The Milky Way's Unusual Companions

May 5, 2015

By most accounts, the Milky Way is a fairly unexceptional galaxy in the Universe at large. However, a team of KIPAC scientists has shown that it has one very unusual feature: its two lesser companions, the Magellanic Clouds.

Images of systems from the SDSS spectroscopic catalog where Milky Way-like hosts have two Magellenic Cloud-like companions

Readily visible to the naked eye in the southern hemisphere, the Large and Small Magellanic Clouds are the two brightest dwarf galaxies that orbit our home galaxy, the Milky Way.  Recent computer-simulation work (including some carried out at KIPAC) suggests that galaxies like the Milky Way are actually very unlikely to have two companions as bright as the Magellanic Clouds.  This notion makes cosmologists a little uncomfortable: nearly all of our current understanding of the universe rests on the Copernican Principle, which says that we are not observing the cosmos from any particularly special or privileged location.  If the Magellanic Clouds make the Milky Way unusual, this basic assumption may be left open to question.
 
To settle the issue, a team of KIPAC scientists set out to address the question with observational data from the Sloan Digital Sky Survey (SDSS), an optical telescope survey of much of the sky.  Led by student Lulu Liu, postdoc Brian Gerke, and professor Risa Wechsler, the team searched for galaxies in the data that were similar to the Milky Way.  To make the cut, a potential Milky-Way analog had to be approximately as bright as the Milky Way and also have no brighter neighbors much nearer than the Milky Way's closest big neighbor, Andromeda.  The team then searched for fainter galaxies near the Milky-Way analogs, with brightnesses like the Magellanic Clouds.
 
The complication is that SDSS was able to measure the distances to the bright galaxies similar to the Milky Way, but not to fainter objects like the Magellanic Clouds.  This means that a faint galaxy nearby one of the Milky-Way analogs could either be a real dwarf satellite of that galaxy, or it could be a bright galaxy in the distant background, or even a very faint galaxy in the foreground.  To account for this, the team devised a means to statistically separate the signal from real satellite galaxies from the noise arising from background objects.  The result was a clean measurement of the likelihood that a galaxy like the Milky Way hosts zero, one, two, or more satellites like the Magellanic Clouds.
 
The result was a ringing confirmation of the simulations: the Magellanic Clouds are indeed quite rare.  Less than 5 percent of galaxies like the Milky Way host two satellites as bright as the Magellanic Clouds, and more than 80 percent host no such satellites at all, in almost perfect agreement with the simulation results.  The simulations themselves involved setting the initial conditions of density fluctuations and allowing the computer to evolve the simulated universe according to the known laws of physics, and were carried out by a team including Liu, Gerke, and Wechsler, as well as KIPAC graduate student Peter Behroozi, former KIPAC postdoctoral scholar Michael Busha, and Anatoly Klypin and Joel Primack of New Mexico State University and the University of California, Santa Cruz respectively.  This consistency between the universe produced by the simulations and the observations or our real one, taken on its own, constitutes a strong endorsement of the standard Cold Dark Matter cosmological paradigm underlying the simulations.
 
But what about the Copernican Principle?  If the Milky Way has such an unusual satellite population, do we need to discard or revise the idea that we don't have a privileged view of the universe?  Not necessarily, says Gerke. In essence, the Copernican Principle implies that the Milky Way should look like a galaxy chosen at random from the general galaxy population.  But since galaxies, like people, are extremely diverse in their properties, it should not be surprising if our particular home galaxy is a bit unusual in at least one respect, just as a randomly selected person would probably have at least one rather odd feature.  In this sense, the Magellanic Clouds may be like the Milky Way's big ears or crooked nose: a bit strange, to be sure, but not necessarily any weirder than what you might find in any other galaxy.
 
This work is based on three papers, one published in the Astrophysical Journal (2011, ApJ, 733, 62) and two submitted to that journal which are available from astro-ph at arXiv:1011.6373 and 1101.2203.  An explanatory video with further information is available from http://risa.stanford.edu/milkyway/.
 
Science Contact:
Brian Gerke
KIPAC
bgerke@slac.stanford.edu
 
Tidbit Author: Brian Gerke and Jack Singal