Galaxy Pairs in the Local Group

0.27 MB·English

by  Azadeh Fattahi

Mon.Not.R.Astron.Soc.000,1–6(2013) Printed1February2013 (MNLATEXstylefilev2.2)
Galaxy Pairs in the Local Group
Azadeh Fattahi1⋆, Julio F. Navarro1, Else Starkenburg1, Christopher R. Barber1,
3 and Alan W. McConnachie2
1
0 1Department of Physics and Astronomy, University of Victoria, PO Box 3055 STN CSC, Victoria, BC, V8W 3P6, Canada
2 2NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada

n
a
J

0 1February2013
3

] ABSTRACT
O
Current models of galaxy formation predict that galaxy pairs of comparable mag-
C nitudes should become increasingly rare with decreasing luminosity. This seems at
. odds with the relatively high frequency of pairingsamong dwarf galaxiesin the Local
h
Group. We use literature data to show that ∼ 30% of all satellites of the Milky Way
p
- and Andromeda galaxies brighter than MV =−8 are found in likely physical pairs of
o comparable luminosity. Besides the previously recognised pairings of the Magellanic
r Clouds and of NGC 147/NGC 185,other candidate pairs include the Ursa Minor and
t
s Dracodwarfspheroidals,aswellastheAndI/AndIIIsatellitesofM31.Thesepairsare
a
much closer than expected by chance if the radial and angular distributions of satel-
[
lites were uncorrelated;in addition, they have very similar line-of-sight velocities and

2 luminosities that differ by less than three magnitudes. In contrast, the same criteria
v pairfewerthan4%ofsatellitesinN-body/semi-analyticmodelsthatmatchtheradial
1 distribution and luminosity function of LocalGroup satellites. If confirmed in studies
6
oflargersamples,the highfrequencyofdwarfgalaxypairingsmayprovideinteresting
1
clues to the formation of faint galaxies in the current cosmologicalparadigm.
3
1. Key words: galaxies:dwarf-galaxies:formation-galaxies:evolution-LocalGroup
1
2
1
:
v 1 INTRODUCTION habithalosofsimilarvirialmass.Inaddition,extrapolating
i such models to the faintest galaxies known indicate that
X The shapes of the galaxy and dark halo mass functions
few, if any, galaxies more massive than a few million solar
r differ substantially in the ΛCDM paradigm (see, e.g.,
massesareexpectedtoforminhaloswithvirialmassbelow
a Benson et al. 2003). This is usually interpreted to imply
1010M⊙.
that the “efficiency” of galaxy formation, as measured by
Recent work has highlighted potential disagreements
the ratio between the stellar mass of a galaxy (M ) and
gal
thevirial1 massofitshosthalo(M200),variesstronglywith between these model predictions and observations, includ-
ing the lack of a characteristic velocity at the faint-end of
virialmass.Inparticular,Mgal/M200shoulddecreasesteeply
blind HI surveys (Zwaan et al. 2010); and the low virial
toward low halo masses in order to match the shallow faint
endofthegalaxyluminosityfunction(see,e.g.,Moster et al. mass(substantiallybelow1010M⊙)inferredfromdynamical
dataforthedwarfspheroidalcompanionsoftheMilkyWay
2010;Behroozi et al. 2010;Guo et al. 2011).
(Boylan-Kolchin et al.2012)andfornearbydwarfirregulars
On the scale of dwarf galaxies, which we define con-
ventionally here as those with Mgal < 109.5M⊙, simple (Ferrero et al. 2012). It could be argued, however, that the
evidenceforsubstantialdisagreementisunconvincing,given
abundance-matchingmodelssuggestadependencenearlyas
steepasM ∝M3 inthedwarfgalaxyregime(Guo et al. that theinferences are eitherindirect (inthecase of theHI
gal 200
velocityfunction)orbasedonsmallandheterogeneoussam-
2010). Such steep scaling would imply that dwarfs span-
ples(inthecaseofthenearbydwarfs;seeWang et al.2012;
ning several decades in stellar mass should nevertheless in-
Vera-Ciro et al. 2012).
It is therefore important to consider further tests of
⋆ Email:[email protected] themodelpredictions.WeexploreherehowthesteepMgal-
1 Wedefineallvirialquantitiesasthosecorrespondingtoasphere M200 relation predicted for dwarfs affects the frequency of
of mean density equal to 200 times the critical density for clo- galaxy pairs of comparable luminosity. Such pairs, when
sure. Furthermore, weshall hereafter refer to the stellar mass of closeenoughtoinhabitthesamedarkmatterhalo(referred
agalaxyas“galaxymass”,forbrevity. tohereafter as “physical pairs”), are expected to berare at
2 Fattahi et al.
allluminosities,butespeciallysointhescaleofdwarfs.This 2 DATASETS
isbecausethefaintercompanionsinphysicalpairstracethe
We use the recent compilation by McConnachie (2012) as
halo substructure, and subhalos are, by and large, far less
the source of the positions, distances, line-of-sight veloci-
massivethanthemainhalo:themostmassivesubhalotypi-
ties, and magnitudes of Local Group dwarfs that we use in
callyhasamassonlyonehundredththatofthemainsystem
ouranalysis. Allvelocities are heliocentric and corrected to
(see, e.g., Springel et al. 2008; Wang et al. 2012).
therest frameoftheGalaxy.Inordertopreventbiases due
Pairsofcomparableluminosityarethereforemorelikely to incompleteness, we consider only satellites brighter than
to form in fairly massive halos, where galaxy formation ef- MV =−8 located within 300 kpc of the Milky Way or An-
ficiency decreases with increasing halo mass, partly com- dromeda (M31) galaxies. The sample consists of 29 dwarfs,
pensating the mass difference between the main halo and 17 of which orbit around M31; the rest are satellites of the
its most massive subhalo. On dwarf galaxy scales the situ- Milky Way.
ation is reversed, and the precipitous decline in galaxy for- For comparison, we have identified analogous samples
mationefficiencywithdecreasinghalomassshouldcurbthe of simulated satellites in the six ∼ 1012M⊙ halos of the
formation of physical pairs of comparable luminosity. More Aquarius Project (Springelet al. 2008) using the model of
generally speaking, isolated associations of dwarf galaxies Starkenburget al. (2012). This is a semi-analytic model
should be rare. They are known to exist (e.g., Tully et al. grafted ontothelevel-2Aquariusruns,which simulateeach
2006; Soares 2007), but their cosmological abundance and halo with several hundredmillion particles, thusensuring a
dependenceon luminosity havenot yet been adequatelyes- resolution high enough to track the formation of all halos
tablished (see Sales et al. 2012, for a recent attempt). and subhalos that might plausibly host the dwarf galaxies
brighter than MV =−8 we use in our analysis. The model
The Local Group offers an interesting environment to
satellites of each Aquarius halo have luminosity and radial
testtheseideas.Advantagesincludethefactthat,awayfrom
distributionsthatarebroadlyconsistentwiththeMilkyWay
the “zone of avoidance” caused by Galactic dust, the cen-
and M31 and therefore provide a useful testbed of the sig-
sus of Milky Way (MW) satellites brighter than MV ∼ −8
nificance of our results for ΛCDM dwarf galaxy formation
is complete (see, e.g., Whitinget al. 2007), and that accu-
models.
rate magnitudes, positions, distances, and line-of-sight ve-
There are a total of 175 simulated satellites brighter
locities are known for all. Many fainter systems in the Lo- than MV = −8 within 300 kpc of the primary galaxies of
cal Group still remain undiscovered, as demonstrated by
all six Aquarius halos (on average 29 per halo). The model
recent discoveries both by the Sloan Digital Sky Survey
provides not only the full 3D position and velocity infor-
in the Milky Way (Koposov et al. 2008), and by the Pan-
mation for all of them, but also allows us to track their
Andromeda Archaeological Survey around M31 (PAndAS;
evolution. Our simulated sample does not include satellites
McConnachie et al. 2009). In the latter the census is likely
whosedarkmatterhaloshavebeenfullydisruptedbytides,
complete within the PAndAS survey area down to a mag-
since their fate is uncertain. We refer the interested reader
nitude limit of MV ∼ −6.5, although some brighter dwarfs
toStarkenburget al.(2012)fordetailsonthesemi-analytic
at larger radius likely have yet to be identified (as exem-
model.
plifiedbythediscoveriesofAndromedaXXVIIIandXXIX;
Bell et al. 2011; Slater et al. 2011).
Further,we know that at least some of thesatellites of 3 ANALYSIS AND RESULTS
the Milky Way and M31 are very likely physically associ-
It has long been noticed that the spatial distribution of
ated and bound to each other. An obvious pairing is that
Milky Way satellites is highly anisotropic (Lynden-Bell
of the Magellanic Clouds (see, e.g., Kallivayalil et al. 2006,
1976),and isoften described as apolarplanewhose signifi-
and references therein). Around M31, there have been sug-
cance has been the matter of much recent debate(see, e.g.,
gestionsthatNGC147andNGC185alsoformaboundpair
Kroupaet al. 2005; Zentneret al. 2005; Libeskind et al.
(van den Bergh 1998). If these pairs are bound,their vicin-
2005; Metz et al. 2007). Around M31, 14 out of 17 satel-
ity to their primary galaxy suggests that we are observing
lites in our sample are in the hemisphere nearer the Milky
them just before they are separated by the tidal field of
Way(McConnachie & Irwin2006).Further,severalhavere-
the main galaxy (Besla et al. 2007; Sales et al. 2011). This
cently been shown to delineate a flattened structure that
impliesveryrecentaccretionandindicatesthattheiroccur-
in total comprises at least half of all known M31 satellites
rence should not beuncommon amongst isolated systems.
(Ibataet al. 2013). These are unlikely configurations for a
Taken at face value, the existence of these two pairs of virialized population and hint strongly at recent accretion.
dwarfsseemsatoddswiththeexpectedrarityofsuchassoci- Our pairing procedure begins by identifying satellites
ations. Weusethisasmotivation tosearch,usingliterature whose nearest neighbour is unusually close when compared
data,forotherdwarfgalaxypairsintheLocalGroup.Wede- with the probability distribution of nearest-neighbour dis-
scribeinSec.2theobservationaldataset andthesimulated tances, dnn, obtained by Monte Carlo sampling a random
satellite dataset we use for comparison. In Sec. 3 we intro- isotropicpopulationofsatelliteswiththesametotalnumber
duce the pairing procedure we have adopted and compare andradialdistribution.WeillustratethisinFig.1,wherewe
the results with those obtained when the same procedure show the dnn distribution expected for two satellites of the
isapplied toahybridN-body/semi-analyticmodelof satel- Milky Way (Draco and Sagittarius), and two of M31 (And
lite galaxy formation applied to N-body simulations from III and And IX).
theAquariusProject.Weconcludewithabriefsummaryin The bottom left panel of Fig. 1 shows that the near-
Sec. 4. estsatellitetoSagittarius(theLMC,52kpcaway)isabout
Galaxy Pairs in the Local Group 3
Figure2.Distributionoftheprobability,P,ofhavinganearest
neighbourascloseasorcloserthanobservedifthesatelliteswere
isotropicallydistributed around each primaryand had the same
radial distribution as that of the Milky Way (solid thick line)
Figure 1. Distribution of nearest-neighbour distances, dnn, to andM31(soliddashedline).Only20%ofsatellitesareexpected
twoMilkyWaysatellites(panelsontheleft)andtwoM31satel- to have P < 0.2, with a very weak dependence on the number
lites (panels on the right) expected if satellites were distributed of satellites and the shape of the radial profile. The probability
isotropicallyabouteachprimarywitharadialdistributionconsis- distribution obtained for a semi-analytic model applied to the
tent withthe observedone. Theprobability,P,that asatellite’s six Galaxy-sized halos of the Aquarius Project is shown by the
nearest neighbour lies, by chance, as close as or closer than ob- dashed blue histogram; 48 out of 175 satellites have P <0.2, or
servedishighlightedbytheshadedregionofeachhistogramand 27%ofthetotal.ThecorrespondingdistributionforLocalGroup
quoted in each panel’s legend. A downward arrow indicates the satellites is shown by the solid red histogram (the contribution
distance to the primary galaxy. The top panels illustrate cases of Milky Way satellites is highlighted by the shaded area of the
where the probability is rather small, indicative of a potential histogram).IntheLocalGroup,morethan40%ofsatelliteshave
physicalassociation.Thebottompanels,ontheotherhand,illus- P <0.2,aresultexpectedtohappenbychanceinfewerthanone
tratetwocaseswherethenearestneighboursarenotsignificantly in100randomrealizations.Theangularandradialdistributions
closerthanexpected atrandom. ofsatellitesthusseemhighlycorrelatedandsuggestthepresence
ofphysically-associatedpairs.
twiceasfarasthedistanceatwhichtheprobabilitydistribu-
tion of nearest-neighbour distances peaks. If the radial and change almost imperceptibly if we confine the Monte Carlo
angular distribution of Milky Way satellites were uncorre- samplestoathree-dimensionalstructureasflatasobserved
lated then Sagittarius would be expected to have a nearest for the Milky Way, i.e., roughly 3:1 in its major-to-minor
neighbour as close or closer than observed in 99 out of 100 axis ratio.
randomrealizations(P =0.99).Sagittariusisthusrelatively On the other hand, these distributions differ markedly
isolated and unlikely to be a memberof a physical pair. from the results for the Milky Way (shaded histogram in
Thesituation reversesforDraco:itsnearestneighbour, Fig. 2) or the combined M31+MW satellites (labelled “Lo-
UrsaMinor,liesonly23kpcaway.Thisismuchcloserthan calGroup”inFig.2).AK-Stestyieldsaprobabilityofless
expectedatrandom;anearestneighbourthatcloseoccursin than 0.3% that the Local Group P distribution is statisti-
fewerthan4outof100randomtrials(P =0.04).Theright- callyconsistentwiththatoftherandomsamples.Thereisa
hand panels of Fig. 1 show as well two analogous examples clearexcessofsmaller-than-expectednearest-neighbourdis-
for the M31 satellite population. In this case, And IX is tances in the Local Group that is difficult to account just
unlikely to bea memberof a pair (P =0.90), whereas And bychance.Forexample,45% ofLocal Groupsatellites have
III is unusually close to And I (P = 0.10), hinting at a P <0.2comparedwiththe20%expectedifthedistribution
possible physical association. was isotropic.
The distribution of the probability, P, that dnn is as Our pairing procedure therefore retains all P < 0.2
smallorsmallerthanobservediftheradialandangulardis- pairs (listed horizontally in the labels of Fig. 2) for further
tributionofMilkyWaysatelliteswereuncorrelatedisshown scrutiny. A true physical pair must also differ little in ve-
by the solid and dotted lines in Fig. 2 for the Milky Way locity, so we impose a maximum difference of 75 km/s in
andM31satellites,respectively.Bothcurvesarerathersim- the line-of-sight velocity difference of the likely members.
ilar, indicating that the P distribution is insensitive to the Thisthresholdismotivatedbythevelocitydifferenceofthe
total number of satellites or their radial distribution. It is Magellanic Clouds, where there is little doubt about their
also insensitive to assuming that the satellite distribution physicalassociation.Weassumeforsimplicitythatthesame
is isotropic. Indeed, the solid and dotted curves in Fig. 2 threshold applies regardless of the luminosity of the pair;
4 Fattahi et al.
Figure3.Leftpanel:GalactocentricvelocityversusdistanceforLocalGroupsatellitesbrighterthanMV =−8.Distancesaremeasured
from the center of each primary: MW satellites are shown as magenta squares; M31’s as green triangles. Velocities for the former
are Galactocentric radial velocities; for the latter they refer to line-of-sight velocities relative to the systemic velocity of M31. Dotted
curves indicate, for reference, the escape velocity from an NFW halo with virial velocity V200 = 250 km/s and concentration c = 10
(Navarroetal. 1997). Filled symbols highlight satellites with a nearest neighbour much closer than expected by chance (P < 0.2, see
Fig. 2). Pairs satisfying additional proximity criteria in velocity (∆V < 75 km/s) and magnitude (∆MV < 3) are joined together by
ellipsestoindicatethattheyarelikelyphysicalpairs.Theseconstitute28%ofthetotalandinclude(i)theMagellanicClouds;(ii)NGC
147 and NGC 185; (iii)UrsaMinor and Draco; and (iv) And I and And III. Right panel: Same as left panel, but for the semi-analytic
satellitepopulationofthesixAquariushalos.Differentcolorscorrespondtodifferenthalos.Notethatthesamecriteriathatpair28%of
LocalGroupsatelliteslinkonlysixsatellitesintheAquariussimulations,orjust3%ofthetotal.
thisisalsoconsistentwiththeideathatmostdwarfsshould The blue dotted histogram in Fig. 2 shows that the P
inhabit halos of similar mass (see Sec. 1) (see Sales et al. distribution for Aquarius satellites differs little from that
2012,for a more thorough discussion). Finally, since we are expected from an isotropic distribution. Only 27% of the
mainlyinterestedinpairsofcomparableluminositieswere- 175 satellites haveP <0.2; of thoseonly3 pairs(i.e., fewer
tain aslikely pairsonly thosewheretheirmagnitudesdiffer than 4%) pass as well the velocity and magnitude criteria.
by ∆MV <3. The three Aquarius pairs (out of six halos) singled out by
Four pairs remain after applying these constraints, as the analysis are shown in the right-hand panel of Fig. 3.
shown in Fig. 3. Around the Milky Way, aside from the Trackingtheirorbitsbackintimerevealsthatnoneofthem
MagellanicClouds,theUrsaMinorandDracopairissingled are actually physically related but that they result simply
out: they are 23 kpc apart, and their velocities differ by from chance, transient associations in position and veloc-
only 12 km/s. Further, they have both approximately the ity space. Note that this does not imply that all satellites
sameluminosity,whichmakesthemespeciallysingular.The havebeenaccretedinisolation.AsdiscussedbyWang et al.
procedure also joins together two pairs of satellites around (2012), a few of the bright satellites in Aquarius were ac-
M31:NGC147andNGC185, aswell asAndIandAndIII. creted in groups, but the accretion happened early and the
Thelatterareseparatedby33kpc;differinvelocitybyjust groups have long been disrupted by the tidal field of the
32 km/s; and in magnitude by 1.7. mainhalo.Thisconfirmsthemodelexpectationthatphysi-
This analysis suggests that nearly 30% of Local Group calassociationsamongstsatellitesshouldbeextremelyrare.
satellites are in likely pairs (8 out of 29). Of the four pairs, TherelativelyhighfrequencyofsatellitepairingsintheLo-
two are almost indisputably associated (the Magellanic calGroupindicatesthattheradialandangulardistributions
Clouds and NGC 147/NGC 185; see, however, Geha et al. ofsatellitesarecorrelated,afactthatisnoteasilyaccounted
2010 for an alternate view of the latter) but the other two forbycurrentdwarfgalaxyformation modelsintheΛCDM
might in principle result from chance close encounters be- paradigm.
tweenunrelatedsatelliteswhereprojectioneffectsreducethe
line-of-sight velocity difference. In order to quantify these
effects we have applied the same pairing procedure to the
satellite populations of thesix Aquariushalos, as identified
by thesemi-analytic model of Starkenburget al. (2012).
Galaxy Pairs in the Local Group 5
4 SUMMARY AND CONCLUSIONS for relic evidence of past interactions between likely pairs
(suchastheMagellanicStreamfortheLMC/SMC(see,e.g.,
We have studied possible pairings amongst the satellites of
Mathewson et al. 1974; Putman et al. 1998); and, finally,
the Milky Way and of M31. Our procedure, which iden-
(iv) by extending this kind of analysis to a volume-limited
tifies unusually close associations in position and velocity
surveyofdwarfgalaxyassociationsinthelocaluniverse(see,
space, suggests that 8 out of the 29 satellites brighter than
e.g., Karachentsev & Makarov 2008). If confirmed, the en-
MV =−8(i.e., nearly∼30%) form 4likelypairsofcompa- hancedclusteringofdwarfsmayofferimportantcluestothe
rable luminosity (∆MV <3). These include the Magellanic formationoffaintgalaxiesthathaveyettobeidentifiedand
Clouds; UrsaMinorandDraco; NGC147and NGC185; as
fully incorporated into galaxy formation models.
well as AndI and And III.
The same pairing procedure applied to a semi-analytic
model of the satellite population in the six halos of the
Aquarius Project yields a likely pair fraction of fewer than ACKNOWLEDGMENTS
4%,eventhoughthemodelsatelliteshaveluminosityandra-
dialdistributionsthatmatchcloselythatoftheLocalGroup We are very grateful to Gabriella De Lucia, Amina Helmi
spirals. Asexpected,noneoftheAquariuspairscorrespond and Yang-Shyang Li for their role in developing the semi-
totruebinarysystems;rather,theyresultfromtransientas- analytic model of galaxy formation used in this paper. We
sociations between otherwise unrelated satellites. The high would like to thank the anonymous referee for comments
pair frequency of the Local Group is unlikely to be just a whichledtotheimprovementofthemanuscript.ESissup-
statisticalfluke:thelikelypairfractionofAquariussatellites ported by the Canadian Institute for Advanced Research
neverexceeds12%inthousandsofrandomtrialswheretheir (CIfAR) Junior Academy and by a Canadian Institute for
magnitudes,angulardirectionsandvelocitiesarereshuffled. Theoretical Astrophysics (CITA) National Fellowship.
We interpret these results as indicative of signifi-
cant clustering in the dwarf galaxy population of the Lo-
cal Group. Although our analysis only considers satel-
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