Spiral Galaxies Stripped Bare

Six spectacular spiral galaxies are seen in a clear new light in images from ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. The pictures were taken in infrared light, using the impressive power of the HAWK-I camera, and will help astronomers understand how the remarkable spiral patterns in galaxies form and evolve.

HAWK-I [1] is one of the newest and most powerful cameras on ESO’s Very Large Telescope (VLT). It is sensitive to infrared light, which means that much of the obscuring dust in the galaxies’ spiral arms becomes transparent to its detectors. Compared to the earlier, and still much-used, VLT infrared camera ISAAC, HAWK-I has sixteen times as many pixels to cover a much larger area of sky in one shot and, by using newer technology than ISAAC, it has a greater sensitivity to faint infrared radiation [2]. Because HAWK-I can study galaxies stripped bare of the confusing effects of dust and glowing gas it is ideal for studying the vast numbers of stars that make up spiral arms.

The six galaxies are part of a study of spiral structure led by Preben Grosbøl at ESO. These data were acquired to help understand the complex and subtle ways in which the stars in these systems form into such perfect spiral patterns.

The first image shows NGC 5247, a spiral galaxy dominated by two huge arms, located 60–70 million light-years away. The galaxy lies face-on towards Earth, thus providing an excellent view of its pinwheel structure. It lies in the zodiacal constellation of Virgo (the Maiden).

The galaxy in the second image is Messier 100, also known as NGC 4321, which was discovered in the 18th century. It is a fine example of a “grand design” spiral galaxy — a class of galaxies with very prominent and well-defined spiral arms. About 55 million light-years from Earth, Messier 100 is part of the Virgo Cluster of galaxies and lies in the constellation of Coma Berenices (Berenice’s Hair, named after the ancient Egyptian queen Berenice II).

The third image is of NGC 1300, a spiral galaxy with arms extending from the ends of a spectacularly prominent central bar. It is considered a prototypical example of barred spiral galaxies and lies at a distance of about 65 million light-years, in the constellation of Eridanus (the River).

The spiral galaxy in the fourth image, NGC 4030, lies about 75 million light-years from Earth, in the constellation of Virgo. In 2007 Takao Doi, a Japanese astronaut who doubles as an amateur astronomer, spotted a supernova — a stellar explosion that is briefly almost as bright as its host galaxy — going off in this galaxy.

The fifth image, NGC 2997, is a spiral galaxy roughly 30 million light-years away in the constellation of Antlia (the Air Pump). NGC 2997 is the brightest member of a group of galaxies of the same name in the Local Supercluster of galaxies. Our own Local Group, of which the Milky Way is a member, is itself also part of the Local Supercluster.

Last but not least, NGC 1232 is a beautiful galaxy some 65 million light-years away in the constellation of Eridanus (the River). The galaxy is classified as an intermediate spiral galaxy — somewhere between a barred and an unbarred spiral galaxy. An image of this galaxy and its small companion galaxy NGC 1232A in visible light was one of the first produced by the VLT (eso9845). HAWK-I has now returned to NGC 1232 to show a different view of it at near-infrared wavelengths.

As this galactic gallery makes clear, HAWK-I lets us see the spiral structures in these six bright galaxies in exquisite detail and with a clarity that is only made possible by observing in the infrared.

Giant Star Goes Supernova -- And Is Smothered By Its Own Dust

A giant star in a faraway galaxy recently ended its life with a dust-shrouded whimper instead of the more typical bang.

Ohio State University researchers suspect that this odd event -- the first one of its kind ever viewed by astronomers – was more common early in the universe.

It also hints at what we would see if the brightest star system in our galaxy became a supernova.

In a paper published online in the Astrophysical Journal, Christopher Kochanek, a professor of astronomy at Ohio State, and his colleagues describe how the supernova appeared in late August 2007, as part of the Spitzer Space Telescope Deep Wide Field Survey.

The astronomers were searching the survey data for active galactic nuclei (AGN), super-massive black holes at the centers of galaxies. AGN radiate enormous amounts of heat as material is sucked into the black hole. In particular, the astronomers were searching for hot spots that varied in temperature, since these could provide evidence of changes in how the material was falling into the black hole.

Because of the alignment of the galaxy with Earth and our sun, astronomers were not able to see what the event looked like to the naked eye while it was happening. But Kochanek believes that we might see the star brighten a decade or so from now. That’s how long it will take for the shockwave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we’ll have something to see here on Earth.

Normally, astronomers wouldn’t expect to find a supernova this way, explained then-Ohio State postdoctoral researcher Szymon Kozlowski. Supernovae release most of their energy as light, not heat.

But one very hot spot, which appeared in a galaxy some 3 billion light years from Earth, didn’t match the typical heat signal of an AGN. The visible spectrum of light emanating from the galaxy didn’t show the presence of an AGN, either – the researchers confirmed that fact using the 10-meter Keck Telescope in Hawaii.

Enormous heat flared from the object for a little over six months, then faded away in early March 2008 – another clue that the object was a supernova.

“Over six months, it released more energy that our sun could produce in its entire lifetime,” Kozlowski said.

The astronomers knew that if the source were a supernova, the extreme amount of energy it emitted would qualify it as a big one, or a “hypernova.” The temperature of the object was around 1,000 Kelvin (about 700 degrees Celsius) -- only a little hotter than the surface of the planet Venus. They wondered -- what could absorb that much light energy and dissipate it as heat?

The answer: dust, and a lot of it.

Using what they learned from the Spitzer survey, the astronomers worked backward to determine what kind of star could have spawned the supernova, and how the dust was able to partly muffle the explosion. They calculated that the star was probably a giant, at least 50 times more massive than our sun. Such massive stars typically belch clouds of dust as they near the end of their existence.

This particular star must have had at least two such ejections, they determined – one about 300 years before the supernova, and one only about four years before it. The dust and gas from both ejections remained around the star, each in a slowly expanding shell. The inner shell – the one from four years ago – would be very close to the star, while the outer shell from 300 years ago would be much farther away.

“We think the outer shell must be nearly opaque, so it absorbed any light energy that made it through the inner shell and converted it to heat,” said Kochanek, who is also the Ohio Eminent Scholar in Observational Cosmology.

That’s why the supernova showed up on the Spitzer survey as a hot dust cloud.

Krzysztof Stanek, professor of astronomy at Ohio State, said that stars probably choked on their own dust much more often in the distant past.

“These events are much more likely to happen in a small, low metallicity galaxy,” he said -- meaning a young galaxy that hadn’t been around long enough for its stars to fuse hydrogen and helium into the more complex chemicals that astronomers refer to as “metals.”

Still, Kozlowski added that more such supernovae will likely be found by NASA’s Wide-field Infrared Explorer (WISE), which was launched in December 2009. “I would expect WISE to see 100 of these events in two years, now that we know what to look for,” he said.

Because of the alignment of the galaxy with Earth and our sun, astronomers were not able to see what the event looked like to the naked eye while it was happening. But Kochanek believes that we might see the star brighten a decade or so from now. That’s how long it will take for the shockwave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we’ll have something to see here on Earth.

We do have at least one chance to see a similar light show closer to home, though.

“If Eta Carinae went supernova right now, this is what it would probably look like,” Kochanek said, referring to the brightest star system in our Milky Way Galaxy.

The two stars that make up Eta Carinae are 7,500 light years away, and they host a distinctive dust shell dubbed the Homunculus Nebula, among other layers of dust. Astronomers believe that the nebula was created when the larger of the two stars underwent a massive eruption around 1840, and that future eruptions are likely.

Growing Galaxies Gently

New observations from ESO’s Very Large Telescope have, for the first time, provided direct evidence that young galaxies can grow by sucking in the cool gas around them and using it as fuel for the formation of many new stars. In the first few billion years after the Big Bang the mass of a typical galaxy increased dramatically and understanding why this happened is one of the hottest problems in modern astrophysics. The results appear in the 14 October issue of the journal Nature.

The first galaxies formed well before the Universe was one billion years old and were much smaller than the giant systems — including the Milky Way — that we see today. So somehow the average galaxy size has increased as the Universe has evolved. Galaxies often collide and then merge to form larger systems and this process is certainly an important growth mechanism. However, an additional, gentler way has been proposed.

A European team of astronomers has used ESO’s Very Large Telescope to test this very different idea — that young galaxies can also grow by sucking in cool streams of the hydrogen and helium gas that filled the early Universe and forming new stars from this primitive material. Just as a commercial company can expand either by merging with other companies, or by hiring more staff, young galaxies could perhaps also grow in two different ways — by merging with other galaxies or by accreting material.

The team leader, Giovanni Cresci (Osservatorio Astrofisico di Arcetri) says: “The new results from the VLT are the first direct evidence that the accretion of pristine gas really happened and was enough to fuel vigorous star formation and the growth of massive galaxies in the young Universe.” The discovery will have a major impact on our understanding of the evolution of the Universe from the Big Bang to the present day. Theories of galaxy formation and evolution may have to be re-written.

The group began by selecting three very distant galaxies to see if they could find evidence of the flow of pristine gas from the surrounding space and the associated formation of new stars. They were very careful to make sure that their specimen galaxies had not been disturbed by interactions with other galaxies. The selected galaxies were very regular, smoothly rotating discs, similar to the Milky Way, and they were seen about two billion years after the Big Bang (at a redshift of around three).

In galaxies in the modern Universe the heavy elements [1] are more abundant close to the centre. But when Cresci’s team mapped their selected distant galaxies with the SINFONI spectrograph on the VLT [2] they were excited to see that in all three cases there was a patch of the galaxy, close to the centre, with fewer heavy elements, but hosting vigorously forming stars, suggesting that the material to fuel the star formation was coming from the surrounding pristine gas that is low in heavy elements. This was the smoking gun that provided the best evidence yet of young galaxies accreting primitive gas and using it to form new generations of stars.

As Cresci concludes: “This study has only been possible because of the outstanding performance of the SINFONI instrument on the VLT. It has opened a new window for studying the chemical properties of very distant galaxies. SINFONI provides information not only in two spatial dimensions, but also in a third, spectral dimension, which allows us to see the internal motions inside galaxies and study the chemical composition of the interstellar gas.”
Notes

[1] The gas filling the early Universe was almost all hydrogen and helium. The first generations of stars processed this primitive material to create heavier elements such as oxygen, nitrogen and carbon by nuclear fusion. When this material was subsequently spewed back into space by intense particle winds from massive young stars and supernova explosions the amounts of heavy elements in the galaxy gradually increased. Astronomers refer to elements other than hydrogen and helium as “heavy elements”.

[2] By carefully splitting up the faint light coming from a galaxy into its component colours using powerful telescopes and spectrographs, astronomers can identify the fingerprints of different chemicals in remote galaxies, and measure the amounts of heavy elements present. With the SINFONI instrument on the VLT astronomers can go one better and get a separate spectrum for each part of an object. This allows them to make a map that shows the quantity of heavy elements present in different parts of a galaxy and also determine where in the galaxy star formation is occurring most vigorously.

G327.1-1.1: Pushing the Envelope

G327.1-1.1 is the aftermath of a massive star that exploded as a supernova in the Milky Way galaxy. A highly magnetic, rapidly spinning neutron star called a pulsar was left behind after the explosion and is producing a wind of relativistic particles, seen in X-rays by Chandra and XMM-Newton (blue) as well as in the radio data (red and yellow). This structure is called a pulsar wind nebula. The likely location of the spinning neutron star is shown in the labeled version. The large red circle shows radio emission from the blast wave, and the composite image also contains infrared data from the 2MASS survey (red, green, and blue) that show the stars in the field.

No clear explanation is yet known for the unusual nature of G327.1-1.1, including the off-center position of the pulsar wind nebula seen in the radio data and the comet-like shape of the X-ray emission. One possibility is that we are seeing the effects of a shock wave bouncing backwards off of the shell of material swept up by the blast wave produced by the explosion, the so-called "reverse shock" from the blast wave. The pulsar is moving upwards, away from the center of the explosion, but the pulsar wind nebula is being swept towards the bottom-left of the image by the reverse shock wave that is also traveling towards the bottom-left. The direction of the pulsar's motion and of the reverse shock are shown in the labeled version.

The X-ray observations allow scientists to estimate the energy released during the supernova explosion and the age of the remnant, as well as the amount of material being swept up as the blast wave from the explosion expands. The faint bubble that the pulsar appears to be creating may also be revealing the fresh pulsar wind being blown into the region cleared out by the reverse shock.

A paper describing these results appeared in The Astrophysical Journal in February 2009 with Tea Temim of the Harvard-Smithsonian Center for Astrophysics (CfA), Patrick Slane (CfA), Bryan Gaensler (University of Sydney), Jack Hughes (Rutgers) and Eric Van Der Swaluw (Royal Netherlands Meterological Institute) as authors.

A distant sparkling eruption of diamonds

You really want to click that to get the very beefy 4000 x 4000 pixel (11 Mb) version. It’ll knock your socks off!

This Hubble image shows NGC 6934, an ancient ball of stars located about 50,000 light years away. Globular clusters are made of stars that are bound to each other gravitationally and orbiting the center on a myriad different paths — think of it as a beehive except with a hundred thousand bees each a million kilometers across. There are about 150 of these guys orbiting the Milky Way, each a dozen or so light years across and containing upwards of a million stars. NGC 6934 is pretty typical of its class, but its great distance dims it to near-obscurity. If it were as close as M 13 or Omega Centauri — both roughly half as far as NGC 6934 — it would be heralded as a gem of the night sky.

Globulars are old. We think they form all at once, with all the stars being born at the same time. Massive stars, which are blue, don’t live long before exploding as supernovae (leaving behind black holes or dense neutrons stars), so they’re all long gone in these billions-of-years-old objects. In fact, in many globulars even stars like the Sun are gone, having used up their fuel and faded away. All that’s left are low mass stars, which means all that remains are red stars.

So why are there so many blue stars in this picture? Ah, it’s false color! It was taken through two filters, one in the red, and the other in the infrared. In the picture, the red filter image is colored blue, and the infrared one is colored red. Note that the brightest stars in the picture are red (meaning they’re bright in the infrared); this is because these are red giants, stars that are nearing the ends of their lives. They’ve swollen up and cooled off, glowing brilliantly.

Even though they’re not actually blue, the ones that look blue in the image are most likely the "normal" stars that are left in the cluster, that is, stars still fusing hydrogen into helium like the Sun is, and are not yet red giants.

Globular clusters like NGC 6934 are incredibly important to our understanding of our galaxy. Because all their stars formed at once and all from the same cloud of gas, they’re a laboratory experiment in astronomy! We don’t have to correct for age or composition of stars (or at least not very much) allowing us to examine other characteristics. They’re located all over the sky, so they’re always around for viewing, and many are isolated in space, making them easy to examine. Much of what we’ve learned about how stars age and die was gleaned from globulars like NGC 6934.

Globular clusters tell us secrets of the Universe, and all we have to do is pay attention. And when they’re as stunningly beautiful as this one, that’s really easy to do.