Entire star cluster thrown out of its galaxy

Entire star cluster thrown out of its galaxy
This artist's illustration shows the hypervelocity star cluster HVGC-1 escaping from the supergiant elliptical galaxy M87. HVGC-1 is the first runaway star cluster discovered by astronomers. It is fated to drift through intergalactic space. Credit: David A. Aguilar (CfA)
(Phys.org) —The galaxy known as M87 has a fastball that would be the envy of any baseball pitcher. It has thrown an entire star cluster toward us at more than two million miles per hour. The newly discovered cluster, which astronomers named HVGC-1, is now on a fast journey to nowhere. Its fate: to drift through the void between the galaxies for all time.

"Astronomers have found runaway stars before, but this is the first time we've found a runaway ," says Nelson Caldwell of the Harvard-Smithsonian Center for Astrophysics. Caldwell is lead author on the study, which will be published in The Astrophysical Journal Letters and is available online.

The "HVGC" in HVGC-1 stands for hypervelocity globular cluster. Globular clusters are relics of the early universe. These groupings usually contain thousands of stars crammed into a ball a few dozen light-years across. The Milky Way galaxy is home to about 150 . The giant elliptical galaxy M87, in contrast, holds thousands.

It took a stroke of luck to find HVGC-1. The discovery team has spent years studying the space around M87. They first sorted targets by color to separate stars and galaxies from globular clusters. Then they used the Hectospec instrument on the MMT Telescope in Arizona to examine hundreds of globular clusters in detail.

A computer automatically analyzed the data and calculated the speed of every cluster. Any oddities were examined by hand. Most of those turned out to be glitches, but HVGC-1 was different. Its surprisingly high velocity was real.

Entire star cluster thrown out of its galaxy
The runaway star cluster HVGC-1 (circled) in a photo from the Canada-France-Hawaii Telescope. The cluster is zooming toward us at a speed of more than two million miles per hour. Credit: CFHT
"We didn't expect to find anything moving that fast," says Jay Strader of Michigan State University, a co-author on the study.

How did HVGC-1 get ejected at such a high speed? Astronomers aren't sure but say that one scenario depends on M87 having a pair of at its core. The star cluster wandered too close to those black holes. Many of its outer stars were plucked off, but the dense core of the cluster remained intact. The two black holes then acted like a slingshot, flinging the cluster away at tremendous speed.

HVGC-1 is moving so fast that it is doomed to escape M87 altogether. In fact, it may have already left the galaxy and be sailing out into intergalactic space.

Entire star cluster thrown out of its galaxy
The monstrous elliptical galaxy M87 is the home of several trillion stars, a supermassive black hole, and a family of 15,000 globular star clusters. One of those globular clusters, HVGC-1, is escaping the galaxy after being flung outward at tremendous speed. Credit: NASA, ESA, and the Hubble Heritage Team

M87, source of the hypervelocity cluster HVGC-1, is a king among galaxies. This supergiant elliptical galaxy weighs as much as 6 trillion Suns, making it one of the most massive galaxies in the nearby universe.

The discovery of HVGC-1 suggests that the core of M87 holds not one but two supermassive . This must be the result of a long-ago collision between two , which merged to form a single giant galaxy. The same fate awaits our own Milky Way, which will collide with the Andromeda galaxy in a few billion years to create an ellilptical galaxy that astronomers have dubbed Milkomeda.

Explore further

Image: Hubble looks into Terzan 7

More information: Paper: arxiv.org/abs/1402.6319
Journal information: Astrophysical Journal Letters

Citation: Entire star cluster thrown out of its galaxy (2014, April 30) retrieved 22 May 2019 from https://phys.org/news/2014-04-entire-star-cluster-thrown-galaxy.html
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Apr 30, 2014
What an amazing find in the nearby Virgo cluster. This globular cluster now holds the record for the largest recorded blueshift of any object wrt the Milky Way. HVGC-1 has an approach velocity of 1026 km/s compared to the Andromeda galaxy (300 km/s) and the previous record holder, the star B030D, in the Andromeda Giant Stream (780 km/s): http://www.scient...niverse/

Of course it may be some time before this cluster is anywhere near the future Milkomeda galaxy, as M 87 is ~55 million light years from us.

Apr 30, 2014
It should be noted that this HVGC-1 is not the only unbound cluster in the Virgo cluster, only the one with the highest velocity wrt the cluster mean velocity. Various studies have found examples of intercluster globular clusters (globular clusters not gravitationally bound to any single galaxy) in Virgo and some studies estimate that as many as ~12,000 of these clusters exist:



The vast majority of these clusters were likely tidally stripped from galaxies passing through the core of the cluster while a few may have been created through tidal interactions of individual galaxies within the Virgo cluster. The anomalous velocity of HVGC-1 points to processes other than tidal stripping or galactic interactions to account for its origin.

Apr 30, 2014
There is one thing I don't understand about this. A runaway star going into another galaxy would be accelerated by the local superblackhole again, and after some of these iterations it would be travelling at a very significant percentage of light speed. Then, let's suppose it is heading to our galaxy at near light speed. From our reference frame, we should measure an acceleration of this star according to the gravity equation, but this would be impossible as it would violate the light speed limit. So what we would see? a star that is accelerating to faster than light speed in our direction or a star that does not obey the gravity law? In the first case, we would see the star entering our solar system before it enters our own galaxy, which sounds weird. But in the second case we would see a star passing by without being affected by gravity (apparently), which is also weird. Please could somebody clarify this?

Apr 30, 2014
At least we have time to think of a better name for our new host galaxy. Anything would be better than Milkomeda. How about Androway?

Apr 30, 2014
. A runaway star going into another galaxy would be accelerated by the local superblackhole again, and after some of these iterations it would be travelling at a very significant percentage of light speed.

It loses that speed again when it comes out the other side (it travels into the gravitational field, trading potential for speed and then back out again trading speed for potential). A kick is only given if it does something like a swing-by maneouver. For that to happen there must be very specific circumstances. It must be going very close to the center of gravity (the black hole(s)) and the relative motion must be in a way that one can gain momentum (while the other loses some).

Getting near light speed is pretty impossible with this as relativistic mass increase (at the latest) will make a swing by where the star gains any speed increasingly unlikely.

May 10, 2014
Primer Fields
I believe these videos clearly show how magnetic fields would launch stars or star clusters across the cosmos without the need for black holes.

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