Galaxy Merger Identification
Some good news and some bad news: first the bad, our Milky Way is going to hit the Andromeda galaxy in the future. The good news is that it will be so far off in the future, there is no need to add it to your calendar. But it will mess the Milky Way and Andromeda up. No more spiral galaxies but one big elliptical is the prediction.
Galaxy collisions are a major renovator of galaxy populations. We’ve suspected it, we have seen examples. The question remains: how often do they happen and how much of an impact. And that is where galaxy evolution and statistics come in. The further we look back in time, and the more red our observations, the better we can estimate how many mergers are happening at any time during the age of the Universe.
How to do so is weirdly harder than it sounds. It’s a collision between two Milky Way sized things! How do you miss that? Quite easily as a matter of fact: they hardly look perturbed until the moment of close encounter.
There are two major techniques to identify merging galaxy pairs: first by identifying pairs of galaxies close enough together that they will likely merge. Benefits are that you can set strict thresholds, motivated by merger simulations. No do they or don’t they. The drawback is that you need an accurate distance. Lots of galaxies are close together on the sky but completely separate along our line of sight. SO accurate distances are the name of the game.
The other major technique looks for a perturbed looking fraction of galaxies. These rely on morphology indicators to identify off-looking galaxies. This can be a little subjective as you can imagine. As much as we have tried to quantify morphology of galaxies, it really depends on the wavelength observed (and originally emitted) and what “perturbed looking” really means. It also means your technique is only sensitive to much closer encounters or even the aftermath.
Enter JWST. Now in the near-infrared, we can examine galaxies in the light they emitted in the optical all the way back to redshift z=5 so most of the history of the Universe. Direct comparison with the local examples!
The paper that appeared this week that I was on used the first technique however! Pairs of galaxies! Using a very clever pairing with the photo-z distances. If there was a clear overlap, it was a pair!
So with just photo-z redshifts, one can create a pair catalog. At a given redshift, you can now reasonably say what fraction of galaxies is undergoing a merger (likely in the next few giga-years aka billions of years).
That gives us an idea how many galaxies are typically getting ready to merge. But how fast is that happening? Is it so slow that the fraction we saw at redshift 4 (12 BILLION years ago) are still going at it? One needs an idea of a timescale to convert it to a merger rate.
This last figure has done exactly that: converted the fraction to a merger rate. And the merger rate in the early Universe is much higher than it is now. It is pushing a few per Gyr! By the time we get to the Milky Way and Andromeda (the here and now) it happens maybe every 100 Gyr?
There were a lot of things to check for this result and the main author Qiao Duan has done a fabulous job of it. It is all clearly laid out and explained. I contributed some minor feedback. Neat paper, just shows what we’ll learn with JWST.