Hidden in a dark cloud
The subject of this week’s Picture of the Week from Hubble is the spiral galaxy IC 4633, located 100 million light-years away from us in the constellation Apus. IC 4633 is a galaxy rich in star-forming activity, as well as hosting an active galactic nucleus at its core. From our point of view, the galaxy is tilted mostly towards us, giving astronomers a fairly good view of its billions of stars.
However, we can’t fully appreciate the features of this galaxy — at least in visible light — because it’s partially concealed by a stretch of dark dust. This dark nebula is part of the Chamaeleon star-forming region, itself located only around 500 light-years from us, in a nearby part of the Milky Way galaxy. The dark clouds in the Chamaeleon region occupy a large area of the southern sky, covering their namesake constellation but also encroaching on nearby constellations, like Apus. The cloud is well-studied for its treasury of young stars, particularly the cloud Cha I, which has been imaged by Hubble and also by the NASA/ESA/CSA James Webb Space Telescope.
The cloud overlapping IC 4633 lies east of the well-known Cha I, II and III, and has been called MW9 or the South Celestial Serpent. A vast, narrow trail of faint gas that snakes over the southern celestial pole, it’s much more subdued-looking than its neighbours. It’s classified as an integrated flux nebula (IFN) — a cloud of gas and dust in the Milky Way galaxy that’s not near to any single star, and is only faintly lit by the total light of all the galaxy’s stars. Hubble has no problem making out the South Celestial Serpent, though this image captures only a tiny part of it. For a showy astronomical object like IC 4633, among the South Celestial Serpent’s coils clearly isn’t a bad place to hide.
[Image Description: A spiral galaxy seen nearly face-on. The disc is made up of many tightly wound spiral arms. They contain small strands of reddish dust, near the centre. On the left side, the disc features glowing patches of star formation. The whole right side, and part of the centre, is obscured by a large cloud of dark grey gas which crosses the image.]
