Pride and Prejudice and Galaxies
Section 1: A Story Often Told & Section 2: The Damage Parameter
Author’s note: this is a rough draft of the first two sections of a piece, or possibly book chapter, about galaxy interactions. For those following my posts sequentially, this is a short interlude: I hope to return to musings about invisible lines next time.
I cannot fix on the hour, or the spot, or the look or the words, which laid the foundation. It is too long ago. I was in the middle before I knew that I had begun. — Jane Austin
Galaxies are surprisingly social creatures. If you make a map of their locations you find they gather near other galaxies; if you look at their inner structure you find evidence of external influence. For not having mouths or brains, their lives are strongly shaped those around them; galaxy encounters are agents of change in a galaxy’s life. This series of posts considers the social worlds of galaxies and the way their interactions and communities shape their lives.
Section 1: A Story Often Told
Galaxy encounters are quite common, and more common than you might think, given all that empty space out there. A standard argument in textbooks points out that if you shrink the Milky Way down to the size of a melon, Andromeda Galaxy, the closest large galaxy would be just 10 feet away, with a number of small galaxies as close as inches away. That is definitely close enough for a chat.
Still, until the 1970s astronomers weren’t convinced interactions between galaxies were common. They argued that sure, galaxies are relatively close to each other, but just like two people randomly walking around in a sizable room aren’t likely to collide often, we don’t expect these galaxies to either. The problem with this argument is that the motions of galaxies aren’t random. Gravitational forces – both individual attractive forces, and the bulk motions from the beginning of the universe – move galaxies toward each other. Two people in a room that see each other are pretty likely to walk toward each other to say hi (well, mostly. Unless of course it’s your ex, your boss, or worse, your ex’s new fling, your boss. They you probably would leave the room. But galaxies don’t have a choice. Gravity is too strong - they always succumb to the attraction).
And what’s more, galaxies are actually a lot bigger than they look (which is saying something, I know). While their stars might stretch 100,000 light years, their invisible dark matter halos can stretch nearly 10x that, reaching nearly to the nearest neighboring galaxy a couple million light years away. Not a lot of room for personal space, when you think about it like that. So even when two galaxies aren’t visibly interacting, their halos are starting to move them together.
Near the end of my junior year of college I was sitting in a car with a good friend of mine, maybe 2 or 3 in the morning, after just dropping off another friend. The dynamics of the situation – boy and girl alone in a vehicle, late at night, slightly slaphappy, tired, and me coming off a rather nasty breakup – forced our hand:
“Just to be clear, I don’t have any interested in dating.”
“Nope, me either. Not in a million years actually.”
“Ok, good.”
“Yup, we’re cool.”
A year later we’d be dating, two and a half years later we’d get married. I guess the point is that we’re often closer than we think, moving toward each other in ways we don’t expect. At least that’s how it is for galaxies, and how it was for us.
Section 2: The Damage Parameter
Galaxy interactions come in many different forms. The most basic distinction is encounters that result in mergers, and encounters that don’t. Sometimes galaxies meet, and then continue on their way. And other times they circle back, around and around, each other in majestic messy swirls until they eventually combine into one larger, brighter galaxy. It’s like the universe’s version of a romance plot, and can be just as intriguing - will NGC 3077 end up with M 81? Will Elizabeth end up with Mr. Darcy? Is it, after all a truth universally acknowledged that every galaxy finding itself in possession of a good number of stars will be in want of a good merger.
So what determines if two galaxies end up together in the end? Just like in a good Jane Austin novel, there are a lot of forces at play. Astronomers quantify the impact that any given encounter will have on the galaxies involved with something called the “damage parameter.” The more extensive the interaction, the larger the damage parameter. There are two key factors that go into calculating this damage. The biggest factor is proximity. The closer two galaxies come to each other, the more they can affect each other. This of course makes sense. Closer galaxies exert more force on each other, they pull more, and exchange more. There’s a reason Sophia drew me a picture of a heart with locks and keys on it, and then another heart broken. It hurts more the closer we get.
But the other major factor is time. A high speed impact, where two galaxies spend just a cosmic evening together will deal far less damage than a slow, long interaction. I barely remember that two-week thing with Emma – was it even a thing? – but I still dress differently, think differently, act differently as a result of dating Elena - my inseparable partner for 2.5 college years. And time is really about relative speed, isn’t it? We when a galaxy is spinning the same way it swings around another galaxy it keeps the same part of it facing that galaxy longer. We call this a prograde interaction. And these really reshape a galaxy. Those stars held next to the other galaxy are like food for the taking. Here’s my heart, have a chomp. If they spin the other way then you only get little glimpses of different parts, it’s harder to penetrate, to pull me apart. The retrograde encounter is a bit more of a game of catch me if you can. Since the galaxy is turning opposite the direction of the encounter, the galaxies spend comparatively little time “face to face,” each turning its face away, in embarrassment, or pain, or laughter - I’m not sure. Regardless, with less eye contact, Those short moments, while impactful, are more easily forgotten, serving a little less damage than a sustained stare.
High damage parameter encounters almost always result in mergers of galaxies. Low damage parameter interactions usually mean a high speed fly by, where each galaxy eventually goes its separate way. And yet both encounters change galaxies, shaping their trajectories, their appearance, their lives.