News 2006

Jun 12, 2006
Simulations show that they can survive giant collisions

Galaxies naines

Many small galaxies called "dwarf galaxies" are found orbiting massive galaxies like our Milky Way. They all contain less than one billion stars, hundred time less than the massive galaxies. Where do these tiny galaxies come from ? In standard cosmological scenarios, they are assumed to be formed early in the history of the Universe but high resolution numerical simulations performed by a team from the Paris Observatory and the Astrophysical Department of CEA/DAPNIA now show that things are not so simple. The results of these simulations demonstrate that a significant fraction of dwarf galaxies are formed and survive after giant colisions between massive galaxies. The existence of these galaxies called "tidal dwarfs" may have a major impact on the cosmological theories of galaxy formation. This work is published in the European journal Astronomy & Astrophysics.

Collisions and Dwarf Galaxies

Massive galaxies like our Milky-Way are surrounded by many dwarf satellite galaxies. Fourteen such dwarf galaxies have already been discovered around the Milky way, two of which very recently in May 2006. Standard cosmological models assume that they were formed very early in the Universe and result from the collapse of primordial fluctuations; they would then have steered clean of the merging process that destroyed many of their congeneric while hierarchically forming the most massive galaxies. Hence, the observed abundance of these dwarf satellites, as well as their spatial distribution, are fundamental tests for cosmological scenarios, that shed light both on the structure of the Universe and the nature of dark matter haloes in which they are orbiting. The number of dwarf galaxies in our Local Group - much lower than expected - is a serious problem for the standard cosmological scenario. Similarly, the observation of an anisotropic distribution of dwarf galaxies around their massive hosts in large surveys, in particular the Sloan Digital Sky Survey (SDSS), has recently nourished many debates.

However, before using dwarf galaxies as a tracer of the evolution of the Universe, one should make sure that they all actually have a primordial cosmological origin. Indeed, another process possibly forming dwarf satellite galaxies has been pointed out for more than a decade: when massive spiral galaxies collide, they build up long Antennae-like tidal tails. In these tails, new dwarf galaxies can form: the so-called « Tidal Dwarf Galaxies ». Whether these tidal dwarfs can survive after the collision of their massive parent galaxies, and subsequently have a noticeable contribution to the total number of dwarf galaxies in the Universe are fundamental questions. Up to now, observational attempts failed to provide satisfying answers. Indeed, it is hard to distinguish dwarf galaxies of tidal origin from primordial dwarfs as soon as they are a few hundred million years old.

May 12, 2006
A camera cooled to 0.3 degree above the absolute zero

 

The first images from a new generation camera called "ArTeMiS-1"  have just been obtained at theGornergrat Observatory, near Zermatt in Switzerland. This camera operates in the still poorly known "sub-millimetre" domain, between the infrared and the millimetre waves. It is an array of  16x16 bolometers, small detectors which measure the light energy by converting it into heat. These detectors need to be cooled at very low temperature, only 300 millidegrees above the absolute zero. These successful observations are the first step toward regular astronomical observations to study star formation and galaxy evolution. The camera is based on the technology developed at the Service d'Astrophysique of CEA/DAPNIA, and LETI/LIR at CEA/Grenoble for the HERSCHEL satellite to be launched in 2007.

 

 

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