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Constraining the Potential of the Milky Way using Stellar Streams

Ejdetjärn, Timmy LU (2017) In Lund Observatory Examensarbeten ASTK02 20171
Lund Observatory
Abstract
The orbital approximation for stellar streams is the assumption that all stars in a stream can be described as following the same orbit. In this thesis, I evaluate this approximation as a method for constraining the potential of the Milky Way. I wrote a program in MATLAB which integrates the orbits of test particles in a potential model which resembles that of the Milky Way's halo and disc. The group of particles are set off with a velocity and position dispersion to act like a typical satellite. By varying the satellite's initial position, velocity and angle of the velocity relative to the disc, a large variety of different streams were created. The formed streams were treated as real observations, to determine how well the method works... (More)
The orbital approximation for stellar streams is the assumption that all stars in a stream can be described as following the same orbit. In this thesis, I evaluate this approximation as a method for constraining the potential of the Milky Way. I wrote a program in MATLAB which integrates the orbits of test particles in a potential model which resembles that of the Milky Way's halo and disc. The group of particles are set off with a velocity and position dispersion to act like a typical satellite. By varying the satellite's initial position, velocity and angle of the velocity relative to the disc, a large variety of different streams were created. The formed streams were treated as real observations, to determine how well the method works on actual streams. Using the position and velocity of the central particle in the satellite, orbits were fitted to the stream by altering the disc and halo masses and calculating the log-likelihood of each orbit. I try to constrain the mass values using a 90% confidence region in a 2D log-likelihood plot. The error in the orbital approximation is clearly visible when plotting the central orbit together with the simulated stream stars. No specific initial parameter (of the ones changed) gave a predictable confidence region, meaning that the approximation fails whatever the details of the stream models. It is found that most streams do not produce a confidence region that contain the masses of the potential they were actually created in. The percentage of confidence regions which had the actual masses inside them were 22.90%, which is very far from the 90% expected (from the confidence region) if the the orbital approximation was accurate. Only using streams with confidence regions entirely in the positive mass region gave a percentage of 12.50%. This means that past studies which used the orbit approximation are likely to have found incorrect answers regarding the shape and mass of the Milky Way. (Less)
Popular Abstract (Swedish)
Om dina ögon vore lika känsliga som astronomers toppmoderna teleskop skulle du kunna se långa band av ljus när du kollar upp mot himlen. Dessa band kallas stjärnströmmar och befinner sig i yttre delen av våran galax, Vintergatan. Strömmarna skapas från en klump av stjärnor, t.ex. en dvärggalax, medan de roterar runt Vintergatan. Det som händer är att stjärnor slits loss från klumpen och dras ut till en lång ström av stjärnor, som, i vissa fall, kan omfamna hela galaxen. Själva processen som drar ut stjärnorna till en lång ström är densamma som får månen att orsaka tidvatten på jorden; stjärnorna i klumpen som är närmre Vintergatan påverkas mer av galaxens gravitation och blir utdragna ur klumpen.

Det tog inte lång tid förrän astronomer... (More)
Om dina ögon vore lika känsliga som astronomers toppmoderna teleskop skulle du kunna se långa band av ljus när du kollar upp mot himlen. Dessa band kallas stjärnströmmar och befinner sig i yttre delen av våran galax, Vintergatan. Strömmarna skapas från en klump av stjärnor, t.ex. en dvärggalax, medan de roterar runt Vintergatan. Det som händer är att stjärnor slits loss från klumpen och dras ut till en lång ström av stjärnor, som, i vissa fall, kan omfamna hela galaxen. Själva processen som drar ut stjärnorna till en lång ström är densamma som får månen att orsaka tidvatten på jorden; stjärnorna i klumpen som är närmre Vintergatan påverkas mer av galaxens gravitation och blir utdragna ur klumpen.

Det tog inte lång tid förrän astronomer kom på hur de kunde använda stjärnströmmar för att lära sig mer om Vintergatan. Exempelvis kan stjärnströmmar användas för att uppskatta galaxens massa, eftersom massan är relaterad till gravitationen som skapade strömmarna. Ett av sätten flera forskare använt för att ta reda på massan är att anta att alla stjärnorna i en stjärnström ligger på samma bana, dvs. stjärnorna följer efter varandra medan de roterar runt galaxen. Man vet att detta antagande inte är exakt rätt, men frågan är: hur fel är det?

I detta projekt undersöker jag ifall stjärnorna i en stjärnström verkligen kan sägas ligga på samma bana. Jag skriver kod och skapar ett program vilket tillåter mig att skjuta iväg en grupp partiklar i ett gravitationssystem, som ska föreställa Vintergatans. Efter att ha låtit partiklarna rotera runt ett tag bildar de stjärnströmmar och jag kan undersöka hur väl dessa följer en bana. Om banan och stjärnorna mestadels inte stämmer överens så vet vi att antagandet inte funkar. Målet är att göra detta för en massvis olika stjärnströmmar och avgöra ifall det finns specifika typer av strömmar för vilka antagandet kan användas. (Less)
Please use this url to cite or link to this publication:
author
Ejdetjärn, Timmy LU
supervisor
organization
course
ASTK02 20171
year
type
M2 - Bachelor Degree
subject
keywords
stellar streams, Milky Way, potential, confidence region, orbital approximation, MW, CR, halo, disc, constrain, mass, orbit, simulation, simulated, fitting, likelihood
publication/series
Lund Observatory Examensarbeten
report number
2017-EXA120
language
English
id
8915063
date added to LUP
2017-06-19 15:55:47
date last changed
2017-06-19 15:55:47
@misc{8915063,
  abstract     = {The orbital approximation for stellar streams is the assumption that all stars in a stream can be described as following the same orbit. In this thesis, I evaluate this approximation as a method for constraining the potential of the Milky Way. I wrote a program in MATLAB which integrates the orbits of test particles in a potential model which resembles that of the Milky Way's halo and disc. The group of particles are set off with a velocity and position dispersion to act like a typical satellite. By varying the satellite's initial position, velocity and angle of the velocity relative to the disc, a large variety of different streams were created. The formed streams were treated as real observations, to determine how well the method works on actual streams. Using the position and velocity of the central particle in the satellite, orbits were fitted to the stream by altering the disc and halo masses and calculating the log-likelihood of each orbit. I try to constrain the mass values using a 90% confidence region in a 2D log-likelihood plot. The error in the orbital approximation is clearly visible when plotting the central orbit together with the simulated stream stars. No specific initial parameter (of the ones changed) gave a predictable confidence region, meaning that the approximation fails whatever the details of the stream models. It is found that most streams do not produce a confidence region that contain the masses of the potential they were actually created in. The percentage of confidence regions which had the actual masses inside them were 22.90%, which is very far from the 90% expected (from the confidence region) if the the orbital approximation was accurate. Only using streams with confidence regions entirely in the positive mass region gave a percentage of 12.50%. This means that past studies which used the orbit approximation are likely to have found incorrect answers regarding the shape and mass of the Milky Way.},
  author       = {Ejdetjärn, Timmy},
  keyword      = {stellar streams,Milky Way,potential,confidence region,orbital approximation,MW,CR,halo,disc,constrain,mass,orbit,simulation,simulated,fitting,likelihood},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {Constraining the Potential of the Milky Way using Stellar Streams},
  year         = {2017},
}