Posted on by Tammy Plotner

A Pure Disk Galaxy Is A Perfect Picture

The bright galaxy NGC 3621, captured here using the Wide Field Imager on the 2.2-metre telescope at ESO’s La Silla Observatory in Chile, appears to be a fine example of a classical spiral. But it is in fact rather unusual: it does not have a central bulge and is therefore described as a pure-disc galaxy.

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What could be more eye-catching than a picture perfect pure disk galaxy? In itself it is untouched – not yet combined with a neighboring elliptical or rouge spiral. This is the way we dream of seeing a distant companion… a virgin galaxy awaiting further growth. In a Universe dominated by clusters of galaxies and violent collisions, just how often does a thin, flat plate of stars occur?

According to the ESO Press Release, NGC 3621 is a spiral galaxy about 22 million light-years away in the constellation of Hydra (The Sea Snake). It is comparatively bright and can be seen well in moderate-sized telescopes. This picture was taken using the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. The data were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition. Joe’s picture of NGC 3621 was ranked fourth in the competition.

This galaxy has a flat pancake shape, indicating that it hasn’t yet come face to face with another galaxy as such a galactic collision would have disturbed the thin disc of stars, creating a small bulge in its center. Most astronomers think that galaxies grow by merging with other galaxies, in a process called hierarchical galaxy formation. Over time, this should create large bulges in the centers of spirals. Recent research, however, has suggested that bulgeless, or pure-disc, spiral galaxies like NGC 3621 are actually fairly common. But just how common?

This galaxy is of further interest to astronomers because its relative proximity allows them to study a wide range of astronomical objects within it, including stellar nurseries, dust clouds, and pulsating stars called Cepheid variables, which astronomers use as distance markers in the Universe. In the late 1990s, NGC 3621 was one of 18 galaxies selected for a Key Project of the Hubble Space Telescope: to observe Cepheid variables and measure the rate of expansion of the Universe to a higher accuracy than had been possible before. In the successful project, 69 Cepheid variables were observed in this galaxy alone.

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This sequence gives a close-up view of the spiral galaxy NGC 3621. This picture was taken using the Wide Field Imager (WFI) at ESO’s La Silla Observatory in Chile. NGC 3621 is about 22 million light-years away in the constellation of Hydra (The Sea Snake). It is comparatively bright and can be well seen in moderate-sized telescopes. The data from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile used to make this image were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition.

One of the fascinating things in viewing this image (for me, at least) is seeing all the star-forming regions on the periphery of the galaxy itself. It reminds me of the NGC objects we see in both M31 and M33 (another pure disk galaxy, too). While smaller backyard telescopes are never going to be able to resolve these kinds of details, I can’t help but wonder what larger, professional level equipment can do on a visual level. While I’m at it, my mind also wonders about what we’ve learned recently of the reliability of Cepheid variables as indicators of distance, too. Is this the end all of information? Nah. Because we’re living in a “pure disk” galaxy. Yeah. You heard me right… The Milky Way fits the model, too!

According to a study done by Juntai Shen (Shanghai Astronomical Observatory), et al: “Bulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here we model the stellar kinematics of the Bulge Radial Velocity Assay (BRAVA), and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar, seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best fit model has a half-length of ~ 4kpc and extends 20 degrees from the Sun-Galactic Center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than ~ 8% of the disk mass. Thus the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy formation is dominated by hierarchical clustering and galaxy mergers.”

Move over, NGC 3621… We’re both commoners.

Many thanks to the European Southern Observatory (ESO) for providing the press release and awesome images!

Posted on by Tammy Plotner

Long Ago and Far, Far Away… Hubble Discovers Most Distant Galaxy Yet!

Hubble Ultra Deep Field - Part D

[/caption]No Princess is sending holographic help messages. No Hans Solo is warming up a Millenium Falcon to jump into hyperdrive. We don’t even have a Death Star waiting around the corner. But, what we do have is evidence that astronomers have pushed the Hubble Space Telescope to its limits and have seen further back in time than ever before. “We are looking back through 96% of the life of the universe, and in so doing, we have found just one galaxy, but it is one, but it is a remarkable object. The universe was only 500 million years old at that time versus it now being thirteen thousand-seven hundred million years old. ” said Garth Illingworth, Ames Research Scientist. We know about the Hubble Ultra Deep Field, but we invite you to boldy go on…

While studying ultra-deep imaging data from the Hubble Space Telescope, an international group of astronomers have found what may be the most distant galaxy ever seen, about 13.2 billion light-years away. “Two years ago, a powerful new camera was put on Hubble, a camera which works in the infrared which we had never really good capability before, and we have now taken the deepest image of the universe ever using this camera in the infrared.” said Garth Illingworth, professor of astronomy and astrophysics at the University of California, Santa Cruz. “We’re getting back very close to the first galaxies, which we think formed around 200 to 300 million years after the Big Bang.” The study pushed the limits of Hubble’s capabilities, extending its reach back to about 480 million years after the Big Bang, when the universe was just 4 percent of its current age. The dim object, called UDFj-39546284, is a compact galaxy of blue stars that existed 480 million years after the Big Bang, only four percent of the universe’s current age. It is tiny. Over one hundred such mini-galaxies would be needed to make up our Milky Way.

The farthest and one of the very earliest galaxies ever seen in the universe appears as a faint red blob in this ultra-deep–field exposure taken with NASA's Hubble Space Telescope. This is the deepest infrared image taken of the universe. Based on the object's color, astronomers believe it is 13.2 billion light-years away. (Credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz), R. Bouwens (University of California, Santa Cruz, and Leiden University), and the HUDF09 Team)

Illingworth and UCSC astronomer Rychard Bouwens (now at Leiden University in the Netherlands) led the study, which will be published in the January 27 issue of Nature. Using infrared data gathered by Hubble’s Wide Field Planetary Camera 3 (WFC3), they were able to see dramatic changes in galaxies over a period from about 480 to 650 million years after the Big Bang. The rate of star birth in the universe increased by ten times during this 170-million-year period, Illingworth said. “This is an astonishing increase in such a short period, just 1 percent of the current age of the universe,” he said. There were also striking changes in the numbers of galaxies detected. “Our previous searches had found 47 galaxies at somewhat later times when the universe was about 650 million years old. However, we could only find one galaxy candidate just 170 million years earlier,” Illingworth said. “The universe was changing very quickly in a short amount of time.”

The Hubble Ultra Deep Field WFC3/IR Image. This Region of the Sky Contains the Deepest Optical and Near-Infrared Images Ever Taken of the Universe and is useful for finding star-forming galaxies at redshifts 8 and 10 (650 and 500 million years after the Big Bang, respectively). At UCSC and Leiden, we are using these data to better understand the properties of the first galaxies. Credit: Bouwen

According to Bouwens, these findings are consistent with the hierarchical picture of galaxy formation, in which galaxies grew and merged under the gravitational influence of dark matter. “We see a very rapid build-up of galaxies around this time,” he said. “For the first time now, we can make realistic statements about how the galaxy population changed during this period and provide meaningful constraints for models of galaxy formation.” Astronomers gauge the distance of an object from its redshift, a measure of how much the expansion of space has stretched the light from an object to longer (“redder”) wavelengths. The newly detected galaxy has a likely redshift value (“z”) of 10.3, which corresponds to an object that emitted the light we now see 13.2 billion years ago, just 480 million years after the birth of the universe. “This result is on the edge of our capabilities, but we spent months doing tests to confirm it, so we now feel pretty confident,” Illingworth said.

The galaxy, a faint smudge of starlight in the Hubble images, is tiny compared to the massive galaxies seen in the local universe. Our own Milky Way, for example, is more than 100 times larger. The researchers also described three other galaxies with redshifts greater than 8.3. The study involved a thorough search of data collected from deep imaging of the Hubble Ultra Deep Field (HUDF), a small patch of sky about one-tenth the size of the Moon. During two four-day stretches in summer 2009 and summer 2010, Hubble focused on one tiny spot in the HUDF for a total exposure of 87 hours with the WFC3 infrared camera.

“NASA continues to reach for new heights, and this latest Hubble discovery will deepen our understanding of the universe and benefit generations to come,” said NASA Administrator Charles Bolden, who was the pilot of the space shuttle mission that carried Hubble to orbit. “We could only dream when we launched Hubble more than 20 years ago that it would have the ability to make these types of groundbreaking discoveries and rewrite textbooks.”

To go beyond redshift 10, astronomers will have to wait for Hubble’s successor, the James Webb Space Telescope (JWST), which NASA plans to launch later this decade. JWST will also be able to perform the spectroscopic measurements needed to confirm the reported galaxy at redshift 10. “It’s going to take JWST to do more work at higher redshifts. This study at least tells us that there are objects around at redshift 10 and that the first galaxies must have formed earlier than that,” Illingworth said.

“After 20 years of opening our eyes to the universe around us, Hubble continues to awe and surprise astronomers,” said Jon Morse, NASA’s Astrophysics Division director at the agency’s headquarters in Washington. “It now offers a tantalizing look at the very edge of the known universe — a frontier NASA strives to explore.” How far back will we go? If you sit around a campfire watching the embers climb skywards and discuss cosmology after an observing night with your astro friends, someone will ultimately bring up the topic of space/time curvature. If you put an X on a balloon and expand it – and trace round its expanse – you will eventually return to your mark. If we see our beginnings, will we also eventually see our end coming up over the horizon? Wow… Pass the marshmallows, please. We’ve got a lot to think about.

Reader Info: Illingworth’s team maintains the First Galaxies website, with information about the latest research on distant galaxies. In addition to Bouwens and Illingworth, the coauthors of the Nature paper include Ivo Labbe of Carnegie Observatories; Pascal Oesch of UCSC and the Institute for Astronomy in Zurich; Michele Trenti of the University of Colorado; Marcella Carollo of the Institute for Astronomy; Pieter van Dokkum of Yale University; Marijn Franx of Leiden University; Massimo Stiavelli and Larry Bradley of the Space Telescope Science Institute; and Valentino Gonzalez and Daniel Magee of UC Santa Cruz. This research was supported by NASA and the Swiss National Science Foundation. Hubble Ultra Deep Field Image and Video courtesy of NASA/STSci.

Posted on by Jon Voisey

What is a galaxy? (Vote now!)

Hubble images of the Omega Centauri starfield from 2002, left, and from 2009, right.

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Classification is key to all sciences, but can often cause debate. Within astronomy, fierce debates have raged over the definition of a planet, both on the low-mass end, as well as the high-mass end. A recent paper explores definitions on a larger scale, pondering the definition of a galaxy, particularly, what separates the smallest of galaxies, the dwarf galaxies, from star clusters.

A working definition for dwarf galaxies was proposed in 1994 based on the brightness of the object in question as well as it’s size. For brightness, the cutoff was taken to be an absolute magnitude (MB) of -16. The size would need to be “more extended than a globular cluster.”

As with many definitions, they seem to work initially, but as new technology became available, objects were discovered around the cutoff line, blurring the distinction. These objects, which were first discovered in the late 90’s, are generally referred to with names like “ultra-faint dwarf spheroidals” (dSphs) and “ultra compact dwarfs” (UCDs). Regarding these small fragments, a 2007 study noted that they may “contain so few stars that they can be fainter than a single bright star and contain less stellar mass than some globular clusters”.

To help reconsider the definition of a galaxy, the authors looked at several commonly used criteria that have been applied (often inconsistently) to these questionable cases previously. This included requirements that the system be gravitationally bound, which would keep stellar streams and other ejected objects from being considered galaxies in their own right. Obviously, most galaxies will slowly bleed away stars due to random interactions, giving rise to hypervelocity stars which will leave the galaxy, so the team proposes a threshold that the galaxy have a “relaxation time” greater than the age of the universe. This would allow dSphs and UCDs to be considered galaxies, but would keep out objects that have generally been considered globular clusters.

Another proposed constraint is based on the size of the object. The team proposes a cutoff where the effective radius be greater than or equal to 100 parsecs. This cutoff would exclude dSphs and UCDs.

The types of stars is another consideration proposed since this can be used to achieve somewhat of an understanding of the history of the object. While clusters usually form in a single instance, galaxies are generally considered to have their own, internal machinations leading to complex stellar populations. Thus, the presence of multiple populations of stars. This would include dSphs and UCDs, but may allow some globular clusters to slip in as well since studies have shown that some of our more massive globular clusters in the Milky Way have interacted with gas clouds, triggering star formation which was absorbed by the clusters.

Dark matter is another criteria that is examined. Since galaxies are proposed to form within dark matter halos and be intrinsically tied into them, the requirement that dark matter be present would fit well with the theory. However, this criteria also poses many difficulties. Firstly, measuring the presence of dark matter in small objects is a challenging task. It is also questionable as to whether or not dSphs and UCDs would contain dark matter as a general rule since their formation is not well understood and the possibility remains that they may have been ejected from our own galaxy during formation and recoalesced, possibly without a dark matter halo.

The last possible criteria is much along the same lines as the nebulous definition for planets that they dominate the local gravitational field. The team considers the possibility that objects would be required to have stellar satellite systems as globular clusters of their own. This would include some dwarf galaxies, but may exclude others.

Even with many of these criteria, classification will still be a treacherous issue. Objects like Omega Centauri may fit some definitions but not others. According to the paper’s lead author, Duncan Forbes, “many amateur astronomers know Omega Cen as massive star cluster, some professional astronomers regard it as a galaxy. This is a stellar system that could be upgraded or downgraded by this exercise, depending on your point of view.”

To help gather opinions on the topic, the authors have set up an online survey to gather opinions on this definition and hope to reach a satisfactory conclusion by collective wisdom. This poll is open to the general public and results will be presented at a future astronomical conferences allowing participants to help take part in the astronomical process. Forbes hopes that this public interaction will help garner public interest in much the same way as the Galaxy Zoo project has.

Posted on by Nancy Atkinson

Swift Survey Finds ‘Missing’ Active Galaxies

From a NASA press release:

Seen in X-rays, the entire sky is aglow. Even far away from bright sources, X-rays originating from beyond our galaxy provide a steady glow in every direction. Astronomers have long suspected that the chief contributors to this cosmic X-ray background were dust-swaddled black holes at the centers of active galaxies. The trouble was, too few of them were detected to do the job.

An international team of scientists using data from NASA’s Swift satellite confirms the existence of a largely unseen population of black-hole-powered galaxies. Their X-ray emissions are so heavily absorbed that little more than a dozen are known. Yet astronomers say that despite the deeply dimmed X-rays, the sources may represent the tip of the iceberg, accounting for at least one-fifth of all active galaxies.

Continue reading “Swift Survey Finds ‘Missing’ Active Galaxies”

Posted on by Nancy Atkinson

Star Birth and Death in the Andromeda Galaxy

M31, or the Andromeda Galaxy seen in a variety of wavelengths by the Herschel and XMM-Newton space observatories. Credits: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendle

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To the naked eye, the Andromeda galaxy appears as a smudge of light in the night sky. But to the combined powers of the Herschel and XMM-Newton space observatories, these new images put Andromeda in a new light! Together, the images provide some of the most detailed looks at the closest galaxy to our own. In infrared wavelengths, Herschel sees rings of star formation and XMM-Newton shows dying stars shining X-rays into space.

During Christmas 2010, the two ESA space observatories targeted Andromeda, a.k.a. M31.

Andromeda is about twice as big as the Milky Way but very similar in many ways. Both contain several hundred billion stars. Currently, Andromeda is about 2.2 million light years away from us but the gap is closing at 500,000 km/hour. The two galaxies are on a collision course! In about 3 billion years, the two galaxies will collide, and then over a span of 1 billion years or so after a very intricate gravitational dance, they will merge to form an elliptical galaxy.

Let’s look at each of the images:

Herschel’s view in far-infrared:

Andromeda in far-infrared from Herschel. Credits: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent

Sensitive to far-infrared light, Herschel sees clouds of cool dust and gas where stars can form. Inside these clouds are many dusty cocoons containing forming stars, each star pulling itself together in a slow gravitational process that can last for hundreds of millions of years. Once a star reaches a high enough density, it will begin to shine at optical wavelengths. It will emerge from its birth cloud and become visible to ordinary telescopes.

Many galaxies are spiral in shape but Andromeda is interesting because it shows a large ring of dust about 75,000 light-years across encircling the center of the galaxy. Some astronomers speculate that this dust ring may have been formed in a recent collision with another galaxy. This new Herschel image reveals yet more intricate details, with at least five concentric rings of star-forming dust visible.

XMM Newton’s view in X-rays

XMM Newton's view in X-Ray. Credits: ESA/XMM-Newton/EPIC/W. Pietsch, MPE

Superimposed on the infrared image is an X-ray view taken almost simultaneously by ESA’s XMM-Newton observatory. Whereas the infrared shows the beginnings of star formation, X-rays usually show the endpoints of stellar evolution.

XMM-Newton highlights hundreds of X-ray sources within Andromeda, many of them clustered around the centre, where the stars are naturally found to be more crowded together. Some of these are shockwaves and debris rolling through space from exploded stars, others are pairs of stars locked in a gravitational fight to the death.

In these deadly embraces, one star has already died and is pulling gas from its still-living companion. As the gas falls through space, it heats up and gives off X-rays. The living star will eventually be greatly depleted, having much of its mass torn from it by the stronger gravity of its denser partner. As the stellar corpse wraps itself in this stolen gas, it could explode.

Together, the infrared and X-ray images show information that is impossible to collect from the ground because these wavelengths are absorbed by Earth’s atmosphere. Visible light shows us the adult stars, whereas infrared gives us the youngsters and X-rays show those in their death throes.

Posted on by Nicholos Wethington

Galactic Mergers Fail to Feed Black Holes

By comparing 140 galaxies that had Active Galactic Nuclei with over 1200 galaxies in a "control group", the likelihood that mergers are the cause of AGN has been brought into doubt. Credit: NASA, ESA, M. Cisternas (Max-Planck Institute for Astronomy)

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The large black holes that reside at the center of galaxies can be hungry beasts. As dust and gas are forced into the vicinity around the black holes, it crowds up and jostles together, emitting lots of heat and light. But what forces that gas and dust the last few light years into the maw of these supermassive black holes?

It has been theorized that mergers between galaxies disturbs the gas and dust in a galaxy, and forces the matter into the immediate neighborhood of the black hole. That is, until a recent study of 140 galaxies hosting Active Galactic Nuclei (AGN) – another name for active black holes at the center of galaxies – provided strong evidence that many of the galaxies containing these AGN show no signs of past mergers.

The study was performed by an international team of astronomers. Mauricio Cisternas of the Max Planck Institute for Astronomy and his team used data from 140 galaxies that were imaged by the XMM-Newton X-ray observatory. The galaxies they sampled had a redshift between z= 0.3 – 1, which means that they are between about 4 and 8 billion light-years away (and thus, the light we see from them is about 4-8 billion years old).

They didn’t just look at the images of the galaxies in question, though; a bias towards classifying those galaxies that show active nuclei to be more distorted from mergers might creep in. Rather, they created a “control group” of galaxies, using images of inactive galaxies from the same redshift as the AGN host galaxies. They took the images from the Cosmic Evolution Survey (COSMOS), a survey of a large region of the sky in multiple wavelengths of light. Since these galaxies were from the same redshift as the ones they wanted to study, they show the same stage in galactic evolution. In all, they had 1264 galaxies in their comparison sample.

The way they designed the study involved a tenet of science that is not normally used in the field of astronomy: the blind study. Cisternas and his team had 9 comparison galaxies – which didn’t contain AGN – of the same redshift for each of their 140 galaxies that showed signs of having an active nucleus.

What they did next was remove any sign of the bright active nucleus in the image. This means that the galaxies in their sample of 140 galaxies with AGN would essentially appear to even a trained eye as a galaxy without the telltale signs of an AGN. They then submitted the control galaxies and the altered AGN images to ten different astronomers, and asked them to classify them all as “distorted”, “moderately distorted”, or “not distorted”.

Since their sample size was pretty manageable, and the distortion in many of the galaxies would be too subtle for a computer to recognize, the pattern-seeking human brain was their image analysis tool of choice. This may sound familiar – something similar is being done with enormous success with people who are amateur galaxy classifiers at Galaxy Zoo.

When a galaxy merges with another galaxy, the merger distorts its shape in ways that are identifiable – it will warp a normally smooth elliptical galaxy out of shape, and if the galaxy is a spiral the arms seem to be a bit “unwound”. If it were the case that galactic mergers are the most likely cause of AGN, then those galaxies with an active nucleus would be more probable to show distortion from this past merger.

The team went through this process of blinding the study to eliminate any bias that those looking at the images would have towards classifying AGN as more distorted. By both having a reasonably large sample size of galaxies and removing any bias when analyzing the images, they hoped to definitively show whether the correlation between AGN and mergers exists.

The result? Those galaxies with an Active Galactic Nucleus did not show any more distortion on the whole than those galaxies in the comparison sample. As the authors state in the paper, “Mergers and interactions involving AGN hosts are not dominant, and occur no more frequently than for inactive galaxies.”

This means that astronomers can’t point towards galactic mergers as the main reason for AGN. The study showed that at least 75% of AGN creation – at least between the last 4-8 billion years – must be from sources other than galactic mergers. Likely candidates for these sources include: “galactic harrassment”, those galaxies that don’t collide, but come close enough to gravitationally influence each other; the instability of the central bar in a galaxy; or the collision of giant molecular clouds within the galaxy.

Knowing that AGN aren’t caused in large part by galactic mergers will help astronomers to better understand the formation and evolution of galaxies. The active nuclei in galaxies that host them greatly influence galactic formation. This process is called ‘AGN feedback’, and the mechanisms and effects that result from the interplay between the energy streaming out of the AGN and the surrounding material in the center of a galaxy is still a hot topic of study in astronomy.

Mergers in the more distant past than 8 billion years might yet correlate with AGN – this study only rules out a certain population of these galaxies – and this is a question that the team plans to take on next, pending surveys by the Hubble Space Telescope and the James Webb Space Telescope. Their study will be published in the January 10 issue of the Astrophysical Journal, and a pre-print version is available on Arxiv.

Source: HST news release, Max Planck Institute for Astronomy, Arxiv paper

Posted on by Ken Kremer

“Astrobiology” Parody Video of Ke$ha’s “We R Who We R”

Wanna get turned on by … “Astrobiology” ?? Are we alone in the universe?

Well check out just this newly-released music video parody of Ke$ha’s hit song “We R Who We R” – “Astrobiology.”

Suspend your disbelief. It’s different. It’s cool. And it’s very clever.

And .. It’s even better the second time around when you listen to the lyrics more closely … combined with the shocking video .. Featuring beautiful maidens and alien dolls galore. Continue reading ““Astrobiology” Parody Video of Ke$ha’s “We R Who We R””

Posted on by Steve Nerlich

Astronomy Without A Telescope – Secular Evolution

M51 - the Whirlpool Galaxy. Credit: NASA

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A traditional galaxy evolution model has it that you start with spiral galaxies – which might grow in size through digesting smaller dwarf galaxies – but otherwise retain their spiral form relatively undisturbed. It is only when these galaxies collide with another of similar size that you first get an irregular ‘train-wreck’ form, which eventually settles into a featureless elliptical form – full of stars following random orbital paths rather than moving in the same narrow orbital plane that we see in the flattened galactic disk of a spiral galaxy.

The concept of secular galaxy evolution challenges this notion – where ‘secular’ means separate or isolated. Theories of secular evolution propose that galaxies naturally evolve along the Hubble sequence (from spiral to elliptical), without merging or collisions necessarily driving changes in their form.

While it’s clear that galaxies do collide – and then generate many irregular galaxy forms we can observe – it is conceivable that the shape of an isolated spiral galaxy could evolve towards a more amorphously-shaped elliptical galaxy if they possess a mechanism to transfer angular momentum outwards.

The flattened disk shape of standard spiral galaxy results from spin – presumably acquired during its initial formation. Spin will naturally cause an aggregated mass to adopt a disk shape – much as pizza dough spun in the air will form a disk. Conservation of angular momentum requires that the disk shape will be sustained indefinitely unless the galaxy can somehow lose its spin. This might happen through a collision – or otherwise by transferring mass, and hence angular momentum, outwards. This is analogous to spinning skaters who fling their arms outwards to slow their spin.

Density waves may be significant here. The spiral arms commonly visible in galactic disks are not static structures, but rather density waves which cause a temporary bunching together of orbiting stars. These density waves may be the result of orbital resonances generated amongst the individual stars of the disk.

Left: Density waves may emerge from gravitational resonances generated by the alignment of stars

It has been suggested that a density wave represents a collisionless shock which has a damping effect on the spin of the disk. However, since the disk is only braking upon itself, angular momentum still has to be conserved within this isolated system.

A galactic disk has a corotation radius – a point where stars rotate at the same orbital velocity as the density wave (i.e. a perceived spiral arm) rotate. Within this radius, stars move faster than the density wave – while outside the radius, stars move slower than the density wave.

This may account for the spiral shape of the density wave – as well as offering a mechanism for the outward transfer of angular momentum. Within the radius of corotation, stars are giving up angular momentum to the density wave as they push through it – and hence push the wave forward. Outside the radius of corotation, the density wave is dragging through a field of slower moving stars – giving up angular momentum to them as it does so.

The result is that the outer stars are flung further outwards to regions where they could adopt more random orbits – rather than being forced to conform to the mean orbital plane of the galaxy. In this way, a tightly-bound rapidly spinning spiral galaxy could gradually evolve towards a more amorphous elliptical shape.

Further reading: Zhang and Buta. Density-Wave Induced Morphological Transformation of Galaxies along the Hubble Sequence.

Posted on by Nancy Atkinson

Clash of the Titan Galaxies

NGC 520 — also known as Arp 157 -- is actually a mashup of two gigantic galaxies. Credit: ESO

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Is this galaxy exploding? Although that’s what it might look like, this is actually two gigantic galaxies crashing into each other. NGC 520 — also known as Arp 157 — is a mashup of two huge galaxies, now combining into one. We can’t really watch the process, as it happens extremely slowly — over millions of years, and the whole process started about 300 million years ago. Apr 157 is about 100,000 light-years across and is now in the middle stage of the merging process, as the two nuclei haven’t come together yet, but the two discs have. The merger features a tail of stars and a prominent dust lane. NGC 520 is one of the brightest interacting galaxies in the sky and lies in the direction of Pisces (the Fish), approximately 100 million light-years from Earth.

This image was taken by the ESO Faint Object Spectrograph and Camera attached to the 3.6-metre telescope at La Silla in Chile.

You’d need about a 4-inch telescope to see this 12th magnitude object yourself. Here’s the location: RA: 1h 24m 35.1s, Declination: +03° 47? 33?. Or put in those coordinates in Google Sky to see it there.

Source: ESO

Posted on by Mark Thompson

Twinkle Twinkle Little Missing Stars, How I Wonder Where you are?

Why is Our Galaxy Called the Milky Way
Why is Our Galaxy Called the Milky Way

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‘Twinkle twinkle little star, how I wonder what you are?’ This nursery rhyme is one of the best loved around the World. For astronomers though, stars can be a bit more of a nightmare, not only in understanding their complex evolutionary processes but also and perhaps more simply, figuring out how many there are. Until now there has been a gross mismatch between the number of stars that are found within our galaxy, the Milky Way and the amount that astronomer think should be there. In short, where are the missing stars?

The Milky Way is joined by about 30 other galaxies that make up our local group of galaxies, including the Andromeda Galaxy and according to current theories there should be about 100 billion stars in each. The calculations are based on the rate of star birth in the Milky Way, about 10 new stars per year. But according to Dr Jan Pflamm-Altenburg of the Argelander Institute for Astronomy at the University of Bonn “Actually, it would give many more stars than we actually see” and therein lies the problem.

The recent study by Dr Pflamm-Altenburg and Dr. Carsten Weidner of the Scottish St. Andrews University suggests that perhaps the estimated rate of star birth being used to calculate the number of stars could simply be too high. With galaxies in our Local Group its relatively easy to just count the number of new stars that can be seen but for more distant galaxies, they are too far away for individual stars to be seen.

By studying the nearby galaxies, Pflamm-Altenburg and Weidner discovered that for every 300 young small stars, there seems to be one large massive new star and fortunately this seems to be universal. Due to the unique nature of the massive young stars, they leave a tell tail sign in the light of distant galaxies so even though they cannot be individually identified they can still be detected and the strength of the signal determines the number of massive stars. Multiply by the number of massive stars by this ratio of 300 and the actual rate of stellar birth can be calculated.

It seems though that this rate has varied over the history of the Universe and dependent on the amount of ‘space’ available in the vicinity of the star formation. If there is a baby boom in star formation then a higher number of heavies seem to form in a theory called ‘stellar crowding’. When stars form, they form as clusters rather than individual stars but it seems that the overall mass of the group is the same, regardless of how many star embryo’s there really are. When star birth is at a high rate, space can be limited so larger more massive stars tend to form compared to smaller stars.

Massive galaxies like this where star birth is booming are called “ultra-compact dwarf galaxies” (UCDs). Sometimes its possible in these galaxies that young stars can even fuse together to form larger stars so the large to small ratio can be around 1:50 instead of 1:300. This means we have been using the wrong figure and estimating far too high.

Using this new found figure, Pflamm-Altenburg and Weidner have recalculated the number of stars that ‘should’ be in a galaxy and compared to those that we can see and rather pleasantly, the numbers match! It seems that the conundrum of the missing stars that has been perplexing astronomers for decades has finally been resolved.

Source: University of Bonn