Archive for December, 2011

December 23, 2011

Cosmic rays from stellar superbubbles

Cosmic rays were discovered almost 100 years ago (1912), yet astrophysicists are still uncertain about where they come from or how they acquire their extremely high energies. Research recently published gives strong evidence that the Cygnus X region, which contains hundreds of very hot, massive, young stars, is a source of cosmic rays and has the means to accelerate them to high energy.

A great deal is known about cosmic rays. They are not electromagnetic radiation (such as gamma rays) but instead consist of charged particles of ordinary matter – electrons, protons, or other atomic nuclei. Of nuclei heavier than hydrogen or helium, the elements represented in cosmic rays occur in proportions close to, but not quite the same as, what is found in typical interstellar gas. A few heavier elements are overrepresented.

The amount of kinetic energy carried by most cosmic rays can range up 1000 TeV (1015 eV), but a small number may be up to 1021 eV. The energy of the highest energy cosmic rays exceeds what could be produced by any known source within our galaxy, so the source is unknown, but likely to be a very energetic active galaxy.

Since cosmic rays are charged particles, their trajectories are bent and twisted by galactic magnetic fields, so there’s no direct way to identify their place of origin by the direction from which they arrive. Possible sources have to be examined individually to determine their ability to produce cosmic rays. For lower energy cosmic rays (under 1000 TeV) the possible sources inside our galaxy include supernova remnants and clusters of very hot, young stars. The new research gives evidence for one instance of the latter.

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December 19, 2011

Possible constraints on dark matter particle mass

Although there is very good indirect evidence for the existence of dark matter, it’s an understatement to say that actual detection of dark matter particles has not been easy. Recent research results from two different teams that both studied gamma rays from dwarf galaxy neighbors of the Milky Way provide an illustration of the difficulty.

Both studies are based on the hypothesis that dark matter consists largely of “WIMPs” (Weakly Interacting Massive Particles). If existing indirect evidence for dark matter can be explained by modifications to the theory of gravity (so dark matter doesn’t actually exist), or if it exists in some form other than WIMPs, then the results of these studies are consistent with either possibility, but otherwise provide no additional information.

A further assumption of the WIMP hypothesis is that WIMPs are Majorana fermions. This is true, at least, if WIMPs are supersymmetric neutralinos. What this means is that WIMPs are their own antiparticles, and hence they are annihilated in any collision with another of their kind.

This annihilation can occur in different ways or “channels”, in which different types of known antiparticle pairs are created, such as b and anti-b mesons or τ+ leptons. These particles subsequently decay and produce gamma rays, which the Fermi Gamma-ray Space Telescope is designed to detect. The latest research shows that gamma-ray emissions that can be detected, from 7 to 10 dwarf satellite galaxies of the Milky Way, are not significantly higher than expected background emissions. Consequently, if indeed dark matter consists of WIMPs, then the individual particles must have masses above some lower bounds.

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December 12, 2011

Long non-coding RNA prevents the death of maturing red blood cells

The main function of RNA was identified early on as being a template – in the form of messenger RNA that reflects the encoding of genes in DNA – for the construction of proteins. A few other forms of RNA were also recognized as playing a well-defined but subsidiary role in this process. However, research within the past five years has established that only 10 to 20% of RNA transcribed from DNA actually serves as a template for proteins.

The function of much of the remaining transcribed RNA is generally unknown. Many of these RNAs are short, with only a few tens of nucleotides, such as microRNA (miRNA). About 1000 different forms of miRNA have beein identified in the human genome, and the effective role of many of these has been discovered.

Longer forms of non-coding RNA, having more that 200 nucleotides, are known simply as long non-coding RNA (lncRNA). Only about 100 have been studied in mammalian tissues so far. The function of only a few of these has been determined. For example, one type is important in regulating stem cells during embryonic development.

Now another lncRNA has been found to play an important role in the maturation of red blood cells.

Long non-coding RNA prevents the death of maturing red blood cells

A long non-coding RNA (lncRNA) regulates programmed cell death during one of the final stages of red blood cell differentiation, according to Whitehead Institute researchers. This is the first time a lncRNA has been found to play a role in red blood cell development and the first time a lncRNA has been shown to affect programmed cell death.

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December 11, 2011

Active star-forming galaxies have substantial halos

Detailed new research shows that there is a distinct correlation between galaxies with large, oxygen-rich gas halos and active ongoing star formation. Although active star formation requires large amounts of available gas, what is surprising is that much, or perhaps even most, of the gas may be in the halo region outside of where most stars are found. Galaxies without substantial halos evidently do not have sufficient gas in the inner regions alone, where most stars exist, to sustain star formation.

Using the Hubble Space Telescope, a survey of 42 galaxies collected information about the distribution of gas in galactic halos that extend far beyond galaxies’ visible stars. Although the gas itself is not visible, its characteristics can be inferred from traces imprinted on the spectra of light from distant quasars. Two other large ground-based telescopes collected information for the same galaxies on their distances, masses due to visible stars, and rates of star formation.

When the two sets of data were compared, some surprises emerged. In galaxies where stars are actively forming, the halos are full of large quantities of oxygen-enriched gas, which may have at least as much mass as is present in the visible stars. However, galaxies not actively forming stars did not have such massive halos.

Keck, Magellan & Hubble Telescopes Find Galactic Recyclers

Among the key findings of the work is that the color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and “feedback” processes by which galaxies acquire gas and then later expel it after processing by stars. The three studies investigated different aspects of the gas recycling phenomenon.

“Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation,” said Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Maryland; a coauthor of one of the Science papers.

The implication is that there must be an efficient process of exchange between gas in a galaxy’s halo, the “circumgalactic medium” (CGM), and the inner region of the galaxy where most stars form and reside: the “interstellar medium” (ISM). Since stars need cold, relatively dense gas in order to form, they cannot form in the warmer, more diffuse CGM, so the gas from there must migrate inward, cool down, and become more dense.

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December 4, 2011

Cygnus X-1 mass and spin determined

Cygnus X-1 was a very puzzling object when it was discovered in 1964, because (as the name suggests) it was an extremely powerful X-ray source. Since X-rays are (fortunately) blocked by the Earth’s atmosphere, the exceptional nature of the object was only recognized when it became possible to do astronomy from above the atmosphere, in this case during a sub-orbital rocket flight. Even today, Cygnus X-1 is one of the strongest persistent X-ray sources known.

At the time of the discovery, black holes were considered to be perhaps nothing more than hypothetical objects. Their existence was allowed for as an admissible solution of the equations of general relativity, but they were considered by many astrophysicists to present such troublesome paradoxes that their actual existence was questionable. For instance, would a “naked singularity” perhaps exist inside a black hole’s event horizon? And would all the information associated with matter falling into a black hole be lost, in contradiction with principles of quantum mechanics?

In the past 50 years, overwhelming evidence has been found for the existence of both stellar-mass black holes (such as the one in Cygnus X-1) and supermassive black holes at the centers of all but the smallest galaxies. Most of the troublesome theoretical paradoxes have also been resolved. There is now abundant observational evidence that Cygnus X-1 is a binary system consisting of a black hole and a blue supergiant star (HDE 226868).


Artist’s conception – credit: Chandra X-Ray Observatory, NASA

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December 2, 2011

Star formation and molecular clouds in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the largest close neighbor of our own galaxy, at a distance of only 160,000 light-years – less than twice the diameter of the Milky Way itself. Its proximity makes it a very useful object to study in connection with the process of star formation, which is generally assumed to occur mainly in giant molecular clouds (GMCs). (The use of the term “cloud” in both cases is coincidental, not indicative of a close similarity. There was other very recent research on GMCs in M33, and the writeup on that provides some background on the topic.)

The diameter of the LMC is about 14,000 light-years, and its mass is only about 1% of the mass of the Milky Way. But it’s still quite a bit larger than the Milky Way’s other satellite galaxies. It’s actually the fourth largest galaxy in the Local Group, in which only the Milky Way, M31, and M33 are larger. It even has some traces of a barred spiral structure, rather than being a pure irregular galaxy.

The LMC hosts active star formation, in spite of its small size compared to the Milky Way, and the research discussed here is the most detailed study so far of the assumed close relationship between star formation and GMCs.

One of the main difficulties of studying star formation is that even though very young stars can be quite hot and luminous, they are also usually enveloped in thick clouds of gas and dust. The typical sign of a hot young star is strong ultraviolet emissions (the Lyman series of hydrogen). These emissions are hidden from view because of their absorption by the gas clouds, yet this same interaction also disperses the clouds. Consequently, by the time very young stars are detectable, the clouds are mostly gone.

However, the dust contained in the clouds is heated by the ultraviolet light that is absorbed, and so there is a characteristic infrared signal from the formation of new stars. Such presumed nascent stars are called “young stellar objects” (YSOs). They can be detected long before the clouds have dispersed. The Spitzer Space Telescope has been the instrument of choice for such studies. A much better correlation between the locations of GMCs and YSOs is to be expected from observations in the infrared than in ultraviolet. That is in fact what has been found, and further supported, in the present study.

GMCs can be detected by millimeter-wavelength emissions of carbon monoxide (CO). The research here represents the highest resolution survey to date of GMCs in the LMC. It was able to identify probable GMCs as small as 45 light-years in size. GMCs are recognized as regions of contiguous CO emissions having the expected structure. Since GMCs can have diameters up to 300 light-years, the obtainable resolution allows for some detection of substructure.

One of the main findings in the present research is that most large GMCs have evidence of active star formation. This implies that star formation begins soon after the formation of the GMC itself. However, there’s still uncertainty about the size and age of observable YSOs and the evidence from CO emissions of GMC characteristics. So there’s more work to do in order to relate the detailed time sequences of GMC and YSO formation. The availability of powerful new millimeter-wave instruments (Atacama Large Millimeter Array) will be a big help.

Another interesting finding is that smaller, less luminous GMCs occur more often than expected.

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December 1, 2011

Magnetic fields may set the stage for birth of new stars

Star formation does not happen as easily as one might suppose from the abundance of stars in a galaxy like the Milky Way, in which around 1000 billion times the mass of the Sun (M) exists in the form of stars. Stars condense out of interstellar gas within the galaxy, but the process is so difficult and complex that in one year, on average, only about 3 M of new stars form. (That could be more than 3 individual stars, since a typical star is less massive than the Sun.)

The process is difficult because most of the available gas is too warm and/or too diffuse for the process to even get started. Astrophysicists have determined that star formation occurs only in gas that is extremely cold, just a few tens of degrees above absolute zero. Because such gas is so cold, much of it exists in the form of molecules such as hydrogen (H2) and carbon monoxide (CO).

The gas needs to be that cold so gravity can overcome the kinetic energy of atoms and molecules in the gas. That allows the gas to become dense enough for the most dense pockets of it to collapse under their own weight and begin the star formation process. In warmer gas the atoms and molecules simply have too much kinetic energy to overcome gravity.

Because of these requirements, most star formation occurs in molecular clouds of gas, in particular the type known as “giant molecular clouds” (GMCs), which contain enough gas to form into stars. Such GMCs are rather large – 30 to 300 light-years in diameter, with masses of 105 to 107 M. The average density of interstellar gas is only about 1 particle (atom or molecule) per cc, but in a GMC the average is 102 to 103 times as high, and can reach 104 to 106 particles per cc in the densest parts (excluding nascent stars).

So the obvious question is: What leads to the formation of a GMC in the first place? New research just published, based on detailed study of the nearby spiral galaxy M33, suggests that galactic magnetic fields may play an important role.

The alignment of molecular cloud magnetic fields with the spiral arms in M33

The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds, thereby regulating cloud accumulation and fragmentation, and affecting the rate and efficiency of star formation. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.

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