Table Of ContentMon.Not.R.Astron.Soc.000,1–6(2009) Printed24January2009 (MNLATEXstylefilev2.2)
Discs of Satellites: the new dwarf spheroidals
Manuel Metz1⋆, Pavel Kroupa1 and Helmut Jerjen2
9 1Argelander-Institut fu¨r Astronomie, Universit¨at Bonn, Auf dem Hu¨gel71, D–53121 Bonn, Germany
0 2Research School of Astronomy and Astrophysics, ANU, Mt.Stromlo Observatory,Weston ACT 2611, Australia
0
2
n Received2008November6/Accepted 2009January12
a
J
2 ABSTRACT
1 The spatial distributions of the most recently discovered ultra faint dwarf satellites
around the Milky Way and the Andromeda galaxy are compared to the previously
]
A reported discs-of-satellites (DoS) of their host galaxies. In our investigation we pay
special attention to the selection bias introduced due to the limited sky coverage of
G
SDSS.WefindthatthenewMilkyWaysatellitegalaxiesfollowcloselytheDoSdefined
h. by the more luminous dwarfs, thereby further emphasizing the statistical significance
p of this feature in the Galactic halo. We also notice a deficit of satellite galaxies with
- Galactocentric distances larger than 100kpc that are away from the disc-of-satellites
o
of the Milky Way. In the case of Andromeda, we obtain similar results, naturally
r
t complementing our previous finding and strengthening the notion that the discs-of-
s
satellites are optical manifestations of a phase-space correlationof satellite galaxies.
a
[
Key words: Galaxies: dwarf, Galaxies: Local Group
1
v
8
5
1 INTRODUCTION Palma et al. 2002), revealed that a correlation is not only
6
1 apparentintheirthree-dimensionaldistribution,butalsoas
In recent years, a new class of ultra faint companion
. apossiblecommonmotion,suchthattheDoSislikelyaro-
1 galaxies with extremely low stellar densities were detected
tationally supported structure(Metz, Kroupa, & Libeskind
0 in the halo of the Milky Way (MW)1 (Willman et al.
2008).
9 2005a,b; Sakamoto & Hasegawa 2006; Zuckeret al.
The satellite system of Andromeda shows an asym-
0 2006a,b; Belokurov et al. 2006, 2007; Walsh et al. 2007;
v: Belokurov et al. 2008) by systematically scanning the metric spatial pattern too (Grebel et al. 1999; Hartwick
2000; Koch & Grebel 2006; Majewski et al. 2007). As for
i SDSS-DR6 photometric catalogue (Yorket al. 2000), more
X the Milky Way, a disc-of-satellites is evident (Metz et al.
thandoublingthenumberofknownsatellitegalaxieswithin
2007), and the entire satellite system is significantly
r the virial radius of the Milky Way. At the same time, new
a shifted in the direction of the barycentre of the Lo-
companions of the Milky Way’s sibling, the Andromeda
cal Group (McConnachie & Irwin 2006). It remains un-
galaxy (M31), were found via deep imaging (Martin et al.
clear whether this structure is also rotationally sup-
2006; Zuckeret al. 2007; Majewski et al. 2007; Ibata et al.
ported. Proper motion data are only available for two
2007;Irwin et al. 2008; McConnachie et al. 2008).
M31 companions, M33 and IC 10, from VLBI observa-
The most luminous Milky Way satellite galaxies
tions of water masers (Brunthaler et al. 2005, 2007), but
have been known for more than three decades to ex-
van derMarel & Guhathakurta (2008) concluded that a
hibit an anisotropic spatial distribution (e.g. Lynden-Bell
possibleintrinsicrotationofthesatellitesystemismarginal
1976, 1983; Majewski 1994; Hartwick 2000). The quan-
from theirstatistical analysis. Alsonodirectpropermotion
titative exploration of this phenomenon found that the
measurement is available for Andromeda itself, but various
dwarf galaxies lie close to a virtual plane, the disc-
attempts have been made to constrain the possible trans-
of-satellites (DoS), highly inclined with respect to the
verse motion of M31 based on statistical properties of the
stellar disc of the Milky Way (Kroupa,Theis, & Boily
motion of its satellite galaxies (e.g. Einasto & Lynden-Bell
2005; Metz, Kroupa, & Jerjen 2007; Li & Helmi 2008).
1982; van derMarel & Guhathakurta 2008) or the persis-
Subsequent studies of kinematical data, radial veloci-
tence of the stellar disc of M33 under the assumption that
ties and proper motions (Lynden-Bell& Lynden-Bell 1995;
it is on a bound orbit to Andromeda(Loeb et al. 2005).
This paper is to follow up the previous work by
⋆ E-mail:[email protected] Metz et al. (2007) (MKJ07 hereafter), supplementing the
1 The discovery of the CVn II dSph was reported twice, by datawithmorerecentfindings:the11+11newlydiscovered
Sakamoto&Hasegawa in the issue 653 of ApJ as a letter and dSph satellite galaxies of the Milky Way and Andromeda
inthesubsequentissueofApJbyBelokurovetal. areaddedtothesampleandcomparedtotheproposeddisc-
2 M. Metz, P. Kroupa and H. Jerjen
Table1. GalactocentriccoordinatesofthesevenultrafaintMW
satellite galaxies reported inthe lastthree years. Four extraob-
jectsarelistedwheretheclassificationasdSphorglobularcluster
isstilluncertain,i.e.UMaII,Willman1,CBe,andBooII.Inthe
forth column the orthogonal distances from the DoS are tabu-
lated. See table 1 in MKJ07 for a listing of all the other MW
satellites.
Name lMW [◦] bMW [◦] R[kpc] dDoS [kpc]
UMaII(a) 159.6 +30.0 36.5 18.5
Wil1(b) 164.7 +47.7 43.0 12.7
CBe(c) 201.8 +75.1 45.2 9.8
BooII(d) 348.1 +78.4 47.6 27.7
Boo(e) 356.6 +77.5 57.6 32.0
UMa(f) 162.0 +50.8 104.9 38.3
Her(c) 30.9 +38.8 134.2 87.2
CVnII(c,g) 132.7 +80.9 150.7 19.8
LeoIV(c) 260.0 +56.2 160.6 56.7
LeoV(h) 256.8 +58.1 180.8 57.3
CVn(i) 86.9 +80.2 219.8 43.4
References: (a)Zucker etal. (2006a); (b)Willmanetal.
(2005a); (c)Belokurovetal. (2006); (d)Walshetal. (2007);
(e)Belokurovetal. (2006); (f)Willmanetal. (2005b);
(g)Sakamoto &Hasegawa (2006); (h)Belokurovetal. (2008);
(i)Zucker etal.(2006b) Figure1.The3DdistributionoftheMilkyWaysatellitegalax-
ies. In the top-left panel an edge-on view onto the fitted disc-
of-satellitesas giveninMKJ07isshown, derivedusingthelarge
of-satellites in Section 2. For the Milky Way, a statistic is circlesanddiamonds.TheMWdisc,located inthecentreofthe
derivedtocalculatetheprobabilitytofindsatellite galaxies plot, is seen edge-on. In the top-right panel, a view rotated by
away from a given reference plane, taking the sky coverage 90◦ about the polar axis of the Milky Way is shown, and in
regionofSDSSintoaccount,andappliedtothemostrecent the lower-right panel a face-on view onto the fitted disc is plot-
discoveries. The results are discussed in Section 3. ted.TheMagellanicCloudsaremarkedbydiamondsymbols,the
dwarfspheroidalsbycircles,wherebythesmallercirclesmarkthe
newlydiscoveredsatellites(Table1).Uncertainties areindicated
bylightgreysticks.Inaddition,theobscurationregion,|b|<5◦,
2 SPATIAL DISTRIBUTION OF THE NEW oftheMWisshownasthedark-shadedregion(thelight-shaded
SATELLITES region being the 15◦ obscuration region). The projected north-
ern sky coverage region of the SDSS is indicated by the yellow
2.1 The Milky Way satellites colouredarea.
InMKJ07weshowedthatthesatellitegalaxiesoftheMilky
Wayarearrangedinapronounceddisc-likedistribution,the creasesto∆=28.5±1.2kpcandthedirectionofthenormal
disc-of-satellites. The direction of the normal vector of the of thefitted plane is (lMW,bMW)=(149.6◦,−5.3◦)±2.2◦.
disc is lMW =157.3◦, bMW =−12.7◦, i.e. theDoS is almost The bootstrap analysis described in detail in MKJ07
perpendiculartotheGalacticplaneandtheclosestdistance is repeated, now also incorporating the additional 11 new
from the Galactic centre is D =8.3kpc. The derived rms- MW satellites. Theresulting shapeand strength parameter
P
height of the disc is ∆=18.5kpc, such that ∆/R =0.15 are γ = 1.5, ζ = 5.0. These two parameters describe the
vir
is therelative thicknessof thedisc, where R =250kpcis distribution of normals fitted to the bootstrapped samples
vir
theapproximatevirialradiusoftheMilkyWay.Theanalysis on the sky: the higher the shape parameter γ, the more
was carried out for the “classical” eleven brightest satellite symmetricthedistribution,andhighervaluesofthestrength
galaxies.Thedifferentprojectionsofthe3-Ddistributionof parameterζindicatemoreconcentrateddistributions.IfHer
theMilkyWaysatellitesareshowninFigure1relativetothe isexcludedfromthebootstrappingwefindγ =1.9,ζ =5.0.
DoS.AllobjectsrecentlydiscoveredinSDSS(Table1)were EventhoughHeriswellawayfromtheDoS,theresultofthe
addedtotheplotassmallercircles.Theprojected northern bootstrapping analysis remains unaffected. The values are
skycoverageregionoftheSDSSishighlightedbytheshaded showninFigure2wheretheconfidencecontoursareplotted
(yellow) area. forthenull-hypothesisthatthesatellitegalaxiesweredrawn
All but one of the newly reported satellite galaxies from an isotropic parent distribution. The random samples
of the Milky Way are found close to the DoS (Table 1). are setup to resemble the observed satellite galaxies of the
The Hercules (Her) dwarf spheroidal, the left-most data- MilkyWay:whatisnewinourapproachhereisthatwetake
point marked by a small circle in the top-left panel of into account two observational constraints: (i) the Zone of
Figure 1, is 87±8kpc away from the DoS at a Galacto- Avoidance is accounted for by not allowing any satellite to
◦
centric distance of 134.2kpc. Without Her, the improved have Galactic latitude |b| < 15 . The 11 classical satellite
fitting results are basically identical to the MKJ07 fit: galaxies are setup with this bias only. (ii) The SDSS sky
◦ ◦ ◦
(lMW,bMW)=(159.7 ,−6.8 )±2.3 ,DP =6.7±1.3kpc,and coverageisaccountedforbyallowingonlysatelliteswithb>
◦
∆=24.9±1.1kpc,1σ uncertaintieswerecalculatedusinga 30 for the remaining ones. The null-hypothesis is rejected
Monte-Carlo method. Including Her, the rms-thickness in- at the (1−α)=99.5% confidence level. This clearly shows
Discs of Satellites: the new dwarf spheroidals 3
Figure 2. Samples of satellite galaxies were constructed from Figure 3. The probability that a satellite galaxy is found in
a spherical isotropic random distribution taking into account i) the SDSS at its derived distance d or closer to the DoS versus
the obscuration due to the MilkyWay disc and ii)the influence their Galactocentric distance R. Galactocentric distance uncer-
of the SDSS sky coverage area. The strength parameter γ and tainties were calculated from Heliocentric distance uncertainties
shapeparameterζ werederivedfor100000randomsampleseach astakenfromtheliterature.Uncertainties inthe FR(d)parame-
individually bootstrapped 10000 times (see text for a definition ter werecomputed by considering sky coverage areas of b>20◦
ofγ andζ).Theplotshowscontourlinesofenclosedvalueswith andb>40◦,respectively.Opensymbolsmarkobjects wherethe
significance levels α as labelled. The filled star marks the value classificationasadSphordisruptedglobularclusterisuncertain.
forallMWsatellites,theopenstarexcludingtheHerculesdwarf
galaxy. The null-hypothesis that the MW satellites are drawn
from a random sample is rejected at the (1−α)=99.5% confi-
SDSS.Forexample,atR=60kpctheprobabilitytofindan
dencelevel.
objectatadistanced=25kpcfromtheDoSincreasesby13
percent from FR=60(25)=0.42 to FR=60,b>+30(25)=0.55.
that, even though the updated DoS is somewhat “thicker” The derived distribution function for a cone region is
than for the classical satellites alone, the currently known strictlyspeakingcorrectonlyforaplaneparalleltotheaxis
MilkyWaysatellite galaxiesarehighlyunlikelydrawnfrom of symmetry. The fact that the DoS is tilted with respect
an isotropic distribution (cf. MKJ07). to this axis by 12.7◦ introduces a correction factor of the
◦
orderof 2 percent,cos(12.7 )≈0.98, that will beneglected
here. Also, the SDSS coverage region is not a perfect cone
2.2 The effect of the SDSS sky coverage
symmetricwithrespecttotheGalacticNorthPole(compare
also figure1 in MKJ07), but only approximately so.
The SDSS survey area covers about 20 percent of the total
sky,beingmerelyasphericalcapattheGalacticNorthpole
withb&+30◦,exceptforthreenarrowstripesinthesouth- The probability that a satellite is found in the SDSS
ernGalacticsky.Certainly,thisobservationalbiasmusthave catalogue at its derived distance d or closer to the DoS is
an influenceon thederivedspatial distribution ofthesatel- plotted in Figure 3 for DP = 3.3kpc, which is the offset
lite galaxies discovered in the SDSS data. Especially, since of the DoS from the Sun (as the Sun is at the vertex of
theDoSisalmostperpendiculartotheMWplane,theprob- the cone-like SDSS sky coverage region). For an isotropic
ability of finding satellite galaxies in the polar region close distribution these values are expected to be uniformly dis-
to theDoS is higherthan at low Galactic latitudes. tributed. For objects with R < 100kpc the probabilities
Onecannowaskwhattheprobabilityistofindasatel- are consistent with a uniform distribution, but for the new
litegalaxyatanorthogonaldistancedfromtheDoS.Forany dwarf spheroidals with R > 100kpc only Hercules is found
givenfixedplaneatadistance06DP <RfromtheGalac- intheupperregion.Incontrast,fiveoftheouterdSphhave
ticcentre,theprobabilitytofindasatellite galaxy at adis- F <0.5.
tance d perpendicularto this planefor a spherical isotropic It is important to recall that FR,b(d) describes a prop-
parent distribution can be analytically expressed. The de- erty of the sky coverage area and does not tell anything
rived distribution function (DF), FR(d), is independent of about the true spatial distribution of the satellite galaxies.
◦
theradialdensitydistributionfunctionofthesatellitegalax- Consider, for example, a sky coverage region of only 10
ies. For a cone-like region (b>b′) about an axis parallel to about the north pole. All satellite galaxies found in such a
thereference-plane,asinthecaseofSDSS,thedistribution region must be very close to a polar DoS independent of
functionFR,b>b′(d)canalsobewrittendown,butneedsnu- whether they are highly an-isotropically distributed or not,
merical integration. An important point to note is that the and the DF may easily have values of order one. Neverthe-
DF rises more steeply for the cone coverage area than for less,fromFigure3wecanconcludethatthereisadeficitof
a full-sky (or full hemisphere) coverage area. This means satellite galaxies far off the DoS at galactocentric distances
that it is more likely to discover a satellite galaxy close to R>100kpc.
theDoSwhenonlyapolarcapisobservedasisdoneforthe It is worthwhile to consider that those objects marked
4 M. Metz, P. Kroupa and H. Jerjen
Table 2. Positions of the recently discovered Andromeda dSph
satellite galaxies in Andromeda-cetric coordinates as defined in
MKJ07 and their distances to the M31 DoS. Heliocentric coor-
dinates and distances were taken from the respective discovery
works,theassumeddistancetoAndromedausedforthetransfor-
mation is 785kpc (McConnachie&Irwin 2006). And XI – XIII
wereallassumedtobelocatedatacommonHeliocentricdistance
and thus appear to be clustered (see also Figure 4), which may
inrealitynotbevalid.WealsolistAndXVIIIhereforcomplete-
ness,butwithadistanceof589kpcthisgalaxyiswelloutsidethe
virialradiusofM31.
Name lM31[◦] bM31[◦] RM31[kpc] dDoS[kpc]
AndXVII(a) 90.6 56.5 45 4
AndX(b) 139.8 −16.7 110 46
AndXII(c) 314.2 −30.6 117 3
AndXI(c) 310.8 −30.3 124 8
AndXX(f) 269.5 13.8 129 116
AndXIII(c) 312.3 −37.6 136 5
AndXIV(d) 250.6 −48.2 162 20
AndXV(e) 172.8 −36.9 176 33
AndXIX(f) 316.9 9.5 188 79
AndXVI(e) 189.7 −27.8 281 27
AndXVIII(f) 356.9 31.7 589 61
Figure 4. The 3D distribution of Andromeda satellite galaxies References: (a)Irwinetal. (2008); (b)Zucker etal. (2007);
(c)Martinetal.(2006);(d)Majewskietal.(2007);(e)Ibataetal.
as inFigure1 forthe MilkyWay. Galaxy types areindicated as
(2007);(f)McConnachieetal.(2008)
follows:dIrrgalaxiesaremarkedbydiamonds,dEsbyhexagons,
anddSphsbycircles,smallercirclesmarkingthenewlydiscovered
dSphs. The location of M33 is marked by the star symbol. The
lightgreysticksaredistanceone-sigmauncertaintiesofthesatel-
lites only. The grey shaded area indicates the projected region
where potentially some satellite detections may be hindered by
ously fitted disc-of-satellites. This is particularly remark-
foregroundMWstructure(seealsoMcConnachie&Irwin2006).
able as the Andromeda system is much less affected by
Agroupofsatellitegalaxies,consistingofAndXI,XII,andXIII,
foreground obscuration due to the disc of the Milky Way
ismarkedbyalightcircle.ThesedSphwereassumedtoallhave
thesameheliocentricdistance. (McConnachie & Irwin2006),anditisalsonotasbiasedas
is the Milky Way system towards the Galactic north pole
region.OnlythetwoAndromedacompanionsreportedvery
with open symbols are likely strongly influenced by tides recently (McConnachie et al. 2008) lie off the disc, whereas
(Gilmore et al. 2008; van den Bergh 2008). If they came the third one, And XVIII at RM31 =589kpc and thus out-
closetotheGalacticdisc,theyarealsolikelypronetopreces- side thevirial radius of Andromeda, is again found close to
sion in the non-spherical potential induced by the Galactic theM31DoS.AsfortheMilkyWay,incorporatingthedata
disc or to scattering events (Zhao 1998; Pen˜arrubia et al. of all the newly discovered dSphs to the fitting methodol-
2002). Only for the B¨ootes dwarf at R = 57.6kpc there ogy of MKJ07 has only a marginal influence on the fitting
seems to be a general agreement that it has to be classified parameters: lM31 = 60.2◦, bM31 = −30.7◦, DP = 15.6kpc,
as a dSph; its total luminosity is about ten times higher and ∆ = 45kpc, whereby the strongest effect is caused by
thanforalltheotherobjectswithR<100kpc.Iftheother those two outlying satellites, And XIX and XX. Without
four objects, UMa II, Willman1, CBe, and Boo II, are not them we find lM31 = 70.2◦, bM31 = −32.9◦, DP = 1.7kpc,
dSphstheymayalsohaveacompletelydifferentparentdis- and∆=39.2kpc,comparedtotheresultsaspreviouslyde-
tribution.Interestingly,alsotheHerdSphhasbeenstrongly rivedinMKJ07:lM31 =73.4◦,bM31 =−31.5◦,DP =1.0kpc,
affected by tides (Coleman et al. 2007). If this is due to a and∆=45.9kpc(lM31 andbM31 arethelongitudesandlati-
recentcloseperigalacticpassage,theorbitofHermighthave tudesofthepoleofthefittedDoSintheAndormedacentric
beeninfluencedbytheMilkyWaydisc,thuspossiblycausing coordinate system as definedin MKJ07).
a drift of the orbit out of theDoS if it were within before.
TheproximityofthedwarfspheroidalstotheM31DoS
is in particular not strongly influenced by the still uncer-
tain distances to them which are of the order 10%, as the
2.3 The Andromeda galaxy
DoS of M31 is seen nearly edge-on from the Sun. Thus, a
For the Andromeda satellites we performed the same steps large distance uncertainty only marginally influences their
as for the Milky Way: The recently discovered companions perpendicular distance to the DoS (see the indicated line-
listed in Table 2 were added to the sample and are plot- of-sight distance uncertainties in the top-left panel of Fig-
ted in Figure 4 with respect to the Andromeda DoS as ure4).ThusalsotheapparentgroupingofAndXI,XII,and
given in MKJ07. For this figure the data-set taken from XIII,markedbythelightcircle,whichiscausedbytheiras-
McConnachie & Irwin(2006)isusedandsupplementedwith sumedcommonheliocentricdistance(Martin et al.2006),is
data as given in Table 2. Again, most of the recently not biasing the finding that these satellites are close to the
discovered satellite galaxies are found close to the previ- M31 DoS.
Discs of Satellites: the new dwarf spheroidals 5
3 CONCLUDING REMARKS thatmultiplegroupscameinfrom thesamedirection along
theintermediate-scalefilamentarystructure.Butthisdirec-
In Figure 1, thebiasing dueto thesky-coveragearea of the tion must then have been different for the Milky Way and
SDSS is obvious. Nevertheless, there are remarkably large Andromeda,andcannothavebeenthesupergalacticplane.
portionsof theskythatareway off theDoS–butnosatel- Not all of the proposed scenarios to explain the satellite
litegalaxies arereported tobelocated there(top-left panel galaxiesasluminousdarkmatterdominatedhaloesmutually
inFig.1).Especially atlargeGalactocentricdistances, R> exclude each other, but it appears to be very controversial
100kpc,thereisadeficitofsatellitesfarofftheDoS(Fig.3). within the CDM community which are the main character-
ThesameistruefortheAndromedasystem,which ismuch istics that makedark matter sub-haloes to beluminous.
lessinfluencedbyobscuration:onlythetwomostrecentlyre-
A possible alternative is that most of the satellite
ported Andromeda companions (McConnachie et al. 2008)
galaxies are of tidal origin (Zwicky 1956; Lynden-Bell
are not found close to the disc, but eight are found very
1983). If the interaction of a gas-rich galaxy with the
close totheDoS.Majewski et al. (2007) notedthat along a
proto Milky Way took place in the early Universe, a large
radial vector from the centre of M31, connecting NGC 147
number of tidal dwarfs would have likely been produced
and And XIV, in total six satellite galaxies are located in
(Okazaki& Taniguchi2000),thatarenaturallyarrangedin
projection, and Irwin et al. (2008) found that And XVII is
adisc-likestructure:duetotheconservationofangularmo-
also on that line. We demonstrated that the new dSphs lie
menta, their orbital poles are determined by the plane of
on a straight line not only in projection, but taking their
the interaction. This process may also explain dwarf galax-
full three-dimensional data into account, we find that they
ies on similar orbits if they were formed in adjacent re-
belong to the same disc-of-satellites as reported in our pre-
gions in the tails (Belokurov et al. 2008). Initially gas-rich,
viouswork (MKJ07). Thisholdstruedespitelarge distance
the TDGs may retain their gas for some time, having pro-
uncertainties.
longed star formation epochs (Recchi et al. 2007), and may
It appears highly unlikely that the disc-of-satellites for evolve into dSph like galaxies we see today (Kroupa 1997;
the two dominant galaxies in the Local Group are the Metz & Kroupa 2007). But it remains to be seen whether
result of observational biases. The bootstrapping analy- Newtonian dynamics can account for the detailed velocity
sis shows, taking the biasing effects due to the SDSS sky distributionfunctionsobservedinthesatellites(L okas2001;
coverage area into account, that it can be excluded at a Gentile et al. 2007).
very high significance level of 99.5% that the MW satel-
Recently, looking at distant host galaxies in the SDSS,
lites are drawn from an isotropic distribution, even then
Bailin et al. (2008) and Steffen & Valenzuela (2008) found
when we include the outlying satellite Hercules. At the
for isolated blue galaxies that their satellites are, in projec-
sameconfidencelevelithasbeenexcludedthattheclassical
tion,isotropicallydistributed(incontrasttowhatHolmberg
11 MW satellites are drawn from an isotropic distribution
1969originallyfound).TheMWandM31seemtobeexcep-
(Metz et al. 2007). The findings presented here strengthen
tions in this case as both have highly an-isotropic satellite
the case that the disc-of-satellites for both, the Milky Way
distributions.Itis,however,worthwhiletoconsiderthatper
and Andromeda, are real and evidence of a spatial (and
hosttypicallyonlyonesinglesatelliteisfoundinthesestud-
kinematical: Lynden-Bell& Lynden-Bell1995; Palma et al.
ies.AllofthedSphs,whichbuilduptheDoS,wouldnotbe
2002; Metz et al. 2008) correlation of the satellite galaxies.
considered in these analyses due to their faintness. That
We emphasise that it has been found that the orbital an-
said, from a statistical point of view the brightest satel-
gular momenta of the innermost classical satellites point in
lites (LMC or M33 type) can be isotropically distributed
a direction close to the pole of the DoS (Metz et al. 2008),
for a large sample of hosts. But each primary individually
which itself is mostly defined by the outer satellites, now
mightwellhaveahighlyan-isotropicallydistributedsatellite
including also the new discoveries. This suggests a strong
galaxy system of dSphs.
phase-space correlation of thesatellites.
The next years will be exciting: new satellite galaxies
Different solutions have been proposed to account for
areexpectedtobediscoveredbyupcomingsearchcampaigns
the satellite galaxies of the Milky Way in the context
suchas theStromlo Missing Satellites Survey(Jerjen 2008)
of them being cold-dark matter (CDM) sub-structures. A
–especially also at lowerGalactic latitudes.Iftheexpected
strongcentralclusteringofsub-haloesthatmimicsa“thin”
ultra faint galaxies turn out to be isotropically distributed,
DoS distribution (Kang et al. 2005; Zentneret al. 2005) is
we are still left with the fact that the most luminous MW
unlikely to be the explanation (MKJ07). The most mas-
satellites are arranged in the disc-of-satellites, whereas the
sive sub-haloes before accretion (Libeskind et al. 2005) do
low luminous systems would be isotropically distributed.
not account for the apparent rotational support of the
However, if they turn out to be located within the disc-
Milky WayDoS (Metz et al. 2008).Othersolutions, not di-
of-satellites, thiswill substantiate thesuggested strong cor-
rectly addressing the DoS problem, range from the most
relation of the satellite galaxies, and it will become more
massive sub-haloes (Stoehret al. 2002) to the least mas-
challenging toexplainthem asluminous,dark matterdom-
sive sub-haloes (Sales et al. 2007), or the earliest forming
inated sub-structures. It is quite clear that dSph satellite
sub-haloes (Strigari et al. 2007) as being the Local Group
galaxies hold important clues not only to the physical pro-
satellites. Recently, it has been proposed that infalling
cesses acting at early cosmological times, but also on the
groups of sub-haloes can account for the satellite galax-
foundation of cosmological theory.
ies (Li& Helmi 2008; D’Onghia & Lake 2008) or that the
dwarfs form in chain structures tracing the dark matter fil-
aments (Ricotti et al. 2008). This would imply that most Acknowledgements
satellite galaxies belong to a single group that came in, or The authors acknowledge financial support from the
6 M. Metz, P. Kroupa and H. Jerjen
Go8/DAAD Australia Germany Joint Research Co- Palma,C.Majewski,S.R.Johnston,K.V.2002,ApJ,564,
operative Scheme. 736
Pen˜arrubia,J.Kroupa,P.Boily,C.M.2002,MNRAS,333,
779
Recchi, S. Theis, C. Kroupa, P. Hensler, G. 2007, A&A,
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