Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/research/LVmeeting/magsys_pres/E+FW_pryor.pdf Дата изменения: Sun Aug 5 03:43:47 2007 Дата индексирования: Sun Dec 23 09:00:57 2007 Кодировка: Поисковые слова: universe

Has the LMC Had Close Encounters With Other Satellite Galaxies?
Carlton (Tad) Pryor ­ Rutgers Slawomir Piatek ­ NJIT/Rutgers


Other collaborators: Ed Olszewski ­ U Arizona Mario Mateo ­ U Michigan Paul Bristow ­ ESO/ECF Chris Tinney ­ Anglo-Australian Obs H. Harris & D. Monet ­ USNO Dante Minniti ­ U Catolica de Chile Heather Morrison ­ Case Western


Questions: · How likely is it that the LMC has passed close to Milky Way substructure? To Milky Way satellites? · We know proper motions, hence approximate orbits, for the LMC, SMC, and six dSphs. Have there been encounters?


A Simple Calculation
· LMC falls in from large radius to Rp= 47 kpc (linear path perpendicular to the radial direction at Rp) · Density of satellites/dark halos, n, given by a spherical NFW halo with Nv inside the virial radius (rvir = 258 kpc, c = 12) · Calculate collision probability: P
coll

= n (RLMC2) ds = (7 x 10-4) (Nv) (R

LMC

/4.4 kpc)2


Some examples: · Nv = 250, RLMC = 4.4 kpc Pcoll = 0.18 · Nv = 250, RLMC = 10.5 kpc Pcoll = 1.0 · Nv = 25, RLMC = 4.4 kpc Pcoll = 0.018 · Nv = 25, RLMC = 33 kpc Pcoll = 1.0


PM Vectors for All 8 Measured Galaxies


Orbital Poles


Some examples: · Nv = 250, RLMC = 4.4 kpc Pcoll = 0.18 · Nv = 250, RLMC = 10.5 kpc Pcoll = 1.0 · Nv = 25, RLMC = 4.4 kpc Pcoll = 0.018 · Nv = 25, RLMC = 33 kpc Pcoll = 1.0 If substructure forms a flattened plane (Kroupa, Theis, & Boily) and the LMC is in the plane: Pcoll by perhaps x4


Have there been encounters? · Take known positions and space velocities and integrate backwards in a Galactic potential (NFW) · Monte Carlo by drawing velocities from distributions given by uncertainties


LMC ­ SMC:


LMC - SMC


· Carina, Draco, Fornax, and Ursa Minor never get closer to the LMC than ~60 kpc


· Carina, Draco, Fornax, and Ursa Minor never get closer to the LMC than ~60 kpc · Sculptor was ~33 kpc away ~0.1 Gyr ago


LMC - Scl


· Carina, Draco, Fornax, and Ursa Minor never get closer to the LMC than ~60 kpc · Sculptor was ~33 kpc away ~0.1 Gyr ago · Sagittarius was ~40 kpc away ~0.1 Gyr ago


LMC-Sgr


· Carina, Draco, Fornax, and Ursa Minor never get closer to the LMC than ~60 kpc · Sculptor was ~33 kpc away ~0.1 Gyr ago · Sagittarius was ~40 kpc away ~0.1 Gyr ago Thus: more close approaches than expected, but no strong interactions.


Cautions: · Sagittarius and Draco motions are preliminary · Uncertain location of the LMC center of mass adds uncertainty to the space velocity A 1.9є change in the location of the center-of-mass on the sky changes the tangential velocity by the uncertainty from the proper motion (~9 km/s)