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CME at Earth

Coronal mass ejections (CMEs), enormous releases of energy from the Sun, can have significant space-weather implications for Earth. Do similar storms from smaller stars — M dwarfs like V374 Peg, or the nearby Proxima Centauri — mean bad news for the planets that these stars host?

Volatile Stars

Habitable zones

Difference in habitable-zone sizes for different stellar types. [NASA]

When plasma is released from the Sun in the form of a CME traveling toward Earth, these storms can be powerful enough to disrupt communications and navigational equipment, damage satellites, and cause blackouts — even with our planetary magnetic field to protect us! How might planets in the habitable zone of M-dwarf stars fare against similar storms?

The first danger for an M dwarf’s planets is that the habitable zone lies much closer to the star: it can range from 0.03 to 0.4 AU (i.e., within Mercury’s orbit). Being so close to the star definitely makes a planet in an M dwarf’s habitable zone vulnerable to storms.

Probability of impact

Colors indicate the probability of CME impact, for different different stellar latitudes where the CME originated vs. orbital inclination of the planet, (a) without any deflection, and (b) taking into account the CME deflection by the star’s magnetic field. Hanging out in an orbit aligned with the current sheet turns out to be a bad idea. [Adapted from Kay et al. 2016]

What about the storms themselves? You might think that because M dwarfs are cooler stars, they would be quieter, releasing fewer CMEs with less energy. Surprisingly, the opposite is true: M dwarfs are significantly more active than solar-type stars, and the CMEs are typically ten times more massive than those released from the Sun. Impacts from these powerful outbursts could easily strip any existing planet atmosphere, making a planet much less likely to be habitable. To make matters worse, M dwarfs can remain magnetically active for billions of years: even a star like Proxima Centauri, which is nearly 5 billion years old, is still relatively active.

Dodging Deflected Storms

Interestingly, an important factor in the survival of an M dwarf’s habitable-zone planet is the plane in which the planet’s orbit lies. A team of scientists led by Christina Kay (NASA Goddard’s Solar Physics Laboratory and Boston University) recently modeled CMEs from V374 Peg, a mid-type M dwarf of roughly a third of the Sun’s mass and radius, to determine how the CMEs propagate and the probability that they’ll impact a hypothetical planet in the star’s habitable zone.

The team shows that traveling CMEs tend to be deflected by the star’s magnetic field. Instead of propagating purely radially outward, the CMEs are pushed toward the astrospheric current sheet — the minimum point of the background magnetic field — which moves around, but is generally located toward the stellar equatorial plane.

Kay and collaborators find that planet orbits roughly aligned with the current sheet therefore have a higher probability of getting hit by a CME: around 10%. In contrast, planets with higher-inclination orbits have CME impact probabilities around 1%. These probabilities translate to an impact rate of about 0.5–5 times per day for a habitable-zone planet around a mid-type M dwarf — which is 2–20 times the average at Earth during solar maximum!

Magnetic field strength needed

Minimum planetary magnetic field strength required to sustain a magnetosphere twice the size of the planetary radius for different CME masses and speeds, for a 1 kG (left) and 20 kG (right) initial CME magnetic field strength. A typical CME requires a field strength of 10–100 G. [Adapted from Kay et al. 2016]

Is There Hope for Planet Habitability?

With this many CME impacts even outside of the current-sheet plane, how can a planet hope to survive? The key lies in having a strong magnetic field to protect the planet. Such a field would deflect the charged particles from the CME, preventing the CME from stripping the planet’s atmosphere.

Kay and collaborators calculate that a habitable-zone mid-type M-dwarf exoplanet would need a planetary magnetic field between tens and hundreds of Gauss — 1 to 2 orders of magnitude more than that of Earth — to protect itself from these CMEs: difficult to muster, but not impossible!

These results provide some interesting food for thought as we continue to discover new exoplanets orbiting M-dwarf stars.

Citation

C. Kay et al 2016 ApJ 826 195. doi:10.3847/0004-637X/826/2/195

filament

A team of scientists has now uncovered half of the entire “skeleton” of the Milky Way, using an automated method to identify large filaments of gas and dust hiding between stars in the galactic plane.

galactic distribution

Galactic distribution of 54 newly discovered filaments, plotted along with colored lines indicating six relevant spiral arms in our galaxy. The upper two plots show the consistency of the filaments’ motion with the spiral arms, while the lower shows their location within the galactic plane. [Wang et al. 2016]

The Search for Nessie and Friends

The Milky Way’s interstellar medium is structured hierarchically into filaments. These structures are difficult to observe since they largely lie in the galactic plane, but if we can discover the distribution and properties of these filaments, we can better understand how our galaxy formed, and how the filaments affect star formation in our galaxy today.

Some of the largest of the Milky Way’s filaments are hundreds of light-years long — like the infrared dark cloud nicknamed “Nessie”, declared in 2013 to be one of the “bones” of the Milky Way because of its position along the center of the Scutum-Centaurus spiral arm.

Follow-up studies since the discovery of Nessie (like this one, or this) have found a number of additional large-scale filaments, but these studies all use different search methods and selection criteria, and the searches all start with visual inspection — by humans — to identify candidates.

What if we could instead automate the detection process and build a homogeneous sample of the large filaments making up the skeleton of the Milky Way?

Automated Detection

This is exactly what a team of astronomers led by Ke Wang (European Southern Observatory) has done. The group used a customization of an algorithm called a “minimum spanning tree” — the technique used to optimize the cost of internet networks, road networks, and electrical grids in our communities — to perform an automated search of data from the Bolocam Galactic Plane Survey. The search was designed to identify long filaments that are coherent both in physical and velocity space.

Using this method, Wang and collaborators found a total of 54 large-scale filaments that met all of their criteria. The survey covered nearly half of the galactic plane, and the team estimates that there may be a total of ~200 large-scale filaments like these in the Milky Way.

masses and lengths of new filaments

Histograms of the mass and length of the newly discovered filaments (N=54). The distributions for the filaments that are bones (N=13) are overplotted in red. [Adapted from Wang et al. 2016]

A Catalog of Bones and More

The authors generated a catalog of the newly discovered filaments, determining properties like their masses (1,000–100,000 solar masses), lengths (30–900 light-years), aspect ratios, temperatures, and more. They then used this catalog to make several statistical observations:

  1. The filaments are widely distributed across the galactic disk, with roughly 50% located within 65 light-years of the galactic plane (for reference, the Sun is 82 light-years “above” the galactic plane).
  2. Roughly a 1/3 of the filaments are part of the Milky Way’s skeleton, lying along the centers of our galaxy’s spiral arms.
  3. Around 1% of the molecular interstellar medium in our galaxy is confined in large filaments like these.
  4. The formation of massive stars occurs more favorably in large filaments, compared to elsewhere in our galaxy.

This catalog is an important building block in our understanding of the structure of the interstellar medium of our galaxy. The authors next plan to extend this census to the rest of our galaxy, providing us with the best picture yet of the skeleton of the Milky Way.

Bonus

Check out all 54 of the filaments discovered by Wang and collaborators in the gif below (or follow the link to the article to view the original images)! Submillimeter dust emission is shown in red, and Spitzer/WISE 24/22 µm emission is shown in cyan. The connected dots show how the filament was identified by the minimum spanning tree algorithm.

gif of all filaments

Citation

Ke Wang (王科) et al 2016 ApJS 226 9. doi:10.3847/0067-0049/226/1/9

Chariklo rings

We’ve recently discovered narrow sets of rings around two minor planets orbiting in our solar system. How did these rings form? A new study shows that they could be a result of close encounters between the minor planets and giants like Jupiter or Neptune.

Unexpected Ring Systems

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Positions of the centaurs in our solar system (green). Giant planets (red), Jupiter trojans (grey), scattered disk objects (tan) and Kuiper belt objects (blue) are also shown. [WilyD]

Centaurs are minor planets in our solar system that orbit between Jupiter and Neptune. These bodies — of which there are roughly 44,000 with diameters larger than 1 km — have dynamically unstable orbits that cross paths with those of one or more giant planets.

Recent occultation observations of two centaurs, 10199 Chariklo and 2060 Chiron, revealed that these bodies both host narrow ring systems. Besides our four giant planets, Chariklo and Chiron are the only other bodies in the solar system known to have rings. But how did these rings form?

Scientists have proposed several models, implicating collisions, disruption of a primordial satellite, or dusty outgassing. But a team of scientists led by Ryuki Hyodo (Paris Institute of Earth Physics, Kobe University) has recently proposed an alternative scenario: what if the rings were formed from partial disruption of the centaur itself, after it crossed just a little too close to a giant planet?

Tidal Forces from a Giant

Hyodo and collaborators first used past studies of centaur orbits to estimate that roughly 10% of centaurs experience close encounters (passing within a distance of ~2x the planetary radius) with a giant planet during their million-year lifetime. The team then performed a series of simulations of close encounters between a giant planet and a differentiated centaur — a body in which the rocky material has sunk to form a dense silicate core, surrounded by an icy mantle.

simulation outcomes

Some snapshots of simulation outcomes (click for a closer look!) for different initial states of the centaur internal structure, its spin, and the distance of closest approach of the centaur to the giant planet. Blue and red represent icy and silicate material, respectively. [Hyodo et al. 2016]

The outcomes of the close encounters are diverse, depending strongly on the internal structure and spin of the minor planet and the geometry of the encounter. But the team finds that, in many scenarios, the centaur is only partially destroyed by tidal forces from the giant as it passes close by.

In these cases the icy mantle and even some of the centaur’s core can be ripped away and scattered, becoming gravitationally bound to the largest remaining clump of the core. The particles travel in highly eccentric orbits, gradually damping as they collide with each other and forming a disk around the remaining core. Further dynamical evolution of this disk could easily shape the rings that we observe today around Chariklo and Chiron.

If Hyodo and collaborators’ scenario is correct, then Chariklo and Chiron are differentiated bodies with dense silicate cores, and their rings are either pure water ice, or a mixture of water ice and a small amount of silicate. Future observations of these minor planets will help to test this model — and observations of other centaurs may discover yet more ring systems hiding in our solar system!

Bonus

Check out this awesome animation from ESO showing an artist’s impression of the ring system around Chariklo! [ESO/L. Calçada/M. Kornmesser]

Citation

Ryuki Hyodo et al 2016 ApJ 828 L8. doi:10.3847/2041-8205/828/1/L8

coronal hole

Coronal holes are where the fast solar wind streams out of the Sun’s atmosphere, sending charged particles on rapid trajectories out into the solar system. A new study examines how the distribution of coronal holes has changed over the last 40 years.

coronal hole schematic

Coronal holes form where magnetic field lines open into space (B) instead of looping back to the solar surface (A). [Sebman81]

Source of the Fast Solar Wind

As a part of the Sun’s natural activity cycle, extremely low-density regions sometimes form in the solar corona. These “coronal holes” manifest themselves as dark patches in X-ray and extreme ultraviolet imaging, since the corona is much hotter than the solar surface that peeks through from underneath it.

Coronal holes form when magnetic field lines open into space instead of looping back to the solar surface. In these regions, the solar atmosphere escapes via these field lines, rapidly streaming away from the Sun’s surface in what’s known as the “fast solar wind”.

Coronal Holes Over Space and Time

Automated detection of coronal holes from image-based analysis is notoriously difficult. Recently, a team of scientists led by Ken’ichi Fujiki (ISEE, Nagoya University, Japan) has developed an automated prediction technique for coronal holes that relies instead on magnetic-field data for the Sun, obtained at the National Solar Observatory’s Kitt Peak between 1975 and 2014. The team used these data to produce a database of 3335 coronal hole predictions over nearly 40 years.

distribution of coronal holes

Latitude distribution of 2870 coronal holes (each marked by an x; color indicates polarity), overlaid on the magnetic butterfly map of the Sun. The low-latitude coronal holes display a similar butterfly pattern, in which they move closer to the equator over the course of the solar cycle. Polar coronal holes are more frequent during solar minima. [Fujiki et al. 2016]

Examining trends in the coronal holes’ distribution in latitude and time, Fujiki and collaborators find a strong correlation between the total area covered by low-latitude coronal holes (holes closer to the Sun’s equator) and sunspot activity. In contrast, the total area of high-latitude coronal holes (those near the Sun’s poles) peaks around the minimum in each solar cycle and shrinks around each solar maximum.

Predicting the Impact of the Solar Wind

Why do these observations matter? Coronal holes are the source of the fast solar wind, so if we can better predict the frequency and locations of coronal holes in the future, we can make better predictions about how the solar wind might impact us here on Earth.

periodicity

Periodicity of high-latitude (orange) and low-latitude (blue) coronal-hole areas, and periodicity of galactic cosmic rays detected at Earth (black). The cosmic rays track the polar coronal-hole area behavior with a 1-year time lag. [Fujiki et al. 2016]

In one example of this, Fujiki and collaborators show that there’s a distinct correlation between polar coronal-hole area and observed galactic cosmic rays. Cosmic rays from within our galaxy have long been known to exhibit a 22-year periodicity. Fujiki and collaborators show that the periodicity of the galactic cosmic-ray activity tracks that of the polar coronal-hole area, with a ~1-year lag time — which is equivalent to the propagation time of the solar wind to the termination shock.

Polar coronal holes are therefore a useful observable indicator of the dipole component of the solar magnetic field, which modulates the incoming cosmic rays entering our solar system. This coronal hole database will be a useful tool for understanding the source of solar wind and the many ways the wind influences the Earth and our solar system.

Citation

K. Fujiki et al 2016 ApJ 827 L41. doi:10.3847/2041-8205/827/2/L41

dark matter

Could the dark matter in our universe be “warm” instead of “cold”? Recent observations have placed new constraints on the warm dark matter model.

What’s the Deal with Cold/Warm/Hot Dark Matter?

MACHOs

An example of cold dark matter: MACHOs, massive objects like black holes that are hiding in the halo of our galaxy. [Alain r]

Nobody knows what dark matter is made of, but we have a few theories. The objects or particles that could make up dark matter fall into three broad categories — cold, warm, and hot dark matter — based on something called their “free streaming length,” or how far they moved due to random motions in the early universe.

Neutrinos are an example of hot dark matter: very light particles with free streaming lengths much longer than the size of a typical galaxy. Cold dark matter could consist of objects like black holes or brown dwarfs, or particles like WIMPs — all of which are very heavy and therefore have free streaming lengths much shorter than the size of a galaxy.

Warm dark matter is what’s in between: middle-mass particles with free streaming lengths roughly the size of a galaxy. There aren’t any known particles that fit this description, but there are theorized particles such as sterile neutrinos or gravitinos that do.

WDM model vs obs 1

Cumulative mass functions at z = 6 for different values of the warm dark matter particle mass mX. The shaded boxs on the left correspond to the observed number density of faint galaxies within different confidence levels. [Menci et al. 2016]

Smoothing Out the Universe

The widely favored model is lambda-CDM, in which cold dark matter makes up the missing matter in our universe. This model nicely explains much of what we observe, but it still has a few problems. The biggest issue with lambda-CDM is that it predicts that there should be many more small, dwarf galaxies than we observe.

While this could just mean that we haven’t yet managed to see all the existing, faint dwarf galaxies, we should also consider alternative models — the warm dark matter model chief among them.

In the early universe, small density perturbations on sub-galactic scales produce dwarf galaxies in the lambda-CDM model. But in the warm dark matter model, the longer free streaming length of the dark matter particles smooth out some of those small perturbations. This results in the formation of fewer dwarf galaxies — which fits better with our current observations.

Limits on Warm Dark Matter

So how can we test this alternative model? The maximum number density of dark-matter halos predicted by the warm dark matter model at a given redshift depends on the mass of the candidate dark matter particle: a larger particle mass means that more halos form. We therefore can set lower limits on the mass of dark matter particles in a two-step process:

  1. Calculate the maximum number density of dark matter halos predicted by models, and
  2. Compare this to the measured abundance of the faintest galaxies at a given redshift.
WDM model vs obs 2

Another way of looking at it: for different values of the dark matter particle mass mX, this shows the maximum number density of dark matter halos predicted at z = 6. The shaded areas represent the observed number density of faint galaxies at different confidence levels. [Menci et al. 2016]

Recently, unprecedented new Hubble observations of ultra-faint, lensed galaxies in the Hubble Frontier Fields at z~6 have allowed for the discovery of more faint galaxies at this redshift than ever before. Now, a team of scientists led by Nicola Menci (INAF Rome) have used these observations to set a new limit on the lowest mass that candidate dark matter particles can have.

Menci and collaborators find that these new observations constrain the particle masses to be above 2.9 keV at the 1σ confidence level. These constitute the tightest constraints on the mass of candidate warm dark matter particles derived to date, and they even allow us to rule out some production mechanisms for theorized particles.

Extending this analysis to other clusters with deep observations will only improve the constraints, bringing us ever closer to understanding what dark matter is made of.

Citation

N. Menci et al 2016 ApJ 825 L1. doi:10.3847/2041-8205/825/1/L1

red dwarf exoplanet

The European Southern Observatory (ESO) is widely expected to address the reports of the discovery of a planet orbiting our nearest stellar neighbor, Proxima Centauri, today. Due to its proximity — 4.25 light-years away — this red dwarf star has been a prime target for exoplanet searches throughout the last couple decades.

Proxima Centauri

Hubble image of Proxima Centauri, our nearest stellar neighbor. [ESA/Hubble]

In anticipation of ESO’s press conference this afternoon, let’s take a look at some of the past work in the search for planetary companions around Proxima Centauri.

The Early Years of Exploring Proxima Centauri

Proxima Centauri was discovered by astronomer Robert Innes in 1915. Studies of this star over the next eighty years primarily focused on better understanding its orbital motion (is it part of the Alpha Centauri star system?) and its flaring nature. But in the 1990s, after the detection of the first exoplanets, Proxima Centauri became a target for its potential to host planet-mass companions.

HST FOS Proxima Centauri

Top: Images of Proxima Centauri on two different days from Hubble’s FOS instrument. The bar across the center is an occulter that partially blocks the light from Proxima Centauri. Middle: Reconstructed images allowing a closer look at a moving feature identified by the authors as a possible companion. Bottom: diagram of the position of the planet candidate (box) relative to Proxima Centauri (star) in the two frames. [Schultz et al. 1998]

1990s: A Possible Planet Detected With Hubble?

In January 1998, a paper led by A.B. Schultz (STScI) reported the possible visual detection of a planetary companion to Proxima Centauri. Observations from Hubble’s Faint Object Spectrograph, which was being used as a coronagraphic camera, revealed excess light that could be interpreted as a substellar object located ~0.5 AU from Proxima Centauri, a small separation that could imply either a short (~1 yr) period or a highly eccentric orbit.

But follow-up observations led by David Golimowski (Johns Hopkins University) were unable to detect this proposed planet. These observations — made by direct imaging with Hubble’s Wide Field Planetary Camera 2 — found no evidence of a companion located 0.12–1.1 AU from Proxima Centauri.

In addition, an astrometric study led by G. Fritz Benedict (McDonald Observatory) the following year also didn’t find any evidence for the proposed companion. Along with prior radial velocity measurements, the astrometry in this study ruled out all companions to Proxima Centauri with a mass of more than 0.8 Jupiter masses and periods between 1 and 1000 days.

Increased Capabilities in Recent Years

With increasing resolution and sensitivity of instruments, as well as better stellar modeling and increased noise-reduction strategies, we are now more likely than ever to be able to detect a planet orbiting Proxima Centauri. Therefore, our continued non-detections have been placing ever more stringent limits on the mass and orbital properties of a hypothetical companion.

In 2014, as part of a long-term study of the solar neighborhood, a team led by John Lurie (University of Washington) published the results of a nearly 13-year campaign that used the Cerro Tololo Inter-American Observatory to obtain astrometric measurements for Proxima Centauri. This detailed study ruled out the possibility of Jupiter-mass companions at orbital periods of 2–12 years.

HARPS-TERRA RV measurements

Radial-velocity measurements of Proxima Centauri from a 2012 study using HARPS-TERRA. No “promising signals” of companions were found. [Anglada-Escudé and Butler 2012]

One of the most advanced instruments currently in the radial-velocity planet search is a spectrometer called the High Accuracy Radial velocity Planet Searcher (HARPS), operated by ESO in La Silla Observatory, Chile. In a study from 2012 led by Guillem Anglada-Escudé (Carnegie Institution of Washington), the team described new data analysis algorithms being used with HARPS. The authors used Proxima Centauri as a test case, finding only a very marginal signal with a period of 5.6 days. The signal’s lack of significance led them to conclude that, “unfortunately, no promising signals are yet detected on Proxima Cen.”

These studies — among others — throughout the last couple decades have placed strict limitations on the mass and orbit of a potential planetary companion to our nearest stellar neighbor. It will be interesting to see what ESO announces this afternoon, and how it fits into the context of these past studies of Proxima Centauri!

Citation

A. B. Schultz et al 1998 AJ 115 345. doi:10.1086/300176
David A. Golimowski and Daniel J. Schroeder 1998 AJ 116 440. doi:10.1086/300437
G. Fritz Benedict et al 1999 AJ 118 1086. doi:10.1086/300975
John C. Lurie et al 2014 AJ 148 91. doi:10.1088/0004-6256/148/5/91
Guillem Anglada-Escudé and R. Paul Butler 2012 ApJS 200 15. doi:10.1088/0067-0049/200/2/15

BH-BH binary

Most theoretical models assume that black holes aren’t charged. But a new study shows that mergers of charged black holes could explain a variety of astrophysical phenomena, from fast radio bursts to gamma-ray bursts.

No Hair

The black hole “no hair” theorem states that all black holes can be described by just three things: their mass, their spin, and their charge. Masses and spins have been observed and measured, but we’ve never measured the charge of a black hole — and it’s widely believed that real black holes don’t actually have any charge.

That said, we’ve also never shown that black holes don’t have charge, or set any upper limits on the charge that they might have. So let’s suppose, for a moment, that it’s possible for a black hole to be charged. How might that affect what we know about the merger of two black holes? A recent theoretical study by Bing Zhang (University of Nevada, Las Vegas) examines this question.

FRB

Intensity profile of a fast radio burst, a sudden burst of radio emission that lasts only a few milliseconds. [Swinburne Astronomy Productions]

Driving Transients

Zhang’s work envisions a pair of black holes in a binary system. He argues that if just one of the black holes carries charge — possibly retained by a rotating magnetosphere — then it may be possible for the system to produce an electromagnetic signal that could accompany gravitational waves, such as a fast radio burst or a gamma-ray burst!

In Zhang’s model, the inspiral of the two black holes generates a global magnetic dipole that’s perpendicular to the plane of the binary’s orbit. The magnetic flux increases rapidly as the separation between the black holes decreases, generating an increasingly powerful magnetic wind. This wind, in turn, can give rise to a fast radio burst or a gamma-ray burst, depending on the value of the black hole’s charge.

GRB

Artist’s illustration of a short gamma-ray burst, thought to be caused by the merger of two compact objects. [ESO/A. Roquette]

Zhang calculates lower limits on the charge necessary to produce each phenomenon. For a 10-solar-mass black hole, he finds that the merger can generate a fast radio burst if the black hole’s charge is more than ~1012 Coulombs (roughly one billion times the charge that travels through a AA battery from full to empty). If its charge is more than ~1016 Coulombs, it can generate a gamma-ray burst.

Limits on Charge

Zhang’s calculations are not just useful in the hypothetical scenario where black holes are charged. They could, in fact, be a way of testing whether black holes are charged.

As we accumulate future gravitational-wave observations (and with two observations by LIGO already announced, it seems likely that there will be many more), we will grow a larger sample of follow-up observations in radio through gamma-ray wavelengths. Our detections — or our lack of detections — of fast radio bursts or gamma-ray bursts associated with these black-hole mergers will allow us to set some of the first real limits on the charge of black holes.

Citation

Bing Zhang 2016 ApJ 827 L31. doi:10.3847/2041-8205/827/2/L31

Smith cloud

What caused the newly discovered “supershell” in the outskirts of our galaxy? A new study finds evidence that a high-velocity cloud may have smashed into the Milky Way’s disk millions of years ago.

Mysterious Gas Shells

GS040.2+00.6–70

A single velocity-channel map of the supershell GS040.2+00.6–70, with red contours marking the high-velocity cloud at its center. [Adapted from Park et al. 2016]

The neutral hydrogen gas that fills interstellar space is organized into structures like filaments, loops, and shells. “Supershells” are enormous shells of hydrogen gas that can have radii of a thousand light-years or more; we’ve spotted about 20 of these in our own galaxy, and more in nearby dwarfs and spiral galaxies.

How do these structures form? One theory is that they result from several supernovae explosions occurring in the same area. But the energy needed to create a supershell is more than 3 x 1052 erg, which corresponds to over 30 supernovae — quite a lot to have exploding in the same region.

There’s an interesting alternative scenario: the supershells might instead be caused by the impacts of high-velocity clouds that fall into the galactic disk.

compact high-velocity cloud

Velocity data for the compact high-velocity cloud CHVC040. The cloud is moving fast enough to create the supershell observed. [Adapted from Park et al. 2016]

The Milky Way’s Speeding Clouds

High-velocity clouds are clouds of mostly hydrogen that speed through the Milky Way with radial velocities that are very different from the material in the galactic disk. The origins of these clouds are unknown, but it’s proposed that they come from outside the galaxy — they might be fragments of a nearby, disrupting galaxy, or they might have originated from flows of accreting gas in the space in between galaxies.

Though high-velocity clouds have long been on the list of things that might cause supershells, we’ve yet to find conclusive evidence of this. But that might have just changed, with a recent discovery by a team of scientists led by Geumsook Park (Seoul National University).

Using the Arecibo radio telescope in Puerto Rico, Park and collaborators have observed a supershell in the outskirts of the Milky Way — and it has a high-velocity cloud at its center! Could this pair of objects be the evidence needed?

A Revealing Pair

The supershell, GS040.2+00.6–70, is roughly 3,000 light-years across, and it’s in the process of expanding outwards. The interior of the shell is filled with a complex structure that looks almost like spokes extending from a central hub. CHVC040, a compact high-velocity cloud, is located right at the central hub; the authors calculate a probability of less than a thousandth of a percent that this alignment is random.

supershell-HVC system

An integrated intensity map (click for a better look!) of neutral hydrogen showing the overall picture of the supershell (left), with the hub-and-spoke complex structure indicated within the shell. Contours in a close-up view (right) shows the location of the high-velocity cloud directly at the central hub. [Park et al. 2016]

Park and collaborators examine the morphology and the velocity data for the shell and the cloud. Based on the authors’ calculations, if CHVC040 were traveling at a typical velocity for high-velocity clouds (several hundred kilometers per second), it would have enough energy to have created the supershell when it slammed into the disk. The parameters of the shell allow the authors estimate when the collision happened: roughly five million years ago.

If this scenario is correct, Park and collaborators’ observations demonstrate that some compact high-velocity clouds can survive their trip through the galactic halo to smash into the galactic disk, forming a supershell on impact. A systematic study of the ~300 known compact high-velocity clouds in the Milky Way may reveal other, similar systems of compact high-velocity clouds coincident with supershells.

Citation

Geumsook Park et al 2016 ApJ 827 L27. doi:10.3847/2041-8205/827/2/L27

radio galaxy

Most radio galaxies exhibit a single pair of radio lobes marking the endpoints of their jets. But the unusual three pairs of radio lobes of a recently observed radio galaxy may reveal information about this galaxy’s past.

A 610 MHz image displaying J1216+0709’s three sets of radio lobes: inner, middle, and outer. These were likely caused by three different episodes of AGN activity. [Adapted from Singh et al. 2016]

A 610 MHz image displaying J1216+0709’s three sets of radio lobes: inner, middle, and outer. These were likely caused by three different episodes of AGN activity. [Adapted from Singh et al. 2016]

Core-Jet-Lobe

Radio galaxies, a subclass of active galactic nuclei (AGN), typically exhibit what’s known as a “core-jet-lobe” structure. A supermassive black hole accreting matter at the galaxy’s core flings material out at the poles, forming two symmetric jets of highly energetic particles. These jets can travel vast distances before spreading out into giant, radio-emitting lobes.

Thousands of these double-lobed radio galaxies have been observed, but a few dozen are unique cases that exhibit two pairs of lobes. These different pairs likely formed during two different phases of AGN activity: the jets were activated long enough to inflate the first lobes, then turned off, and then turned back on again and inflated the second lobes.

Now, the third-ever case of a triple set of lobes has been discovered: the radio galaxy J1216+0709, located roughly 2 billion light-years away.

Clues from Morphology

A spectral image map between the 325 and 620 MHz GMRT observations. There’s no signature of compact hot-spot structures in the outer lobes, indicating that the supply of jet material to the outer lobes stopped long ago. [Adapted from Singh et al. 2016]

A spectral image map between the 325 and 620 MHz GMRT observations. There’s no signature of compact hot-spot structures in the outer lobes, indicating that the supply of jet material to the outer lobes stopped long ago. [Adapted from Singh et al. 2016]

J1216+0709 is an early-type elliptical galaxy hosting a supermassive black hole of several billion solar masses at its core. The galaxy’s unusual radio structure was discovered by a team of scientists led by Veeresh Singh (Physical Research Laboratory in Ahmedabad, India), using India’s Giant Metrewave Radio Telescope (GMRT).

The radio lobes detected in J1216+0709 consist of an inner pair ~310 thousand light-years across, a nearly coaxial middle pair ~770 thousand light-years across, and an outer pair ~2.7 million light-years across. Singh and collaborators note several important observations about the galaxy’s morphology:

  1. The outer pair of lobes is much fainter than the inner pairs, and it doesn’t contain any hot spots. This makes sense if the outer lobes are the oldest, as expected, and are no longer being actively fed.
  2. The inner pairs of lobes are both brighter and longer on their eastern sides than on their western sides, suggesting that the jets are intrinsically asymmetric.
  3. The outer pair of lobes is bent with respect to the inner jets. This could mean that the material is interacting with the surrounding environment, which may have a large-scale density gradient. Alternatively, it could mean that the galaxy moved in between the two cycles of AGN activity.

Interaction as a Trigger

What could be triggering the bursts of jet activity? Singh and collaborators reference J1216+0709 against a catalog of galaxies and clusters, and find that the host galaxy is part of a small group of three galaxies.

Though there’s no visible disturbance in the host galaxy’s morphology, minor interactions with two nearby dwarf galaxies could be triggering the sporadic AGN activity. In the future, more sensitive optical data may be able to confirm this model.

Citation

Veeresh Singh et al 2016 ApJ 826 132. doi:10.3847/0004-637X/826/2/132

MWA

The modern search for extraterrestrial intelligence, known as SETI, began at radio observatories more than 50 years ago. Now scientists at the Murchison Widefield Array (MWA) in Western Australia have launched a new SETI endeavor — the first to look for signals in the low-radio-frequency regime.

New Approach

Because radio waves are such an important part of human communication, many SETI experiments search for signals from alien civilizations in radio bands — typically in the 1.4–1.7 GHz frequency range. But recently, new radio observatories are being developed in the lower-frequency range, including the MWA at 80–300 MHz.

The primary goals of the MWA are to detect neutral atomic hydrogen from the Epoch of Reionization and to study the Sun, heliosphere, and Earth’s ionosphere. In July of 2014 it was undertaking a spectral line survey of the galactic plane, when a team of scientists led by Steven Tingay (ICRAR, Curtin University, Australia; National Institute for Astrophysics, Italy) realized that this data could also be used for SETI purposes.

The Hunt for Communication

known exoplanet systems

Distribution of the 38 known exoplanet systems in the field of view of this study. [Tingay et al. 2016]

The MWA presents a unique opportunity for SETI: it’s on an extremely radio-quiet site, it’s a high-sensitivity array operating in a unique frequency range, and it has access to the southern hemisphere and an exceptionally large field of view.

Tingay and collaborators examined observations from the MWA that consisted of spectra in the 103–133 MHz range for each part of the sky in a 400-square-degree field of view. The authors compared this field to the Kepler catalog, identifying 45 planets in 38 known planetary systems within the field. They then examined the MWA spectra at the locations of each planetary system, searching for narrowband radio signals coming from the systems.

sample spectrum

Sample spectrum from one of the closest stars in the MWA field. No signals from alien civilizations are evident here. [Adapted from Tingay et al. 2016]

No Civilizations Yet

No such signals were found, and Tingay and collaborators used their observations to set upper limits on the power of any isotropic emission coming from each system. As an example, they found that no signals in the 103–133 MHz range are being broadcast from the planets around GJ 667C — which is 22 light-years away — with a power greater than ~1013 W.

This limit, however, is still 1000 times more powerful than the most powerful transmission ever deliberately broadcast into space by humans: a message sent from the Arecibo observatory with an equivalent isotropic transmission of ~1010 W. This means that these observations from the MWA may not be sensitive enough to detect messages from hypothetical alien civilizations.

Proof of Future Capabilities

SKA

Artist’s impression of the Square Kilometer Array, a future radio observatory. [SKA/Swinburne Astronomy Productions]

Tingay and collaborators are undeterred, however, arguing that the results motivate a deeper and larger search. This initial study covered a limited part of the sky at limited frequencies; the authors suggest that the next step is to perform a SETI experiment to the same depth as these observations, but over the full MWA frequency range and the full sky accessible from Western Australia. This would require a feasible ~1 month of observing time.

Longer observing time still could search even deeper, and future arrays observing in similar frequency ranges, like the Square Kilometer Array, will be even more sensitive. This study demonstrates the utility of such arrays for conducting SETI experiments in this new low-frequency band.

Citation

S. J. Tingay et al 2016 ApJ 827 L22. doi:10.3847/2041-8205/827/2/L22

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