Astronomers Detect Water in Atmosphere of Distant Planet

Artist’s rendering of the planetary system HR 8799 at an early stage in its evolution, showing the planet HR 8799c, a disk of gas and dust, and interior planets.  Credit: Image courtesty of Dunlap Institute for Astronomy & Astrophysics; Mediafarm.

One of the discovery images of the system obtained at the Keck II telescope using the adaptive optics system and NIRC2 Near-Infrared Imager. The rectangle indicates the field-of-view of the OSIRIS instrument for planet C.  Credit: Image courtesy of NRC-HIA, C. Marois and Keck Observatory.

KAMUELA, HI – A team of international scientists using the W. M. Keck Observatory has made the most detailed examination yet of the atmosphere of a Jupiter-size planet beyond our Solar System. 

According to lead author Quinn Konopacky, an astronomer with the Dunlap Institute for Astronomy & Astrophysics, University of Toronto and a former Lawrence Livermore National Laboratory (LLNL) postdoc, “We have been able to observe this planet in unprecedented detail because of Keck Observatory’s advanced instrumentation, our ground-breaking observing and data processing techniques, and because of the nature of the planetary system.” The paper appears online March 14th in Science Express, and March 22nd in the journal Science.

“This is the sharpest spectrum ever obtained of an extrasolar planet,” said co-author Bruce Macintosh, an astronomer at LLNL. “This shows the power of directly imaging a planetary system—the exquisite resolution afforded by these new observations has allowed us to really begin to probe planet formation.” 

The team, using the OSIRIS instrument fitted on the mighty Keck II telescope on the summit of Mauna Kea, Hawaii, has uncovered the chemical fingerprints of specific molecules, revealing a cloudy atmosphere containing water vapor and carbon monoxide. “With this level of detail,” says coauthor Travis Barman, an astronomer at the Lowell Observatory, “we can compare the amount of carbon to the amount of oxygen present in the atmosphere, and this chemical mix provides clues as to how the planetary system formed.”

There has been uncertainty about how planets in other solar systems formed, with two leading models, called core accretion and gravitational instability. When stars form, they are surrounded by a planet-forming disk. In the first scenario, planets form gradually as solid cores slowly grow big enough to start absorbing gas from the disk. In the latter, planets form almost instantly as parts of the disk collapse on themselves. Planetary properties, like the composition of a planet’s atmosphere, are clues as to whether a system formed according to one model or the other.  

Although the planet’s atmosphere shows clear evidence of water vapor, that signature is weaker than would be expected if the planet shared the composition of its parent star. Instead, the planet has a high ratio of carbon to oxygen—a fingerprint of its formation in the gaseous disk tens of millions of years ago.  As the gas cooled with time, grains of water ice form, depleting the remaining gas of oxygen. Planetary formation began when ice and solids collected into planetary cores—very similar to how our solar system formed. 

“Once the solid cores grew large enough, their gravity quickly attracted surrounding gas to become the massive planets we see today,” said Konopacky. “Since that gas had lost some of its oxygen, the planet ends up with less oxygen and less water than if it had formed through a gravitational instability.”

The planet is one of four gas giants known to orbit a star called HR 8799, 130 light-years from Earth. The authors and their collaborators previously discovered this planet, designated HR 8799c, and its three companions back in 2008 and 2010. Unlike most other planetary systems, whose presence is inferred by their effects on their parent star, the HR8799 planets can be individually seen. 

“We can directly image the planets around HR 8799 because they are all large, young, and very far from their parent star. This makes the system an excellent laboratory for studying exoplanet atmospheres,” said coauthor Christian Marois, an astronomer at the National Research Council of Canada and another former LLNL postdoc. “Since its discovery, this system just keeps on surprising us.”

Although the planet does have water vapor, it’s incredibly hostile to life—like Jupiter, it has no solid surface, and it has a temperature of more than a thousand degrees Fahrenheit as it glows with the energy of its original formation. Still, this discovery provides clues as to the possibility of other Earth-like planets in other solar systems. “The fact that the HR8799 giant planets may have formed the same way our own giant planets did is a good sign—that same process also made the rocky planets close to the Sun,” said Dr. Macintosh. 

The HR 8799 four planet system:
All four planets are more massive than any in our Solar System, with masses three to seven times that of Jupiter. Their orbits are similarly large when compared to our system. The system is believed to be young, of the order of 30 million years. HR 8799c orbits 40 times farther from its parent star than the Earth orbits from the Sun; in our Solar System that would put it beyond the realm of Neptune.

The OSIRIS instrument:
The team analyzed the distant giant’s atmosphere using a high-resolution imaging spectrograph called OSIRIS. Just as Keck’s adaptive optics technology gives astronomers a sharp image of HR 8799c, OSIRIS enables an extremely detailed analysis of the spectrum of the light from the planet—much more detailed than ever before—and allows astronomers to separate the star’s light from the planet’s. This in turn provides a more detailed understanding of the composition of the gas giant’s atmosphere. 

The telescope’s adaptive optics system corrects for distortion caused by the Earth’s atmosphere, making the infrared view through Keck II sharper than through the Hubble Space Telescope.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

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E-mail: stark8@llnl.gov

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Source: W. M. Keck Observatory

Astronomers Open Window Into Europa’s Ocean

Based on new data from the W. M. Keck Observatory about Jupiter's moon Europa, astronomers hypothesize that chloride salts bubble up from the icy moon's global liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiter's largest moon, Io. This illustration of Europa (foreground), Jupiter (right) and Io (middle) is an artist's concept. Credit: NASA/JPL-Caltech. Hi-Res Image

KAMUELA, Hawaii—With data collected from the mighty W. M. Keck Observatory, California Institute of Technology (Caltech) astronomer Mike Brown — known as the Pluto killer for discovering a Kuiper-belt object that led to the demotion of Pluto from planetary status — and Kevin Hand from the Jet Propulsion Laboratory (JPL) have found the strongest evidence yet that salty water from the vast liquid ocean beneath Europa’s frozen exterior actually makes its way to the surface.

The data suggests there is a chemical exchange between the ocean and surface, making the ocean a richer chemical environment, and implies that learning more about the ocean could be as simple as analyzing the moon’s surface. The work is described in a paper that has been accepted for publication in the Astronomical Journal.

The findings were derived from spectroscopy delivered from the Keck Observatory, which operates the largest and most scientifically productive telescopes on Earth.

“We now have the best spectrum of this thing in the world,” Brown says. “Nobody knew there was this little dip in the spectrum because no one had the resolution to zoom in on it before.”

Ten-meter Keck II, fitted with Adaptive Optics (AO) to adjust for the blurring effect of Earth’s atmosphere, and its OH-Suppressing Infrared Integral Field Spectrograph (OSIRIS) produced details not capable of collection when NASA’s Galileo mission (1989–2003) was sent to study Jupiter and its moons. 

“We now have evidence that Europa’s ocean is not isolated—that the ocean and the surface talk to each other and exchange chemicals,” says Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy at Caltech. “That means that energy might be going into the ocean, which is important in terms of the possibilities for life there. It also means that if you’d like to know what’s in the ocean, you can just go to the surface and scrape some off.”

“The surface ice is providing us a window into that potentially habitable ocean below,” says Hand, deputy chief scientist for solar system exploration at JPL.

Since the days of the Galileo mission, when the spacecraft showed that Europa was covered with an icy shell, scientists have debated the composition of Europa’s surface. The infrared spectrometer aboard Galileo was not capable of providing the detail needed to definitively identify some of the materials present on the surface. Now, using current technology on ground-based telescopes, Brown and Hand have definitively identified a spectroscopic feature on Europa’s surface that indicates the presence of a magnesium sulfate salt, a mineral called epsomite, that could only originate from the ocean below.

“Magnesium should not be on the surface of Europa unless it’s coming from the ocean,” Brown says. “So that means ocean water gets onto the surface, and stuff on the surface presumably gets into the ocean water.”

Europa’s ocean is thought to be 100 kilometers deep and covers the entire globe. The moon remains locked in relation to Jupiter, with the same hemisphere always leading and the other trailing in its orbit. The leading hemisphere has a yellowish appearance, while the trailing hemisphere seems to be splattered and streaked with a red material.

The spectroscopic data from that red side has been a cause of scientific debate for 15 years. It is thought that one of Jupiter’s largest moons, Io, spews volcanic sulfur from its atmosphere, and Jupiter’s strong magnetic field sends some of that sulfur hurtling toward the trailing hemisphere of Europa, where it sticks. It was also clear from Galileo’s data that there is something other than pure water ice on the trailing hemisphere’s surface. The debate has focused on what that other something is—i.e., what has caused the spectroscopic data to deviate from the signature of pure water ice.

“From Galileo’s spectra, people knew something was there besides water. They argued for years over what it might be—sodium sulfate, hydrogen sulfate, sodium hydrogen carbonate, all these things that look more or less similar in this range of the spectrum,” says Brown. “But the really difficult thing was that the spectrometer on the Galileo spacecraft was just too coarse.”

Brown and Hand decided that the latest spectrometers on ground-based telescopes could improve the data pertaining to Europa, even from a distance of about 400 million miles. Using the Keck II telescope on Mauna Kea, they first mapped the distribution of pure water ice versus anything else on the moon. The spectra showed that even Europa’s leading hemisphere contains significant amounts of nonwater ice. Then, at low latitudes on the trailing hemisphere—the area with the greatest concentration of the nonwater ice material—they found a tiny dip in the spectrum that had never been detected before.

The two researchers racked their brains to come up with materials that might explain the new spectroscopic feature, and then tested everything from sodium chloride to Drano in Hand’s lab at JPL, where he tries to simulate the environments found on various icy worlds. “We tried to think outside the box to consider all sorts of other possibilities, but at the end of the day, the magnesium sulfate persisted,” Hand says.

Some scientists had long suspected that magnesium sulfate was on the surface of Europa. But, Brown says, “the interesting twist is that it doesn’t look like the magnesium sulfate is coming from the ocean.” Since the mineral he and Hand found is only on the trailing side, where the moon is being bombarded with sulfur from Io’s, they believe that there is a magnesium-bearing mineral everywhere on Europa that produces magnesium sulfate in combination with sulfur. The pervasive magnesium-bearing mineral might also be what makes up the nonwater ice detected on the leading hemisphere’s surface.

Brown and Hand believe that this mystery magnesium-bearing mineral is magnesium chloride. But magnesium is not the only unexpected element on the surface of Europa. Fifteen years ago, Brown showed that Europa is surrounded by an atmosphere of atomic sodium and potassium, presumably originating from the surface. The researchers reason that the sodium and potassium chlorides are actually the dominant salts on the surface of Europa, but that they are not detectable because they have no clear spectral features.

The scientists combined this information with the fact that Europa’s ocean can only be one of two types—either sulfate-rich or chlorine-rich. Having ruled out the sulfate-rich version since magnesium sulfate was found only on the trailing side, Brown and Hand hypothesize that the ocean is chlorine-rich and that the sodium and potassium must be present as chlorides.

Therefore, Brown says, they believe the composition of Europa’s sea closely resembles the salty ocean of Earth. “If you could go swim down in the ocean of Europa and taste it, it would just taste like normal old salt,” he says.

Hand emphasizes that, from an astrobiology standpoint, Europa is considered a premier target in the search for life beyond Earth; a NASA-funded study team led by JPL and the Johns Hopkins University Applied Physics Laboratory have been working with the scientific community to identify options to explore Europa further.  “If we’ve learned anything about life on Earth, it’s that where there’s liquid water, there’s generally life,” Hand says. “And of course our ocean is a nice salty ocean. Perhaps Europa’s salty ocean is also a wonderful place for life.”

The Astronomical Journal paper is titled “Salts and radiation products on the surface of Europa.” The work was supported by the Richard and Barbara Rosenberg Professorship at Caltech and by the NASA Astrobiology Institute through the Astrobiology of Icy Worlds node at JPL.

The W. M. Keck Observatory operates the two biggest and most scientifically productive telescopes on Earth. The twin 10-meter optical/infrared telescopes located on the summit of Mauna Kea on the Island of Hawaii, feature a world-leading suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

  
Science Contact:

Contact:
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(626) 395-3227
debwms@caltech.edu

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Steve Jefferson

Adaptive Optics allows Earth-based monitoring of Io’s Fiery show

Quiescent activity of Io observed in 2010 and 2011 showing several quasi-permanent eruptions at 3.8 microns [bottom] and the absence of bright, hotter outbursts at 2.1 microns. 

Kamuela, Hawaii – Watching active volcanic eruptions should be done from a safe distance, and a group of California researchers has figured out how to do it from, ironically, Mauna Kea – one of Earth’s tallest volcanoes – using the W. M. Keck Observatory. Employing an ingenious combination of telescopic surveys and archival data, they have gathered nearly 40 distinct snapshots of effusive (slow) volcanic eruptions and high temperature outbursts on Jupiter’s tiny moon, Io, showing details as small as 100 km (60 miles) on the moon’s surface. 

While space-based telescopes were once required for viewing surface details on Io – similar in size to our Moon, but more than 1,600 times distant – adaptive optics (AO), pioneered at Keck, allows teams like that led by Franck Marchis, a researcher at the Carl Sagan Center of the SETI Institute, to collect fascinating data on the wild show from Earth. Marchis presented results from ground-based telescopic monitoring of Io’s volcanic activity over the past decade this week, at the 2012 Division of Planetary Sciences Meeting of the American Astronomical Society.

Erupting volcanoes on Io cannot be seen well from beneath the Earth’s atmosphere using classical astronomical techniques. Io is a relatively small satellite with a 3,600 km diameter, more than 630 million kilometers away. In 1979, Voyager 1 visited the Jovian system, revealing Io’s dynamic volcanic activity from the first close-up pictures of its surface, capturing bizarre volcanic terrains, active plumes and hot spots. The Galileo spacecraft remained in orbit in the Jovian system from 1995 to 2003 and observed more than 160 active volcanoes and a broad range of eruption styles. Several outstanding questions remained in the post-Galileo era, and the origin and long-term evolution of Io’s volcanic activity is still not fully understood. 

In the meantime, astronomers designed instruments to break the “seeing barrier” and improve the image quality of ground-based telescopes. The blurring (“seeing”) introduced by the constant motion of the Earth’s atmosphere can be measured and corrected in real time using adaptive optics (AO), providing an image with a resolution close to the theoretical “diffraction limit” of the telescope. The W. M. Keck Observatory has used adaptive optics since 1999. 

“Since our first observation of Io in 2001 using the Keck II 10-meter telescope and its AO system from Mauna Kea in Hawaii, our group became very excited about the technology.  We also began using AO at the Very Large Telescope in Chile, and at the Gemini North telescope in Hawaii.  The technology has improved over the years, and the image quality and usefulness of these AO systems have made them part of the essential instrument suite for large telescopes,” said Marchis.

Since 2003, combining their own observing programs with archival data, the team led by Marchis has gathered approximately 40 epochs of observations of Io in the near-infrared. These images show details as small as 100 km (60 miles) on the surface of the satellite. 

Their observations have revealed young and energetic eruptions called outbursts.  These are easily detectable from their immense thermal emission at shorter wavelengths, implying a high eruption temperature. The team observed the awakening of the volcano Tvashtar simultaneously with the New Horizons spacecraft, which flew past Jupiter on its way to Pluto. From a combined survey based on three large telescopes, they reported that the eruption was detectable from April 2006 to September 2007. Older observations from the Galileo spacecraft and the W. M. Keck Observatory show that this volcano previously displayed a similar “fire fountain” eruption which started in November 1999 and lasted for 15 months. Similarly, Pillan, an energetic eruption detected with the Galileo spacecraft from 1996 to 1999, had sporadic activity again in August 2007 that was reported by the team using the Keck II telescope.

“The episodicity of these volcanoes points to a regular recharge of magma storage chambers” said Ashley Davies a volcanologist at the Jet Propulsion Laboratory, California Institute of Technology, and a member of the study.  “This will allow us to model the eruption process and understand how heat is removed from Io’s deep interior by this particular style of volcanic activity.”

Four additional young eruptions were detected during this survey including an extremely active volcano located at a region that had never shown activity in the past. The new activity was seen in May 2004 and had a total output of 10% the average Io thermal output. This was more energetic than Tvashtar in 2001, implying a fire fountain style eruption. Interestingly, the team did not observe any “mega-outburst” during this survey, with an energetic output similar to the eruption on Surt in 2001, the most energetic eruption ever witnessed in the Solar System. They conclude that such outbursts are rare and short-lived, typically lasting only a few days.

The team and several others groups continued to monitor Io’s volcanic activity. They noticed that since September 2010, Io’s volcanic activity has been globally quiescent. A dozen permanent, low temperature eruptions, which represent less dramatic “effusive” activity, are still detected across the surface of Io, but recent observations of the satellite show an absence of young bright eruptions.

“Spacecraft have only been able to capture fleeting glimpses of Io’s volcanoes, Voyager for a few months, Galileo a few years, and New Horizons a few days.  Ground-based observations, on the other hand, can continue to monitor Io’s volcanoes over long time-scales. The more telescopes looking at Io, the better time coverage we can obtain,” said Julie Rathbun from Redlands University, a planetary scientist not directly involved in this study but who has monitored Io for more than 15 years.. “AO observations from 8- to 10-meter class telescopes are a dramatic improvement in spatial resolution over previous ground-based observations.  Soon they will not only be our only way to monitor Io’s volcanoes, but the best way.  We should be making these observations more often.”

The monitoring of Io’s volcanism will continue to build a timeline of activity and thermal emission variability, which will be further complemented by data obtained by other missions to the Jupiter system (such as the ESA mission JUICE, or a future dedicated Europa or Io mission). Until these missions, however, the large, AO-enabled ground-based telescopes will shoulder the task of monitoring Io’s volcanic activity.

The next generation of AO systems will provide even better image quality and open the visible wavelength range to planetary astronomers. These systems are currently under development and will have their first light in the coming years. Colorful surface changes due to volcanic activity, such as plume deposits or lava flow fields, will be detectable from the ground.

 “The understanding and characterization of volcanoes on Io is one of the many very exciting applications of the current Keck AO systems,” said Peter Wizinowich, Optical Systems Manager at W. M Keck Observatory. “Marchis’ simulations of what Keck’s proposed Next Generation AO system (NGAO) could do for the field of solar system astronomy remind us that there is a lot more breakthrough science awaiting the delivery of NGAO.” 

The W. M. Keck Observatory operates two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Earliest Spiral Galaxy Surprises Astronomers

HST/Keck false colour composite image of galaxy BX442.

Credit: David Law; Dunlap Insitute for Astronomy & Astrophysics

Artist's rendering of galaxy BX442 and companion

Credit: Dunlap Institute for Astronomy & Astrophysics; Joe Bergeron

Kinematic velocity and velocity dispersion maps of BX442
Credit: David Law; Dunlap Institute for Astronomy & Astrophysics

Kamuela, HI – In the beginning, galaxies were hot and clumpy – too hot to settle down and form grand spirals like the Milky Way and other galaxies seen in the nearby universe today. But astronomers have now been surprised by the discovery of a solitary grand design spiral galaxy in the early universe which could hold clues to how spirals start to take shape. The find was announced in a report in the July 19 edition of the journal Nature.

The ancient spiral, called BX442, was found by astronomers who first surveyed 300 distant galaxies using the Hubble Space Telescope, then followed up and confirmed using detailed observations and analyses from the W. M. Keck Observatory in Hawaii.

“As you go back in time to the early universe — about three billion years after the Big Bang; the light from this galaxy has been travelling to us for about 10.7 billion years —galaxies look really strange, clumpy and irregular, not symmetric” said astronomer Alice Shapley of UCLA. “The vast majority of old galaxies look like train wrecks. Our first thought was, why is this one so different, and so beautiful?”

Not only was the spiral shape clearly visible, but by using Keck’s OSIRIS instrument (OH-Suppressing Infrared Imaging Spectrograph), astronomers were able to study different parts of BX442 and determine that it is, in fact, rotating and not just two unrelated disk galaxies along the same line of sight that give the appearance of being a single spiral galaxy.

“We first thought this could just be an illusion and that perhaps we were being led astray by the picture,” said Shapley, a coauthor on the Nature paper. “What we found when we took spectra of this galaxy is that the spiral arms do belong to this galaxy; it wasn’t an illusion. Not only does it look like a rotating spiral disk galaxy; it really is. We were blown away.”

Using laser adaptive optics (AO) to cancel out much of the Earth’s atmospheric distortions, the Keck II Telescope is able to get equal or better resolution than the Hubble Space Telescope. This was critical in this case, said astronomer David Law of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto and the lead author on the paper.

“Galaxies at this distance appear super, super faint and super, super tiny,” said Law. “We needed every inch of Keck’s light collecting area, exquisite image quality from the AO system, and a sensitive instrument to not only detect the galaxy but chop up its light into 3,600 pieces to analyze. OSIRIS is really one of the only instruments in the world that could do what we needed, and everything came together beautifully.”

In the end, it took thirteen hours over three nights with the Keck II Telescope to gather the spectra from BX422 needed to confirm the nature of the distant and early spiral.

“We got a beautiful map that told us this thing is a rotating disk,” said Shapley.

What also sets BX442 apart from other galaxies of its epoch is that it appears to be in the process of merging with another galaxy. That, in fact, could be the reason it is beginning to form a spiral.

“Indeed, many of the most well-known grand design spiral galaxies in the nearby universe (e.g., M51, M81, M101) are observed to have nearby companions, and small satellites such as the Sagittarius dwarf galaxy may even be partly responsible for producing spiral patterns in our own Milky Way galaxy,” the researchers wrote in their paper.

They tested the idea with a simulation and found that the spiral pattern could be formed by such a merger. The simulations indicate that its glory may be fleeting though; the spiral may dissipate again in just 100 million years.

“BX442 represents a link between early galaxies that are much more turbulent and the rotating spiral galaxies that we see around us,” Shapley said. “Indeed, this galaxy may highlight the importance of merger interactions at any cosmic epoch in creating grand design spiral structure.”

***

Co-authors are Charles Steidel, the Lee A. DuBridge Professor of Astronomy at the California Institute of Technology; Naveen Reddy, assistant professor of physics and astronomy at UC Riverside; Charlotte Christensen, postdoctoral scholar at the University of Arizona, and Dawn Erb, assistant professor of physics at the University of Wisconsin, Milwaukee.

Shapley’s research is funded by the David and Lucile Packard Foundation.

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth’s turbulent atmosphere. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

(Partially adapted from a press release by UCLA)

Found: Heart of Darkness

This is the portion of sky in which astronomers found the Segue 1 dwarf galaxy. Can you see it? Credit: Marla Geha

Using the DEIMOS instrument on the Keck II telescope, astronomers could identify which stars were moving together as a group. They are circled here in green. Credit: Marla Geha

By subtracting out all the other objects in the image and leaving the Segue I member stars, the “darkest galaxy” emerges. Credit: Marla Geha

All three images above are combined in this captioned mosaic

Credit: Marla Geha, Keck Observatory

Kamuela, HI – Astronomers using the 10-meter Keck II telescope in Hawaii have confirmed in a new paper that a troupe of about 1,000 small, dim stars just outside the Milky Way comprise the darkest known galaxy, as well as something else: a treasure trove of ancient stars.

By “dark” astronomers are not referring to how much light the galaxy, called Segue 1, puts out, but the fact that the dwarf galaxy appears to have 3,400 times more mass than can be accounted for by its visible stars. In other words, Segue 1 is mostly an enormous cloud of dark matter decorated with a sprinkling of stars.

The initial announcement of the “Darkest Galaxy” was made two years ago by Marla Geha, a Yale University astronomer, Joshua Simon from the Carnegie Institution of Washington, and their colleagues. This original claim was based on data from the Sloan Digital Sky Survey and the Keck II telescope. Those observations indicated the stars were all moving together and were a diverse group, rather than simply a cluster of similar stars that had been ripped out of the nearby and more star-rich Sagittarius dwarf galaxy. A competing group of astronomers at Cambridge University were, however, not convinced.

So Simon, Geha and their group returned to Keck and went to work with the telescope’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) to measure how the stars move not just in relation to the Milky Way, but also in relation to each other.

If the 1,000 or so stars were all there was to Segue 1, with just a smidgeon of dark matter, the stars would all move at about the same speed, said Simon. But the Keck data show they do not. Instead of moving at a steady 209 km/sec relative to the Milky Way, some of the Segue 1 stars are moving at rates as slow as 194 kilometers per second while others are going as fast as 224 kilometers per second.

“That tells you Segue 1 must have much more mass to accelerate the stars to those velocities,” Geha explained. The paper confirming Segue 1’s dark nature appeared in the May 2011 issue of The Astrophysical Journal.

The mass required to cause the different star velocities seen in Segue 1 has been calculated at 600,000 solar masses. But there are only about 1,000 stars in Segue 1, and they are all close to the mass of our Sun, Simon said. Virtually all of the remainder of the mass must be dark matter.

Stellar Old Folks Home

Equally exciting news from Segue 1 is its unusual collection of nearly primordial stars. One way to tell how long ago a star formed is by its heavy element content, which can be gleaned from the characteristic absorption features in the star’s spectrum. Very old or primitive stars come from a time when the universe was young and few large stars had yet grown old enough to fuse lightweight atoms like hydrogen and helium into heavier elements like iron and oxygen. Early, and therefore ancient, stars that formed from early gas clouds are therefore very low in heavy elements.

The researchers managed to gather iron data on six stars in Segue 1 with the Keck II telescope, and a seventh Segue 1 star was measured by an Australian team using the Very Large Telescope. Of those seven, three proved to have less than one 2,500th as much iron as our own Sun.

“That suggests these are some of the oldest and least evolved stars that are known,” said Simon.

Searches for such primitive stars among the Milky Way’s billions have yielded less than 30.

“In Segue 1 we already have 10 percent of the total in the Milky Way,” Geha said. “For studying these most primitive stars, dwarf galaxies are going to be very important.”

Dark Matter Demolition Derby

The confirmation of the large concentration of dark matter in Segue 1 underscores the importance of other research that has focused on Segue 1. In particular, some researchers have been looking with the space-based Fermi Gamma Ray Telescope in hopes of catching sight of a faint glimmer of gamma rays which could be created, theoretically, by the collision and annihilation of pairs of dark matter particles.

So far the Fermi telescope has not detected anything of the sort, which isn’t entirely surprising and doesn’t mean the dark matter isn’t there, said Simon.

“The current predictions are that the Fermi telescope is just barely strong enough or perhaps not quite strong enough to see these gamma rays from Segue 1,” Simon explained. So there are hopes that Fermi will detect at least the hint of a collision.
“A detection would be spectacular,” said Simon. “People have been trying to learn about dark matter for 35 years and not made much progress. Even a faint glow of the predicted gamma rays would be a powerful confirmation of theoretical predictions about the nature of dark matter.”

In the meantime, astronomers suspect there are other, perhaps even darker dwarf galaxies hovering around the Milky Way, waiting to be discovered. “We’d like to find more objects like Segue 1,” Simon said.

Source: W.M.Keck Observatory