Andromeda is certainly one of the most photographed celestial objects, being the largest and nearest galaxy to our own Milky Way. Nobody, however, has seen it quite like this before. NASA’s Swift satellite, in its tireless pursuit of cosmic explosions, has taken a sensational image of Andromeda in all its ultraviolet beauty. The picture is commendable because it is the sharpest high resolution image in this wavelength of M31, as Andromeda is frequently known due to its position at number 31 in Charles Messier’s famous catalogue. “Swift has revealed about 20 thousand ultraviolet sources in M31, especially hot young stars and dense star clusters,” said Stefan Immer, scientist at NASA’s Goddard Space Flight Center. “The fact that we have mapped the galaxy with three ultraviolet filters is particularly significant. This has allowed us to study the stellar formation processes in M31 in greater detail than in the past.”
M31 is two and a half million light years away (i.e. the distance that light takes two and a half million years to travel); it spreads across 220 thousand light years and contains hundreds of billions of stars. On a clear night, it is visible with the naked eye and is the most distant celestial object visible without instruments. The image of M31 released to the public by NASA is in fact a composition of 330 photos taken between 25 May and 26 July 2008 by the ultraviolet optical telescope on Swift at three different wavelengths (192.8, 224.6 and 260 nanometres). The final mosaic was obtained by assembling a formidable mass of data, equal to 85 gigabytes of images, a job that the NASA computers took 10 weeks to carry out.
Paolo Giommi, director of the ASI Science Data Center, explains why it was decided to observe Andromeda in ultraviolet: “The light emitted in the various energy bands that are now accessible on Earth from space, such as optical ultraviolet, infrared, and X-rays, provide very different information on the celestial sources under observation. In this case the ultraviolet rays emitted in abundance by very young stars give us a clear visualisation of the zones where new generations of stars are forming in Andromeda.”
What have we discovered that is new? What primarily emerges, writes NASA, is the striking difference between the distension at the centre of the galaxy and its spiral arms. The nucleus is smoother and redder because it is crowded with cold, ancient stars. “Very few new stars are formed in this central zone because most of the material necessary for their formation has been depleted,” said Immer. Beyond the nucleus, dense clusters of blue, young and hot stars shine. M31 is a spiral galaxy like the Milky Way and in the same way the disc and spiral arms of M31 contain gas and dust in large quantities. The enormous disc is 150 thousand light years across and is a veritable factory of new stars: “a ring of fire” that churns out stars at an unusually intense rate. However, exactly what stokes this stellar forge is still a mystery. A possible explanation proposed by scientists involves the tides raised by the small satellite galaxies orbiting M31 which reinforce the interactions inside the gas clouds. “Swift is also sounding out the galaxies near M31 to give astronomers a better understanding of the causes of stellar formation,” said Neil Gehrels, the principal investigator.
Swift is a NASA mission with a fundamental contribution from the Italian Space Agency. Italy supplied and calibrated the mirror for the XRT telescope; the scientific software and the archives are at the ASI Science Data Center; and the ASI base at Malindi is the mission’s Earth station, where Italian teams actively participate in the mission’s scientific projects. Since Swift was launched into orbit in November 2004, the satellite has collected more than 400 gamma-ray bursts, high-energy flashes associated with the birth of black holes.