The moment a galactic light went out
The further away scientists look, the further back in time they see. Astronomers use this method to study the evolution of the Universe by analyzing nearby and more distant galaxies and comparing their features.
NASA's Hubble is particularly well suited for this type of observation because of its extremely high resolution and strategic position above the blurring effects of the earth's atmosphere. This has allowed it to detect many of the most distant galaxies known, as well as making detailed images of faraway objects.
Comparing galaxies in the distant past with those around us today, astronomers have observed that the nearby galaxies are far quieter and calmer than their distant brethren seen earlier in their respective lives.
Indeed, nearby galaxies (although not our Milky Way) are often large, elliptical galaxies with little or no ongoing star formation, while their stars tend to be elderly and red in color. As astronomers would say, these galaxies are "red and dead."
However, this isn't the case for galaxies further away, which typically show more vigorous star birth. Perhaps this is because as the Universe has aged, galaxies have often collided and merged together, thereby disrupting gas clouds within them.
A merger will usually be a trigger for such intense star formation that the supply of gas is used up, meaning there is none left for additional star formation. The merged elliptical galaxy then creeps into old age, getting redder as its stars get older. This is expected to happen to the Milky Way when it merges with the nearby Andromeda Galaxy, some four billion years from now.
The galaxy in the image above, catalogued as 2MASX J09442693+0429569, marks a transitional phase in this process as young, star-forming galaxies settle to become massive, red and dead.
The galaxy has tail-like features extending from it, typical of a galaxy that has recently undergone a merger. Studying the properties of the light, astronomers see no sign of ongoing star formation; meaning the merger likely triggered an event which used up all the gas.
However, the observations suggest star formation was quite active until the very recent past, ceasing only within the last billion years.