The Mystery of Dark Matter Deepens

Like all galaxies, our Milky Way is home to a strange substance called dark matter. Dark matter is invisible, betraying its presence only through its gravitational pull. Without dark matter holding them together, our galaxy's speedy stars would fly off in all directions. The nature of dark matter is a mystery -- a mystery that a new study has only deepened.

"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.

The standard cosmological model describes a universe dominated by dark energy and dark matter. Most astronomers assume that dark matter consists of "cold" (i.e. slow-moving) exotic particles that clump together gravitationally. Over time these dark matter clumps grow and attract normal matter, forming the galaxies we see today.

 

Cosmologists use powerful computers to simulate this process. Their simulations show that dark matter should be densely packed in the centers of galaxies. Instead, new measurements of two dwarf galaxies show that they contain a smooth distribution of dark matter. This suggests that the standard cosmological model may be wrong.

"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.

Dwarf galaxies are composed of up to 99 percent dark matter and only one percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.

Walker and his co-author Jorge Peñarrubia (University of Cambridge, UK) analyzed the dark matter distribution in two Milky Way neighbors: the Fornax and Sculptor dwarf galaxies. These galaxies hold one million to 10 million stars, compared to about 400 billion in our galaxy. The team measured the locations, speeds and basic chemical compositions of 1500 to 2500 stars.

"Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy," explained Peñarrubia. "That makes it much more challenging to determine the distribution of dark matter."

Their data showed that in both cases, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This contradicts the prediction that the density of dark matter should increase sharply toward the centers of these galaxies.

"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 Peñarrubia.

Some have suggested that interactions between normal and dark matter could spread out the dark matter, but current simulations don't indicate that this happens in dwarf galaxies. The new measurements imply that either normal matter affects dark matter more than expected, or dark matter isn't "cold." The team hopes to determine which is true by studying more dwarf galaxies, particularly galaxies with an even higher percentage of dark matter.

by "environment clean generations"

Galactic Danger Zone

 Not every place within a galaxy experiences the same conditions for habitability - some parts are lethal thanks to supernovae, whilst others do not possess enough heavy elements to allow rocky planets and life to develop. Credit: The Hubble Heritage Team, AURA/STScI/NASA

We know for certain that life exists in the Milky Way galaxy: that life is us. Scientists are continually looking to understand more about how life on our planet came to be and the conditions that must be met for its survival, and whether those conditions can be replicated elsewhere in the Universe. It turns out that looking at our entire Galaxy, rather than focusing just on life-giving properties of our planet or indeed the habitability of regions of our own Solar System, is a good place to start.  

How far our planet orbits from the Sun, along with other factors such as atmospheric composition, a carbon cycle and the existence of water, has told astronomers much about the conditions that are required for life to not only originate, but to survive on rocky worlds. 

This distance from a star is referred to, quite simply, as the ‘Habitable Zone’ or sometimes the ‘Goldilocks Zone’ because conditions here are neither too hot or too cold for water to be liquid on the planet’s surface -- conditions just right for life as we know it to thrive. 

Copernican theory tells us that our world is a typical rocky planet in a typical planetary system. This concept has spurred some astronomers to start thinking bigger, way beyond the simplicity of any one planetary system and instead towards much grander scales. Astronomers are exploring whether there is a Galactic Habitable Zone (GHZ) in our Galaxy – a region of the Milky Way that is conducive to forming planetary systems with habitable worlds. The Galactic Habitable Zone implies that if there are conditions just right for a planet around a star, then the same must go for a galaxy.  

This concept was first introduced by geologist and paleontologist Peter Ward and Donald Brownlee, an astronomer and astrobiologist, in their book, ‘Rare Earth’. The idea of a GHZ served as an antagonistic view point to the Copernican principle. 

Despite scientists such as Carl Sagan and Frank Drake favoring the theory of mediocrity based on the Copernican model, which supports the probability of the Universe hosting other forms of complex life, Ward and Brownlee were certain our Earth and the conditions within our Galaxy that allowed such life to evolve are both extremely rare. 

Their answer to the famous Fermi paradox – if extraterrestrial aliens are common, why is their existence not obvious? – is that alien life more complex than microbes is not very common at all, requiring a number of factors, each of low possibility, to come into play. In short, Ward and Brownlee were suggesting that much of the Galaxy was inhospitable to complex life. In their view, only a narrow belt around the Galaxy was fertile: the Galactic Habitable Zone.

Since then, many astronomers have looked at the idea of the GHZ. Not all believe that it necessarily supports Ward and Brownlee’s Rare Earth hypothesis.

One recent assessment of the GHZ, by Michael Gowanlock of NASA’s Astrobiology Institute, and his Trent University colleagues David Patton and Sabine McConnell, has suggested that while the inner sector of the MIlky Way Galaxy may be the most dangerous, it is also most likely to support habitable worlds. 

Their paper, accepted for publication in the journal Astrobiology, modeled habitability in the Milky Way based on three factors: supernova rates, metallicity (the abundance of heavy elements, used as a proxy for planet formation) and the time taken for complex life to evolve. They found that although the greater density of stars in the inner galaxy (out to a distance of 8,100 light years from the galactic center) meant that more supernovae exploded, with more planets becoming sterilized by the radiation from these exploding stars, the chances of finding a habitable planet there was ten times more likely than in the outer Galaxy. 

This contradicts previous studies that, for example, suggested the GHZ to be a belt around the Galaxy between distances of 22,800 light years (7 kiloparsecs) and 29,300 light years (9 kiloparsecs) from the galactic center. What’s noticeable is that our Sun orbits the Galaxy at a distance of about 26,000 light years (8 kiloparsecs) – far outside GHZ proposed by Gowanlock’s team. Why is their proposed galactic habitable zone so different? 

“We assume that metallicity scales with planet formation,” says Gowanlock. Heavy elements are produced by dying stars, and the more generations of stars there have been, the greater the production of these elements (or ‘metals’ as they are termed by astronomers). Historically, the greatest amount of star formation has occurred in the inner region of the Milky Way. “The inner Galaxy is the most metal-rich, and the outer Galaxy is the most metal-poor. Therefore the number of planets is highest in the inner Galaxy, as the metallicity and stellar density is the highest in this region.”  

    A supernova sterilizes an alien world in this artist's impression. Credit: David A Aguilar (CfA)

However, amongst so much star formation lurks a danger: supernovae. Gowanlock’s team modeled the effects of the two most common forms of supernovae – the accreting white dwarfs that produce type Ia supernovae, and the collapsing massive stars of type II supernovae. 

Measurements of the galactic abundance of the isotope aluminum-26, which is a common by-product of type II supernovae, have allowed astronomers to ascertain that a supernova explodes on average once every 50 years. Meanwhile, previous studies have indicated that a supernova can have a deleterious effect on any habitable planet within 30 light years. 

“In our model, we assume that the build-up of oxygen and the ozone layer is required for the emergence of complex life,” says Gowanlock. “Supernovae can deplete the ozone in an atmosphere. Therefore, the survival of land-based complex life is at risk when a nearby supernova sufficiently depletes a great fraction of the ozone in a planet's atmosphere.” 

The team discovered that at some time in their lives, the majority of stars in our Galaxy will be bathed in the radiation from a nearby supernova, whereas around 30% of stars remain untouched or unsterilized. “Sterilization occurs on a planet that is roughly [at a distance] between 6.5 to 98 light years, depending on the supernovae,” says Gowanlock. “In our model, the sterilization distances are not equal, as some supernovae are more lethal than others.” 

Although the outer regions of the Galaxy, with their lower density of stars and fewer supernovae, are generally safer, the higher metallicity in the inner Galaxy means that the chances of finding an unsterilized, habitable world are ten times greater, according to Gowanlock’s model. However, their model does not stipulate any region of the Galaxy to be uninhabitable, only that it’s less likely to find habitable planets elsewhere.

This explains why our Solar System can reside far outside of the inner region, and it also gives hope to SETI – Gowanlock’s model proposes that there are regions of the Galaxy even more likely to have life, and many SETI searches are already targeted towards the galactic center. 

However, not all are in favor of the new model. Ward and Brownlee noted that the Sun’s position in the Galaxy is far more favorable because planets that dance around stars that are too close to the galactic center are more likely to suffer from a perturbed orbit by the gravity of another star that has wandered too close. Others question some of the assumptions made in the research, such as the accuracy of the percentage of planets that are habitable in the galaxy (1.2 percent), or that tidally-locked worlds can be habitable.

 An artist's impression of a potentially habitable planet around a Sun-like star. The habitability of such worlds not only depends on conditions on the planet and its distance from the star, but may also depend on where in the Galaxy it is located. Credit: ESO/M Kornmesser

“The authors may be making some assumptions that aren’t too well justified,” says Professor Jim Kasting of Penn State University and author of How to Find a Habitable Planet. “They seem well ahead of the rest of us who are still pondering these questions.” 

However, others believe that the research is promising. “This is one of the most complete studies of the Galactic Habitable Zone to date,” says Lewis Dartnell, an astrobiologist at University College London. “The results are intriguing, finding that white dwarf supernovae are over five times more lethal to complex life on habitable worlds than core collapse supernovae.” 

The GHZ isn’t static; the research paper written by Gowanlock’s team points out that over time the metallicity of the Galaxy will begin to increase the farther out one travels from the galactic center.

“This is why stars that form at a later date have a greater chance of having terrestrial planets,” says Gowanlock. As a result, perhaps the heyday for life in our Galaxy is yet to come.

by "environment clean generations"

The Most-Studied Star Explosion Ever

New Supernova Type Ia supernova PTF 11kly, the youngest ever detected, is seen above over three successive nights. The left image, taken Aug. 22, shows the event before it exploded supernova, approximately 1 million times fainter than the human eye can detect. The center image, from Aug. 23, shows the supernova at about 10,000 times fainter than the human eye can detect. The right image, from Aug. 24, shows that the event is 6 times brighter than the previous day.

Astronomers just spotted a brand-new supernova mere hours after it exploded, thanks to a robotic telescope and some smart computer algorithms. Now they’re scrambling to use as many telescopes as possible, on Earth and in space, to observe the star’s death throes. 

New supernovae are not terribly rare, but this one is unique because it is so close — 21 million light years away — and it’s of a type that is crucial to astronomical measurements. The supernova, PTF 11kly, is the youngest ever detected.

It showed up in the spiral galaxy M101, the Pinwheel Galaxy, a rather large spiral (10 times the size of the Milky Way) located in the constellation Ursa Major, known to its friends as the Big Dipper. It’s a Type Ia supernova, a very bright type that is used for gauging distances among galaxies. The use of Type Ia supernovae as standard candles helped astronomers prove how rapidly the universe is expanding, and led to the discovery of dark energy. So it’s an important type, and the discovery of a super-new, superclose supernova is tantalizing news for astronomers.

 The Palomar Transient Factory (PTF) survey, which is designed to observe and discover astronomical events as they happen, spotted the supernova earlier this week, according to a news release from the Lawrence Berkeley National Laboratory. First, a robotic observation system mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory scans the sky, and feeds data to a supercomputer at Berkeley Lab. 

The computers use machine learning algorithms to comb through the PTF data and flag interesting astronomical phenomena. Within a couple hours of spotting PTF 11kly, the system sent its coordinates to telescopes around the world so others could check it out, according to LBL.

Three hours later, telescopes in the Canary Islands captured the supernova’s spectral signature, and 12 hours later, astronomers using the Keck and Lick observatories determined it was a Type Ia. This makes the Canary Islands spectra the earliest Type Ia spectra ever taken. Afterward, astronomers sent an emergency request to NASA to use the Hubble Space Telescope, which will observe the supernova this weekend.

Over at Bad Astronomy, Phil Plait describes that Type Ia supernovae occur when a super-dense white dwarf siphons material off a companion star. If the white dwarf siphons off enough material, it can start to fuse hydrogen into helium, and the whole star will explode. This ginormous energy release makes the supernovae very bright, which makes them useful for gauging intergalactic distances. Type Ia are all thought to explode in similar ways, allowing them to be used as standard benchmarks — standard candles in astronomical parlance. (Click through to Bad Astronomy for a full rundown of what this supernova may mean to astronomy.)

In a fortuitous find, astronomers may actually have a picture of the supernova progenitors. Hubble Space Telescope images taken back in 2002 show two red giant stars very close to the location of PTF 11kly, Plait points out. If a white dwarf was nearby, it could have siphoned material from one of those red giants, sparking the runaway fusion event that led to supernova. Follow-up observations will help prove this.

Catching this supernova so early will give astronomers a glimpse into its outermost layers, which will tell them about the exploded star’s characteristics, according to astronomer Andrew Howell of UC Santa Barbara and Las Cumbres Global Telescope Network. “When you catch them this early, mixed in with the explosion you can actually see unburned bits from star that exploded! It is remarkable,” he said in a news release. “We are finding new clues to solving the mystery of the origin of these supernovae that has perplexed us for 70 years. Despite looking at thousands of supernovae, I’ve never seen anything like this before.”

You can see it, too. The supernova is getting brighter every night, and it should be visible with a decent pair of binoculars within the next couple weeks, according to astronomers at LBL and Oxford University. The best time to see it will be just after evening twilight in the Northern hemisphere.

by "environment clean generations"