NGC 4394 is the archetypal barred spiral galaxy, with bright spiral arms emerging from the ends of a bar that cuts through the galaxy’s central bulge. These arms are peppered with young blue stars, dark filaments of cosmic dust, and bright, fuzzy regions of active star formation. At the centre of NGC 4394 lies a region of ionised gas known as a LINER. LINERs are active regions that display a characteristic set of emission lines in their spectra — mostly weakly ionised atoms of oxygen, nitrogen and sulphur.
Although LINER galaxies are relatively common, it’s still unclear where the energy comes from to ionise the gas. In most cases it is thought to be the influence of a black hole at the centre of the galaxy, but it could also be the result of a high level of star formation. In the case of NGC 4394, it is likely that gravitational interaction with a nearby neighbour has caused gas to flow into the galaxy’s central region, providing a new reservoir of material to fuel the black hole or to make new stars.
