GOODS field containing distant dwarf galaxies forming stars at an incredible rate. Image Credit: ESO
Massive galaxies in the early Universe formed stars at a much faster clip than they do today — creating the equivalent of a thousand new suns per year. This rate reached its peak 3 billion years after the Big Bang, and by 6 billion years, galaxies had created most of their stars.
New observations from the Hubble Space Telescope show that even dwarf galaxies — the small, low mass clusters of several billion stars — produced stars at a rapid rate, playing a bigger role than expected in the early history of the Universe.
Today, we tend to see dwarf galaxies clinging to larger galaxies, or sometimes engulfed within, rather than existing as blazing collections of stars alone. But astronomers have suspected that dwarfs in the early Universe could turn over stars quickly. The trouble is, most images aren’t sharp enough to reveal the faint, faraway galaxies we need to observe.
“We already suspected that dwarf starbursting galaxies would contribute to the early wave of star formation, but this is the first time we’ve been able to measure the effect they actually had,” said lead author Hakim Atek of the École Polytechnique Fédérale de Lausanne (EPFL) in a press release. “They appear to have had a surprisingly significant role to play during the epoch where the Universe formed most of its stars.”
Previous studies of starburst galaxies in the early Universe were biased toward massive galaxies, leaving out the huge number of dwarf galaxies that existed in this era. But the highly sensitive capabilities of Hubble’s Wide Field Camera 3 have now allowed astronomers to peer at low-mass dwarf galaxies in the distant Universe.
This image represents the data that comes from using the NASA/ESA Hubble Space Telescope’s highly-sensitive Wide Field Camera 3 in its grism spectroscopy mode. Image Credit: NASA / ESA
Atek and colleagues looked at 1000 galaxies from roughly three billion years to 10 billion years after the Big Bang. They dug through their data, in search of the H-alpha line: a deep-red visible spectral line, which occurs when a hydrogen electron falls from its third to second lowest energy level.
In star forming regions, the surrounding gas is continually ionized by radiation from the newly formed stars. Once the gas is ionized, the nucleus and removed electron can recombine to form a new hydrogen atom with the electron typically in a higher energy state. This electron will then cascade back to the ground state, a process that produces H-alpha emission about half the time.
So the H-alpha line is an effective probe of star formation and the brightness of the H-alpha line (which is much easier to detect than the faint, almost invisible, continuum) is an effective probe of the star formation rate. From this single line, Attek and colleagues found that the rate at which stars are turning on in early dwarfs is surprisingly high.
“These galaxies are forming stars so quickly that they could actually double their entire mass of stars in only 150 million years — this sort of gain in stellar mass would take most normal galaxies 1-3 billion years,” said co-author Jean-Paul Kneib, also of EPFL.
The team doesn’t yet know why these small galaxies are producing such a vast number of stars. In general, bursts of star formation are thought to follow somewhat chaotic events like galactic mergers or the shock of a supernova. But by continuing to study these dwarf galaxies, astronomers hope to shed light on galactic evolution and help paint a consistent picture of events in the early Universe.
The paper has been published today in the Astrophysical Journal and may be viewed here. The latest Hubblecast (below) also covers this exciting result.
This new image shows the Large Magellanic Cloud galaxy in infrared light as seen by the Herschel Space Observatory, a European Space Agency-led mission with important NASA contributions, and NASA's Spitzer Space Telescope. Image credit: ESA/NASA/JPL-Caltech/STScI
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When we think of stars, we might think of their building blocks as white hot… But that’s not particularly the case.The very “stuff” that creates a sun is cold dust and in this combined image produced by the Herschel Space Observatory, a European Space Agency-led mission with important NASA contributions; and NASA’s Spitzer Space Telescope, we’re taking an even more incredible look into the environment which forms stars. This new image peers into the dusty arena of both the Large and Small Magellanic Clouds – just two of our galactic neighbors.
Through the infra-red eyes of the Herschel-Spitzer observation, the Large Magellanic Cloud would almost appear to look like a gigantic fireball. Here light-years long bands of dust permeate the galaxy with blazing fields of star formation seen in the center, center-left and top right (the brightest center-left region is called 30 Doradus, or the Tarantula Nebula. The Small Magellanic Cloud is much more disturbed looking. Here we see a huge filament of dust to the left – known as the galaxy’s “wing” – and, to the right, a deep bar of star formation.
This new image shows the Small Magellanic Cloud galaxy in infrared light from the Herschel Space Observatory a European Space Agency-led mission with important NASA contributions, and NASA's Spitzer Space Telescope. Image credit: ESA/NASA/JPL-Caltech/STScI
What makes these images very unique is that they are indicators of temperature within the Magellanic Clouds. The cool, red areas are where star formation has ceased or is at its earliest stages. Warm areas are indicative of new stars blooming to life and heating the dust around them. “Coolest areas and objects appear in red, corresponding to infrared light taken up by Herschel’s Spectral and Photometric Imaging Receiver at 250 microns, or millionths of a meter. Herschel’s Photodetector Array Camera and Spectrometer fills out the mid-temperature bands, shown in green, at 100 and 160 microns.” says the research team. “The warmest spots appear in blue, courtesy of 24- and 70-micron data from Spitzer.”
Both the LMC and SMC are the two largest satellite galaxies of the Milky Way and are cataloged as dwarf galaxies. While they are large in their own right, this pair contains fewer essential star-forming elements such as hydrogen and helium – slowing the rate of star growth. Although star formation is generally considered to have reached its apex some 10 billion years ago, some galaxies were left with less basic materials than others.
“Studying these galaxies offers us the best opportunity to study star formation outside of the Milky Way,” said Margaret Meixner, an astronomer at the Space Telescope Science Institute, Baltimore, Md., and principal investigator for the mapping project. “Star formation affects the evolution of galaxies, so we hope understanding the story of these stars will answer questions about galactic life cycles.”
A map of the sky showing the numbers of stars counted in the Sagittarius streams. The dotted red lines trace out the Sagittarius streams, and the blue ellipses in the center show the current location of the Sagittarius Dwarf Galaxy. Image credit: S. Koposov and the SDSS-III collaboration
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Galactic interactions can have big effects on the shapes of the disks of galaxies. So what happens when a small galaxy intermingles with the outer part of our own larger Milky Way Galaxy? It’s not pretty, as rivers of stars are being sheared off from a neighboring dwarf galaxy, Sagittarius, according to research by a team of astronomers led by Sergey Koposov and Vasily Belokurov (University of Cambridge).
Analyzing data from the latest Sloan Digital Sky Survey (SDSS-III), the team found two streams of stars in the Southern Galactic hemisphere that were torn off Sagittarius dwarf galaxy. This new discovery also connects newly found streams with two previously discovered streams in the Northern Galactic hemisphere.
Describing the phenomenon, Koposov said, “We have long known that when small dwarf galaxies fall into bigger galaxies, elongated streams, or tails, of stars are pulled out of the dwarf by the enormous tidal field.”
Wyn Evans, one of the other team members commented, “Sagittarius is like a beast with four tails.”
At one time, the Sagittarius dwarf galaxy was one of the brightest of our Galaxy’s satellites. Now its remains are on the other side of our Galaxy, and in the process of being broken apart by immense tidal forces. Estimates show that the Sagittarius dwarf galaxy lost half its stars and gas over the past billion years.
Before the SDSS-III data analysis, it was known that Sagittarius had two tails – one in front of and one behind the remnant. This discovery was made by using previous SDSS imaging, specifically a 2006 study which found the Sagittarius tidal tail in the Northern Galactic sky appears to be split in two.
Commenting on the previous discovery, Belokurov added, “That was an amazing discovery, but the remaining piece of the puzzle, the structure in the South, was missing until now.”
Analyzing density maps of over 13 million stars in the SDSS-III data, Koposov and his team found that the Sagittarius stream in the South is also split into two. One stream is thicker and brighter, while the other is thinner and fainter. According to the paper, the fainter stream is simpler and more metal-poor, while the brighter stream is more complex and metal-rich.
The deduction makes sense since each successive generation of stars will create and distribute (via supernovae) more metals into the next generation of star formation.
An artist's impression of the four tails of the Sagittarius Dwarf Galaxy (the orange clump on the left of the image) orbiting the Milky Way. The bright yellow circle to the right of the galaxy's center is our Sun (not to scale). Image credit: Amanda Smith (University of Cambridge)
While the exact cause of the tidal tail split is unknown, astronomers believe that the Sagittarius dwarf may have been part of a binary galactic system, much like the Large and Small Magellanic Clouds, visible in our Southern hemisphere. Despite the nature of the tidal tail split being presently unknown, astronomers have known that over time, many smaller galaxies have been torn apart or absorbed by our Milky Way Galaxy, as well as other galaxies in the Universe.
The movie (below) shows multiple streams produced by the disruption of the Sagittarius dwarf galaxy in the Milky Way halo. Our Sun is depicted by the orange sphere. The Sagittarius dwarf galaxy is in the middle of the stream. The area shown in the movie is roughly 200,000 parsecs (about 600,000 light-years.) Movie credit: S. Koposov and the SDSS-III collaboration.
If you’d like to learn more, you can read the full scientific paper at: arxiv.org
Galaxy Cluster MACS J0329.6-0211 lenses several background galaxies including a distant dwarf galaxy. CREDIT: A. Zitrin, et al.
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Gravitational lensing is a powerful tool for astronomers that allows them to explore distant galaxies in far more detail than would otherwise be allowed. Without this technique, galaxies at the edge of the visible universe are little more than tiny blobs of light, but when magnified dozens of times by foreground clusters, astronomers are able to explore the internal structural properties more directly.
Recently, astronomers at the University of Heidelberg discovered a gravitational lensed galaxy that ranked among the most distant ever seen. Although there’s a few that beat this one out in distance, this one is remarkable for being a rare quadruple lens.
The images for this remarkable discovery were taken using the Hubble Space Telescope in August and October of this year, using a total of 16 different colored filters as well as additional data from the Spitzer infrared telescope. The foreground cluster, MACS J0329.6-0211, is some 4.6 billion light years distant. In the above image, the background galaxy has been split into four images, labelled by the red ovals and marked as 1.1 – 1.4. They are enlarged in the upper right.
Assuming that the mass of the foreground cluster is concentrated around the galaxies that were visible, the team attempted to reverse the effects the cluster would have on the distant galaxy, which would reverse the distortions. The restored image, also corrected for redshift, is shown in the lower box in the upper right corner.
After correcting for these distortions, the team estimated that the total mass of the distant galaxy is only a few billion times the mass of the Sun. In comparison, the Large Magellanic Cloud, a dwarf satellite to our own galaxy, is roughly ten billion solar masses. The overall size of the galaxy was determined to be small as well. These conclusions fit well with expectations of galaxies in the early universe which predict that the large galaxies in today’s universe were built from the combination of many smaller galaxies like this one in the distant past.
The galaxy also conforms to expectations regarding the amount of heavy elements which is significantly lower than stars like the Sun. This lack of heavy elements means that there should be little in the way of dust grains. Such dust tends to be a strong block of shorter wavelengths of light such as ultraviolet and blue. Its absence helps give the galaxy its blue tint.
Star formation is also high in the galaxy. The rate at which they predict new stars are being born is somewhat higher than in other galaxies discovered around the same distance, but the presence of brighter clumps in the restored image suggest the galaxy may be undergoing some interactions, driving the formation of new stars.
NGC7600 is an elliptical galaxy and is around 50 Mpc in distance. This image shows an interleaved system of shells that are described in this Astronomical Journal Letters here. These types of structures around elliptical galaxies were first revealed by Malin & Carter in 1980. This deep image of NGC7600 shows faint features not previously seen. Credit: Ken Crawford
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As a professional astronomy journalist, I read a lot of science papers. It hasn’t been all that long ago that I remember studying about galaxy groups – with the topic of dark matter and dwarf galaxies in particular. Imagine my surprise when I learn that two of my friends, who are highly noted astrophotographers, have been hard at work doing some deep blue science. If you aren’t familiar with the achievements of Ken Crawford and R. Jay Gabany, you soon will be. Step inside here and let us tell you why “it matters”…
According to Ken’s reports, Cold Dark Matter (or CDM) is a theory that most of the material in the Universe cannot be seen (dark) and that it moves very slowly (cold). It is the leading theory that helps explain the formation of galaxies, galaxy groups and even the current known structure of the universe. One of the problems with the theory is that it predicts large amounts of small satellite galaxies called dwarf galaxies. These small galaxies are about 1000th the mass of our Milky Way but the problem is, these are not observed. If this theory is correct, then where are all of the huge amounts of dwarf galaxies that should be there?
Enter professional star stream hunter, Dr. David Martinez-Delgado. David is the principal investigator of the Stellar Tidal Stream Survey at the Max-Planck Institute in Heidelberg, Germany. He believes the reason we do not see large amounts of dwarf galaxies is because they are absorbed (eaten) by larger galaxies as part of the galaxy formation. If this is correct, then we should find remnants of these mergers in observations. These remnants would show up as trails of dwarf galaxy debris made up mostly of stars. These debris trails are called star streams.
“The main aim of our project is to check if the frequency of streams around Milky Way-like galaxies in the local universe is consistent with CDM models similar to that of the movie.” clarifies Dr. Martinez-Delgado. “However, the tidal destruction of galaxies is not enough to solve the missing satellite problem of the CDM cosmology. So far, the best given explanation is that some dark matter halos are not able to form stars inside, that is, our Galaxy would surround by a few hundreds of pure dark matter satellites.”
Enter the star stream hunters professional team. The international team of professional astronomers led by Dr. David Martinez-Delgado has identified enormous star streams on the periphery of nearby spiral galaxies. With deep images he showed the process of galactic cannibalism believed to be occurring between the Milky Way and the Sagittarius dwarf galaxy. This is in our own back yard! Part of the work is using computer modeling to show how larger galaxies merge and absorb the smaller ones.
This image has been inverted and contrast enhanced to help display the faint shell features and debris fragments. The farthest fragment is 140 kpc in projection from the center of the galaxy. Credit: Ken Crawford“Our observational approach is based on deep color-magnitude diagrams that provide accurate distances, surface brightness, and the properties of stellar population of the studied region of this tidal stream.” says Dr. Martinez-Delgado (et al). “These detections are also strong observational evidence that the tidal stream discovered by the Sloan Digitized Sky Survey is tidally stripped material from the Sagittarius dwarf and support the idea that the tidal stream completely enwraps the Milky Way in an almost polar orbit. We also confirm these detections by running numerical simulations of the Sagittarius dwarf plus the Milky Way. This model reproduces the present position and velocity of the Sagittarius main body and presents a long tidal stream formed by tidal interaction with the Milky Way potential.”
Enter the team of amateurs led by R. Jay Gabany. David recruited a small group of amateur astrophotographers to help search for and detect these stellar fossils and their cosmic dance around nearby galaxies, thus showing why there are so few dwarf galaxies to be found.
“Our observations have led to the discovery of six previously undetected, gigantic, stellar structures in the halos of several galaxies that are likely associated with debris from satellites that were tidally disrupted far in the distant past. In addition, we also confirmed several enormous stellar structures previously reported in the literature, but never before interpreted as being tidal streams.” says the team. “Our collection of galaxies presents an assortment of tidal phenomena exhibiting strikingly diverse morphological characteristics. In addition to identifying great circular features that resemble the Sagittarius stream surrounding the Milky Way, our observations have uncovered enormous structures that extend tens of kiloparsecs into the halos of their host’s central spiral. We have also found remote shells, giant clouds of debris within galactic halos, jet-like features emerging from galactic disks and large-scale, diffuse structures that are almost certainly related to the remnants of ancient, already thoroughly disrupted satellites. Together with these remains of possibly long defunct companions, our survey also captured surviving satellites caught in the act of tidal disruption. Some of these display long tails extending away from the progenitor satellite very similar to the predictions forecasted by cosmological simulations.”
The .5 meter Ritchey-Chretien Telescope of the Blackbird Observatory is situated at 7300 ft.(2225 meters) elevation under spectacularly clear and dark skies in the south central Sacramento Mountains of New Mexico, near Mayhill. Photo credit: R. Wodaski
Can you imagine how exciting it is to be part of deep blue science? It is one thing to be a good astrophotographer – even to be an exceptional astrophotographer – but to have your images and processing to be of such high quality as to be contributory to true astronomical research would be an incredible honor. Just ask Ken Crawford…
“Several years ago I was asked to become part of this team and have made several contributions to the survey. I am excited to announce that my latest contribution has resulted in a professional letter that has been recently accepted by the Astronomical Journal.” comments Ken. “There are a few things that make this very special. One, is that Carlos Frenk the director of the Institute for Computational Cosmology at Durham University (UK) and his team found that my image of galaxy NGC7600 was similar enough to help validate their computer model (simulation) of how larger galaxies form by absorbing satellite dwarf galaxies and why we do not see large number of dwarf galaxies today.”
Dr. Carlos Frenk has been featured on several television shows on the Science and Discovery channels, to name a few, to explain and show some of these amazing simulations. He is the director of the Institute for Computational Cosmology at Durham University (UK), was one of the winners of the 2011 Cosmology Prize of The Peter and Patricia Gruber Foundation.
“The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability.” says Frenk (et al). “Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations.”
The Rancho Del Sol Observatory is located in the foothills of the northern California's Sierra Mountains approximately one hour north of Sacramento. It houses a .5 meter Ritchey-Chretien Telescope. Credit: Ken CrawfordAnd it requires very accurate depictions of studies. According to the team, this pilot survey was conducted with three privately owned observatories equipped with modest sized telescopes located in the USA and Australia. Each observing site features very dark, clear skies with seeing that is routinely at and often below 1.5 arcseconds. These telescopes are manufactured by RC Optical Systems and follow a classic Ritchey-Chretien design. The observatories are commanded with on-site computers that allow remote operation and control from any global location with highband web accesses. Each observatory uses proven, widely available remote desktop control software. Robotic orchestration of all observatory and instrument functions, including multiple target acquisition and data runs, is performed using available scripting software. Additional use of a wide field instrument was employed for those galaxies with an extended angular size. For this purpose, they selected the Astro Physics Starfire 160EDF6, a short focal length (f/7) 16 cm aperture refractor that provides a FOV of 73.7 × 110.6 arcmin. But, it’s more than just taking a photograph. The astrophotographer needs to completely understand what needs to be drawn out of the exposure. It’s more than just taking a “pretty picture”… it’s what matters.
“The galaxy I want to show you has some special features called ‘shells’. I had to image very deep to detect these structures and carefully process them so you can see the delicate structures within.” explains Crawford. “The galaxy name is NGC7600 and these shell structures have not been captured as well in this galaxy before. The movie above shows my image of NGC7600 blending into the simulation at about the point when the shells start to form. The movie below shows the complete simulation.”
“What is ground breaking is that the simulation uses the cold dark matter theory modeling the dark matter halos of the galaxies and as you can see, it is pretty convincing.” concludes Crawford. “So now you all know why we do not observe lots of dwarf galaxies in the Universe.”
But, we can observe some very incredible science done by some very incredible friends. It’s what matters…
The circled cluster of stars is the dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered. The bright star within the circle is a foreground star within our own Milky Way galaxy. This image was obtained with the Gemini Multi-Object Spectrograph at the Gemini North telescope in Hawaii. Credit: Gemini Observstory/AURA/Eric Bell
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Yep. It’s that time of year again. Time to enjoy the Andromeda Galaxy at almost every observing opportunity. But now, rather than just look at the nearest spiral to the Milky Way and sneaking a peak at satellites M32 and M110, we can think about something more when we peer M31’s way. There are two newly discovered dwarf galaxies that appear to be companions of Andromeda!
Eric Bell, an associate professor in astronomy, and Colin Slater, an astronomy Ph.D. student, found Andromeda 28 and Andromeda 29 by utilizing the Sloan Digital Sky Survey and a recently developed star counting technique. To back up their observations, the team employed data from the Gemini North Telescope in Hawaii. Located at 1.1 million and 600,000 light-years respectively, Andromeda XXVIII and Andromeda XXIX have the distinction of being the two furthest satellite galaxies ever detected away from the host – M31. Can they be spotted with amateur equipment? Not hardly. This pair comes in about 100,000 fainter than Andromeda itself and can barely be discerned with some of the world’s largest telescopes. They’re so faint, they haven’t even been classified yet.
“With presently available imaging we are unable to determine whether there is ongoing or recent star formation, which prevents us from classifying it as a dwarf spheroidal or a dwarf irregular.” explains Bell.
The dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered, is clustered toward the middle of this image, obtained with the Gemini North telescope in Hawaii. Credit: Gemini Observstory/AURA/Eric Bell
In their work – published in a recent edition of the edition of the Astrophysical Journal Letters – the team of Bell and Slater explains how they were searching for dwarf galaxies around Andromeda to help them understand how physical matter relates to theoretical dark matter. While we can’t see it, hear it, touch it or smell it, we know it’s there because of its gravitational influence. And when it comes to gravity, many astronomers are convinced that dark matter plays a role in organizing galaxy structure.
“These faint, dwarf, relatively nearby galaxies are a real battleground in trying to understand how dark matter acts at small scales,” Bell said. “The stakes are high.”
Right now, current consensus has all galaxies embedded in surrounding dark matter… and each “bed” of dark matter should have a galaxy. Considering the volume of the Universe, these predictions are pretty much spot on – if we take only large galaxies into account.
“But it seems to break down when we get to smaller galaxies,” Slater said. “The models predict far more dark matter halos than we observe galaxies. We don’t know if it’s because we’re not seeing all of the galaxies or because our predictions are wrong.”
“The exciting answer,” Bell said, “would be that there just aren’t that many dark matter halos.” Bell said. “This is part of the grand effort to test that paradigm.”
Right or wrong… pondering dark matter and dwarf galaxies while observing Andromeda will add a whole new dimension to your observations!
For Further Reading: Andromeda XXVIII: A Dwarf Galaxy more than 350 kpc from Andromeda and Andromeda XXIX: A New Dwarf Spheroidal Galaxy 200 kpc from Andromeda.
This artist's conception shows a dwarf galaxy seen from the surface of a hypothetical exoplanet. A new study finds that the dark matter in dwarf galaxies is distributed smoothly rather than being clumped at their centers. This contradicts simulations using the standard cosmological model known as lambda-CDM. Credit: David A. Aguilar (CfA)
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Dark matter… If it can’t be seen, then how do we know it’s there? If it wasn’t for the effects of gravity, we wouldn’t. We’d have a galaxy filled with runaway stars and no galaxy would exist for long. But how it behaves and how it is distributed in one of the biggest cosmic cryptograms of all. Even with new research, there seems to be more questions than answers!
“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.
It is generally accepted that our Universe is predominately composed of dark matter and dark energy. Of the former, it is considered to be “cold”, stately exotic particles which coalesce through gravitation. As they evolve, these dark matter “clumps” then attract “normal” matter which forms present day galaxy structures. Through computer modeling, astronomers have simulated this growth process which concludes that galactic centers should be dense with dark matter. However, these models aren’t consistent with findings. By measuring two dwarf galaxies, scientists have found a even distribution instead.
“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” Walker stated.
Why study a dwarf instead of a spiral? In this case, the dwarf galaxy is a perfect candidate because of its composition – 99% dark matter and 1% stars. Walker and his co-author Jorge Penarrubia (University of Cambridge, UK) chose two nearby representatives – the Fornax and Sculptor dwarfs – for their study. In comparison to the Milky Way’s estimated 400 billion stars, this pair averages around 10 million instead. This allowed the team to take a comprehensive sample of around 1500 to 2500 stars for location, speed and basic chemical composition. But even at a reduced amount, this type of stellar accounting isn’t exactly easy picking.
“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” explained Penarrubia. “That makes it much more challenging to determine the distribution of dark matter.”
What the team found was somewhat surprising. According to the modeling techniques, dark matter should have clumped at the core. Instead they found it evenly distributed over a distance measuring several hundred light years across.
“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.
It is hypothesized that interactions between normal and dark matter might be responsible for the distribution, but the computer simulations say it shouldn’t happen to a dwarf. New queries to new findings? Yes. This revelation may suggest that dark matter isn’t always “cold” and that it could be impacted by normal matter in unexpected ways.
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?
Astronomers detected supermassive black holes in 28 distant, low-mass galaxies, including the four shown in these Hubble Space Telescope images. Image credit: A. Koekemoer, Space Telescope Science Institute.
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The Hubble Space Telescope has done it again. By utilizing a slitless grism, the Wide Field Camera 3 has uncovered evidence that supermassive black holes are right at home in some very small galaxies. Apparently these central black holes began their life when their host galaxies were first forming!
“It’s kind of a chicken or egg problem: Which came first, the supermassive black hole or the massive galaxy? This study shows that even low-mass galaxies have supermassive black holes,” said Jonathan Trump, a postdoctoral researcher at the University of California, Santa Cruz. Trump is first author of the study, which has been accepted for publication in the Astrophysical Journal.
It’s another cosmic conundrum. As we’ve learned, large galaxies are host to central supermassive black holes and many of them are the AGN variety. But the real puzzle is why do some smaller galaxies contain them when most do not? By taking a closer look at dwarf galaxies some 10 billion light-years away, astronomers are reaching back in time to when the Universe was about an estimated quarter of its current age.
“When we look 10 billion years ago, we’re looking at the teenage years of the universe. So these are very small, young galaxies,” Trump said.
If your mind is still wondering what a “slitless grism” is, then wonder no more. It’s part of Hubble’s WFC3 infrared camera that provides spectroscopic information. Thanks to highly detailed information on the different wavelengths of light, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) team could achieve separate spectra from each sector of the candidate galaxies and identify emissions from black hole sources.
“This is the first study that is capable of probing for the existence of small, low-luminosity black holes back in time,” said coauthor Sandra Faber, University Professor of astronomy and astrophysics at UC Santa Cruz and CANDELS principal investigator. “Up to now, observations of distant galaxies have consistently reinforced the local findings–distant black holes actively accreting in big galaxies only. We now have a big puzzle: What happened to these dwarf galaxies?”
It’s possible they are forerunners of the massive galaxies we see today. “Some may remain small, and some may grow into something like the Milky Way,” Trump said. But this theory is a juxtaposition in itself. According to Faber, “To become big galaxies today, the dwarf galaxies would have to grow at a rate much faster than standard models predict. If they remain small, then nearby dwarf galaxies should also have central black holes. There might be a large population of small black holes in dwarf galaxies that no one has noticed before.”
But these distant little dwarfs aren’t quiet – they are actively forming new stars. According to Trump, “Their star formation rate is about ten times that of the Milky Way. There may be a connection between that and the active galactic nuclei. When gas is available to form new stars, it’s also available to feed the black hole.”
But the Hubble wasn’t the only instrument interested in the 28 small galaxy studies. The team also employed x-ray data acquired by NASA’s Chandra X-ray Observatory. To help refine their information on such small, faint objects, the data was combined to improve the signal-to-noise ratio.
“This is a powerful technique that we can use for similar studies in the future on larger samples of objects,” Trump said. “Together the compactness of the stacked OIII spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei.”
Hubble's view of NGC 4214, Canes Venatici (The Hunting Dogs). Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
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Lots of activity taking place inside NGC 4214, and Hubble has peered inside this dwarf galaxy to see stars in all stages of their evolution, as well as gas clouds with huge cavities blown out by stellar winds. Wow! Also visible are bright stellar clusters and complex patterns of glowing hydrogen, some forming a candy-cane-like structure in the upper right of this optical and near-infrared image. NGC 4214 is located in the constellation of Canes Venatici (The Hunting Dogs), about 10 million light-years away. Hubble scientists say this galaxy is an ideal laboratory to research the triggers of star formation and evolution.
Observations of this dwarf galaxy have also revealed clusters of much older red supergiant stars. Additional older stars can be seen dotted all across the galaxy. The variety of stars at different stages in their evolution indicates that the recent and ongoing starburst periods are not the first, and the galaxy’s abundant supply of hydrogen means that star formation will continue into the future.
