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Tom Mazanec
2015-Nov-19, 06:54 PM
What decides if a spiral is barred or classic normal? Are they formed that way, with some difference in formation? Or do spirals switch from one to the other over millions of millennia?

Noclevername
2015-Nov-19, 07:05 PM
According to the WP article on Spiral Galaxies (https://en.wikipedia.org/wiki/Spiral_galaxy):


The proportion of barred spirals relative to their barless cousins has changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.[3]

And since the number of galaxies has not increased since then, I'd assume that means that unbarred spirals become barred over time.

As far as how they are classified:

https://en.wikipedia.org/wiki/Galaxy_morphological_classification

And how they form: https://en.wikipedia.org/wiki/Barred_spiral_galaxy


The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure.[5]

Bars are thought to be temporary phenomena in the lives of spiral galaxies; the bar structures decay over time, transforming galaxies from barred spirals to more "regular" spiral patterns. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars.[6] Since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years.[7]

John Mendenhall
2015-Nov-19, 07:08 PM
IIRC somebody is running a large participation project to classify as many galaxies as possible I don't recall the details.

Grashtel
2015-Nov-21, 09:07 PM
IIRC somebody is running a large participation project to classify as many galaxies as possible I don't recall the details.
Are you thinking of Galaxy Zoo (http://www.galaxyzoo.org/)?

Cougar
2015-Nov-21, 10:05 PM
...And since the number of galaxies has not increased since then, I'd assume that means that unbarred spirals become barred over time.

Yes, that's good evidence for that.


As far as how they are classified:

https://en.wikipedia.org/wiki/Galaxy_morphological_classification

I'm surprised the article does not mention the fact that the standard Hubble tuning fork diagram does not represent anything about the evolution of galaxies. It's purely a listing of types. Here are a couple comments I've read that I think are pertinent... :D

Madore: "Who goes in and classifies what turbulent eeddies look like? No physicist in their right mind would classify that sort of thing. And maybe that's all we're doing . We have this frozen picture in time of things that don't look like what they used to look like, probably won't look the same in the future, and don't even look like what they [I]do look like when you look at them in another wavelength! It may well be that every galaxy's structure is so contingent on the last event or environment that it found itself in, that what you see now in this fine little slice in time and this other tiny slice in wavelength have nothing to do with its global history... to reduce a whole galaxy down to two letter and a number seems to trivialize the whole thing."


For Simon Driver, a galaxy specialist at Australia's Mount Stromlo Observatory, the Hubble tuning fork is just butterfly counting. "The name of the game is to find something to replace it with."



Both from Jeff Kanipe, author of Chasing Hubble's Shadows, The Search for galaxies at the Edge of Time (2006).

ngc3314
2015-Nov-23, 05:27 PM
Simulations and analytical stability analyses show that the formation of a bar is greatly enhanced by mass distributions which give rotation patters that are unstable to a so-called m=2 mode (which is to say, a bar). Interactions can make a formerly nonbarred galaxy barred, although the duration of the bar may not be very long. In today's Universe, roughly equal fractions of spirals are strongly barred weakly barred (including small bars near the nucleus), and nonbarred. Simulations also show that a bar may slowly dissolve into a ring as it transfers angular momentum between different stars' orbits; many bars are accompanied by such rings, and there are a handful of spirals with strong rings but no bars, making them candidates for evaporated bars.

Bars clearly appear over cosmic time; the fraction of spirals with a bar has increased, after a surprisingly early first appearance.

I gotta say this - Barry and Simon were being deliberately provocative and they knew it. The annoying thing, which keeps the Hubble classes in wide use after 80+ years, is that they correlate pretty well with physically interesting things such as the star-forming history and gas content of galaxies, which is why there is to this day no fully satisfactory replacement except for direct measurement of the quantity of interest. Certainly we expect details of arm patterns to change, and there is a real issue as to what level of detail it makes sense to classify since these details can't be long-term constants of a galaxy - but no better, simple first sorting of galaxies has emerged. So there.