False color image of the Lockman-hole area of the sky at infrared wavelengths as imaged by the Herschel Space Observatory. Credit: ESA/SPIRE Consortium/HerMES Consortium
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When it comes to forming stars, the size of a galaxy does matter, according to research out today in the online version of Nature.
But it doesn’t have to be as massive as we once thought.
Alexandre Amblard, an astrophysicist at the University of California, Irvine, and his colleagues used new data from the Herschel Space Observatory to peer into Lockman Hole area of the sky, where extragalactic light comes from star-forming galaxies out of reach for even the world’s most powerful telescopes.
The Lockman Hole is a patch of the sky, 15 square degrees, lying roughly between the pointer stars of the Big Dipper.
Called submillimetre galaxies, the study subjects emit light at wavelengths between the radio and infrared parts of the spectrum, so studying them requires novel approaches borrowing from both radio and optical astronomy. The galaxies by themselves are too blurry to be resolved with individual far-infrared telescopes – but their average properties can be observed and analyzed, which is exactly what Amblard and his colleagues did.
The authors measured variations in the intensity of extragalactic light at far-infrared wavelengths, and derived statistics for the level of clustering of light halos. They assume that the clustering reflects the underlying distribution of dark matter, and fit the data to a halo model of galaxy formation, which connects the spatial distribution of galaxies in the Universe to that of dark matter.
Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA
Amblard and his colleagues discovered an enormous fact: the ‘haloes’ of dark matter that surround the Universe’s most active star-forming galaxies are each more massive than about 300 billion solar masses.
What’s even more interesting is that the new threshold for star formation is actually smaller than some previous estimates.
“I think there was one prediction that put the number around 5000 billion times that of the sun, but that was just a prediction from a theory of galaxy formation.“ said Asantha Cooray, also an astrophysicist at UC Irvine and second author on the new paper. “The general consensus was that it may be between 100 to 1000 billion times the sun. We now have a more precise answer from this work.”
Cooray said he’s most excited “that we can look at a detailed image of the sky showing distant, star-forming galaxies and infer not only details about the stars and gas in those galaxies but also about the amount of dark matter needed to form such galaxies. Beyond inferring the presence, we still don’t know exactly what dark matter is.”
The results appear online ahead of print today on Nature’s website.
Schematic representation of a thick disc structure. The thick disc is formed of stars that are typically much older than those in the thin disc, making it an ideal probe of galactic evolution (Credit: Amanda Smith, IoA graphics officer)
A team of astronomers from the UK, the US and Europe have identified a thick stellar disc in the nearby Andromeda galaxy for the first time. The discovery and properties of the thick disc will constrain the dominant physical processes involved in the formation and evolution of large spiral galaxies like our own Milky Way.
By analyzing precise measurements of the velocities of individual bright stars within the Andromeda galaxy using the Keck telescope in Hawaii, the team have managed to separate out stars tracing out a thick disc from those comprising the thin disc, and assess how they differ in height, width and chemistry.
Optical image of The Andromeda galaxy (M31) (credit Robert Gendler)
Spiral structure dominates the morphology of large galaxies at the present time, with roughly 70% of all stars contained in a flat stellar disc. The disc structure contains the spiral arms traced by regions of active star formation, and surrounds a central bulge of old stars at the core of the galaxy. “From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar discs, both a ‘thin’ and a ‘thick’ disc,” explains the leader of the study, Michelle Collins, a PhD student at Cambridge’s Institute of Astronomy. The thick disc consists of older stars whose orbits take them along a path that extends both above and below the more regular thin disc. “The classical thin stellar discs that we typically see in Hubble imaging result from the accretion of gas towards the end of a galaxy’s formation, whereas thick discs are produced in a much earlier phase of the galaxy’s life, making them ideal tracers of the processes involved in galactic evolution.”
Currently, the formation process of the thick disc is not well understood. Previously, the best hope for comprehending this structure was by studying the thick disc of our own Galaxy, but much of this is obscured from our view. The discovery of a similar thick disk in Andromeda presents a much cleaner view of spiral structure. Andromeda is our nearest large spiral neighbor — close enough to be visible to the unaided eye — and can be seen in its entirety from the Milky Way. Astronomers will be able to determine the properties of the disk across the full extent of the galaxy and look for signatures of the events connected to its formation. It requires a huge amount of energy to stir up a galaxy’s stars to form a thick disc component, and theoretical models proposed include accretion of smaller satellite galaxies, or more subtle and continuous heating of stars within the galaxy by spiral arms.
Ages and orientations of the stellar components of disc galaxies. The halo (or spheroid) contains the oldest populations, followed by the thick stellar disc. The thin disc typically contains the youngest generations of stars. (Credit: RAVE collaboration)
“Our initial study of this component already suggests that it is likely older than the thin disc, with a different chemical composition” commented UCLA Astronomer, Mike Rich. “Future more detailed observations should enable us to unravel the formation of the disc system in Andromeda, with the potential to apply this understanding to the formation of spiral galaxies throughout the Universe.”
“This result is one of the most exciting to emerge from the larger parent survey of the motions and chemistry of stars in the outskirts of Andromeda,” said fellow team member, Dr. Scott Chapman, also at the Institute of Astronomy. “Finding this thick disc has afforded us a unique and spectacular view of the formation of the Andromeda system, and will undoubtedly assist in our understanding of this complex process.”
This study was published in Monthly Notices of the Royal Astronomical Society by Michelle Collins, Scott Chapman and Mike Irwin from the Institute of Astronomy, together with Rodrigo Ibata from L’Observatoire de Strasbourg, Mike Rich from University of California, Los Angeles, Annette Ferguson from the Institute for Astronomy in Edinburgh, Geraint Lewis from the University of Sydney, and Nial Tanvir and Andreas Koch from the University of Leicester.
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!
[/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.
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.
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.
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.
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.
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.
