NGC 4654 - An Asymmetric Spiral Galaxy
Hubble recently returned this amazing image of a spiral galaxy 55 million light years from us. At first glance you can see the spiral arms, the dust lanes and the bright blue/pink areas of star formation that go all the way towards the centre.
But look to the back of this image, and you'll spot a distinct lack of dust lanes and stars, it's as if the galaxy had a blanked thrown over half of it.
Two distinct processes are at play here, one is a close neighbour NGC 4639, which 500 million years ago came close and stripped the galaxy of much of the dust and gas as it passed by, on the side where we see a lack of stars.
The other is related to where this galaxy sits, as part of the Virgo supercluster, it's moving through the clusters intergalactic medium, which causes a kind of ram pressure that pulls out the hydrogen gas that normally would be creating those dust lanes and in turn, stars.
However, that doesn't appear to be effecting half of this galaxy, with a fair amount of star formation, meaning the entire galaxy levels out at a normal rate of star formation for a spiral galaxy of it's size.
While scientists aren't entirely certain why this is the case, the fact it has recently undergone a close pass on another galaxy may be responsible, like supernova, it pushes the gas and dust in a wave which then concentrates at the edges, creating a huge amount of star formation as it does. On the galactic scale, it may be responsible for pushing the gas and dust to one side, increasing it's density sufficiently to currently be forming many stars.
If this is the case, then over the next couple of 100 million years, this galaxy will slowly lose it's cold hydrogen, and the move to a lenticular shape, similar to the galaxy it passed by.
