A simple survey yields a cosmic conundrum
Quasars are thought to be powered by accretion of material onto supermassive black holes in the centers of distant galaxies. Gamma-ray bursts, the death throes of massive stars, are the most energetic explosions in the universe. But there is no reason to expect galaxies in the foreground to have any association with these background light sources.
"The result contradicts our basic concepts of cosmology, and we are struggling to explain it," said Jason X. Prochaska, associate professor of astronomy and astrophysics at the University of California, Santa Cruz.
Prochaska and graduate student Gabriel Prochter led the survey, which used data from NASA's Swift satellite to obtain observations of the transient, bright afterglows of long-duration gamma-ray bursts (GRBs). They described their findings in a paper accepted for publication in Astrophysical Journal Letters. The paper, which could have strange cosmological implications, has been a source of significant debate among astronomers throughout the world.
The study is based on a fairly straightforward concept. When light from a GRB or a quasar passes through a foreground galaxy, the absorption of certain wavelengths of light by gas associated with the galaxy creates a characteristic signature in the spectrum of light from the distant object. This provides a marker for the presence of a galaxy in front of the object, even if the galaxy itself is too faint to observe directly.
Prochter and Prochaska analyzed 15 GRBs in the new study and found strong absorption signatures indicating the presence of galaxies along 14 GRB sightlines. They had previously used data from the Sloan Digital Sky Survey (SDSS) to determine the incidence of galaxies along the sightlines to quasars. Based on the quasar study, they would have predicted only 3.8 galaxies instead of the 14 detected along the GRB sightlines.
The quasar analysis was based on more than 50,000 SDSS observations, so the data for quasars are much more robust statistically than the data for GRBs, Prochaska said. Nevertheless, the probability that their results are just a statistical fluke is less than about one in 10,000, he said.
The researchers examined three potential explanations for the inconsistency. The first is obscuration of some quasars by dust in galaxies. The idea is that if a quasar is behind a dusty galaxy it wouldn't be seen, and this could skew the results. "The counter argument is that with this huge database of quasar observations, the effect of dust has been well characterized and it should be minimal," Prochter said.
Another possibility is that the absorption lines in the GRB spectra are from gas ejected by the GRBs themselves, rather than from gas in intervening galaxies. But in nearly every case when researchers have taken a closer look in the direction of the GRB, they have in fact found a galaxy at the same position as the gas.
The third idea is that the intervening galaxy may act as a gravitational lens, enhancing the brightness of the background object, and that this effect is somehow different for GRBs than for quasars. Although Prochaska said he prefers this explanation, several factors make strong lensing of the GRBs seem unlikely.
"Those who know more about gravitational lensing than I do tell me it's unlikely to be the answer," Prochaska said....
NASA Scientists Conduct Census of Nearby Hidden Black Holes
The observation implies that if these hidden black holes exist---and most scientists are convinced they do---they must be from the more distant, earlier universe, a concept that has interesting implications for galaxy evolution.
This work constitutes the first census of the highest-energy part of the X-ray sky, where the most dust-enshrouded black holes are thought to shine. A team from NASA's Goddard Space Flight Center in Greenbelt, Md., conducted the census, comprised of nearly two years of continuous data from the European Space Agency's International Gamma Ray Astrophysics Laboratory, or INTEGRAL, satellite.
"Naturally it is difficult to find something we know is hiding well and which has eluded detection so far," said Volker Beckmann of Goddard and the University of Maryland, Baltimore County, lead author on a report in an upcoming issue of The Astrophysical Journal. "INTEGRAL is a telescope that should see nearby hidden black holes, but we have come up short." . . .
There is, of course, a thread in the Main News forum covering the suggestion that black holes don't form at all but rather another exotic object known as the MECO does instead.
Supernovae make dark matter bloat
More than 80 per cent of the matter in the cosmos is in some strange undetected form. We can see the gravitational effect of this dark matter, but cannot identify it. Computer simulations have reconstructed how dark matter clumped together with normal gas in the early universe to form small galaxies and how these small galaxies merged over billions of years to create huge star systems like the Milky Way.
The simulations raised one mystery, however. They showed that the density of dark matter should spike sharply at the centre of galaxies, while observations of the motions of stars reveal that the dark matter cores of galaxies today are much more puffed out, with densities that are constant over thousands of light years. "We have known about this problem for more than 10 years," says Sergey Mashchenko from McMaster University in Hamilton, Ontario, Canada.
Astronomers have suggested several possible fixes for this anomaly. For instance, they thought that dark matter might puff outwards because some kind of exotic force between the particles - other than gravity - makes them collide and scatter off each other like snooker balls. Now Mashchenko and his colleagues have shown that the smoothing of the density of dark matter is down to the explosions of massive stars at the end of their lives. At their peak, these supernovae can outshine their host galaxies.
Mashchenko thought that shock waves from supernovae should churn up interstellar gas in a galaxy, and that the gravitational disturbances created by this sloshing gas should in turn smooth out the spike of dark matter at the centre. To test this, his team used a supercomputer to simulate the evolution of a small, primordial galaxy that started off with a central spike in the density of dark matter. Sure enough, just 80 supernovae explosions per million years - typical of values expected in dwarf galaxies today - were enough to smooth out the dark matter spike to match observations if they continued for at least 100 million years or so (Nature, vol 443, p 539).
The same model can also solve another conundrum. The cosmos has far fewer dwarf galaxies - which contain several billion stars compared with the hundreds of billions in larger galaxies - than cosmological simulations predict. In Mashchenko's simulations, because supernovae smoothed out the spike in dark matter density at the centre, dwarf galaxies were less tightly bound together by their own gravity. Any encounters with bigger galaxies would therefore have easily torn many dwarfs apart, and this may explain their paucity