Elliptical Galaxies
Elliptical galaxies have obtained their designation because their stellar light exhibits an elliptical shape, without any salient structures within the ellipses. In extreme cases, e.g. in the central regions of galaxy clusters, they can reach much larger sizes and become much more massive than spiral galaxies. Elliptical galaxies have formed via gravitational mergers of spiral and dwarf galaxies. In this process, the gas is strongly stirred up and compressed in the merging galaxies, which gives rise to intense star formation within a short period of time. The most massive stars explode as supernovae after a short time (a few million years), which quickly heat up the left-over gas. Hence, elliptical galaxies possess only hot gas, out of which stars cannot form anymore. Hence, star formation in elliptical galaxies has ceased!
NGC 4486 (M 87)
Maksutov-Newton, Canon 1100Da,
6.2 hours
The biggest and most massive galaxy in the Virgo Cluster is the elliptical galaxy M
87. With its ca. 2.5 thousand billion stars it is one of the biggest galaxies in the known universe. Over billions of years, it has swallowed quite a few galaxies of different size via gravitational merging. A supermassive black hole (ca. 6.5 billion solar masses) is residing in the galaxy centre. Jets with highly relativistic particles are ejected from its immediate surroundings, which emit synchrotron radiation. This radiation can be seen in the radio regime as spectacular structures (see Galaxy Overlays). The above optical image also shows a hint at one of the two jets. Close to the centre, a protusion is visible in the direction of about 2 o'clock. The light of this structure does not originate from stars, but is rather synchrotron radiation from relativistic electrons, which are moving in a magnetic field at nearly the speed of light. Only one of the jets is seen here, namely that moving towards us - it is relativistically amplified by so-called Doppler boosting. The counter-jet, which ought to be seen in the direction of about 8 o'clock, is strongly dimmed by the same process as it moves away from us, i.e. by Doppler deboosting, and is hence not visible in the optical regime. The small insert at the top right shows a black-and-white version of the central region of the galaxy after subtracting the entire (elliptically distributed) stellar light, with the residual jet emission remaining.
NGC4374 (M 86)
Maksutov-Newton, Canon 1100Da,
18.8 hours
M 84 is one of the very bright and massive elliptical galaxies in the Virgo Cluster. Its distance is about 43 million light years. Its centre hosts a supermassive black hole, from which - similar to M 87 - highly energetic particles are ejected as twin jets. In the radio domain, these jets are observed as synchrotron-emitting structures (see Galaxy Overlays). The elliptical distribution of the stellar light is featureless.
NGC4406 (M 84)
Maksutov-Newton, Canon 1100Da,
18.8 hours
M 86, too, is a rather bright and massive, strongly elliptical galaxy in the Virgo Cluster. Its distance is about 53 million light years. Contrary to M 84 and M 87, it lacks any radio radiation.
NGC 4594 (M 104)
Maksutov-Newton, Canon 1100Da,
15.1 hours
M
104, the so-called 'Sombrero Galaxy', is about 30 million light years distant. Its morphology is eye-catching: a nearly spherical distribution of stars (a 'halo'), as is usually known in elliptical galaxies (which is why I put the galaxy into this category), encloses a disk of dust and gas. New stars are still being formed within this disk, whereas star formation has ceased in the halo. This giant halo has most likely formed billions of years ago via the collision of two or more galaxies. The dust/gas disk is a relic of such a period of gravitational galaxy merging. The image on the right shows the superposition of the colour image with a contrast-enhanced black-and-white negative. The stellar halo becomes spherical! Its diameter measures about 200000 light years, twice that of our Milky Way!