A Hubble image of
a field of distant galaxies. The CANDELS project has analyzed these and
other data to study how galaxies form and evolve in the early universe.
Credit: NASA/Hubble. Low Resolution Image (jpg)
Galaxies today come very roughly in two
types: reddish, elliptically shaped collections of older stars, and
bluer, spiral shaped objects dominated by young stars. The conventional
wisdom is that the two
types are related to one another, ellipticals representing an older,
more evolved stage of galaxies. Astronomers have discovered during the
past decade that these two categories seem also to apply to galaxies in
the early universe. In particular, galaxies so distant from us that
their light has been traveling for about eleven and one-half billion
years, 84% of the age of the universe, also generally fall into these
two groups.
A major puzzle about these early galaxy types involves their specific
properties: Red elliptical galaxies today are generally large in
diameter, but in the distant cosmos the corresponding galaxies are much
smaller - perhaps five times smaller than local ones of the same mass
and much smaller than their blue, star-forming colleagues. If galaxies
gain in mass with time, through collisions or other processes, they
would be expected also to increase in size with time. Therefore, if the
early red galaxies really do represent older stages of bluer objects,
then as a class they should be more massive and larger, not smaller.
The CANDELS project (Cosmic Assembly Near-Infrared Deep Extragalactic
Legacy Survey) has acquired a very large database of optical and
infrared observations of distant galaxies. Writing in one of their new
papers this month, CfA astronomer Matt Ashby and the CANDELS team
propose a solution to the dilemma. They studied a set of galaxies whose
light has been en route for between about nine and twelve billion years.
Based on the measured rates of star formation in blue galaxies inferred
from the radiation, they conclude that they have undergone collisions
that induce star formation. That's what makes them shine exceptionally
brightly. After a billion years or so, however, these starbursts leave
many of them depleted in fuel, and as a result the galaxies shrink in
size to become the compact red galaxies that are so puzzling. In the
later universe star-forming galaxies have grown to considerably larger
sizes, and their star formation is consequently spread across much
larger volumes, so that the same quenching mechanism does not take
place, leaving them to retain their sizes as ellipticals.
