Do we know (have we observed and cataloged) any rogue star, being not part of galaxy, but drifting somewhere in inter-galactic space?

I know that determining if a star is a part of galaxy or not is a matter of definition, but let's take those, that inter-galactic space is where the gravitational influence of any galaxy isn't dominating.


Different types of observations have been made:

  • hypervelocity stars: there are stars in the halo of our Galaxy (it is kind of the suburbs of the Milky Way) with such velocity that they are escaping it, and aiming to the intergalactic medium (IGM). There not yet fully in the IGM, but the're on their way. (Reference)
  • molecular clouds in the IGM: there are molecular clouds detected in the IGM, that are forming stars. Stars are not directly observed, but these star forming molecular clouds are proof of their existence. (Reference)

It's hard to do better, because it is hard to observe directly stars outside of the Milky Way.

  • $\begingroup$ +1 I completely forget about molecular clouds in the IGM. It would be really wild to find an entire planetary system that developed in the IGM. Maybe one day we will find a way to observe them. $\endgroup$ – called2voyage Oct 16 '13 at 12:28
  • $\begingroup$ Well, most of the time you can expect to have a planetary system when you form a star, whatever the environment. Basically, all you need is a protostellar (and later protoplanetary) disk to form, and disks are a quite common feature of star formation (and they form by angular momentum conservation so you don't really care of the environment). So, ISM or IGM, as soon as you form stars, you should form planets as well! $\endgroup$ – MBR Oct 16 '13 at 12:37
  • $\begingroup$ Right that is what is so exciting...to imagine a world that formed completely outside of a galaxy. Could such a world form life? What would it be like? So many possibilities. $\endgroup$ – called2voyage Oct 16 '13 at 12:38
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    $\begingroup$ It would partly depends on the composition of your molecular clouds. I don't know much about it, but it seems that most of these clouds result from galaxies interaction (and are therefore made of gas from galaxies, not just H and He, the main components of IGM), so you could expect to have planets in habitable zone with enough oxygen, carbon, and all those elements you'd need to see life forms poping out. $\endgroup$ – MBR Oct 16 '13 at 12:45
  • $\begingroup$ Intergalactic stars are well established en.wikipedia.org/wiki/Intergalactic_star#Discovery $\endgroup$ – ProfRob Mar 16 '15 at 22:59

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