File:Galaxy cluster MACS J2129-0741 and lensed galaxy MACS2129-1 (35317438752).jpg

The galaxy, called MACS 2129-1, is considered ÒdeadÓ because it is no longer making stars. The existence of dead galaxies so earlyÑwhen the universe was just one-quarter its current ageÑhas long been a puzzle, as the Universe at that time was full of gas and at the peak of the cosmic star formation history. The leading theory has been that they formed in galaxy collisions that efficiently drove all the gas into the center of the collision and turned it into stars.

ÒPerhaps we have been blind to the fact that early ÒdeadÓ galaxies could in fact be disks, simply because we havenÕt been able to resolve them,Ó said study leader Sune Toft of the Dark Cosmology Centre at the Niels Bohr Institute, University of Copenhagen. ÒThis new insight may force us to rethink the whole cosmological context of how galaxies burn out early on and evolve into local elliptical-shaped galaxies.Ó

When the universe was just 3 billion years old, half of the most massive galaxies were extremely compact and had already completed their star formation. Astronomers believe that they ultimately grew into the most massive elliptical galaxies seen in the nearby universe today. Scientists theorize they did this through mergers with small companion galaxies, which added to the stars on the galaxyÕs outskirts.