Categorized | Sci-Tech

Double header: Firsts for brown dwarf, cosmic filament

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Two teams of scientists score big with Keck Observatory this week. The first group discovered the first known cosmic web filament while the second directly imaged a brown dwarf for the first time.

Rare brown dwarf discovery provides benchmark for future exoplanet research (Photo courtesy of Crepp et al. 2014 | APJ)

Rare brown dwarf discovery provides benchmark for future exoplanet research (Photo courtesy of Crepp et al. 2014 | APJ)

Rare Brown Dwarf Discovery Provides Benchmark for Future Exoplanet Research

A team of researchers led by Justin R. Crepp, the Freimann Assistant Professor of Physics at the University of Notre Dame, has directly imaged a very rare type of brown dwarf that can serve as a benchmark for studying objects with masses that lie between stars and planets.

Their paper on the discovery was published recently in Astrophysical Journal.

Initial data came from the TRENDS (TaRgetting bENchmark-objects with Doppler Spectroscopy) high-contrast imaging survey that uses adaptive optics and related technologies to target older, faint objects orbiting nearby stars, and precise measurements were made at the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii. Brown dwarfs emit little light because they do not burn hydrogen and cool rapidly. Crepp said they could provide a link between our understanding of low-mass stars and smaller objects such as planets.

HD 19467 B, a T-dwarf, is a very faint companion to a nearby Sun-like star, more than 100,000 times as dim as its host. Its distance is known precisely, and the discovery also enables researchers to place strong constraints on important factors such as its mass, orbit, age, and chemical composition without reference to the spectrum of light received from its surface.

Precise radial velocity measurements were obtained using the HIRES instrument installed on Keck Observatory’s 10-meter, Keck I telescope. The observations, which span 17 years starting from 1996, show a long-term acceleration, indicating that a low-mass companion was “tugging” on the parent star. Follow-up high-contrast imaging observations were then taken in 2012 using the NIRC2 instrument on the Keck II telescope with the adaptive optics system revealing the companion as shown above. Observations were granted through each of the Keck Observatory consortium members, including NASA, the California Institute of Technology, and the University of California.

While scientists understand the light received from stars relatively well, the spectra from planets is complicated and little understood. Understanding brown dwarfs, such as HD 19467 B, could be a step towards a fuller understanding of exoplanets.

“This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date,” Crepp said. “With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made out of, what its mass is, radius, age, etc., basically all relevant physical properties.”

HIRES (the High-Resolution Echelle Spectrometer) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding planets orbiting other stars. Astronomers also use HIRES to study distant galaxies and quasars, finding clues to the Big Bang.

NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.

Computer simulations suggest that matter in the universe is distributed in a "cosmic web" of filaments, as seen in the image above from a large-scale dark-matter simulation. The inset is a zoomed-in, high-resolution image of a smaller part of the cosmic web, 10 million light-years across, from a simulation that includes gas as well as dark matter. The intense radiation from a quasar can, like a flashlight, illuminate part of the surrounding cosmic web (highlighted in the image) and make a filament of gas glow, as was observed in the case of quasar UM287. (Image courtesy of S. Cantalupo - UCSC | Joel Primack - UCSC | Anatoly Kylpin - NMSU)

Computer simulations suggest that matter in the universe is distributed in a “cosmic web” of filaments, as seen in the image above from a large-scale dark-matter simulation. The inset is a zoomed-in, high-resolution image of a smaller part of the cosmic web, 10 million light-years across, from a simulation that includes gas as well as dark matter. The intense radiation from a quasar can, like a flashlight, illuminate part of the surrounding cosmic web (highlighted in the image) and make a filament of gas glow, as was observed in the case of quasar UM287. (Image courtesy of S. Cantalupo – UCSC | Joel Primack – UCSC | Anatoly Kylpin – NMSU)

UCSC Scientists Capture First Cosmic Web Filaments

Astronomers have discovered a distant quasar illuminating a vast nebula of diffuse gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic web.

Researchers at the University of California, Santa Cruz, led the study, published Jan. 19 in the journal, Nature.

Using the 10-meter Keck I telescope at the W. M. Keck Observatory in Hawaii, the researchers detected a very large, luminous nebula of gas extending about 2 million light-years across intergalactic space.

“This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” said Sebastiano Cantalupo, first author of the paper and a postdoctoral fellow at UC Santa Cruz.

The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which (about 84 percent) is invisible dark matter.

This web is seen in the results from computer simulations of the evolution of structure in the universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of filaments that connect them.

Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionized gas are expected to trace a pattern similar to that seen in dark matter simulations.

Until now, these filaments have never been seen. Intergalactic gas has been detected by its absorption of light from bright background sources, but those results don’t reveal how the gas is distributed.

In this study, the researchers detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar.

“This quasar is illuminating diffuse gas on scales well beyond any we’ve seen before, giving us the first picture of extended gas between galaxies,” said J. Xavier Prochaska, coauthor and professor of astronomy and astrophysics at UC Santa Cruz. “It provides a terrific insight into the overall structure of our universe.”

The hydrogen gas illuminated by the quasar emits ultraviolet light known as Lyman alpha radiation.

The distance to the quasar is so great (about 10 billion light-years) that the emitted light is “stretched” by the expansion of the universe from an invisible ultraviolet wavelength to a visible shade of violet by the time it reaches the Keck telescope and the LRIS (Low Resolution Imaging Spectrometer) used for this discovery.

Knowing the distance to the quasar, the researchers calculated the wavelength for Lyman alpha radiation from that distance and built a special filter for LRIS to get an image at that wavelength.

“We have studied other quasars this way without detecting such extended gas,” Cantalupo said. “The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow. We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar.”

A quasar is a type of active galactic nucleus that emits intense radiation powered by a supermassive black hole at the center of the galaxy.

In an earlier survey of distant quasars using the same technique to look for glowing gas, Cantalupo and others detected so-called “dark galaxies,” the densest knots of gas in the cosmic web. These dark galaxies are thought to be either too small or too young to have formed stars.

“The dark galaxies are much denser and smaller parts of the cosmic web. In this new image, we also see dark galaxies, in addition to the much more diffuse and extended nebula,” Cantalupo said. “Some of this gas will fall into galaxies, but most of it will remain diffuse and never form stars.”

The researchers estimated the amount of gas in the nebula to be at least ten times more than expected from the results of computer simulations.

“We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models,” Cantalupo said.

In addition to Cantalupo and Prochaska, the coauthors of the paper include Piero Madau, professor of astronomy and astrophysics at UC Santa Cruz, and Fabrizio Arrigoni-Battaia and Joseph Hennawi at the Max Planck Institute for Astronomy in Heidelberg, Germany.

This research was supported by grants from the National Science Foundation (AST-1010004, OIA-1124453) and NASA (NNX12AF87G).

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and world-leading laser guide star adaptive optics systems.

The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

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