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Galactic Bar Formation?
It's been a while since I've explored this topic. Years ago no one
had posited this model and I was told it was wrong. I thought I'd check if anything has changed since. Has anyone posited the following model for creating a galactic bar? Two galaxies merge. A pair of massive black holes (and stars) become trapped in orbit at some kpc separation (the ends of the bar). A bar of stars forms between them, probably having orbits that are co rotating and longitudinal. In a frame rotating with the forming bar, it would appear the stars are moving essentially along a line within the bar. At some point enough stars are within the bar that the gravity of the stars between the BH's causes the massive black holes to slide down the bar to the center, where they orbit and merge. We see the bar and we see a line of dust down each bar, that curves into what appears to be a circular trajectory near the center. See: https://en.wikipedia.org/wiki/Barred_spiral_galaxy to see the dust lanes. Any aspects matching (or refuting) this model been observed? Thanks, rt [[Mod. note -- One big problem with this model is that massive black holes aren't massive enough to dominate the stellar dynamics at kiloparsec (= galactic-bar-size) distances. That is, on the size scale of a galactic bar, even a "supermassive" black hole is still only a tiny fraction of the mass in stars, interstellar gas/dust, and maybe dark matter. -- jt]] |
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Galactic Bar Formation?
On Thursday, February 23, 2017 at 7:47:38 AM UTC-8, Martin Brown wrote:
On 22/02/2017 20:03, wrote: It's been a while since I've explored this topic. Years ago no one had posited this model and I was told it was wrong. I thought I'd check if anything has changed since. No. It's still wrong. The BH influence is confined to the small region nearest the centre of the galaxy where they are in mutual orbit out to a= modest multiple of their separation. Yes agreed / understood. Moreover numerical simulations of the evolution spirals into barred spiral galaxies tends to suggest that they are an inevitable consequence= of small pertubations of the whole ensemble as it tries to find a lowest= energy state after being shaken up by an interaction. eg. https://arxiv.org/abs/1211.6754 Thanks for the reference, I'll check it out..........but, we have Dark Matter and Dark Energy that are NOT accurately accounted for in our theoretical models. It's possible they fit the ideas currently used within such models, and, it's possible that the models are wrong and new equations need to be discovered. If the latter, then the simulations are wrong, period. So we need to be open minded in our quest for what's really going on. [[Mod. note -- One big problem with this model is that massive black holes aren't massive enough to dominate the stellar dynamics at kilopar= sec (=3D galactic-bar-size) distances. That is, on the size scale of a gal= actic bar, even a "supermassive" black hole is still only a tiny fraction of the mass in stars, interstellar gas/dust, and maybe dark matter. -- jt]] Indeed. Yes.........Indeed, I fully agree. But that's not the point I am going after. Let's slightly change the idea to make more clear what I'm wondering if anyone has seen. Suppose in some way, two elliptical galaxies were in an orbit with separation 100kpc so that they just have their outermost stars coming into contact. Their orbital velocity is such that they are locked in orbit and we'll ignore 3 body issues (maybe we suppose the ejection of a 3rd galaxy allowed them to come into a close stable orbit). Anyway, we begin with two elliptical galaxies of 100kpc diameter each with 100kpc separation distance. What would happen? As these two galaxies orbit, some stars from either can fall toward the other. This (highly idealized model) would naturally evolve with a lot of time, into a bar. A real merger is messy, but, the core of two galaxies might undergo this sort of thing and that's what I'm looking for to test whether this might be so. The stars within the bar wouldn't really be moving along a line, because the bar would be rotating. so the stars moving along the bar would be moving in ellipses....can you see this? Eventually, after a lot of stars had fallen down into the bar orbital path, (which by the way becomes much like stellar orbits within a globular cluster where stars are moving along approximately radial lines, and not in circular orbits) then the mutual galactic gravitational potential would favor objects falling down along the bar.....including the BH's. So the BH's could just fall down along the bar that the stars had formed...or more likely if this is so, the bar the stars were beginning to form. Make better sense? Anyone seen anything that looks at all like stages in the above model? rt [[Mod. note -- Models of this sort have a long history. See, e.g., the classic 1972 paper by Toomre & Toomre, http://adsabs.harvard.edu/abs/1972ApJ...178..623T I've never heard of such a model producing a *bar* (they tend to produce "bridges" and "tails", as in the Toomre & Toomre paper), but I am not an expert on this area of research. -- jt]] |
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