When one looks at a deep image of the distant universe using the Hubble Space Telescope, a myriad of galaxies fill the field of view. Some galaxies sport elegant spiral shapes, others take on giant 3D oval (ellipsoidal) shapes, and still other have no discernible shape at all.
On small scales galaxies appear to be put down haphazardly with all possible orientations and distances away from us. It is only when one takes a step back to view this panoply of objects thousands or even millions at a time, that we see that the arrangement is far from random.
On larger scales, galaxies form a kind of 3D spider web which we call the “cosmic web.” These objects with 10 billion stars each have a tendency to collect at the junctures or “nodes” of the cosmic web. These galaxies that "grow up in the city" are seen to have a very different course of evolution compared to those that reside in more sparsely populated regions.
Somewhat surprisingly, at some point about 2-3 billion years after the Big Bang, galaxies in clusters suddenly started forming stars at the maximum possible rate. Such “urban”
Galaxies become veritable star forming machines, cranking out 1000 or more stars per year. Such galaxies are caught in a state of near-constant fireworks displays lasting at least 10 million years, as new stars pop into existence at an average rate of three per day!
Interestingly, these objects stop forming stars equally suddenly, through a process astronomers call “quenching," or even "strangulation" (although no one is getting hurt that we know of). After all the dust has settled from this ultra-productive star forming period, these galaxies are found to have used up nearly all of their resource. As a result they will be unable to produce additional stars in their long foreseeable futures.
As an analogy, it is as if galaxies found in such densely-populated "urban" environments undergo a kind of Industrial Revolution. As can happen on this planet too, it is only following such a period that the realization hits that resources were used up too quickly to make a healthy recovery.
Understanding the cause of this enhanced period of star formation in galaxy clusters is important for piecing together the history of the Milky Way. In the next blog, we will trace a more likely history for the Milky Way, which after all 'grew up' more in the suburbs.