NGC 694

Transcription

Hubble has snapped a spectacular view of the largest “player” in the Leo Triplet, a galaxy with an unusual anatomy: it displays asymmetric spiral arms and an apparently displaced core. The peculiar anatomy is most likely caused by the gravitational pull of the other two members of the trio. The unusual spiral galaxy, Messier 66, is located at a distance of about 35 million light-years in the constellation of Leo. Together with Messier 65 and NGC 3628, Messier 66 is one third of the Leo Triplet, a trio of interacting spiral galaxies, part of the larger Messier 66 group. Messier 66 wins out in size over its fellow triplets — it is about 100 000 light-years across. Messier 66 is the proud owner of exclusive asymmetric spiral arms which seem to climb above the galaxy’s main disc and an apparently displaced nucleus. This asymmetry is unusual; most often, dense waves of gas, dust and newly born stars wind about the galaxy’s centre in a symmetric way. Astronomers believe that Messier 66’s once orderly shape has most likely been distorted by the gravitational pull of its two neighbours. Hubble has imaged Messier 66’s striking dust lanes and bright star clusters along the spiral arms in fine detail. Star clusters — pictured in the blue and pinkish regions of the image — are key tools for astronomers since they are used as indicators of how the parent galaxies assembled over time. Messier 66 boasts a remarkable record of supernovae explosions. The spiral galaxy has hosted three supernovae since 1989, the latest one occurring in 2009. A supernova is a stellar explosion that may momentarily outshine its entire host galaxy. It then fades away over a period lasting several weeks or months. During its very short life the supernova radiates as much energy as the Sun would radiate over a period of about 10 billion years. A recent NASA/ESA Hubble Space Telescope image captures what appears to be one very bright and bizarre galaxy, but is actually the result of a pair of spiral galaxies, like our own Milky Way, smashing together at incredible speeds. This new image of NGC 2623, which is located in the constellation of Cancer, was taken by Hubble’s Advanced Camera for Surveys, or ACS, before the recent Hubble Servicing Mission. Once two distinct galaxies, NGC 2623 is a nearly complete merger, with elements of both original galaxies affecting its appearance and behaviour. Colliding galaxies engage in a powerful exchange of material — they literally throw matter and gas toward one another as they race together. Stretching out from the centre of NGC 2623 are two tidal tails of young stars showing that a merger has taken place. The dramatic exchange of mass and gases initiates star formation, seen there in both the tails. The prominent lower tail is richly populated with bright star clusters. These star clusters may have formed as part of a loop of stretched material associated with the northern tail, or they may have formed from debris falling back onto the nucleus. In addition to this active star-forming region, both galactic arms harbour very young stars in the early stages of their evolutionary journey. The core, or nucleus, of this merged galaxy is exceedingly bright. Often these mergers (including this case) result in an active galactic nucleus, where one of the supermassive black holes found at the centres of the two original galaxies is stirred into action. Matter is pulled toward the black hole, forming an accretion disc. The energy released by the frenzied motion heats up the disc, causing it to emit across a wide range of the electromagnetic spectrum. Although rare in our solar neck of the woods, extremely bright galaxies like NGC 2623 exist at higher redshifts or, in other words, at an earlier stage of the Universe’s evolution when these systems were dominant. Their brightness signals furious activity – new star formation and a rambunctious core. Galaxy evolution is a hot topic in astronomy because it is fundamental to our understanding of the Universe. Studying merging galaxies such as NGC 2623, allows astronomers to witness the shaping of new, combined galaxies and the subsequent birth of stars. This could even provide essential information about our own Milky Way that has cannibalised other small galaxies and could, one day, collide with our neighbour, the Andromeda Galaxy. One of the big mysteries in astronomy is how galaxies grow and evolve over time. Collisions between galaxies are thought to be key events that shape their development. A stunning collection of 59 new images of colliding galaxies has been released to mark the 18th anniversary of the NASA/ESA Hubble Space Telescope. They give us a unique insight into how galaxies merge to form larger galaxies. Astronomers think that the first galaxies formed out of clumps of gas in the early Universe. These proto-galactic clumps then went on to coalesce and to merge together to form bigger and bigger galaxies as time goes on and their gravity attracts each other. So what exactly happens when two big galaxies collide? That is precisely what Hubble is showing us in these new images where it has captured some of these ―merger events in the act. These cosmic collisions beautifully portrayed in the new Hubble images are not as swift and ferocious as we might think. They are not like collisions between everyday objects. They are slow stately affairs and may take hundreds of millions of years to take place. In this series of images, snapshots of different pairs of interacting galaxies are seen representing the different stages in this process — a gradual waltz of stars and gas choreographed by gravity. Despite taking hundreds of millions of years to eventually merge, the two galaxies of a pair actually approaches each other at very high velocities, up to millions of kilometres per hour. Also, because the distances between individual stars are so large, it is actually very rare for two stars to hit each other. The hundreds of million of stars in each of the galaxies all move individually, following the pull of gravity from all the other stars and the dark matter in the galaxies. This interwoven tidal forces can produce the most intricate and varied effects as galaxies pass close to each other and eventually merge. The new collection of Hubble images illustrates how galaxy collisions produce a remarkable variety of intricate and delicately interwoven galactic shapes. Typically the first signs of an interaction appears in the first encounter of the galaxies. The tug of gravity teases out stars and gas from the passing galaxies, often building bridges between them. During subsequent encounters, long streams of gas and dust, known as tidal tails, stretch out and sweep back to wrap around the cores. These long and spectacular tidal tails are the clear signature of an interaction, and can persist long after the main action is over. As the galaxy cores finally approach each other, the gas and dust clouds are buffeted and accelerated dramatically by the push and pull of matter from various directions. These forces can result in shockwaves rippling through the interstellar clouds. Gas and dust are siphoned into the active central regions, fuelling bursts of star formation that appear as characteristic blue knots of young stars. As the clouds of dust build they are heated so that they radiate strongly, becoming some of the brightest infrared objects in the sky. Some of the galaxies show striking highly distorted features, dust lanes crossing between galaxies and long filaments of stars and gas extending far beyond the central regions. We see beautifully interwoven galaxies twisted by these gargantuan encounters. Triggered by the colossal and violent interaction between the galaxies, stars form from large clouds of gas in dramatic and brilliant bursts of stellar fireworks, creating new blue star clusters. Here in this image of NGC 6670 a pair of overlapping galaxies – seen edge-on on the Hubble image – resembles a leaping dolphin. Scientists believe that it has already experienced at least one close encounter and is now in an early stage of a second. The next image shows an astonishing pair of galaxies, composed of spiral galaxies of similar mass and size, NGC 5257 and NGC 5258. The galaxies are clearly interacting with each other creating a bridge of dim stars connecting the two galaxies, almost like two dancers holding hands while performing a pirouette. Both galaxies harbour supermassive black holes in their centres and are actively forming new stars in their discs. This galaxy pair, dubbed IC 694 and NGC 3690, is thought to have experienced one close pass some 700 million years ago. As a result of this interaction, the system underwent a fierce burst of star formation. In the last fifteen years or so six supernovae have gone off in the outer reaches of the galaxy, making this system a notable supernova factory. The image of this beautiful interacting pair shows long tails sweeping out from the centre of the galaxies. These are tidal tails of stars and gas that have been pulled from the distorted discs of the merging galaxies.