Thursday, 9 April 2015

Featured Object - The Needle Galaxy

Galaxies are often very faint objects. It can be difficult to see the fine detail without very long exposures. Not the case with edge-on galaxies. With the stars of the galaxy in-line, the galaxies appear much brighter (though with less area).

Here we can see NGC 4565, otherwise known as the needle galaxy. This galaxy is easy to observe not only because it is edge on but also because it is a very bright galaxy to begin with. It is more luminous than either the Milky Way or the Andromeda Galaxy.

Being edge-on, it can be quite difficult to say very much about it's structure, other than it is a spiral galaxy (since it is a flattened disk). Using precise measurements of the brightness as well as the rotation rates of stars at different distances from the centre of the galaxy, it is possible to infer some things. The luminosity profile of the central bulge suggests that this is a barred spiral galaxy (like our own).

It's central bulge is also slightly box shaped (rather than spheroid). This is another feature it shares with our galaxy and may also be further evidence that this is a barred spiral galaxy as there is some evidence that these two features may be linked.

The most obvious feature of the galaxy is its dark band of dust, obscuring light around the edge of the galaxy. This is a common feature of edge-on galaxies. It also means that any observer within NGC 4565 would be unable to see the Milky Way as our galaxy would be obscured by that dust, just as certain areas of our sky are blocked by the Milky Way's dust lanes.

Those interested in other edge-on galaxies might like to try:
  • NGC 3628 (visible November-June)
  • NGC 4244 (visible November-June)
  • NGC 5907 (visible December-August)
  • NGC 891 (visible June-February - so not at time of writing)

Observing Tips

  • Use Galaxy Cam
  • Exposure times of 120 000 ms or higher
  • A clear/mono filter will yield the best detail