Universe Today

Today at the 13th Cambridge Workshop on “Cool Stars, Stellar Systems, and the Sun,” Dr. Kevin L. Luhman (Harvard-Smithsonian Center for Astrophysics) announced the discovery of a unique pair of newborn brown dwarfs in orbit around each other. Brown dwarfs are a relatively new class of objects discovered in the mid-1990s that are too small to ignite hydrogen fusion and shine as stars, yet too big to be considered planets. “Are brown dwarfs miniature failed stars, or super-sized planets, or are they altogether different from either stars or planets?” asks Luhman. The unique nature of this new brown dwarf pair has brought astronomers a step closer to the answer.

One possible explanation for the origin of brown dwarfs is that they are born in the same way as stars. Stars form in huge interstellar clouds in which gravity causes clumps of gas and dust to collapse into “seeds,” which then steadily pull in more and more material until they grow to become stars. However, when this process is studied in detail by computer, many simulations fail to produce brown dwarfs. Instead, all the seeds grow into full-fledged stars. This result led some astronomers to wonder if brown dwarfs and stars are created in different ways.

“In one alternative that has been proposed recently,” explains Luhman, “the seeds in an interstellar cloud pull on each other through their gravity, causing a slingshot effect and ejecting some of the seeds from the cloud before they have a chance to grow into stars. These small bodies are what we see as brown dwarfs, according to that hypothesis.”

Testing these ideas for the birth of brown dwarfs is hampered by the fact that brown dwarfs are normally extremely faint and hard to detect in the sky. For most of their lives, they are not hot enough to ignite hydrogen fusion, so they do not shine brightly like stars, and instead are relatively dim like planets. However, for a short time immediately following their birth, brown dwarfs are relatively bright due to the leftover heat from their formation. As a result, brown dwarfs are easiest to find and study at an age of around 1 million years, which is newborn compared to the 4.5 billion year age of our Sun.

Taking advantage of this fact, Luhman searched for newborn brown dwarfs in a cluster of young stars located 540 light-years away in the southern constellation of Chamaeleon. Luhman conducted his search using one of the two 6.5-meter-diameter Magellan telescopes at Las Campanas Observatory in Chile, which are among the largest telescopes in the world.

Of the two dozen new brown dwarfs found, most were isolated and floating in space by themselves. However, Luhman discovered one pair of brown dwarfs orbiting each other at a remarkably wide separation. All previously known pairs of brown dwarfs are relatively close to each other, less than half the distance of Pluto from the Sun. But the brown dwarfs in this new pair are much farther apart, about six times the distance of Pluto from the Sun.

Because these brown dwarfs are so far apart, they are very weakly bound to each other by gravity, and the slightest tug would permanently separate them. Therefore, Luhman concludes, “The mere existence of this extremely fragile pair indicates that these brown dwarfs were never subjected to the kind of violent gravitational pulls that they would undergo if they had formed as ejected seeds. Instead, it is likely that these baby brown dwarfs formed in the same way as stars, in a relatively gentle and undisturbed manner.”

Dr. Alan P. Boss (Carnegie Institution) agrees, stating, “Luhman’s discovery strengthens the case for the formation mechanism of brown dwarfs being similar to that of stars like the Sun, and hence for brown dwarfs being worthy of being termed ‘stars,’ even if they are too low in mass to be able to undergo sustained nuclear fusion.”

The discovery of this binary brown dwarf will be published in an upcoming issue of The Astrophysical Journal. The discovery paper currently is online in PDF format at http://cfa-www.harvard.edu/~kluhman/paper.pdf

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

The Magellan telescopes are operated by the Carnegie Institution of Washington, the University of Arizona, Harvard University, the University of Michigan, and the Massachusetts Institute of Technology.

Las Campanas Observatory is operated by the Carnegie Observatories, which was founded in 1904 by George Ellery Hale. It is one of six departments of the private, nonprofit Carnegie Institution of Washington, a pioneering force in basic scientific research since 1902.

Original Source: Harvard CfA News Release

Current theories of the formation of galaxies are based on the hierarchical merging of smaller entities into larger and larger structures, starting from about the size of a stellar globular cluster and ending with clusters of galaxies. According to this scenario, it is assumed that no massive galaxies existed in the young universe.

However, this view may now have to be revised. Using the multi-mode FORS2 instrument on the Very Large Telescope at Paranal, a team of Italian astronomers have identified four remote galaxies, several times more massive than the Milky Way galaxy, or as massive as the heaviest galaxies in the present-day universe. Those galaxies must have formed when the Universe was only about 2,000 million years old, that is some 12,000 million years ago.

The newly discovered objects may be members of a population of old massive galaxies undetected until now.

The existence of such systems shows that the build-up of massive elliptical galaxies was much faster in the early Universe than expected from current theory.

Hierarchical merging
Galaxies are like islands in the Universe, made of stars as well as dust and gas clouds. They come in different sizes and shapes. Astronomers generally distinguish between spiral galaxies – like our own Milky Way, NGC 1232 or the famous Andromeda galaxy – and elliptical galaxies, the latter mostly containing old stars and having very little dust or gas. Some galaxies are intermediate between spirals and ellipticals and are referred to as lenticular or spheroidal galaxies.

Galaxies are not only distinct in shape, they also vary in size: some may be as “light” as a stellar globular cluster in our Milky Way (i.e. they contain about the equivalent of a few million Suns) while others may be more massive than a million million Suns. Presently, more than half of the stars in the Universe are located in massive spheroidal galaxies.

One of the main open questions of modern astrophysics and cosmology is how and when galaxies formed and evolved starting from the primordial gas that filled the early Universe. In the most popular current theory, galaxies in the local Universe are the result of a relatively slow process where small and less massive galaxies merge to gradually build up bigger and more massive galaxies. In this scenario, dubbed “hierarchical merging”, the young Universe was populated by small galaxies with little mass, whereas the present Universe contains large, old and massive galaxies – the very last to form in the final stage of a slow assembling process.

If this scenario were true, then one should not be able to find massive elliptical galaxies in the young universe. Or, in other words, due to the finite speed of light, there should be no such massive galaxies very far from us. And indeed, until now no old elliptical galaxy was known beyond a radio-galaxy at redshift 1.55 that was discovered almost ten years ago.

The K20 survey
In order to better understand the formation process of galaxies and to verify if the hierarchical merging scenario is valid, a team of Italian and ESO astronomers used ESO’s Very Large Telescope as a “time machine” to do a search for very remote elliptical galaxies. However, this is not trivial. Distant elliptical galaxies, with their content of old and red stars, must be very faint objects indeed at optical wavelengths as the bulk of their light is redshifted into the infrared part of the spectrum. Remote elliptical galaxies are thus among the most difficult observational targets even for the largest telescopes; this is also why the 1.55 redshift record has persisted for so long.

But this challenge did not stop the researchers. They obtained deep optical spectroscopy with the multi-mode FORS2 instrument on the VLT for a sample of 546 faint objects found in a sky area of 52 arcmin2 (or about one tenth of the area of the Full Moon) known as the K20 field, and which partly overlaps with the GOODS-South area. Their perseverance paid off and they were rewarded by the discovery of four old, massive galaxies with redshifts between 1.6 and 1.9. These galaxies are seen when the Universe was only about 25% of its present age of 13,700 million years.

For one of the galaxies, the K20 team benefited also from the database of publicly available spectra in the GOODS-South area taken by the ESO/GOODS team.

A new population of galaxies
The newly discovered galaxies are thus seen when the Universe was about 3,500 million years old, i.e. 10,000 million years ago. But from the spectra taken, it appears that these galaxies contain stars with ages between 1,000 and 2,000 million years. This implies that the galaxies must have formed accordingly earlier, and that they must have essentially completed their assembly at a moment when the Universe was only 1,500 to 2,500 million years old.

The galaxies appear to have masses in excess of one hundred thousand million solar masses and they are therefore of sizes similar to the most massive galaxies in the present-day Universe. Complementary images taken within the GOODS (“The Great Observatories Origins Deep Survey”) survey by the Hubble Space Telescope show that these galaxies have structures and shapes more or less identical to those of the present-day massive elliptical galaxies.

The new observations have therefore revealed a new population of very old and massive galaxies.

The existence of such massive and old spheroidal galaxies in the early Universe shows that the assembly of the present-day massive elliptical galaxies started much earlier and was much faster than predicted by the hierarchical merging theory. Says Andrea Cimatti (INAF, Firenze, Italy), leader of the team: “Our new study now raises fundamental questions about our understanding and knowledge of the processes that regulated the genesis and the evolutionary history of the Universe and its structures.”

Original Source: ESO News Release

In the last few decades, scientists have discovered that there is a lot more to the universe than meets the eye: The cosmos appears to be filled with not just one, but two invisible constituents-dark matter and dark energy-whose existence has been proposed based solely on their gravitational effects on ordinary matter and energy.

Now, theoretical physicist Robert J. Scherrer has come up with a model that could cut the mystery in half by explaining dark matter and dark energy as two aspects of a single unknown force. His model is described in a paper titled “Purely Kinetic k Essence as Unified Dark Matter” published online by Physical Review Letters on June 30 and available online at http://arxiv.org/abs/astro-ph/0402316.

“One way to think of this is that the universe is filled with an invisible fluid that exerts pressure on ordinary matter and changes the way that the universe expands,” says Scherrer, a professor of physics at Vanderbilt University.

According to Scherrer, his model is extremely simple and avoids the major problems that have characterized previous efforts to unify dark matter and dark energy.

In the 1970s, astrophysicists postulated the existence of invisible particles called dark matter in order to explain the motion of galaxies. Based on these observations, they estimate that there must be about 10 times as much dark matter in the universe as ordinary matter. One possible explanation for dark matter is that it is made up of a new type of particle (dubbed Weakly Interacting Massive Particles, or WIMPs) that don’t emit light and barely interact with ordinary matter. A number of experiments are searching for evidence of these particles.

As if that weren’t enough, in the 1990s along came dark energy, which produces a repulsive force that appears to be ripping the universe apart. Scientists invoked dark energy to explain the surprise discovery that the rate at which the universe is expanding is not slowing, as most cosmologists had thought, but is accelerating instead. According to the latest estimates, dark energy makes up 75 percent of the universe and dark matter accounts for another 23 percent, leaving ordinary matter and energy with a distinctly minority role of only 2 percent.

Scherrer’s unifying idea is an exotic form of energy with well-defined but complicated properties called a scalar field. In this context, a field is a physical quantity possessing energy and pressure that is spread throughout space. Cosmologists first invoked scalar fields to explain cosmic inflation, a period shortly after the Big Bang when the universe appears to have undergone an episode of hyper-expansion, inflating billions upon billions of times in less than a second.

Specifically, Scherrer uses a second-generation scalar field, known as a k-essence, in his model. K-essence fields have been advanced by Paul Steinhardt at Princeton University and others as an explanation for dark energy, but Scherrer is the first to point out that one simple type of k-essence field can also produce the effects attributed to dark matter.

Scientists differentiate between dark matter and dark energy because they seem to behave differently. Dark matter appears to have mass and to form giant clumps. In fact, cosmologists calculate that the gravitational attraction of these clumps played a key role in causing ordinary matter to form galaxies. Dark energy, by contrast, appears to be without mass and spreads uniformly throughout space where it acts as a kind of anti-gravity, a repulsive force that is pushing the universe apart.

K-essence fields can change their behavior over time. When investigating a very simple type of k-essence field-one in which potential energy is a constant-Scherrer discovered that as the field evolves, it passes through a phase where it can clump and mimic the effect of invisible particles followed by a phase when it spreads uniformly throughout space and takes on the characteristics of dark energy.

“The model naturally evolves into a state where it looks like dark matter for a while and then it looks like dark energy,” Scherrer says. “When I realized this, I thought, ‘This is compelling, let’s see what we can do with it.'”

When he examined the model in more detail, Scherrer found that it avoids many of the problems that have plagued previous theories that attempt to unify dark matter and dark energy.

The earliest model for dark energy was made by modifying the general theory of relativity to include a term called the cosmological constant. This was a term that Einstein originally included to balance the force of gravity in order to form a static universe. But he cheerfully dropped the constant when astronomical observations of the day found it was not needed. Recent models reintroducing the cosmological constant do a good job of reproducing the effects of dark energy but do not explain dark matter.

One attempt to unify dark matter and dark energy, called the Chaplygin gas model, is based on work by a Russian physicist in the 1930s. It produces an initial dark matter-like stage followed by a dark energy-like evolution, but it has trouble explaining the process of galaxy formation.

Scherrer’s formulation has some similarities to a unified theory proposed earlier this year by Nima Arkani-Hamed at Harvard University and his colleagues, who attempt to explain dark matter and dark energy as arising from the behavior of an invisible and omnipresent fluid that they call a “ghost condensate.”

Although Scherrer’s model has a number of positive features, it also has some drawbacks. For one thing, it requires some extreme “fine-tuning” to work. The physicist also cautions that more study will be required to determine if the model’s behavior is consistent with other observations. In addition, it cannot answer the coincidence problem: Why we live at the only time in the history of the universe when the densities calculated for dark matter and dark energy are comparable. Scientists are suspicious of this because it suggests that there is something special about the present era.

Original Source: Vanderbilt University News Release