
h
p
ABSTRACT
-
o We use EAGLE toexploretheeffectgalaxymergershaveonthestellar specificangularmo-
r mentumofgalaxies,jstars.We characterisemergersinto:dry(gas-poor)/wet(gas-rich),ma-
t jor/minor,andbydifferentspinalignmentsandorbitalparameters.Ourwet(dry)mergershave
s
a anaverageneutralgasfraction(i.e.theratiobetweentheneutralgasandthestellarmassesof
[ the mergingsystem) of 1.1 (0.02),while minor(major)mergersare those with stellar mass

ratiosbetweenthesecondaryandprimarygalaxyintherange0.1−0.3(>0.3).Wecorrelate
1
v thepositionsofgalaxiesinthejstars-stellarmassplaneatz =0withtheirmergerhistory,and
7 findthatgalaxiesoflowspinssuffereddrymergers,whilegalaxiesofnormal/highspinssuf-
0 feredpredominantlywetmergers,ifanyatall.Theradialjstars profilesofgalaxiesthatwent
4 throughdrymergersaredeficientby≈0.3dexatr .10r50comparedtogalaxiesthatwent
4 throughwetmergers.Bystudyinggalaxiesbeforeandaftermergers,wefindthatdrymergers
0 reducejstars by ≈ 30%, while wetmergersincreaseit by ≈ 10%, onaverage.The latter is
.
1 connected to the build-up of the central stellar over-density by newly formed stars of high
0 rotationalspeed.Movingfromminortomajormergersmostlyaccentuatestheeffectsabove.
7 Whenthespinvectorsofthegalaxiespriortothedrymergeraremisaligned,jstars decreases
1 toagreatermagnitude,whileinwetmergers,co-rotationandhighorbitalangularmomentum
v: leadto thelargestjstars increase.We makepredictionsforwhatwouldbe theobservational
i signaturesinthemeanjstarsprofilesdrivenbydrymergers:(i)shallowradialprofilesand(ii)
X profilesthatcontinuetorisebeyond≈ 10r50,bothofwhicharesignificantlydifferentfrom
r spiralgalaxies.
a
Key words: galaxies: formation- galaxies: evolution- galaxies:fundamentalparameters-
galaxies:structure
1 INTRODUCTION Fall(1983)presentedthefirstobservationalcompilationofthespe-
Galaxy mergers are a natural consequence of the hierarchical cific AM of the stellar component of galaxies, jstars, and its re-
growthofstructures(White&Rees1978)andsinceearlyonhave lation with stellar mass, Mstars. Fall (1983) found that elliptical
andspiralgalaxiesfollowparallelsequences,withtheformerhav-
been posed to be a key physical process in ther morphologi-
cal transformation (e.g. Toomre&Toomre 1972; Toomre 1977; ing jstars a factor of ≈ 6 lower than the latter. Fall (1983) con-
White 1978; Farouki&Shapiro 1982; Barnes 1988). Since then, cludedthatinhierarchicalcosmologiesthejstars valuesofspirals
and ellipticals could be understood if spirals roughly conserve j
galaxy mergers have become an essential process in cosmologi-
in their formation process (see also Moetal. 1998), while ellip-
calgalaxy formationmodels(e.g.Coleetal.2000;Springeletal.
ticals need efficient j dissipation. Galaxy mergers are a natural
2001; DeLuciaetal. 2006; Boweretal. 2006; Lagosetal. 2008;
dissipativephenomenon whichcouldaccount forthegalaxypop-
seeBaugh2006forareview).
ulation of low spins. Early simulations (e.g. Barnes&Efstathiou
In the context of the angular momentum (AM) of galaxies,
1987;Navarro&White1994;Heyletal.1996;Zavalaetal.2008)
showed that dynamical friction can efficiently move high j ma-
⋆ E-mail:[email protected] terial to the outer regions of galaxies, effectively lowering the
2 Claudiadel P.Lagos etal.
jstars ofthestellarcomponentthatiseasilymeasurable.Lateron, maticchangeinthemorphologyandspinofgalaxies,butultimately
Romanowsky&Fall(2012),viaidealisedmodelswithintheΛcold mergers are one of many physical processes at play, and contin-
dark matter (ΛCDM) paradigm, showed that galaxy mergers can uing gas accretion and star formation can reshape that remnant
naturallyexplainthepositionsofellipticalgalaxiesinthejstars− morphology andkinematics. Recently, Sparre&Springel (2016a)
Mstars plane,andthatdisksandbulgesfollowfundamentallydif- found,usingcosmologicalhydrodynamicalsimulationsthatgalaxy
ferentjstars−Mstarsrelations(seealsoObreschkow&Glazebrook remnantsof majormergersevolveintostar-formingdiskgalaxies
2014). Recently, Lagosetal. (2017) found using the EAGLE hy- unless sufficiently strong feedback is present to prevent the disk
drodynamical simulations that galaxies can have low jstars and regrowth.Thisfeedbackisanessentialmechanisminthenewgen-
lie in the sequence of elliptical galaxies without having had any erationofcosmological hydrodynamical simulations,suchas EA-
mergers. This can happen if galaxies suffer quenching before GLE (Schayeetal. 2015), Illustris (Vogelsbergeretal. 2014) and
theirhalosreachtheirmaximumexpansion(i.e.‘turnover’epoch; Horizon-AGN(Duboisetal.2016),andmostlikelyplaysamajor
Catelan&Theuns1996),andthusretainarelicspecificAMtheir roleinreproducingthemorphologicaldiversityseeningalaxies.
haloandgashadveryearlyon.However,Lagosetal.(2017)also Althoughthereisextensiveliteratureforhowdifferentmerger
showedthatthisonlyhappensto≈2%ofthegalaxies,andthatthe configurations can affect galaxies (see above), cosmological hy-
preferred scenario for having alow jstars continued to be galaxy drodynamical simulations are necessary to realistically repre-
mergers.Inthelatter,Lagosetal.(2017)showedthatgalaxiesstill sent the frequency of them in a galaxy population, and thus
needtobequenchedbyfeedback,otherwisethecontinuinggasac- it is the best way of shedding light on why galaxies display
cretion and star formation dominates over the negative effect of the diversity of jstars seen in observations, especially as mod-
mergers,andgalaxiesendupwithhighspinsatz =0. ern simulations reproduce the observations well (Zavalaetal.
Recentobservationalmeasurementsofjstarscomingfromin- 2016;Tekluetal.2015;Pedrosa&Tissera2015;Geneletal.2015;
tegral field units (IFU) galaxy surveys, such as the Sydney-AAO Lagosetal.2017;Sokolowskaetal.2016).Thisisthemotivation
Multi-objectIntegralfieldspectrograph(SAMI;Croometal.2012) of our work. We use the EAGLE (Schayeetal. 2015) cosmologi-
byCorteseetal.(2016),havesuggestedthatgalaxiesformacon- cal hydrodynamical simulations to statistically study how galaxy
tinuous sequence inthejstars −Mstars plane, insteadof thetwo mergersdrivethepositionsofgalaxiesinthejstars−Mstarsplane.
sequences originally found by Fall (1983), and thus that there We also study the main parameters determining how much spin-
may not be a fundamental distinction between bulges and disks. up or down occurs, and the cumulative effect mergers may have
Corteseetal. (2016) found that the positions of galaxies in the in the z = 0 galaxy population. EAGLE has a good compro-
jstars − Mstars plane were strongly correlated with the Hubble mise between volume, (100Mpc)3, and resolution, 700 pc, that
morphological type, Se`rsic index and the spin parameter of the allowstoushaveastatisticallysignificantsampleofgalaxies(sev-
starsλR,whichprovidesameasurement ofhowrotationallysup- eral thousands with Mstar > 109M⊙) and with enough struc-
portedagalaxyis(Emsellemetal.2007).Corteseetal.(2016)con- tural detail to be able to study mean radial jstars profiles. The
cludedthatthelarge-scalemorphologyofgalaxiesisregulatedby latter is defined as j(r) = |J(< r)|/M(< r), where J(< r)
theirmassanddynamicalstate.Thismaynot,however,bethefull and M(< r) are the angular momentum vector and mass en-
story.Emsellemetal.(2011)showedthatearly-typegalaxies,from closed in r. EAGLE has now been extensively tested against lo-
theATLAS3Dsurvey,havealargevarietyofλR valuesandthus cal and high-redshift observations of galaxy sizes (Furlongetal.
theycannotbeseenasoneuniformtypeofgalaxy.Emsellemetal. 2015a),colours(Trayfordetal.2015),stellarmassesandstarfor-
(2011)foundtwobroadclassificationsforearly-typegalaxies:fast mationrates(SFRs;Schayeetal.2015;Furlongetal.2015b),and
andslowrotators.SomeimportanttrendsfoundbyEmsellemetal. the gas contents of galaxies (Bahe´etal. 2016; Lagosetal. 2015,
(2011)andextendedrecentlytohigherstellarmassesbyVealeetal. 2016; Crainetal. 2016), among other properties, with high suc-
(2017),isthatthefractionofslow rotatorsincreasessteeplywith cess.ThegalaxypopulationinEAGLEthereforelooksrealisticand
stellarmass,andthatthevastmajorityofS0galaxiesarefastrota- thusitgivesussomeconfidencethatwecanuseittolearnabout
tors.Alltheseobservationsmeasurekinematicsofgalaxieswithin theroleofgalaxymergersinthejstars−Mstarsplane.Theadvent
a relatively small area of the galaxy (typically within 1 effective of IFU surveys, such asSAMI, MaNGA (Bundyetal. 2015) and
radius), which leavesopen the question of whether galaxies with MUSE (Baconetal. 2010), and the first global measurements of
lowspinsaretheresultofamajorlossof totaljstars or simplya jstarsathighredshift(Burkertetal.2016;Swinbanketal.submit-
rearrangementofjstars inspiteoftotalj conservation.Thesefor- ted),makeourstudyverytimely.
mation scenarios are not mutually exclusive, and thus one has to This paper is organised as follows. § 2 briefly describes the
askwhatgivesrisetosuchvarietyofobserveddynamicalstatesin detailsofthe EAGLEsimulationandintroducestheparametersof
galaxies,andparticularly,early-types. mergerswe willbe studying. Herewealso present a comparison
Jesseitetal.(2009),Boisetal.(2011)andNaabetal.(2014) withobservationalmeasurementsofmergerrates,toshowthatthe
found that the formation paths of slow and fast rotators can be frequencyofmergersiswellrepresentedinEAGLE.In§3westudy
very varied. For example, Naabetal. (2014) showed that slow thecumulative effectof galaxymergersasseen bydissectingthe
rotators could be formed as a result of wet major mergers, dry z = 0galaxypopulation.Wealsocomparethemeanradialjstars
major mergers and dry minor mergers. In the case of wet merg- profilesinEAGLEwithobservationsofearly-typegalaxies.Wethen
ers,theremnant canbe either fastor slow rotators, or even disks focusontheeffectgalaxymergershaveonthenetvalueofjstars
(e.g.Bekki1998;Springel2000;Coxetal.2006;Robertsonetal. aswellasitsradialdistributioningalaxies,splittingmergersinto
2006; Johanssonetal. 2009; Peiranietal. 2010; Lotzetal. 2010; minor/major,wet/dryandinspinandorbitalparameters.Herewe
Naabetal. 2014; Morenoetal. 2015; Sparre&Springel 2016b). alsoconnect thechangeinjstars withchanges inthestellarmass
DiMatteoetal. (2009) showed that even dry major mergers of distribution,andanalysethedistributionofthestellarcomponents
pressuresupportedgalaxiescanresultinarotation-supporteddisk ofthegalaxiespriortothemergerandintheremnant.Wepresent
if the orbital AM is large enough and efficiently transferred into adiscussionofourresultsandourmainconclusionsin§4.Finally,
the orbits of stars. Many of these mergers may result in a dra- inAppendixAweanalysetherobustnessofourresultagainstthe
Effect ofmergers onangularmomentum 3
2.1 Mergerparametersstudied
Table 1. Features ofthe Ref-L100N1504 EAGLEsimulation usedinthis
paper. The row list: (1) comoving box size, (2) number of particles, (3)
We identify mergers using the merger trees available in the EA-
initial particle masses of gas and (4) dark matter, (5) comoving gravita-
GLEdatabase(McAlpineetal.2015).Thesemergertreeswerecre-
tional softening length, and (6) maximum physical comoving Plummer-
atedusingtheD−TreesalgorithmofJiangetal.(2014).Quetal.
equivalent gravitational softeninglength.Unitsareindicated ineachrow.
EAGLEadopts(5)asthesofteninglengthatz>2.8,and(6)atz<2.8. (2017)describehowthisalgorithmwasadaptedtoworkwithEA-
GLEoutputs. Galaxiesthatwentthroughmergershavemorethan
oneprogenitor,andforourpurpose,wetrackthemostmassivepro-
genitors of merged galaxies, and compare the properties of those
Property Units Value
withthatofthemergerremnanttoanalysetheeffectonjstars.The
(1) L [cMpc] 100 trees stored in the public database of EAGLE connect 29 epochs.
(2) #particles 2×15043 The time span between snapshots can range from ≈ 0.3 Gyr to
(3) gasparticlemass [M⊙] 1.81×106 ≈1Gyr.Weusethesesnapshotstoanalysetheevolutionofjstars
(4) DMparticlemass [M⊙] 9.7×106 in galaxies under the effect of mergers. We consider the interval
(5) Softeninglength [ckpc] 2.66
betweenoutputsappropriate,asourpurposeistoanalysegalaxies
(6) max.gravitationalsoftening [pkpc] 0.7
beforeandafter,ratherthanduringthemerger.Inordertoestimate
therobustnesstothetimeintervalbetweenoutputsusedinthesim-
ulationsforourresults,inAppendixAweanalysemergersusing
timeresolutionofthemainsimulationusedhere,whileAppendixB amuch finer timeintervals (i.e.snipshots; S15) and findthat our
presentsadditionalplotsthataidtheinterpretationofourresults. calculationsarerobustanddonotsensitivelydepend onhowfine
thetimeintervalbetweenoutputsare.
Wesplitmergersintomajor and minor mergers. Theformer
arethosewithastellarmassratiobetweenthesecondary andthe
2 THEEAGLESIMULATION
primarygalaxy> 0.3,whileminormergershaveamassratiobe-
TheEAGLEsimulationsuite1 (describedindetailbySchayeetal. tween 0.1 and 0.3. Lower mass ratios areconsidered tobe unre-
2015,hereafterS15,andCrainetal.2015,hereafterC15)consists solved and thus are classified to be simply accretion (Crainetal.
ofalargenumberofcosmologicalhydrodynamicsimulationswith 2016).
different resolutions, cosmological volumes and subgrid models, Inadditiontodefiningminorandmajormergers,weestimate
adopting the PlanckCollaboration (2014) cosmological parame- thegasfractionofthemergerwiththeaimofthenclassifyingthem
ters. S15 introduced a reference model, within which the param- asgas-rich(wet)orgas-poor(dry)mergers.Thisfractionisdefined
etersofthesub-gridmodelsgoverningenergyfeedbackfromstars as:
andaccretingblackholes(BHs)werecalibratedtoensureagood
Ms +Mp
mpraetscehntt-odathyedzisk=g0al.a1xgieasla.xystellarmassfunctionandthesizesof fgas,merger ≡ Mneussttarrasl+Msnpteaurtsral, (1)
InTable1wesummarisetheparametersofthesimulationused
whereMs andMp aretheneutralgasmassesofthesec-
in this work, including the number of particles, volume, particle neutral neutral
ondaryandprimarygalaxies,respectively,whileMs andMp
masses, and spatial resolution. Throughout the text we use pkpc stars stars
arethecorrespondingstellarmasses.Bothsetofmassesaremea-
todenoteproperkiloparsecsandcMpctodenotecomovingmega-
sured within an aperture of 30 pkpc. Neutral gas masses in EA-
parsecs.AmajoraspectoftheEAGLEprojectistheuseofstate-of-
GLE are calculated asin Lagosetal. (2015). Inshort, weuse the
the-artsub-gridmodelsthatcaptureunresolved physics.Thesub-
fitting function of Rahmatietal. (2013), who studied the neutral
gridphysicsmodulesadoptedbyEAGLEare:(i)radiativecooling
gasfractionincosmological hydrodynamical simulationsbycou-
andphotoheating,(ii)starformation,(iii)stellarevolutionanden-
plingthemtoafullradiativetransfercalculationusingTRAPHIC
richment,(iv)stellarfeedback,and(v)blackholegrowthandactive
(Pawlik&Schaye2008).Thisfittingfunctionconsiderscollisional
galacticnucleus(AGN)feedback(seeS15fordetailsonhowthese
ionisation, photo-ionisation by a homogeneous ultraviolet (UV)
aremodelledandimplementedinEAGLE).Inaddition,thefraction
background and by recombination radiation. Weadopt themodel
of atomic and molecular gas in each gas particle is calculated in
ofHaardt&Madau(2001)todescribetheUVbackground. Here,
post-processingfollowingLagosetal.(2015).
neutralgasreferstoatomicplusmoleculargas.
The EAGLE simulations were performed using an exten-
sively modified version of the parallel N-body smoothed parti- Fig. 1 shows the distribution of fgas,merger in three redshift
cle hydrodynamics (SPH) code GADGET-3 (Springeletal. 2008; binsin EAGLE.Wefindthatthedistributionsaremostlybimodal,
Springel 2005). Among those modifications are updates to the andweusethisfacttodefinegas-rich(fgas,merger >0.5)andgas-
SPH technique, which are collectively referred to as ‘Anarchy’ poor (fgas,merger 6 0.2) mergers, as shown by the vertical dot-
ted lines. From now on, we name these two sub-samples as wet
(see Schalleretal. 2015 for an analysis of the impact of these
and dry mergers, respectively. At 0 6 z 6 3, these two sam-
changesonthepropertiesofsimulatedgalaxiescomparedtostan-
plesaremadeof2,677and1,775mergers,respectively,andhave
dard SPH).Weuse SUBFIND(Springeletal. 2001; Dolagetal.
2009)toidentifyself-boundoverdensitiesofparticleswithinhalos medianfgas,merger of1.1and0.02,respectively.Intheliterature,
‘dry’ mergers usually refer to galaxies completely devoid of gas
(i.e.substructures).ThesesubstructuresarethegalaxiesinEAGLE.
(e.g. Makino&Hut 1997; Naabetal. 2006a; Taranuetal. 2013).
However, the reason behind that definition was mostly technical:
1 See http://eagle.strw.leidenuniv.nl and mergers were studied with collisionless simulations. However, in
http://www.eaglesim.org/forimages,moviesanddataproducts. realitygalaxiesareexpectedtohavesomegas,evenintheregime
A database with many of the galaxy properties in EAGLE is publicly of ‘red and dead’ passive galaxies, as shown by the observations
availableanddescribedinMcAlpineetal.(2015). ofATLAS3D(Youngetal.2011;Serraetal.2012).EAGLEallows
4 Claudiadel P.Lagos etal.
us to define ‘dry’ mergers in a more realistic fashion, by simply 0.4
imposing them to be gas poor. When we dissect fgas,merger into
all mergers
thecontributionsfromtheprimary(theonewiththehigheststellar
mass)andsecondarygalaxies,wefindthatatanyredshiftthetotal dry wet z≤0.5
gasfractionisdominatedbytheprimarygalaxy.InEAGLEwefind ncy 0.3 0.5<z≤1.2
agoodcorrelationbetweenthegasfractionsoftheprimaryandthe ue 1.2<z≤2.5
q
secondarygalaxy,whichisstrongerathighredshift.Thiscorrela- e
r
tionisaconsequenceofthe‘conformity’ofthegalaxypopulation ed f 0.2
(i.e.gas-richgalaxiestendtobesurrounded bygas-richgalaxies; s
Kauffmannetal.2013;Wangetal.2015;Hearinetal.2016). mali
Throughout this paper we study j of the stars in three or
n
apertures, using all the star particles within the half-mass ra- 0.1
dius of the stars, jstars(r50) (close to what most observers have;
e.g. Emsellemetal. 2011; Corteseetal. 2016; Vealeetal. 2017),
within5timesr50,jstars(5r50),andusingallofthestarparticles 0.0
inthesubhalo,jstars(tot),aswellasthefullradialprofiles.Wecal- 0.0 0.2 0.4 0.6 0.8 1.0
culateradialj profilesasinLagosetal.(2017),whicheffectively log (1+f )
10 gas,merger
meansthatwearemeasuringamass-weightedaveragejstarswithin
r (i.e. ≡ |J(< r)|/M(< r)). We will refer to these measure- Figure1.GasfractiondistributionofmergersinEAGLEin3redshiftbins,
mentsas‘meanradialjstars profiles’.Lagosetal.(2017)showed aslabelled.Distributionsaremostlybimodal,andweusethistodefinegas-
that jstars(r50) converges in EAGLE at Mstars & 109.5M⊙, and rich(wet)andgas-poor(dry)mergersinEAGLE(shownasdottedlines).
thuswelimitoursampleonlytogalaxieswithstellarmassesabove
thatthreshold.Wecalculatetwoanglesthatdeterminehowmerg-
ersareoriented:thefirst,θspin,istheanglesubtendedbetweenthe
~jstars(tot)vectorsofthetwogalaxiesthatareabouttomerge,and
the second, θorb, is the angle between~jstars(tot) of the primary EAGLEthatperpendicularmergersare≈ 1.3timesmorecommon
galaxyandtheorbitalAMvector, than co-rotating mergers, but counter-rotating mergers are ≈ 3.4
and≈ 2.6timeslesscommonthanperpendicular andco-rotating
θspin =acos ˆjsstars(tot)·ˆjpstars(tot) , (2) mergers,respectively.Inthecaseoforbitalalignments,wedefine
h i
co-planarmergersasthosewith|cos(θ )| > 0.7,whileperpen-
and orb
dicular mergers have |cos(θ )| 6 0.3. Here we find that co-
orb
θ =acos ˆj ·ˆjp (tot) , (3) planarmergersare≈ 1.5morefrequent thanperpendicular ones.
orb orbital stars
h i Thetrendsweseeherefor θspin andθorb areconsistent withthe
where~jsstars(tot)and~jsptars(tot)arethenormalisedjstars vectors trendspresentedbyWelkeretal.(2015)usingtheHorizon-AGN
ofthesecondaryandprimarygalaxies,respectively,and~j = simulation. Welkeretal. (2015) showed that satellite galaxies on
orbital
~r×~v. Here,~r and~v are the position and velocity vectors of the adecayingorbittowardsthecentralgalaxytendtoalignwiththe
secondary galaxyintherest-frameof theprimaryone, calculated galacticplaneofthecentralinawaythat,bythetimetheymerge,
inthelastsnapshotthetwogalaxieswereidentifiedasseparateob- aremostlikelytocomeinanorbitalignedwiththegalacticplaneof
jects.Galaxygrowthproducedbygasaccretionandstarformation theprimary,whichherewouldappearasahighvalueofcos(θ ).
orb
willbetermed‘smoothaccretion’duringtherestofthepaper. Welkeretal. (2015) also found that mergers taking place in fila-
The top and middle panels of Fig. 2 show the merger rate mentsaremorelikelytobeofgalaxieswithcos(θspin) ≈ 0ifthe
density of minor and major mergers in (primary) galaxies with primarygalaxyisapassive,spheroidalgalaxy,whileco-rotationis
Mstars > 109.5M⊙ as a function of redshift, respectively, and expectediftheprimarygalaxyisaspiral,star-forminggalaxy.The
split into wet and dry mergers. The frequency of minor and ma- frequencieswereportinthebottompanelsofFig.2areconsistent
jor mergers are relatively noisy due to the small volume of the withthispicture.
simulation and the relatively high stellar mass threshold we are Fig.3comparesthemajormergerrateofEAGLEgalaxieswith
applying toour galaxysample. Thefrequency of dry mergersin- Mstars > 1011M⊙ atdifferentredshiftsagainstacompilationof
creases from z = 2.5 down to z = 1 in both minor and ma- observations.Hereweemploy3differentstellarmassratiostode-
jor mergers, with a approximately constant frequency at z < 1, finemajormergers:> 1: 5,> 1: 4and>1: 3,withtheaimof
while the frequency of wet mergers decreases steadily towards showingthesystematicvariationsproducedbythedefinitionofma-
z = 0. The latter is driven by the EAGLE galaxies having neu- jormerger(i.e.observationsadoptslightlydifferentmassratiosto
tralgasfractionsthatdecreasewithtime(Lagosetal.2015,2016). definemajormergers).TheobservationsofFig.3presentmeasure-
InthebottompanelsofFig.2weshow thefrequencyofmergers mentsofmergerratesfromthecharacterisationof pairfrequency
split by their spin orientation and orbital alignment, respectively. (Bundyetal. 2009; Blucketal. 2009; Lo´pez-Sanjuanetal. 2012;
In the case of spin alignments, we define co-rotating, perpendic- Robothametal.2014),aswellasfromtheidentificationofgalaxies
ularandcounter-rotatingmergersasthosewithcos(θspin) > 0.7 withdisturbedmorphologies(Blucketal.2012).Bothsetofmea-
(angles between 0−45 degrees), −0.15 < cos(θspin) < 0.15 surements agree remarkably well. We find that the major merger
(angles between 81− 99 degrees) and cos(θspin) < −0.7 (an- rateofmassivegalaxiesisinexcellentagreementwiththeobserva-
glesbetween135−180degrees),respectively.Randomlyoriented tions.Forourpurposethismeansthattheeffectofgalaxymergers
mergers in three dimensions would imply a uniform distribution ontheAMofgalaxiesisnotgoingtobeover(under)-represented,
incos(θspin);hencethenumber ofmergersintheseequalranges whichwouldbethecaseifEAGLEwaspredictingahigher(lower)
(0.3incos(θspin)directlyshowtheirrelativefrequency.Wefindin mergerratecomparedtoobservations.
Effect ofmergers onangularmomentum 5
10-3 10.00
M ≥1011 M
stars O •
1:3
10-4 1:4
1:5
10-5 -1Gyr1.00
minor mergers e/
wet at
1100--63 dry er r
g
er0.10
m
10-4 Lopez-Sanjuan+12
Bundy+0.9
-1yr Bluck+09
G Bluck+12
-3c 10-5 Robotham+14
Mp major mergers 0.01
c wet 0.0 0.5 1.0 1.5 2.0 2.5 3.0
sity/ 1100--63 dry redshift
n
ate de 10-4 FGiLgEuraes3a.fMunecrtgioenr roaftereidnshgifatl.axWieesswhoitwh mMesrtgaerrsra>tios1>0111M:⊙3in(soElAid-
er r line), > 1 : 4 (short-dashed line) and > 1 : 5 (long-dashed line).
g We also show the observational measurements of Bundyetal. (2009),
er
m 10-5 spin alignment Blucketal. (2009), Lo´pez-Sanjuanetal. (2012), Blucketal. (2012) and
co-rotating Robothametal.(2014).Mostoftheseobservational worksassumemajor
perpendicular mergersarethosewithstellarmassratios> 1 : 4.EAGLEpredictsmajor
1100--63 counter-rotating mergerratesofmassivegalaxies thatareinexcellent agreementwiththe
observationsintheentireredshiftrangewheremeasurementsareavailable.
10-4
10-5
orbital alignment
co-planar
10-6 perpendicular
0.0 0.5 1.0 1.5 2.0 2.5
redshift
Figure2.MergerratedensityasafunctionofredshiftinEAGLE.Thetop
panelshowsminormergersandthesubsamplesofwetanddryminormerg-
ers,aslabelled.Themiddlepanelisthesamebutformajormergers,while
the bottom panels show mergers split into spin (i.e. co-rotating, perpen-
dicularandcounter-rotating)andorbital(i.e.co-planarandperpendicular)
alignments,respectively.
3 THEEFFECTOFMERGERSONTHESTELLAR
SPECIFICAMOFGALAXIESTHROUGHOUT
COSMICTIME
In this section we analyse the effect galaxy mergers of different
nature have on the specific AM of galaxies. In § 3.1, we present
an analysis of the accumulated effect of mergers by studying the
galaxypopulationatz=0.In§3.2weanalysetheeffectofmerg-
ersbycomparingmeasurementsofgalaxypropertiesbeforeandaf-
terthemergers,andhowthesedependonthenatureofthemerger.
In§3.3weanalysetheradialrearrangementofjstarsasaresultof
mergers. Figure 5. Visualisation of the optical morphology of galaxies in the
jstars(r50)-Mstars plane atz = 0.Werandomly select galaxies in 10
bins of log10(jstars(r50)) and 8 bins of log10(Mstars) in the range
3.1 Theneteffectofgalaxymergersseenatz =0 100.5−103pkpckms−1and1010−1012M⊙,respectively,andshow
heretheirsyntheticg,r,iface-onopticalimages.Onlybinswith>3galax-
The top-left panel of Fig. 4 shows how the galaxy merger rate iesareshownhere.Theseimagesare60pkpconasidearearepublicly
changes with the position of galaxies in the jstars(r50)-Mstars availablefromtheEAGLEdatabase(McAlpineetal.2015).Thesolidand
plane.Wedefinetheaveragemergerrateofindividualgalaxiesas dottedlinesshowthemedianandthe16thto84thpercentilerange.
the number of mergers divided by the stellar-massweighted age.
Herewedonotdistinguishrecentfromfarinthepastmergers,but
6 Claudiadel P.Lagos etal.
log (average merger rate/Gyr-1) fraction of the mergers fraction of the mergers
10
-1.00 -0.83 -0.67 -0.50 -0.33 -0.17 0.00 0.15 0.29 0.43 0.58 0.72 0.86 1.00 0.15 0.29 0.43 0.58 0.72 0.86 1.00
z=0 minor mergers major mergers
3.0 all mergers
-1s)
m 2.5
k
c
p
k
p
g(j(r)/ 10stars50 12..50
o
l
1.0
0.1 0.08 -0.01 -0.08 0.01
0.5
9.0 9.5 10.0 10.5 11.0 11.5 9.0 9.5 10.0 10.5 11.0 11.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
log (M / M ) log (M / M ) log (M / M )
10 stars O • 10 stars O • 10 stars O •
fraction of the mergers fraction of the mergers
0.00 0.17 0.33 0.50 0.67 0.83 1.00 0.00 0.17 0.33 0.50 0.67 0.83 1.00
dry mergers wet mergers
3.0
-1s)
m 2.5
k
c
p
k
p
g(j(r)/ 10stars50 12..50
o
l
1.0
-0.15 0.14 -0.2 0.03
0.5
9.0 9.5 10.0 10.5 11.0 11.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
log (M / M ) log (M / M )
10 stars O • 10 stars O •
Figure4.Toppanels:ThespecificAMofthestarsmeasuredwithalltheparticleswithinthehalf-massradiusofthestellarcomponentasafunctionofstellar
massatz=0forgalaxieswithMstellar >109M⊙inEAGLE.Thelinewitherrorbarsinallofthepanelsshowthemedianandthe16thto84thpercentile
rangeofallgalaxiesatz = 0.Intheleftpanelwecolourbins(with> 5galaxies)inwhichmorethan50%ofthegalaxieshavesufferedmergers,bythe
averagemergerratepergalaxytheyhadduringtheirlifetimes.Inthemiddleandrightpanelswecoloredthosesamebinsbythefractionofthemergersthat
wereminorandmajor,respectively.Byconstruction,thefractionsofthemiddleandrightpanelsineach2-dimensionalbinaddupto1.Bottompanels:As
inthetopmiddleandrightpanels,butforthefractionofthemergersthatweredryandwet,respectively.Thearrowsineachpanelindicatethedirectionsin
whichthefrequencyoftherespectivemergertypeincreasesandthenumbernexttothearrowsshowthebestfitpower-lawindexfortherelations:average
mergerrate∝Mα,mergerfraction∝Mαandmergerfraction∝jα.
justcounttheiroccurrence.Wecolouronlythosebinsinwhichat (bestpower-law fitis∝ M0.1 ),andnoclear correlationisseen
stars
least50%ofthegalaxieshaveundergonemergersduringtheirlife- withjstars(r50)atfixedstellarmass.
times. Thisiswhy below Mstars ≈ 1010M⊙ there arevery few InthetopmiddleandrightpanelsofFig.4wecalculatethe
colouredbins,i.e.mostgalaxiesheredidnotundergomergers.At
fractionofthemergersshownintheleftpanelthatwereminorand
1010M⊙ . Mstars . 1010.5M⊙ mostlygalaxieswithlowspins major, respectively. Wealso performed power-law best fitstothe
have a significant contribution from mergers. These galaxies are
relationshipbetweenthemergerfractionandMstarsandjstars(r50)
hostedbyhalosthatareonaverage20%moremassivethanthose
toquantifythetrends.Thebestfitpower-lawindicesareshownin
ofgalaxiesofthesamestellarmassbutthatneverhadmergers.At
eachpanelofFig.4.
Mstars &1010.5M⊙thevastmajorityofgalaxieshadatleastone
mergerbyz = 0.Themergerrateincreaseswithincreasingmass Thefractionofmajorandmajormergersveryweaklyincrease
anddecrease,respectively,withincreasingstellarmass(seepower-
Effect ofmergers onangularmomentum 7
1.000
-1s) 3.0 11.5
m
k 2.5 11
kpc log 0.100
og(j/p10stars 12..50 NNN≥≥=012 1100.510stars(M/M j/jstarshalo
l 1.0 1z=0010 .5a Mll mO •<eMrgstearrss<1011 MO • 9.5 O) • 0.010

-1s) 3.0 11.5
m 0.001
k 2.5 11
kpc log 3.0
og(j/p10stars 12..50 1100.510stars(M/M -1c km s) 2.5
l 1.0 minor mergers 9.5 O) • pkp
-1m s) 3.0 11.5 og(j/10stars 12..50 NMN=≥M01W98
k 2.5 11 l MMW98
og(j/pkpc 10stars 12..50 1100.510starslog(M/M 1.00.3 0.5 1 z1=0010 .5d2 MryO •m<3Merrs4gt/arre5s5r0<s1011 M10O • 20 30 50
l 1.0 major mergers 9.5 O) •
Figure7.Toppanel:AsinFig.6forthesubsampleofcentralgalaxiesat
z = 0that have not suffered a merger (black solid line) and those that

-1s) 3.0 r/r 11.5 wentthroughatleastonedrymerger(redsolidline), andthathavestel-
m 50 larmassesintherange1010.5M⊙−1011M⊙.Hereweshowjstars in
kpc k 2.5 11 log duanritksmofatttheersppaerctiicfilcesAwMithoifnththeedvairrkiamlraattdeirush.aGloa,ljahxaielos,tchaaltcduildatendotwhiathvealal
og(j/p10stars 12..50 1100.510stars(M/M taFmotieg1≈r.0g6errb05ut0.y2t,pfwjiochrhaaitlllhloeyegihsnaualbvateshxaeijmessstpaatlmhresaeotifnhrgcaaarddeliaaiaxstriianleensgagssfehtr,ooomwonnen≈dianrvy0eth.rm1aegeaterotg.preB5rpo0agtnotteoolfm.r≈oTmph0ae.n≈4deajl:0shh.aA0el2dos
l 1.0 dry mergers 9.5 O) • linesshowthepredictionfromMoetal.(1998)forself-gravitatingdisksin
Navarroetal.(1997)DMhaloprofiles(Eq.4).Wefindthatthesubsample
-1m s) 3.0 11.5 tohfegsaulabxsiaemspwleithofnodrmyemrgeergrserfsolhloaws≈the0.t6hedoerxetliecsasljesxtpaercst,adtieovniactlionsgeflyro,mwhtihlee
k 2.5 11 idealisedcaseofspecificAMconversation.
pc lo
k g
og(j/p10stars 12..50 1100.510stars(M/M ljaswtarsin(dr5ic0e)s: aint fiFxige.d4s)t.eWllaeramlsaosssetheeafsreliqguhetlnycystroofnmgearjotrreannddwmitih-
l 1.0 wet mergers 9.5 O) • nor mergers decrease and increase, respectively with increasing
jstars(r50). Thedirections inwhich thefrequency of mergersin-
0.3 0.5 1 2 3 4 5 10 20 creaseareshownasarrowsinFig.4.
r/r50 In the bottom panels of Fig. 4 we split the mergers into dry
and wet, following the definition of Fig. 1. Here wesee stronger
Fsuigreudrew6it.hiTnoprapsanaeflu:njcsttiaornso(fsorliidnluinneitss)oafnrd50Mfsotragrsal(adxaisehseadtzlin=es)0mweitah- trendswithbothMstars and jstars(r50).Inthecaseof drymerg-
total stellar masses inthe range1010.5M⊙ −1011M⊙ thathavegone ers, wefind an increase intheir frequency withincreasing stellar
mass,andweidentifyasignificantgradientofanincreasingfrac-
throughdifferentnumbersofgalaxymergers,aslabelled.Linesshowthe
medianoftheprofilesofgalaxies,whiletheshadedregionsshowthe16th tionofdrymergerswithdecreasingjstars(r50)atfixedstellarmass
to84thpercentilerange,plottedonlyforNmergers =0,>2,forclarity. (seepower-lawindicesinthebottompanelofFig.4).Inthecase
ThescaleofjstarsandMstarsaremarkedintheleftandrightaxis,respec- ofwetmergers,wefindthattheirfractionincreaseswithdecreas-
tively. Otherpanels:Asinthetoppanel, butdistinguishing betweenmi- ingstellar massand increasing jstars(r50).Thelatter though isa
nor,major,dryandwetmergers,aslabelled.Thisfigureshowsthatgalaxy veryweaktrend.Fig.4indicatesthatthegasfractioninvolvedin
mergersgenerallyleadtoadeficitofjstarsatr.10r50,withdrymergers the merger (i.e. whether it is wet or dry) is more important than
causingpronounceddeficitsof≈0.5dex.Atsufficientlylargeradii,jstars the mass ratio (i.e. whether it is a major or a minor merger) for
convergestoavaluesetbythedarkmatterhalo.
thecurrentjstars(r50)ofgalaxies.Weexaminethesameplotsfor
8 Claudiadel P.Lagos etal.
jstars measured within5r50 andfind thesametrends (not shown verysimilarevenwhenwedonotnormalisethex-axisbyr50.By
here),andthusconcludethatthesetrendsarevalidforjstarsinside splittingmergersintominorandmajor(secondandthirdpanelsof
theinnerandouterregionsofgalaxies.Theseresultssuggestthat Fig.6)wefindthatgalaxiesthathadonemajormergercanhavea
galaxymergerscanhaveadevastatingeffectonthespecificAMof deficitinjstarssimilartothosethathadtwominormergers,andin-
galaxies,butwiththeexacteffectstronglydependingonthenature creasingthefrequencyofmajormergersdoesnotseemtohavethe
ofthemerger. cumulativeeffect that isseenfor minor mergers. EAGLEgalaxies
Lagosetal.(2017)foundthatthepositionsofgalaxiesinthe that hadmajor mergerscould havehad minor mergerstoo, while
jstars(r50)-Mstars plane are strongly correlated with a galaxy’s forthesampleofminormergers,weremoveallgalaxiesthathadat
gas fraction, stellar age, stellar concentration, optical colour and leastonemajormerger.
V/σ, all of which are usually used to distinguish early and late Wethenanalysemergerssplitintodryandwetinthebottom
type galaxies. In Fig. 5 we explicitly show how the morphology panelsofFig.6andfindthatdrymergershaveacatastrophicef-
ofgalaxieschangesinthisplane.Toconstructthisfigureweran- fectonjstars fromthecentreallthewayto≈ 20r50.Thedeficit
domly selected galaxies in 10 bins of log10(jstars(r50)) and 8 of jstars compared to galaxies without a merger is as large as
binsoflog10(Mstars)intheranges100.5−103pkpckms−1and ≈ 0.5 dex. Also note that the stellar mass cumulative profile is
1010 − 1012M⊙, respectively, and selected their synthetic g, r, muchshallowerforgalaxiesthatwentthroughadrymerger.Inthe
i images from the EAGLE database, described in McAlpineetal. caseofwetmergersweseetheexactopposite.Verylittledifference
(2015). These images are face-on views of the selected galaxies isfound between galaxiesthat did not suffer amerger, and those
and are 60 pkpc on a side (see McAlpineetal. 2015 for a de- that suffered one, two or more wet mergers. This reinforces the
scriptionofhow theseimageswereproduced). Thisfigureshows conclusionthattojstars ofagalaxy,whatmattersmostiswhether
thatatfixedMstars,galaxiesgofrombeingredspheroidalsatlow themergerisdryorwet.WewillshowinFig.10thatthisisalso
jstars(r50)tobeinggrand-designspiralsathighjstars(r50)inthe true when we study jstars before and after the merger. Note that
stellarmassrange1010M⊙ .Mstars .1011M⊙.Athigherstel- inthecaseofdrymergers,westillseethatthemeanradialjstars
larmasses,galaxieswithhighjstars(r50)appearmorelikedefunct profileconvergesatsufficientlylargeradiitoajstars(tot)thatdoes
spirals,withlittlestarformationandagingdisks.Ifwefollowthe notstronglydependonthemerginghistoryofgalaxies.
median jstars(r50), one sees that galaxies go from being prefer-
entiallyspiral/disk-dominatedatMstars ≈ 1010M⊙ tospheroids 3.1.1 Thegalaxy/halospecificAMconnection
atMstars & 1011.5M⊙.Giventhestrongcorrelationbetweenthe
positionsof galaxiesinthejstars(r50)-Mstars planewiththefre- Whenwecompare jstars of thegalaxies withthe specificAMof
quencyofwet/drymergers,andwiththemorphologiesofgalaxies, their dark matter halos in the top panel of Fig. 7 we find that
onewouldexpectmorphologiestobeconnectedtowet/drymerg- galaxies that went through at least one dry merger on average
ers.Rodriguez-Gomezetal.(2016)showedthatthemorphologies have a jstars(tot) that is 5 times smaller than that of their halo,
of galaxies are connected with the merger history in the Illustris while galaxies that did not go through a merger typically retain
simulation, but they could only determine a clear correlation in ≈40−50%oftheirhaloj.Thislatterresultagreesverywellwith
massivegalaxies,Mstar > 1011M⊙,duetopredominanceofdry the prediction by Stevensetal. (2016a) for spiral galaxies. With
mergersandex-situformationofthestars.Rodriguez-Gomezetal. asemi-analyticmodel,thoseauthorsevolvedtheone-dimensional
(2016)didnotdissectgalaxymergersinmanyparameters,andthus structureofdiscsinaseriesofannulioffixedspecificAM.They
they were unable to find finer correlations between mergers and assumedthatwhengascooledoraccretedontoagalaxythatitcar-
morphology.OurresultsinEAGLEsuggestthatthemorphologyof ried the same total j of the halo at that time in both magnitude
galaxies, aswellastheirjstars,sensitivelydepend onthetypeof anddirection,andthatiswasdistributedexponentially.Starswere
the merger (dry/wet minor/major merger, and on the spin align- formedinannuli thatwereToomreunstableor hadsufficientH .
2
mentandorbitalparameters).Wewillexplorethisconnectionand Atz = 0,theyfoundspiralgalaxies(whichhadnotsuffereddry
theeffectonjstars(r50)in§3.2. mergers) had jstars/jhalo = 0.4 ±0.29, independent of galaxy
Wenowexaminethemeanradialjstarsprofilesofgalaxiesat mass(seetheirFig.13).Despitethecompletelydifferentmethod-
z = 0in Fig.6in anarrow range of stellar mass, 1010.5M⊙ . ology,thisalignsalmostperfectlywiththeresultofEAGLEgalaxies
Mstars . 1011M⊙.InthesameFigurewealsoshowthecumula- thathavenotparticipatedinadrymerger.
tiveradialprofileofMstars.Inthetoppanelweshowhowdifferent Fall (1983) and Romanowsky&Fall (2012) suggested that
theradialjstarsprofileisingalaxiesthatsuffereddifferentnumbers spiral galaxies needed to have ajstars close to that of their halo,
ofmergers,withoutyetdistinguishingthenatureofthemerger.In- while ellipticals had to lose 90% of their j, postulating a funda-
creasingthefrequencyofmergershastheeffectofflatteningthera- mental difference between the two galaxy populations. The con-
dialjstarsprofile,inawaythatgalaxiesthatwentthroughahigher clusionsreachedbytheseauthorswerebiasedbytheavailableob-
numberofmergershaveadeficitofjstars at0.5r50 .r .10r50 servations, that in the best case went out to ≈ 10r50. According
aslargeas≈ 0.3dexcomparedtotheircounterpartsofthesame to EAGLE,early-type galaxies only reach ≈ 0.1 of the expected
massbutthatdidnotgothroughmergers.Atsufficientlylargeradii, haloj atr ≈ 10r50,onaverage.EAGLEgalaxiesshowthatjstars
jstars convergessothatgalaxieswithdifferentnumberofmergers continuestoincreaseouttomuchlargerradiiduetotheeffectof
have a similar jstars(tot). This suggests that the most important adding halo stars. EAGLE predicts that this difference shrinks at
effectofmergersisintheradialstructureofjstars ratherthanthe largerradii,although stillnot converging toafractionof jhalo as
totaljstars.Thestellarmasscumulativeprofileofgalaxiesisalso highasgalaxieswithnomergersintheirlifetime.Earlysimulations
shallowerwhenthefrequencyofmergersincreases,whichmeans ofmergerspredictedthatdynamicalfrictioncouldredistributeAM
thatalargerfractionofthestellarmassislockedupintheoutskirts fromtheinnertotheouterregions(e.g.Barnes&Efstathiou1987;
of galaxies. Although there is a small tendency for galaxies that Navarro&White1994;Heyletal.1996).Fromthosesimulations
went through more mergers to have a larger r50, the trends here onewouldexpectanetweakconservationofj.Ourfindingswith
arenotaffectedbythis,asthedifferencesintheradialprofilesare EAGLEshowasignificantdisparitybetweenthestellarandthehalo
Effect ofmergers onangularmomentum 9
3.5 3.5
3.0 3.0
2.5 2.5
2.0 Romanowsky & Fall (2012) 2.0 ATLAS3D
1010.5 M <M <1011.5 M 1010.5 M <M <1011.5 M
1.5 O • stars O • 1.5 O • stars O •
) 2.6<log (j (r )/pkpc km s-1)<2.8 ) 2.6<log (j (r )/pkpc km s-1)<2.8
-1s 1.0 10 stars 50 -1s 1.0 10 stars 50
m 3.5 m 3.5

k 3.0 k 3.0
c c
p 2.5 p 2.5
k k
p p
(j/0stars 12..50 2.3<log (j (r )/pkpc km s-1)<2.6 (j/0stars 12..50 2.3<log (j (r )/ApTkLpAc Sk3mD s-1)<2.6
g1 1.0 10 stars 50 g1 1.0 10 stars 50
o 3.5 o 3.5
l l

3.0 3.0
2.5 2.5
2.0 2.0
ATLAS3D
1.5 1.5
2<log (j (r )/pkpc km s-1)<2.3 2<log (j (r )/pkpc km s-1)<2.3
1.0 10 stars 50 1.0 10 stars 50
0.3 0.5 1 2 3 4 5 10 20 30 50 0.3 0.4 0.5 1 2 3 4 5
r/r r/r
50 50
Figure8.MeanradialjstarsprofilesforgalaxiesinEAGLEatz = 0and Figure9.MeanradialjstarsprofileforthesameEAGLEgalaxiesofFig.8.
with stellar masses in the range 1010.5M⊙ −1011.5M⊙ in 3 bins of Forthesimulatedgalaxies,weshowthemedianandthe16thand84thper-
jstars(r50),aslabelledineachpanel.Indottedlinesweshowallgalaxies centileranges.IndividualdottedlinesshowATLAS3Dgalaxies thathave
in that range, while the thick and thin solid lines show the median, and stellarmassesintherange1010.5M⊙−1011.5M⊙andhavejstars(r50)
the16thand84thpercentileranges,respectively.Weshowobservationsof intherangesshownineachpanel.TheagreementbetweenEAGLEandthe
early-typegalaxiesfromRomanowsky&Fall(2012)asdashedlines.Their observationsisexcellent.
sampledgalaxieshavestellarmassesintherangeweselectedhere,andwe
showeachgalaxyintheircorrespondingbinofjstars(r50).Hereweonly
showthemedianmeasurement,buterrorbarsaroundthosemeasurements with weak conservation of AM (to within ≈ 50%; see their Fig.
canbeaslargeas≈0.5dex,particularlyatr&3r50. 15).However, thesubsampleofgalaxiesthathadatleastonedry
mergersignificantlydeviatesfromthetheoreticalprediction,with
a jstars that is on average lower by ≈ 0.6 dex and displaying a
j, but that is not as large as suggested by some of the idealised steeperincreasewithradiithanthatexpectedfromEq.4.Thisisa
models(Romanowsky&Fall2012). consequenceofsignificantlossesofAMduethedissipativenature
We compare the mean radial jstars profiles to the theoreti- ofmergers(e.g.Romanowsky&Fall2012).Wecomebacktothis
cal expectation of Moetal. (1998) for a self-gravitating disk in in§4.
aNavarroetal. (1997) dark matter profileinthebottom panel of
Fig.7.Inthismodel,thehaloisassumedtorespondadiabatically
3.1.2 Comparisonwithobservationsofearly-typegalaxies
to the assembly of the disk, and thus the AM of individual dark
matter particlesisconserved. No bulgecomponent isincluded in We compare EAGLE galaxies with low jstars with the observa-
thismodel.Moetal.(1998)describedthemeanradialAMprofile tionsofRomanowsky&Fall(2012)inFig.8.Romanowsky&Fall
ofdisksas (2012) presented mean radial jstars profiles for 7 ellipticals and
r S0 galaxies in the stellar mass range of 1010.5M⊙ . Mstars .
Jd(<r)=Z Vc(R)RΣ(R)2πRdR, (4) 1011.5M⊙. We took these 7 galaxies and separated them into 3
0 bins of log10(jstars(r50)/pkpckms−1), 2−2.3, 2.4−2.6 and
where Vc(R) = GM/R, with M = MDM(R)+Mstars(R) 2.6−2.8, which in EAGLEwould correspond to galaxies below,
beingthestellarpplusdarkmattermassenclosedwithinR,andΣ closetoandabovethemedianjstars(r50)atthatstellarmass,and
the stellar surface density at R. Here we are implicitly neglect- compare them with EAGLE in Fig. 8. Note that in EAGLE most
ingtheeffectofthegasinthepotentialwellofthegalaxy,which galaxies of such stellar mass are expected to be early-type (see
makessenseasweareonlyanalysingrelativelymassivegalaxies, Fig.5).Wefindthatatlowjstars(bottompanelofFig.8)thescatter
1010.5M⊙ .Mstars .1011M⊙,whichhavelowgasfractionsin inthemeanradial profilesbecomesincreasingly largercompared
EAGLE(Lagosetal.2015;Crainetal.2016). togalaxiesofhigherjstars,andgalaxieswithflatmeanjstars pro-
Wefind that the subsample of galaxies that never suffered a filesbecomemorecommon.EAGLEdescribesthevarietyofmean
mergerfollowsthetheoreticalexpectationofMoetal.(1998)rel- radialjstars profilesobservedbyRomanowsky&Fall(2012)very
ativelyclosely,withaslightlysteeperslopeatr .2r50andshal- well,eventhecaseswherejstarsceasestoincreaseatr&3r50.
lower at r & 4r50, reflecting the fact that these galaxies are not WiththeaimoftestingEAGLEwithalargernumberofgalax-
puredisksandthattheydonotfollowpreciselytheassumptionof ies, we extracted mean radial jstars profiles for every ATLAS3D
exponential thin disks (e.g. Stevensetal. 2016b). This is consis- galaxy(Cappellarietal.2011),followingtheproceduredescribed
tentwiththeconclusionofLagosetal.(2017)thatEAGLEgalaxies inLagosetal.(2017).Theseprofilessampleupto≈ 2r50 inthe
on average follow the expectation of isothermal collapsing halos bestcases,buttheyinformusoftheshapeoftheradialjstars pro-
10 Claudiadel P.Lagoset al.
file in the inner regions of galaxies. Fig. 9 shows the compari- shotscover different timescales), and alsodue togalaxieshaving
son between EAGLE and ATLAS3D in the same stellar mass and verydifferentsizesatdifferentcosmicepochs.InAppendixBwe
jstars(r50)rangesofFig.8.Fromtoptobottom,eachpanelshows showthatthebiasintroducedbystudyingmergersatdifferentcos-
8,10and15ATLAS3Dgalaxies,respectively.Theagreementwith micepochsandtakingplaceingalaxiesofdifferentstellarmasses
theobservationsisexcellent.ATLAS3Ddisplaysatrendofthescat- isminimal,andthatthedifferenceseenherebetweenminor/major,
ter intheradial jstars profilesincreasing when going to low spin wet/drymergersisofthesamemagnitudeinsubsamplesatdiffer-
galaxies of a very similar magnitude to the one seen in EAGLE. entredshiftsandstellarmasses.Fromhereon,weanalysegalaxy
This gives us confidence that the simulation not only reproduces mergerstogether,regardlessthecosmicepochandthestellarmass
theaverage j-mass relation, as shown by Lagosetal.(2017),but ofthegalaxyinwhichtheyoccur,unlessotherwisestated.
alsotheradialprofilesofgalaxies,whereobservationsexists.The Given the importance of wet/dry mergers over minor/major
number of galaxiesintheUniverseinwhichthiscomparison can mergers, weexplore the effect of spinalignments and orbital pa-
bedoneisstillverysparse,buttheadventofIFUinstruments(e.g. rametersinthesubsamplesofdryandwetmergersintherightpart
SAMI,MaNGA,MUSE)islikelytochangethis. ofFig.10.Wefirstmeasuretheeffectofco-rotating(cos(θspin)>
0.7), perpendicular (−0.15 < cos(θspin) < 0.15) and counter-
rotating (cos(θspin) < −0.7) mergers (middle symbols and bars
3.2 jstarsbeforeandaftergalaxymergers in Fig. 10). We find that wet mergers between co-rotating galax-
We analyse how much jstars(r50) changes between two consec- ies lead to a larger and more frequent increase of jstars(r50),
utive snapshots for galaxies with Mstars > 109.5M⊙ and in the while perpendicular wet mergers tend to produce little changes
redshift range 0 6 z 6 2.5, separating galaxies into those that injstars(r50).64%oftheco-rotatingdryminormergersincrease
hadanddidnothaveamerger,andsplittingmergersintodifferent jstars(r50),afrequencythatisevenhigherthansmoothaccretion.
types:minor/major,wet/dryandwithdifferentspinalignmentsand Theeffectofcounter-rotatingmergersisinbetweentheco-rotating
orbital parameters. Thisisshown inthetoppanel of Fig.10(the and perpendicular mergers. Perpendicular mergers are the most
equivalentforjstars(5r50)isshowninFig.B1).Thebottompanel common configuration in EAGLE(seeFig.2) andthat iswhythe
ofFig.10showsthefractionthatdisplayanincreaseinjstars(r50) bars for wet minor and major mergers are weighted more heav-
forthesamecasesanalysedinthetoppanel.Thefirst3datapoints ilytowards the results of perpendicular rather than toco-rotating
compare the change in jstars(r50) due tosmooth accretion and a mergers.Fordrymergerswefindthesametrend:co-rotatingmerg-
minor or major merger. In the latter case, star formation and gas erstendtobelessdamagingthanperpendicularorcounter-rotating
accretionmaycontinue,andthus,wecannotfullyseparatetheef- mergersforjstars(r50).
fectofthemerger.Galaxiesthatdidnotsuffermergersonaverage In the rightmost part of Fig. 10 we analyse the effect of the
increasetheirjstars(r50)by≈15%inbetweensnapshots,andare orbitalparameters.Particularly,weanalyseco-planarandperpen-
likelytoundergoanimportantincreaseinjstars(r50)(i.e.≈ 35% dicularmergers,thesubsamplewithhighjorbital (i.e.higherthan
ofthegalaxiesatleastdoubletheirjstars(r50)inasnapshot).On the median), and with high and low jorbital/jstars(5r50) (above
thecontrary, galaxy mergersare morelikely tonot change or re- andbelow themedian, respectively). §2.1presents thedefinition
ducetheirjstars(r50),dependingonwhethertheyareminororma- ofjorbital,andherewecomparejorbital tojstars(5r50)ofthepri-
jor mergers, respectively. From the bottom panel of Fig. 10, one mary galaxy prior tothe merger. jstars(5r50) isa good measure-
seesthatsmoothaccretionincreasesjstars(r50)≈60%ofthetime, ment of the galaxy’s total jstars (see Fig. 6). We do not find a
whileminorandmajormergersonlydothisin≈54%and≈43% strong effect of the orientation of themergers on jstars(5r50), as
ofthecases,respectively. both thedistributions of theco-planar and perpendicular mergers
In§3weconcludedthatdryandwetmergershaveverydif- are statisticallyindistinguishable (the KS p-value is 0.56). When
ferentneteffectsonjstars,soinFig.10wesplitmergersintosev- comparingmergersofhighandlowjorbital,wefindasignificant
eralsubsamplestopindownthecircumstancesinwhichjstars de- difference (with a KS p-value of 10−5) in which mergers with
creases/increasesthemost.Wefirsttakealloftheminorandmajor highjorbital preferentially result inanincrease injstars(5r50) of
mergers and split them into dry and wet (shown from the 4th to ≈ 15%. The largest systematic is found when we separate wet
the7thsymbolsandbarsinFig.10).Wefindthatwetminormerg- mergers by their jorbital/jstars(5r50) (the p-value comparing the
ersproduce asimilarincrease of jstars(r50) tosmooth accretion, twosubsamplesofhigh/low jorbital/jstars(5r50)is10−24).High
with a smaller percentage of galaxies going through a major in- values of jorbital/jstars(5r50) efficiently spin-up the galaxy, in-
creaseinjstars(r50)(≈20%ofwetminormergersproduceanin- creasing jstars(5r50) by ≈ 22%, on average, and in 60% of the
creaseofmorethanafactorof2).Dryminormergers,ontheother cases.Thissuggeststhatgalaxiesspin-upbecausepartoftheorbital
hand, display a strong preference for decreasing jstars(r50). For AMistransferredtotheremnantgalaxy.Westudythesubsampleof
majormergersthetrendsaresimilarbutwithalargerdifferencebe- wet,co-rotating(cos(θspin) > 0.7)andhighjorbital/jstars(5r50)
tweendryandwetmergers.Drymajormergersreducejstars(r50) mergers,andfindthattheyincreasejstars(5r50)in≈ 70%ofthe
in≈ 75% of the cases, whichshows that thisisone of themost cases,by≈ 44%onaverage, andthusthisformof mergeristhe
catastrophicformsofmergersfortheAMbudgetofgalaxies.Note most efficientat spinning-up galaxies. Inthecase of drymergers
thatinEAGLEthegasfractionofthemergerismoreimportantthan wedonotfindastrongdependenceonanyoftheorbitalparame-
themassratio.WecalculatetheKolmogorov-Smirnovp-valuebe- tersstudiedhere.
tweendryandwetmergersforthecasesofminorandmajormerg- When studying jstars(5r50) (Fig. B2) we find very simi-
ersandfindthatthereisnegligibleprobability,<10−15,thatthey lar results. The only major difference is that dry mergers show
aredrawnfromthesamepopulation. a stronger dependence on the orbital parameters, with high
So far we have stacked all of the galaxy mergers that take jorbital/jstars(5r50) and co-planar mergers leading to a higher
place in galaxies with Mstars > 109.5M⊙ and in the redshift fraction of galaxies displaying and increase in jstars(5r50). One
range0 6 z 6 2.5. Thismayintroducesignificantbiasesdueto could conclude that the AM in the inner parts of galaxies during
thetimeintervalbetweenoutputsofthesimulation(differentsnap- drymergersisnotgreatlyaffectedbytheorbitalparametersofthe