In The End

As galaxies age, they tend to become quiescent. The bulk of their free gas has been used up, there is little or no new star formation, and their existing stars tend to be more aged and red in color. When we look at nearby galaxies, we see many large elliptical with these very characteristics. Astronomers sometimes refer to these galaxies as “red and dead”

But when we look further away, we are looking back in time. We see younger galaxies, laden with gas, stellar nurseries and bright, blue, young stars. By comparing the younger more distant galaxies with our closer and older neighbors, astronomers can study the course of the evolution of the Universe.

One of the aging factors in galaxies, aside from the passage of time, seems to be galactic interaction. When galaxies collide and merge, it usually triggers a period of intense star formation, using up the supply of the gas clouds within them. After this new star genesis slows or ceases, the merged galaxy then slides into old age, getting redder as its stars grow older.

This Hubble image shows an elliptical galaxy catalogued as 2MASX J09442693+0429569 in a transitional phase of this process. The galaxy has an irregular shape and tail-like features extending from it, signs that it has recently undergone a merger. Astronomers studying the light from the galaxy see no sign of ongoing star formation, another indication of a collision that disrupted gas clouds, triggered star formation and used up the supply of star forming material. Observations suggest that star formation ceased only within the last billion years.