Milky Way Galaxy Seeks New Dwarf Companion

By Doug on July 18, 2008 at 12:00 pm | In Blog Posts |

It is interesting times for hunters of low-luminosity galaxies in the Local Group - our local concentration of galaxies. The low-hanging fruit has all been picked. Anything you could discover by visually examining a Schmidt plate or CCD mosaic image has been found. Is it the end of times for explorers of the Local Group?

Dr. Vasily Belokurov of Cambridge University and his collaborators reveal the answer to be no in their recent submission entitled “Leo V: A Companion of a Companion of the Milky Way Galaxy” found
here. Interestingly, the head-shot of this newly discovered dwarf galaxy shows … nothing! The foreground field stars greatly outnumber the few much more distant evolved stars in the cluster and there is no visible concentration on the image.

Seems like a hard sell, but it isn’t. Belokurov’s team has mined a vast and very influential database of object brightnesses, colors, and spectra known as the Sloan Digital Sky Survey and then obtained follow-up observations to confirm their discovery. The database provides the ability to select out stars from the only-slightly-fuzzy, much-more-distant background galaxies and also precise color information. This latter capability was key to the success of locating the new dwarf galaxy - its stars were sufficiently metal-poor that many of the evolved ones were so-called “blue horizontal branch” stars. The concentration of BHB stars on the sky does make this patch of sky stand out - it indicates a grouping of stars of similar age and metal abundance.

The final clincher was getting radial velocities for the handful of brightest, coolest stars in the galaxy - the so-called “red giant branch” (RGB) stars. Objects within the dwarf galaxy are only moving a few km/sec relative to each other. On the other hand, foreground field stars from the Milky Way have radial velocity differences of tens to hundreds of km/sec since they are orbiting the Milky Way’s much more massive center at various distances and on a variety of orbits. Belokurov’s team obtained 247 spectra on the 6.5m MMT telescope on Mount Hopkins in Arizona and found five RGB stars close to the dwarf galaxy center with near-identical velocities. Score!

The new pup - not to be confused with pope - christened “Leo V” is 180 kpc (about 600,000 light years) distant and is moving away from us at a speed of 132 km/sec. But that radial velocity includes a component of the Sun’s motion around the center of the Milky Way galaxy. When solar orbital component is removed, Leo V ends up moving only about 60 km/sec relative to the center of mass of Local Group galaxies.

Intriguingly, Leo V is found projected on the sky only three degrees away from a very similar beast with the very distinctive name Leo IV - also discovered by Dr. Belokurov and his collaborators! The researchers point out that the proximity of these objects in Local Group space may foreshadow additional discoveries along a stream of such apparently faint and intrinsically low-luminosity objects which could then inform our ideas of the formation of the Milky Way galaxy.

I must point out another very cool use of Local Group galaxy data. You may recall from high school or college physics that if you know velocities and positions of objects and the forces acting on them, you can predict where they were in the past and where they will be in the future. For galaxies, the force is gravity and you can get good estimates of their masses from their brightnesses (corrected by a dark matter fraction). You can also assume that at some time around 10 billion years ago, all of the present-day galaxies were essentially at rest with respect to each other. Given 1) their three-dimensional positions now, 2) their radial velocities now, and the assumption of zero initial velocities way back when, you have enough “boundary conditions” to solve each of their paths in the interim. But - and this is a big but - there is no simple way to do it! One has to try out many, many configurations of starting locations, run the system forward and see if the radial velocities and positions you end up with are similar to those we see now. If not, throw the galaxies back in the box, shake and try again! If you are interested in such games, check out the references and citations in this paper. Leo V can now be added to the list of objects used, so we now have an excuse to re-run these models!