Unlocking the secrets of the universe

Long ago, about 400,000 years after the beginning of the universe (the Big Bang), the universe was dark. There were no stars or galaxies, and the universe was filled primarily with neutral hydrogen gas.

This artist's rendering shows the universe's first, massive, blue stars embedded in gaseous filaments, with the cosmic
microwave background just visible at the edges. Using radio observations of the distant universe, NSF-funded researchers
Judd Bowman of Arizona State University, Alan Rogers of MIT and their colleagues discovered the influence of such early
stars on primordial gas. Although they can't directly see the light from the massive stars, Bowman's team was able to infer
their presence from dimming of the cosmic microwave background (CMB), a result of the gaseous filaments absorbing the
stars' UV light. The CMB is dimmer than expected, indicating that the filaments may have been colder than expected,
possibly from interactions with dark matter [Credit: N.R.Fuller, National Science Foundation]

Then, for the next 50-100 million years, gravity slowly pulled the densest regions of gas together until ultimately the gas collapsed in some places to form the first stars.

What were those first stars like and when did they form? How did they affect the rest of the universe? These are questions astronomers and astrophysicists have long pondered.

Now, after 12 years of experimental effort, a team of scientists, led by ASU School of Earth and Space Exploration astronomer Judd Bowman, has detected the fingerprints of the earliest stars in the universe. Using radio signals, the detection provides the first evidence for the oldest ancestors in our cosmic family tree, born by a mere 180 million years after the universe began.

"There was a great technical challenge to making this detection, as sources of noise can be a thousand times brighter than the signal -- it's like being in the middle of a hurricane and trying to hear the flap of a hummingbird's wing." says Peter Kurczynski, the National Science Foundation program officer who supported this study. "These researchers with a small radio antenna in the desert have seen farther than the most powerful space telescopes, opening a new window on the early universe."

Radio Astronomy

To find these fingerprints, Bowman's team used a ground-based instrument called a radio spectrometer, located at the Australia's national science agency (CSIRO) Murchison Radio-astronomy Observatory (MRO) in Western Australia. Through their Experiment to Detect the Global EoR Signature (EDGES), the team measured the average radio spectrum of all the astronomical signals received across most of the southern-hemisphere sky and looked for small changes in power as a function of wavelength (or frequency).

In each instrument, radio waves are collected by an antenna consisting of two rectangular metal panels mounted
horizontally on fiberglass legs above a metal mesh. The EDGES detection required the radio quietness at the Murchison
Radio-astronomy Observatory, as Australian national legislation limits the use of radio transmitters near the site. This
discovery sets the stage for follow-up observations with other powerful low-frequency facilities, including HERA
and the forthcoming SKA-low [Credit: CSIRO Australia]

As radio waves enter the ground-based antenna, they are amplified by a receiver, and then digitized and recorded by computer, similar to how FM radio receivers and TV receivers work. The difference is that the instrument is very precisely calibrated and designed to perform as uniformly as possible across many radio wavelengths.

The signals detected by the radio spectrometer in this study came from primordial hydrogen gas that filled the young universe and existed between all the stars and galaxies. These signals hold a wealth of information that opens a new window on how early stars -- and later, black holes, and galaxies -- formed and evolved.

"It is unlikely that we'll be able to see any earlier into the history of stars in our lifetimes," says Bowman. "This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries."

This detection highlights the exceptional radio quietness of the MRO, particularly as the feature found by EDGES overlaps the frequency range used by FM radio stations. Australian national legislation limits the use of radio transmitters within 161.5 miles (260 km) of the site, substantially reducing interference which could otherwise drown out sensitive astronomy observations.

The results of this study have been recently published in Nature by Bowman, with co-authors Alan Rogers of the Massachusetts Institute of Technology's Haystack Observatory, Raul Monsalve of the University of Colorado, and Thomas Mozdzen and Nivedita Mahesh also of ASU's School of Earth and Space Exploration.

Unexpected results

The results of this experiment confirm the general theoretical expectations of when the first stars formed and the most basic properties of early stars.

A timeline of the universe, updated to show when the first stars emerged. This updated timeline of the universe reflects the
recent discovery that the first stars emerged by 180 million years after the Big Bang. The research behind this timeline
was conducted by Judd Bowman of Arizona State University and his colleagues, with funding from
 the National Science Foundation [Credit: N.R.Fuller, National Science Foundation]

"What's happening in this period," says co-author Rogers of MIT's Haystack Observatory, "is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It's causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies. This is the first real signal that stars are starting to form, and starting to affect the medium around them."

The team originally tuned their instrument to look later in cosmic time, but in 2015 decided to extend their search. "As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature," Rogers says. "We see this dip most strongly at about 78 megahertz, and that frequency corresponds to roughly 180 million years after the Big Bang," Rogers says. "In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest."

The study also revealed that gas in the universe was probably much colder than expected (less than half the expected temperature). This suggests that either astrophysicists' theoretical efforts have overlooked something significant or that this may be the first evidence of non-standard physics: Specifically, that baryons (normal matter) may have interacted with dark matter and slowly lost energy to dark matter in the early universe, a concept that was originally proposed by Rennan Barkana of Tel Aviv University.

"If Barkana's idea is confirmed," says Bowman, "then we've learned something new and fundamental about the mysterious dark matter that makes up 85 percent of the matter in the universe, providing the first glimpse of physics beyond the standard model."

The next steps in this line of research are for another instrument to confirm this team's detection and to keep improving the performance of the instruments, so that more can be learned about the properties of early stars. "We worked very hard over the last two years to validate the detection," says Bowman, "but having another group confirm it independently is a critical part of the scientific process."

Bowman would also like to see an acceleration of efforts to bring on new radio telescopes like the Hydrogen Epoch of Reionization Array (HERA) and the Owens Valley Long Wavelength Array (OVRO-LWA).

"Now that we know this signal exists," says Bowman, "we need to rapidly bring online new radio telescopes that will be able to mine the signal much more deeply."

The antennas and portions of the receiver used in this experiment were designed and constructed by Rogers and the MIT Haystack Observatory team. The ASU team and Monsalve added the automated antenna reflection measurement system to the receiver, outfitted the control hut with the electronics, constructed the ground plane and conducted the field work for the project. The current version of EDGES is the result of years of design iteration and ongoing detailed technical refinement of the calibration instrumentation to reach the levels of precision necessary for successfully achieving this difficult measurement.

Source: Arizona State University [February 28, 2018]

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Stars around the Milky Way: Cosmic space invaders or victims of galactic eviction?

An international team of astronomers led by the Max Planck Institute for Astronomy (MPIA) has made a surprising discovery about the birthplace of groups of stars located in the halo of our Milky Way galaxy.

The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. 
The locations of the observed stars above and below the disc, which are used to test the perturbation scenario, 
are indicated [Credit: T.Mueller/NASA/JPL-Caltech]

These halo stars are grouped together in giant structures that orbit the center of our galaxy, above and below the flat disk of the Milky Way. Researchers thought they may have formed from debris left behind by smaller galaxies that invaded the Milky Way in the past.

But in a study published today in the journal Nature, astronomers now have compelling evidence showing that some of these halo structures actually originate from the Milky Way's disk itself, but were kicked out.

"This phenomenon is called galactic eviction," said co-author Judy Cohen, Kate Van Nuys Page Professor of Astronomy at Caltech. "These structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk. This passage causes oscillations, or waves, that eject stars from the disk, either above or below it depending on the direction that the perturbing mass is moving."

"The oscillations can be compared to sound waves in a musical instrument," said lead author Maria Bergemann of MPIA. "We call this 'ringing' in the Milky Way galaxy 'galactoseismology,' which has been predicted theoretically decades ago. We now have the clearest evidence for these oscillations in our galaxy's disk obtained so far!"

For the first time, Bergemann's team presented detailed chemical abundance patterns of these halo stars using the W. M. Keck Observatory on Maunakea, Hawaii."

The analysis of chemical abundances is a very powerful test, which allows, in a way similar to the DNA matching, to identify the parent population of the star. Different parent populations, such as the Milky Way disk or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately link them to their parent populations," said Bergemann.

With Andromeda no longer considered the Milky Way's big brother, new simulations are needed to find out what will happen when the two galaxies eventually collide.

Dr Kafle used a similar technique to revise down the weight of the Milky Way in 2014, and said the latest finding had big implications for our understanding of our nearest galactic neighbours.

"It completely transforms our understanding of the local group," he said.

"We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed.

"It's really exciting that we've been able to come up with a new method and suddenly 50 years of collective understanding of the local group has been turned on its head."

University of Sydney astrophysicist Professor Geraint Lewis said it was exciting to be at a time when the data was getting so good.

"We can put this gravitational arms race to rest," he said.

Source: International Centre for Radio Astronomy Research [February 14, 2018]

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Supermassive black hole model predicts characteristic light signals at cusp of collision

A new simulation of supermassive black holes -- the behemoths at the centers of galaxies -- uses a realistic scenario to predict the light signals emitted in the surrounding gas before the masses collide, said Rochester Institute of Technology researchers.

Magnetic field lines emanate from a pair of supermassive black holes nearing merger within a large gas disk in a simulation
by RIT scientists. Periodic light signals in the gas disk could someday help scientists locate supermassive binary
black holes [Credit: RIT Center for Computational Relativity and Gravitation]

The RIT-led study represents the first step toward predicting the approaching merger of supermassive black holes using the two channels of information now available to scientists -- the electromagnetic and the gravitational wave spectra -- known as multimessenger astrophysics. The findings appear in the paper "Quasi-periodic Behavior of Mini-disks in Binary Black Holes Approaching Merger," published in the Astrophysical Journal Letters.

"We've performed the first simulation in which an accretion disk around a binary black hole feeds individual accretion disks, or mini-disks, around each black hole in general relativity and magnetohydrodynamics," said Dennis Bowen, lead author and postdoctoral researcher at RIT's Center for Computational Relativity and Gravitation.

Unlike their less massive cousins, first detected in 2016, supermassive black holes are fed by gas disks that surround them like doughnuts. The strong gravitational pull of the black holes that inspiral toward one another heats and disrupts the flow of gas from disk to black hole and emits periodic signals in the visible to X-ray portions of the electromagnetic spectrum.

"We have not yet seen two supermassive black holes get this close," Bowen said. "It provides the first hints of what these mergers will look like in a telescope. The filling and refilling of mini-disks affect the light signatures."

Two supermassive black holes at the center of a large gas disk are on a collision course in a time sequence simulated
by RIT scientists. An alternating flow of gas fills and depletes mini disks feeding the black holes, shown above.
Characteristic light signals emitted in the gas could mark the location of the invisible masses.
(Note: The dot at the center of the image is not part of the simulation)
[Credit: RIT Center for Computational Relativity and Gravitation]

The simulation models supermassive black holes in a binary pair, each surrounded by its own gas disks. A much larger gas disk encircles the black holes and disproportionately feeds one mini-disk over another, leading to the filling-and-refilling cycle described in the paper.

"The evolution is long enough to study what the real science outcome would look like," said Manuela Campanelli, director of the Center for Computational Relativity and Gravitation and a co-author on the paper.

Binary supermassive black holes emit gravitational waves at lower frequencies than stellar-mass black holes. The ground-based Laser Interferometer Gravitational-wave Observatory, in 2016, detected the first gravitational waves from stellar mass black holes collisions with an instrument tuned to higher frequencies. LIGO's sensitivity is unable to observe the gravitational wave signals produced by supermassive black hole coalescence.

The launch of the space-based Laser Interferometer Space Antenna, or LISA, slated for the 2030s, will detect gravitational waves from colliding supermassive black holes in the cosmos. When operational in the 2020s, the ground-based Large Synoptic Survey Telescope, or LSST, under construction in Cerro Pachón, Chile, will produce the widest, deepest survey of light emissions in the universe. The pattern of signals predicted in the RIT study could guide scientists to orbiting pairs of supermassive black holes.

Two supermassive black holes at the center of a large gas disk are on a collision course in a time 

sequence simulated by RIT scientists. An alternating flow of gas fills and depletes mini disks feeding 

the black holes, shown above. Characteristic light signals emitted in the gas could mark the location

 of the invisible masses. (Note: The dot at the center of the image is not part of the simulation) 

[Credit: RIT Center for Computational Relativity and Gravitation]

"In the era of multimessenger astrophysics, simulations such as this are necessary to make direct predictions of electromagnetic signals that will accompany gravitational waves," Bowen said. "This is the first step toward the ultimate goal of simulations capable of making direct predictions of the electromagnetic signal from binary black holes approaching merger."

Bowen and his collaborators combined simulations from RIT's Black Hole Lab computer clusters and the Blue Waters supercomputer at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, one of the largest supercomputers in the United States.

Astrophysicists from RIT, Johns Hopkins University and NASA Goddard Space Flight Center collaborated on the project. The publication is based on Bowen's Ph.D. dissertation at RIT and completes research begun by a co-author, Scott Noble, a former RIT post-doctoral researcher, now at NASA Goddard. Their research is part of a collaborative National Science Foundation-funded project led by Campanelli. Co-authors include Vassilios Mewes, RIT postdoctoral researcher; Miguel Zilhao, former RIT post-doctoral researcher, now at Universidade de Lisboa, in Portugal; and Julian Krolik, professor of physics and astronomy at Johns Hopkins University.

In an upcoming paper, the authors will explore further the correlation between gas flowing in and out of the accretion disks and fluctuating light emissions. They will present predictions of light signatures scientists can expect to see with advanced telescopes when looking for supermassive black holes approaching merger.

Author: Susan Gawlowicz | Source: Rochester Institute of Technology [Febraury 14, 2018]

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New models give insight into the heart of the Rosette Nebula

A hole at the heart of a stunning rose-like interstellar cloud has puzzled astronomers for decades. But new research, led by the University of Leeds, offers an explanation for the discrepancy between the size and age of the Rosetta Nebula's central cavity and that of its central stars.

Rosette Nebula image is based on data obtained as part of the INT Photometric H-Alpha Survey of the Northern
Galactic Plane, prepared by Nick Wright, Keele University, on behalf of the IPHAS Collaboration
[Credit: Nick Wright, Keele University]

The Rosette Nebula is located in the Milky Way Galaxy roughly 5,000 light-years from Earth and is known for its rose-like shape and distinctive hole at its centre. The nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases with several massive stars found in a cluster at its heart.

Stellar winds and ionising radiation from these massive stars affect the shape of the giant molecular cloud. But the size and age of the cavity observed in the centre of Rosette Nebula is too small when compared to the age of its central stars.

Through computer simulations, astronomers at Leeds and at Keele University have found the formation of the Nebula is likely to be in a thin sheet-like molecular cloud rather than in a spherical or thick disc-like shape, as some photographs may suggest. A thin disc-like structure of the cloud focusing the stellar winds away from the cloud's centre would account for the comparatively small size of the central cavity.

3-D visualization of the simulated nebula, showing the dense disc-like molecular cloud in red, the tenuous
stellar wind focused away from the disc in blue and the magnetic field lines in grey. The magnetic
field is of key importance in forming a disc-like, not spherical, molecular cloud
[Credit: C. J. Wareing et al., 2018, MNRAS]

Study lead author, Dr Christopher Wareing, from the School of Physics and Astronomy said: "The massive stars that make up the Rosette Nebula's central cluster are a few millions of years old and halfway through their lifecycle. For the length of time their stellar winds would have been flowing, you would expect a central cavity up to ten times bigger.

"We simulated the stellar wind feedback and formation of the nebula in various molecular cloud models including a clumpy sphere, a thick filamentary disc and a thin disc, all created from the same low density initial atomic cloud.

"It was the thin disc that reproduced the physical appearance - cavity size, shape and magnetic field alignment -- of the Nebula, at an age compatible with the central stars and their wind strengths.

Slice through the simulation of the Rosette Nebula, perpendicular to the disc of the molecular cloud. The disc
of the molecular cloud (shown in red) is clearly focussing the wind from the central star (shown in blue)
away from the cloud and into the surroundings of the cloud (shown in green)
[Credit: C. J. Wareing et al., 2018, MNRAS]

"To have a model that so accurately reproduces the physical appearance in line with the observational data, without setting out to do this, is rather extraordinary.

"We were also fortunate to be able to apply data to our models from the ongoing Gaia survey, as a number of the bright stars in the Rosette Nebula are part of the survey.

Applying this data to our models gave us new understanding of the roles individual stars play in the Rosette Nebula. Next we'll look at the many other similar objects in our Galaxy and see if we can figure out their shape as well."

The simulations, published in the Monthly Notices of the Royal Astronomical Society, were run using the Advanced Research Computing centre at Leeds. The nine simulations required roughly half a million CPU hours -- the equivalent to 57 years on a standard desktop computer.

Martin Callaghan, a member of the Advanced Research Computing team, said: "The fact that the Rosette Nebula simulations would have taken more than five decades to complete on a standard desktop computer is one of the key reasons we provide powerful supercomputing research tools. These tools enabled the simulations of the Rosette Nebula to be done in a matter of a few weeks."

Source: University of Leeds [February 13, 2018]

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