Sky and Telescope - July 2017 - 22
by Keith Cooper
Half the stars in the universe live in "dead" galaxies, where star formation
has ground to a halt. What great force has stopped it in its tracks?
p BLACK DEATH The supermassive black hole ensconced in a galaxy's center may ultimately be responsible for its demise.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
altogether; they're now composed of stars that are cool, red,
and old compared to more massive and short-lived stars. Such
galaxies are evocatively described as "red and dead."
Part of the reason for this is that there's less gas available
for star formation today than there was 10 billion years ago.
Yet three-quarters of red-and-dead galaxies have no shortage of gas - it's just that it's too warm to make stars. These
galaxies contain half the stars in the universe yet will make
hardly any more - something seems to have happened to
them. An unknown force, powerful enough to affect entire
galaxies, is on a killing spree. Three suspects have been implicated, each with its own compelling means and opportunity.
Death by Blunt Force
Most red-and-dead galaxies are found in clusters, which house
ellipticals near their centers and dusty, disk-shaped lenticular
galaxies cruising through their suburbs. Could it be that the
environment around these galaxies determined their fate?
ESA / ATG MEDIA L A B
If we could travel back in time 10 billion
years, we would experience a universe at the height of fertility. Galaxies underwent tremendous growth spurts as stars
formed at prodigious rates: Hundreds or even thousands of
them ignited each year in every galaxy. Deep within these
galaxies, black holes turned into cosmic lighthouses, gorging
on so much gas that the material they were trying to consume
grew bright enough to be seen across the universe as quasars.
Since then, there's been a decline. Many black holes have
shut off their surrounding lighthouses, while in general
the creation of new stars has become less efﬁcient. Our
own Milky Way struggles to turn dense clouds containing
hundreds of solar masses of molecular gas into a measly
Sun's worth of stars each year. But at least our Milky Way is
still forming stars. Some galaxies have ceased star formation