Discovery News - 2010 Mar 03
Judging from the latest Hubble image released today, some galaxies are a lot like schoolyard bullies. I mean, there are more than a hundred billion galaxies in our observable universe, and most of them are fairly "social": they clump together in galactic clusters, possibly forming their own cliques (jocks, nerds, stoners, brainy over-achievers, and the ubiquitous Gothlings).
And then there are the loners: galaxies that don't seem to have many neighboring galaxies to socialize with, poor things. They just hang out in a comparatively empty section of the night sky, surrounded by dark matter (which isn't very sociable) and hot gas, pretending it's all cool, and really, they like being alone... while secretly hungering for peer approval.
But before you feel too sorry for them, consider this: their isolation may well be their own damn fault. One of the prevailing models for galactic evolution is the so-called "bottom up" model, wherein galaxies are pulled together by gravity and over time, the smaller ones get cannibalized by the bigger ones. In fact, Ian O'Neill reports today on new evidence favoring this model for our own Milky Way. Maybe those loner galaxies aren't so much hungry for approval, as for the tasty stars belonging to any smaller and weaker galactic brethren that get too close. That's definitely a bit of a social liability (just ask the zombies). No wonder they're largely alone in the sky.
The latest Hubble image showcases ESO 306-17, a large elliptical galaxy that belongs to one such "fossil group" (the term astronomers use to refer to loner galaxies). The name arises because scientists aren't sure if these galaxies really are fossils and represent the remnants of a once vibrant galactic community, or whether they offer compelling evidence that ESO 306-17 is a ravenous cannibal that devoured its closest neighbors -- an extreme version of what may have happened in the Milky Way.
Truly, gravity is a harsh mistress; there are no cosmic bodyguards to protect the small from the large in the great big universal schoolyard. But there are glimmers of hope for the put-upon smaller galaxies, or at least their remnants. Zoom in closely enough on ESO 306-17 and you can barely make out faint clusters of stars through the galactic halo. These are globular clusters: stars that have banded together to fight off the larger "bully" galaxy and avoided being completely cannibalized. Astronomers are hopeful they may even be able to find a few ultra-compact dwarf galaxies nearby -- the core that is left when smaller galaxies are consumed by larger ones. Intriguing evidence that even in our universe, it's possible for a smaller galaxy to find the strength to ward off schoolyard bullies.