Most big galaxies contain big black holes.
Not just big, supersized, with millions of
times the sun's mass. Some of these black
holes are actively devouring gas. This drives
particle jets that can spew matter millions
of light-years into space, and it also makes
the holes a source of penetrating, or hard,
X-rays. At these energies, the sky glows in
every direction, even far away from bright
sources.
Astronomers have long suspected that active
supermassive black holes in galaxies were
responsible, but they just couldn't find enough
of them to account for the X-ray glow, especially
the peak of the energy spectrum. Now, astronomers
using NASA's Swift satellite confirm that
a largely unseen population of black-hole-powered
galaxies is out there.
There are so many that scientists say they
might fully account for the cosmic X-ray background.
What emission we detect from an active black
hole is a function of how we see it, whether
we're looking face-on and into one of it's
jets, or viewing it from the side, through
the disk of gas and dust that surrounds it.
The brightest active black holes, which include
quasars and blazars, are those we see face-on.
But as the viewing angle increases, the surrounding
disk absorbs increasing amounts of radiation.
Astronomers have always assumed that many
active galaxies were oriented edgewise to
us, but because the disk of gas smothers most
of their X-rays, these sideways black holes
just weren't detected.
And that's where Swift comes in. Since 2004,
the satellites Burst Alert Telescope has been
building up the largest, most sensitive X-ray
map of the sky. Using these data, astronomers
found that the most heavily absorbed galaxies
create the energy peak in the cosmic X-ray
background.
What does it all mean? When the universe was
about half its present age, about 7 billion
years ago, galaxies crashed together more
frequently and these collisions produced gas
rich galaxies with heavily obscured black
holes. The Swift survey shows that galaxy
mergers helped activate these black holes
by feeding them torrents of fresh gas.
The new findings are consistent with idea
that the X-ray background peaked around this
time, when our own galaxy was young and before
our solar system was born.
