Orphan supernovae?

November 14, 2011 By Steve Nerlich
Supernova G292.0+1.8. Like most supernovae it detonated within a host galaxy - in fact our galaxy. But are there orphan supernovae without host galaxies? Credit: Chandra.

For some years now astronomers have been scratching their heads over the appearance of supernovae that detonate out in the middle of nowhere – rather than within a host galaxy.

Various hypotheses have been proposed, notably that they might be hypervelocity  - which are stars flung out of their host galaxy due to an unfortunate coincidence of gravitational interactions. It’s thought that such interactions may accelerate those stars up to a velocity of more than 100 kilometers a second – that is, more than the escape velocity of your average galaxy.

But Zinn et al suggest a more mundane suggestion for their particular orphan of interest, which is SN 2009z. They propose that it is in a galaxy, it’s just a galaxy that is very difficult to see.

They propose the supernova actually detonated within a low surface brightness galaxy, N271. From the images they have produced, this seems a reasonable claim – it’s just that low surface brightness galaxies (or LSBs) aren’t meant to have supernovae.

Since galaxies can appear as extended objects, rather than as point-like stars, we refer to them as having ‘surface brightness’ - which can vary across the object’s apparent surface. LSB’s are generally isolated field galaxies, rather than being grouped in amongst dense galaxy clusters. They are most often dwarf galaxies as well, but at least one spiral LSB has been identified.

The dimness of LSB galaxies is suggestive of them having almost no active star formation – either being too old, with no free hydrogen remaining for new star formation – or just not dense enough for much star formation to ever have taken off.

Left frame: Sloan Digital Sky Survey image showing the location of the Type IIB supernovae SN 2009z. Right frame: Close-up of the rectangular area taken by the New Technology Telescope (ESO), showing the location of SN 2009z at the cross marks. It seems closely associated with the small galaxy N271, even though such a galaxy is not usually thought capable of supporting massive star formation. Credit: Zinn et al.

But here you have supernova SN 2009z that was most likely was contained within LSB galaxy N271. And SN 2009z was a Type II supernova – a massive and short-lived star that underwent core collapse. Indeed, it was a Type IIb with only a small shell of hydrogen when it detonated. Type IIb supernovae are probably massive stars which lose most, but not all, of their hydrogen shell through having it stripped off by a companion star in a binary system.

This all seems quite unusual behaviour for a galaxy that does not support active star formation. Zinn et al propose that LSB must go through short bursts of active followed by long quiescent phases of almost no activity. This then suggests that the progenitor star of supernova SN 2009z was formed in the previous starburst period, before N271 quietened down again.

Of course, none of this need suggest that hypervelocity stars don’t exist – indeed several have been discovered since the first confirmed finding in 2005. All those known are associated with the Milky Way, since finding a single isolated hypervelocity star ejected by a distant galaxy is probably beyond the detection of our current technology – unless of course they go supernovae.

But given what we know so far:

• a hypervelocity star arises from a binary system’s unfortunate interaction with a galaxy’s central supermassive black hole;

• one binary member is captured, the other flung violently outwards at escape velocity.

• but, massive stars that go supernovae only have a main sequence life span of the order of millions of years;

• so, even at more than 100 kilometers a second, it’s unlikely that any are going to make it across the many light years distance from the center of a galaxy to its outer boundary before they detonate.

Putting all this together... orphan supernovae? Busted (well, unless we find one anyway).

Explore further: Hubble Snaps Images of a Pinwheel-Shaped Galaxy

More information: Zinn et al. Supernovae without host galaxies? The low surface brightness host of SN 2009Z.

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1 / 5 (4) Nov 14, 2011
What about time dilation for the high speed ejected star? Our time is not it's time.
5 / 5 (2) Nov 14, 2011
10^5m/s isn't a relativistic speed, typicalguy.
3.7 / 5 (3) Nov 14, 2011
Could multiple galaxies colliding with each other and then separating again cause entire clouds of stars, basically mini-galaxies to spin off into space, taking supernova candidates with them?
5 / 5 (2) Nov 14, 2011
MorituriMax, sometimes sufficiently massive galaxies undergoing an interaction produce smaller, less massive "tidal dwarf" galaxies as collisional debris. Given that the redshifts of the 3 identified galaxies in the illustration (and N271) are all similar, perhaps a past interaction among the group produced the LSB galaxy N271. This in turn may have initiated star formation in this 3 billion solar-mass LSB galaxy leading to the formation of SN 2009z.

Although not mentioned in the paper, the tidal dwarf scenario is consistent with the elevated star formation rate seen in this dwarf galaxy.
not rated yet Nov 14, 2011
In every galactic collision simulation I have ever seen, vast numbers of stars are flung out into space never to return.

I fail to see why there should be any head scratching. There should be large numbers of stars in the voids between galaxies.
not rated yet Nov 14, 2011
No love for an idea that clouds of gas were expelled from a galaxy(ies) and later collapsed into star(s) (which later supernovaed)?
not rated yet Nov 14, 2011
As stated in the article single stars ejected from galaxies tend to take a long time to get anywhere. Longer than they take to turn into supernovas generally. Therefore the idea of hard to see galaxies that can contain and form supernovae is more appealing. Although as pointed out the other type could possibly happen.... maybe.
not rated yet Nov 14, 2011
What an incredibly lonely place that must be.

Any intelligent race that grows up around a star out in that void is pretty much doomed to solitude. At least we have some kind of hope that we can get out and explore the galaxy and maybe even meet some other races.

On the other hand, maybe they won't be tempted to waste valuable resources on space exploration that may not even pay off for us in the end. If we can find an economical way to travel faster than 5-10% the speed of light, then perhaps the same fate holds true for us as well.
1 / 5 (1) Nov 14, 2011
In the timescales you're talking about, Decimatus, I think we have a good chance of exploring everywhere and meeting everyone.. that is if we can get off this rock and spread our eggs over many many planets.
not rated yet Nov 14, 2011
In the timescales you're talking about, Decimatus, I think we have a good chance of exploring everywhere and meeting everyone.. that is if we can get off this rock and spread our eggs over many many planets.

We do yes, compared to the beings stuck out in the void. If we can go the speed of light, it has been estimated that we could colonize the entire galaxy ina bout 60 million years(balls to the wall expansion).

But I did say "economical method of travel". There is a pretty fair chance that the laws of physics will not allow our frail bodies to use some exotic form of travel such as warp, hyperspace, or wormholes. If that is the case, then the best system of travel might be antimatter powered with the speed of light being the limit.

The question becomes, can our(future) society affod the costs of spitting out colonists in all directions wihtout any decent expectation of return on investment?
not rated yet Nov 15, 2011
In the 1400's, the royalty who bankrolled the Atlantic crossings could reasonably expect that opening new trade routes and eventual resource acquisition would make for worthwhile investments.

Our children won't have it so easy, probably.

Imagine if Adam and Eve were the first colonists of a longshot effort of past humans to get off their old rock. They just fired a ship out to the nearest habitable planet and wished them good luck. Barely enough resources to afford sending a colony to earth, let alone a return flight. How would the mother colony look today? Wishing they hadn't spent valuable resources on a venture they will never benefit from? I know we benefit, but without benefit to the mother colony such ventures are less likely.

Of course, any number of things could change the picture completely, not least of which are advanced propulsion technologies. But in the end, the harder space colonization is, the less likely it is to happen.
not rated yet Nov 15, 2011
No love for an idea that clouds of gas were expelled from a galaxy(ies) and later collapsed into star(s) (which later supernovaed)?

No, sorry. If that cloud is tiny enough to have mass sufficient for only a star or two AND it is ejected out of the galaxy at escape velocities... Then no way its relatively tiny gravity can hold it together. Much less coalesce into a star.
1 / 5 (3) Nov 15, 2011
First this Steve Nerlich guy does not know the difference between supernova and supernovae. Then he believes that finding a supernova in a dim galaxy proves that no orphan supernovae exist.

In another ten years, we'll probably see entirely science-challenged people ignorantly writing science articles, relying on some kind of auto-correct-your-science-writing software.

Oh, well, maybe watching Mythbusters is the closest to science he's ever been.

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