NASA JPL-Caltech | 2011 Feb 16
Herschel finds less dark matter but more starsThe Herschel Space Observatory has revealed how much dark matter it takes to form a new galaxy bursting with stars. Herschel is a European Space Agency cornerstone mission supported with important NASA contributions.
The findings are a key step in understanding how dark matter, an invisible substance permeating our universe, contributed to the birth of massive galaxies in the early universe.
"If you start with too little dark matter, then a developing galaxy would peter out," said astronomer Asantha Cooray of the University of California, Irvine. He is the principal investigator of new research appearing in the journal Nature, online on Feb. 16 and in the Feb. 24 print edition. "If you have too much, then gas doesn't cool efficiently to form one large galaxy, and you end up with lots of smaller galaxies. But if you have the just the right amount of dark matter, then a galaxy bursting with stars will pop out."
The right amount of dark matter turns out to be a mass equivalent to 300 billion of our suns.
Herschel launched into space in May 2009. The mission's large, 3.5-meter (11.5-foot) telescope detects longer-wavelength infrared light from a host of objects, ranging from asteroids and planets in our own solar system to faraway galaxies.
"This remarkable discovery shows that early galaxies go through periods of star formation much more vigorous than in our present-day Milky Way," said William Danchi, Herschel program scientist at NASA Headquarters in Washington. "It showcases the importance of infrared astronomy, enabling us to peer behind veils of interstellar dust to see stars in their infancy."
Cooray and colleagues used the telescope to measure infrared light from massive, star-forming galaxies located 10 to 11 billion light-years away. Astronomers think these and other galaxies formed inside clumps of dark matter, similar to chicks incubating in eggs.
Giant clumps of dark matter act like gravitational wells that collect the gas and dust needed for making galaxies. When a mixture of gas and dust falls into a well, it condenses and cools, allowing new stars to form. Eventually enough stars form, and a galaxy is born.
Herschel was able to uncover more about how this galaxy-making process works by mapping the infrared light from collections of very distant, massive star-forming galaxies. This pattern of light, called the cosmic infrared background, is like a web that spreads across the sky. Because Herschel can survey large areas quickly with high resolution, it was able to create the first detailed maps of the cosmic infrared background.
"It turns out that it's much more effective to look at these patterns rather than the individual galaxies," said Jamie Bock of NASA's Jet Propulsion Laboratory in Pasadena, Calif. Bock is the U.S. principal investigator for Herschel's Spectral and Photometric Imaging Receiver instrument used to make the maps. "This is like looking at a picture in a magazine from a reading distance. You don't notice the individual dots, but you see the big picture. Herschel gives us the big picture of these distant galaxies, showing the influence of dark matter."
The maps showed the galaxies are more clustered into groups than previously believed. The amount of galaxy clustering depends on the amount of dark matter. After a series of complicated numerical simulations, the astronomers were able to determine exactly how much dark matter is needed to form a single star-forming galaxy.
"This measurement is important, because we are homing in on the very basic ingredients in galaxy formation," said Alexandre Amblard of UC Irvine, first author of the Nature paper. "In this case, the ingredient, dark matter, happens to be an exotic substance that we still have much to learn about."
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ESA Space Science | 2011 Feb 16
Herschel quantifies the dark matter threshold for starburst galaxiesESA’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.
The galaxies are far away and each boasts some 300 billion times the mass of the Sun. The size challenges current theory that predicts a galaxy has to be more than ten times larger, 5000 billion solar masses, to be able form large numbers of stars.
The new result is published today in a paper by Alexandre Amblard, University of California, Irvine, and colleagues.
Most of the mass of any galaxy is expected to be dark matter, a hypothetical substance 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.
Current models of the birth of galaxies start with the accumulation of large amounts of dark matter. 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.
“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.
This discovery was made by analysing 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 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.
There are so many galaxies in Herschel’s images that they overlap, creating a fog of infrared radiation known as the cosmic infrared background. The galaxies 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.
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.
ESA Science & Technology | 2011 Feb 16
Submillimetre galaxies reside in dark matter haloes with masses greater than 3 × 1011 solar masses - A Amblard et alHow much dark matter is needed to trigger a starburst in the cosmic cribs where galaxies are born? A new study, based on data from ESA's Herschel Space Observatory, has revealed that dark matter halos with a mass larger than 300 billion times the Sun's are particularly efficient at igniting massive starbursts, as they house the most active star-forming galaxies in the Universe. Astronomers have discovered this key threshold by measuring small fluctuations in the Cosmic Infrared Background, the integrated diffuse emission produced by the dust from every galaxy that ever existed. These fluctuations trace the distribution of otherwise mostly unresolved star-forming galaxies and of the dark matter halos that enshroud them. These results are reported in the 24 February 2011 issue of Nature and are published online today.
Modern astronomers have established that there is much more to galaxies than meets the eye, as the stars, gas and dust - perceivable by telescopes across the entire electromagnetic spectrum - make up only about 10 per cent of their total mass; the rest is believed to reside in a considerably larger assembly, or halo, of invisible dark matter. These halos are, according to the most commonly accepted theory for the formation of cosmic structure the sites where galaxies take shape. In this scenario, 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.
The study focussed on starburst galaxies, the most prolific stellar factories in the history of the Universe, which give birth to hundreds or even thousands of stars per year - as a comparison, our Galaxy produces, on average, only one star per year. It is during these intense bursts, lasting from tens to a few hundred million years, that certain types of galaxies are believed to acquire most of their stars. "Our analysis has led to the first observational estimate of the minimum mass of a halo in which a large number of stars could ignite suddenly, creating a starbursting galaxy. This is a key ingredient to improve our current understanding of galaxy formation and evolution," adds Cooray.
The team addressed the issue by studying the maps in the Herschel Multi-tiered Extra-galactic Survey (HerMES). The galaxies in these maps shine brightly in the far-infrared and submillimetre portion of the spectrum, as their dust component absorbs a significant fraction of starlight and radiates it again at these longer wavelengths. As the Universe expands, this emission is shifted to longer wavelengths and, for the very distant galaxies, it peaks in the longer wavelength channels of the SPIRE instrument aboard Herschel. Corresponding to redshifts of about z~2–3, these galaxies thus appear as they were when the Universe was only a few billion years old.
Extreme star-forming galaxies like these in the distant Universe have been a real puzzle for over a decade. They are rather challenging to detect individually, as source confusion effects arise because of the relatively poor angular resolution achievable at the wavelengths at which they radiate the bulk of their light. "As previous results obtained with early Herschel data show, even with deep observations it is possible to directly resolve only about 15 per cent of them," notes co-author Seb Oliver from the University of Sussex, United Kingdom, who coordinates the HerMES Key Programme together with Jamie Bock from Caltech, USA. "But this is not the end of the story: with the large maps of HerMES and the sensitivity and resolution of Herschel, we have finally been able to trace them," adds Oliver.
The infrared light radiated collectively by all faint, star-forming galaxies creates a diffuse background, known as the Cosmic Infrared Background (CIB). "The CIB exhibits fluctuations which closely reflect the clustering pattern of the galaxies responsible for it," says Alexandre Amblard from UC Irvine and currently at NASA Ames, first author of the Nature paper. Observing these fluctuations, which span a wide range of angular scales, is one of the very few methods available to explore the properties of the unresolved population of galaxies contributing to the CIB. "Thanks to Herschel's extraordinary data, we have been able, for the first time, to pin the fluctuations down to very small scales - about 1 arc minute," he adds.
The team of astronomers used HerMES observations of two fields, the Lockman Hole 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 scrutinise 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 Göran Pilbratt, Herschel Project Scientist at ESA.
In order to extract information about these galaxies and how they formed, the data need to be properly modelled. "The tool we employed for this is known as the halo model," says co-author Paolo Serra, also from UC Irvine and currently at NASA Ames. The halo model is a statistical approach to describe the dark matter distribution, which is viewed as an ensemble of discrete objects - the halos. Within this framework, galaxies and their clustering properties can be studied in relation to the dark matter halos to which they belong. "By applying the halo model to the fluctuations of the CIB measured by Herschel on both large and small scales, it was possible to characterise correlations in the distribution not only of galaxies that reside in different halos, but also of galaxies occupying the same halo," adds Serra.
The latter effect is the key to constraining differences in the way galaxies and dark matter halos tend to be grouped: it is, in fact, at the small scales that the two distributions deviate the most from each other, as the least massive amongst dark matter halos do not host galaxies. "This analysis has demonstrated that the starbursting galaxies we targeted tend to reside in halos with a mass greater than 3×1011 times that of the Sun, thus leading to a firm estimate of the minimum mass needed by a halo for a starburst galaxy to form within it," notes Amblard. "This value singles out the halos where star formation takes place at its highest efficiency, and is ten times lower than the one predicted by semi-analytical models for galaxy formation," adds Serra.
Theory suggests that star formation may set in even within dark matter halos with masses smaller than this value, but it is expected to be promptly suppressed by supernova explosions of the most massive amongst these first stars: these would blow away the surrounding gas which, in the absence of the strong gravitational potential of a massive dark matter halo, would leave the halo barren. At the other end of the scale, if the dark matter halo is too massive, the cooling of gas is inefficient and cannot happen rapidly enough to produce a starburst.
"From a theoretical perspective, the interplay between these two mechanisms, and possibly with other processes as well, is not yet fully understood," notes Cooray. "In this context, our measurements provide strong evidence for a mass scale where these effects are balanced, resulting in a very powerful star formation activity. This value will now have to be taken into account by all theoretical efforts trying to model the properties of starbursting galaxies, and will certainly shed new light on some still nebulous details about galaxy formation," concludes Cooray.
- Nature 470(7335) 510 (2011 Feb 24) DOI: 10.1038/nature09771
- Physics Reports 372(1) (Dec 2002) DOI: 10.1016/S0370-1573(02)00276-4
arXiv.org > astro-ph > arXiv:astro-ph/0206508 > 28 Jun 2002