Galaxy Starbursts Triggered by Dark Matter -“An Enigma Inside a Mystery”

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Most of the mass of any galaxy is expected to be "dark matter," an elusive X Factor that has yet to be detected but which astronomers believe must exist to provide sufficient gravity to prevent galaxies ripping themselves apart as they rotate. But ESA’s Herschel space observatory has discovered a population of dust-enshrouded galaxies that do not need as much "dark matter" as previously thought to collect gas and burst into star formation.


“Herschel is showing us that we don’t need quite so much dark matter as we thought to trigger a starburst,” says Asantha Cooray, University of California, Irvine, a co-author on today’s paper. Current models of the birth of galaxies start with the accumulation of large amounts of dark matter believed to reside in a considerably larger assembly, or halo. Its gravitational attraction drags in ordinary atoms. If enough atoms accumulate, a ‘starburst’ is ignited, in which stars form at rates 100–1000 times faster than in our own galaxy does today.

Dark matter halos are, according to the most commonly accepted theory for the formation of cosmic structure, the sites where galaxies take shape. In this theory, tiny fluctuations in the early Universe grew, under the attractive effect of gravity, into a complex network of dark matter sheets and filaments – the so-called cosmic web; later, gas accumulating in the densest knots of the cosmic web began to cool, giving rise to clumps where the first stars formed and which would later assemble into galaxies.

Since the cosmic web constitutes the skeleton supporting the later emergence of stars and galaxies, the distribution of galaxies is expected to follow, and thus trace, that of the dark matter. Whereas the growth of dark matter structures is only regulated by gravity, a number of additional phenomena affect star and galaxy formation, resulting in two different clustering trends. Astronomers refer to this by saying that galaxies are biased tracers of the dark matter distribution.

An interesting feature in this context is that all galaxies are to be found within dark matter halos, with one or more galaxies inhabiting a halo, but not all halos are expected to harbour a galaxy.

"The formation of a galaxy is simply not efficient enough in halos with masses that are either too large or too small," explains Asantha Cooray from the University of California, Irvine, USA, who directed the study based on Herschel data that has revealed new details about the most efficient sites for galaxy formation.

This dark matter discovery was made by analyzing infrared images taken by Herschel’s SPIRE (Spectral and Photometric Imaging Receiver) instrument at wavelengths of 250, 350, and 500 microns. These are roughly 1000 times longer than the wavelengths visible to the human eye and reveal galaxies that are deeply enshrouded in a fog of dust.

“With its very high sensitivity to the far-infrared light emitted by these young, enshrouded starburst galaxies, Herschel allows us to peer deep into the Universe and to understand how galaxies form and evolve,” says Göran Pilbratt, the ESA Herschel project scientist.

The team of astronomers used HerMES observations of two fields, the Lockman Hole (see below) and the GOODS North. "With its large telescope and its unprecedented resolution and sensitivity at these far-infrared wavelengths, Herschel is a unique facility that has now made it possible to scrutinize the CIB fluctuations down to scales that reveal really interesting information about the emergence of starburst galaxies around the peak of the star formation history of the Universe," comments Goran Pilbratt, Herschel Project Scientist at ESA.

The galaxies in the Hershel images are not distributed randomly but follow the underlying pattern of dark matter in the Universe, and so the fog has a distinctive pattern of light and dark patches.
 
Analysis of the brightness of the patches in the SPIRE images has shown that the star-formation rate in the distant infrared galaxies is 3–5 times higher than previously inferred from visible-wavelength observations of similar, very young galaxies by the Hubble Space Telescope and other telescopes.

Further analysis and simulations have shown that this smaller mass for the galaxies is a sweet spot for star formation. Less massive galaxies find it hard to form more than a first generation of stars before fizzling out. At the other end of the scale, more massive galaxies struggle because their gas cools rather slowly, preventing it from collapsing down to the high densities needed to ignite star formation.

But at this newly identified ‘just-right’ mass of a few hundred billion solar masses, galaxies can make stars at prodigious rates and thus grow rapidly.

“This is the first direct observation of the preferred mass scale for igniting a starburst,” says Dr Cooray.

The image below shows the patch of the sky known as the 'Lockman Hole', as observed by the SPIRE instrument on board Herschel. Located in northern constellation of Ursa Major, The Great Bear, the 'Lockman Hole' is a field on the sky almost devoid of foreground contamination and thus ideally suited for observations of galaxies in the distant Universe.

Almost every dot in the image is an entire galaxy, each containing billions of stars and appearing as they did 10-12 billion years ago, when the Universe was only a couple of billion years old. The blue, green and red colours represent the three far-infrared wavelengths used for Herschel's observations: 250, 350 and 500 micron, respectively.

The galaxies shown in white have equal intensity in all three wavebands and are the ones forming the most stars. Detecting these galaxies individually is particularly challenging, as they are both extremely faint and numerous, so many of them overlap in Herschel's images. This creates a fog of infrared radiation known as the Cosmic Infrared Background (CIB), which reflects the clustering pattern of the galaxies responsible for this fog. Studying the CIB and its fluctuations is thus an extremely powerful tool to explore the way galaxies tend to be grouped on both small and large scales.

Models of galaxy formation can now be adjusted to reflect these new results, and astronomers can take a step closer to understanding how galaxies – including our own –came into being.

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The Daily Galaxy via ESA

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