Archive for February, 2012

February 27, 2012

The dwarf satellite galaxy problem

Simulations of galaxy formation based on the Lambda-Cold Dark Matter (ΛCDM) cosmological model predict that a large galaxy such as the Milky Way should have many dwarf satellite galaxies, perhaps thousands. However, only about 20 or 30 have been identified. Where are the rest? Are they really there? That question alludes to the “dwarf galaxy problem“.

Astrophysicists suspect that most satellite galaxies are much smaller than the galaxies they orbit. And, in addition, such dwarf galaxies may consist mainly of dark matter, with few visible stars, so they should be very difficult to detect, even if there are a lot of them. Since dwarf galaxies consisting mainly of dark matter are so difficult to find by visible light, there could be enough of them to reconcile the large number of dwarf galaxies that simulations predict to exist with the small number actually observed.

Surprisingly, recent research (Vegetti, et al) has been able to detect a very distant dwarf satellite galaxy by gravitational lensing effects – and from that it is possible to infer that a large number should exist.

The image shows an Einstein ring, which consists of a foreground galaxy (JVAS B1938+666) in the middle, and the distorted image of a more distant galaxy making up most of the ring. A detailed mathematical analysis of the image has confirmed that a minor irregularity in the ring is caused by the presence of a dwarf satellite galaxy of the lens galaxy.

February 18, 2012

An Alzheimer’s disease protein seems to spread via synapses

For what seems like forever, medical scientists have been trying to figure out the relative disease-causing importance of two types of protein often found in the brains of Alzheimer’s disease (AD) victims: tau and amyloid-beta (Aβ). While the larger question is still unresolved, recent research may have made a significant discovery about the tau protein.

The research indicates mechanisms involved in the very early stages of AD, so it may lead to diagnostic tests that can signal the beginnings of the disease long before symptoms become apparent. This might allow for timely applications of preventive therapies, and might even contribute to the discovery of such therapies.

Tau is associated with Alzheimer’s because it is found in the form of neurofibrillary tangles (NFTs) in the neurons of the autopsied brains of Alzheimer’s victims. (Such tangles are also found in neurons affected by other neurodegenerative diseases. Collectively these diseases are called tauopathies.)

February 8, 2012

How did some early black holes get so big so fast?

The supermassive black holes (SMBHs) found in the centers of large galaxies can be astonishingly large. The closest example to us is in the giant elliptical galaxy M87, and it’s estimated to be 6.6 billion solar masses (M). More distant examples can be even larger, more than 10 billion M (at distances ~300 million light-years).

Those are extremes. 1 or 2 billion M SMBHs are a little more common in our neighborhood, though still rare. Rather surprisingly, however, SMBHs that large can be found even in the very early universe. The largest yet discovered is about 2 billion M, and it’s 12.9 billion light-years away, at a redshift z=7.085. That SMBH reached its observed size only 765 million years after the big bang, i. e. perhaps 500 million years after the very first stars formed. It’s been a difficult problem to understand how SMBHs that large could have formed so quickly. A recently announced computer simulation of a large part of the very early universe may have come up with a good answer.

It is only barely possible to detect very bright objects (such as quasars or large galaxies) at redshifts z~7 with the best telescope technology today, and impossible to detect less bright objects (even the brightest stars) or objects at higher redshifts. So direct observation of the earliest stars – which may have begun to form as early as z~30, 100 million years after the big bang – is currently impossible, and computer simulations must be used to understand their properties and the process in which they formed.