Stars made from galactic recycling material

Mar 15, 2012
Recycling in space: Images of the six galaxies with detected inflows taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Most of these galaxies have a disk-like, spiral structure, similar to that of the Milky Way. Star formation activity occurring in small knots is evident in several of the galaxies' spiral arms. Because the spirals appear tilted in the images, Rubin et al. concluded that we are viewing them from the side, rather than face-on. This orientation meshes well with a scenario of 'galactic recycling' in which gas is blown out of a galaxy perpendicular to its disk, and then falls back in at different locations along the edge of the disk. Credit: K. Rubin, MPI for Astronomy

(PhysOrg.com) -- Ordinary galaxies such as our own Milky Way contain a plethora of gas and dust. Nevertheless, there is not nearly enough matter to explain how galaxies produce new stars at the observed rates for long. As a solution, a matter cycle on gigantic scales has been proposed, for which concrete traces exist in our local galactic neighbourhood. Now, a study led by Kate Rubin of the Max Planck Institute for Astronomy has found the first direct evidence of such a key part of "galactic recycling" also in distant galaxies gas flowing back into distant galaxies.

Star formation regions, such as the , create some of the most beautiful astronomical sights. It is estimated that in our home galaxy, the Milky Way, on average one solar mass's worth of matter per year is turned into stars. Yet a survey of the available raw material, clouds of gas and dust, shows that, using only its own resources, our galaxy could not keep up this rate of star formation for longer than a couple of billion years.

Our own Milky Way, however, is significantly older than this and still active. Why is this the case? Is our home galaxy currently undergoing a rather special, short-lived era of star formation? Both stellar age determinations and comparison with other spiral show that not to be the case. One per year is a typical star formation rate, and the problem of insufficient raw matter appears to be universal as well.

Evidently, additional matter finds its way into galaxies. One possibility is an inflow from huge low-density filling the intergalactic voids; there is, however, very little evidence that this is happening. Another possibility, closer to home, involves a gigantic cosmic matter cycle. Gas is observed to flow away from many galaxies, and may be pushed by several different mechanisms, including violent supernova explosions (which are how end their lives), and the sheer pressure exerted by light emitted by bright stars on gas in their cosmic neighbourhood.

As this gas drifts away, it is pulled back by the galaxy's gravity, and could re-enter the same galaxy in time scales of one to several billion years. This process might solve the mystery: the gas we find inside galaxies may only be about half of the raw material that ends up as fuel for star formation. Large amounts of gas are caught in transit, but will re-enter the galaxy in due time. Add up the galaxy's gas and the gas currently undergoing cosmic recycling, and there is a sufficient amount of raw matter to account for the observed rates of star formation.

There was, however, uncertainty about the viability of this proposal for cosmic recycling. Would such gas indeed fall back, or would it more likely reach the galaxy's escape velocity, flying ever further out into space, never to return? For local galaxies out to a few hundred million light-years in distance, there had indeed been studies showing evidence for inflows of previously-expelled gas. But what about more distant galaxies, where outflows are known to be much more powerful – would gravity still be sufficient to pull the gas back? If no, astronomers might have been forced to radically rethink their models for how is fuelled on galactic scales.

Now, a team of astronomers led by Kate Rubin (MPIA) has used the Keck I telescope on Mauna Kea, Hawai'i, to examine gas associated with a hundred galaxies at distances between 5 and 8 billion light-years (z ~ 0.5 – 1), finding, in six of those galaxies, the first direct evidence that gas adrift in intergalactic space does indeed flow back into star-forming galaxies.

As the observed rate of inflow might well depend on a galaxy's orientation relative to the observer, and as Rubin and her team can only measure average gas motion, the real proportion of galaxies with this kind of inflow is likely to be higher than the 6% directly suggested by their data, and could be as high as 40%. This is a key piece of the puzzle and important evidence that cosmic recycling ("galactic fountains") could indeed solve the mystery of the missing raw matter.

Explore further: Image: Chandra's view of the Tycho Supernova remnant

More information: Kate H. R. Rubin et al., The Direct Detection of Cool, Metal-Enriched Gas Accretion onto Galaxies at z ~ 0.5, Astrophysical Journal Letters, Bd. 747 (2012), S. 26ff.

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Pressure2
1 / 5 (1) Mar 15, 2012
Could a galaxy's magnetic field be the reason for the gases spiraling back into the galaxy's outer edges?

Gas ions could be expelled at the galaxy's poles by a magnetic field and reenter at its outer edges.
kevinrtrs
1 / 5 (6) Mar 15, 2012
It is estimated that in our home galaxy, the Milky Way, on average one solar mass's worth of matter per year is turned into stars

Where and how did they get this estimate? That would be interesting to know because it would explain why they are suddenly so concerned about the missing mass. What presuppositions were made in getting to that estimate?

Questions do pop up though with the mention of billions of years age for the Milky way. Why does it still have spiral arms after all these years? Surely those should have wound up by now, given the rate at which the galaxy&arms are rotating - you can correct me if I'm wrong.

AS for a magnetic field: Why does the galaxy have one in the first place? No secular theoretical model has been able to adequately and satisfactorily explain where it comes from so far. It appears to be another one of those vexing unsolvable conundrums for the big bang model.
barakn
not rated yet Mar 15, 2012
Could a galaxy's magnetic field be the reason for the gases spiraling back into the galaxy's outer edges?

Gas ions could be expelled at the galaxy's poles by a magnetic field and reenter at its outer edges.
Ions trapped in a magnetic dipole tend to get stuck on a field line and bounce from pole to pole.

Tachyon8491
1 / 5 (2) Mar 15, 2012
Gravitational inflow of previously ejected gas and dust would be logical in this ontogenic galactic cycle - what is missing in the article is the actual symptomology of this inflow - what is the form of this evidence, what are its dynamic attributes? I dare presume that it involves differential red/blue doppler shift in certain segments of the circumspherical galactic aura.
Pressure2
not rated yet Mar 15, 2012
Barakn, wouldn't the ions follow the magnetic lines? A rotating galaxy's magnetic field would be shaped just like a washer magnet. The positive ions should be expelled from one pole, negative ions out from the other.
roboferret
5 / 5 (3) Mar 16, 2012
Questions do pop up though with the mention of billions of years age for the Milky way. Why does it still have spiral arms after all these years? Surely those should have wound up by now, given the rate at which the galaxy&arms are rotating - you can correct me if I'm wrong.

http://en.wikiped...e_theory very neatly explains many of the observed properties of spiral galaxies. If windup was an issue, it would be one for creationists too. The starlight - time problem they have (we can see things >6000LY away) is explained by saying that the curvature of spacetime was different then, the earth was in a "time bubble" which allowed the starlight to get here - but they forget this would allow millions and billions of years to pass in distant galaxies. They have no problems using contradictory arguments though, like Kev, they're not smart enough to notice.