The Coma cluster is a large group of galaxies roughly 320 million light-years from Earth. Over 1,000 galaxies are among its members, and Hubble recently focused in on one in particular. Galaxy D100 garnered such attention because it's plunging into the core of the cluster, losing its gas as it does so.
The phenomenon is called “ram pressure stripping” and it is aging D100 prematurely, as the precious fuel for the formation of new stars is taken away. This gas has ended up in a long tail stretching out of the galaxy, which is being compressed by gravity and pressure and forming new stars. In a tiny blue region among the red-looking plume, 200,000 new stars have been born. The observations are reported in the Astrophysical Journal.
"This galaxy stands out as a particularly extreme example of processes common in massive clusters, where a galaxy goes from being a healthy spiral full of star formation to a 'red and dead galaxy,'" lead author William Cramer of Yale University said in a statement. "The spiral arms disappear, and the galaxy is left with no gas and only old stars. This phenomenon has been known about for several decades, but Hubble provides the best imagery of galaxies undergoing this process."
The gas loss began at the edge of the galaxy and is now coming from the central region of D100. Despite that, the tail of gas is quite peculiar and not like your regular clumpy ram pressure stripping. It is neat and thin, stretching for over 200,000 light-years (twice the size of the Milky Way) while being just 7,000 light-years across. This whole process started roughly 300 million years ago.
"The tail is remarkably well-defined, straight and smooth, and has clear edges," explained team member Jeffrey Kenney, also from Yale. "This is a surprise because a tail like this is not seen in most computer simulations. Most galaxies undergoing this process are more of a mess. The clean edges and filamentary structures of the tail suggest that magnetic fields play a prominent role in shaping it. Computer simulations show that magnetic fields form filaments in the tail's gas. With no magnetic fields, the tail is more clumpy than filamentary."
This stunning picture from Hubble has another important feature related to ram pressure stripping, a sort of glimpse of the future if you like. A galaxy called D99, seen in red in the picture, underwent this process long before D100. It is now a red and dead galaxy, a similar fate that awaits D100 in a few hundred million years.