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We know there are rouge stars floating in intergalactic space, thought to be caused by galactic collision. What other other classes of celestial object could be found floating around in intergalactic space?

Within galaxies there are the following types of object:

  • Nebulae
  • Stars
  • Brown Dwarfs
  • White Dwarfs
  • Black Holes
  • Neutron Stars
  • Comets
  • Asteroids
  • Interstellar rogue planets
  • Planetary Systems
  • Star Systems
  • Star Clusters
  • Debris Disks

Which of these can possibly exist in intergalactic space? Asteroids and comets are probably quite common, but is it known for entire planetary systems to exist in intergalactic space? Even star clusters?

Out of all the possible types, could I also have a link / designation to each object thats been observed, floating in intergalactic space.

For example, here's an intergalactic star HE_0437-5439

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You ask two very different questions with an implied equivalence between them: which can possibly be in intergalactic space and which have been seen in intergalactic space. Any object that is sufficiently bound to be ejected from a galaxy is a candidate for intergalactic space. From that point of view the most challenging are star clusters and nebulae. Given a star that is going to be ejected, at least some of the planets/companions can be close enough to the primary star that they can be ejected with it. Objects far from the primary can be ejected on their own. That doesn't mean that they have been seen. I would wager a substantial amount that no brown dwarf has been seen outside the galaxy, but that doesn't mean there aren't any.

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  • $\begingroup$ I didn't I said out of the possible types, could I have a reference to those that happen to have also been discovered. $\endgroup$ – Starkers Jul 19 '14 at 3:23
  • $\begingroup$ Maybe I wasn't utterly clear, but the proof is more of an added bonus rather than being integral to the main question $\endgroup$ – Starkers Jul 19 '14 at 3:23
  • $\begingroup$ My point is that I think all of these can be ejected from galaxies, but some will be hard to see at long distance. For weakly bound ones, you need to make sure that the tidal forces do not disrupt the system. When you do that, you may lose the outer reaches, but you may eject enough to count as whatever category you are interested in. Certainly we can't see asteroids outside the galaxy. $\endgroup$ – Ross Millikan Jul 19 '14 at 3:37
  • $\begingroup$ No I wouldn't think so! Even planetary systems? That's pretty amazing, have we observed star wobble on rogue stars? $\endgroup$ – Starkers Jul 19 '14 at 5:47
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I think Ross answers the question well. Pretty much all of these things could exist between galaxies.

I know of no direct detections of many of them (especially planets, comets, asteroids, rogue planets, debris disks, brown dwarfs - all of which are too faint).

Here are some decent reference to get going with: NB. these refer to "intracluster" objects, because they have mostly been detected in clusters of galaxies (between the galaxies), particularly in Virgo and Fornax.

The detection of type Ia supernovae in "intergalactic space" (hence impying white dwarfs there too). http://adsabs.harvard.edu/abs/2003AJ....125.1087G

Intergalactic red giants http://adsabs.harvard.edu/abs/1998Natur.391..461F

Intergalactic globular clusters http://adsabs.harvard.edu/abs/2007ApJ...654..835W

Intergalactic planetary nebulae (i.e. in between red giants and white dwarfs) http://adsabs.harvard.edu/abs/2002AJ....123..760A

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