Oy vey
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 from 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 well do average population
properties reflect those of individual systems? Do galaxies evolve in a manner “stable”
enough to support taking this approach at all? I spend most of my time thinking about
these issues and how to get around them — if indeed that’s possible. Science is an
adventure: we might be wrong, but we will learn something by peering into our own
pasts, either way.
To me at least, there’s a life lesson here. Be undaunted by provisos: Look out, look up,
think hard, and reap the rewards.
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As with scars on people, however, certain details of galaxies’ pasts are “encoded” in
astronomical snapshots. ↩
Louis Abramson 