blue compact dwarf galaxy (BCD)
The lowest luminosity type of starburst
galaxy. A blue compact dwarf (BCD) is a small galaxy, about one-tenth
the size of a typical large spiral such as the Milky Way, that appears blue
by virtue of containing large clusters of hot, massive stars, which ionize
the surrounding interstellar gas with their intense ultraviolet radiation.
These massive blue stars are very young by stellar standards – under
ten million years or so old. They were created in a huge starburst, a violent
episode of star formation that in
some cases engulfs an entire galaxy.
|NGC 1705, a very active blue dwarf compact galaxy
at a distance of about 6 Mpc. It hosts a young (about 10 Myr) super
star cluster with a mass of around a million solar masses, which may
be the main source of mechanical energy for the outflow. This picture
mixes three images: neutral Hydrogen (HI) in blue, broad band optical
in green, and ionized hydrogen in red. Image credit: Gerhardt Muerer
Observations of the young universe show a much larger population of blue
galaxies at early epochs, suggesting that they might play a key role in
the early history of galaxy formation.
Local BCDs, such as Haro 3 (NGC 3353), may be the galactic equivalent of
the coelacanth – primeval structures surviving into a mature cosmos
that form laboratories for the study of galactic evolution.