The toughest thing about studying galaxy evolution is that we don’t get to watch it happen (kinda like biological evolution). Telescopes only reveal galaxies at one instant of their lives, and from this we must use “context clues” to help us make educated guesses about how they got where they are, and why they look the way they do.

Fortunately, telescopes are not the only tools at our disposal.

Using computers, we can generate hypothetical histories of simulated galaxies. Using models and assumptions, we can similarly try to infer from each galaxy’s data snapshots various facts about its past, such as how old it is, or (maybe) how many stars it has formed over various intervals of time.

Below are two movies showing how three pretty important characteristics of at least one sample of galaxies might’ve changed over the past ~11 billion years (about 80% of the age of the Universe). The movie on the left shows how galaxies’ star formation rates (SFRs; how many stars a galaxy is currently forming) changes as a function of its current stellar mass (M; how many total stars it has formed previous to that point). I’ve colored the trajectories of things that end up being the same mass as today’s Milky Way, i.e., the galaxy we live in. The colors represent when a galaxy reached half its mass today, with things that grew up early being red, and things that grew up late being blue.

You can see the rainbow of paths that these galaxies will take to their final destination, and, while they all end up at about the same mass, many billions of years ago, they spread out over a huge range. Some galaxies raced ahead early on. These tend to have low star formation rates today, and so appear “red and dead.” Conversely, some galaxies grew more leisurely. These tend to have higher star formation rates, and are therefore bluer, like our own Milky Way.

The movie on the right shows how the same set of galaxies might have evolved in size (r, for “radius”) as they grew in mass (same x-axis as the other movie). Here, objets that end up at the Milky Way’s mass are in grey, and you can see that the diversity in their SFR histories is also reflected in their size histories. Not surprisingly, all galaxies get bigger in size as they get more massive, but, interestingly, at any given mass, galaxies were smaller in the past. Indeed, the smallest objects tend also to be the oldest; i.e., the “red and dead” ones/the objects with red tracks in the other movie. You can see this clearly from the colored bands, which show the size–mass relation for red and blue galaxies over time from this paper; the orange band (red galaxies) is lower than the blue one (for blue galaxies) over most masses.

It’s possible that the physics that controls the diversity of mass-growth paths in the left hand movie might be linked to the fact that small galaxies are old through the matter density of the Universe (see these two papers, which kinda just reiterate some awesome stuff from the ’60s). Because this was always higher in the past, there was simply more gas to be turned into stars per unit volume, so the same mass could fit into a smaller sphere a long time ago than it can today. There’s probably a lot of other forces going on, but I suppose it’s part of my deep intellectual interest to figure out how much of the story of galaxy evolution is due to just that simple fact.