* Astronomy

Title: Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks
Authors: Frederic Bournaud (1), Chanda J. Jog (2), Francoise Combes (3) ((1)AIM, CEA-Saclay (2) IISc Bangalore (3) Observatoire de Paris LERMA)

Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occurred several times for most present-day galaxies. However the effect of repeated minor mergers on galactic structure and evolution has not been studied systematically so far. In this paper, we present a numerical study of multiple, subsequent, minor galaxy mergers, with various mass ratios ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star formation. We study the morphological and kinematical properties of the remnants, and show that several so-called "minor" mergers can lead to the formation of elliptical-like galaxies, that have global morphological and kinematical properties similar to that observed in real elliptical galaxies. The properties of these systems are compared with that of elliptical galaxies produced by the standard scenario of one single major merger. We thus show that repeated minor mergers can theoretically be a process to form elliptical galaxies without major mergers, and can be more frequent than major mergers in particular at moderate redshift. This process must then have formed some elliptical galaxies seen today, and we find in particular that it could explain the high boxiness of massive ellipticals, and some fundamental relation observed in ellipticals. In addition, because repeated minor mergers, even at high mass ratios, destroy disks into spheroids, these results indicate that spiral galaxies cannot have grown only by a succession of minor mergers.

Read more   (635kb, PDF)

...In the category of "useful if you're an astrophysicist" comes a set of animations based on computer models of major astronomical events, such as the collisions of galaxies. The astronomer involved, John Dubinski, decided that the visuals his models were producing were so compelling, they belonged on film. So, he collaborated with a composer to score the models in action, posted a number on the web, and put those and others on a DVD.
Read more

A new study by a researcher at The University of Nottingham has provided the first observational evidence of how massive galaxies formed. The results of this study have major implications for many other areas of research and are being used by astronomers to explain seemingly unrelated processes such as how massive black holes and the universe's stars came to be.

The research, led by Dr. Christopher J. Conselice of the University's School of Physics and Astronomy, is published in the February 20th edition of the Astrophysical Journal.
It uses the deepest images taken by the Hubble Space Telescope to study galaxies when they were only two billion years old. His team has found that the majority of the most massive galaxies in the early universe are undergoing multiple and spectacular mergers.
These mergers lead to the creation of new stars from colliding gas clouds and likely feed and grow the masses of black holes lurking in the centre of all galaxies.
The work is helping to definitively confirm what scientists have long hoped for - massive galaxies form when smaller galaxies merge together - a major and previously unconfirmed prediction of the cosmological standard model.

"The results show us that the most massive galaxies we see in today's universe, which are passive and old, were once undergoing rapid mergers with each other, which it turns out is how they form" - Dr. Christopher J. Conselice.

While distant galaxies have been studied for over a decade, it has until now remained a mystery how they evolved into the galaxies we see today. Young galaxies have very low masses and astronomers have long been puzzled by how these systems turn into massive galaxies in the local universe.

The Conselice results demonstrate that a typical massive galaxy in today's universe has undergone four to five mergers with other galaxies to transform from these young low mass systems into the giant galaxies.


Examples of galaxies in the early universe, a few billion years after the Big Bang. These galaxies are forming through violent mergers, which are likely driving the star formation and black hole formation in these galaxies.
Credit The University of Nottingham

These mergers are very rare today, with only about one per cent of galaxies merging, whereas 10 billion years ago, nearly all massive galaxies were undergoing mergers. An analysis technique developed by Conselice over a period of more than 10 years was used on the deepest images ever taken of the universe to make these discoveries.
The results further show that massive galaxies did not form rapidly, within a few million years after the Big Bang, neither did they form gradually over an extended period of time. In a surprising finding, almost all of this merger activity occurred from the birth of the universe to about six billion years ago.

"Perhaps the most amazing thing about these results is that massive galaxy formation is largely over when the universe is half its current age. This means that all this merging activity was somehow curtailed by an unknown process" - Dr. Christopher J. Conselice.

The research may hold clues about the formation of our own galaxy. The Milky Way contains spiral arms, which are not thought to form through the merger process. However, at the centre of our galaxy is a spherical system of stars called a bulge - a high-density region featuring many old stars and a massive black hole, which probably formed as a result of these mergers.
The research could also help astronomers to see into the Milky Way's future - it is possible that our galaxy will itself merge with Andromeda, our nearest neighbouring large galaxy in around a billion years from now. This would see the destruction of the spiral disk that surrounds the bulge and change dramatically the shape of our galaxy, as well as significantly altering the positions of stars we see in the night sky.

Source

-- Edited by Blobrana at 12:52, 2006-02-20