Tag Archives: radio

BIGHORNS_Sky_avg_SN_integr

Ionospheric effects not detrimental to EoR detection from ground : CAASTRO Research

The Epoch of Reionisation (EoR) is the time in the early Universe when the first stars and galaxies formed and re-ionised the neutral hydrogen. Indirect information about the EoR has been obtained from the Cosmic Microwave Background and spectra of the distant quasars. However, the bulk of information about the physical parameters of the EoR is encoded in the 21cm line (1420 MHz) from neutral hydrogen redshifted into the low radio frequency range 200 – 50 MHz, for redshifts of 6 < z < 30.

The observational approaches range from large interferometer arrays to single antenna experiments. The latter, so-called global EoR experiments, spatially average the signal from the entire visible sky and try to identify the tiny signature of the EoR (of order 100 milliKelvin, which is a few orders of magnitude smaller than the Galactic foregrounds) in the sky-averaged spectrum. This extremely challenging precision requires very long observations (hundreds of hours) to achieve a sufficiently high signal-to-noise ratio. Moreover, ground-based global EoR experiments are affected by frequency-dependent effects (i.e. absorption and refraction) due to the propagation of radio-waves in the Earth’s ionosphere. The amplitude of these effects changes in time. There has therefore been an ongoing discussion in the literature on the importance of ionospheric effects and whether the global EoR signature can feasibly be detected from the ground.BIGHORNS_Sky_avg_SN_integr

The team of CAASTRO researches at Curtin University, led by Dr Marcin Sokolowski, used three months’ worth of 2014/2015 data collected with the BIGHORNS system with a conical log-spiral antenna deployed at the Murchison Radio-astronomy Observatory to study the impact of the ionosphere on its capability to detect the global EoR signal. Comparison of data collected on different days at the same sidereal time enabled the researchers to infer some properties of the ionosphere, such as electron temperature (Te≈470 K at night-time) and amplitude and variability of ionospheric absorption of radio waves. Furthermore, the data sample shows that the sky-averaged spectrum indeed varies in time due to fluctuations of these ionospheric properties. Nevertheless, the data analysis indicates that averaging over very long observations (several days or even several weeks) suppresses the noise and leads to an improved signal-to-noise ratio. Therefore, the ionospheric effects and fluctuations are not fundamental impediments that prevent ground-based instruments, such as BIGHORNS, from integrating down to the precision required for global EoR experiments, provided that the ionospheric contribution is properly accounted for in the data analysis.

 

Publication details:

The impact of the ionosphere on ground-based detection of the global Epoch of Reionisation signal

Source: CAASTRO

Radio-optical overlay image of galaxy J1649+2635. Yellow is visible-light image; Blue is the radio image, indicating the presence of jets.

Credit: Mao et al., NRAO/AUI/NSF, Sloan Digital Sky Survey

Strange Galaxy Perplexes Astronomers

With the help of citizen scientists, a team of astronomers has found an important new example of a very rare type of galaxy that may yield valuable insight on how galaxies developed in the early Universe. The new discovery technique promises to give astronomers many more examples of this important and mysterious type of galaxy.

The galaxy they studied, named J1649+2635, nearly 800 million light-years from Earth, is a spiral galaxy, like our own Milky Way, but with prominent “jets” of subatomic particles propelled outward from its core at nearly the speed of light. The problem is that spiral galaxies are not supposed to have such large jets.

“The conventional wisdom is that such jets come only from elliptical galaxies that formed through the merger of spirals. We don’t know how spirals can have these large jets,” said Minnie Mao, of the National Radio Astronomy Observatory (NRAO).

Radio-optical overlay image of galaxy J1649+2635. Yellow is visible-light image; Blue is the radio image, indicating the presence of jets. Credit: Mao et al., NRAO/AUI/NSF, Sloan Digital Sky Survey
Radio-optical overlay image of galaxy J1649+2635. Yellow is visible-light image; Blue is the radio image, indicating the presence of jets.
Credit: Mao et al., NRAO/AUI/NSF, Sloan Digital Sky Survey



J1649+2635 is only the fourth jet-emitting spiral galaxy discovered so far. The first was found in 2003, when astronomers combined a radio-telescope image from the Karl G. Jansky Very Large Array (VLA) and a visible-light image of the same object from the Hubble Space Telescope. The second was revealed in 2011 by images from the Sloan Digital Sky Survey and the VLA, and the third, found earlier this year, also was discovered by combining radio and visible-light images.

“In order to figure out how these jets can be produced by the ‘wrong’ kind of galaxy, we realized we needed to find more of them,” Mao said.

To do that, the astronomers looked for help. That help came in the form of large collections of images from both radio and optical telescopes, and the hands-on assistance of volunteer citizen scientists. The volunteers are participants in an online project called the Galaxy Zoo, in which they look at images from the visible-light Sloan Digital Sky Survey and classify the galaxies as spiral, elliptical, or other types. Each galaxy image is inspected by multiple volunteers to ensure accuracy in the classification.

So far, more than 150,000 Galaxy Zoo participants have classified some 700,000 galaxies. Mao and her collaborators used a “superclean” subset of more than 65,000 galaxies, for which 95 percent of those viewing each galaxy’s image agreed on the classification. About 35,000 of those are spiral galaxies. J1649+2635 had been classified by 31 Galaxy Zoo volunteers, 30 of whom agreed that it is a spiral.

Next, the astronomers decided to cross-match the visible-light spirals with galaxies in a catalog that combines data from the NRAO VLA Sky Survey and the Faint Images of the Radio Sky at Twenty Centimeters survey, both done using the VLA. This job was done by Ryan Duffin, a University of Virginia undergraduate working as an NRAO summer student. Duffin’s cross-matching showed that J1649+2635 is both a spiral galaxy and has powerful twin radio jets.

“This is the first time that a galaxy was first identified as a spiral, then subsequently found to have large radio jets,” Duffin said. “It was exciting to make such a rare find,” he added.

Jets such as those seen coming from J1649+2635 are propelled by the gravitational energy of a supermassive black hole at the core of the galaxy. Material pulled toward the black hole forms a rapidly-rotating disk, and particles are accelerated outward along the poles of the disk. The collision that presumably forms an elliptical galaxy disrupts gas in the merging galaxies and provides “fuel” for the disk and acceleration mechanism. That same disruption, however, is expected to destroy any spiral structure as the galaxies merge into one.

J1649+2635 is unusual not only because of its jets, but also because it is the first example of a “grand design” spiral galaxy with a large “halo” of visible-light emission surrounding it. 

“This galaxy presents us with many mysteries. We want to know how it became such a strange beast,” Mao said. “Did it have a unique type of merger that preserved its spiral structure? Was it an elliptical that had another collision that made it re-grow spiral arms? Is its unique character the result of interaction with its environment?”

“We will study it further, but in addition, we need to see if there are more like it,” Mao said.

“We hope that with projects like the Galaxy Zoo and another called Radio Galaxy Zoo, those thousands of citizen scientists can help us find many more galaxies like this one so we can answer all our questions,” Mao said. Mao and her colleagues have dubbed these rare galaxies “Spiral DRAGNs,” an acronym for the technical description, “Double-lobed Radio sources Associated with Galactic Nuclei.”

Mao and Duffin worked with Frazer Owen, Emmanuel Momjian, and Mark Lacy, also of the NRAO; Bill Keel of the University of Alabama; Glenn Morrison of the University of Hawaii and the Canada-France-Hawaii Telescope; Tony Mroczkowski of the Naval Research Laboratory; Susan Neff of NASA’s Goddard Space Flight Center; Ray Norris of CSIRO Astronomy and Space Science in Australia; Henrique Schmitt of the Naval Research Laboratory; and Vicki Toy and Sylvain Veilleux of the University of Maryland. The scientists are reporting their findings in theMonthly Notices of the Royal Astronomical Society. 

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Source: NRAO

This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. The observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.In this picture the features seen in the HL Tauri system are labelled.

Credit:

ALMA (ESO/NAOJ/NRAO)

Revolutionary ALMA Image Reveals Planetary Genesis

This new image from ALMA, the Atacama Large Millimeter/submillimeter Array, reveals extraordinarily fine detail that has never been seen before in the planet-forming disc around a young star. These are the first observations that have used ALMA in its near-final configuration and the sharpest pictures ever made at submillimetre wavelengths. The new results are an enormous step forward in the observation of how protoplanetary discs develop and how planets form.

This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. The observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.In this picture the features seen in the HL Tauri system are labelled. Credit: ALMA (ESO/NAOJ/NRAO)
This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. The observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.In this picture the features seen in the HL Tauri system are labelled.
Credit:
ALMA (ESO/NAOJ/NRAO)

For ALMA’s first observations in its new and most powerful mode, researchers pointed the antennas at HL Tauri — a young star, about 450 light-years away, which is surrounded by a dusty disc [1]. The resulting image exceeds all expectations and reveals unexpectedly fine detail in the disc of material left over from star birth. It shows a series of concentric bright rings, separated by gaps [2].

“These features are almost certainly the result of young planet-like bodies that are being formed in the disc. This is surprising since such young stars are not expected to have large planetary bodies capable of producing the structures we see in this image,” said Stuartt Corder, ALMA Deputy Director.

When we first saw this image we were astounded at the spectacular level of detail. HL Tauri is no more than a million years old, yet already its disc appears to be full of forming planets. This one image alone will revolutionise theories of planet formation,” explained Catherine Vlahakis, ALMA Deputy Program Scientist and Lead Program Scientist for the ALMA Long Baseline Campaign.

HL Tauri’s disc appears much more developed than would be expected from the age of the system. Thus, the ALMA image also suggests that the planet-formation process may be faster than previously thought.

Such high resolution can only be achieved with the long baseline capabilities of ALMA and provides astronomers with new information that is impossible to collect with any other facility, even the NASA/ESA Hubble Space Telescope. “The logistics and infrastructure required to place antennas at such distant locations required an unprecedented coordinated effort by an expert international team of engineers and scientists,” said ALMA Director, Pierre Cox. “These long baselines fulfill one of ALMA’s major objectives and mark an impressive technological, scientific and engineering milestone.”

Young stars like HL Tauri are born in clouds of gas and fine dust, in regions which have collapsed under the effects of gravitation, forming dense hot cores that eventually ignite to become young stars. These young stars are initially cocooned in the remaining gas and dust, which eventually settles into a disc, known as a protoplanetary disc.

Through many collisions the dust particles will stick together, growing into clumps the size of sand grains and pebbles. Ultimately, asteroids, comets and even planets can form in the disc. Young planets will disrupt the disc and create rings, gaps and holes such as those seen in the structures now observed by ALMA [3].

The investigation of these protoplanetary discs is essential to our understanding of how Earth formed in the Solar System. Observing the first stages of planet formation around HL Tauri may show us how our own planetary system may have looked more than four billion years ago, when it formed.

Most of what we know about planet formation today is based on theory. Images with this level of detail have up to now been relegated to computer simulations or artist’s impressions. This high resolution image of HL Tauri demonstrates what ALMA can achieve when it operates in its largest configuration and starts a new era in our exploration of the formation of stars and planets,” says Tim de Zeeuw, Director General of ESO.

Notes

[1] Since September 2014 ALMA has been observing the Universe using its longest ever baselines, with antennas separated by up to 15 kilometres. This Long Baseline Campaign will continue until 1 December 2014. The baseline is the distance between two of the antennas in the array. As a comparison, other facilities operating at millimetre wavelengths provide antennas separated by no more than two kilometres. The maximum possible ALMA baseline is 16 kilometres. Future observations at shorter wavelengths will achieve even higher image sharpness.

[2] The structures are seen with a resolution of just five times the distance from the Sun to the Earth. This corresponds to an angular resolution of about 35 milliarcseconds — better than what is routinely achieved with the NASA/ESA Hubble Space Telescope.

[3] In visible light, HL Tauri is hidden behind a massive envelope of dust and gas. ALMA observes at much longer wavelengths, which allows it to study the processes right at the core of this cloud.