|Hubble Goes 'Deep' to Sample Young Galaxies||
In one of its most memorable observations, NASA's Hubble Space Telescope
gazed into a "keyhole" piece of sky and didn't blink for 10 days. The
telescope's unwavering "eyes" uncovered a galactic gold mine: a
bewildering assortment of galaxies stretching back billions of
years. Astronomers dubbed the area the Hubble Deep Field.
How could Hubble top the landmark Hubble Deep Field observations? How
about by viewing a piece of the sky equal to 60 Hubble Deep Fields? The
telescope's powerful Advanced Camera for Surveys did just that, sampling
galaxies in an area over one-third the size of the full moon. The
galactic census is called the Great Observatories Origins Deep Survey
(GOODS), the most ambitious study of the early universe yet undertaken
with the Hubble telescope.
Hubble is one of three "great observatories" involved in this survey.
The other two are the Chandra X-ray Observatory and the Space Telescope
Infrared Facility (SIRTF), which will be launched in August 2003.
Astronomers are using Chandra to search the GOODS fields for the
earliest black holes in the universe. SIRTF will hunt for more stars.
The GOODS survey targeted two seemingly empty spots in the sky -- one in
the northern sky, the other in the southern sky. But Hubble proved that
these small scraps of sky are teeming with galactic life. The telescope
discovered roughly 50,000 galaxies in both regions combined. Astronomers
have identified more than 2,000 of them as infant galaxies, observed
when the cosmos was less than about 2 billion years old. With these
Hubble snapshots, astronomers are creating a scrapbook of galaxy
evolution from infancy to adulthood.
The myriad galaxies in the Hubble Deep Field represented the first big
step for Hubble astronomers to understand galaxy evolution. But studying
galaxy evolution in the Hubble Deep Field is like trying to understand
the population of a country by sampling a small village. Astronomers
don't know if the galaxies in that village are representative of the
universe's galactic population. The GOODS survey, on the other hand, is
akin to sampling the population of a large city to make inferences about
galaxies in the cosmos.
In preliminary results, Hubble astronomers report that the sizes of
galaxies clearly increase continuously from when the universe was about
1 billion to 6 billion years old. (Many astronomers believe that the
universe is about 13.7 billion years old.) GOODS astronomers also find
that the star-birth rate rose slightly between the time the universe was
about 1 billion and 1.5 billion years old, and remained high until about
7 billion years ago, when it quickly dropped. This is further evidence
that major galaxy building trailed off when the universe was about half
its current age.
The growth of black holes in galaxies is part of the galaxy evolution
scrapbook. For the GOODS survey, astronomers used the X-ray "eyes" of
Chandra to hunt for these mysterious objects. One of the fascinating
findings in this "deepest" X-ray image ever taken is the discovery of
seven mysterious black holes that do not correspond to the galaxies seen
in the Hubble image. Astronomers suspect that these objects are the most
distant black holes ever detected. An alternative theory is that the
galaxies in which they reside cannot be seen because they are heavily
enshrouded in dust.
When comparing the Hubble and Chandra fields, astronomers also found
that active black holes in distant, relatively small galaxies were more
rare than expected. This may be due to the effects of early generations
of massive stars that exploded as supernovas, evacuating galactic gas
and thus reducing the supply of gas needed to feed a supermassive black