Archive for November, 2011

November 23, 2011

Most stars in dwarf galaxies formed early in the universe

A study based on a survey of very distant galaxies selected 69 individual objects that had exceptionally bright emissions lines, suggesting that they were galaxies very actively forming new stars. Out of that number, four objects for which the necessary measurements could be made proved to have strong lines due to ionized oxygen. In these cases it was possible to conclude that the objects were relatively small galaxies whose mass was about 100 million solar masses (M) – and they were forming stars so rapidly that the mass due to stars would double in just 10 million years – an average of 10 M per year. That compares to a rate of about 3 M per year at present in the Milky Way – even though it is 1000 times as massive as the early galaxies studied.

The galaxies studied were at a redshift z~1.7, which corresponds to a light-travel distance of 11.5 billion light-years, so the galaxies are seen as they were about 2.2 billion years after the big bang. Other surveys have suggested that this era was when the rate of star formation everywhere was near its peak. From the observed numbers of small galaxies of ~108 M and the star formation rates at z~1.7 it can be estimated that most of the stars in present-day dwarf galaxies (from 108 M to 109 M) formed in a few short bursts from 1 to 5 billion years after the big bang.

Simulations of star formation in dwarf galaxies suggests that rapid star formation activity occurs in bursts. This is because when star formation occurs most rapidly the combined energy emitted by the hot young stars quickly either heats gas inside the galaxy to a temperature too high for new stars to form or expels the gas entirely. Only after enough time has passed following the burst does the gas cool off enough and fall back into the galaxy to restart the cycle. So the 4 galaxies most carefully studied are probably not representative, since they were more likely to have been noticed than similar galaxies not in the middle of a burst of star formation.

According to the research paper:

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

Our discovery of an abundant galaxy population at z ~ 1.7 with extremely high emission line equivalent widths implies that many high-redshift, low-mass galaxies form many of their stars in extreme starbursts. We propose that we have observed an important formation mode for dwarf galaxies: a small number of strong starbursts that occur at early epochs (z > 1) each form ~108 M in stars in a very short time span (~30 Myr) to build up the bulk of the stellar components of present-day dwarf galaxies. This is in quantitative agreement with ’archaeological’ studies of present-day dwarf galaxies, which have shown that their star formation histories are burst-like and that the ages of their stellar populations suggest formation redshifts z > 1. Under the reasonable assumption based on ΛCDM predictions for galaxy growth that the observed galaxies grow in mass by less than an order of magnitude up to the present day, our observations provide direct evidence for such an early formation epoch and, in particular, that short-lived bursts contribute much or even the majority of star formation in dwarf galaxies.

Further reading:

Hubble Uncovers Tiny Galaxies Bursting with Starbirth in Early Universe

Population explosion in dwarf galaxies

Tiny galaxies bursting with stars

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

November 21, 2011

Neutron star research points to different classes of supernovae

Classifying things is the starting point for almost all scientific fields – from flowers to fundamental particles. Once one has classes the next step is to find subclasses, and then sub-subclasses. Finding correlations between different classification schemes, then, often leads to significant understandings.

Neutron stars are not stars in the normal sense. They are remnants composed entirely of neutrons left after a star larger than the Sun, but not too large, explodes as a supernova at the end of its life as a star. There are different types of both neutron stars and supernovae.

Consider neutron stars first. When a star whose inner core is more massive than the Chandrasekhar limit – about 1.4 M (M denoting the mass of the Sun) – exhausts its nuclear fuel it collapses as a supernova because it is too heavy to support itself through degeneracy pressure. If the material remaining after the explosion is less than about 3 M (the Tolman–Oppenheimer–Volkoff limit) the remnant is a neutron star. Otherwise the result is a black hole. Typically, the total mass of the progenitor of a neutron star is in the range of 5 to 15 M.

Since a neutron star can no longer release energy from thermonuclear reactions, it may radiate very little electromagnetic energy. Consequently it may be rather difficult to detect, like a black hole, unless it’s a member of a binary system, so that there are visible gravitational effects on the companion.

However, some neutron stars may have energy sources that allow them to emit electromagnetic radiation at wavelengths all the way from radio to X-rays or even gamma rays. If the neutron star also has a strong magnetic field, such emissions may be observable in periodic pulses occurring at frequencies from a thousandth of a second on up. In this case the neutron star is a pulsar.

There are different types of pulsars too, depending on the energy source.

November 20, 2011

Researchers Create a Pituitary Gland from Scratch

The development of a complex multicellular organism from a single-cell zygote into a complete animal (or plant) is somewhat of a neat trick. It’s impressive that from the zygote perhaps a couple hundred different types of cells can emerge, with each of the trillions of final cells at the right time and place. What’s just as surprising is that the whole process is so neatly programmed by evolution into an organism’s genome that it happens automatically as the appropriate set of genes gets turned on at just the right time.

Developmental biologists have been studying the process for decades. What they’ve found is that cells at any particular stage not only make up specific types of embryonic tissue, but are also programmed to turn on genes for the next generation of cells based on the types of tissue and nearby tissue they occur in.

Biologists have now learned enough about the details of this program that they can make it work – for certain tissues and organs – in a lab dish instead of a complete embro, starting from pluripotent stem cells.

In research just published, the organ was a mouse pituitary gland, a very small organ, but with complex function. In humans it’s about the size of a pea and weighs only half a gram. But it secretes dozens of different endocrine hormones.

It’s particularly important that the gland has a 3-dimensional structure that’s essential to its function. Being able to grow a pituitary gland from stem cells is a very significant achievement towards eventual regenerative medicine, in which larger and more complex organs such as kidneys or even hearts can be grown from stem cells.

Self-organized pituitary-like tissue from mouse ES cells

The possibility that functional, three-dimensional tissues and organs may be derived from pluripotent cells, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), represents one of the grand challenges of stem cell research, but is also one of the fundamental goals of the emerging field of regenerative medicine. Developmental biology has played a central role in informing such efforts, as it has been shown that stem cell differentiation can be directed to follow a given lineage pathway by culturing stem cells in conditions that recapitulate the specific cellular and molecular environment from which such cells normally emerge during embryogenesis. Intriguingly, recent work has shown that when ES cells are cultured under the appropriate conditions, they can be driven to self-organize into complex, three-dimensional tissue-like structures that closely resemble their physiological counterparts, a remarkable advance for the field.

New work by Hidetaka Suga of the Division of Human Stem Cell Technology, Yoshiki Sasai, Group Director of the Laboratory for Organogenesis and Neurogenesis, and others has unlocked the most recent achievement in self-organized tissue differentiation, steering mouse ESCs to give rise to tissue closely resembling the hormone-secreting component of the pituitary, known as the adenohypophysis, in vitro.

Not only did the lab-grown pituitary tissue have much of the appropriate physiological activity, but when transplanted into mice whose pituitary gland had been removed, the mice survived much better than controls.

Further reading:

A Gland Grows Itself

Researchers Create a Pituitary Gland from Scratch

Pituitary glands from stem cells

Self-formation of functional adenohypophysis in three-dimensional culture

November 19, 2011

Overactive Neurons May Tangle the Senses… But There’s More to It

Synesthesia is pretty interesting. In the most common form certain colors are perceived when particular associated letters or numerals are seen (even if they are not actually colored themselves). Other more unusual forms may (for example) associate colors with the performance of certain types of swimming strokes, or involve seeing two distinct colors at the same time.

When this happens, it’s as though some wires are crossed in the brain. Research just published suggests that an effect consistent with that idea and somewhat but not entirely like it is actually happening. It seems to occur in people in whom some of their perceptual neurons are too easily excitable. Consequently, higher levels in the perceptual system register more stimuli than are actually present, as the brain attempts to make sense of the inconsistent lower level data.

The research considered the commmon type of synesthesia – grapheme-color synesthesia – where colors are associated with particular letters and numbers.

Synaesthesia sends visual cortex crazy

Terhune and colleagues, stimulated the visual cortex of six grapheme-colour synaesthetes by applying a magnetic coil to the scalp to produce a weak magnetic field.

They found non-synaesthetes required three times greater magnetic stimulation to their visual cortex than synaesthetes in order to experience phosphenes, transient flashes of light or other visual disturbances.

Terhune says the study is the first to show that synaesthesia is linked to hyperexcitability in the area of the brain known as the visual cortex.

“One of the really interesting things about this study is the difference in the level of excitability [between synaesthetes and non-synaesthetes] is so great,” says Terhune.

But there must be more to synesthesia than hyperactive neurons.

November 19, 2011

Favored Higgs hiding spot remains after most complete search yet

Are we there yet? Well, no, not quite. Just be patient and try counting the cows, or something.

Despite a number of rumors and preliminary results, the Higgs boson hasn’t yet been found. There’s not even a clear signal yet that could be the real thing, with only a bit more data needed for better statistical confidence.

However, the possible range in energy space where the Higgs could hide keeps shrinking. The search parties are closing in. And it seems meaningful that so far everyone sees the same range where the Higgs might be.

Favored Higgs hiding spot remains after most complete search yet

The CMS and ATLAS experiments at the Large Hadron Collider have backed the Standard Model Higgs boson, if it exists, into a corner with their first combined Higgs search result.

The study, made public today, eliminates several hints the individual experiments saw in previous analyses but leaves in play the favored mass range for the Higgs boson, between 114 and 141 GeV. ATLAS and CMS ruled out at a 95 percent confidence level a Higgs boson with a mass between 141 and 476 GeV. …

“I think it could be an interesting message the data is telling us,” said physicist Eilam Gross of the Weizmann Institute of Science, who shares leadership of the ATLAS experiment’s Higgs group. “Any discovery starts with the inability to exclude.”

Several related measurements indirectly suggest a Standard Model Higgs boson exists at the lower end of the mass range.

It is possible, however, that some answer, based on data already collected, may come out before the end of the year.

Higgs hunt enters endgame

Analysis of the very latest data from this autumn — which Murray isn’t yet ready to share — will scour the range that remains. If it turns out to be empty, physicists may have to accept that the particle simply isn’t there. Working around the clock, the detector teams hope to have this larger data set analysed before the end of December. “We’ll know the outcome within weeks,” says Guido Tonelli, spokesman for the CMS detector.

November 19, 2011

Climate panel says prepare for weird weather

This certainly won’t change many skeptics’ minds… but for everyone else it further emphasizes the dangers (as if more were needed).

What mainly motivates the skeptics, it seems, is fears about the costs of reducing emissions of greenhouse gases, especially CO2. That will certainly entail costs, but even those are not easy to predict. Ironically, and unfortunately for the companies involved, the cost of photovoltaic energy has been plummeting, as the required materials (especially polysilicon) have become much cheaper. This, along with limitations on demand due to economic turbulence (especially in Europe), has put a number of solar companies out of business. But the trend is clear: photovoltaic energy, and other forms of solar energy, just keep getting more affordable. The exact opposite is true for fossil fuels, even without taking account of global warming and other externalities.

It’s not so hard to predict that there will be significant effects of accelerated global warming, but it’s harder to predict the timing and magnitude of the effects and their costs. Some costly effects seem pretty likely, such as extended periods of drought in many places, loss of habitable land, due to sea level rise, where many millions of people now live, poorer health and more deaths due to higher temperatures and severe shortages of food and potable water. And so forth.

And then there are the wildcards associated with extreme weather – severe flooding, more frequent and more intense hurricanes and tornadoes, forest fires resulting from prolonged heat waves.

Some of these extremes appear to be nearly certain, while others remain unpredictable possibilities. But even the possibilities, and their costs, need to be accounted for in terms of increased statistical expectation. You don’t know how much you’ll lose during a weekend at the Las Vegas casinos, but if you look at the odds (and the profits the casinos make), you can calculate the expected loss.

Climate panel says prepare for weird weather

Extreme weather, such as the 2010 Russian heat wave or the drought in the horn of Africa, will become more frequent and severe as the planet warms, the Intergovernmental Panel on Climate Change (IPCC) warns in a report released today. Some areas could become “increasingly marginal as places to live in”, the report concludes.

It is “virtually certain” — meaning 99–100% probability in IPCC terminology — that the twenty-first century will see an increase in the frequency and magnitude of warm temperature extremes and a decrease in cold extremes.

It is much less clear, however, how climate change will affect rainfall, flood risk and storminess.

And these weather events aren’t independent things either. Extremes in one part of the global weather system can lead to extremes in other parts. For example, the 2010 Russian heat wave and the severe flooding in Pakistan. (References: here, here)

It’s unfortunate that people don’t have too much trouble understanding the need to prepare for unpredictable events like earthquakes or even asteroid impacts – yet there’s so much reluctance to do anything about far more predictable outcomes associated with extreme weather.

Further reading:

Science panel: Get ready for extreme weather

IPCC Report: Global Warming—and Changing Population—Will Worsen the Toll of Extreme Weather

U.N. Panel Finds Climate Change Behind Some Extreme Weather Events

IPCC says it again – more confidently – expect extreme weather more often

November 13, 2011

Learning why BRCA1 mutations lead to breast cancer

BRCA1 is a gene whose name is among the most familiar to the general public of all genes – for the unfortunate reason that mutations of BRCA1 are associated with significantly increased hereditary risk for breast cancer (as the name suggests). Strangely, however, it’s still not known exactly how mutations of BRCA1 confer this greater risk for cancer. But recent research has narrowed down the possibilities.

The protein that the gene codes for, BRCA1, is a fairly long chain of 1863 amino acids. BRCA1 is known to be involved in repair of damaged DNA. If BRCA1 is defective (due to a gene mutation), the failure to properly repair DNA (which can become damaged for many possible reasons) can lead to a cell becoming cancerous.

Previous research has identified around 1500 different mutations of BRCA1 associated with cancer risk. But most of these mutations directly affect the amino acids of only two different regions of BRCA1. Every protein has an amine group at one end of its chain and a carboxyl group at the other end. One of the two regions in which mutations lead to cancer is at the amino end and is called the “RING domain”. The other region, near the carboxyl end, is called BRCT (BRCA1 carboxyl-terminal tandem repeats).

The RING domain is known to have a role in the process that attaches a marker called ubiquitin to a protein in order to identify the protein as ready for recycling. The BRCT region is associated with the process of protein phosphorylation, which is a key element of chemical signaling within cells. What hasn’t been known is what aspects of DNA repair are disrupted by defects in one or both of these regions.

November 11, 2011

Galaxy interactions accelerate the growth of supermassive black holes

It’s now well-known that there’s a rough correlation between a galaxy’s size and the size of its central supermassive black hole (SMBH). The correlation is even better (for spiral galaxies) between the black hole size and the size of the central bulge of the galaxy. It’s been found that the mass of a SMBH is generally close to 1/1000th of the mass of the central bulge. So in some way or other, it seems that galaxies and their black holes grow together, although it’s still unclear whether the galaxy or the black hole takes the lead.

The question can be asked in a different way. Which factors contribute to galaxy growth and which to black hole growth? The problem is that black holes usually can’t be observed, or even detected, directly. About the only way to find SMBHs is when they are part of an “active galaxy”, in the form of an active galactic nucleus (AGN). AGNs represent a stage in which large amounts of gas and dust are being swept into the vicinity of a black hole, before actually falling into the black hole. This is a fairly unusual state, which occupies perhaps only about 1% of a galaxy’s life time, if it occurs at all. Nevertheless, AGN are commonly used as indicators of a growing SMBH. It’s certainly true that SMBHs grow while its galaxy is “active”.

One obvious mechanism for the simultaneous growth of galaxies and their SMBHs is merger between two (or more) galaxies. This has been the favored theory for some time, and a number of studies provide some evidence for its occurrence. Recent examples: here, here, here, here, here, here.

However, more recently, other studies suggest that mergers may not be the most common cause of AGNs: here, here.

Cleary, this is still an issue that’s as yet very much unresolved. The latest research to appear adds more evidence that galaxy mergers – and probably even close encounters – do contribute to the growth of SMBHs and stimulate flare-ups of AGNs.

November 10, 2011

Controlling cancer cells by weakening their defense against reactive oxygen species

Reactive oxygen species (ROS, sometimes loosely referred to as “free radicals”) are generally harmful to cells. Persistent high levels of ROS may damage cells and, eventually, cause cancer. But ROS are also harmful to cancer cells – which, therefore, often develop means of defending themselves against ROS.

Shielding Cancer Cells from Damage

By activating an enzyme involved in the breakdown of glucose, researchers were able to slow the growth of lung cancer cells and increase the harm inflicted by reactive oxygen species (ROS)—byproducts of normal metabolism that can cause damage to the cell in high concentrations. The findings, published Thursday (November 3) in Science Express, could one day be used to make cancer cells more susceptible to cancer treatments and minimize tumor growth.

It’s long been known that cancer cells often have an alternate form of the enzyme pyruvate kinase M1 (PKM1) – PKM2. The new research shows why this is: cancer cells can control the activity of PKM2, which is not possible with PKM1, and this control helps them defend against ROS. Cancer cells need such a defense, because their metabolism differs from normal cell metabolism and is more active, tending to result in higher ROS levels.

The way this works is that both PKM1 and PKM2 play an important role in the glycolysis process, which metabolizes glucose. Glutathione is an antioxidant molecule that can help neutralize ROS. But it also participates in glycolysis, which diverts it from acting as an antioxidant. Cancer cells favor PKM2 over PKM1, because they can lower its level of activity, and therefore leave more glutathione available for antioxidant work.

The researchers equipped cancer cells with a mutant form of PKM2 that, like PKM1, the cells could not control. Such cells were found to sustain more damage from ROS. And when altered cells were implanted in mice, they grew smaller tumors than unmodified cancer cells.

There are fairly obvious ways that these findings may help develop better cancer therapies.

November 9, 2011

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

According to observations of a very distant gamma-ray burst (GRB) recently reported, it appears that a couple of galaxies in the early universe, only 1.8 billion years after the big bang, contain a higher concentration of some elements heavier than hydrogen and helium than the Sun does. This is rather surprising, since the Sun is about 4.5 billion years old, and formed out of gas and dust in which heavy elements had been accumulating for about 9 billion years.

Isaac Asimov supposedly remarked, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’, but ‘That’s funny …'” That may be appropriate in this case. How did those galaxies have all those heavy elements so early?

The trigger for the observation was a GRB detected in March 2009 and designated GRB 090323. This was the type of GRB known as a “long” GRB, because the brightest phase lasts more than two seconds, and there is a diminishing afterglow that may last days. (The “short” type lasts less than two seconds and shows no afterglow.) Long GRBs are thought to result from supernova events in which a jet of relativistic particles is aimed in our direction.

Although the burst was determined to be quite distant (redshift z=3.57, corresponding to a light travel time of about 11.9 billion years), there was nothing about it at first that seemed especially unusual. However, an analysis of the spectrum of the afterglow indicated that the light from the GRB had passed through two galaxies relatively close to each other, one of which may have been the host of the GRB. What was especially odd was that the spectrum showed the presence of higher concentrations of zinc and sulfur than occur in the Sun.

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323, was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The authors of the research paper point out that “These are the highest metallicities ever measured in galaxies at z > 3.” (“Metallicity” refers to the relative abundance of heavy elements.) But how unusual is this, really?