A Ring of Dark Matter

Dark matter has to be made up of some sort of elementary particle, but we know astoundingly little about it. Its existence can be inferred from its necessary effects — something we can ‘t see seems to be holding galaxy clusters together, because the gravity from the stars we do observe in them isn’t sufficient to do the job. That makes gathering any evidence for dark matter’s behavior — indeed, for its very existence — a crucial goal for astrophysicists. And today we have the strongest supporting evidence yet that dark matter is real.

A ring of dark matter

The work comes via the Hubble Space Telescope, used by a team of astronomers to locate what appears to be a ring of dark matter in the cluster ZwCl0024+1652, some five billion light years from our Solar system. The ring is 2.6 million light years across, and this detection appears to be unique. Says M. James Jee (Johns Hopkins): “This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster.” Lee was a member of the team that found the dark matter ring.

Image: The galaxy cluster Cl 0024+17 (ZwCl0024+1652) as seen by Hubble’s Advanced Camera for Surveys. The image displays faint faraway background galaxies that had their light bent by the cluster’s strong gravitational field. By mapping the distorted light and using it to deduce how dark matter is distributed in the cluster, astronomers spotted the ring of dark matter. One of the background galaxies is located about two times further away than the yellow cluster galaxies in the foreground, and has been multiple-imaged into five separate arc-shaped components, seen in blue. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

A possible cause for the ring: The collision between two galaxy clusters some 1 to 2 billion years ago. Using earlier evidence of such a collision, the team’s computer simulations modeled the event, showing how any associated dark matter might react. In a way, says Holland Ford (also of Johns Hopkins), “Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models.”

Let’s be clear on the method here. Dark matter, by its nature, cannot be seen. Astronomers infer its existence by observing how its gravity bends the light of distant background galaxies. We’ve looked at that phenomenon, called gravitational lensing, before. It’s a powerful tool not only in the study of cosmological structure but is also helpful in exoplanet detections. Such lensing is a major way to learn about dark matter, but in this case the substance — whatever it is — is widely separated from the gas and galaxies that make up the clusters themselves.

That gives astronomers a chance to home in on qualities particular to dark matter, the things that distinguish it from the ordinary matter — stars, planets, people — that make up the 4 percent of the universe we can actually see. And what has shown up is an unexpected ‘rippling’ effect that caused ripples of its own among team members:

“I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” Jee explained. “I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.”

Jee goes on to explain the ripple effect:

“The collision between the two galaxy clusters created a ripple of dark matter which left distinct footprints in the shapes of the background galaxies. It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.”

So now we’re seeing dark matter in a new kind of distribution, offering further clues about its nature. I sometimes think of dark matter (and the equally mysterious dark energy) as needed correctives to the notion that we are within a few years (or decades, perhaps) of truly understanding the cosmos. That point of view, which settles in every few centuries only to be disrupted by new scientific breakthroughs, now needs the adjustment supplied by a simple fact: We have no good explanation for more than a fraction of the matter that pervades the universe, nor do we fathom the relentless acceleration now thought to fuel its expansion.

An excellent video on these findings is available here. Younger readers who are contemplating a career in astrophysics should rush to get into work like this. We are entering an era of unprecedented discovery.

Looking Hard at Gliese 581

We’d all like to know more about Gliese 581 c, the most talked about exoplanet of them all because of the possibility — however controversial — that it may be habitable. One way to learn more would be to observe a transit, which is what the Canadian space telescope called MOST is now attempting to do. The odds are roughly one in thirty, according to MOST principal investigator Jaymie Mark Matthews, but even the few observations ahead for MOST will tell us more about the star in question.

Matthews’ thoughts are reported in an article in the British Columbian alternative daily The Tyee, along with a nice backgrounder on the planet by writer Monte Paulsen. Evidently the Swiss team behind the Gliese 581 c announcement, which includes Michel Mayor and Didier Queloz (the first to identify an exoplanet, in 1995), had contacted the MOST controllers at the University of British Columbia before going public with their latest work. They hoped a transit could verify the existence of the new planet and sharpen up our knowledge of its parameters.

From the story:

“We had our first chance earlier this week,” Matthews told The Tyee. “We’ll have another intense stakeout in less than two weeks.”

If MOST does catch a transit, astronomers will be able to combine MOST’s data on the planet’s size and speed with HARP’s observations of mass. “We would be the first to measure the density of an Earth-like planet. No one’s ever been able to do that,” Matthews said. “We would be able to tell whether it was an ocean, or rocky.”

But in the absence of a transit, MOST can still be helpful in looking at how active Gliese 581 actually is. The mission — Microvariability and Oscillations of STars — is uniquely qualified for that project, being designed to conduct seismic probes of stellar structures and ages. A closer look at the star’s flare activity may tell us something about conditions in its habitable zone, whether or not Gliese 581 c is actually within that zone. According to Matthews, MOST will release preliminary findings on the investigation some time next month.

The Search for Vulcan

40 Eridani is a triple-star system some 16 light years from Earth. If it rings a faint bell, that’s probably because of its association with Star Trek. In the universe of the show, 40 Eridani is home to Vulcan, birthplace of the inscrutable Mr. Spock (Gene Roddenberry himself signed off on the idea). Not so long ago, the existence of planets would have been doubted in such a system, but we’re learning that double and even triple star systems can and do support planets. So maybe there is a ‘Vulcan’ out there after all, though doubtless sans humanoids with pointy ears.

View of two habitable zones

In any case, the elements of this system are widely spaced, and 40 Eridani A is a K-class star not so different from Centauri B, a star that could well support Earth-mass planets. Recently Angelle Tanner (Caltech) embarked on simulations designed to show whether or not the Space Interferometery Mission (known as SIM PlanetQuest) might be able to detect such a world. Tanner’s work confirmed that a planet like this in the habitable zone (about 0.6 AU) would be detectible. But it will have to wait for later missions, perhaps armed with sunshade or advanced coronagraph technology, to make the key spectroscopic measurements that would reveal the presence of biomarkers in its atmosphere.

The image above (click to enlarge) offers a comparison of the larger habitable zone around our Sun and that of 40 Eridani A (Credit: JPL). The Jet Propulsion Laboratory also offers a nifty animation of ‘Spock’s home’ here. Tanner’s work is slated for Publications of the Astronomical Society of the Pacific (and thanks to Hans Bausewein for the heads-up). The future of SIM PlanetQuest itself is more problematic. As of May 1, the word is “Launch deferred indefinitely by NASA headquarters.”

In Search of Ancient Stars

We’ve seen recently how difficult it can be to pin down the age of a star. Even the Alpha Centauri system is problematic, with age ranges for Centauri A and B varying from slightly less than four billion years to as many as eleven (depending on which star we’re talking about, and which of several methods was used for the calculation). But one thing that helps with stars that are older than the ordinary is the chemical composition of the star in question. “Surprisingly, it is very hard to pin down the age of a star,” says Anna Frebel (University of Texas), “although we can generally infer that chemically primitive stars have to be very old.”

Frebel’s work has led her to a star that is old indeed. It is HE 1523-0901, now pegged thanks to the work of Frebel’s team at the astounding age of 13.2 billion years, meaning it would have formed not all that long after the Big Bang. The researchers were able to study radioactive elements in the star to create a precise calculation. A spectrograph on one of the 8.2-meter instruments at the European Southern Observatory’s Very Large Telescope in Chile measured uranium and thorium, matching up their indications with readings of europium, osmium and iridium. The star’s amazing spectrum pinned down its age with unusual accuracy.

Diagram of the cosmic clock

Image (click to enlarge): Astronomer Anna Frebel of the the University of Texas at Austin McDonald Observatory and her colleagues have deduced the star’s age based on the amounts of radioactive elements it contains compared to certain other “anchor” elements, specifically europium, osmium, and iridium. The study of the star’s chemical make-up was made using the UVES spectrograph on the Kueyen Telescope, part of ESO’s Very Large Telescope, at Paranal, in Chile. Credit and copyright: European Southern Observatory.

HE 1523-0901 thus becomes a laboratory for the creation of elements not long after the Big Bang, which is thought to have occurred some 13.7 billion years ago. Finding it was tricky work indeed, for the abundance of a given radioactive isotope decreases over time. “Actual age measurements are restricted to the very rare objects that display huge amounts of the radioactive elements thorium or uranium,” says Norbert Christlieb, co-author of the paper on this work, and that makes finding this ancient star in the first place a remarkable occurrence, or maybe we can just call it, as Frebel does, ‘informed serendipity.’

The paper is Frebel et al., “Discovery of HE 1523-0901, a Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium”, Astrophysical Journal Letters 660:L117-L120, 2007 (May 10, 2007). Abstract available, with full text here.

Titan’s Tholins: Precursors of Life?

Tholins are interesting molecules, large and complex. They’re organic aerosols — particles small enough to remain suspended in the atmosphere for some time — formed from methane and nitrogen. Their presence on Titan is intriguing because they’re thought to contain some of the chemical precursors of life. That makes studying how they form there a preoccupation with those wanting insight into how life appears.

Titan is a wonderful laboratory for such studies. We already knew that nitrogen and methane dominated its atmosphere. New measurements from Cassini now show that tholins form much higher in that atmosphere than was previously believed. The most recent Cassini flybys, though, have also demonstrated the presence of benzene, a key component in the formation of aromatic hydrocarbon compounds. Moreover, Cassini’s Ion Beam Spectrometer (IBS) and Electron Spectrometer (ELS) have picked up the presence of large positive and negative ions.

Here’s Andrew Coates (University College, London) on the unexpected find:

“An additional surprising point is the large numbers of negative ions we see during Cassini’s lowest flybys above the surface. This newly discovered, and important, population represents a highly significant proportion of the whole ionosphere at these locations.”

To which Hunter Waite (leader of Cassini’s Ion Neutral mass Spectrometer team) adds:

“Our analysis suggests that the organic compounds are formed through ion-neutral chemical processes, which then give rise to the complex negative ions found by the ELS.”

Tholin formation diagram

Negative ions, then, seem to have a role to play in the process that makes tholins out of their carbon-nitrogen precursors. And if this sounds completely theoretical, bear in mind that one theory for life’s formation on Earth involves the planet’s bombardment by tholin-rich comets that hauled in the needed raw materials. Tholins are plentiful on the surface of the icy bodies found in the outer Solar System. Clearly, the more we learn about these interesting substances, the more insights we’ll have into the movement of life’s precursors throughout planetary systems around our own and other stars.

Image: The tholin formation process. Credit: Southwest Research Institute.

Carl Sagan, who did major work on tholins (and, in fact, came up with the term) would have been fascinated with these results. The paper is Waite et al., “The Process of Tholin Formation in Titan’s Upper Atmosphere,” in Science Vol. 316 (May 11, 2007), pp. 870-875, with abstract here.