Posted on by Nancy Atkinson

Astronomers Find a “Spine” Along Spiral Arms of the Milky Way

Researchers have identified the first "bone" of the Milky Way - a long tendril of dust and gas that appears dark in this infrared image from the Spitzer Space Telescope. Running horizontally along this image, the "bone" is more than 300 light-years long but only 1 or 2 light-years wide. It contains about 100,000 suns' worth of material. Credit: NASA/JPL/SSC

Astronomers have found what may be considered a piece of a galactic skeleton; a dark structure of gas and dust that might provide a backbone on which one of the spiral arms extend from the central bar of the Milky Way galaxy.

“This ‘bone’ is likely made from high density gas — the type that forms stars — and while the feature that we see is a sinuous distinction you get from dust, there is a huge amount of gas,” said Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics (CfA) at a press conference at the American Astronomical Society meeting in Long Beach, California today. “But we just don’t know yet what it is.”

A flipped image of IC342, in which a 'backbone' structure can be seen in the spiral arms. Credit: Jarret et al. 2012; WISE Enhanced Resolution Galaxy Atlas.
A flipped image of IC342, in which a 'backbone' structure can be seen in the spiral arms. Credit: Jarret et al. 2012; WISE Enhanced Resolution Galaxy Atlas.

While this is the first time such a structure has been seen in our own galaxy, other spiral galaxies seemingly display internal “endoskeletons.” Observations, especially at infrared wavelengths of light, have found long skinny features jutting between galaxies’ spiral arms. These relatively straight structures are much less massive than the curving spiral arms.

Goodman said that since we view the Milky Way from the inside, its exact structure is difficult to determine, but it is thought to have a central bar and two major spiral arms that wrap around its disk.

A team of astronomers first spotted the galactic bone while studying a dust cloud nicknamed “Nessie,” since its shape is reminiscent of the Loch Ness monster. The central part of the “Nessie” bone was discovered in Spitzer Space Telescope data in 2010 by James Jackson (Boston University). With further analysis, Goodman’s team determined the dark cloud goes way beyond the original section that was first found, and is as much as eight times longer than Jackson’s original sighting.

Radio emissions from molecular gas show that the feature is not a chance projection of material on the sky, but instead a real feature. Not only is “Nessie” in the galactic plane, but also it extends much longer than anyone anticipated. This slender bone of the Milky Way is more than 300 light-years long but only 1 or 2 light-years wide. It contains about 100,000 suns’ worth of material, and now looks more like a cosmic snake.

“This bone is much more like a fibula – the long skinny bone in your leg – than it is like the tibia, or big thick leg bone,” Goodman said.

It lies along the plane of the Milky Way, and since our vantage point is just above the the plane, Goodman and her team are hopeful that the skeleton may be able to be mapped.

“It’s possible that the ‘Nessie’ bone lies within a spiral arm, or that it is part of a web connecting bolder spiral features. Our hope is that we and other astronomers will find more of these features, and use them to map the skeleton of the Milky Way in 3-D,” she said.

The team’s paper is not quite finished yet, but it is online on the new open source scientific collaboration site, Authorea.

For more information, visit http://milkywaybones.org.

Source: CfA, AAS

Posted on by John Williams

Hubble Census Unveils Galaxies Shining Near Cosmic Dawn

This new image of the Hubble Ultra Deep Field (HUDF) 2012 campaign reveals a previously unseen population of seven faraway galaxies, which are observed as they appeared in a period 350 million to 600 million years after the Big Bang. Credit: NASA, ESA, R. Ellis (Caltech), and the UDF 2012 Team

Astronomers using NASA’s Hubble Space Telescope have spotted some of the most distant, dim and ancient galaxies ever detected in a new survey. The images, taken with Hubble’s Wide Field Camera 3 (WFC 3) looks further back in time than any previous Hubble observation, providing information about the conditions in the early Universe.

“This is like a scientific version of the story of Genesis,” said astronomer Avi Loeb from Harvard University.

The seven distant galaxies represent a previously unseen population of galaxies that formed more than 13 billion years ago, when the Universe was less than 3 percent of its present age. In these deepest images to date from Hubble, astronomers were able to take a sample of the amount of galaxies at the time. The results show a smooth decline in the number of galaxies with increasing look-back time to about 450 million years after the Big Bang.

The data provides the first reliable census of this uncharted period of cosmic history, according to the scientists. As astronomers look even deeper into the Universe, galaxy numbers appear to drop off smoothly leading them to believe that the “cosmic dawn” was gradual, not a dramatic event.

“Observations of the microwave afterglow from the Big Bang tell us that reionization happened more than about 13 billion years ago,” said Brant Robertson of the University of Arizona in Tucson, a member of the survey team. “Our data confirms that reionization was a drawn-out process occurring over several hundred million years with galaxies slowly building up their stars and chemical elements. There wasn’t a single dramatic moment when galaxies formed; it was a gradual process.”

These galaxies were found as part of an ambitious Hubble survey of an intensively studied patch of sky known as the Ultra Deep Field (UDF), which was originally taken in 2003-2004, focusing in on a small area in the sky in the constellation Fornax. In the new 2012 campaign, called UDF 2012, a team of astronomers led by Richard Ellis of the California Institute of Technology used the WFC3 to peer deeper into space in near-infrared light than any previous Hubble observation. The observations were made over a period of six weeks during August and September 2012, and the first scientific results are now appearing in a series of scientific papers. The UDF 2012 team is publicly releasing these unique data, after preparing them for other research groups to use.

“Hubble is achieving just great science,” said John Grunsfeld, former astronaut and NASA’s associate administrator for science, speaking at a briefing about the new survey. “This is an origins story, where we’re going back to the beginning, back to the first stars that appeared in the Universe. This validates that when we get James Webb Space Telescope online it will have a lot to look at and a lot to do.”

The James Webb Space Telescope is slated to launch in 2018.

Astronomers detected seven galaxies in the time period 400-600 million years after the Big Bang. All extremely distant, they ranged in distance with redshifts from 8.6 to nearly 12.

Astronomers study the distant universe in near-infrared light because the expansion of space stretches ultraviolet and visible light from galaxies into infrared wavelengths, a phenomenon called “redshift.” The more distant a galaxy, the higher its redshift.

Notably, one of the galaxies may be a distance record breaker, observed 380 million years after the birth of our universe in the Big Bang, corresponding to a redshift of 11.9. This is the galaxy UDFj-39546284, which was previously detected and was originally suggested as the most distant object ever found nearly two years ago by Hubble. Later observations put it at a redshift of 10.3, but the newly refined observations put it even more distant.

A timeline of the Universe and our observations of it. Credit: University of Arizona.

Scientists think that the universe began with the Big Bang about 13.7 billion years ago. Hydrogen formed about 400,000 years later but with no stars, spacetime was dark. About 200 million years later, hydrogen clouds collapsed forming the first stars and galaxies; what astronomers call the “cosmic dawn.” Light from these new stars began breaking down hydrogen into protons and electrons during a time period called cosmic reionization. In the present universe, scientists see galaxies growing in mass and size with the synthesis of elements, leading to the formation of complex molecules including the components to create life. Our Sun and solar system formed just over 4 billion years ago.

“The team pushed Hubble to its limits. This is probably the farthest back Hubble can look, according to the study leader, Richard Ellis. “We are pushing Hubble well beyond what it was designed to do.”

Read more about the findings and the HUDF 2012 Campaign at the HubbleSite.

Read the team’s paper: The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12: New Results from the 2012 Hubble Ultra Deep Field Campaign

Additional Sources: CalTech ESA Hubble

Posted on by Jason Major

Do We Really Need Dark Matter?

Hubble mosaic of massive galaxy cluster MACS J0717.5+3745, thought to be connected by a filament of dark matter. Credit: NASA, ESA, Harald Ebeling (University of Hawaii at Manoa) & Jean-Paul Kneib (LAM)

Even though teams of scientists around the world are at this very moment hot on the trail of dark matter — the “other stuff” that the Universe is made of and supposedly accounts for nearly 80% of the mass that we can’t directly observe (yet) —  and trying to quantify exactly how so-called “dark energy” drives its ever-accelerating expansion, perhaps one answer to these ongoing mysteries is maybe they don’t exist at all.

This is precisely what one astronomer is suggesting in a recent paper, submitted Dec. 3 to Astrophysical Journal Letters.

In a paper titled “An expanding universe without dark matter and dark energy” (arXiv:1212.1110) Pierre Magain, a professor at Belgium’s Institut d’Astrophysique et de Géophysique, proposes that the expansion of the Universe could be explained without the need for enigmatic material and energy that, to date, has yet to be directly measured.

In addition, Magain’s proposal puts a higher age to the Universe than what’s currently accepted. With a model that shows a slower expansion rate during the early Universe than today, Magain’s calculations estimate its age to be closer to 15.4 – 16.5 billion years old, adding a couple billion more candles to the cosmic birthday cake.

The benefit to a slightly older Universe, Magain posits, is that it’s not so uncannily close to the apparent age of the most distant galaxies recently found — such as MACS0647-JD, which is 13.3 billion light-years away and thus (based on current estimates, see graphic at right) must have formed when the Universe was a mere 420 million years old.

Read more: Now Even Further: Ancient Galaxy is Latest Candidate for Most Distant

Using accepted physics of how time behaves based on Einstein’s theory of general relativity — namely, how the passage of time is relative to the position and velocity of the viewer (as well as the intensity of the gravitational field the viewer is within) — Magain’s model allows for an observer located within the Universe to potentially be experiencing a different rate of time than a hypothetical viewer located outside the Universe. Not to be so metaphysical as to presume that there are external observers of our Universe but merely to say that an external point would be a fixed one against which one could benchmark a varying passage of time inside the Universe, Magain calls this universal relativity.

A viewer experiencing universal relativity would, Magain claims, always measure the curvature of the Universe to be equal to zero. This is what’s currently observed, a “flatness problem” that Magain insinuates is strangely coincidental.

By attributing an expanding Universe to dark energy and the high velocities of stars along the edges of galaxies (as well as the motions of galaxy clusters themselves) to dark matter, we may be introducing ad hoc elements to the Universe, says Magain. Instead, he proposes his “more economical” model — which uses universal relativity — explains these apparently accelerating, increasingly expanding behaviors… and gives a bigger margin of time between the Big Bang and the formation of the first galactic structures.

Read more: First Images in a New Hunt for Dark Energy

There’s quite a bit of math involved, and since I never claimed to understand physics equations you can check out the original paper here.

While intriguing, the bottom line is that dark energy and dark matter have still managed to elude science, existing just outside the borders of what can be observed (although the gravitational lensing effects of what’s thought to be dark matter filaments have been observed by Hubble) and Magain’s paper is merely putting another idea onto the table — one that, while he recognizes needs further testing and relies upon very specific singular parameters, doesn’t depend upon invisible, unobservable and mysteriously dark “stuff”. Whether it belongs on the table or not will be up to other astrophysicists to decide.

Prof. Magain’s research was supported by ESA and the Belgian Science Policy Office.

At right: Artist’s impression of dark matter (h/t to Steve Nerlich)

Note: this is “just” a submitted paper and has not been selected for publication yet. Any hypotheses proposed are those of the author and are not endorsed by this site. (Personally I like dark matter. It’s fascinating stuff… even if we can’t see it. Want an astrophysicist’s viewpoint on the existence of dark matter? Check out Ethan Siegel’s blog response here.)

Posted on by Jason Major

The Brightest Galaxies in the Universe Were Invisible… Until Now

Hubble images of six of the starburst galaxies first found by ESA’s Herschel Space Observatory (Keck data shown below each in blue)

Many of the brightest, most actively star-forming galaxies in the Universe were actually undetectable by Earth-based observatories, hidden from view by thick clouds of opaque dust and gas. Thanks to ESA’s Herschel space observatory, which views the Universe in infrared, an enormous amount of these “starburst” galaxies have recently been uncovered, allowing astronomers to measure their distances with the twin telescopes of Hawaii’s W.M. Keck Observatory. What they found is quite surprising: at least 767 previously unknown galaxies, many of them generating new stars at incredible rates.

Although nearly invisible at optical wavelengths these newly-found galaxies shine brightly in far-infrared, making them visible to Herschel, which can peer through even the densest dust clouds. Once astronomers knew where the galaxies are located, they were able to target them with Hubble and, most importantly, the two 10-meter Keck telescopes — the two largest optical telescopes in the world.

By gathering literally hundreds of hours of spectral data on the galaxies with the Keck telescopes, estimates of their distances could be determined as well as their temperatures and how often new stars are born within them.

“While some of the galaxies are nearby, most are very distant; we even found galaxies that are so far that their light has taken 12 billion years to travel here, so we are seeing them when the Universe was only a ninth of its current age,” said Dr. Caitlin Casey, Hubble fellow at the UH Manoa Institute for Astronomy and lead scientist on the survey. “Now that we have a pretty good idea of how important this type of galaxy is in forming huge numbers of stars in the Universe, the next step is to figure out why and how they formed.”

A representation of the distribution of nearly 300 starbursts in one 1.4 x 1.4 degree field of view.

The galaxies, many of them observed as they were during the early stages of their formation, are producing new stars at a rate of 100 to 500 a year — with a mass equivalent of several thousand Suns — hence the moniker “starburst” galaxy. By comparison the Milky Way galaxy only births one or two Sun-mass stars per year.

The reason behind this explosion of star formation in these galaxies is unknown, but it’s thought that collisions between young galaxies may be the cause.

Another possibility is that galaxies had much more gas and dust during the early Universe, allowing for much higher star formation rates than what’s seen today.

“It’s a hotly debated topic that requires details on the shape and rotation of the galaxies before it can be resolved,” said Dr. Casey.

Still, the discovery of these “hidden” galaxies is a major step forward in understanding the evolution of star formation in the Universe.

“Our study confirms the importance of starburst galaxies in the cosmic history of star formation. Models that try to reproduce the formation and evolution of galaxies will have to take these results into account.”

– Dr. Caitlin Casey, Hubble fellow at the UH Manoa Institute for Astronomy

“For the first time, we have been able to measure distances, star formation rates, and temperatures for a brand new set of 767 previously unidentified galaxies,” said Dr. Scott Chapman, a co-author on the studies. “The previous similar survey of distant infrared starbursts only covered 73 galaxies. This is a huge improvement.”

The papers detailing the results were published today online in the Astrophysical Journal.

Sources: W.M. Keck Observatory article and ESA’s news release.

Image credits: ESA–C. Carreau/C. Casey (University of Hawai’i); COSMOS field: ESA/Herschel/SPIRE/HerMES Key Programme; Hubble images: NASA, ESA. Inset image courtesy W. M. Keck Observatory.

Posted on by Jason Major

A Virtual Galactic Smash-Up!

An online simulator for galactic collisions (Adrian Price-Whelan/Columbia University)

Have you ever had the desire to build your own galaxies, setting your own physical parameters and including as many stars as you want, and then smash them together like two toy cars on a track? Well, now you can do just that from the comfort of your own web browser (and no waiting billions of years for the results!)

This interactive online app by Adrian Price-Whelan lets you design a galaxy, including such parameters as star count, radius and dispersion rate, and then create a second galaxy to fling at it. Clicking and dragging on the black area will send the invading galaxy on its course, letting you watch the various results over and over again. (If those SMBH’s hit, look out!)

In reality many galaxies have gone through (or are going through, from our perspective) such collision events, our own Milky Way being no exception. In fact, the Milky Way is on course to collide with the Andromeda Galaxy… in about 4 billion years.

So while we wait patiently for that, this is just a bit of addictive fun to distract you from your Cyber Monday shopping spree. Enjoy!

(Source: Columbia University Astronomy & Physics)

Inset image: Hubble interacting galaxies UGC 9618, 450 million light-years away. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Posted on by Ray Sanders

New Online Astronomy Course from CosmoQuest

For those of you who’d like to brush up on your Astronomy knowledge, or never took Astronomy 102, CosmoQuest has a new online course offering for you!

Following the success of the initial 101-level course (CQX 001: Solar System Science), the newest course offering is “CQX 003: Galaxies and Galaxy Clusters”. Just like the previous course offering, CQX003 is an 8-session, 4-week course, which will explore galaxies, galaxy clusters, and brief introduction to cosmology.

“Planets are cool and all, but I’m an extragalactic girl at heart. There is just NO comparison for studying the way that billions of stars interact in some of the largest gravitationally bound structures in the universe.” said Nicole Gugliucci (CosmoQuest) via the CosmoQuest Blog. “This class will cover all of that as well as what active galaxies are all about, another one of my favorite subjects. Then it will round up with a brief introduction to cosmology which is truly the study of EVERYTHING.”

Once again, the course will be a hybrid online course with lectures taking place via Google+ hangouts, with course assignments and homework assigned via Moodle. The instructor will once again be yours truly, Ray Sanders. For those not familiar with me, I’m a research assistant at Arizona State University, and have written for Universe Today in the past. I also blog when I have time over at “Dear Astronomer”.

In addition to my lectures, there may also be “guest” appearances from astronomers Dr. Pamela Gay, and Dr. Nicole Gugliucci.

“I love my solar system and its amazing planets and moons, but this class will give you a chance to expand your understanding beyond the solar system and explore the limits of what we know about the universe.” adds Georgia Bracey (CosmoQuest). “Beginning back when the idea of other galaxies was still a matter of debate, you’ll journey forward to examine our present-day understanding of how galaxies are formed and evolve, including a look at some of the hot topics in astronomy like dark matter, dark energy, active galactic nuclei, and the geometry of the universe.”

CosmoQuest has additional courses in the works for students interested in Cosmology, Data Reduction, Geology/Planetary Science, and more.

The cost for the class is $240, and the class is limited to 8 participants, with the possibility for an additional 5 participants. CQX003: Galaxies and Galaxy Clusters begins on November 26th 2012. More information, and a sign up link is at: http://cosmoquest.org/Classes

Don’t miss this opportunity to combine the convenience of an online class with the lively interaction of a small group of astronomers and astronomy enthusiasts!

Posted on by Nancy Atkinson

Now Even Further: Ancient Galaxy is Latest Candidate for Most Distant

It seems that every few months or so comes a new discovery of a new “most distant galaxy ever found.” It’s not really a surprise that new benchmarks are reached with such an amazing frequency as our telescopes get better and astronomers refine their techniques for observing faraway and ancient objects. This latest “most distant” is pretty interesting in that it was found by combining observations from two space telescopes – Hubble and Spitzer – as well as using massive galaxy clusters as gravitational lenses to magnify the distant galaxy behind them. It’s also extremely small and may not even be a fully developed galaxy at the time we are seeing it.

While this galaxy, named MACS0647-JD, appears as a diminutive blob in the new images, astronomers say it offers a peek back into a time when the universe was just 3 percent of its present age of 13.7 billion years. This newly discovered galaxy was observed 420 million years after the Big Bang, and its light has traveled 13.3 billion years to reach Earth.

“This object may be one of many building blocks of a galaxy,” said Dan Coe of the Space Telescope Science Institute, lead author of a new paper on the observations. “Over the next 13 billion years, it may have dozens, hundreds, or even thousands of merging events with other galaxies and galaxy fragments.”

The discovery comes from the Cluster Lensing And Supernova Survey with Hubble (CLASH), a program that combines the power of space telescopes with the natural zoom of gravitational lensing to reveal distant galaxies in the early Universe. Observations with Spitzer’s infrared eyes allowed for confirmation of this object.

The light from MACS0647-JD was magnified by a massive galaxy cluster named MACS J0647+7015, and without the cluster’s magnification powers, astronomers would not have seen the remote galaxy. Because of gravitational lensing, the CLASH research team was able to observe three magnified images of MACS0647-JD with the Hubble telescope. The cluster’s gravity boosted the light from the faraway galaxy, making the images appear about eight, seven, and two times brighter than they otherwise would that enabled astronomers to detect the galaxy more efficiently and with greater confidence.

“This cluster does what no manmade telescope can do,” said Marc Postman, also from STScI. “Without the magnification, it would require a Herculean effort to observe this galaxy.”

MACS0647-JD is just a fraction of the size of our Milky Way galaxy, and is so small it may not even be a fully formed galaxy. Data show the galaxy is less than 600 light-years wide. Based on observations of somewhat closer galaxies, astronomers estimate that a typical galaxy of a similar age should be about 2,000 light-years wide. For comparison, the Large Magellanic Cloud, a dwarf galaxy companion to the Milky Way, is 14,000 light-years wide. Our Milky Way is 150,000 light-years across.

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The galaxy was observed with 17 filters, spanning near-ultraviolet to near-infrared wavelengths, using Hubble’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). Coe discovered the galaxy in February while poring over a catalogue of thousands of gravitationally lensed objects found in Hubble observations of 17 clusters in the CLASH survey. But the galaxy appeared only in the two reddest filters.

“So either MACS0647-JD is a very red object, only shining at red wavelengths, or it is extremely distant and its light has been ‘redshifted’ to these wavelengths, or some combination of the two,” Coe said. “We considered this full range of possibilities.”

The CLASH team identified multiple images of eight galaxies lensed by the galaxy cluster. Their positions allowed the team to produce a map of the cluster’s mass, which is primarily composed of dark matter. Dark matter is an invisible form of matter that makes up the bulk of the universe’s mass. “It’s like a big puzzle,” said Coe. “We have to arrange the mass in the cluster so that it deflects the light of each galaxy to the positions observed.” The team’s analysis revealed that the cluster’s mass distribution produced three lensed images of MACS0647-JD at the positions and relative brightness observed in the Hubble image.

Coe and his collaborators spent months systematically ruling out these other alternative explanations for the object’s identity, including red stars, brown dwarfs, and red (old or dusty) galaxies at intermediate distances from Earth. They concluded that a very distant galaxy was the correct explanation.

Redshift is a consequence of the expansion of space over cosmic time. Astronomers study the distant universe in near-infrared light because the expansion of space stretches ultraviolet and visible light from galaxies into infrared wavelengths. Coe estimates MACS0647-JD has a redshift of 11, the highest yet observed.

Images of the galaxy at longer wavelengths obtained with the Spitzer Space Telescope played a key role in the analysis. If the object were intrinsically red, it would appear bright in the Spitzer images. Instead, the galaxy barely was detected, if at all, indicating its great distance. The research team plans to use Spitzer to obtain deeper observations of the galaxy, which should yield confident detections as well as estimates of the object’s age and dust content.

MACS0647-JD galaxy, however, may be too far away for any current telescope to confirm the distance based on spectroscopy, which spreads out an object’s light into thousands of colors. Nevertheless, Coe is confident the fledgling galaxy is the new distance champion based on its unique colors and the research team’s extensive analysis. “All three of the lensed galaxy images match fairly well and are in positions you would expect for a galaxy at that remote distance when you look at the predictions from our best lens models for this cluster,” Coe said.

The new distance champion is the second remote galaxy uncovered in the CLASH survey, a multi-wavelength census of 25 hefty galaxy clusters with Hubble’s ACS and WFC3. Earlier this year, the CLASH team announced the discovery of a galaxy that existed when the universe was 490 million years old, 70 million years later than the new record-breaking galaxy. So far, the survey has completed observations for 20 of the 25 clusters.

The team hopes to use Hubble to search for more dwarf galaxies at these early epochs. If these infant galaxies are numerous, then they could have provided the energy to burn off the fog of hydrogen that blanketed the universe, a process called re-ionization. Re-ionization ultimately made the universe transparent to light.

Read the team’s paper (pdf).

Sources: HubbleSite, ESA Hubble

Posted on by Nancy Atkinson

Astronomers Uncover a Crime of Galactic Proportions

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. Credit: ESO/Y. Beletsky

A previously undetected heist of stars was uncovered by astronomers who were actually looking for why an unexpected amount of microlensing events were being seen around the outskirts of the Milky Way. Instead, they found the Large Magellanic Cloud (LMC) had been stealing stars from its neighbor, the Small Magellanic Cloud (SMC), leaving behind a trail of stars. Although the crime was likely committed hundreds of milllions of years ago during a collision between the two galaxies, the new information is helping astronomers to understand the history of these two galaxies that are in our neighborhood.

“You could say we discovered a crime of galactic proportions,” said Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.

The Large Magellanic Cloud almost got away with it, if it wasn’t for those meddling astronomers….

Astronomers were originally monitoring the LMC to hunt for the reason for the unexpected microlensing events. Their initial hypothesis was that massive compact halo objects, or MACHOs were causing the effect, where a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. The MACHOs were thought to be faint objects, roughly the mass of a star, but not much is known about them. Several surveys looked for MACHOs in order to find out if they could be a major component of dark matter – the unseen stuff that holds galaxies together.

In order for MACHOs to make up dark matter, they must be so faint that they can’t be directly detected. So, the team of astronomers hoped to see MACHOs within the Milky Way by lensing distant LMC stars.

“We originally set out to understand the evolution of the interacting LMC and SMC galaxies,” said lead author of a new paper on the results, Gurtina Besla of Columbia University. “We were surprised that, in addition, we could rule out the idea that dark matter is contained in MACHOs.”

“Instead of MACHOs, a trail of stars removed from the SMC is responsible for the microlensing events,” said Loeb.

Only a fast-moving population of stars could yield the observed rate and durations of the microlensing events. The best way to get such a stellar population is a galactic collision, which appears to have occurred in the LMC-SMC system.

“By reconstructing the scene, we found that the LMC and SMC collided violently hundreds of millions of years ago. That’s when the LMC stripped out the lensed stars,” said Loeb.

Their research also supports recent findings suggesting that both Magellanic Clouds are on their first pass by the Milky Way.

However, this isn’t a closed case. The evidence for the trail of lensed stars is persuasive, but they haven’t been directly observed yet. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds.

The simulation results will be published in the Monthly Notices of the Royal Astronomical Society.

Read the team’s paper: The Origin of the Microlensing Events Observed Towards the LMC and the Stellar Counterpart of the Magellanic Stream

Source: CfA

Posted on by John Williams

Stirred, Not Shaken. Black Hole Antics Puff Up Whopper of a Galaxy

Its massive gravitational field warping space, the huge elliptical galaxy A2261-BCG, seems to have a diffuse halo of stars instead of a bright central galactic core. Image credit: NASA/ESA Hubble

Bloated far beyond the size of normal galaxies, one or more black holes may have puffed up an elliptical galaxy to a whopping size, according to astronomers. To their surprise, however, the black holes are missing.

Normally, scientists measure a concentrated peak of light surrounding the central black hole surrounded by a fuzzy halo of stars. Instead, astronomers, using NASA’s Hubble Space Telescope, find that the galaxy, known as A2261-BCG, is just a diffuse, bloated foggy patch of light. The intensity of starlight remains even across the entire galaxy. Past Hubble observations show supermassive black holes, each weighing billions of times more than our Sun, reside at the cores of nearly all galaxies.

“Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach,” explained astronomer and co-author Tod Lauer in a press release. Lauer is with the National Optical Astronomy Observatory in Tucson, Ariz. “With this Hubble observation, we cut into the biggest peach and we can’t find the pit. We don’t know for sure that the black hole is not there, but Hubble shows that there’s no concentration of stars in the core.”

So where are the black holes?

Astronomers, in a paper that appeared in the September 10 issue of The Astrophysical Journal, have two ideas, both involving galactic billiards, for the galaxy’s puffy appearance. In one scenario, a pair of merging black holes gravitationally stir up then scatter the galaxy’s stars. In another, the merging black holes are ejected leaving the swarm of stars with no gravitational anchor allowing them to wander outward.

Galaxy cores tend to be sized proportionally to the wheeling expanse of the host galaxy. In the case of A2261-BCG, which spans about a million light-years (10 times that of our Milky Way Galaxy), the central region is three times larger than other very luminous galaxies, according to the paper. The monster galaxy is the most massive and brightest galaxy in the Abell 2261 galaxy cluster.

Team leader Marc Postman of the Space Telescope Science Institute in Baltimore, Md., said in the press release that the galaxy stood out in the Hubble image. “When I first saw the image of this galaxy, I knew right away it was unusual,” Postman explained. “The core was very diffuse and very large. The challenge was then to make sense of all the data, given what we knew from previous Hubble observations, and come up with a plausible explanation for the intriguing nature of this particular galaxy.”

The team admits the ejected black-hole ideas sound far-fetched, “but that’s what makes observing the universe so intriguing — sometimes you find the unexpected,” said Postman.

As a follow-up, the team is searching for the sound of material falling into the black hole using the Very Large Array (VLA) radio telescope in New Mexico. Comparing the VLA data with Hubble images will allow the researchers to confirm the existence of a black hole and map its location.

Source: Hubblesite

Posted on by Nancy Atkinson

Astronomers Find Ultimate Oxymoron: A Small Supermassive Black Hole

There’s jumbo shrimp and accurate rumors; now there’s even a mini supermassive black hole. Astronomers have identified the smallest supermassive black hole ever observed, and while it’s considered a shrimp as far as supermassive black holes go, this guy is still pretty big: the mass of the black hole in galaxy NGC 4178 is estimated to be about 200,000 times the mass of our Sun. But it was a surprise that this galaxy had a black hole at all.

Astronomers using the Chandra X-Ray Observatory in conjunction with other observatories took a look at NGC 4178, a late-type spiral galaxy located about 55 million light years from Earth. It does not contain a bright central concentration, or bulge, of stars in its center, and so it was thought that perhaps this galaxy was one of the few that didn’t harbor a black hole.

With using Chandra’s X-Ray vision, as well as infrared data the NASA’s Spitzer Space Telescope and radio data from the Very Large Array, Nathan Secrest, from George Mason University and his team identified a weak X-ray source at the center of the galaxy, and also saw varying brightness at infrared wavelengths, suggesting that a black hole was actually in the center of NGC 4178 and was pulling in material from its surroundings. The same data also suggested that light generated by this infalling material is heavily absorbed by gas and dust and was therefore surrounding a black hole.

They were able to estimate the size of the black hole by using the known relationship between the mass of a black hole and the amount of X-rays and radio waves it generates.

While this is the lowest mass supermassive black holes ever observed, astronomers admit this is probably near the extreme low-mass end of being in the “supermassive” range. And as the team pointed out in their paper, there is increasing evidence that several late-type galaxies do host supermassive black holes, and that a classical bulge is not a requirement for a supermassive black hole to form and grow.

Read the team’s paper.

Source: NASA