Posted on by Steve Nerlich

Astronomy Without A Telescope – Green Peas

A Green Pea galaxy - which may be an local analogue of the univere's first galaxies. Credit: Galaxy Zoo/SDS.

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The ground-breaking discovery of a new class of galaxies, Green Peas, in 2009 by a group of Galaxy Zoo volunteers – have recently been followed up by further observations in the radio spectrum.

The Green Peas were first identified from Sloane Digital Sky Survey data – and then in Hubble Space Telescope archive images. Now radio observations of Green Pea galaxies (from GMRT and VLA) have led to some new speculation on the role of magnetic fields in early galaxy formation.

Green Pea galaxies were so named from their appearance as small green blobs in Galaxy Zoo images. They are low mass galaxies, with low metallicity and high star formation rates – but, surprisingly, are not all that far away. This is surprising given that their low metallicity means they are young – and being not very far away means they formed fairly recently (in universal timeframe terms).

Most nearby galaxies reflect the 13.7 billion year old age of the universe and have high metallicity resulting from generations of stars building elements heavier than hydrogen and helium through fusion reactions.

But Green Peas do seem to have formed from largely unsullied clouds of hydrogen and helium that have somehow remained unsullied for much of the universe’s lifetime. And so, Green Peas may represent a close analogue of what the universe’s first galaxies were like.

Their green color comes from strong OIII (ionised oxygen) emission lines (a common consequence of lots of new star formation) within a redshift (z) range around 0.2. A redshift of 0.2 means we see these galaxies as they were when the universe was about 2.4 billion years younger (according to Ned Wright’s cosmology calculator). Equivalent early universe galaxies are most luminous in ultraviolet at a redshift (z) between 2 and 5 – when the universe was between 10 and 12 billion years younger than today.

Spectroscopic data from Green Pea galaxy 587739506616631548 - demonstrating the prominent OIII emission lines which are characteristic of Green Pea galaxies. Credit: Galaxy Zoo.

Anyhow, studying Green Peas in radio has yielded some interesting new features of these galaxies.

With the notable exception of Seyfert galaxies, where the radio output is dominated by emission from supermassive black holes, the bulk radio emission from most galaxies is a result of new star formation, as well as synchrotron radiation arising from magnetic fields within the galaxy.

Based on a number of assumptions, Chakraborti et al are confident they have discovered that Green Peas have relatively powerful magnetic fields. This is surprising given their youth and smaller size – with magnetic field strengths of around 30 microGauss, compared with the Milky Way’s approximately 5 microGauss.

They do not offer a model to explain the development of Green Pea magnetic fields, beyond suggesting that turbulence is a likely underlying factor. Nonetheless, they do suggest that the strong magnetic fields of Green Peas may explain their unusually high rate of star formation – and that this finding suggests that the same processes existed in some of the first galaxies to appear in our 13.7 billion year old universe.

Further reading:
Chakraborti et al Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies
Cardamone et al Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies.

Posted on by Tammy Plotner

Are The Milky Way’s First Stars Responsible For Destroying Its Satellite Galaxies?

About a decade ago, standard cosmological models encountered a slight problem when applied to the Milky Way… missing satellite galaxies. While the calculations predicted as many as 500, only 10 are documented and modern figures state as many as 20. So what happened to the other 480 that should be out there? Either they don’t exist – or we can’t see them for some reason. Thanks to research done by the LIDAU project and two researchers from Observatoire Astronomique de Strasbourg, we might just have an answer.

About 150 million years after the Big Bang, the Universe’s first stars began to appear out of the cold, electrically neutral hydrogen and helium gas which filled it. As their intense light cut through the hydrogen atoms, it returned them to their plasma state in a process called reionisation. Things really began to heat up from there… gas began escaping the gravity of low-mass galaxies and as a consequence, they lost their star-forming abilities. By computing the observable consequences of this process, Pierre Ocvirk and Dominique Aubert demonstrated that the Milky Way’s first stars had the power of reionisation and it “is indeed an essential process in the standard model of galaxy formation.” This photo-evaporation state neatly explains the sparsity and age of Milky Way companions and offers up the reason satellite galaxies are rare in this neighborhood.

“On the other hand, their sensitivity to UV radiation means satellite galaxies are good probes of the reionisation epoch. Moreover, they are relatively nearby, from 30000 to 900000 light-years, which allows us to study them in great details, especially with the forthcoming generation of telescopes.” says Ocvirk. “In particular, the study of their stellar content with respect to their position could give us precious insight into the structure of the local UV radiation field during the reionisation.”

Current theory states this photo-evaporation was simply caused by nearby galaxies, resulting in a uniform event – but the new model built by the two French researchers proves this assumption wrong. Their high resolution numerical simulation accounts for the dynamics of the dark matter haloes from beginning to end, as well as their resultant gas impacted star formation and UV radiation.

“It is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the center of the Milky way, on its satellite galaxies. Indeed, contrary to previous models, the radiation field produced in this configuration is not uniform, but decreases in intensity as one moves away from the source.” explains Ocvirk. “On one hand, the satellite galaxies close to the galactic center see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. On the other hand, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous.”

Where did initial assumptions fall short? In previous models reionisation was thought to occur over an evenly distributed UV background, but the MIlky Way’s first stars had already done its damage by consuming its satellites. As the study suggests, our own galaxy is responsible for the lack of smaller companions.

Says Ocvirk; “This new scenario has deep consequences on the formation of galaxies and the interpretation of the large astronomical surveys to come. Indeed, satellite galaxies are affected by our galaxy’s tidal field, and can be slowly digested into our galaxy’s stellar halo. They can also be stretched into filaments and form stellar streams.”

It’s a very interesting new concept and will be one of the main science goals of the Gaia space mission, scheduled for launch in 2013. Until then, the Observatoire Astronomique de Strasbourg team will continue in their efforts to further understand radiative processes during reionisation.

Original Story Source: Observatoire Astronomique de Strasbourg Press Release. For Further Reading: A signature of the internal reionisation of the Milky Way and LIDAU collaboration (Light In the Dark Ages of the Universe).

Posted on by Jon Voisey

Is M85 Missing a Black Hole?

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The conventional wisdom of galaxies is that they should have a central massive black hole (CMBH). The presence of such objects has been confirmed in our own galaxy as well as numerous other galaxies, including the Andromeda galaxy (M31) and even some dwarf galaxies. The mass of these objects, several million times the mass of the Sun, has been found to be related to many properties of galaxies as a whole, indicating that their presence may be critical in the formation and evolution of galaxies as a whole. As such, finding a massive galaxy without a central black hole would be quite surprising. Yet a recent study by astronomers from the University of Michigan Ann Arbor seems to have found an exception: The well known M85.

To determine the mass of the CMBH, the team used the spectrograph on board the Hubble Space Telescope to examine the pull the central object had on stars in the nearby vicinity. The higher this mass is, the more quickly the stars should orbit. This orbital velocity is detected as a shift in the color of the light, blue as the stars move towards us, red as they move away. The amount the light is shifted is dependent on just how fast they move.

Doppler shift of gas and dust caused by M84's supermassive black hole. Image Credit: Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team, and NASA
Doppler shift of gas and dust caused by M84's supermassive black hole. Image Credit: Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team, and NASA
This technique has been used previously in other galaxies, including another large elliptical of similar brightness in the Messier catalog, M84. This galaxy had its CMBH probed by Hubble in 1997 and was determined to have a mass of 300 million solar masses.

When this method was applied to M85 the team did not discover a shift that would be indicative of a black hole with a mass expected for a galaxy of such size. Using another, indirect method of determining the CMBH mass by looking at the the amount of overall light from the galaxy, which is generally correlated with black hole mass, would indicate that M85 should contain a black hole of 300 million to 2 billion solar masses. Yet this study indicates that, if M85 contains a central black hole at all, the upper limit for the black hole would be around 65 million solar masses.

This study is not the first to report a non-detection for the galaxy, a 2009 study led by Alessandro Capetti from Osservatorio Astronoimco di Torino in Italy, searched M85 for signs of radio emission from the black hole region. Their study was unable to detect any significant radio waves from the core which, if M85 had a significant black hole, should be present, even with a small amount of gas feeding into the core.

Overall, these studies demonstrate a significant shortcoming in secondary methods of black hole mass estimation. Such indirect methods have been previously used with confidence and have even been the basis for studies drawing the connection between galaxy evolution and black hole mass. If cases like M85 are more common that previously thought, it may prompt astronomers to rethink just how connected black holes and a galaxies properties really are.

Posted on by Tammy Plotner

New Dark Matter Census – The Hubble Survey

This image of galaxy cluster MACS J1206.2-0847 is part of a broad survey with NASA's Hubble Space Telescope. Credit: NASA, ESA, M. Postman (STScI), and the CLASH Team

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Way off in the constellation of Virgo, galaxy cluster MACS J1206.2-0847 -or MACS 1206 for short – is making news as the forerunner of a brand new Hubble Space Telescope survey. What’s new for the aging telescope? Now astronomers are able to assemble a highly detailed dark matter map… one that involves more galaxy clusters than ever before.

These “dark matter” maps are proving their worth by allowing astronomers to test some theories. In this case, it’s some unusual findings which suggest dark matter is more densely packed inside galaxy clusters than some models predict. If this holds true, it may point to evidence that galaxy clusters pulled together sooner than once predicted. The multiwavelength survey, called the Cluster Lensing And Supernova survey with Hubble (CLASH), takes an unprecedented look at the distribution of dark matter in 25 massive clusters of galaxies.

“The era when the first clusters formed is not precisely known, but is estimated to be at least 9 billion years ago and possibly as far back as 12 billion years ago.” says the Hubble team. “If most of the clusters in the CLASH survey are found to have excessively high accumulations of dark matter in their central cores, then it may yield new clues to the early stages in the origin of structure in the universe.”

To date, the CLASH team has finished their observations of six of the 25 clusters. Of these, MACS 1206 has a distance of about 4.5 billion light-years and was photographed with Hubble’s Advanced Camera for Surveys and the Wide Field Camera 3 in April 2011 through July 2011. What are the strange shapes? These “distortions” are where the light is bent by the extreme gravitation of dark matter.

“Lensing effects can also produce multiple images of the same distant object, as evident in this Hubble picture. In particular, the apparent numbers and shapes of distant galaxies far beyond a galaxy cluster become distorted as the light passes through, yielding a visible measurement of how much mass is in the intervening cluster and how it is distributed.” says the team. “The substantial lensing distortions seen are proof that the dominant component of clusters is dark matter. The distortions would be far weaker if the clusters’ gravity came only from the visible galaxies in the clusters.”

Original Story Source: Hubble News Release.

Posted on by Tammy Plotner

Early Galaxies – Clearing The “Cosmic Fog”

Scientists have used ESO’s Very Large Telescope to probe the early Universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history — known as reionisation — occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionisation for the first time. They have also demonstrated that this phase must have happened quicker than astronomers previously thought.

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The seasons are changing for both hemispheres and it’s not uncommon to wake up to wonderful, mysterious swirls of fog. What we experience here on Earth is water vapor, but the Universe was once filled with a fog of hydrogen gas. As the hours progress, the Sun slowly burns it off – quietly revealing trees, houses and the road ahead. In time after expansion began, the electrically neutral hydrogen gas was slowly swept away by the light of ultraviolet radiation from early galaxies…

Using the Very Large Telescope (VLT) like a “time machine”, a team of astronomers cut through the cosmic cloud layer to view some of the most distant galaxies recorded so far – a look back between 780 million and a billion years after the Big Bang. These antediluvian galaxies excited the gas, making it electrically charged (ionised), it gradually became transparent to ultraviolet light. While you may argue this process is technically known as reionization, there is theorized to be a brief timeline when hydrogen was also ionised.

“Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil. Astronomers can go one better: we can look directly into the remote past and observe the faint light from different galaxies at different stages in cosmic evolution,” explains Adriano Fontana, of INAF Rome Astronomical Observatory who led this project. “The differences between the galaxies tell us about the changing conditions in the Universe over this important period, and how quickly these changes were occurring.”

As we know from spectroscopy, each element has its own signature – the emission lines – and the strongest in ultraviolet is the Lyman-alpha line generated from hydrogen. This bold spectral signature is easily recognizable – even at a vast distance. By observing the Lyman-alpha line for five very remote galaxies, the team was able to establish two critical factors: their distance through redshift and how soon they could be detected. Through this process, the astronomers were then able to establish how much the Lyman-alpha emission was reabsorbed by the neutral hydrogen fog and create a timeline… A whole lot like recording what minute each landmark reappears when terrestrial fog clears and seeing the long road ahead.

“We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample,” says lead author Laura Pentericci of INAF Rome Astronomical Observatory. “When the Universe was only 780 million years old this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe’ volume. But only 200 million years later the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionization must have happened quicker than astronomers previously thought.”

As always, there’s a bit more to the story. In this case, by understanding the rate at which the ancient absorbent obstruction began fading, scientists could also deduce the source of the powerful ultraviolet radiation. Could it be first generation stars – or even the work of primeval black holes?

“The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed,” says Eros Vanzella of the INAF Trieste Observatory, a member of the research team. “These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent.”

To prove anything, it’s going to take a lot more research and some very accurate measurements – ones that are already in the planning stage for the future ESO European Extremely Large Telescope. But, in the meantime, the team used the great light-gathering power of the 8.2-metre VLT to carry out spectroscopic observations, targeting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.

Original Story Source: ESO Press Release. For Further Reading: Probing The Earliest Galaxies And The Epoch Of Reionization.

Posted on by Ray Sanders

Even the Early Universe Had the Ingredients for Life

The optical image of TN J0924-2201, a very distant radio galaxy at (redshift) z = 5.19, obtained with the Hubble Space Telescope. (c) NASA/STScI/NAOJ.

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For us carbon-based life forms, carbon is a fairly important part of the chemical makeup of the Universe. However, carbon and oxygen were not created in the Big Bang, but rather much later in stars. How much later? In a surprising find, scientists have detected carbon much earlier in the Universe’s history than previously thought.

Researchers from Ehime University and Kyoto University have reported the detection of carbon emission lines in the most distant radio galaxy known. The research team used the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope to observe the radio galaxy TN J0924-2201. When the research team investigated the detected carbon line, they determined that significant amounts of carbon existed less than a billion years after the Big Bang.

How does this finding contribute to our understanding of the chemical evolution of the universe and the possibilities for life?

To understand the chemical evolution of our universe, we can start with the Big Bang. According to the Big Bang theory, our universe sprang into existence about 13.7 billion years ago. For the most part, only Hydrogen and Helium ( and a sprinkle of Lithium) existed.

So how do we end up with everything past the first three elements on the periodic table?

Simply put, we can thank previous generations of stars. Two methods of nucleosythesis (element creation) in the universe are via nuclear fusion inside stellar cores, and the supernovae that marked the end of many stars in our universe.

Over time, through the birth and death of several generations of stars, our universe became less “metal-poor” (Note: many astronomers refer to anything past Hydrogen and Helium as metals”). As previous generations of stars died out, they “enriched” other areas of space, allowing future star-forming regions to have conditions necessary to form non-star objects such as planets, asteroids, and comets. It is believed that by understanding how the universe created heavier elements, researchers will have a better understanding of how the universe evolved, as well as the sources of our carbon-based chemistry.

So how do astronomers study the chemical evolution of our universe?

By measuring the metallicity (abundance of elements past Hydrogen on the periodic table) of astronomical objects at various redshifts, researchers can essentially peer back into the history of our universe. When studied, redshifted galaxies show wavelengths that have been stretched (and reddened, hence the term redshift) due to the expansion of our universe. Galaxies with a higher redshift value (known as “z”) are more distant in time and space and provide researchers information about the metallicity of the early universe. Many early galaxies are studied in the radio portion of the electromagnetic spectrum, as well as infra-red and visual.

The research team from Kyoto University set out to study the metallicity of a radio galaxy at higher redshift than previous studies. In their previous studies, their findings suggested that the main era of increased metallicity occurred at higher redshifts, thus indicating the universe was “enriched” much earlier than previous believed. Based on the previous findings, the team then decided to focus their studies on galaxy TN J0924-2201 – the most distant radio galaxy known with a redshift of z = 5.19.

The deep optical spectrum of TN J0924-2201 obtained with FOCAS on the Subaru Telescope. The red arrows point to the carbon emission line.

The research team used the FOCAS instrument on the Subaru Telescope to obtain an optical spectrum of galaxy TN J0924-2201. While studying TN J0924-2201, the team detected, for the first time, a carbon emission line (See above). Based on the detection of the carbon emission line, the team discovered that TN J0924-2201 had already experienced significant chemical evolution at z > 5, thus an abundance of metals was already present in the ancient universe as far back as 12.5 billion years ago.

If you’d like to read the team’s findings you can access the paper Chemical properties in the most distant radio galaxy – Matsuoka, et al at: http://arxiv.org/abs/1107.5116

Source: NAOJ Press Release

Posted on by Tammy Plotner

Looking Into The Eye Of A Monster – Active Galaxy Markarian 509

Active galaxy Markarian 509 as seen by the Hubble Space Telescope's WFPC2. Credits: NASA, ESA, J. Kriss (STScI) and J. de Plaa (SRON)

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“The world is a vampire, sent to drain… Secret destroyers, hold you up to the flames…” Ah, yes. It’s the biggest vampire of all – the supermassive black hole. In this instance, it’s not any average, garden-variety black hole, but one that’s 300 million times the mass of the Sun and growing. Bullet with butterfly wings? No. This is more a case of butterfly wings with bullets.

An international team of astronomers using five different telescopes set their sites on 460 million light-year distant Markarian 509 to check out the action surrounding its huge black hole. The imaging team included ESA’s XMM-Newton, Integral, NASA/ESA Hubble Space Telescope, NASA’s Chandra and Swift satellites, and the ground-based telescopes WHT and PARITEL. For a hundred days they monitored Markarian 509. Why? Because it is known to have brightness variations which could mean turbulent inflow. In turn, the inner radiation then drives an outflow of gas – faster than a speeding bullet.

“XMM-Newton really led these observations because it has such a wide X-ray coverage, as well as an optical monitoring camera,” says Jelle Kaastra, SRON Netherlands Institute for Space Research, who coordinated an international team of 26 astronomers from 21 institutes on four continents to make these observations.

And the vampire reared its ugly head. Instead of the previously documented 25% changes, it jumped to 60%. The hot corona surrounding the black hole was spattering out cold gas “bullets” at speeds in excess of one million miles per hour. These projectiles are torn away from the dusty torus, but the real surprise is that they are coming from an area just 15 light years away from the center. This is a lot further than most astronomers speculate could happen.

“There has been a debate in astronomy for some time about the origin of the outflowing gas,” says Kaastra.

But there’s more than just bullets here. These new observations at multiple wavelengths are showing the coolest gas in the line of sight toward Markarian 509 has 14 different velocity components – all from different locations at the galaxy’s heart. What’s more, there’s indications the black hole accretion disc may have a shield of gas harboring temperatures ranging in the millions of degrees – the motivating force behind x-rays and gamma rays.

An artist's impression of the central engine of an active galaxy. A black hole is surrounded by matter waiting to fall in. Fearsome radiation from near the black hole drives an outflow of gas. Credits: NASA and M. Weiss (Chandra X-ray Center)

“The only way to explain this is by having gas hotter than that in the disc, a so-called ‘corona’, hovering above the disc,” Jelle Kaastra says. “This corona absorbs and reprocesses the ultraviolet light from the disc, energising it and converting it into X-ray light. It must have a temperature of a few million degrees. Using five space telescopes, which enabled us to observe the area in unprecedented detail, we actually discovered a very hot ‘corona’ of gas hovering above the disc. This discovery allows us to make sense of some of the observations of active galaxies that have been hard to explain so far.”

To make things even more entertaining, the study has also found the signature of interstellar gas which may have been the result of a one-time galaxy collision. Although the evidence may be hundreds of thousands of light years away from Mrk 509, it may have initially triggered this activity.

“The results underline how important long-term observations and monitoring campaigns are to gain a deeper understanding of variable astrophysical objects. XMM-Newton made all the necessary organisational changes to enable such observations, and now the effort is paying off,” says Norbert Schartel, ESA XMM-Newton Project Scientist.

Ah, Markarian 509… “Despite all my rage… I am still just a rat in cage.”

Original Story Source: ESA News. For Further Reading: Multiwavelength Campaign on Mrk 509 VI. HST/COS Observations of the Far-ultraviolet Spectrum.

Posted on by Tammy Plotner

Holmberg II – Forever Blowing Bubbles

Hubble’s famous images of galaxies typically show elegant spirals or soft-edged ellipses. But these neat forms are only representative of large galaxies. Smaller galaxies like the dwarf irregular galaxy Holmberg II come in many shapes and types that are harder to classify. This galaxy’s indistinct shape is punctuated by huge glowing bubbles of gas, captured in this image from the NASA/ESA Hubble Space Telescope.

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“I’m forever blowing bubbles… Pretty bubbles in the air…” Its name is Holmberg II, and it’s a dwarf galaxy that’s only 9.8 million light-years away. It’s part of the M81 Galaxy Group and one of the few that isn’t distracted by gravity from nearby peers. Holmberg II is an active little galaxy and one that’s full of holes – the largest of which spans 5500 light years wide. But what makes this one really fascinating is that it’s expelling huge bubbles of gas…

Here the remnants of mature and dying stars have left thick waves of dust and gas, carved into shape by stellar winds. Some ended their lives as supernovae – sending rippling shockwaves through the thinner material to hang in space like fantasy ribbons. With no dense nucleus to deform it like an elliptical galaxy, nor distorting arms like a spiral, this irregular star-forming factory is the perfect place for astronomers to take a close look stellar formation in a new way.

Keep thinking bubbles, because Holmberg II is the perfect example of the “champagne” model of starbirth – where new stars create even newer ones. How does it work? When a bubble is created by stellar winds, it moves outwards until it reaches the edge of the molecular cloud that spawned it. At the exterior edge, dust and gas have been compressed and form a nodule similar to a blister. Here another new star forms.. and triggers again… and triggers again… similar to the chain reaction which happens when you open a bottle of champagne.

And fill the glass again, because Holmberg II is also known as Arp 268. While Halton Arp certainly knows his stuff when it comes to unusual galaxies, there’s even more. According to the Hubble team, our little dwarf also has an ultraluminous X-ray source in the middle of three gas bubbles which appears in the image’s upper right hand corner. No one is quite sure of what it just might be! Maybe black hole bubbles?

“They fly so high… Nearly reach the sky. Then in my dreams they fade and die…” Perhaps Dean Martin?

Original Story Source: Hubble News.

Posted on by Jon Voisey

Homeless Supernovae

NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF
NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF

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In a post earlier this month, we looked at a team of astronomers searching for stars that were on ejected from their birthplaces in clusters. These stars could receive the needed kick from a gravitational swing by the core of the cluster to achieve a velocity of a few tens of km/sec. But a similar mechanism can function in the cores of galaxies giving stars a speed of roughly 1,000 km/sec, enough to leave their parent galaxies. a new study asks whether we have ever witnessed any of these stellar cast offs explode as supernovae.

The team, led by Peter-Christian Zinn at Ruhr University in Bochum, Germany, searched through roughly 6,000 supernovae listed in the Sternbarg Astronomical Institute Supernova Catalog, for which no host galaxy was apparent, yet weren’t too distant from any known galaxy. The latter criteria was added because, even at the high velocities, stars still couldn’t get too far before they reached the end of their fuses. The team imposed a rough inner cut off of around 10 kiloparsecs (roughly 1/3 of the width of the disk of the Milky Way). They expected stars should be at least this distance from the cores of the parent galaxy.

The initial list contained five candidate stars, dating back as far as 1969. The first step the team used to determine if the supernova was truly in a galaxy or not, was to take long exposure images of the immediate area, to draw out potential low surface brightness hosts. The team also used archival data in the far ultraviolet as well as the x-ray spectrum to determine whether or not the nearby galaxies from which the supernovae could potentially be ejected had an extended disk, invisible in the visible portion of the spectrum that would have allowed the progenitor star to form in the outskirts of the galaxy. These wavelengths are tracers of ongoing star formation which are sites in which high mass stars that would lead to core-collapse supernovae, would likely be found.

The oldest candidate, SN 1969L, was located near the flocculent spiral NGC 1058. While the deep exposures did not show a host galaxy, the x-ray and UV images both showed some extended structure of the parent galaxy at the distance of the supernova. This led to the conclusion that this supernova, while far removed from its host galaxy, was still gravitationally tied to it.

With the second candidate, SN 1970L, the team again failed to find any faint host galaxy. However, the supernova was situated between two galaxies, NGC 2968 and a faint elliptical, NGC 2970. A 1994 study had revealed a faint bridge of matter connecting the two, implying that they had had an interaction in the past. This interaction would likely have pulled off gas and stars, of which SN 1970L could have been one.

SN 1997C was the third candidate and also lacked a discernible host galaxy, even with long exposures. This one also did not have an indication of an extended disk of which the supernova could have been part. Given the characteristics of the supernova, the team estimated that it had an original mass of 15 times that of the Sun. Given its projected distance and the lifetime of such stars, the team noted that this would correspond to a velocity of some 3,000 km/sec, which is several times the speed of the highest confirmed hypervelocity star. As such, the team expected that this star would have to be ejected in a similar manner to SN 1970L, using an interaction between galaxies. Given that the host galaxy is known to be one in a small cluster and the disk shows some signs of perturbation, they suggested this was likely.

The fourth candidate, SN 2005nc, the team selected because there was no nearby galaxy they could assign as a possible parent. They suggested this was due to an extremely distant host galaxy, too faint to resolve with previous studies. The basis for this assertion was that the supernova came with a gamma ray burst that indicated an origin some 5-6 billion light years distant. Due to the associated GRB, the Hubble telescope swung in to take a look. These archival pictures failed to reveal any objects that could readily be identified as host galaxies leaving the team to presume the host was simply too far away to resolve.

The last candidate was SN 2006bx located near the galaxy UGC 5434. This supernova did not appear to be in a faint background galaxy and did not have hints of being formed in an extended disk. The estimated velocity from the projected distance was ~850 km/sec which placed it in the realm of plausible speeds for stars ejected by gravitational assists from the supermassive black hole at the center of galaxies.

Posted on by Jon Voisey

Globular Clusters on a Plane

Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist's impression. Credit: Jon Lomberg

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Globular clusters are generally some of the oldest structures in our galaxy. Many of the most famous ones formed around the same time as our galaxy, some 13 billion years ago. However, some are distinctly younger. While many classification schemes are used, one breaks globular clusters into three groups: an old halo group which includes the oldest of the clusters, those in the disk and bulge of the galaxy which tend to have higher metallicity, and a younger population of halo clusters. The latter of these provides a bit of a problem since the galaxy should have settled into a disk by the time they formed, depriving them of the necessary materials to form in the first place. But a new study suggests a solution that’s not of this galaxy.

The new study looked at the distribution of these younger clusters around our Milky Way. Of the three classifications for globular clusters discussed, the young halo clusters are scattered well beyond the range of the other populations. The young halo extends to as much as 120 kiloparsecs (400 thousand light years) while the old halo clusters tend to lie within 30 kiloparsecs (100 thousand light years). Additionally, the young clusters don’t appear to be rotating with the disk of the galaxy whereas the old halo slowly orbits in the same direction as the disk.

In looking more carefully at the positions of these satellites, the team, led by Stefan Keller at the Australian National University, found that the younger population tends to lie in a wide plane that is tilted from the rotational axis of our galaxy by a mere 8°.

This plane is strikingly similar to another recognized grouping of objects: Many of the known dwarf galaxies lie in a nearly identical plane, known as the Plane of Satellites (PoS). This finding suggests that this population of globular clusters is a relic of cannibalized galaxies. Even more interesting is that, while these objects are younger than the distinctly “old” population, there is still a large variation in their ages. This implies that this plane wasn’t created by the accretion of one, or even a few minor galaxies, but a consistent feeding of small galaxies onto the Milky Way for much of the history of the universe, and all from the same direction. Studies of the distribution of satellites around our nearest major neighbor, M31, the Andromeda galaxy, has turned up a similar preferred plane, tilted some 59° from its disk.

One explanation for this is that this is a preferred direction that traces invisible filaments of dark matter. While dark matter distributions are difficult to predict, models haven’t accounted for such strong filamentary structure on such small scales. Rather, in the neighborhood of our galaxy, the overall distribution is described as an oblate spheroid. One of the reasons astronomers believe our own dark matter halo is so nicely shaped is the way it is affecting the Sagittarius dwarf galaxy which is slowly being accreted onto our own. If the dark matter were more wispy, it should be stretched out in different manners.

Another possibility the authors consider is that the objects were created in a preferred plane “from the break up of a large progenitor at early times”. In other words, the filament could be a fossil of larger structure before our galaxy formed along which these dwarf galaxies formed and from which these galaxies could have been slowly accreting over the history of the galaxy.