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earth-like planet

The first challenge in the hunt for life elsewhere in our universe is to decide where to look. In a new study, two scientists examine whether Sun-like stars or low-mass M dwarfs are the best bet for hosting exoplanets with detectable life.

Ambiguity of Habitability

habitable zones

The habitable zones of cool M-dwarf stars lie much closer in than for Sun-like stars, placing habitable-zone planets around M dwarfs at greater risk of being affected by space weather.

Most exoplanet scientists will freely admit frustration with the term “habitability” — it’s a word that has many different meanings and is easily misinterpreted when it appears in news articles. Just because a planet lies in a star’s habitable zone, for instance, doesn’t mean it’s necessarily capable of supporting life.

This ambiguity, argue authors Manasvi Lingam and Abraham Loeb (Harvard University and Harvard-Smithsonian Center for Astrophysics), requires us to take a strategic approach when pursuing the search for primitive life outside of our solar system. In particular, we risk losing the enthusiasm and support of the public (and funding sources!) when we focus on the general search for planets in stellar habitable zones, rather than specifically searching for the planets most likely to have detectable signatures of life.

Sun vs M dwarf

Illustration of the difference between a Sun-like star and a lower-mass, cooler M-dwarf star. [NASA’s Goddard Space Flight Center/S. Wiessinger]

Weighing Two Targets

So how do we determine where best to look for planets with detectable biosignatures? To figure out which stars make the optimal targets, Lingam and Loeb suggest an approach based on standard cost-benefit analyses common in economics. Here, what’s being balanced is the cost of an exoplanet survey mission against the benefit of different types of stellar targets.

In particular, Lingam and Loeb weigh the benefit of targeting solar-type stars against that of targeting stars of any other mass (such as low-mass M-dwarfs, popular targets of many current exoplanet surveys). The advantage of one type of target over the other depends on two chief factors:

  1. the probability that the targeted star hosts planets with life, and 
  2. the probability that biosignatures arising from this life are detectable, given our available technology.

Promise of Sun-Like Stars

relative benefit of searches around different stars

Relative benefit of searching for signatures of life around stars with varying masses, assuming a transmission spectroscopy survey mission; results are similar for a direct-imaging mission. Green curve assumes a flat prior; red and blue curves assume priors in which habitability is suppressed around low-mass stars. [Lingam & Loeb 2018]

Taking observational constraints into account, Lingam and Loeb’s results depend on what is known in statistics as a “prior” — an assumption that goes into the calculation. The two possible outcomes are:

  1. If we assume a flat prior — i.e., that the probability of life is the same for any choice of star — then searching for life around M-dwarfs proves the most advantageous, because the detection of biosignatures becomes much easier.
  2. If we assume a prior in which habitability is suppressed around low-mass stars, then it is more advantageous to search for life around solar-type stars.

So which of these priors is correct? There is mounting evidence, particularly based on considerations of space weather, that the habitability of Earth-like planets around M dwarfs might be much lower than their counterparts around solar-like stars.

If this turns out to be true, then Lingam and Loeb argue exoplanet survey missions should target Sun-like stars throughout our galaxy for the best chances of efficiently detecting life beyond our solar system.

Citation

Manasvi Lingam and Abraham Loeb 2018 ApJL 857 L17. doi:10.3847/2041-8213/aabd86

chromosphere

The best-studied star — the Sun — still harbors mysteries for scientists to puzzle over. A new study has now explored the role of tiny magnetic-field hiccups in an effort to explain the strangely high temperatures of the Sun’s upper atmosphere.

solar temperatures

Schematic illustrating the temperatures in different layers of the Sun. [ESA]

Strange Temperature Rise

Since the Sun’s energy is produced in its core, the temperature is hottest here. As expected, the temperature decreases further from the Sun’s core — up until just above its surface, where it oddly begins to rise again. While the Sun’s surface is ~6,000 K, the temperature is higher above this: ~10,000 K in the outer chromosphere.

So how is the chromosphere of the Sun heated? It’s possible that the explanation can be found not amid high solar activity, but in quiet-Sun regions.

In a new study led by Milan Gošić (Lockheed Martin Solar and Astrophysics Laboratory, Bay Area Environmental Research Institute), a team of scientists has examined a process that quietly happens in the background: the cancellation of magnetic field lines in the quiet Sun.

Activity in a Supergranule

IRIS quiet-Sun observations

Top left: SDO AIA image of part of the solar disk. The next three panels are a zoom of the particular quiet-Sun region that the authors studied, all taken with IRIS at varying wavelengths: 1400 Å (top right), 2796 Å (bottom left), and 2832 Å (bottom right). [Gošić et al. 2018]

The Sun is threaded by strong magnetic field lines that divide it into supergranules measuring ~30 million meters across (more than double the diameter of Earth!). Supergranules may seem quiet inside, but looks can be deceiving: the interiors of supergranules contain smaller, transient internetwork fields that move about, often resulting in magnetic elements of opposite polarity encountering and canceling each other.

For those internetwork flux cancellations that occur above the Sun’s surface, a small amount of energy could be released that locally heats the chromosphere. But though each individual event has a small effect, these cancellations are ubiquitous across the Sun.

This raises an interesting possibility: could the total of these internetwork cancellations in the quiet Sun account for the overall chromospheric heating observed?

Simultaneous Observations

To answer this question, Gošić and collaborators explored a quiet-Sun region in the center of a supergranule, making observations with two different telescopes:

  1. The Swedish 1 m Solar Telescope (SST), which provides spectropolarimetry that lets us watch magnetic elements of the Sun as they move and change, and
  2. The Interface Region Imaging Spectrograph (IRIS), a spacecraft that takes spectra in three passbands, allowing us to probe different layers of the solar atmosphere.

Simultaneous observations of the quiet-Sun region with these two telescopes allowed the scientists to piece together a picture of chromospheric heating: as SST observations showed opposite-polarity magnetic-field regions approach each other and then disappear, indicating a field cancellation, IRIS observations often showed brightening in the chromosphere.

Falling Short

SST quiet-Sun observations

SST observations, including the continuum intensity map (upper left), magnetogram showing the magnetic field elements (upper right), and intensity maps in the core of the Ca II 8542 Å line (lower left) and Hα 6563 Å line (lower right). [Gošić et al. 2018]

By careful interpretation of their observations, Gošić and collaborators were able to estimate the total energy contribution from the hundreds of field cancellations they detected. The authors determined that, while the internetwork cancellations can significantly heat the chromosphere locally, the apparent number density of these cancellations falls an order of magnitude short of explaining the overall chromospheric heating observed.

Does this mean quiet-Sun internetwork fields aren’t the cause of the strangely warm temperatures in the chromosphere? Perhaps … or perhaps we don’t yet have the telescope power to detect all of the internetwork field cancellations. If that’s the case, upcoming telescopes like the Daniel K. Inouye Solar Telescope and the European Solar Telescope will let us answer this question more definitively.

Citation

M. Gošić et al 2018 ApJ 857 48. doi:10.3847/1538-4357/aab1f0

Betelgeuse in infrared

What happens on the last day of a massive star’s life? In the hours before the star collapses and explodes as a supernova, the rapid evolution of material in its core creates swarms of neutrinos. Observing these neutrinos may help us understand the final stages of a massive star’s life — but they’ve never been detected.

MiniBooNE neutrino detector

A view of some of the 1,520 phototubes within the MiniBooNE neutrino detector. Observations from this and other detectors are helping to illuminate the nature of the mysterious neutrino. [Fred Ullrich/FNAL]

Silent Signposts of Stellar Evolution

The nuclear fusion that powers stars generates tremendous amounts of energy. Much of this energy is emitted as photons, but a curious and elusive particle — the neutrino — carries away most of the energy in the late stages of stellar evolution.

Stellar neutrinos can be created through two processes: thermal processes and beta processes. Thermal processes — e.g., pair production, in which a particle/antiparticle pair are created — depend on the temperature and pressure of the stellar core. Beta processes — i.e., when a proton converts to a neutron, or vice versa — are instead linked to the isotopic makeup of the star’s core. This means that, if we can observe them, beta-process neutrinos may be able to tell us about the last steps of stellar nucleosynthesis in a dying star.

But observing these neutrinos is not so easily done. Neutrinos are nearly massless, neutral particles that interact only feebly with matter; out of the whopping ~1060 neutrinos released in a supernova explosion, even the most sensitive detectors only record the passage of just a few. Do we have a chance of detecting the beta-process neutrinos that are released in the final few hours of a star’s life, before the collapse?

Neutrino luminosities

Neutrino luminosities leading up to core collapse. Shortly before collapse, the luminosity of beta-process neutrinos outshines that of any other neutrino flavor or origin. [Adapted from Patton et al. 2017]

Modeling Stellar Cores

To answer this question, Kelly Patton (University of Washington) and collaborators first used a stellar evolution model to explore neutrino production in massive stars. They modeled the evolution of two massive stars — 15 and 30 times the mass of our Sun — from the onset of nuclear fusion to the moment of collapse.

The authors found that in the last few hours before collapse, during which the material in the stars’ cores is rapidly upcycled into heavier elements, the flux from beta-process neutrinos rivals that of thermal neutrinos and even exceeds it at high energies. So now we know there are many beta-process neutrinos — but can we spot them?

Neutrino fluxes

Neutrino and antineutrino fluxes at Earth from the last 2 hours of a 30-solar-mass star’s life compared to the flux from background sources. The rows represent calculations using two different neutrino mass hierarchies. Click to enlarge. [Patton et al. 2017]

Observing Elusive Neutrinos

For an imminent supernova at a distance of 1 kiloparsec, the authors find that the presupernova electron neutrino flux rises above the background noise from the Sun, nuclear reactors, and radioactive decay within the Earth in the final two hours before collapse.

Based on these calculations, current and future neutrino observatories should be able to detect tens of neutrinos from a supernova within 1 kiloparsec, about 30% of which would be beta-process neutrinos. As the distance to the star increases, the time and energy window within which neutrinos can be observed gradually narrows, until it closes for stars at a distance of about 30 kiloparsecs.

Are there any nearby supergiants soon to go supernova so these predictions can be tested? At a distance of only 650 light-years, the red supergiant star Betelgeuse should produce detectable neutrinos when it explodes — an exciting opportunity for astronomers in the far future!

Citation

Kelly M. Patton et al 2017 ApJ 851 6. doi:10.3847/1538-4357/aa95c4

CR7

Thirteen billion years ago, early galaxies ionized the gas around them, producing some of the first light that brought our universe out of its “dark ages”. Now the Atacama Large Millimeter/submillimeter Array (ALMA) has provided one of the first detailed looks into the interior of one of these early, distant galaxies.

Sources of Light

reionization

Artist’s illustration of the reionization of the universe (time progresses left to right), in which ionized bubbles that form around the first sources of light eventually overlap to form the fully ionized universe we observe today. [Avi Loeb/Scientific American]

For the first roughly hundred million years of its existence, our universe expanded in relative darkness — there were no sources of light at that time besides the cosmic microwave background. But as mass started to condense to form the first objects, these objects eventually shone as the earliest luminous sources, contributing to the reionization of the universe.

To learn about the early production of light in the universe, our best bet is to study in detail the earliest luminous sources — stars, galaxies, or quasars — that we can hunt down. One ideal target is the galaxy COSMOS Redshift 7, known as CR7 for short.

Targeting CR7

CR7 is one of the oldest, most distant galaxies known, lying at a redshift of z ~ 6.6. Its discovery in 2015 — and subsequent observations of bright, ultraviolet-emitting clumps within it — have led to broad speculation about the source of its emission. Does this galaxy host an active nucleus? Or could it perhaps contain the long-theorized first generation of stars, metal-free Population III stars?

To determine the nature of CR7 and the other early galaxies that contributed to reionization, we need to explore their gas and dust in detail — a daunting task for such distant sources! Conveniently, this is a challenge that is now made possible by ALMA’s incredible capabilities. In a new publication led by Jorryt Matthee (Leiden University, the Netherlands), a team of scientists now reports on what we’ve learned peering into CR7’s interior with ALMA.

ALMA-detected metals in CR7

ALMA observations of [C II] (white contours) are overlaid on an ultraviolet image of the galaxy CR7 taken with Hubble (background image). The presence of [C II] throughout the galaxy indicate that CR7 does not primarily consist of metal-free gas, as had been previously proposed. [Matthee et al. 2017]

Metals yet No Dust?

Matthee and collaborators’ deep spectroscopic observations of CR7 targeted the far-infrared dust continuum emission and a gas emission line, [C II]. The authors detected [C II] emission in a large region in and around the galaxy, including near the ultraviolet clumps. This clearly indicates the presence of metals in these star-forming regions, and it rules out the possibility that CR7’s gas is mostly primordial and forming metal-free Pop III stars.

The authors do not detect far infrared continuum emission from dust, which sets an unusually low upper limit on the amount of dust that may be present in this galaxy. This limit allows them to better interpret their measurements of star formation rates in CR7, providing more information about the galaxy’s properties. 

Lastly, Matthee and collaborators note that the [C II] emission is detected in multiple different components that have different velocities. The authors propose that these components are accreting satellite galaxies. If this is correct, then CR7 is not only a target to learn about early sources of light in the universe — it’s also a rare opportunity to directly witness the build-up of a central galaxy in the early universe.

Citation

J. Matthee et al 2017 ApJ 851 145. doi:10.3847/1538-4357/aa9931

Now that the hubbub of GW170817 — the first coincident detection of gravitational waves and an electromagnetic signature — has died down, scientists are left with the task of taking the spectrum-spanning observations and piecing them together into a coherent picture. Researcher Iair Arcavi examines one particular question: what caused the blue color in the early hours of the neutron-star merger?

kilonova

Observations of the GW170817 kilonova by Hubble over a ~week-long span. [ESA/Hubble]

Early Color

When the two neutron stars of GW170817 merged in August of last year, they produced not only gravitational waves, but a host of electromagnetic signatures. Chief among these was a flare of emission thought to be powered by the radioactive decay of heavy elements formed in the merger — a kilonova.

The emission during a kilonova can come from a number of different sources — from the heavy-element-rich tidal tails of the disrupting neutron stars, or from fast, light polar jets, or from a wind or a disk outflow — and each of these components could reveal different information about the original neutron stars and the merger.

It’s therefore important that we understand the sources of the emission that we observed in the GW170817 kilonova. In particular, we’d like to know where the early blue emission came from that was spotted in the first hours of the kilonova.

light curve of the GW170817

The combined ultraviolet–optical–infrared light curve of the GW170817 kilonova. The rise in the emission occurs on roughly a day-long timescale. [Arcavi 2018]

Comparing Models

To explore this question, Iair Arcavi (Einstein Fellow at University of California, Santa Barbara and Las Cumbres Observatory) compiled infrared through ultraviolet observations of the GW170817 kilonova from nearly 20 different telescopes. To try to distinguish between possible sources, Arcavi then compared the resulting combined light curves to a variety of models.

Arcavi found that the light curves for the GW170817 kilonova indicate an initial ~24-hour rise of emission. This rise is best matched by models in which the emission is produced by radioactive decay of ejecta with lots of heavier elements (likely from tidal tails). The subsequent decline of the emission, however, is fit as well or better by models that include lighter, faster outflows, or additional emission due to shock-heating from a wind or a cocoon surrounding a jet.

optical and ultraviolet lightcurves

Optical and ultraviolet light curves for the first 3 days after merger, as compared to four different emission models. Observations at earlier times, where the models differ more substantially, could provide stronger constraints for future mergers. [Arcavi 2018]

Missing Ultraviolet

The takeaway from Arcavi’s work is that we can’t yet eliminate any models for the GW170817 kilonova’s early blue emission — we simply don’t have enough data.

Why not? It turns out we had some bad luck with GW170817: a glitch in one of the detectors slowed down localization of the source, preventing earlier discovery of the kilonova. The net result was that the electromagnetic signal of this merger was only found 11 hours after the gravitational waves were detected — and the ultraviolet signal was detected 4 hours after that, when the kilonova light curves are already decaying.

If we had ultraviolet observations that tracked the earlier, rising emission, Arcavi argues, we would be able to differentiate between the different emission models for the kilonova. So while this may be the best we can do with GW170817, we can hope that with the next merger we’ll have a full set of early observations — allowing us to better understand where its emission comes from.

Citation

Iair Arcavi 2018 ApJL 855 L23. doi:10.3847/2041-8213/aab267

NGC 1052-DF2

You may have seen recent news about NGC 1052–DF2, a galaxy that was discovered to have little or no dark matter. Now, a new study explores what NGC 1052–DF2 does have: an enigmatic population of unusually large and luminous globular clusters.

NGC 1052–DF2 cluster spectra

Keck/LRIS spectra (left and right) and HST images (center) of the 11 clusters associated with NGC 1052–DF2. The color images each span 1” × 1”. [van Dokkum et al. 2018]

An Unusual Dwarf

The ultra-diffuse galaxy NGC 1052–DF2, originally identified with the Dragonfly Telescope Array, has puzzled astronomers since the discovery that its dynamical mass — determined by the motions of globular-cluster-like objects spotted within it — is essentially the same as its stellar mass. This equivalence implies that the galaxy is strangely lacking dark matter; the upper limit set on its dark matter halo is 400 times smaller than what we would expect for such a dwarf galaxy.

Led by Pieter van Dokkum (Yale University), the team that made this discovery has now followed up with detailed Hubble Space Telescope imaging and Keck spectroscopy. Their goal? To explore the objects that allowed them to make the dynamical-mass measurement: the oddly bright globular clusters of NGC 1052–DF2.

cluster sizes

Sizes (circularized half-light radii) vs. absolute magnitudes for globular clusters in NGC1052–DF2 (black) and the Milky Way (red). [Adapted from van Dokkum et al. 2018]

What’s Up with the Globular Clusters?

Van Dokkum and collaborators spectroscopically confirmed 11 compact objects associated with the faint galaxy. These objects are globular-cluster-like in their appearance, but the peak of their luminosity distribution is offset by a factor of four from globular clusters of other galaxies; these globular clusters are significantly brighter than is typical.

Using the Hubble imaging, the authors determined that NGC 1052–DF2’s globular clusters are more than twice the size of the Milky Way’s globular clusters in the same luminosity range. As is typical for globular clusters, they are an old (~9.3 billion years) population and metal-poor.

Rethinking Formation Theories

The long-standing picture of galaxies has closely connected old, metal-poor globular clusters to the galaxies’ dark-matter halos. Past studies have found that the ratio between the total globular-cluster mass and the overall mass of a galaxy (i.e., all dark + baryonic matter) holds remarkably constant across galaxies — it’s typically ~3 x 10-5. This has led researchers to believe that properties of the dark-matter halo may determine globular-cluster formation.

cluster luminosity function

The luminosity function of the compact objects in NGC 1052–DF2. The red and blue curves show the luminosity functions of globular clusters in the Milky Way and in the typical ultra-diffuse galaxies of the Coma cluster, respectively. NGC 1052–DF2’s globular clusters peak at a significantly higher luminosity. [Adapted from van Dokkum et al. 2018]

NGC 1052–DF2, with a globular-cluster mass that’s >3% of the mass of the galaxy (~1000 times the expected ratio!), defies this picture. This unusual galaxy therefore demonstrates that the usual relation between globular-cluster mass and total galaxy mass probably isn’t due to a fundamental connection between the dark-matter halo and globular-cluster formation. Instead, van Dokkum and collaborators suggest, globular-cluster formation may ultimately be a baryon-driven process.

As with all unexpected discoveries in astronomy, we must now determine whether NGC 1052–DF2 is simply a fluke, or whether it represents a new class of object we can expect to find more of. Either way, this unusual galaxy is forcing us to rethink what we know about galaxies and the star clusters they host.

Citation

Pieter van Dokkum et al 2018 ApJL 856 L30. doi:10.3847/2041-8213/aab60b

Mars asteroids

Like evidence left at a crime scene, the mineral olivine may be the clue that helps scientists piece together Mars’s possibly violent history. Could a long-ago giant impact have flung pieces of Mars throughout our inner solar system? Two researchers from the Tokyo Institute of Technology in Japan are on the case.

A Telltale Mineral

olivine

Olivine, a mineral that is common in Earth’s subsurface but weathers quickly on the surface. Olivine is a major component of Mars’s upper mantle. [Wilson44691]

Olivine is a major component of the Martian upper mantle, making up 60% of this region by weight. Intriguingly, olivine turns up in other places in our solar system too — for instance, in seven out of the nine known Mars Trojans (a group of asteroids of unknown origin that share Mars’s orbit), and in the rare A-type asteroids orbiting in the main asteroid belt.

How did these asteroids form, and why are they so olivine-rich? An interesting explanation has been postulated: perhaps this olivine all came from the same place — Mars — as the result of a mega impact billions of years ago.

Evidence for Impact

Mars bears plenty of signs pointing to a giant impact in its past. The northern and sourthern hemispheres of Mars look very different, a phenomenon referred to as the Mars hemisphere dichotomy. The impact of a Pluto-sized body could explain the smooth Borealis Basin that covers the northern 40% of Mars’s surface. 

Mars topography

This high-resolution topographic map of Mars reveals the dichotomy between its northern and sourthern hemispheres. The smooth region in the northern hemisphere, the Borealis basin, may have been formed when a giant object impacted Mars billions of years ago. [NASA/JPL/USGS]

Other evidence piles up: Mars’s orbit location, its rotation speed, the presence of its two moons — all could be neatly explained by a large impact around 4 billion years ago. Could such an impact have also strewn debris from Mars’s mantle across the solar system?

To test this theory, we need to determine if a mega impact is capable of producing enough ejecta — and with the appropriate compositions and orbits — to explain the Mars trojans and the A-type asteroids we observe. Tackling this problem, researchers Ryuki Hyodo and Hidenori Genda have performed numerical simulations to explore the ejecta from such a collision.

Distributing Debris

Hyodo and Genda examine the outcomes of a Mars mega impact using smoothed particle hydrodynamics simulations. They test different impactor masses, impactor speeds, angles of impact, and more to determine how these properties affect the properties of the Martian ejecta that result.

Mars mega-impact ejecta

Debris ejected in a Mars mega impact, at 20 hours post-impact. Blue particles are from the impactor, red particles are from Mars, yellow particles are clumps of >10 particles. [Hyodo & Genda 2018]

The authors find that a large amount of debris can be ejected from Mars during such an impact and distributed between ~0.5–3 AU in the solar system. Roughly 2% of this debris could originate from Mars’s olivine-rich, unmelted upper mantle — which could indeed be the source of the olivine-rich Mars Trojan asteroids and rare A-type asteroids.

How can we further explore this picture? Debris from a Mars mega impact would not just have been the source of new asteroids; the debris likely also collided with pre-existing asteroids — or even transferred to early Earth. Signatures of a Mars mega impact may therefore be recorded in main-belt asteroids or in meteorites found on Earth, providing tantalizing targets for future studies in the effort to map out Mars’s past.

Citation

Ryuki Hyodo and Hidenori Genda 2018 ApJL 856 L36. doi:10.3847/2041-8213/aab7f0

S106 star-forming region

Deep within giant molecular clouds, hidden by dense gas and dust, stars form. Unprecedented data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the intricate magnetic structures woven throughout one of the most massive star-forming regions in the Milky Way.

How Stars Are Born

Horsehead Nebula

The Horsehead Nebula’s dense column of gas and dust is opaque to visible light, but this infrared image reveals the young stars hidden in the dust. [NASA/ESA/Hubble Heritage Team]

Simple theory dictates that when a dense clump of molecular gas becomes massive enough that its self-gravity overwhelms the thermal pressure of the cloud, the gas collapses and forms a star. In reality, however, star formation is more complicated than a simple give and take between gravity and pressure. The dusty molecular gas in stellar nurseries is permeated with magnetic fields, which are thought to impede the inward pull of gravity and slow the rate of star formation.

How can we learn about the magnetic fields of distant objects? One way is by measuring dust polarization. An elongated dust grain will tend to align itself with its short axis parallel to the direction of the magnetic field. This systematic alignment of the dust grains along the magnetic field lines polarizes the dust grains’ emission perpendicular to the local magnetic field. This allows us to infer the direction of the magnetic field from the direction of polarization.

Magnetic field vectors

Magnetic field orientations for protostars e2 and e8 derived from Submillimeter Array observations (panels a through c) and ALMA observations (panels d and e). Click to enlarge. [Adapted from Koch et al. 2018]

Tracing Magnetic Fields

Patrick Koch (Academia Sinica, Taiwan) and collaborators used high-sensitivity ALMA observations of dust polarization to learn more about the magnetic field morphology of Milky Way star-forming region W51. W51 is one of the largest star-forming regions in our galaxy, home to high-mass protostars e2, e8, and North.

The ALMA observations reveal polarized emission toward all three sources. By extracting the magnetic field orientations from the polarization vectors, Koch and collaborators found that the molecular cloud contains an ordered magnetic field with never-before-seen structures. Several small clumps on the perimeter of the massive star-forming cores exhibit comet-shaped magnetic field structures, which could indicate that these smaller cores are being pulled toward the more massive cores.

These findings hint that the magnetic field structure can tell us about the flow of material within star-forming regions — key to understanding the nature of star formation itself.

sin omega maps

Maps of sin ω for two of the protostars (e2 and e8) and their surroundings. [Adapted from Koch et al. 2018]

Guiding Star Formation

Do the magnetic fields in W51 help or hinder star formation? To explore this question, Koch and collaborators introduced the quantity sin ω, where ω is the angle between the local gravity and the local magnetic field.

When the angle between gravity and the magnetic field is small (sin ω ~ 0), the magnetic field has little effect on the collapse of the cloud. If gravity and the magnetic field are perpendicular (sin ω ~ 1), the magnetic field can slow the infall of gas and inhibit star formation.

Based on this parameter, Koch and collaborators identified narrow channels where gravity acts unimpeded by the magnetic field. These magnetic channels may funnel gas toward the dense cores and aid the star-formation process.

The authors’ observations demonstrate just one example of the broad realm ALMA’s polarimetry capabilities have opened to discovery. These and future observations of dust polarization will continue to reveal more about the delicate magnetic structure within molecular clouds, further illuminating the role that magnetic fields play in star formation.

Citation

Patrick M. Koch et al 2018 ApJ 855 39. doi:10.3847/1538-4357/aaa4c1

bent jets

Powerful jets emitted from the centers of distant galaxies make for spectacular signposts in the radio sky. Can observations of these jets reveal information about the environments that surround them?

Signposts in the Sky

seven bent DLRGs

VLA FIRST images of seven bent double-lobed radio galaxies from the authors’ sample. [Adapted from Silverstein et al. 2018]

An active supermassive black hole lurking in a galactic center can put on quite a show! These beasts fling out accreting material, often forming intense jets that punch their way out of their host galaxies. As the jets propagate, they expand into large lobes of radio emission that we can spot from Earth — observable signs of the connection between distant supermassive black holes and the galaxies in which they live.

These distinctive double-lobed radio galaxies (DLRGs) don’t all look the same. In particular, though the jets are emitted from the black hole’s two poles, the lobes of DLRGs don’t always extend perfectly in opposite directions; often, the jets become bent on larger scales, appearing to us to subtend angles of less than 180 degrees.

Can we use our observations of DLRG shapes and distributions to learn about their surroundings? A new study led by Ezekiel Silverstein (University of Michigan) has addressed this question by exploring DLRGs living in dense galaxy-cluster environments.

Projected density of DLRG–central galaxy matches (black) compared to a control sample of random positions–central galaxy matches (red) for different distances from a cluster center. DLRGs have a higher likelihood of being located close to a cluster center. [Silverstein et al. 2018]

Living Near the Hub

To build a sample of DLRGs in dense environments, Silverstein and collaborators started from a large catalog of DLRGs in Sloan Digital Sky Survey quasars with radio lobes visible in Very Large Array data. They then cross-matched these against three galaxy catalogs to produce a sample of 44 DLRGs that are each paired to a nearby massive galaxy, galaxy group, or galaxy cluster.

To determine if these DLRGs’ locations are unusual, the authors next constructed a control sample of random galaxies using the same selection biases as their DLRG sample.

Silverstein and collaborators found that the density of DLRGs as a function of distance from a cluster center drops off more rapidly than the density of galaxies in a typical cluster. Observed DLRGs are therefore more likely than random galaxies to be found near galaxy groups and clusters. The authors speculate that this may be a selection effect: DLRGs further from cluster centers may be less bright, preventing their detection.

Bent Under Pressure

bent vs. unbent DLRGs

The angle subtended by the DLRG radio lobes, plotted against the distance of the DLRG to the cluster center. Central galaxies (red circle) experience different physics and are therefore excluded from the sample. In the remaining sample, bent DLRGs appear to favor cluster centers, compared to unbent DLRGs. [Silverstein et al. 2018]

In addition, Silverstein and collaborators found that location appears to affect the shape of a DLRG. “Bent” DLRGs (those with a measured angle between their lobes of 170° or smaller) are more likely to be found near a cluster center than “unbent” DLRGs (those with angles of 170°–180°). The fraction of bent DLRGs is 78% within 3 million light-years of the cluster center, and 56% within double that distance — compared to a typical fraction of just 29% in the field.

These results support the idea that ram pressure — the pressure experienced by a galaxy as it moves through the higher density environment closer to the center of a cluster — is what bends the DLRGs.

What’s next to learn? This study relies on a fairly small sample, so Silverstein and collaborators hope that future deep optical surveys will increase the completeness of cluster catalogs, enabling further testing of these outcomes and the exploration of other physics of galaxy-cluster environments.

Citation

Ezekiel M Silverstein et al 2018 AJ 155 14. doi:10.3847/1538-3881/aa9d2e

Galactic center

Far from the galactic suburbs where the Sun resides, a cluster of stars in the nucleus of the Milky Way orbits a supermassive black hole. Can chemical abundance measurements help us understand the formation history of the galactic center nuclear star cluster?

Studying Stellar Populations

Stellar populations comparison

Metallicity distributions for stars in the inner two degrees of the Milky Way (blue) and the central parsec (orange). [Do et al. 2018]

While many galaxies host nuclear star clusters, most are too distant for us to study in detail; only in the Milky Way can we resolve individual stars within one parsec of a supermassive black hole. The nucleus of our galaxy is an exotic and dangerous place, and it’s not yet clear how these stars came to be where they are — were they siphoned off from other parts of the galaxy, or did they form in place, in an environment rocked by tidal forces?

Studying the chemical abundances of stars provides a way to separate distinct stellar populations and discern when and where these stars formed. Previous studies using medium-resolution spectroscopy have revealed that many stars within the central parsec of our galaxy have very high metallicities — possibly higher than any other region of the Milky Way. Can high-resolution spectroscopy tell us more about this unusual population of stars?

Spectral Lines on Display

Tuan Do (University of California, Los Angeles, Galactic Center Group) and collaborators performed high-resolution spectroscopic observations of two late-type giant stars located half a parsec from the Milky Way’s supermassive black hole.

Spectral comparison

Comparison of the observed spectra of the two galactic center stars (black) with synthetic spectra with low (blue) and high (orange) [Sc/Fe] values. Click to enlarge. [Do et al. 2018]

In order to constrain the metallicities of these stars, Do and collaborators compared the observed spectra to a grid of synthetic spectra and used a spectral synthesis technique to determine the abundances of individual elements. They found that while one star is only slightly above solar metallicity, the other is likely more than four times as metal-rich as the Sun.

The features in the observed and synthetic spectra generally matched well, but the absorption lines of scandium, vanadium, and yttrium were consistently stronger in the observed spectra than in the synthetic spectra. This led the authors to conclude that these galactic center stars are unusually rich in these metals — trace elements that could reveal the formation history of the galactic nucleus.

Old Stars, New Trends?

[Sc/Fe] trend

Scandium to iron ratio versus iron abundance for stars in the disk of the Milky Way (blue) and the stars in this sample (orange). The value reported for this sample is a 95% lower limit. [Do et al. 2018]

For stars in the disk of the Milky Way, the abundance of scandium relative to iron tends to decrease as the overall metallicity increases, but the stars investigated in this study are both iron-rich and anomalously high in scandium. This hints that the nuclear star cluster might represent a distinct stellar population with different metallicity trends.

However, it’s not yet clear what could cause the elevated abundances of scandium, vanadium, and yttrium relative to other metals. Each of these elements is linked to a different source; scandium and vanadium are mainly produced in Type II and Type Ia supernovae, respectively, while yttrium is likely synthesized in asymptotic giant branch stars. Future observations of stars near the center of the Milky Way may help answer this question and further constrain the origin of our galaxy’s nuclear star cluster.

Citation

Tuan Do et al 2018 ApJL 855 L5. doi:10.3847/2041-8213/aaaec3

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