Gemini and Keck Put New Spin on Galaxy Formation

[bystander]

Gemini and Keck Put New Spin on Galaxy Formation
Gemini Observatory | 2015 Dec 14

[img3="Regular spiral galaxies, such as the 'whirlpool galaxy' on the left, form far fewer stars than the clumpy galaxy on the right. The blue regions have the least star-forming gas and red-yellow regions have the most. Credit: Dr Danail Obreschkow, ICRAR. Image uses data from the Hubble Space Telescope."]http://www.icrar.org/__data/assets/imag ... /Image.jpg[/img3][hr][/hr]

A team of Australian researchers used two Maunakea-based observatories – Gemini North and W. M. Keck Observatory – to discover why some galaxies are clumpy rather than spiral in shape and it appears that low spin is to blame. The finding challenges an earlier theory that high levels of gas cause clumpy galaxies, and sheds light on the conditions that brought about the birth of most of the stars in the Universe. The finding was published today in The Astrophysical Journal.

A combination of integral field spectroscopy data from Keck Observatory and Gemini Observatory was the key to obtaining measurements for a galaxy’s spin. Keck Observatory’s OSIRIS instrument collected data high spatial resolution in the galaxy centers, and the Gemini Multi-Object Spectrograph (GMOS) collected data for high surface brightness sensitivity out to large radii. ...

A New Spin on Star-Forming Galaxies
International Center for Radio Astronomy Research | 2015 Dec 14

Low Angular Momentum in Clumpy, Turbulent Disk Galaxies - Danail Obreschkow et al

• Astrophysical Journal 815(2):97 (2015 Dec 20) DOI: 10.1088/0004-637X/815/2/97
  arXiv.org > astro-ph > arXiv:1508.04768 > 19 Aug 2015 (v1), 16 Nov 2015 (v2)

[Ann]

That's fascinating!

My immediate reaction is the thought that a high spin ought to even out bumps and clumps in galaxies. But bumps and clumps is what feeds star formation in galaxies.

Click to view full size image

Another little tidbit I remembered is that starforming clouds in the Milky Way are typically elongated, often very elongated. If they were more "round" and not so "stretched out", they might be able to produce more stars. The picture at left by Rogelio Bernal Andreo shows a typically elongated starforming molecular cloud in Orion.

Ann

[Ann]

I just remembered another interesting take on this.

Click to view full size image

Here you can see Adam Block's latest image of early Sb-type spiral galaxy NGC 488. (Full size can be seen here.)

Many people consider NGC 488 to be extraordinarily beautiful, and I agree. It is so perfectly symmetrical. But there is little star formation here, and the galaxy's colors are red: B-V is 0.87, and U-B is 0.35. If you check out the full size version of Adam Block's picture of this galaxy, you'll see that the emission nebulas in this galaxy, visible as pink dots, are extremely small and faint.

The almost perfectly symmetrical shape of NGC 488 suggests that this galaxy is rotating fast. That, indeed, appears to be the case.

C. J. Peterson wrote:
The rotation curve of NGC 488 has been measured out to a radius of 20 kpc, at which point the velocity is 363 km s^{- 1} and continuing to increase with radius. This rotational velocity is the largest known for an Sb galaxy.

The high rotational speed of NGC 488 and its low rate of star formation really seem to be connected.

Ann

[neufer]

High spin = more dark matter

[Ann]

High spin = more dark matter

Sounds plausible, but is it really reasonable to presume that the galaxies had less dark matter during earlier epochs, when most of the star formation in the universe took place?

Doesn't it seem more probable that an increasing rotational velocity in spiral galaxies over time is an evolutionary thing?

Ann

[geckzilla]

I don't see why rotational velocity would always increase over time. Depending on the arrangement of mass as the galaxy proceeds through its evolution, it seems like it could either increase, decrease, or remain nearly the same over time.

[Ann]

Art, you may be right. The amount of dark matter can't very well have increased over time (unless dark matter has properties that no one has guessed at until now), but if a particularly large helping of dark matter surrounded a clump of baryonic matter, that in itself may have caused the baryonic matter to collapse particularly quickly and easily into a rotating disk. This disk might then tend to rotate faster and faster. Still, this increase in rotation could happen slowly, and the galaxy might well be an efficient star factory in its youth.

So dark matter could well be a factor here.

Ann

[rstevenson]

... but if a particularly large helping of dark matter surrounded a clump of baryonic matter, that in itself may have caused the baryonic matter to collapse particularly quickly and easily into a rotating disk. This disk might then tend to rotate faster and faster. ...

Well, "rotate faster and faster" is a description of a change in velocity. What energy source is causing that change in velocity?

Rob

[Ann]

... but if a particularly large helping of dark matter surrounded a clump of baryonic matter, that in itself may have caused the baryonic matter to collapse particularly quickly and easily into a rotating disk. This disk might then tend to rotate faster and faster. ...

Well, "rotate faster and faster" is a description of a change in velocity. What energy source is causing that change in velocity?

Rob

Good question, Rob, and one I certainly can't answer. But how did spiral galaxies get their original spin anyway? It seems to me that they really did "speed up" as they collapsed into disks. Am I wrong about that?

A disk galaxy can probably spin up if it merges with another, preferably smaller galaxy in just the right way. A galaxy with a lot of dark matter might be better than other galaxies at attracting other galaxies to merge with.

Ann

[rstevenson]

... But how did spiral galaxies get their original spin anyway? It seems to me that they really did "speed up" as they collapsed into disks. Am I wrong about that?

There's a fairly good explanation at... http://trenchesofdiscovery.blogspot.ca/ ... otate.html

Since the mass of the visible galaxy plus its dark matter is what starts the rotation in the first place, no doubt a galaxy with an unusually large amount of dark matter would collapse and spin faster -- to begin with. But it seems to me that this process should settle down into a stable state regardless of how much mass the galaxy contains, unless and until that stability is interrupted by the acquisition of more mass from a merger.

Rob