I said here that understanding galaxy evolution entails the work of many people. Why? Because there are big obstacles standing between us and understanding galaxy star-formation histories.
Fundamentally, we are hindered by the fact that we can only observe a galaxy at one point in its evolution. That is, we see only “snapshots” of the lives of galaxies — cosmic time unfolds too slowly for us to measure real-time changes in all but the nearest galaxies. This clearly limits what we can infer: imagine being asked to figure out someone’s life story given just one photograph!1
How we make progress
We try to overcome this problem by exploiting the fact that looking into space is looking back in time: the speed of light is finite, so the farther away a galaxy is, the farther back in time it emitted the light we collect. Thus, if we’re bold (scientists are bold), we can associate distinct galaxies at different distances with different epochs in the evolution of a single (average) galaxy.
Of course, there are many caveats to this approach: How representative are average properties of individual systems? Do galaxies evolve in a manner “stable” enough to support taking this approach at all? Science is an adventure: we might be wrong, but we will learn something either way! So — with these caveats in mind — we look out and attempt to peer into our own pasts.
- As with scars on people, however, certain details of galaxies’ pasts are “encoded” in astronomical snapshots. ↩