Free Astronomy Magazine January-February 2015

SPACE CHRONICLES find out how and why galaxies in clusters evolve from blue to red over a very short period of time,” says Fumagalli. “Catching a galaxy right when it switches from one to the other allows us to investigate how this happens.” Observing this cosmic spectacle, how- ever, is no mean feat. The Norma Cluster lies close to the plane of our own galaxy, the Milky Way, so it is hidden behind copious amounts of galactic dust and gas. With the help of MUSE, which is mounted on one of the VLT’s 8-me- tre Unit Telescopes at the Paranal Observatory in Chile, scientists could not only detect the gas in and around the galaxy, but were able to see how it moves. The new instru- ment is so efficient that a single hour of observing time was sufficient to ob- tain a high resolu- tion image of the galaxy as well as the distribution and mo- tion of its gas. The observations show that the out- skirts of ESO 137-001 are already complete- ly devoid of gas. This is a result of the clus- ter gas — heated to millions of degrees — pushing the cool- er gas out of ESO 137-001 as this drives towards the centre of the cluster. This happens first in the spiral arms where the stars and matter are more thinly spread than at the centre, and gravity has only a rela- tively weak hold over the gas. In the centre of the galaxy, however, the gravitational pull is strong enough to hold out longer in this cos- mic tug-of-war and gas is still observed. Eventually, all of the galactic gas will be swept away into bright streaks behind ESO 137-001 — telltale rem- nants of this dramatic robbery. The gas that is torn away from the galaxy is mixed with the hot cluster gas to form magnificent tails extending to a dis- tance of over 200,000 light-years. The team had a closer look at these streams of gas to better understand the turbulence created by the interaction. Surprisingly the new MUSE observations of this gas plume show that the gas continues to rotate in same way the galaxy does, even after being swept out into space. Furthermore, researchers were able to determine that the ro- tation of stars in ESO 137-001 re- mains unchanged. This provides further evidence for the cluster gas, not gravity, being responsible for stripping the galaxy (if gravity were to play a role in the stripping pro- cess, the researchers would have ex- pected to see disruptions within the galaxy). Matteo Fossati (Universi- täts-Sternwarte München and Max- Planck-Institut für extraterrestrische Physik, Garching, Germany) and a co-au thor of the paper concludes: “With the details revealed by MUSE we are getting closer to fully under- standing the processes that go on in such collisions. We see the motions of the galaxy and the gas in detail — something that wouldn’t be pos- sible without the new and unique MUSE instrument. These and future observations will help us develop a better idea of what is driving the evolution of galaxies.” T his view shows how the new MUSE instrument on ESO’s Very Large Telescope gives a three-di- mensional depiction of the galaxy ESO 137-001 as it falls into the vast Norma Galaxy Cluster and is strip- ped of its gas. For each part of the galaxy the light has been split up into its component colours — reveal- ing not only the motions of different parts of the gal- axy and its long tail but also clues to its chemical composition and other properties. At certain wave- lengths the emission from the tail of the galaxy is clearly seen, as well as the different velocities of dif- ferent parts. During the subsequent analysis the astronomer can move through the data and study dif- ferent views of the object at different wavelengths, just like tuning a television to different channels at different frequencies. In this sequence the speed is adjusted to highlight the glow from star formation regions in the tail. [ESO/M. Fumagalli/L. Calçada] T his video zooms in on spiral galaxy ESO 137-001, which is undergoing ram pressure stripping. The se- quence begins with a view of the night sky near the con- stellation of Triangulum Australe (The Southern Triangle). It then zooms through observations from the Digitized Sky Survey 2, and ends with a view of the galaxy obtain- ed by Hubble. [NASA, ESA, Digitized Sky Survey 2. Ac- knowledgements: Ming Sun (UAH) and Serge Meunier] n