Dwarf Galaxy Structures in the Local GroupNotes for the seminar "Themen aus der aktuellen astronomischen Forschung"

Institute of Astronomy, University of Vienna

Albert Georg Passegger

October 31, 2014

Abstract

We discuss different phase space correlated structures of dwarf galaxies and as-sociated objects in the Local Group and briefly summarize some possible formationscenarios.

Contents

1 Introduction 2

2 The Disc of Satellites 2

3 The Vast Polar Structure (VPOS) 33.1 Satellite Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.2 Globular Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3 Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 The Great Plane of Andromeda (GPoA) 6

5 Planes of Non-Satellites in the LG 7

6 The Magellanic Stream 8

7 Possible Origins 97.1 Accretion of Primordial Dwarf Galaxies . . . . . . . . . . . . . . . . . . . 97.2 Formation of Second-Generation Dwarf Galaxies . . . . . . . . . . . . . . 10

1

1 INTRODUCTION 2

1 Introduction

There are various objects in the Local Group (LG) besides the two major galaxies, theMilky Way (MW) and Andromeda (M31). A great number of objects are different typesdwarf galaxies, which turn out to be unevenly distributed in the LG. Two main questionsnaturally arise:

• How are the various objects, including dwarf galaxies, globular clusters and streams,distributed? Put differently, are there any distinctive anisotropic structures in theLG?

• If there are any anisotropies, what is their origin and what processes led to theirpresent form?

In this thesis we will summarize the most important results of the attempts to answerthese questions.

2 The Disc of SatellitesLet us consider first the vicinity of the MW. It has been known for decades that there areseveral objects in the vicinity of the MW. The most important objects, which seem to berelated to the MW, are the so-called satellite galaxies. These are dwarf galaxies, which aregravitationally bounded to the MW. Originally, the 11 classical satellite galaxies Carina,Draco, Fornax, Leo I, Leo II, Sagittarius, Sculptor, Sextans, Ursa Minor and the Largeand Small Magellanic Clouds (LMC, SMC) have been assumed to be such objects.

As confirmed by [Kroupa et al. (2005)], these dwarf galaxies are phase-space correlatedand are aligned in a prominent anisotropic substructure, the so-called disc of satellites(DoS). This common disc is highly inclined to the MW disc (see fig. 1). Using data fromthe Sloan Digital Sky Survey, it has been shown that at least 13 fainter satellite galaxiesalso lie in this plane. It is guessed that these satellite galaxies are luminous DM halos,however, there are less observed bright satellite galaxies than predicted from the models.It is an open question whether this is consistent with cold or warm DM cosmology.

Figure 1: The disc of satellites edge-on (left) and face-on (right). The MW disc is indicated asblack horizontal line through the origin. [Fouquet et al. (2012)]

3 THE VAST POLAR STRUCTURE (VPOS) 3

Most of the satellites in the DoS are co-orbiting, which leads to an angular momentumsupport of the disc. The directions of the angular momenta are given by the normals tothe orbital planes of the satellites. In a galactic coordinate plot one therefore sees aclustering of angular momentum directions indicating the rotational stabilization of theDoS. [Pawlowski et al. (2012)]

There are different scenarios how the DoS could have been formed. The commonphase-space distribution of the satellites is usually interpreted as evidence that theyformed in one common group. Two hypotheses based on usual cosmology are, that thesatellites fell into the MW as a group or they were accreted as filaments. Both scenariosare problematic for different reasons. Most important, the resulting disc would be parallelto the MW disc, which is almost perpendicular to the real orientation of the DoS.

Circumventing these problems, another possible formation scenario is that the MWsatellite galaxies originate from DM-free tidal dwarf galaxies (TDGs) formed in an inter-action of the MW with another gas-rich galaxy, or even a merger event at the location ofM31. This model is discussed later.

Of course, it is a priori possible that the DoS is a chance result and not a real substruc-ture of the MW formed by certain dynamical processes. The probability for the DoS tobe a randomly formed structure could be decreased, if the orbital planes of other objectsthan satellite galaxies are found to align with the DoS, for example streams and globularclusters. If the DoS has a tidal origin, these objects could have been formed together withdwarf galaxies. If, on the other hand, no other independent objects are found within theDoS, the tidal scenario is doubtful.

Therefore, it is convenient to analyze the distributions of globular clusters and streams,i.e. disrupted debris of clusters or galaxies, to obtain more informations on satellite orbits.This turns out to lead to a so-called vast polar structure (VPOS), which does not onlycontain satellite galaxies of the MW, but also other types of objects.

3 The Vast Polar Structure (VPOS)

3.1 Satellite Galaxies

First we discuss the extension of the DoS beyond the 11 classical dwarf galaxies by takingall known dwarf galaxies in the LG into account. In [Pawlowski et al. (2013)], the LGgalaxy data set from the catalogue by [McConnachie (2012)] is used, which includes infor-mations on galaxies within 3 Mpc from the sun. For simplicity, the considered coordinatesystem has its origin in the mid-point between MW and M31 (center of the LG). Thez-axis points towards the Galactic north pole, the x-axis from the sun to the galacticcenter and the y-axis points in the galactic spin direction. All galaxies within 1.5 Mpc ofthe chosen origin are considered as LG members. This is a justified choice, because thisis approximately the doubled distance between MW and M31. Outside this radius thereare the next galaxy groups.

The considered sample contains 78 galaxies, which are divided into three subsets: thehost galaxies MW and M31, the satellite galaxies around both hosts, and non-satellitegalaxies. To make such a categorization well defined, one has to fix a selection criterion forsatellite galaxies. The usual convention is to consider all galaxies within a radius of 300kpc of the respective host galaxy as satellite galaxies. All other galaxies are non-satellites.Indeed, there are no known galaxies around either host galaxy between approximately 270kpc and 320 kpc. Furthermore, the cumulative number distribution of the dwarf galaxiesbecomes shallower with increasing distance from the respective host galaxy (see fig. 2).

3 THE VAST POLAR STRUCTURE (VPOS) 4

Figure 2: The cumulative number of galaxies plotted against min(dMW , dM31), where dMW anddM31 are the distances from the center of the MW and M31, respectively. [Pawlowski et al. (2013)]

Finally, 300 kpc is the approximate virial radius of the DM halos around MW and M31.So one can assume, that the gravitational force beyond 300 kpc is too weak to influenceany dwarf galaxies.

To determine a best fitting plane of a chosen group of galaxies with known positions,the method by [Metz et al. (2007)] is usually used. Irrespective of the concrete fittingtechnique, it is not clear whether the chosen group defines the most dominant plane inthe sample. Probably there is a dominant plane of a subset. To verify this, so-called 4-galaxy-normal density plots are used [Pawlowski et al. (2013)]: All possible combinationsof four galaxies from the sample are determined, a plane is fitted to each combination andtheir normal directions are calculated. If galaxies in a sample lie in a common plane, the4-galaxy-normal directions of combinations of four galaxies in this sample will be verysimilar. In a (usually galactic coordinate) plot, this can be identified with an overdensityof normal directions. Additionally, each 4-galaxy-normal is weighted with

log(

a + b

c

),

where a, b and c are the long, intermediate and shortest axis of the 4-galaxy-plane,respectively. Finally, it is determined which galaxies are members of a certain overdensity,i.e. of a certain dominant plane. If a galaxy does not contribute to any overdensity, it doesnot lie within the respective plane. The contribution of a galaxy to a 4-galaxy-normaldensity peak is determined by the weight of each associated galaxy within a defined angleof the peak. For all 4-galaxy normals close to the density peak, all associated plane weightsof each galaxy are summed up and the weight contribution of the most dominant galaxyis normalized to one. The relative weight of each galaxy then tells about its contributionto different peaks.

With these methods, consider the satellite galaxies of the MW. The full sample consistsof 27 satellites around the MW, the most distant with dMW = 260 kpc. A fit of allsatellites leads to a best fitting plane called ’VPOSall’ in [Pawlowski et al. (2013)]. It

3 THE VAST POLAR STRUCTURE (VPOS) 5

is also verified whether this is the most dominant plane, i.e. if there are any subsetsdefining a dominant plane: The 4-galaxy-normal density plot shows two peaks, one closeto the normal of VPOSall (but inclined by 20 degrees), and the second shallower peakcoinciding with the normal direction of the young halo globular cluster (YHGC) plane,which is discussed below. All galaxies except Leo I, Hercules and Ursa Major (whichhave the largest vertical distance to the best fitting plane of VPOSall) contribute to Peak1. Excluding these three galaxies leads to a sample called ’VPOS-3’, whose best fittingplane normal points very close to Peak 1, as well as to the average orbital pole and theMagellanic stream normal (see fig. 4).

3.2 Globular Clusters

There are three groups of globular clusters, i.e. tightly bound spherical clusters of starsin a galaxy’s halo, according to their metallicity and morphology: old halo (OH) clusters,bulge/disc (BD) clusters and young halo clusters (YHGC). For the analysis of the DoS,especially the YHGCs are interesting objects, since it is assumed that they have beenformed due to accretion. In a tidal formation scenario of the DoS, the YHGCs would havebeen formed together with the satellite galaxies. Thus it is expected that the YHGCsshow a similar planar distribution, which is aligned with the DoS, whereas the OH andBD clusters should have no correlation with the DoS. More concretely, the BD clustersshould align with the MW disc.

In [Pawlowski et al. (2012)], the positions of 30 YH, 70 OH and 37 BD globularclusters are considered. A best fitting plane and its normal vector can be calculated foreach sample. The OH and BD positions are also taken into account as a consistencycheck.

Indeed, the normal vector to the disc of YHGCs (called the DoGCYH) is less than 13◦away from the DoS normal vector, which turns out to be a highly significant agreement.Thus the YHGC distribution coincides with the DoS. Fitting the 10 outermost YHGCs(having galactocentric distances larger than 20 kpc) separately shows that this sub-sampledominates the DoGCYH, as the corresponding plane normal has an angular distance ofonly 9.5◦ from the DoS normal. The 20 remaining YHGCs lead to a normal vector withan angular distance of 13.4◦ from the DoS normal. [Pawlowski et al. (2012)]

As expected, the best-fitting normal vector for the OH globular clusters is 66◦ awayfrom the DoS normal, which confirms that these clusters are not correlated with theDoS. Moreover, the best-fitting plane of the BD clusters lies within the MW disc. Thiscorroborates that the DoS together with the DoGCYH is not a random phenomenon, buta significant anisotropic structure with a common origin.

3.3 Streams

Finally, the distribution of streams around the MW is considered. Assuming a tidal originfor the DoS, these streams would originate from debris of the tidal formation of dSphsand globular clusters in a tidal tail. Extended streams are good tracers for the path ofclusters and satellite galaxies in a tidal scenario. The tidal tail possibly deviates from thepath of the satellite, however it always remains in the satellite’s orbital plane.

Fig. 3 illustrates how the normal vector of a stream is determined in [Pawlowski etal. (2012)]. Two points s and e are chosen, which are either start and end points oroverdensities of the stream. Then a plane is defined with these two points and the MWcenter, which is assumed to be the orbital center of the streams. The vectors s and e arethen transformed from galactic, heliocentric coordinates to galactic, cartesian coordinates

4 THE GREAT PLANE OF ANDROMEDA (GPOA) 6

Figure 3: Illustration of the stream plane normal determination. [Pawlowski et al. (2012)]

and shifted by the distance of the sun from the galactic center, yielding vectors s′ and e′.The stream normal vector to the constructed plane is

n = s′ × e′

|s′ × e′|,

where × denotes the usual cartesian cross product.

In [Pawlowski et al. (2012)], 14 MW streams are analyzed this way, including theMagellanic Stream (MS). 7 of these streams, namely GCN, NGC 5053, Styx, Magellanic,Cocytos, Palomar 5 and Lethe, have plane normal vectors less than 32◦ to the DoS normalvectors. Their anchor points have galactocentric distances of approximately 11 kpc, whichsuggests that these streams lie within the DoS and DoGCYH in a common plane. Sincethis plane consists of different types of objects, it is called the Vast Polar Structure(VPOS) around the MW.

Of course, it is a puzzling result that some stream-normals are clustered in the vicin-ity of the DoS, which is completely independently defined. Thus a natural questionis, whether this could be a chance result. [Pawlowski et al. (2012)] tested, using aKolmogorov-Smirnov test, how probable this common clustering is when an isotropic dis-tribution is assumed. They found that the probability to find the observed distributionis less than 0.24 per cent. This confirms the significance of the results.

In fig. 4, 5 and 6, the VPOS and its constituents are shown in different plots.

4 The Great Plane of Andromeda (GPoA)

Following the analysis of [Pawlowski et al. (2013)], the full sample consists of 34 dwarfsatellites around M31. The best fitting plane to all of these galaxies shows, that theyare only weakly anisotropic. Again the 4-galaxy-normal distribution of this sample canbe analyzed. The normal direction to the GPoA coincides with a strong peak in the4-galaxy-normal distribution of all 34 satellites. However, the normal to the best fittingplane of all satellites does not lie within an overdensity of 4-galaxy-normals. So there isno single preferred plane for all satellites.

However, there is a sub-sample of 15 galaxies which lie in a thin plane [Ibata et al.(2013)]. The probability for a randomly aligned structure is 0.13 percent, so this is aquite significant structure. The respective plane is called the Great Plane of Andromeda(GPoA). It is inclined by 50.5 degrees to the M31 disc. Adding galaxies, which are closeto this plane, like NGC 205, M32, IC 10 and LGS 3, leads to a quite similar best fitting

5 PLANES OF NON-SATELLITES IN THE LG 7

Figure 4: A plot of normal directions of the different DoS samples (in [Pawlowski et al. (2013)]misleadingly called VPOS) discussed in subsection 3.1, including the 4-galaxy-normal densities.The YHGC planes discussed in subsection 3.2 are also shown. [Pawlowski et al. (2013)]

plane. In [Pawlowski et al. (2013)], these galaxies are therefore added to the GPoA (seefig. 7). Similar to the VPOS, the GPoA coherently rotates, as the line-of-sight velocitiesin the rest frame of M31 indicate [Ibata et al. (2013)].

5 Planes of Non-Satellites in the LG

With the criterion of [Pawlowski et al. (2013)], there are 15 non-satellite galaxies in thesample, i.e. galaxies which are more than 300 kpc away from the MW and M31. The bestfitting plane is not planar, but triaxial ellipsoidal and crosses both host galaxies. Thenormal vector points to (l, b) = (227.2◦,−35.2◦), which is approximately the directionof the short axis of the ellipsoidal plane. Its root mean square (rms) height is almost300 kpc, which is much wider than the VPOS and GPoA. This motivates to look for asub-sample of non-satellites within a much thinner plane.

In a LG galaxy distribution plot with the mid-point between MW and M31 as origin,MW and M31 lying on the equator and the normal of the full non-satellite plane pointingto the north pole, one observes two distinctive groupings for the non-satellite galaxies (seefig. 8). One band-like structure consisting of 9 galaxies (LG Galaxy Plane 1, LGP1) isabove the equator, and a second, smaller structure consisting of 5 galaxies (LG GalaxyPlane 2, LGP2) is below. The only unrelated galaxy is the Pegasus dIrr galaxy, whichseems to be very close to M31 satellites in the M31 galactic disc plane. A fit to thetwo groups of non-satellites shows that there seem to be two thin planar structures,which contain all known non-satellites, except Pegasus dIrr lying between the two planes.Moreover, both planes have similar properties:

• rms heights of LGP1 and LGP2 are 55 kpc and 66 kpc, respectively.

• Both planes have similar distances from the MW, which coincide with the distancesfrom M31, namely 150 kpc in average. This shows, that LGP1 and LGP2 areparallel to the line connecting MW and M31.

6 THE MAGELLANIC STREAM 8

Figure 5: A plot of the different normal directions of the samples discussed in [Pawlowski et al.(2012)].

• Viewed edge-on, with the cross product of the two plane normals as the line of sight(which coincides with the line connecting MW and M31, MW and M31 overlappingin the center), the host galaxies and most of their satellites lie within a wedgebetween LGP1 and LGP2, which cross at the position of Leo A (if Pegasus dIrr isexcluded from LGP2, see fig. 9).

• LGP1 and LGP2 both cross the most outer parts of the satellite distribution of MWand M31.

For consistency, [Pawlowski et al. (2013)] also determined the 4-galaxy-normal dis-tribution for the 15 non-satellite galaxies. The LGP1 normal direction coincides with apeak in this distribution, but there are two other very strong nearby peaks. The LGP2normal direction only lies in a shallow overdensity of 4-galaxy-normals. Hence, LGP1 andLG2 are not the only possible planar structures in the LG, but they are the only onescontaining almost all known non-satellites and have similar properties. The statisticalsignificance of these structures has to be studied further.

6 The Magellanic Stream

Not only the two LG planes, but also the Magellanic Stream (MS) seems to connect MWand M31 like a bridge. The MS is a gaseous stream in the Southern hemisphere, whichreaches from LMC and SMC to M31, with an extension of approximately 150◦. Its originis not clear. It might have been formed by a stripping of gas from the LMC and SMCinteracting with the MW by tidal forces or ram-pressure stripping due to the hot halo gasof the MW. Proper motion measurements however show that the Magellanic Clouds areprobably on their first infall towards the MW, which contradicts usual stream formationmodels [Pawlowski et al. (2013)].

The MS, LMC and SMC orbits align with the VPOS. This possibly indicates that theLMC, SMC and VPOS members are tidal dwarf galaxies (TDGs) formed during a mergerin M31. The MW satellites even may be TDGs formed during an interaction betweena larger LMC progenitor and the MW, as suggested by [Pawlowski, Kroupa, de Boer

7 POSSIBLE ORIGINS 9

Figure 6: The positions of the VPOS members in cartesian coordinates, with the y-axis pointingtowards the galactic north pole. The 11 classical satellites (yellow), 13 fainter satellites (green),YHGCs (blue) and red curves connecting stream anchor points are shown in the plot. The light-red shaded regions illustrate the corresponding planes spanned by the stream anchor points andthe galactic center. The stream coordinates are magnified by a factor of 3. The left panel showsan edge-on view, whereas the right panel is rotated by 90◦, thus face-on. [Pawlowski et al. (2012)]

(2011)]. The satellites around the MW almost align with the MS. The VPOS-3 plane andthe MS are inclined by only 11◦. The GPoA is almost parallel to the MS (only inclined by27◦). The members of LGP1 and two LGP2 members close to the MS follow the velocitytrend of the MS. Moreover, the MS is connected to the M31 by satellites in the M31 discplane. In position, velocity and orientation, the MS seems to connect the MW and M31.This indicates a common, possibly tidal origin [Pawlowski et al. (2013)].

7 Possible OriginsThere are different possible scenarios, which could have formed these anisotropic struc-tures in the LG.

7.1 Accretion of Primordial Dwarf Galaxies

The VPOS is a very pronounced, thin planar structure. The YHGCs are thought to bebrought in along with dwarf galaxies in a accretion scenario, which results in a similarspatial distribution of both types of objects. The streams form by tidal disruption in theMW halo. Due to the common orbital angular momentum of the objects in the VPOS,an accretion of individually infalling objects can be excluded.

Alternatively, there might have been a group accretion on to the MW forming thesesatellites. However, the stripped off GCs would have similar orbits as their former hostdwarf galaxy. If dwarf galaxies do not orbit within the VPOS, their produced GCs willneither do. This means that most YHGCs around the MW must originate from dwarfgalaxies in the VPOS, which implies that YHGCs stripped off from galaxies with unrelatedorbits are missing. Moreover, the dwarf galaxies must have been close together beforeaccretion, which is an unlikely scenario due to the large diameters (1 − 2 Mpc) of theplanes [Pawlowski et al. (2012)].

The filamentary dark matter halo distributions possibly have a large influence on theformation of planar structures in the LG. However, the filaments have the size of the virial

7 POSSIBLE ORIGINS 10

Figure 7: A plot of normal directions of the full satellite galaxy sample of M31 and the GPoA.The 4-galaxy-normal density distribution shows that no common plane for all satellites exists.[Pawlowski et al. (2013)]

radius (300 kpc) and are therefore too extended for the formation of thin planar structureswith much smaller rms heights. DM sub-halos would be accreted nearly isotropic on tothe host galaxies, because the sizes of the filaments and the host galaxy halos are almostthe same [Pawlowski et al. (2013)].

The large-scale alignment of the different types of objects in the VPOS and GPoA isyet not consistently explained within an infall scenario in standard cosmological models.

7.2 Formation of Second-Generation Dwarf Galaxies

The phase space correlation of satellite galaxies, GCs and streams is easily explained in atidal scenario. In such a scenario, the dwarf galaxies are formed in a merger or fly-by oftwo galaxies, resulting in the formation of satellite galaxies as descendants of tidal dwarfgalaxies (TDGs). One possible process is the collision of two almost perpendicular discgalaxies colliding on a polar orbit, tidally stripping off material, which is accreted in apolar oriented tidal tail. In this tail, clumps form due to tidal forces, which finally formTDGs. This is a possible scenario for the formation of, for example, the VPOS, since theformation is thought to have happened 9− 12 Gyr ago [Pawlowski et al. (2012)].

M31 is a possible candidate for the formation of the VPOS in an ancient MW fly-bydue to its large amount of matter. The satellite distributions in the VPOS and the GPoAfit into this picture: The GPoA is seen almost edge-on from the MW and also extends inthe north-south direction, similar to the VPOS [Pawlowski et al. (2012)].

In [Lynden-Bell (1976)] it is suggested that a tidal interaction of a large Magellanicgalaxy (progenitor of LMC) with the MW formed some MW satellites in the MS plane.Simulations show that the resulting tidal debris can arrange in a planar distribution.

[Hammer et al. (2010)] proposed a tidal origin of the LMC and the GPoA. A mergerof M31 with a large, gaseous progenitor could have formed young and gas-rich galaxiesand TDGs about 9 Gyr ago. The GPoA then could have been formed from the tidaldebris. In simulations, parts of the tidal tail escape and build a tail pointing to the MW.

7 POSSIBLE ORIGINS 11

Figure 8: An all-sky plot of the two group planes of non-satellite galaxies, LGP1 (yellow) andLGP2 (green), with respect to the midpoint between MW and M31. The pluses and crossesindicate the positions of the satellite galaxies of VPOS and GPoA, respectively. The Pegasus dIrrlies in the plane of M31 and is possibly an outer member of the GPoA. [Pawlowski et al. (2013)]

This possibly indicates that the LMC and SMC are TDGs formed during this merger.Also the morphological and kinematic properties of the disc, bulge and halo streams ofM31 are signs of a merger event 9 Gyr ago [Pawlowski et al. (2012)].

However, it is also possible that the VPOS evolved from TDGs in a merger at the M31location, as suggested by [Fouquet et al. (2012)]. A tidal tail containing several TDGs wasthen expelled towards the MW, where the galaxies were stripped off in a polar alignedplane. The Magellanic stream between MW and M31 indicates this possible scenario.Simulations show at least one tidal tail connecting both host galaxies. This tidal tailmight consist of LGP1 and the MS, because the LGP1 members and the MS have similarposition and velocity, and is accreted on to the MW, where the MS could be a tidal tailconnecting MW and M31 and containing gas expelled by tidal or ram-pressure processesfrom the Magellanic clouds.

7 POSSIBLE ORIGINS 12

Figure 9: The two non-satellite galaxy planes LGP1 (yellow) and LGP2 (green) viewed edge-on.MW and M31 are indicated as ellipses. Most satellite galaxies of the VPOS and GPoA (plusesand crosses) lie within a wedge with LGP1 and LGP2 as boundaries. [Pawlowski et al. (2013)]

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