The consequences of dwarf galaxies colliding with the Milky Way
Abstract: I simulate the collision of satellite galaxies with the Milky Way and observe the effects that this has on the orbits of the globular cluster populations within both the Milky Way and the satellite galaxies. This is done in order to investigate whether some of the Milky Way's globular clusters could have been donated from satellite galaxies which have been tidally stripped, since it is believed that the Sagittarius dwarf spheroidal galaxy is being tidally disrupted and that some of its globular clusters have been tidally stripped from it (such as Pal 12 and NGC 4147). It is also believed that there have been other Sagittarius-like satellite galaxies in the past which have donated globular clusters to the Milky Way. From a simulation of the 63 Casetti-Dinescu globular clusters orbiting in the Milky Way, I find that most of the encounters between the globular clusters occur at separations between 0.1 kpc and 10 kpc. I also find that there are approximately 24 (0.64%) collisions that may occur between the globular clusters in the next 10 Gyr. However some of these 24 collisions involve the same globular cluster (for example NGC 2808 which has 7 collisions). I vary the initial position values of the globular cluster pair which has the closest encounter, and I find that the collision is very dependant upon the initial conditions of the globular clusters. I then include the classical satellite galaxies in a simulation with the 63 globular clusters, and I find that most of the encounters between the satellite galaxies and the globular clusters occur at separations between 1 kpc and 10 kpc. However there is one encounter with a separation of 227 pc, this is between the SMC and NGC 7006, which has a probability of 1 for occurring in the next 10 Gyr. I also find that there are 10 (1.44%) collisions between the satellite galaxies and the globular clusters that may occur in the next 10 Gyr. The only satellite galaxies that are involved however are the Sagittarius dwarf galaxy, the LMC and the SMC. I generate 500 random Sagittarius-like satellite galaxies based upon the initial conditions of the Sagittarius dwarf galaxy, and I generate 500 random globular clusters. I simulate 10 randomly selected Sagittarius-like satellite galaxies, each containing 10 randomly selected globular clusters which are initially on circular orbits around their host galaxy, and the globular clusters have spacings from their host galaxy in steps of 0.45 kpc. The simulated dwarf galaxies use a decreasing mass function, and I find that after 1 Gyr they have lost all of their globular clusters to the Milky Way. I continue this simulation for a further 10 Gyr, and upon investigation of some of the globular clusters' properties (eccentricity, j_z, orbital energy, and pericentre-apocentre distances), and compare them to the globular clusters from the Casetti-Dinescu database. I find that there are 5 (8%) Casetti-Dinescu globular clusters which have similar properties to the accreted globular clusters from the Sagittarius-like satellite galaxies. These are NGC 1851, NGC 3201, NGC 4590, NGC 7006, and Pal 13. In appendix 3 I run a more realistic simulation and find that there are 6 (~10%) Casetti-Dinescu globular clusters which have similar properties to the accreted globular clusters from the Sagittarius-like satellite galaxies. These are NGC 1851, NGC 4147, NGC 4590, NGC 5024, NGC 6205, NGC 6284. I also calculate that if we have two Sagittarius-like satellite galaxies orbiting within the Milky Way every 5 Gyr and they each donate 9 globular clusters, then this means that 36 - 45 (23 - 29%) of the Milky Way's globular cluster population may have come from tidal stripping events (i.e. 4 - 5 Sagittarius-like satellite galaxies). This is in good agreement with Forbes et al., (2010), van den Bergh (2000), and Mackey & Gilmore (2004), who suggested that there were 27 - 47, 35, and 41 globular clusters which were accreted from satellite galaxies respectively. I believe that the 9 globular clusters previously mention are members of the 36 - 45 globular clusters that have been accreted in the Milky Way's history.
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