Looking past its long spiral arms filled with stars and the dark threads of dust crossing it, your eye might be caught by the shining point at the centre of UGC 3478, the spiral galaxy starring in this Hubble Picture of the Week. This point is the galaxy’s nucleus, and indeed there is something special about it: it is a growing giant black hole which astronomers call an active galactic nucleus, or AGN.
UGC 3478, located in the constellation Camelopardalis, is what is
known as a Seyfert galaxy. This is a type of galaxy with an AGN at its
core. Like all such “active galaxies”, the brightness that you see here
hides a supermassive black hole
at the centre of the galaxy. A disc of gas spirals into this black
hole, and as the material crashes together and heats up it emits very
strong radiation. The spectrum
of this radiation includes hard X-ray emission, which clearly mark it
out from the stars in the galaxy. Despite the strong brightness of the
compact central region, we can still clearly see the disc of the galaxy
around it, which makes the galaxy a Seyfert galaxy.
Many active galaxies are known to astronomers at vast distances from
Earth, thanks to the great brightness of their nuclei highlighting them
next to other, dimmer galaxies. At 128 million light-years from Earth,
UGC 3478 is positively neighbourly to us. The data used to make this
image comes from a Hubble survey of nearby powerful AGNs found in
relatively high-energy X-rays, like this one, which it is hoped can help
astronomers to understand how the galaxies interact with the
supermassive black holes at their hearts.
