[Two earlier articles referred to in this article are included with their dates in this post. Most necessary links are given at their first instance in any of the three articles.]
Aug 30, 2014
Source: University College of London
Bex Caygill Tel: +44 (0)20 3108 3846
New research shows we will soon uncover the origin of the ultraviolet light that bathes the cosmos, helping scientists understand how galaxies were built.
The study published this month in The Astrophysical Journal Letters by UCL cosmologists Dr Andrew Pontzen and Dr Hiranya Peiris (both UCL Physics & Astronomy), together with collaborators at Princeton and Barcelona Universities, shows how forthcoming astronomical surveys will reveal what lit up the cosmos.
A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across). Astronomers will soon know whether or not these kinds of computer models give an accurate portrayal of light in the real cosmos.
Credit: Andrew Pontzen/Fabio Governato
“Which produces more light? A country’s biggest cities or its many tiny towns?” asked Dr Pontzen, lead author of the study. “Cities are brighter, but towns are far more numerous. Understanding the balance would tell you something about the organisation of the country. We’re posing a similar question about the universe: does ultraviolet light come from numerous but faint galaxies, or from a smaller number of quasars?”
Quasars are the brightest objects in the Universe; their intense light is generated by gas as it falls towards a black hole. Galaxies can contain millions or billions of stars, but are still dim by comparison. Understanding whether the numerous small galaxies outshine the rare, bright quasars will provide insight into the way the universe built up today’s populations of stars and planets. It will also help scientists properly calibrate their measurements of dark energy, the agent thought to be accelerating the universe’s expansion and determining its far future.
The new method proposed by the team builds on a technique already used by astronomers in which quasars act as beacons to understand space. The intense light from quasars makes them easy to spot even at extreme distances, up to 95% of the way across the observable universe. The team think that studying how this light interacts with hydrogen gas on its journey to Earth will reveal the main sources of illumination in the universe, even if those sources are not themselves quasars.
Two types of hydrogen gas are found in the universe – a plain, neutral form and a second charged form which results from bombardment by UV light. These two forms can be distinguished by studying a particular wavelength of light called ‘Lyman-alpha’ which is only absorbed by the neutral type of hydrogen. Scientists can see where in the universe this ‘Lyman-alpha’ light has been absorbed to map the neutral hydrogen.
Since the quasars being studied are billions of light years away, they act as a time capsule: looking at the light shows us what the universe looked like in the distant past. The resulting map will reveal where neutral hydrogen was located billions of years ago as the universe was vigorously building its galaxies.
An even distribution of neutral hydrogen gas would suggest numerous galaxies as the source of most light, whereas a much less uniform pattern, showing a patchwork of charged and neutral hydrogen gas, would indicate that rare quasars were the primary origin of light.
Current samples of quasars aren’t quite big enough for a robust analysis of the differences between the two scenarios; however, a number of surveys currently being planned should help scientists find the answer.
Chief among these is the DESI (Dark Energy Spectroscopic Instrument) survey which will include detailed measurements of about a million distant quasars. Although these measurements are designed to reveal how the expansion of the universe is accelerating due to dark energy, the new research shows that results from DESI will also determine whether the intervening gas is uniformly illuminated. In turn, the measurement of patchiness will reveal whether light in our universe is generated by ‘a few cities’ (quasars) or by ‘many small towns’ (galaxies).
Co-author Dr Hiranya Peiris, said: “It’s amazing how little is known about the objects that bathed the universe in ultraviolet radiation while galaxies assembled into their present form. This technique gives us a novel handle on the intergalactic environment during this critical time in the universe’s history.”
Dr Pontzen, said: “It’s good news all round. DESI is going to give us invaluable information about what was going on in early galaxies, objects that are so faint and distant we would never see them individually. And once that’s understood in the data, the team can take account of it and still get accurate measurements of how the universe is expanding, telling us about dark energy. It illustrates how these big, ambitious projects are going to deliver astonishingly rich maps to explore. We’re now working to understand what other unexpected bonuses might be pulled out from the data.”
See this article here.
Milky Way May Have Formed ‘Inside-Out’
Jan 20, 2014
Gaia provides new insight into Galactic evolution
A breakthrough using data from the [ESA]/Gaia-ESO project has provided evidence backing up theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way’s disc – the vast collection of giant gas clouds and billions of stars that give our Galaxy its ‘flying saucer’ shape.
By tracking the fast-produced elements, specifically magnesium in this study, astronomers can determine how rapidly different parts of the Milky Way were formed. The research suggests that stars in the inner regions of the Galactic disc were the first to form, supporting ideas that our Galaxy grew from the inside-out.
Using data from the 8-m VLT in Chile, one of the world’s largest telescopes, an international team of astronomers took detailed observations of stars with a wide range of ages and locations in the Galactic disc to accurately determine their ‘metallicity’: the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.
Immediately after the Big Bang, the Universe consisted almost entirely of hydrogen and helium, with levels of “contaminant metals” growing over time. Consequently, older stars have fewer elements in their make-up – so have lower metallicity.
“The different chemical elements of which stars – and we – are made are created at different rates – some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes,” said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.
This is a figure illustrating latest Gaia-ESO research findings. (Click image for larger size) Credit: Amanda Smith/Institute of Astronomy
Massive stars, which have short lives and die as ‘core-collapse supernovae’, produce huge amounts of magnesium during their explosive death throes. This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.
The team have shown that older, ‘metal-poor’ stars inside the Solar Circle – the orbit of our Sun around the centre of the Milky Way, which takes roughly 250 million years to complete – are far more likely to have high levels of magnesium. The higher level of the element inside the Solar Circle suggests this area contained more stars that “lived fast and die young” in the past.
The stars that lie in the outer regions of the Galactic disc – outside the Solar Circle – are predominantly younger, both ‘metal-rich’ and ‘metal-poor’, and have surprisingly low magnesium levels compared to their metallicity.
This discovery signifies important differences in stellar evolution across the Milky Way disc, with very efficient and short star formation timescales occurring inside the Solar Circle; whereas, outside the Sun’s orbit, star formation took much longer.
“We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form,” said Maria Bergemann from Cambridge’s Institute of Astronomy, who led the study.
“This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out.”
The findings offer new insights into the assembly history of our Galaxy, and are the part of the first wave of new observations from the Gaia-ESO survey, the ground-based extension to the Gaia space mission – launched by the European Space Agency at the end of last year – and the first large-scale survey conducted on one the world’s largest telescopes: the 8-m VLT in Paranal, Chile.
The study is published online today through the astronomical database Astro-ph, and has been submitted to the journal Astronomy and Astrophysics.
This digitally enhanced double-exposure was taken in May 2003 over the Kofa Mountains in Arizona, USA. Dark dust, millions of stars, and bright glowing red gas highlight the plane of our Milky Way Galaxy. Photo credit: Richard Payne (Arizona Astrophotography)
The new research also sheds further light on another much debated “double structure” in the Milky Way’s disc – the so-called ‘thin’ and ‘thick’ discs.
“The thin disc hosts spiral arms, young stars, giant molecular clouds – all objects which are young, at least in the context of the Galaxy,” explains Aldo Serenelli from the Institute of Space Sciences (Barcelona), a co-author of the study. “But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity.”
During the latest research, the team found that:
- Stars in the young, ‘thin’ disc aged between 0 – 8 billion years all have a similar degree of metallicity, regardless of age in that range, with many of them considered ‘metal-rich’.
- There is a “steep decline” in metallicity for stars aged over 9 billion years, typical of the ‘thick’ disc, with no detectable ‘metal-rich’ stars found at all over this age.
- But stars of different ages and metallicity can be found in both discs.
“From what we now know, the Galaxy is not an ‘either-or’ system. You can find stars of different ages and metal content everywhere!” said Bergemann. “There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs – that’s how we know these two discs probably exist – but they could have very different origins.”
Added Gilmore: “This study provides exciting new evidence that the inner parts of the Milky Way’s thick disc formed much more rapidly than did the thin disc stars, which dominate near our Solar neighbourhood.”
In theory, say astronomers, the thick disc – first proposed by Gilmore 30 years ago – could have emerged in a variety of ways, from massive gravitational instabilities to consuming satellite galaxies in its formative years. “The Milky Way has cannibalised many small galaxies during its formation. Now, with the Gaia-ESO Survey, we can study the detailed tracers of these events, essentially dissecting the belly of the beast,” said Greg Ruchti, a researcher at Lund Observatory in Sweden, who co-leads the project.
With upcoming releases of Gaia-ESO, an even better handle on the age-metallicity relation and the structure of the Galactic disc is expected, say the team. In a couple of years, these data will be complemented by positions and kinematics provided by the Gaia satellite and together will revolutionise the field of Galactic astronomy.
See this article here.
The Universe in 3D
May 3, 2011
The biggest 3-D map of the distant universe ever made, using light from 14,000 quasars – supermassive black holes at the centers of galaxies billions of light years away – has been constructed by scientists with the third Sloan Digital Sky Survey (SDSS-III).
The map is the first major result from the Baryon Oscillation Spectroscopic Survey (BOSS), SDSS-III’s largest survey, whose principal investigator is David Schlegel of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). The huge new map was presented at the April meeting of the American Physical Society in Anaheim, CA, by AnÅ¾e Slosar of Brookhaven National Laboratory.
BOSS is extending the existing Sloan Digital Sky Survey map of the universe based on galaxies, center, into the realm of intergalactic gas in the distant universe, using the light from bright quasars (blue dots). Credit: Sloan Digital Sky Survey – See more at: http://www.astrobio.net/topic/deep-space/cosmic-evolution/the-universe-in-3d/#sthash.aQus5jw9.dpuf
BOSS is the first attempt to use baryon acoustic oscillation (BAO) as a precision tool to measure dark energy. Baryon oscillation refers to how matter clumps in a regular way throughout the Universe, a physical manifestation of the expansion of the Universe. Until now, 3-D maps showing this oscillation have been based on the distribution of visible galaxies. BOSS is the first survey to map intergalactic hydrogen gas as well, using distant quasars whose light is produced by supermassive black holes at the centers of active galaxies.
“Quasars are the brightest objects in the Universe, which we use as convenient backlights to illuminate the intervening hydrogen gas that fills the Universe between us and them,” Slosar says. “We can see their shadows, and the details in their shadows” – specifically, the absorption features in their spectra known as the Lyman-alpha forest – “allowing us to see how the gas is clumped along our line of sight. The amazing thing is that this allows us to see the Universe so very far away, where measuring positions of individual galaxies in large numbers is impractical.”
“BOSS is the first attempt to use the Lyman-alpha forest to measure dark energy,” says principal investigator Schlegel. “Because the Sloan Telescope has such a wide field of view, and because these quasars are so faint, there was no one who wasn’t nervous about whether we could really bring it off.”
By using 14,000 of the quasars collected by the Sloan Telescope at Apache Point Observatory in New Mexico during the first year of BOSS’s planned five-year run, the new map demonstrates that indeed it is possible to determine variations in the density of intergalactic hydrogen gas at cosmological distances and thus to measure the effects of dark energy at those distances.
Slosar, who leads BOSS’s Lyman-alpha cosmology working group, says that while similar measurements have been made with individual quasars or small groups of quasars in the past, “These have given only one-dimensional information about fluctuations in density along the line of sight. Before now there has never been enough density of quasars for a 3-D view.”
The distance scale of the new map corresponds to an early time in the history of the Universe, when the distribution of matter was nearly uniform. Any effects of dark energy detected so early would settle basic questions about its nature.
Measuring the expansion history of the Universe
Baryon acoustic oscillation is cosmologists’ shorthand for the periodic clustering (oscillation) of matter (baryons), which originated as pressure (acoustic) waves moving through the hot, opaque, liquid-like early universe. The pressure differences resulted in differences in density and left their signature as small variations in the temperature of the cosmic microwave background. Later – because the denser regions formed by the pressure waves seeded galaxy formation and the accumulation of other matter – the original acoustic waves were echoed in the net-like filaments and voids of the clustering of galaxies and in variations in the density of intergalactic hydrogen gas.
CMB from ESA/Planck
A 2-D slice through BOSS´s full 3-D map of the universe to date. The black dots going out to about 7 billion light years are relatively nearby galaxies. The colored region beginning at about 10 billion light years is intergalactic hydrogen gas; red areas have more gas and blue areas have less. The blank region between is inaccessible to the Sloan Telescope, but the proposed BigBOSS survey would be able to observe it. Credit: AnÅ¾e Slosar and BOSS Lyman-alpha cosmology working group
The oscillations repeat at about 500-million-light-year intervals, and because this scale is firmly anchored in the cosmic microwave background it provides a ruler – a very big one – to measure the history of the expanding universe. With this cosmic yardstick it will be possible to determine just how fast the Universe was expanding at the redshift of the objects in the BOSS survey – in other words, how the expansion rate has changed over time. (Redshift is the degree to which the light from an object speeding away from the viewer is shifted toward the red end of the spectrum.) Knowing whether expansion has accelerated at a constant rate or has varied over time will help decide among the major theories of dark energy.
Over its five-year extent, BOSS is using two distinct methods to calibrate the markings on the cosmic yardstick. The first method, well tested, will precisely measure 1.5 million luminous red galaxies at “low” redshifts around z = 0.7 (z stands for redshift). The second method will eventually measure the Lyman-alpha forest of 160,000 quasars with high redshifts around z = 2.5. These redshifts correspond to galaxies at distances of 2 to 6 billion light years and quasars at 10 to 11 billion light years.
Lyman-alpha is the name given to a line in the spectrum of hydrogen, marking the wavelength of light emitted when an excited hydrogen electron falls back to its ground state; it’s a strong signal in the light from quasars. As the quasar’s light passes through intervening clouds of hydrogen gas, additional lines accumulate where the gas clouds absorb the signal, echoing it but shifting by different degrees according to factors including the redshift of the gas cloud and its density. The spectrum of a distant quasar may have hundreds of lines, clumped and blended into a messy, wiggly structure in the spectrum: this is what astronomers call the Lyman-alpha forest.
“In theory, you can turn any of these absorption lines directly into redshifts and locate the gas cloud precisely,” says Bill Carithers of Berkeley Lab’s Physics Division, who concentrates on extracting relevant information from the noisy data that comes straight from the telescope. “But in practice only the spectra of the very brightest quasars are clean enough to make things that simple.”
Carithers says that “while a very long exposure could improve the signal-to-noise ratio, that comes at a price. We need lots and lots of quasars to make a map. We can only afford to spend so much telescope time on each.”
Since the heart of BAO is the correlation distance among density oscillations, the trick turns out to be not overconcentrating on individual spectra but instead measuring the correlations among them. “For any correlation distance, many quasars will contribute,” says Carithers, “so the noise will average and the signal will get stronger. We can say, ‘I’ll use my data, noise and all.’”
If the attempt to measure density variations in the intergalactic gas is indeed successful, what will the BAO correlation signal from the Lyman-alpha forest look like? Shirley Ho of Berkeley Lab’s Physics Division, working with Slosar and Berkeley Lab’s Martin White, developed simulations to find out.
Zooming in on the map slice shows areas with more gas (red) and less gas (blue) as revealed by correlations of the Lyman-alpha forest data from the spectra of thousands of quasars. A distance of one billion light years is indicated by the scale bar. Credit: AnÅ¾e Slosar and BOSS Lyman-alpha cosmology working group
“”We modeled what you would see when you have a BOSS-like data set, and through the simulations we understand the possible sources of systematics when we try with real data to detect the acoustic peak from the Lyman-alpha forest, the signature of baryon acoustic oscillations,” Ho says. Comparing the real data to the simulation confirms whether the search is working as hoped.
With Peter Nugent, who heads the Computational Cosmology Center at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), Ho established a 30-terabyte BOSS Project Directory to store the Lyman-alpha simulations, plus the entire Lyman-alpha raw data set as it arrives. The directory also contains a subset of galaxy data and is available to all BOSS collaborators and to the public. The total BOSS data set is stored in a dedicated cluster of computers nicknamed Riemann.
Targeting the search
The wide-field Sloan Telescope covers a wide expanse of sky at moderate magnification. To measure both galaxies and quasars, a thousand targets for each BOSS exposure are selected in advance from existing surveys. At the telescope’s focal plane, “plug plates” are precision-machine-drilled with tiny holes at positions of known galaxies and quasars. These holes are plugged with optical fibers that channel the light from each chosen galaxy or quasar to a spectrograph, which isolates the spectrum of each individual object. Schlegel credits Berkeley Lab’s Nicholas Ross for doing much of the “incredibly hard work” involved in this targeting.
Slosar says, “Our exploratory paper includes less than a tenth of the 160,000 quasars that BOSS will study, but already that’s enough to establish a proof of the concept. This is a potentially revolutionary technique for mapping the very distant universe. We’re paving the way for future BAO experiments like BigBOSS to follow suit.” BigBOSS is a proposed survey that will find precise locations for 20 million galaxies and quasars and go beyond BOSS to encompass 10 times the volume of the finished BOSS map.
“By the time BOSS ends, we will be able to measure how fast the Universe was expanding 11 billion years ago with an accuracy of a couple of percent,” says Patrick McDonald of Berkeley Lab and Brookhaven, who pioneered techniques for measuring the Universe with the Lyman-alpha forest. “Considering that no one has ever measured the cosmic expansion rate so far back in time, that’s a pretty astonishing prospect.”
Says Slosar, “We now know we can use the Lyman-alpha forest to look at the dark energy. There is all this structure at the distant universe that has never been seen before. Sometimes I feel like an adventuring cartographer from the Middle Ages!”
Studying the nature of the Universe can help astrobiologists identify the conditions in which habitable planets are most likely to form around distant stars.
See this article here.
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