* Astronomy

Astronomers studying new images of a nearby galaxy cluster have found evidence that high-speed collisions between large elliptical galaxies may prevent new stars from forming, according to a paper to be published in a November 2008 issue of The Astrophysical Journal Letters.
Led by Jeffrey Kenney, professor and chair of astronomy at Yale, the team saw a spectacular complex of warm gas filaments 400,000 light-years-long connecting the elliptical galaxy M86 and the spiral galaxy NGC 4438 in the Virgo galaxy cluster, providing striking evidence for a previously unsuspected high-speed collision between the galaxies.

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An international team of researchers led by Mederic Boquien of the University of Massachusetts Amherst has shown that debris formed when two galaxies collide makes a simpler, more accessible laboratory for studying the process of star formation. The team presented their results at a press conference Monday, June 2 at the American Astronomical Society meeting in St. Louis, Missouri.

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Title: HST/ACS Observations of Star Formation Driven Outflows in Nearby Edge-on Spiral Galaxies: Dependence of Halo Morphology on Star Formation Activity
Authors: Joern Rossa (University of Florida), Michael Dahlem (CSIRO/ATNF), Ralf-Juergen Dettmar (AIRUB), Roeland P. van der Marel (STScI)

 We present new high spatial resolution narrowband imaging observations of extraplanar diffuse ionized gas (eDIG) in four late-type, actively star forming edge-on spirals, obtained with ACS on-board HST. Our F658N (H-alpha) observations reveal a multitude of structures on both small and large scales. Whereas all four galaxies have been studied with ground-based telescopes before, here the small scale structure of the extended emission line gas is presented for the first time at a spatial resolution of 0.05", corresponding to 5.0 pc at the mean distance to the target galaxies. The eDIG morphology is very different for all four targets, probably as a result of their different levels of star formation activity. We find that the morphology of the eDIG, in particular the break-up of diffuse emission into filaments in galaxy halos, shows a strong dependence on the level of star formation activity per unit area, and eDIG can be arranged into a morphological sequence. NGC4634 and NGC5775 have the highest SF rate per unit area in our sample and the observed morphology suggests that the break-up of the smooth eDIG layer into individual resolved filaments occurs only above a certain threshold of SF activity per unit area. Combined with ground-based data for samples that span a larger range of galaxy mass our results indicate that the gravitational potential also plays an important role in the eDIG morphology. In low-mass galaxies the gas can be expelled due to shallow gravitational potentials more easily and couple with strong star formation driven outflows on a local scale. This is in contrast to the more massive galaxies, which show smooth eDIG layers, unless they are powered by a superwind, as in the case of nucleated starburst galaxies.

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 Life and death in the universe      
Bonn Astronomers simulate the formation and disintegration of star clusters     
Stars always evolve in the universe in large groups, known as clusters. Astronomers distinguish these formations by their age and size. The question of how star clusters are created from interstellar gas clouds and why they then develop in different ways has now been answered by researchers at the Argelander Institute for Astronomy at the University of Bonn with the aid of computer simulations. The scientists have solved `at least at a theoretical level` one of the oldest astronomical puzzles, namely the question of whether star clusters differ in their internal structure. The findings have now been published in the science journal "Monthly Notices of the Royal Astronomical Society" (MNRAS 380, 1589).

Astronomical observations have shown that all stars are formed in star clusters. Astronomers distinguish between, on the one hand, small and, by astronomical standards, young star clusters ranging in number from several hundred to several thousand stars and, on the other, large high-density globular star clusters consisting of as many as ten million tightly packed stars which are as old as the universe. No one knows how many star clusters there might be of each type, because scientists have not previously managed to fully compute the physical processes behind their genesis.
Stars and star clusters are formed as interstellar gas clouds collapse. Within these increasingly dense clouds, individual "lumps" emerge which, under their own gravitational pull, draw ever closer together and finally become stars. Similar to our "solar wind", the stars send out strong streams of charged particles. These "winds" literally sweep out the remaining gas from the cloud. What remains is a cluster that gradually disintegrates until its component stars can move freely in the interstellar space of the Milky Way.
Scientists believe that our own sun arose within a small star cluster which disintegrated in the course of its development.

"Otherwise our planetary system would probably have been destroyed by a star moving close by" -  Professor Dr. Pavel Kroupa of the Argelander Institute for Astronomy at Bonn University.

 In order to achieve a better understanding of the birth and death of stellar aggregations Professor Kroupa and Dr. Holger Baumgardt have developed a computer programme that simulates the influence of the gases remaining in a cluster on the paths taken by stars.
The main focus of this research has been on the question of what the initial conditions must look like if a new-born star cluster is to survive for a long time. The Bonn astronomers discovered that clusters below a certain size are very easily destroyed by the radiation of their component stars. Heavy star clusters, on the other hand, enjoy significantly better "survival chances".
For astronomers, another important insight from this work is that both light and heavy star clusters do have the same origins. As Professor Kroupa explains,

"It seems that when the universe was born there were not only globular clusters but also countless mini star clusters. A challenge now for astrophysics is to find their remains" - Dr. Pavel Kroupa.

The computations in Bonn have paved the way for this search by providing some valuable theoretical pointers.
The Argelander Institute has recently been equipped with five "GRAPE Computers", which operate at speeds 1,000 times higher than normal PCs. They are being deployed not only in research but also for research-related teaching: "Thanks to the GRAPE facilities, our students and junior academics are learning to exploit the power of supercomputers and the software developed specially for them."
The Argelander Institute is regarded world-wide as a Mecca for the computation of stellar processes. Despite their enormous calculating capacity, the machines require several weeks to complete the simulation.

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