NSF VLA Maps a Hidden Hydrogen Shell Around the Orion Nebula

Radio emission from neutral hydrogen atoms in the direction of the Orion Nebula, the most nearby regions of high-mass star formation. The red colors show the 21-cm emission from hydrogen, resolved for the first time at this level of detail by observations from the Neutral Atomic Hydrogen in the Solar Neighborhood (NeAtHood) project, led by Juan Diego Soler from the University of Vienna. The cyan colors show the emission from warm interstellar dust in near-infrared light. Credit: Juan D. Soler, University of Vienna, with data from the NRAO's Jansky VLA and NASA's Wide-field Infrared Survey Explorer (WISE).

Hi-Res File

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New view of a familiar nebula

The Orion Nebula is perhaps the best known nebula, yet new discoveries continue to emerge from observations across all wavelengths of the electromagnetic spectrum. Long known to be a region of active star formation, astronomers using the U.S. National Science Foundation Very Large Array (NSF VLA) have resolved the emission from neutral atomic hydrogen that elucidates how the young stars in Orion are shaping their surrounding neighborhood.

A team of astronomers led by Juan D. Soler of the University of Vienna used the NSF VLA, operated by the U.S. National Science Foundation’s National Radio Astronomy Observatories (NSF NRAO), to obtain high-resolution observations of the emission from neutral hydrogen atoms (HI) at 21 centimeters wavelength. Neutral atomic hydrogen, or H I, is traced by faint radio emission at a wavelength of 21 centimeters. Mapping that emission at high angular resolution is challenging, but the NSF VLA’s interferometric design makes it possible to resolve the structure of nearby star-forming regions.The NSF VLA, therefore, proved to be the best possible instrument for the task, Soler explains. “The NSF VLA is a very unique instrument. It is fundamental, offering the best resolution that we can get in HI from the northern hemisphere. We cannot do this with any other instrument.”

Measuring the bubble directly

When Soler and his team looked deep within the Orion Nebula, they focused on a previously identified expanding bubble formed by young stars. These past observations used tracers such as CII as a proxy for the hydrogen and offered an estimate of the mass associated with star formation. Because Soler’s team was able to observe the HI directly, their data provided a more direct estimate of the bubble’s mass,improving measurements by a factor of ten. “Measuring mass is fundamental,” Soler says, “because it tells us how efficiently these newly formed stars shape their environment with wind and radiation.”

Furthermore, the data obtained by Soler’s team also map the neutral atomic hydrogen in the vast molecular clouds within the Orion Nebula. Soler describes this interrelationship: “Talking about molecular clouds without talking about HI, is like talking about islands without ever mentioning the sea. It turns out that as we’re resolving the sea, we’re finding phenomena like this bubble that are shaping and connecting those molecular clouds. We imagine them as separate objects, not as if they were islands, completely isolated; in fairness, they’re more like archipelagos.”

First result from NeAtHood

These results, published in Astronomy & Astrophysics, are the first within a larger project called Neutral Atomic Hydrogen in the Solar System Neighborhood (NeAtHood), which aims to continue observing other nearby molecular clouds within star-forming regions visible from the northern hemisphere to produce arcminute-resolution HI maps. Soler looks to the possibility of future observations both within this program and beyond, saying, “[The bubble] is the kind of thing that should be there, but we were not expecting to see this so clearly. So now I wonder, what other surprises are hiding in the data? And in the future, the Next-Generation VLA (ngVLA) is going to target even more distant regions in the Milky Way.”

Source: National Radio Astronomy Observatory (NRAO)/News

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Links: Scientific Paper

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About NRAO

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Wanted: the rotating radio emission of the Milky Way

Fig. 1: Radio map of the Milky Way. This map shows the amount of electrons with nearly the speed of light times combined with the magnitude of the transverse component of the galactic magnetic field, projected along each line of sight through the Milky Way. © MPA

The magnetic fields of the Milky Way cause electrons with nearly the speed of light to rotate and to emit radio waves. As consequence, this radiation should also "rotate" in some way, i.e. the polarization of the radiation will change. This very weak circular polarization of the Milky Way, however, has not been observed so far. Researchers at the Max Planck Institute for Astrophysics and colleagues have now predicted some properties of this polarization and created a "wanted poster" to allow targeted searches. A measurement of the circular polarization would provide important insights into the structure of the galactic magnetic fields and confirm that electrons - and not positrons - are the source of this radio emission in the Milky Way.

The vast space in between the Milky Way stars is not empty; it is filled with gas, dust, magnetic fields, and particles with almost the speed of light - the so-called cosmic radiation. This consists of atomic nuclei, electrons and small amounts of antimatter, especially positrons and antiprotons. Part of the cosmic radiation reaches the earth directly, but it can also be detected indirectly. The ultra-fast electrons and positrons emit radiation, which has already been detected and measured (Fig. 1). So far, however, it is almost impossible to distinguish whether this radio emission originates from electrons or positrons.

The circularly polarized radiation could tip the scientists off, since electrons and positrons rotate in opposite directions. However, this radiation is less than one thousandth of the galactic radio emission; researchers therefore have been unable to detect it. Moreover, astronomers do not have a clear idea of what patterns to look for in the sky, they do not know what this radiation should look like. This gap has now been filled by Torsten Enßlin and his colleagues. The astrophysicists show that the current information about the magnetic field of the Milky Way is enough to estimate the circular polarization.

Three conditions must be fulfilled for a region in space to radiate circular polarized light. First, there must be an excess of electrons (or positrons) with almost the speed of light, so that the rotation of these particles in the magnetic field will be in a preferred direction. Second, the magnetic field has to be at least partially aligned with the observer so that the direction of rotation is visible in the sky projection. And third, the magnetic field must not be completely in the direction of the line of sight, since the radio waves are mainly emitted transversely to the magnetic field.

Information about both the amount of electrons and positrons with almost the speed of light and the transverse component of the magnetic field is given by the radio map of the Milky Way (Fig. 1). In general, it is assumed that this emission is generated mainly by electrons with only a small contribution by positrons.

Fig. 2: Map of the Faraday effect in the Milky Way. This map shows the line-of-sight component of the galactic magnetic field weighted with the amount of thermal electrons, projected along each line of sight through the Milky Way. Regions where the magnetic field is mainly directed at us are red and regions in which it points away from us are blue. © MPA

Information on the line of sight component of the magnetic field comes from measurements of the so-called Faraday effect. Linearly polarized light, radiated from radio-galaxies outside the Milky Way, is being rotated as it traverses the galactic magnetic field. This rotation depends on both the intensity and the orientation of the magnetic field along the line of sight. Radio waves interact with slow thermal electrons in the galactic gas, which perform circular motions in the magnetic fields. The rotation of the linear polarization of the lightwaves is in the opposite direction as the rotation of these electrons. Since the magnitude of the Faraday effect varies with the frequency of the radiation, it can be detected and mapped. In this way, a Faraday map of the sky was produced already in 2012 by Niels Oppermann working with Torsten Enßlin (Fig. 2, MPA Highlight November 2012). This shows the summed up magnetic field component that is aligned with any given line-of-sight.

Thus, all three necessary components are known: the number of electrons at nearly the speed of light, and the strengths of the two magnetic field components involved. The information from observations, however, is always given only as a projection along a line of sight. For an accurate prediction of the circular polarization, further data is needed to describe how these three components are distributed along the lines of sight.

Fig. 3: Map of the intensity and direction of rotation of the circular polarization of the radio emission. Regions in which the polarization is predominantly clockwise are red, and regions with counterclockwise polarization are blue. This prediction was made by combining the intensity maps of galactic radio emission (Figure 1), the Faraday effect (Figure 2), and a rough model of the 3D distribution of galactic electrons. The details of the true circular polarization will differ, but the map should show the correct direction of rotation more often than not if our knowledge of particles at near the speed of light in the Milky Way is more or less correct. © MPA

For an estimation of this distribution in the third dimension, Torsten Enßlin used both known and plausible statistical properties of turbulent magnetic fields. Thus, he was able to show that the exact details of the statistics do not have much influence on the results, as long as the magnetic fields do exhibit any improbable structure. The PhD student Sebastian Hutschenreuter then made a prediction of the circular polarization using a coarse model of the distribution of both thermal and highly energetic electrons, as well as the magnetic energy contained in the Milky Way from observed radio and Faraday maps (Figure 3).

The details of the prediction will not be accurate in all details as there were uncertain assumptions. However, the map should indicate the preferred direction of rotation of the actual circular polarization more often than not. This statistical prediction therefore is suitable in searches for the extremely weak circular polarization signal.

The next step will be to look for the predicted small circular polarization pattern in the data of both existing and soon-to-come terrestrial radio telescopes. If astronomers were able to actually detect the "rotating radiation", astronomers could draw important conclusions about the galactic magnetic field and confirm that electrons and not positrons are the source of this radiation in the Milky Way.

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 Original Publication

1. Torsten A. Enßlin, Sebastian Hutschenreuter, Valentina Vacca, and  Niels Oppermann
 
The Galaxy in circular polarization: all-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays

submitted
 
Source

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Authors

Enßlin, Torsten
Scientific Staff 
Phone: 2243  
Email: tensslin@mpa-garching.mpg.de
Links: personal homepage (the institute is not responsible for the contents of personal homepages)

 
Hutschenreuter, Sebastian  
PhD student 

Source: Max Planck Institute for Astrophysics

Hitomi Mission Charts Hot Winds of a Galaxy Cluster for the First Time

Measurements of unprecedented detail returned by Japan's Hitomi satellite have allowed scientists to track the motion of X-ray-emitting gas at the heart of the Perseus cluster of galaxies for the first time. The results showcase the long-awaited premiere of a next-generation X-ray instrument whose key components were developed at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

A revolutionary instrument aboard the ill-fated Hitomi satellite returned the most detailed measurements yet made of the million-degree atmosphere at the core of a galaxy cluster. Watch the video to learn more. Credits: NASA's Goddard Space Flight Center. Download the video in HD at NASA's Scientific Visualization Studio

Led by the Japan Aerospace Exploration Agency (JAXA), Hitomi was launched on Feb. 17. Following the successful activation of the observatory and instruments, Hitomi suffered a mission-ending spacecraft anomaly on March 26.

Before its demise, though, Hitomi was able to peer into the Perseus cluster of galaxies, an assemblage of thousands of galaxies bound together by gravity. Located about 240 million light-years away and named for its host constellation, the Perseus galaxy cluster contains a vast amount of extremely hot gas. At temperatures averaging 90 million degrees Fahrenheit (50 million degrees Celsius), the gas glows brightly in X-rays. Prior to Hitomi's launch, astronomers lacked the capability to measure the detailed dynamics of this gas, particularly its relationship to bubbles of gas expelled by an active supermassive black hole in the cluster's core galaxy, NGC 1275.

"For the first time, we have mapped the motion of the X-ray-emitting gas in a cluster of galaxies and determined its velocity structure over a wide range of spatial scales," said Goddard's Richard Kelley, the U.S. principal investigator for the Hitomi collaboration. "Although this gas is continually stirred by fast outflows from the central black hole, its velocities are small on astronomical scales and show evidence for only minor levels of turbulence."

The Perseus galaxy cluster is filled with hot X-ray-emitting gas, as seen in this image from NASA's Chandra X-ray Observatory. Astronomers using the Soft X-ray Spectrometer aboard the Hitomi satellite have, for the first time, mapped the motion of this gas and determined its velocity structure across a large part of the cluster. The square overlay, which spans about 195,000 light-years at the cluster's distance, shows the area observed by Hitomi. Colors correspond to the detected gas speeds, with bluer colors indicating faster motion toward Earth and redder colors showing greater velocities in the opposite direction. Credits: NASA Goddard and NASA/CXC/SAO/E. Bulbul, et al. Download additional visuals at NASA's Scientific Visualization Studio

Hitomi's revolutionary Soft X-ray Spectrometer (SXS), developed and built by Goddard scientists working closely with colleagues from several institutions in the United States, Japan, and the Netherlands, spent more than two and a half days observing a wide area in the cluster's core. The resulting X-ray spectrum, which provides 30 times the detail of the best previous observation, unveiled a rich landscape of X-ray emission lines from iron, nickel, chromium, and manganese -- metals forged in billions of massive stars in the cluster's galaxies and dispersed when they exploded as supernovae.

The SXS studied a square patch of the sky measuring about 195,000 light-years on a side at the cluster's distance. The total range of gas velocities directed toward or away from Earth within this square was found to be about 365,000 miles an hour (590,000 kilometers per hour) -- enormous by human standards but surprisingly modest on cosmic scales. The observed velocity range indicates that turbulence is responsible for only about 4 percent of the total gas pressure. This result is of particular interest to astrophysicists. Turbulent pressure was a previously unmeasured quantity that could significantly impact estimates of the cluster's mass. The SXS measurements show that only minor corrections are needed.

"I'm surprised the hot gas is absorbing the power output of the black hole so quickly, so efficiently. The gas is relatively stable and isn’t getting pushed around as much as we thought," said team member Brian McNamara, a professor of physics and astronomy at the University of Waterloo in Ontario. "Hitomi's Perseus observation tells us that we can probably weigh distant galaxy clusters to greater accuracy than we can weigh our own Milky Way galaxy."

In visible light, the core of the Perseus galaxy cluster looks completely different. The cluster gas so prominent at X-ray wavelengths gives way to a clutch of fuzzy galaxies viewed through a scrim of faint stars in our own. This image shows the same sky area as the Chandra image above. The bizarre object at the center is NGC 1275, the cluster's central galaxy and itself a prominent source of X-ray and radio emission. At the galaxy's core lies a supermassive black hole activated by infalling matter. Glowing filaments extending from the galaxy trace bubbles of gas blown violently outward by the black hole. Despite this activity, Hitomi measurements show that hot cluster gas in the vicinity is moving at cosmically modest speeds. Credits: R. Jay GaBany. Copyright © 2008 by R. Jay GaBany, used with permission

The Hitomi spacecraft, then known as ASTRO-H, as it appeared on Nov. 27, 2015, at Tsukuba Space Center in Japan. The open compartment visible at the spacecraft's lower left houses the Soft X-ray Spectrometer.Credits: JAXA

A paper from the Hitomi collaboration details these findings in the July 7 edition of the journal Nature.

To determine motion at cosmic scales, astronomers typically disperse light into a rainbow-like spectrum. Discrete bright or dark lines in the spectrum correspond to wavelengths characteristically emitted or absorbed by specific chemical elements. For gas moving away from or toward Earth, these lines will be displaced toward the red or blue end of the spectrum, respectively, by an amount depending on the gas speed. Gas motion may also blur or broaden these lines, providing another clue to the cluster's environment.

Hitomi's SXS employed a revolutionary approach for measuring X-ray "colors" called microcalorimetry. The technique produces unprecedented spectral resolution without blurring the spectrum when looking at sources occupying a relatively large part of the sky, which occurs with traditional methods. The SXS microcalorimeter measured heat produced when individual particles of light, called photons, struck the detector. This resulted in a tiny temperature increase that precisely determined the energy of the incident X-ray light.

During the SXS check-out phase, a door in its line of sight remained closed to protect the microcalorimeter from potential contaminants as materials aboard the spacecraft emitted residual gas into the vacuum of space. This door was designed to transmit X-rays at a reduced level, but it necessarily absorbed some lower-energy X-rays before they could reach the detector.

"The SXS, a technological marvel, worked spectacularly well, exceeding its performance goal despite being partially obstructed," said co-author Robert Petre, chief of Goddard's X-ray Astrophysics Laboratory and the U.S. project scientist for Hitomi.

The Perseus observation provides a tantalizing glimpse of the tremendous advance that X-ray microcalorimetry will bring to astrophysics. U.S. researchers pioneered development of the technology in the 1980s, but Hitomi's all-too-brief run represents its most successful space application to date.

"This is the first time we've looked at a galaxy cluster with an instrument capable of resolving the components of various atomic emission lines, and we immediately saw contradictions with current models," said team member Maxim Markevitch at Goddard. "This is a long-awaited tool for diagnosing the conditions in cosmic plasmas that we can finally apply to galaxy clusters, and there will be a number of papers to come based on these data."

Hitomi, which translates to "pupil of the eye," was known before launch as ASTRO-H. The mission was developed by the Institute of Space and Astronautical Science, a division of JAXA. It was built jointly by an international collaboration led by JAXA, with contributions from NASA Goddard and other institutions in the U.S., Japan, Canada and Europe.

By Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.
Editor: Ashley Morrow

Source:  NASA/Hitomi