Wrapping Around The Mystery Of Spiral Galaxy Arms

How disk galaxies form their spiral arms have been puzzling astrophysicists for almost as long as they have been observing them. With time, they have come to two conclusions… either this structure is caused by differences in gravity sculpting the gas, dust and stars into this familiar shape, or its just a random occurrence which comes and goes with time.

Now researchers are beginning to wrap their conclusions around findings based on new supercomputer simulations – simulations which involve the motion of up to 100 million “stellar particles” that mimic gravitational and astrophysical forces which shape them into natural spiral structure. The research team from the University of Wisconsin-Madison and the Harvard-Smithsonian Center for Astrophysics are excited about these conclusions and report the simulations may hold the essential clues of how spiral arms are formed.

“We show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” says UW-Madison astrophysicist Elena D’Onghia, who led the new research along with Harvard colleagues Mark Vogelsberger and Lars Hernquist.

“The spiral arms are self-perpetuating, persistent, and surprisingly long lived,” adds Vogelsberger.

When it comes to spiral structure, it’s probably the most widely occurring of universal shapes. Our own Milky Way galaxy is considered to be a spiral galaxy and around 70% of the galaxies near to us are also spiral structured. When we think in a broader sense, just how many things take on this common formation? Whisking up dust with a broom causes particles to swirl into a spiral shape… draining water invokes a swirling pattern… weather formations go spiral. It’s a universal happening and it happens for a reason. Apparently that reason is gravity and something to perturb it. In the case of a galaxy, it’s a giant molecular cloud – the star-forming regions. Introduced into the simulation, the clouds, says D’Onghia, a UW-Madison professor of astronomy, act as “perturbers” and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

“We find they are forming spiral arms,” explains D’Onghia. “Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there.”

So, what of companion galaxies? Can spiral structure be caused by proximity? The new research also takes that into account and models for “stand alone” galaxies as well. However, that’s not all the study included. According to Vogelsberger and Hernquist, the new computer-generated simulations are focusing on clarifying observational data. They are taking a closer look at the high-density molecular clouds and the “gravitationally induced holes in space” which act as ” the mechanisms that drive the formation of the characteristic arms of spiral galaxies.”

Until then, we know spiral structure isn’t just a chance happening and – to wrap things up – it’s probably the most common form of galaxy in our Universe.

Original Story Source: Harvard-Smithsonian Center for Astrophysics.

Speca – An Intriguing Look Into The Beginning Of A Black Hole Jet

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Its name is SPECA – a Spiral-host Episodic radio galaxy tracing Cluster Accretion. That’s certainly a mouthful of words for this unusual galaxy, but there’s a lot more going on here than just its name. “This is probably the most exotic galaxy with a black hole, ever seen. It is like a ‘missing-link’ between present day and past galaxies. It has the potential to teach us new lessons about how galaxies and clusters of galaxies formed in the early Universe,” said Ananda Hota, of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), in Taiwan and who discovered this exotic galaxy.

Located about 1.7 billion light-years from Earth, Speca is a radio source that contains a central supermassive black hole. As we have learned, galaxies of this type produce relativistic “jets” which are responsible for being bright at the radio frequencies, but that’s not all they create. While radio galaxies are generally elliptical, Speca is a spiral – reason behind is really unclear. As the relativistic jets surge with time, they create lobes of sub-atomic material at the outer edges which fan out as the material slows down… and Speca is one of only two galaxies so far discovered to show this type of recurrent jet activity. Normally it occurs once – and rarely twice – but here it has happened three times! We are looking at a unique opportunity to unravel the mysteries of the beginning phase of a black hole jet.

“Both elliptical and spiral galaxies have black holes, but Speca and another galaxy have been seen to produce large jets. It is also one of only two galaxies to show that such activity occurred in three separate episodes.” explains Sandeep Sirothia of NCRA-TIFR. “The reason behind this on-off activity of the black hole to produce jets is unknown. Such activities have not been reported earlier in spiral galaxies, which makes this new galaxy unique. It will help us learn new theories or change existing ones. We are now following the object and trying to analyse the activities.”

Dr. Hota and an international team of scientists reached their first conclusions while studying combined data from the visible-light Sloan Digital Sky Survey (SDSS) and the FIRST survey done with the Very Large Array (VLA) radio telescope. Here they discovered an unusually high rate of star formation where there should be none and they then confirmed their findings with ultraviolet data from NASA’s GALEX space telescope. Then the team dug even deeper with radio information obtained from the NRAO VLA Sky Survey (NVSS). At several hundred million years old, these outer lobes should be beyond their reproductive years… Yet, that wasn’t all. GMRT images displayed yet another, tiny lobe located just outside the stars at the edge of Speca in plasma that is just a few million years old.

“We think these old, relic lobes have been ‘re-lighted’ by shock waves from rapidly-moving material falling into the cluster of galaxies as the cluster continues to accrete matter,” said Ananda. “All these phenomena combined in one galaxy make Speca and its neighbours a valuable laboratory for studying how galaxies and clusters evolved billions of years ago.”

As you watch the above galaxy merger simulation created by Tiziana Di Matteo, Volker Springel, and Lars Hernquist, you are taking part in a visualization of two galaxies combining which both have central supermassive black holes and the gas distribution only. As they merge, you time travel over two billion years where the brightest hues indicate density while color denotes temperature. Such explosive process for the loss of gas is needed to understand how two colliding star-forming spiral galaxies can create an elliptical galaxy… a galaxy left with no fuel for future star formation. Outflow from the supernovae and central monster blackholes are the prime drivers of this galaxy evolution.

“Similarly, superfast jets from black holes are supposed to remove a large fraction of gas from a galaxy and stop further star formation. If the galaxy is gas-rich in the central region, and as the jet direction changes with time, it can have an adverse effect on the star formation history of a galaxy. Speca may have once been part of such a scenario. Where multiple jets have kicked out spiral arms from the galaxy. To understand such a process Dr Hota’s team has recently investigated NGC 3801 which has very young jet in very early-phase of hitting the host galaxy. Dust/PAH, HI and CO emission shows an extremely warped gas disk. HST data clearly showa outflow of heated-gas. This gas loss, as visualised in the video, has possibly caused the decline of star formation. However, the biggest blow from the monster’s jets are about to give the knock-down punch the galaxy.

“It seems, we observe this galaxy at a rare stage of its evolutionary sequence where post-merger star formation has already declined and new powerful jet feedback is about to affect the gaseous star forming outer disk within the next 10 million years to further transform it into a red-and-dead early-type galaxy.” Dr. Hota says.

The causes behind why present day radio galaxies do not contain a young star forming disks are not clear. Speca and NGC 3801 are ideal laboratories to understand black hole galaxy co-evolution processes.

Original Research Paper: Caught in the act: A post-merger starforming early-type galaxy with AGN-jet feedback. For Further Reading: Various press releases and news on the discovery of Speca. This article has been changed slightly from its original publication to reflect more information from Dr. Hota.

The Care And Feeding Of Teenage Galaxies… And By The Way, They Need Gas

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Got a teenager? Then you know the story. Go to look for your favorite bag of chips and they’re gone. You eat one portion of meat and they need three. If you like those cookies, then you better have a darn good place to stash them. And, while you’re at it, their car needs gas. Apparently there’s a reason for the word “universal”, because teenage galaxies aren’t much different. Thanks to some new studies done by ESO’s Very Large Telescope, astronomers have been able to take a much closer look at adolescent galaxies and their “feeding habits” during their evolution. Some 3 to 5 billion years after the Big Bang they were happiest when just provided with gas, but later on they developed a voracious appetite… for smaller galaxies!

Scientists have long been aware that early galaxy structures were much smaller than the grand spirals and hefty ellipticals which fill the present Universe. However, figuring out exactly how galaxies put on weight – and where the bulk supply comes from – has remained an enigma. Now an international group of astronomers have taken on more than a hundred hours of observations taken with the VLT to help determine how gas-rich galaxies developed.

“Two different ways of growing galaxies are competing: violent merging events when larger galaxies eat smaller ones, or a smoother and continuous flow of gas onto galaxies.” explains team leader, Thierry Contini (IRAP, Toulouse, France). “Both can lead to lots of new stars being created.”

The undertaking is is MASSIV – the Mass Assembly Survey with the VIsible imaging Multi-Object Spectrograph, a powerful camera and spectrograph on the VLT. It’s incredible equipment used to measure distance and properties of the surveyed galaxies Not only did the survey observe in the near infrared, but also employed a integral field spectrograph and adaptive optics to refine the images. This enables astronomers to map inner galaxy movements and content, as well as leaving room for some very surprising results.

“For me, the biggest surprise was the discovery of many galaxies with no rotation of their gas. Such galaxies are not observed in the nearby Universe. None of the current theories predict these objects,” says Benoît Epinat, another member of the team.

“We also didn’t expect that so many of the young galaxies in the survey would have heavier elements concentrated in their outer parts — this is the exact opposite of what we see in galaxies today,” adds Thierry Contini.

These results point towards a major change during the galactic “teenage years”. At some time during the young Universe state, smooth gas flow was a considerable building block – but mergers would later play a more important role.

“To understand how galaxies grew and evolved we need to look at them in the greatest possible detail. The SINFONI instrument on ESO’s VLT is one of the most powerful tools in the world to dissect young and distant galaxies. It plays the same role that a microscope does for a biologist,” adds Thierry Contini.

The team plans on continuing to study these galaxies with future instruments on the VLT as well as using ALMA to study the cold gas in these galaxies. However, their work with gas isn’t the only “station” on the block. In a separate study led by Kate Rubin (Max Planck Institute for Astronomy), the Keck I telescope on Mauna Kea, Hawaii, has been used to examine gas associated with a hundred galaxies at distances between 5 and 8 billion light-years – the older teens. They have found initial evidence of gas flowing back into distant galaxies that are actively forming new stars.

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

Apparently, like a teenager with the munchies, matter finds its way into those galactic tummies. One feeding theory is an inflow from huge low-density gas reservoirs filling the intergalactic voids… another is huge cosmic matter cycle. While there is very little evidence to support either hypothesis, gases have been observed to flow away from some galaxies and may be moshed around by several different sources – such as supernovae events or peer pressure from gigantic stars.

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

It might very well be a case of cosmic recycling… but I’d feel safer hiding my cookies.

Original Story Sources: ESO News Release and MPIA Science News Release. For Further Reading: Research Paper 1, Research Paper 2, Research Paper 3 and Research Paper 4.

Galactic Archaeology: NGC 5907 – The Dragon Clash

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The sprawling northern constellation of Draco is home to a monumental galactic merger which left a singular spectacle – NGC 5907. Surrounded by an ethereal garment of wispy star trails and currents of stellar material, this spiral galaxy is the survivor of a “clash of the dragons” which may have occurred some 8 to 9 billion years ago. Recent theory suggests galaxies of this type may be the product of a larger galaxy encountering a smaller satellite – but this might not be the case. Not only is NGC 5907 a bit different in some respects, it’s a lot different in others… and peculiar motion is just the beginning.

“If the disc of many spirals is indeed rebuilt after a major merger, it is expected that tidal tails can be a fossil record and that there should be many loops and streams in their halos. Recently Martínez-Delgado et al. (2010) have conducted a pilot survey of isolated spiral galaxies in the Local Volume up to a low surface brightness sensitivity of ~28.5 mag/arcsec2 in V band. They find that many of these galaxies have loops or streams of various shapes and interpret these structures as evidence of minor merger or satellite infall.” says J. Wang of the Chinese Academy of Sciences. “However, if these loops are caused by minor mergers, the residual of the satellite core should be detected according to numerical simulations. Why is it hardly ever detected?”

The “why” is indeed the reason NGC 5907 is being intensively studied by a team of six scientists of the Observatoire de Paris, CNRS, Chinese Academy of Sciences, National Astronomical Observatories of China NAOC and Marseille Observatory. Even though NGC 5907 is a member of a galactic group, there are no galaxies near enough to it to be causing an interaction which could account for its streamers of stars. It is truly a warped galaxy with gaseous and stellar disks which extend beyond the nominal cut-off radius. But that’s not all… It also has a peculiar halo which includes a significant fraction of metal enriched stars. NGC 5907 just doesn’t fit the patterns.

“For some of our models, we assume a star formation history with a varying global efficiency in transforming gas to stars, in order to preserve enough gas from being consumed before fusion.” explains the research team. “Although this fine-tuned star formation history may have some physical motivations, its main role is also to ensure the formation of stars after the emergence of the gaseous disc just after fusion.”

On left, the NGC 5907 galaxy. It is compared to the simulations, on right. Both cases show an edge-on galactic disk surrounded by giant loops of old stars, which are witnessing of a former, gigantic collision. (Jay Gabany, cosmotography.com / Observatoire de Paris / CNRS / Pythéas / NAOC)

Now enter the 32- and 196-core computers at the Paris Observatory center and the 680-core Graphic Processor Unit supercomputer of Beijing NAOC with the capability to run 50000 billion operations per second. By employing several state of the art, hydrodynamical, and numerical simulations with particle numbers ranging from 200 000 to 6 millions, the team’s goal was to show the structure of NGC 5907 may have been the result of the clash of two dragon-sized galaxies… or was it?

“The exceptional features of NGC 5907 can be reproduced, together with the central galaxy properties, especially if we compare the observed loops to the high-order loops expected in a major merger model.” says Wang. “Given the extremely large number of parameters, as well as the very numerous constraints provided by the observations, we cannot claim that we have already identified the exact and unique model of NGC 5907 and its halo properties. We nevertheless succeeded in reproducing the loop geometry, and a disc-dominated, almost bulge-less galaxy.”

In the meantime, major galaxy merger events will continue to be a top priority in formation research. “Future work will include modelling other nearby spiral galaxies with large and faint, extended features in their halos.” concludes the team. “These distant galaxies are likely similar to the progenitors, six billion years ago, of present-day spirals, and linking them together could provide another crucial test for the spiral rebuilding disc scenario.”

And sleeping dragons may one day arise…

Original Story Source: Paris Observatory News. For Further Reading: Loops formed by tidal tails as fossil records of a major merger and Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907.

The Genesis of Galaxy Eris…

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In as much time as it takes to give birth to human life, a supercomputer and a team of researchers at the University of California, Santa Cruz, and the Institute for Theoretical Physics in Zurich have given rise to the first simulation of the physics involved in galaxy formation that produced the Milky Way. They named their child Eris…

“Previous efforts to form a massive disk galaxy like the Milky Way had failed, because the simulated galaxies ended up with huge central bulges compared to the size of the disk,” said Javiera Guedes, who recently earned her Ph.D. in astronomy and astrophysics at UC Santa Cruz and is first author of a paper which has been accepted for publication in the Astrophysical Journal.

This comparison shows the Eris simulation (top) and the Milky Way (bottom). Credit: S. Callegari, J. Guedes, and the 2MASS collaboration.
Like the Milky Way, Eris is a lovely barred spiral galaxy – her figure and star content as identical as modeling can make it. By studying our own galaxy and others like it, this simulation fits the mold from every angle. “We dissected the galaxy in many different ways to confirm that it fits with observations,” Guedes said.

And “seven sisters” were involved in the project, too. NASA’s state-of-the-art Pleiades supercomputer took on the task of 1.4 million processor-hours. But the calculations didn’t stop there. Simulations on supercomputers at UCSC and the Swiss National Supercomputing Center were involved, too. “We took some risk spending a huge amount of supercomputer time to simulate a single galaxy with extra-high resolution,” Madau said.

For over two decades, attempts at creating the evolution of a Milky Way type galaxy have been just outside the grasp of researchers. They just weren’t able to produce the proper shape, size and population to fit known properties. Thanks to this new breakthrough, support for the “cold dark matter” theory has predominated and the Big Bang theory supported. What gave Eris the edge? Try our now better understanding star formation.

“Star formation in real galaxies occurs in a clustered fashion, and to reproduce that out of a cosmological simulation is hard,” Madau said. “This is the first simulation that is able to resolve the high-density clouds of gas where star formation occurs, and the result is a Milky Way type of galaxy with a small bulge and a big disk. It shows that the cold dark matter scenario, where dark matter provides the scaffolding for galaxy formation, is able to generate realistic disk-dominated galaxies.”

Giving birth to Eris wasn’t an easy task. Through low-resolution simulations, researchers began assembling clumps of dark matter – shaping them into galactic halos. From there they selected information on a halo with similar mass and merger history to our own and “rewound the tape” to its infancy. By focusing on a small area, they were able to add additional particle information and step up the resolution.

“The simulation follows the interactions of more than 60 million particles of dark matter and gas. A lot of physics goes into the code–gravity and hydrodynamics, star formation and supernova explosions–and this is the highest resolution cosmological simulation ever done this way,” said Guedes, who is currently a postdoctoral researcher at the Swiss Federal Institute of Technology in Zurich (ETH Zurich).

What sets Eris apart from its predecessors is the ability to “see” in high resolution / high density. This allows for a more pragmatic approach to star formation and placement. It’s an important consideration, because supernova occur in high density regions and high resolution allows them to be taken into account.

“Supernovae produce outflows of gas from the inner part of the galaxy where it would otherwise form more stars and make a large bulge,” Madau said. “Clustered star formation and energy injection from supernovae are making the difference in this simulation.”

Arise, Eris… Your time has come!

Original Story Source: University of Santa Cruz News. For Further Reading: Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris Simulation.

Galactic Mergers Fail to Feed Black Holes

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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

Astronomy Without A Telescope – Secular Evolution

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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.

Taking a Galaxy’s Temperature

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The role that supermassive black holes play in the formation of galaxies is a “hot” topic in astronomy. Using the Chandra X-Ray Observatory, an international team of astronomers have been able to create a temperature map of one galaxy, NGC 5813, which is located in the Virgo III Group of galaxies. The new map shows in unprecedented detail the history of various periods of activity of the Active Galactic Nucleus (AGN), which is associated with a supermassive black hole that resides at its center. They found that regular outbursts of the AGN maintained the temperature of the gas in the region of the galaxy, continually reheating the gas that would otherwise have cooled down.

Paper co-author Dr. Scott Randall of the Chandra Mission Planning Team at the Harvard-Smithsonian Center for Astrophysics said, “Although there are other systems that show AGN outburst shocks, this is still the only system where unambiguous shocks from multiple outbursts are seen. This allows us to directly measure the heating from shocks, and directly observe how often these shocks take place. Thus, at present NGC 5813 is *uniquely* well suited to the study of AGN heating.”

By studying images taken by the Chandra X-Ray Observatory, and combining these observations with those taken by the Giant Metrewave Radio Telescope (GMRT) and the Southern Astrophysical Research Telescope (SOAR), they were able to make out large cavities produced by periods of activity in the supermassive black hole. The researchers were able to determine that there were three pairs of large cavities, which corresponded to active outbursts of the galactic nucleus 3 million, 20 million and 90 million years ago (from our perspective here on Earth).

What makes the galaxy NGC 5813 especially suited to this study is its relative isolation from other galaxies that could influence the formation of these cavities – it is an older galaxy that is relatively undisturbed, allowing for these cavities in the gas to persist over such a long time period.

Current models of galaxy formation must take into account just how much of an influence the output of the supermassive black hole at the center of a galaxy has on the formation of stars within the galaxy, and the evolution of the shape and size of the galaxy as a whole. This process of “AGN feedback” has a dramatic influence on how the galaxy takes shape. The research by Dr. Randall, et. al shows an intimate portrait of this process.

Dr. Randall explained, “This is an important result for stellar formation and galaxy evolution. The AGN heats the gas, preventing it from cooling and forming large amounts of stars. There have been several galaxy evolution models proposed that require this kind of “AGN feedback” near the centers of galaxies to explain the observed differences in galaxies. Here we show explicitly that this kind of feedback can and does take place, at least in this system.”

A labeled image of the various shock waves and cavities formed by the activity of the AGN. Image Credit: Credit: NASA/CXC/SAO/S.Randall et al.

As you can see in the image directly above, various outbursts of the AGN create shock waves in the gas near the center of the galaxy. As these shock waves expanded and the galaxy evolved over millions of years, the heat generated by the shocks spread outwards and into the gas surrounding NGC 5813. The gas between all of the galaxies in a cluster is called the intracluster medium (ICM). The heat – which is produced by the friction of the gases at the edge of each of the shock waves – radiates outward into the surrounding gas, increasing its temperature.

The output of the jets streaming from the supermassive black hole in the center vary over a span of roughly 10 million years, and the amount of energy that each outburst puts out is rather variable – the difference between the last two largest outbursts, for example, is almost an order of magnitude.

This process is cyclical, though the details of the mechanisms involved are still a topic that isn’t completely understood.

Dr. Randall explained this process as follows:

“…the gas cools radiatively, and flows in towards the AGN. The cool gas is rapidly accreted by the black hole, dirving [sic] an energetic outburst. The outburst heats the gas (via shocks), stopping the inflow and starving the AGN. The gas is then able to cool once more, and the cycle repeats, with, in this case, a period of about 10 million years. However, the fine details of how the jet and the ICM interact are not currently well uderstood [sic], and it is not clear how well this simple model describes reality. Our goal with the upcoming deep Chandra observation is to better understand the details of this process, most likely through comparisons with detailed numerical simulations.”

Further observations of NGC 5813 in the fall of 2011 using Chandra are in the works, Dr. Randall said. The results of their analysis will be published in the Astrophysical Journal. A preprint version of the paper, “Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback,” is available on Arxiv.

Sources: Chandra press release, Arxiv paper, email interview with Dr. Scott Randall

Astronomy Without A Telescope – Black Hole Evolution

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While only observable by inference, the existence of supermassive black holes (SMBHs) at the centre of most – if not all – galaxies remains a compelling theory supported by a range of indirect observational methods. Within these data sources, there exists a strong correlation between the mass of the galactic bulge of a galaxy and the mass of its central SMBH – meaning that smaller galaxies have smaller SMBHs and bigger galaxies have bigger SMBHs.

Linked to this finding is the notion that SMBHs may play an intrinsic role in galaxy formation and evolution – and might have even been the first step in the formation of the earliest galaxies in the universe, including the proto-Milky Way.

Now, there are a number of significant assumptions built into this line of thinking, since the mass of a galactic bulge is generally inferred from the velocity dispersion of its stars – while the presence of supermassive black holes in the centre of such bulges is inferred from the very fast radial motion of inner stars – at least in closer galaxies where we can observe individual stars.

For galaxies too far away to observe individual stars – the velocity dispersion and the presence of a central supermassive black hole are both inferred – drawing on the what we have learnt from closer galaxies, as well as from direct observations of broad emission lines – which are interpreted as the product of very rapid orbital movement of gas around an SMBH (where the ‘broadening’ of these lines is a result of the Doppler effect).

But despite the assumptions built on assumptions nature of this work, ongoing observations continue to support and hence strengthen the theoretical model. So, with all that said – it seems likely that, rather than depleting its galactic bulge to grow, both an SMBH and the galactic bulge of its host galaxy grow in tandem.

It is speculated that the earliest galaxies, which formed in a smaller, denser universe, may have started with the rapid aggregation of gas and dust, which evolved into massive stars, which evolved into black holes – which then continued to grow rapidly in size due to the amount of surrounding gas and dust they were able to accrete.

Distant quasars may be examples of such objects which have grown to a galactic scale. However, this growth becomes self-limiting as radiation pressure from an SMBH’s accretion disk and its polar jets becomes intense enough to push large amounts of gas and dust out beyond the growing SMBH’s sphere of influence. That dispersed material contains vestiges of angular momentum to keep it in an orbiting halo around the SMBH and it is in these outer regions that star formation is able to take place. Thus a dynamic balance is reached where the more material an SMBH eats, the more excess material it blows out – contributing to the growth of the galaxy that is forming around it.

The almost linear correlation between the SMBH mass (M) and velocity dispersion (sigma) of the galactic bulge (the 'M-sigma relation') suggests that there is some kind of co-evolution going on between an SMBH and its host galaxy. The only way an SMBH can get bigger is if its host galaxy gets bigger - and vice versa. The left chart shows data points derived from different objects in a galaxy - the right chart shows data points derived from different types of galaxies. Credit: Tremaine et al. (2002).

To further investigate the evolution of the relationship between SMBHs and their host galaxies – Nesvadba et al looked at a collection of very red-shifted (and hence very distant) radio galaxies (or HzRGs). They speculate that their selected group of galaxies have reached a critical point – where the feeding frenzy of the SMBH is blowing out about as much material as it is taking in – a point which probably represents the limit of the active growth of the SMBH and its host galaxy.

From that point, such galaxies might grow further by cannibalistic merging – but again this may lead to a co-evolution of the galaxy and the SMBH – as much of the contents of the galaxy being eaten gets used up in star formation within the feasting galaxy’s disk and bulge, before whatever is left gets through to feed the central SMBH.

Other authors (e.g. Schulze and Gebhardt), while not disputing the general concept, suggest that all the measurements are a bit out as a result of not incorporating dark matter into the theoretical model. But, that is another story…

Further reading: Nesvadba et al. The black holes of radio galaxies during the “Quasar Era”: Masses, accretion rates, and evolutionary stage.

Astronomy Without A Telescope – Not So Ordinary

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Sorry – a bit of southern sky bias in this one. But it does seem that our favourite down under naked eye objects are even more unique than we might have thought. The two dwarf galaxies, the Large and Small Magellanic Clouds, orbit the Milky Way and have bright star forming regions. It would seem that most satellite galaxies, in orbit around other big galaxies, don’t. And, taking this finding a step further, our galaxy may be one of a declining minority of galaxies still dining on gas-filled dwarf galaxies to maintain a bright and youthful appearance.

We used to think that the Sun was an ordinary, unremarkable star – but these days we should acknowledge that it’s out of statistical mid-range, since the most common stars in the visible universe are red dwarfs. Also, most stars are in binary or larger groups – unlike our apparently solitary one.

The Sun is also fortunately positioned in the Milky Way’s habitable zone – not too close-in to be constantly blasted with gamma rays, but close-in enough for there to be plenty of new star formation to seed the interstellar medium with heavy elements. And the Milky Way itself is starting to look a bit out of the ordinary. It’s quite large as spiral galaxies go, bright with active star formation – and it’s got bright satellites.

The Lambda Cold Dark Matter (CDM) model of large scale structure and galaxy formation has it that galaxy formation is a bottom-up process, with the big galaxies we see today having formed from the accretion of smaller structures – including dwarf galaxies – which themselves may have first formed upon some kind of dark matter scaffolding.

Through this building-up process, spinning spiral galaxies with bright star forming regions should become common place – only dimming if they run out of new gas and dust to feast on, only losing their structure if they collide with another big galaxy – first becoming a ‘train wreck’ irregular galaxy and then probably evolving into an elliptical galaxy.

The  Lambda CDM model suggests that other bright spiral galaxies should also be surrounded by lots of gas-filled satellite galaxies, being slowly draw in to feed their host. Otherwise how is it that these spiral galaxies get so big and bright? But, at least for the moment, that’s not what we are finding – and the Milky Way doesn’t seem to be a ‘typical’ example of what’s out there.

The relative lack of satellites observed around other galaxies could mean the era of rapidly accreting and growing galaxies is coming to a close – a point emphasised by the knowledge that we observe distant galaxies at various stages of their past lives anyway. So the Milky Way may already be a relic of a bygone era – one of the last of the galaxies still growing from the accretion of smaller dwarf galaxies.

Supernova 1987a, which exploded near the Tarantula Nebula of the Large Magellanic Cloud. Credit: Anglo-Australian Observatory.

On the other hand – maybe we just have some very unusual satellites. To a distant observer, the Large MC would have nearly a tenth of the luminosity of the Milky Way and the Small MC nearly a fortieth – we don’t find anything like this around most other galaxies. The Clouds may even represent a binary pair which is also fairly unprecedented in any current sky survey data.

They are thought to have passed close together around 2.5 billion years ago – and it’s possible that this event may have set off an extended period of new star formation. So maybe other galaxies do have lots of satellites – it’s just that they are dim and difficult to observe as they are not engaged in new star formation.

Either way, using our galaxy as a basis for modelling how other galaxies work might not be a good idea – apparently it’s not so ordinary.

Further reading: James, P. A. And Ivory C.F. On the scarcity of Magellanic Cloud-like satellites.