Barred Spiral Galaxies

[William]

I have been reviewing recent observations and hypotheses concerning the formation and evolution of galaxies. The recent finding that the Milky Way is a barred spiral leads to the question:

Why do barred spiral galaxies form?

(See next comment as to the time evolution in the percentage of barred spirals galaxies.)

http://geology.wcupa.edu/mgagne/ess111/notes/milky.ppt


http://en.wikipedia.org/wiki/Barred_spiral_galaxy

http://www.news.wisc.edu/11405

[William]

Why would there be an increase in the number of barred spiral galaxies?

What is the "bar" in a barred spiral galaxy?


http://www.astronomy.com/asy/default.aspx?c=a&id=7245

A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts...

Bars have been forming steadily over the last 7 billion years, more than tripling in number. "The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," Sheth explains. "They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today."

The findings, Sheth continues, have important ramifications for galaxy evolution. "We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time," he says.

[Neverfly]

I'm a little confused... The references you gave did not answer your question?

Barred Spirals are a product of galactic evolution. Essentially, a spiral galaxy should grow a bar given time.

The bar is formed due to the immense gravity of the spiral hub. As stars move in their orbits, instability eventually leads to elliptical orbits. As more and more stars develop elliptical orbits, the bar becomes more pronounced.

The Milky Way will lose it's bar spiral status in about 5 billion years.

[William]

In reply to Neverfly, see Original Post: "I'm a little confused... The references you gave did not answer your question?

Barred Spirals are a product of galactic evolution. Essentially, a spiral galaxy should grow a bar given time.

The bar is formed due to the immense gravity of the spiral hub. As stars move in their orbits, instability eventually leads to elliptical orbits. As more and more stars develop elliptical orbits, the bar becomes more pronounced.

The Milky Way will lose it's bar spiral status in about 5 billion years."

Hi Neverfly,

Mass for Spiral Galaxy's Bars?
I do not see how gas can move from the mid point of a galaxy to form the bars. Where is the source of the gas? Extra galactic?

Remember the spiral arms for some odd reason have gas that has not formed stars.

As other authors have noted it is puzzling that the Milky Way's bulge has no gas whereas the spirals that move about a plane in the Milky Way have so much gas they create an optical zone of avoidance for astronomical observation. (i.e. There is no issue with astronomical observations through the Milky Way's bulge.)

As per the paper I quoted, the percentage of barred spiral galaxies increases with time, however, the percentage of barred galaxies, only increases for small galaxies. (i.e. The number of barred galaxies for large galaxies does not change with time.)

Spiral galaxies reach a maximum size
As I noted previously for some odd reason spiral galaxies reach a maximum size.

Event can remove spiral galaxy bars
As noted in the paper some event can happen that causes barred spiral galaxies to loss their bars.

What cause mass to move into galaxy centre?
Besides the issue of where does the mass come from to create the spiral galaxy bars there is the problem of the mechanism by which mass can move toward the centre of the galaxy.

In the formation of the sun and solar system there is a single large cloud of dust. Proto planets form and move toward the centre as there is a continuous disk of gas and particles that can interact as tidal forces slows down the proto planets.

In a galaxy there are vast spaces between stars and gas clouds. There appears to be no mechanism to dissipate the angular momentum and to pull the stars and matter into the centre of the galaxy.

What should form is an elliptical galaxy where there is a random distribution of angular momentum, rather than a spiral galaxy that has bars that revolve in a single plane.

There is quite obviously a different mechanism that is creating and constraining the spiral galaxy.

Comment:
Besides the above issues I thought it was interesting that spiral galaxy's that have bars can suddenly become star burst galaxies which is relevant as our solar system is in this particular galaxy.

As noted below it appears a group of galaxies have some peculiar property where by they can stop star formation in a lone galaxy that approaches them. Interesting the time required for the group of galaxies to stop star formation in the lone galaxy that approaches a group of galaxies is depend on the mass of the lone galaxy.

Strangulation Of Spiral Galaxies: ‘Missing Link’ Discovered

http://www.sciencedaily.com/releases...1124194936.htm

[Neverfly]

William,
I don't have much of solid answers to most of what you just said.
Hopefully some of the much more knowledgeable folks here can provide greater insight.

I'm going off some things I had read in books some time ago and don't have those books handy for referencing. Sadly.

[StupendousMan]

William,

You'll find the answers to many of your questions in a good graduate textbook on galactic astronomy. Please go read one. I suggest Binney and Tremaine, "Galactic Dynamics."

Understanding the formation of bars in galaxies due to self-gravitational instabilities requires a good deal of mathematical background. If you find the graduate-level texts too difficult, try starting with some advanced undergraduate texts, such as Shu's "The Physics of Astrophysics."

After you've read the chapter on spiral structure in one of these books, please come back and ask us any questions which remain.

[William]

Cosmological theory must of course explain observations. One hint a theory is incorrect is when there are sets of related observational evidence that it cannot explain and that appear to be connected.

Theories are formed based on certain assumptions concerning the fundamental mechanisms. Assumptions are not facts and can obviously be incorrect.

As many have noted it is possible due to the lack of parameter constraints and the difficulty in modelling complex systems to create models that make predictions but that have no basis in physical reality.

This is interesting, in addition to the observation of spiral galaxy strangulation. (See my comment above on spiral galaxy strangulation.)

http://www.sciencedaily.com/releases...1124102701.htm


Mystery Of Missing Hydrogen: Apparent Absence From Distant Galaxies Puzzles Astronomers

Something vital is missing in the far distant reaches of the universe: hydrogen -- the raw material for stars, planets and possible life…. If anything, hydrogen was expected to be more abundant so early in the life of the universe because it had not yet been consumed by the formation of all the stars and galaxies we know today.

P.S.
S.M. I am not sure what the point of your comment was.

Have you read anything about the brilliant Lord Kevin who regardless of observations that contradicted his beliefs continued to his death to angrily defend his position. Lord Kevin appeared incapable of responding to observations (developing and considering alternative hypotheses) when it was painfully clear that there there was something fundamental incorrect in his hypothesis.

I do not understand what invokes the anger. Science is not religion.

[Buttercup]

Barred spiral galaxies are my favorite. Awesomely pretty.

[StupendousMan]

P.S.
S.M. I am not sure what the point of your comment was.

You asked a question about a complicated subject -- the formation of bars in spiral galaxies. I replied that, in order to understand this subject properly, you would need to apply some sophisticated math and physics to the issue of gravitational instabilities in a rotating disk of gas. I gave you two references to books which describe this issue in detail, since I have no intention of typing in all that information.

If you want to learn about this topic, you'll read those books (or similar ones). If you just want to post rambling attacks on the current astronomical community without trying to understand the material, you'll continue posting in this thread, and create new threads whenever you can find some press release which makes hype-worthy claims.

Your choice.

[Neverfly]

P.S.
S.M. I am not sure what the point of your comment was.

Have you read anything about the brilliant Lord Kevin who regardless of observations that contradicted his beliefs continued to his death to angrily defend his position. Lord Kevin appeared incapable of responding to observations (developing and considering alternative hypotheses) when it was painfully clear that there there was something fundamental incorrect in his hypothesis.

I do not understand what invokes the anger. Science is not religion.

I hope you are referring to Lord Kelvins anger and not saying Stupendous Man was angry...
Lord Kelvins inability to accept evidence over his belief would be a personal issue regarding Kelvin- it does not mean all scientists do that or that you can whip that out every time someone refutes your argument.

Stupendous Man made a helpful suggestion.
Realistically, I am not sure anyone can post a solid answer to you on a forum that will make it all beautifully clear to you.

[William]

Hello Stupendousman,

I see you missed the thread “Spiral Galaxies Rotate, Elliptical Galaxies do not. Why?”

Do you have an answer to that question?

A) Spiral Galaxies Rotate, Elliptical Galaxies do not. Why?

http://www.bautforum.com/questions-a...ml#post1233771


B) Spiral Galaxies have new stars forming in them Elliptical galaxies do not. Why?

S.M. do you have an answer to that question?

Comment:
Perhaps the answer to that question can be found by explaining what creates a red spiral galaxy. (There is a very recent finding that star formation is halted when a spiral galaxy approaches a group of spiral galaxies. Odd.)


B-1) Unusual red spiral galaxies

http://www.universetoday.com/2008/11...ies-strangled/

Usually a galaxy's shape corresponds with its color as well as its age. Spiral galaxies appear blue because they are still dynamically forming hot young stars. Elliptical galaxies, on the other hand, are mostly old, dead, and red. But surprisingly, two international collaborations have both separately identified a population of unusual red spiral galaxies.

P.S.
S.M. I accept your criticism concerning the presentation and organization of the observations. I am working away on the following and will come back later with a concerted effort to try to link the observations.

- Starburst Galaxies

- Blue compact galaxies (BCGs)

- Wolf-Rayet galaxies

[Spaceman Spiff]

Have a look at this, from a world's expert on secular evolution of spiral galaxies, and bars therein (Francoise Combes), written for Scientific American (because most people reading this will find even Shu's book tough slogging, never mind Binney & Tremaine). Go to Francoise Combes' web page for a gif animation of the evolution of spiral galaxy structure due to gas inflows. The RHS is stars, the LHS is gas within the galaxy, in 200 Myr time steps.

To William -- Stupendous Man is right. This (and nearly all topics in astrophysics) are not just "bull sessions" over a few beers, but are incredibly detailed in physics and steeped mathematics and often supported by intersecting mountain ranges of evidence (not that this last point means that we understand all -- far from it). I'll second Stupendous Man again. Asking honest questions is great. Posting disingenuous ones isn't cool and gets to be a drag real fast.

[William]

In reply to Spaceman Spiff.

Thanks for your comment, concerning barred galaxies.

Did you see this thread.

Spiral Galaxies have net angular momentum, elliptical galaxies do not.

http://www.bautforum.com/questions-a...ml#post1233771

[Spaceman Spiff]

In reply to Spaceman Spiff.

Thanks for your comment, concerning barred galaxies.

Did you see this thread.

Spiral Galaxies have net angular momentum, elliptical galaxies do not.

http://www.bautforum.com/questions-a...ml#post1233771

Yes, briefly. But you started this by linking to a very rudimentary, undergraduate, explanation of the properties of galaxies. As is typical there are some over simplifications, including the one that elliptical galaxies have ZERO angular momentum. But what has this to do with the present thread or with anything at all?

[William]

Why is the question ("Spiral Galaxies have net angular momentum, Elliptical galaxies do not. Why?"), interesting?

Do you have an answer?

P.S.
You are quite correct I have nothing interesting to say concerning barred spiral galaxies.

The observation that star production is stopped in lone spiral galaxies that approach a galaxy cluster is interesting. Do you have any thoughts?


http://www.universetoday.com/2008/11...ies-strangled/

[StupendousMan]

Why is the question ("Spiral Galaxies have net angular momentum, Elliptical galaxies do not. Why?"), interesting?

Let's try moving this discussion to quantitative terms. William, if you can find a small sample of measured angular momenta for, say, 10 spiral galaxies, and 10 elliptical galaxies, then it might help us to focus the comments. Simple statements with no observations to support them don't help very much.

If anyone else wants to contribute a simple list of measured angular momenta for some galaxies, please do.

[trinitree88]

Spaceman...that's a pretty spiffy animation of the formation of a barred spiral. Thanks. pete

[parejkoj]

If anyone else wants to contribute a simple list of measured angular momenta for some galaxies, please do.

I don't really want to get into this conversation (except to demand that Shu update his textbook! Though I've heard rumors that this is in the works). But there have been a few astro-ph postings about the angular momentum structure of galaxies. Here's one.

[PraedSt]

Have a look at this, from a world's expert on secular evolution of spiral galaxies, and bars therein (Francoise Combes), written for Scientific American (because most people reading this will find even Shu's book tough slogging, never mind Binney & Tremaine). Go to Francoise Combes' web page for a gif animation of the evolution of spiral galaxy structure due to gas inflows. The RHS is stars, the LHS is gas within the galaxy, in 200 Myr time steps.

Could you please clear something up for me Spaceman? I was given to understand that:

1. Galaxy spiral arms presented scientists with a 'winding up' problem.
2. This was resolved by theorising that the arms were permanent structures formed by gravity waves. The simplistic traffic jam analogy- the arms have a relatively fixed structure, and stars move in and out of them.

From the animations on that site, it looks to me as if the arms do indeed wind up. I can see inner arms catching up with, and subsequently merging with, the outer arms. The entire structure is preserved through time with the fresh generation of new arms close to the core.

Of course it's quite likely I'm making a mistake. Staring at a rotating pin-wheel is hard; I feel quite dizzy at the moment...

[Spaceman Spiff]

Could you please clear something up for me Spaceman? I was given to understand that:

1. Galaxy spiral arms presented scientists with a 'winding up' problem.
2. This was resolved by theorising that the arms were permanent structures formed by gravity waves. The simplistic traffic jam analogy- the arms have a relatively fixed structure, and stars move in and out of them.

From the animations on that site, it looks to me as if the arms do indeed wind up. I can see inner arms catching up with, and subsequently merging with, the outer arms. The entire structure is preserved through time with the fresh generation of new arms close to the core.

Of course it's quite likely I'm making a mistake. It's hard to stare at a rotating pin-wheel; I feel quite dizzy at the moment...

Spiral arms, as bars, are best understood as "patterns in motion" -- waves of locally enhanced density and/or star formation within molecular clouds (note: "gravity waves" is a loaded phrase, and without proper context can confuse the uninformed reader). They are definitely NOT a fixed structure, nor is the pattern a constant of time. There are different types of disturbances which affect the wave pattern as functions of time and spatial location. I hope that helps some; for more, read the Scientific American article -- it's all in there at a level that the non-specialist can appreciate.

Also, try having a look at the simulations called "Bar destruction and reformation" on her webpage. The second link moves into the rotating frame of the bar.

[PraedSt]

Spiral arms, as bars, are best understood as "patterns in motion" -- waves of locally enhanced density and/or star formation within molecular clouds (note: "gravity waves" is a loaded phrase, and without proper context can confuse the uninformed reader). They are definitely NOT a fixed structure, nor is the pattern a constant of time. There are different types of disturbances which affect the wave pattern as functions of time and spatial location. I hope that helps some; for more, read the Scientific American article -- it's all in there at a level that the non-specialist can appreciate.

Thanks, all cleared up. That sciam article is superb. My phrase to descibe the spiral structure: a choreographed precession (?) of partly aligned elliptical orbits.

That traffic jam analogy was poor. Completely gave me the wrong idea.

[William]

I had reviewed different reference sources for the thread "Spiral Galaxies Rotated, Elliptical Galaxies do not. Why?".

The following statement is consistent with all sources.

http://en.wikipedia.org/wiki/Elliptical_galaxy

Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. The motion of stars in elliptical galaxies is predominately radial, unlike the disks of spiral galaxies, which are dominated by rotation.

Furthermore, there is very little interstellar matter (neither gas nor dust), which results in low rates of star formation, few open cluster, and few young stars; rather elliptical galaxies are dominated by old stellar populations, giving them red colours. ….

The properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they are formed by the same physical processes, although this remains controversial.

[StupendousMan]

Thank you for that summary, William.

There are two main over-arching explanations for the differences between elliptical and spiral galaxies. One focuses on the very early evolution of the giant cloud of gas which eventually formed a galaxy: if that cloud remained gaseous while collapsing down into a smaller unit, the dissipation between gas molecules would lead to a flattened disk galaxy, in which stars would form later. On the other hand, if stars formed first, while the cloud was still big and spherical, then the lack of dissipation in star-star gravitational interactions would lead to an elliptical galaxy.

The other explanation suggests that elliptical galaxies form when two galaxies collide. In the collision, much of the gas in the original (spiral) galaxies is turned into stars or flung into deep space. The orbits of the stars which result from the collision are consistent with those in an elliptical galaxy.

I'm not sure which explanation is currently more fashionable. Both may be true and important to some degree.

For more details, one can read the references mentioned earlier in this thread. If there are very specific questions which remain after having read those questions, please ask them here.

[transreality]

I think a third explanation concerns the seperation of the baryonic component from the rotating dark matter halo during galactic interactions.

Concerning the formation of a bar the dark matter halo may also have a role. From the sci-am article mentioned above the bar results from the orbital shells of the outer stars disconnecting from the orbits of the inner stars; ie; the two groups of stars are in different dynamic realms. If the DM halo has started to seperate from the baryonic component by in general moving radially from the centre, forming a donut around the outer galaxy, then the stars of the inner galaxy, in the hole of the DM halo, are free to self-gravitate and merge to form a singular bar. Meanwhile the outer stars still embedded in the halo move in orbits dominated by the angular momentum of the halo.

[William]

In reply to Stupendous Man's comment:There are two main over-arching explanations for the differences between elliptical and spiral galaxies. One focuses on the very early evolution of the giant cloud of gas which eventually formed a galaxy: if that cloud remained gaseous while collapsing down into a smaller unit, the dissipation between gas molecules would lead to a flattened disk galaxy, in which stars would form later. On the other hand, if stars formed first, while the cloud was still big and spherical, then the lack of dissipation in star-star gravitational interactions would lead to an elliptical galaxy.

Elliptical
The early monolithic collapse theory explains the elliptical galaxy, as the net angular momentum of an elliptical galaxy is typically zero. The primitive gas cloud's motions are random and do not have net angular momentum about any specific axis. The monolithic collapse theory is support by observational analysis that shows uniform age of the stars (old) in an elliptical.

Comment:
One problem with the early monolithic collapse theory, is it supports theoretical analysis that shows gas clouds effectively cool thereby collapse quickly (as compared to the age of the galaxies). Where is the source of new gas coming from for the spiral galaxies? (i.e. Why does the spiral galaxy gas not monotonically collapse?)

Spiral
For the case of spiral galaxies there is significant angular momentum, which is used by the mechanism to create and destroy bars etc;

Start the explanation of how a spiral galaxy forms at the gas cloud stage. If the gas cloud does not have net angular momentum it will not have angular momentum after collapse. How does nature determine it is time to form an elliptical galaxy as opposed to a spiral galaxy. Is the initial primitive gas cloud uniform?

A second issue with spiral galaxies is how do they avoid transformation from spiral to elliptical after mergers. Statistical analysis shows the number of spirals to ellipticals remains approximately the same. Also observational evidence indicates ellipticals are consistently old. Some mechanism appears to enable spiral galaxies to remain spiral galaxies even after major mergers.

In the case of the Milky Way the Milky Way's bulge is older than its disk. It seems difficult to argue for a monolithic collapse to form the bulge and at some later date gas to create a disk.

[slang]

Start the explanation of how a spiral galaxy forms at the gas cloud stage. If the gas cloud does not have net angular momentum it will not have angular momentum after collapse.

Warning, amateur musing ahead: perhaps a difference could be in the way the collapse happens. One 'focal point' for the collapse might give a uniform elliptical, but what happens if the cloud starts collapsing in two locations? I would guess that if the locations are sufficiently far apart you might end up with two collapsing clouds, but what if they are closer? What if there are many points where collapse starts, and local density differences determine whether any motions and interactions average out to a uniform system, or to a more dynamic system with momentum in abundance? What if single collapse starts either close to the center, or closer to the 'edge' of a cloud?

[Spaceman Spiff]

Elliptical
The early monolithic collapse theory explains the elliptical galaxy, as the net angular momentum of an elliptical galaxy is typically zero. The primitive gas cloud's motions are random and do not have net angular momentum about any specific axis. The monolithic collapse theory is support by observational analysis that shows uniform age of the stars (old) in an elliptical.

Comment:
One problem with the early monolithic collapse theory, is it supports theoretical analysis that shows gas clouds effectively cool thereby collapse quickly (as compared to the age of the galaxies). Where is the source of new gas coming from for the spiral galaxies? (i.e. Why does the spiral galaxy gas not monotonically collapse?)

Spiral
For the case of spiral galaxies there is significant angular momentum, which is used by the mechanism to create and destroy bars etc;

Start the explanation of how a spiral galaxy forms at the gas cloud stage. If the gas cloud does not have net angular momentum it will not have angular momentum after collapse. How does nature determine it is time to form an elliptical galaxy as opposed to a spiral galaxy. Is the initial primitive gas cloud uniform?

A second issue with spiral galaxies is how do they avoid transformation from spiral to elliptical after mergers. Statistical analysis shows the number of spirals to ellipticals remains approximately the same. Also observational evidence indicates ellipticals are consistently old. Some mechanism appears to enable spiral galaxies to remain spiral galaxies even after major mergers.

In the case of the Milky Way the Milky Way's bulge is older than its disk. It seems difficult to argue for a monolithic collapse to form the bulge and at some later date gas to create a disk.

Scientists are underwhelmed by arguments of personal incredulity. Instead we use physics to investigate the workings of nature and we observe nature to confront our models. The Millennium Simulation Project is our first real go at understanding the history of galaxies and large scale structure over cosmic time. See this paper for an example of how this work illuminates the history of MW type galaxies, and this one as one of many investigations into the origin of galaxy morphology in the local universe. (Both papers are to be found at the bottom of the MSP web page linking a long list of papers that have used their "mock universe".)

Our understanding of galaxy evolution has a long way to go yet, but staring into our navels isn't what's happening.

[01101001]

That traffic jam analogy was poor. Completely gave me the wrong idea.

Were you thinking of one location where all traffic slowed due to some obstruction or thing to see?

The waves I picture for that analogy moved backward (or I suppose possibly forward under weird circumstances), opposite traffic flow.

Example: for some reason, say something blows across the highway, people brake; people behind see the brake lights and brake. People behind them brake; and so on and the dense bit of slowed cars moves backward through traffic.

When I'm driving along and suddenly have to slow for no apparent reason except that people ahead slowed, I always think I'm participating in a traveling compression wave.

I expect these waves continue moving backward until they reach a lesser density of traffic, where people see the brakelights ahead but are trailing so much that they just slow instead of hitting the brakes, and then the people behind them don't slow at all, and the wave dies.

[PraedSt]

Were you thinking of one location where all traffic slowed due to some obstruction or thing to see?

Yes!

Well, more or less. I knew the arms were rotating, but I thought they were permanent structures. To throw another analogy into this- like hands on a watch.

What I hadn't understood was this: because orbital speeds vary along an elliptical orbit, if you align several of these ellipses in the right way, each slightly offset from the one previous, it looks as if a compression wave has formed.

I'm all happy now...

[parejkoj]

While cleaning out my backlog of links and filling the papers into Papers¹, I re-ran into Barred Galaxies: an Observer's Perspective by Dimitri Gadotti. Figured it was relevant here.

1. If you do research and have a Mac, go get it right now! It's great - it understands ADS and arXiv and can perform searches within them, keeps track of what you've read and what you haven't, can produce BibTeX files from collections and "smart collections" which are like smart folders in Finder, iTunes or iPhoto and has a--currently limited--note-taking capability which is planned to be expanded into full hi-lighting and context-sensitive notes in the future!