Globular clusters rotate at heart

May 09, 2014
The core of the globular cluster Messier 13 is just 25 000 light-years away and measures about 145 light-years in diameter. It lies in the constellation Hercules and sometimes can even be seen with small binoculars. Credit: ESA / Hubble und NASA

(Phys.org) —Astronomers from the University of Texas at Austin and Germany's Max Planck Institute for Extraterrestrial Physics (MPE) recently found a surprise when studying some of the oldest star clusters in our galaxy. The stars at the centers of these clusters are rotating around a common axis. It was previously thought any central rotation would have been long erased, leaving the central stars to random orbits. The work has been accepted for publication in the Astrophysical Journal Letters.

These "globular clusters" are ancient collections of up to a million old with simple chemical compositions, tightly bound together by gravity. Globular clusters orbit most galaxies, including our own Milky Way. Due to these clusters' old age and fairly spherical shape, with a strong concentration of stars towards the center, they have historically been viewed as simple systems. However, new observations keep revealing surprising results.

The team, led by MPE's Maximilian Fabricius and including Texas' Eva Noyola, observed 11 globular clusters from the University of Texas at Austin's McDonald Observatory with the Harlan J. Smith Telescope. They found that all of the clusters show this central rotation.

This result is "astonishing," Fabricius says. "We did not expect this; originally we observed these globular clusters to measure their central velocity dispersions"—that is, the random motions of stars within a cluster.

The 2.7-meter (107-inch) Harlan J. Smith Telescope at the University of Texas McDonald Observatory. Photo by Marty Harris/McDonald Observatory.

Noyola adds, "Theory and numerical simulations of globular clusters indicate that any central rotation should be erased on relatively short timescales. Because these globular clusters were formed billions of years ago, we would expect that any rotation signature would have been eradicated by now. Even though previous measurements showed some rotation in a handful of systems, they only probed the motion of stars in the outer regions."

The astronomers are about halfway through their project of studying 27 of the Milky Way's approximately 150 globular clusters. Their findings raise interesting questions about the formation history and evolution of globular clusters. None of the current theoretical models predict such a ubiquitous and strong rotation.

However, it is important to note that the 11 globular clusters studied so far do not include any so-called "core-collapsed" . Core-collapse is a process that might eradicate rotation. Future observations of the remaining 16 clusters the team plans to study will shed light on this and other questions, such as a possible correlation between rotation and the position of a globular cluster inside our galaxy.

Velocity field of the globular cluster Messier 13, where the velocities range from -5km/s (away from the observer – blue) to +5km/s (towards the observer – red). The solid line shows the rotation axis, while the green arcs indicate the flattening direction measured from the distribution of the stars (from archive data). Credit: From Fabricius et al. 2014

The new measurements of a these globular cluster cores were only possible with the help of the MPE-built instrument VIRUS-W, which was used in conjunction with the 2.7-meter Harlan J. Smith Telescope for this research. VIRUS-W allows the scientists to simultaneously measure more than 260 spectra in their field of view, determining the motion of stars to an accuracy of a few kilometers per second. That means that for a given globular cluster, one night at the Smith Telescope with an observing time of a few hours is enough to determine the velocity field at the core of a cluster.

Explore further: Entire star cluster thrown out of its galaxy

More information: "Central Rotations of Milky Way Globular Clusters," Maximilian H. Fabricius et al., Astrophysical Journal Letters. Preprint: arxiv.org/abs/1405.1722

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User comments : 11

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Tuxford
1.3 / 5 (13) May 09, 2014
They found that all of the clusters show this central rotation. This result is "astonishing,"

Not so if one applies just a bit of logic, instead of simple-minded merger-mania conventional wisdom. If globulars grow from within by the spontaneous nucleation of new matter from deep within the core of the mother star, and eventually grow into galaxies which themselves are observed to have similar rotational orbits of stars, it seems that this mechanism might well be consistent from the early stages of globular formation.
Pejico
May 09, 2014
This comment has been removed by a moderator.
Tuxford
1 / 5 (10) May 09, 2014
@Pejico

You must be new to my posts, else you would know not to ask, since the answer is disturbing to conventional wisdom.

LaViolette's SubQuantum Kinectics predicts such growth models. And now, if you are paying attention, the observational evidence is becoming more evident. It is only a matter of time....

His physics model is recently published.

http://starburstf...l_G.html

And now the system dynamics model is available online, so one can experiment with initial conditions that exist deep within the core stars. Hence we see massive outflows of gas from active galaxies.

http://starburstf...mulator/
cantdrive85
1 / 5 (9) May 10, 2014
It was previously thought any central rotation would have been long erased

Yep.

This result is "astonishing," Fabricius says. "We did not expect this

Sounds like a falsification of the theory.

"Theory and numerical simulations of globular clusters indicate that any central rotation should be erased on relatively short timescales. Because these globular clusters were formed billions of years ago, we would expect that any rotation signature would have been eradicated by now.

Yep, definitely a falsification.

However, it is important to note that the 11 globular clusters studied so far do not include any so-called "core-collapsed" globular clusters. Core-collapse is a process that might eradicate rotation.

Can you say ad hoc explanation? Sad state of affairs in "science" these days...

cantdrive85
May 10, 2014
This comment has been removed by a moderator.
Rustybolts
5 / 5 (1) May 10, 2014
Astonishing because we didn't understand it in the first place.
vidyunmaya
2 / 5 (4) May 10, 2014
Very useful and Interesting. Do you see Magnetic fields around these clusters and a central axis ?
vidyunmaya
2 / 5 (4) May 10, 2014
Very useful and Interesting. Do you see Magnetic fields around these clusters and a central axis ?
omatwankr
5 / 5 (2) May 10, 2014
don't worry, its nothing a liberal sprinkling of fair dust , ooops, I mean dArK MAtTeR won't be able to fix

ranter out
katesisco
1 / 5 (2) May 10, 2014
I wonder if 'mother star;' is a euphemism for a black hole too small to be identified? I also support the idea of a magnetic field. These clusters, born into a strong magnetic field, retain this field forever?
thingumbobesquire
5 / 5 (3) May 10, 2014
Seems that this taken with the recent finding that younger stars are at the center of clusters overturns some assumptions. I.e., new hypo-thesis.
denise
3 / 5 (2) May 11, 2014
Uh Tuxford..a little advice...I know this is a physics site, but articles get picked up from different sources and people from all over can be introduced to the site. So yes, we can be new here...so enough with the condescending sarcasm to show off your obvious higher level of knowledge of star clusters. In response to your snide comment to Pejico, a simple polite educational response would have been much more professional and appreciated. While I don't have a degree in physics I do love science and have heard that some star clusters have evidence of black holes at their centers...but I didn't know they could pop out new stars like gremlins. You learn something new everyday.
barakn
5 / 5 (3) May 12, 2014
They found that all of the clusters show this central rotation. This result is "astonishing,"

Not so if one applies just a bit of logic, instead of simple-minded merger-mania conventional wisdom. If globulars grow from within by the spontaneous nucleation of new matter from deep within the core of the mother star, and eventually grow into galaxies which themselves are observed to have similar rotational orbits of stars, it seems that this mechanism might well be consistent from the early stages of globular formation. Tuxford

So not only does your fantastic "theory" violate conservation of energy/mass but now also violates conservation of angular momentum?

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