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August 12

Spiral galaxy

Recently I was watching a PBS entry on the subject of the Milky Way and on galaxies in general. One question that I can't find a concrete answer to is what is it that determines the direction of the spin of a spiral galaxy? If you make a rough analogy with a hurricane, the direction of spin is determined by the direction of the earth's rotation, i.e. by external forces. So are there external forces acting on galaxies which influence which direction they spin? Or is it just the net effect of all manner of random gravitational pulls within the galaxy? Or both? (Maybe the article explains it, but I'm as dense as a neutron star so I might have overlooked it.) ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:51, 12 August 2022 (UTC)[reply]

While not directly answering your question, it seems that observations of spin direction show marked asymmetry - see discussion here. Mikenorton (talk) 21:17, 12 August 2022 (UTC)[reply]

(It's always fun to read up on astronomy articles and see how outdated my understanding is.) There's a couple distinct concepts in spiral galaxies: the oribts of stars around a general galactic center, which is separate from the rotation of spiral arms (and of course there's plenty of other motions in galaxies too, but those are the two you're asking about). The first concept is that if you were to take a naive model of a flat disk galaxy and say that all the stars orbit around a dense center similar to how planets orbit around a star, then by simply looking at Kepler's third law for a little while you'd see that even if you started with stars grouped in spiral arms, they'd quickly dissipate (differential rotation or the "wrapping problem"). But there's a kicker in that if you look at how stars orbit a galaxy (the galaxy rotation curve), it doesn't neatly behave as if they were all following Kepler's laws and Newtonian gravity, and that's due to the dark matter halo (or at least that's how the effects of observed invisible mass effects are currently conceptualized -- dark matter itself is not yet understood). But dark matter would not be enough to stabilize spiral arms either, and while I've seen alternative models thrown around, the essential takeaway is that spiral arms are not a function of the orbits of stars themselves, but of star formation, which is catalyzed by enormous dense clouds of gas. In both models of spiral arms featured in the spiral galaxy article (of which I'm only really somewhat familiar with density wave theory) the spiral arms are a propagation of areas of high gas density, which trigger formation of massive, hot, and short-lived stars -- more importantly in other models the activity of these stars and the tendency to supernova is essential to the propagation of the spiral arms as well. At the end of the day the apparent pattern rotation of the spiral arms will be much slower than that of the individual stars.

Now, where does the original rotation, the angular momentum, come from? Well this is where I find out my astrophysics knowledge is outdated, because it looks like people no longer like the top-down models of galaxy formation, in which you have giant clouds of gas collapsing into smaller clouds that individually collapse into stars -- the kicker being that any tiny amount of net angular momentum in your giant cloud starts to appear to magnify in terms of angular velocities as the cloud collapses, similar to say a skater or dancer spinning faster as they pull in their arms/legs -- this also causes the initially spherical-ish cloud to flatten out like a wad of pizza dough. The bottom-up models by contrast works by collisions of smaller bodies, of which I have seen many simulations forming pretty spiral shapes (and residual angular momentum just like if you imagine taking two adhesive billiard balls and knocking them into each other at an angle, they'll stick and continue to spin together -- ok, not an everyday occurrence, but I'm sure you've seen paired figure skaters do something similar where one grabs their partner skating in at a skew and they start spinning, no additional application of force required), but those were pretty primitive simulations at the time, so they've obviously gotten a lot better. (Here's a decent some-astronomy-class-level non-math overview of models of galaxy formation.) It's all detailed with more pretty pictures in the galaxy formation article linked. Hopefully this answer didn't get too long and just repeat what's in the articles. SamuelRiv (talk) 22:21, 12 August 2022 (UTC)[reply]

The initial direction of spin is, as far as I know, just believed to be random, just as it is with star systems. Once this gets established, it's "locked-in". Everything subject to the system's gravitational effect is going to be induced to orbit in the same direction and aligned with the same plane, unless lots of energy winds up changing that, because the gravitation of everything in the system will tug it in that direction. In our own Solar System, see Uranus, which spins "on its side", with its poles almost perpendicular to the ecliptic, unlike pretty much everything else in the Solar System. Generally this is believed to be the result of one or more giant impacts in its history that flipped it, just as Earth's axial tilt is believed to be the result of an impact that formed the Moon. At the scale of the whole universe, the prevailing theory holds that its large-scale structure is random, the result of randomness at the quantum scale that then got "blown up" massively in size as the universe expanded due to cosmic inflation. This resulted in the initial distribution of stuff post-inflation, and since then the universe has just been evolving under primarily the influence of gravity. --47.147.118.55 (talk) 05:09, 14 August 2022 (UTC)[reply]

So current thinking is that it's random. Good enough. Thank you all. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:04, 16 August 2022 (UTC)[reply]

In the broadest sense, the universe is largely uniform with small anisotropies. If there were a net angular momentum in the universe (so if the expanding universe with all the random galaxy collisions had an ever-so-slight preference for which way everyone started moving or spinning at the end of the day), then our fundamental understanding of how the universe and physics works would be changed, because that means that there's a preferred direction in space. (This wouldn't necessarily be catastrophic -- there's (apparently) a preferred direction in time (different but sorta similar idea), and possibly in neutrino handedness and some other particle physics quantities -- but it'd be mega-mclarge-huge.) SamuelRiv (talk) 04:25, 16 August 2022 (UTC)[reply]

Resolved