CfA: New Insights on How Spiral Galaxies Get Their Arms

[bystander]

New Insights on How Spiral Galaxies Get Their Arms
Harvard-Smithsonian Center for Astrophysics | 2013 Apr 02

[img3="Powerful new computer simulations are allowing astronomers to understand how spiral arms in galaxies form and survive. These simulations suggest that the arms arise as a result of the influence of giant molecular clouds - star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds act as "perturbers" and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely. In this frame from one such simulation, more than 100 million "stellar particles" form the familiar shape of a spiral galaxy. (Credit: Thiago Ize & Chris Johnson (Scientific Computing and Imaging Institute))"]http://www.cfa.harvard.edu/image_archiv ... /lores.jpg[/img3]

Click to play embedded YouTube video.

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Spiral galaxies are some of the most beautiful and photogenic residents of the universe. Our own Milky Way is a spiral. Our solar system and Earth reside somewhere near one of its filamentous arms. And nearly 70 percent of the galaxies closest to the Milky Way are spirals.

But despite their common shape, how galaxies like ours get and maintain their characteristic arms has proved to be an enduring puzzle in astrophysics. How do the arms of spiral galaxies arise? Do they change or come and go over time?

The answers to these and other questions are now coming into focus as researchers capitalize on powerful new computer simulations to follow the motions of as many as 100 million "stellar particles" as gravity and other astrophysical forces sculpt them into familiar galactic shapes. A team of researchers from the University of Wisconsin-Madison and the Harvard-Smithsonian Center for Astrophysics reports simulations that seem to resolve long-standing questions about the origin and life history of spiral arms in disk galaxies.

"We show for the first time that stellar spiral arms are not transient features, as claimed for several decades," says UW-Madison astrophysicist Elena D'Onghia, who led the new research along with Harvard colleagues Mark Vogelsberger and Lars Hernquist.

"The spiral arms are self-perpetuating, persistent, and surprisingly long lived," adds Vogelsberger.

The origin and fate of the emblematic spiral arms in disk galaxies have been debated by astrophysicists for decades, with two theories predominating. One holds that the arms come and go over time. A second and widely held theory is that the material that makes up the arms - stars, gas and dust - is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods.

The new results fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds - star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds act as "perturbers" and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

"We find they are forming spiral arms," explains D'Onghia. "Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there."

The new study modeled stand-alone disk galaxies, those not influenced by another nearby galaxy or object. Some recent studies have explored the likelihood that spiral galaxies with a close neighbor (a nearby dwarf galaxy, for example) get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor.

According to Vogelsberger and Hernquist, the new simulations can be used to reinterpret observational data, looking at both the high-density molecular clouds as well as gravitationally induced "holes" in space as the mechanisms that drive the formation of the characteristic arms of spiral galaxies.

Puzzle of how spiral galaxies get their arms comes into focus
University of Wisconsin, Madison | 2013 Apr 02

Self-Perpetuating Spiral Arms in Disk Galaxies - Elena D'Onghia, Mark Vogelsberger, Lars Hernquist

• Astrophysical Journal 766(1) 34 (2013 Mar 20) DOI: 10.1088/0004-637X/766/1/34
  arXiv.org > astro-ph > arXiv:1204.0513 > 02 Apr 2012 (v1), 28 Jan 2013 (v2)

[geckzilla]

So which came first, the spiral or the elliptical?

[neufer]

So which came first, the spiral or the elliptical?

The spiral.

[geckzilla]

What's this smooth thing they've got in the simulation there prior to the arms forming called?

[Chris Peterson]

What's this smooth thing they've got in the simulation there prior to the arms forming called?

A protogalaxy?

[geckzilla]

Why is it so smooth, anyway? How can we tell if we are looking at elliptical galaxies or protogalaxies? Galaxy formation just makes no sense at all to me.

[Chris Peterson]

Why is it so smooth, anyway? How can we tell if we are looking at elliptical galaxies or protogalaxies? Galaxy formation just makes no sense at all to me. :oops:

I think it's a matter of evolution. Spirals evolve into ellipticals after interacting with other galaxies. While protogalaxies may look a lot like ellipticals in terms of structure, I guess they're quite different in terms of composition, and certainly much older- gone after the first billion or so years of the Universe.

[geckzilla]

I've read that ellipticals are the end stage for a galaxy and that when it reaches that point it doesn't have a lot of star formation going on and just stays at this uninteresting, nondescript blob of stars. I wonder why it wouldn't eventually coalesce like this simulation and take some kind of shape once again?

[bystander]

http://asterisk.apod.com/viewtopic.php?t=31104

[Ann]

I've read that ellipticals are the end stage for a galaxy and that when it reaches that point it doesn't have a lot of star formation going on and just stays at this uninteresting, nondescript blob of stars. I wonder why it wouldn't eventually coalesce like this simulation and take some kind of shape once again?

http://www.cfa.harvard.edu/news/2013/pr201310.html wrote:
The new results fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds - star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds act as "perturbers" and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

So giant molecular clouds are essential for forming spiral arms. But elliptical galaxies have no giant molecular clouds. Their arms have been destroyed one way or another, possibly through violent mergers. Also, their gas reserves have been mostly used up. A merger often leads to runaway star formation, which quickly uses up much of the galaxies' available gas. Many of the massive stars that formed in this starburst then go supernova more or less simultaneously, blowing much of the available gas away from the galaxies.

Very massive elliptical galaxies can pull their gas back in even if it has been blown into space by a string of supernovae. However, these massive ellipticals almost invariably have very massive central black holes. Presumably their black holes have grown through the many mergers that elliptical galaxies are likely to have undergone. Mergers will send large helpings of gas in the direction of the galaxy's central black hole, and much of the gas will end up in the hole, making it more massive.

This picture of the center of M87 is a perfect example of why elliptical galaxies don't form spiral arms again. In this huge elliptical galaxy without obvious gas clouds, gas can nevertheless be seen whirling around the galaxy's supermassive central black hole. Much of this gas is being funneled into the black hole. An incredible jet is blown out from just outside the Schwarzschild radius of the black hole. This incredible jet stirs up all of the gas in the entire galaxy, making it impossible for the gas of M87 to settle down and form giant molecular clouds.

Also remember that the amount of "free gas" in the universe is steadily declining. Some of this gas has disappeared into supermassive black holes like the one in M87. Much more of this gas, however, is locked up inside low-mass stars. Consider the Pleiades. The blue brilliance of the massive stars is like the tip of the iceberg of this cluster. The eight most massive and luminous stars of the Pleiades probably contain an average of about four solar masses each, which means that around thirty solar masses of gas are locked up inside these brilliant beacons. But the Pleiades cluster as a whole is estimated to contain 800 solar masses. Most of the rest of the circa 770 solar masses are probably locked up inside red and brown dwarfs.

Most stars that form in any star formation event are low-mass stars, stars that contain 50% of the mass of the Sun or less. Solar-mass stars eventually evolve into planetary nebulae, and then they give back much of their gas to the surrounding universe. Provided this gas can be captured and contained in a self-gravitating gas cloud, the gas can be recycled and turned into a new generation of stars. But the low-mass stars, those that contain most of the mass of the original gas cloud, may persist as long as the universe will exist. For all practical purposes, at least as long as we are talking about a universe that is less than a hundred billion years old, not a single red dwarf may ever give back its gas to the universe.

So the universe is generally running out of gas, because of the ever growing number of greedy gas-hoarding red dwarfs. Large elliptical galaxies almost invariably have large and hungry black holes, which regularly have the kind of outbursts that prevent giant molecular clouds from forming. And without large molecular clouds, spiral arms will not form in the first place.

Ann

[rstevenson]

Perhaps shape also enters into it. Ellipticals form from the merger of two or more galaxies, probably spirals. The total amount of gas in those two galaxies is still within the elliptical, but it is now spread throughout a roughly spherical shape instead of confined to two very thin disks, so it will have less influence on the galaxy's gravitational dynamics from then on.

Rob

[MargaritaMc]

Ann,
Your post was fascinating - thanks a lot.
Can you give the date of the Apod for this image
http://apod.nasa.gov/apod/image/1004/ant_hst_big.jpg, which is a link about planetary nebulae in your penultimate paragraph, as I'd like very much to read more about it.

Many thanks
Margarita

[neufer]

[b][color=#FF0000]Uninteresting, nondescript reddish blob on elliptical trainer[/color][/b]

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I've read that ellipticals are the end stage for a galaxy and that when it reaches that point it doesn't have a lot of star formation going on and just stays at this uninteresting, nondescript blob of stars.

<<Originally, Edwin Hubble thought that elliptical galaxies may evolve into spiral galaxies, which later turned out to be false. Most elliptical galaxies are composed of low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters. Current thinking is that an elliptical galaxy may be the result of a long process where two galaxies of comparable mass, of any type, collide and merge. It has been theorized that an elliptical galaxy may result from a merger of the Milky Way and M31.

The motion of stars in elliptical galaxies is predominantly 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 star clusters, and few young stars; rather elliptical galaxies are dominated by old stellar populations, giving them red colours.>>

[bystander]

Can you give the date of the Apod for this image
http://apod.nasa.gov/apod/image/1004/ant_hst_big.jpg

http://apod.nasa.gov/apod/ap100425.html

You can find it through the APOD Calendar. The 1004 in the image url is year and month. Bring up the calendar for April 2010 and there it is on the 25th.

[MargaritaMc]

Thanks, bystander. It's so OBVIOUS once you've pointed out how the number is allocated... I really have a blank where codes are concerned!
M