The spiral rebuilding scenario

Schematic representation of the newly proposed scenario of "spiral galaxy rebuilding": galaxies collide (1), then merge (2), inducing a burst of stellar formation activity. After the merging, the gas and the stars fall towards the centre in a very compact structure (3). Part of the gas which did not fall back initially, gradually rebuilds a disc around the compact structure, making a new spiral galaxy (4 and 5). The images are pictures of distant galaxies at various redshifts taken by the Hubble Space Telescope. The central panel displays the star formation rate as a function of time. The numbers coincide with the numbers shown on the images.