April 16, 2012
Sometimes the universe seems like something from Dr. Seuss. In recent weeks astronomers have demonstrated this by announcing the discovery of juggling galaxies as well as a square, emerald-cut, gem of a galaxy. With 100 billion or so more galaxies to explore out there, who knows what else they will find? Green eggs and ham, perhaps?
Let?s start with the jugglers. Here is the situation: When astronomers add up all the gas and dust contained in ordinary galaxies like our own Milky Way, they run into a serious problem. There?s not nearly enough matter to account for rates at which stars are actually being created and born. If all the matter seen is all there is, stars should have all burned out eons ago and very few newer stars would light up the heavens.
Yet the Milky Way and other galaxies have clearly not burned out. So what gives?
Here is a little more detail: In our Milky Way it?s estimated that every year about one solar mass (an amount of matter equal to that of our Sun) worth of dust and gas is turned into stars. A survey of the available raw materials shows that our galaxy could not keep up this rate of star formation for longer than a couple of billion years ? only a fraction of the age of our galaxy. Star ages and comparisons with other spiral galaxies show that one solar mass per year is a typical star formation rate. So the problem appears to be universal.
One solution might be that a lot of matter is hanging out just outside of galaxies, blown there by the stellar winds of billions of stars and the occasional supernova blast. If that matter eventually tumbles back into galaxies, over multi-billion-year timescales, it could be part of a gigantic juggling act, or galactic fountain, which recycles matter and balances the galactic ledgers.
But there?s the rub: Outflows of matter from galaxies have been documented, but no one has evidence of matter was falling back in. Until now.
In March a team of astronomers led by Kate Rubin of the Max Planck Institute for Astronomy in Germany announced that they used the Keck I telescope on Mauna Kea to find evidence of inflows in distant spiral galaxies. Rubin and her team examined gas associated with a hundred galaxies at distances between 5 and 8 billion light-years with the Keck I telescope?s Low Resolution Imaging Spectrograph (LRIS). They found in six of those galaxies the first direct evidence that gas adrift in intergalactic space does indeed flow back into star-forming galaxies.
It appears that as the gas from the six galaxies drifts away it is, indeed, pulled back by the galaxy?s gravity and could re-enter the same galaxy on timescales of one to several billion years. If so, then the gas we find inside galaxies at any given time may only be about half of the raw material that ends up as fuel for star formation.
It?s a juggling act with about half the balls in the air. Added all together, there is a sufficient amount of raw matter to account for the observed rates of star formation.
In March an international team of astronomers using both Keck and Subaru telescopes announced and explained a rare square galaxy with a striking resemblance to an emerald cut diamond. The astronomers - from Australia, Germany, Switzerland and Finland - discovered the rectangular shaped galaxy within a group of 250 galaxies some 70 million light years away.
?In the Universe around us, most galaxies exist in one of three forms: spheroidal, disc-like, or lumpy and irregular in appearance,? said astrophysicist Alister Graham from Swinburne University of Technology in Sydney, Australia. A rectangular-shaped galaxy is a very unusual object.
False-color image of LEDA 074886. The central contrast has been adjusted to reveal the inner disk/bar-like component. For reference, the major axis of the boxy outer red annulus spans 3.2?3.8 kpc, while the outer edge of the outermost blue annulus has a major axis of 5.2 kpc. Central rotation was measured with Keck Observatory?s Echellette Spectrograph and Imager (ESI), from a slit placed across the disk of the galaxy. The image was taken with Suprime-Cam at the Subaru Telescope.
?It?s one of those things that just makes you smile because it shouldn?t exist, or rather you don?t expect it to exist,? said Graham. ?It?s a little like the?discovery of some exotic new species which at first glance appears to defy the laws of nature.?
The unusually shaped galaxy was detected in a wide field-of-view image taken with the Japanese Subaru Telescope for an unrelated program by Swinburne astrophysicist Lee Spitler.
The astronomers suspect it is unlikely the galaxy is shaped like a cube. Instead, they believe that it may resemble an inflated disc seen side on, like a short cylinder.
Support for this scenario comes from observations with Keck Observatory?s Echellette Spectrograph and Imager (ESI), which revealed a rapidly spinning, thin disc with a side-on orientation lurking at the center of the galaxy. The outermost measured edge of this galactic disc is rotating at a speed in excess of 100,000 kilometers per hour.
?One possibility is that the galaxy may have formed out of the collision of two spiral galaxies,? said Swinburne?s Duncan Forbes, a co-author of the research. ?While the pre-existing stars from the initial galaxies were strewn to large orbits creating the emerald cut shape, the gas sank to the mid plane where it condensed to form new stars and it?s the disc that we have observed.?
?Curiously, if the orientation was just right,? said Graham, ?when our own disc-shaped galaxy collides with the disc-shaped Andromeda galaxy about three billion years from now we may find ourselves the inhabitants of a square looking galaxy.?
So there you have it: jugglers and a square. It?s enough to humble every Who in Whoville.