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26 February 2018

PALFA Discovers Neutron Stars on a Collision Course

Got any plans in 46 million years? If not, you should keep an eye out for PSR J1946+2052 around that time — this upcoming merger of two neutron stars promises to be an exciting show!

Survey Success

Average profile for PSR J1946+2052 at 1.43 GHz from a 2 hr observation from the Arecibo Observatory. [Stovall et al. 2018]

It seems like we just wrote about the dearth of known double-neutron-star systems, and about how new surveys are doing their best to find more of these compact binaries. Observing these systems improves our knowledge of how pairs of evolved stars behave before they eventually spiral in, merge, and emit gravitational waves that detectors like the Laser Interferometer Gravitational-wave Observatory might observe.

Today’s study, led by Kevin Stovall (National Radio Astronomy Observatory), goes to show that these surveys are doing a great job so far! Yet another double-neutron-star binary, PSR J1946+2052, has now been discovered as part of the Arecibo L-Band Feed Array pulsar (PALFA) survey. This one is especially unique due to the incredible speed with which these neutron stars orbit each other and their correspondingly (relatively!) short timescale for merger.

An Extreme Example

The PALFA survey, conducted with the enormous 305-meter radio dish at Arecibo, has thus far resulted in the discovery of 180 pulsars — including two double-neutron-star systems. The most recent discovery by Stovall and collaborators brings that number up to three, for a grand total of 16 binary-neutron-star systems (confirmed and unconfirmed) known to date.

The 305-m Arecibo Radio Telescope, built into the landscape at Arecibo, Puerto Rico. [NOAO/AURA/NSF/H. Schweiker/WIYN]

The newest binary in this collection, PSR J1946+2052, exhibits a pulsar with a 17-millisecond spin period that whips around its compact companion at a terrifying rate: the binary period is just 1.88 hours. Follow-up observations with the Jansky Very Large Array and other telescopes allowed the team to identify the binary’s location to high precision and establish additional parameters of the system.

PSR J1946+2052 is a system of extremes. The binary’s total mass is found to be ~2.5 solar masses, placing it among the lightest binary-neutron-star systems known. Its orbital period is the shortest we’ve observed, and the two neutron stars are on track to merge in less time than any other known neutron-star binaries: in just 46 million years. When the two stars reach the final stages of their merger, the effects of the pulsar’s rapid spin on the gravitational-wave signal will be the largest of any such system discovered to date.

More Tests of General Relativity

What can PSR J1946+2052 do for us? This extreme system will be especially useful as a gravitational laboratory. Continued observations of PSR J1946+2052 will pin down with unprecedented precision parameters like the Einstein delay and the rate of decay of the binary’s orbit due to the emission of gravitational waves, testing the predictions of general relativity to an order of magnitude higher precision than was possible before.

As we expect there to be thousands of systems like PSR J1946+2052 in our galaxy alone, better understanding this binary — and finding more like it — continue to be important steps toward interpreting compact-object merger observations in the future.

Citation

K. Stovall et al 2018 ApJL 854 L22. doi:10.3847/2041-8213/aaad06

23 February 2018

Our Galactic Neighbor Hosts Complex Organic Molecules

For the first time, data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the presence of methyl formate and dimethyl ether in a star-forming region outside our galaxy. This discovery has important implications for the formation and survival of complex organic compounds — important for the formation of life — in low-metallicity galaxies both young and old.

No Simple Picture of Complex Molecule Formation

ALMA, pictured here with the Magellanic Clouds above, has observed organic molecules in our Milky Way Galaxy —
and beyond. [ESO/C. Malin]

Complex organic molecules (those with at least six atoms, one or more of which must be carbon) are the precursors to the building blocks of life. Knowing how and where complex organic molecules can form is a key part of understanding how life came to be on Earth — and how it might arise elsewhere in the universe. From exoplanet atmospheres to interstellar space, complex organic molecules are ubiquitous in the Milky Way.

In our galaxy, complex organic molecules are often found in the intense environments of hot cores — clumps of dense molecular gas surrounding the sites of star formation. However, it’s not yet fully understood how the complex organic molecules found in hot cores come to be. One possibility is that the compounds condense onto cold dust grains long before the young stars begin heating their natal shrouds. Alternatively, they might assemble themselves from the hot, dense gas surrounding the blazing protostars.

Composite infrared and optical image of the N 113 star-forming region in the LMC. The ALMA coverage is indicated by the gray line. Click to enlarge. [Sewiło et al. 2018]

Detecting Complexity, a Galaxy Away

Using ALMA, a team of researchers led by Marta Sewiło (NASA Goddard Space Flight Center) recently detected two complex organic molecules — methyl formate and dimethyl ether — for the first time in our neighboring galaxy, the Large Magellanic Cloud (LMC). Previous searches for organic molecules in the LMC detected small amounts of methanol, the parent molecule of the two newly-discovered compounds. By revealing the spectral signatures of dimethyl ether and methyl formate, Sewiło and collaborators further prove that organic chemistry is hard at work in hot cores in the LMC.

This discovery is momentous because dwarf galaxies like the LMC tend to have a lower abundance of the heavy elements that make up complex organic molecules — most importantly, oxygen, carbon, and nitrogen. Beyond lacking the raw materials necessary to create complex molecules, the gas of low-metallicity galaxies does a poorer job preventing the penetration of high-energy photons. The impinging photons warm dust grains, resulting in a lower probability of forming and maintaining complex organic molecules. Despite this, organic molecules appear to be able to develop and persist — which has exciting implications for organic chemistry in low-metallicity environments.

ALMA observation of emission by methyl formate in a hot core in the LMC.
[Adapted from Sewiło et al. 2018]

A Lens into the Past

In the early universe, before the budding galaxies have had time to upcycle their abundant hydrogen into heavier elements, organic chemistry is thought to proceed slowly or not at all. The discovery of complex organic molecules in a nearby low-metallicity galaxy upends this theory and propels us toward a better understanding of the organic chemistry in the early universe.

Citation

Marta Sewiło et al 2018 ApJL 853 L19. doi:10.3847/2041-8213/aaa079

21 February 2018

Driving Jets from the Sun

Could coronal mass ejections (bottom panel) be driven by the same mechanism as the much smaller coronal jets (top panel)? [NASA]

What launches small jets and enormous coronal mass ejections (CMEs) from the Sun’s surface? New simulations explore how changing magnetic fields can drive these powerful eruptions.

Different Sizes, Same Jets?

Coronal jets are frequent, short-lived eruptions of plasma that are launched from low in the Sun’s atmosphere and travel outward through the corona. These ejections occur frequently across the Sun’s surface, lasting for ~10 minutes at a time and reaching lengths of ~50,000 km — a few times the Earth’s diameter, but still tiny compared to their enormous cousins, CMEs.

Despite the difference in size scales, a team of scientists led by Peter Wyper of Durham University has proposed that both coronal jets and CMEs are launched by the same mechanism, a process known as “magnetic breakout”. In a recent publication, Wyper and collaborators show the results of a series of 3D magnetohydrodynamic simulations of coronal jets to see what we would expect to observe from magnetic-breakout-driven eruptions.

Breaking Free

In the magnetic breakout model, magnetic field lines above filaments break and reconnect, removing confinement and allowing the filament to erupt from the Sun’s surface. Wyper and collaborators simulate this process by modeling a small bipolar structure on the Sun’s surface, embedded in a background magnetic field. They then observe how the magnetic fields rearrange themselves over time.

Schematic of the evolutionary sequence that produces breakout jets. The growing, twisted flux rope is shown by the yellow field lines. [Wyper et al. 2017]

The coronal jets that form in these simulations have four main stages:

Filament channel formation
Free energy is stored in a mini-filament or flux rope embedded within a background larger-scale magnetic field.
Breakout
Slow reconnection of the magnetic field lines above the growing flux rope eventually leads to a critical point where the flux rope can rapidly reconnect with the external field, breaking out.
Eruptive jet
As the filament escapes, it rapidly unwinds its twist and accelerates surrounding plasma along with it, causing the sudden eruption of an energetic, helical jet.
Relaxation
After several minutes, the jet propagates away, the reconnection subsides, and the fields relax into a new equilibrium similar to the starting point.

A Match to Observations

Wyper and collaborators run three sets of simulations with differing inclinations of the background magnetic fields, and they show that magnetic breakout in all three cases can lead to the production of broad, twisting jets with features consistent with what we’ve observed on the Sun.

In particular, the differing background field inclinations lead to a diversity of reconnection outflows preceding the jets. For highly inclined fields, for instance, a fast, dense outflow is driven with an inverted Y shape; for vertical fields, the outflows form much weaker spires.

All of these behaviors have been observed, and Wyper and collaborators’ model ties them together into a unified picture of coronal jet launching that has also been proposed to describe CMEs on a larger scale. If this picture is correct, then it may be possible that the complexities of these different solar eruptions can all be boiled down to one underlying process.

Eruption sequence for an inclined background field at a) 16 minutes, b) 24 minutes, and c) 31 minutes. [Wyper et al. 2018]

Citation

P. F. Wyper et al 2018 ApJ 852 98. doi:10.3847/1538-4357/aa9ffc

16 February 2018

An Eccentric Planet Skims a Giant Star

As part of a major survey of evolved stars, scientists have discovered the most eccentric planet known to orbit a giant. What can we learn from this unusual object before it’s eventually consumed by its host?

Planetary Diversity

An example of the diversity of just a few of the planetary systems discovered by the Kepler mission. [NASA]

In the early stages of exoplanet science, it was easy to assume that all systems around other stars would be similar to our own solar system: rocky worlds close in, gas giants further out — and all with co-planar, low-eccentricity orbits.

As we observed the first exoplanets and learned about their properties, however, it quickly became apparent that most other systems don’t resemble our own. The more exoplanets we observe, the more we become aware of the diversity of planetary systems — with planet compositions, masses, and orbits unlike any in the solar system.

Orbit of HD 76920b, oriented properly and overlaid with the solar system inner planets’ orbits to scale. A comet and asteroid from our solar system are shown as having comparably eccentric orbits. [Wittenmyer et al. 2017]

Relative Sizes Matter

Some systems are easier to study than others. Since exoplanet detection and characterization techniques rely on looking for the imprint of planets on stellar signals, systems consisting of a small star and large planet are favored. For this reason, exoplanets orbiting solar-like or dwarf stars are especially well studied — but we don’t have nearly as much information about planets orbiting massive, hot stars.

To combat this lack of data, several teams have begun surveys particularly targeting evolved, massive stars. One of these is known as the Pan-Pacific Planet Search, a survey that uses the 3.9m Anglo-Australian Telescope in Australia to study the spectra of metal-rich subgiants in the southern hemisphere. Fresh among the discoveries from this survey is a planet orbiting HD 76920, reported on in a recent publication led by Robert Wittenmyer (University of Southern Queensland and University of New South Wales, Australia).

An Extreme Orbit

Orbital eccentricity vs. planet’s periastron distance for the 116 confirmed planets orbiting giant stars. HD 76920b, the most eccentric of them, is shown with the red dot. [Wittenmyer et al. 2017]

Wittenmyer and collaborators conducted follow-up spectroscopy with two additional telescopes to confirm the properties of HD 76920. The team reports that HD 76920b — a giant planet of perhaps 4 Jupiter masses, with a period of 415 days and an eccentricity of e = 0.86 — is the most eccentric planet ever discovered orbiting a giant star.

How did HD 76920b achieve its extreme orbit? The go-to explanation for such an orbit is gravitational influence from a distant, massive stellar companion — and yet the authors find no evidence in their observations for a second star in the system. Instead, the team suggests that HD 76020b arrived on its current orbit via planet–planet scattering interactions earlier in the system’s lifetime.

Artist’s impression of a planet being engulfed by its host star. [NASA/ESA/G. Bacon]

Toasty Future

Lastly, Wittenmyer and collaborators use modeling to explore HD 76020b’s future. This planet’s orbit is already so extreme that it nearly skims the surface of its host, dipping to within 4 stellar radii of the star’s surface at its closest approach. The authors show that the planet will be engulfed by its host on a timescale of ~100 million years due to a combination of the star’s expanding radius and tidal interactions.

Gathering more observations of this extreme planet — and hunting for others like it — will help us to continue to learn about the formation and evolution of the diverse planetary systems our universe houses.

Citation

Robert A. Wittenmyer et al 2017 AJ 154 274. doi:10.3847/1538-3881/aa9894

14 February 2018

Galactic Pairs in the Early Universe

In the spirit of Valentine’s Day, today we’ll be exploring apparent pairs of galaxies in the distant, early universe. How can we tell whether these duos are actually paired galaxies, as opposed to disguised singles?

Real Pair, or Trick of the Light?

In the schematic timeline of the universe, the epoch of reionization is when the first galaxies and quasars began to form and evolve. [NASA]

The statistics of merging galaxies throughout the universe reveal not only direct information about how galaxies interact, but also cosmological information about the structure of the universe. While we’ve observed many merging galaxy pairs at low redshift, however, it’s much more challenging to identify these duos in the early universe.

A merging pair of galaxies at high redshift appears to us as a pair of unresolved blobs that lie close to each other in the sky. But spotting such a set of objects doesn’t necessarily mean we’re looking at a merger! There are three possible scenarios to explain an observed apparent duo:

It’s a pair of galaxies in a stage of merger.
It’s a projection coincidence; the two galaxies aren’t truly near each other.
It’s a single galaxy being gravitationally lensed by a foreground object. This strong lensing produces the appearance of multiple galaxies.

Hubble photometry of one of the three galaxy groups identified at z ~ 8, with the galaxies in the image labeled with their corresponding approximate photometric redshifts. [Adapted from Chaikin et al. 2018]

Hunting for Distant Duos

In a recent study led by Evgenii Chaikin (Peter the Great St. Petersburg Polytechnic University, Russia), a team of scientists has explored the Hubble Ultra Deep Field in search of high-redshift galaxies merging during the epoch of reionization, when the first galaxies formed and evolved.

Using an approach called the “dropout technique”, which leverages the visibility of the galaxies in different wavelength filters, Chaikin and collaborators obtain approximate redshifts for an initial sample of 7,000 objects. They find that roughly 50 have a redshift of z ~ 7, and 22 have a redshift of z ~ 8. None of the galaxies at z ~ 7 are in pairs, but the sample at z ~ 8 includes three groups for which the distance between galaxies is less than ~1 arcsecond.

But are these three pairs actual merging galaxies?

Conclusions from Statistics

Top: Gas density at z ~ 7.7 in the authors’ simulation output. Bottom: Mock observations of this output with Hubble’s WFC3 (left) and JWST’s NIRCam (right). [Adapted from Chaikin et al. 2018]

To answer this question, the authors next perform numerical simulations of galaxy formation and produce mock observations showing what the simulated field would look like in an equivalent deep Hubble exposure.

Based on their simulation statistics, Chaikin and collaborators argue that the three pairs at z ~ 8 do represent an unusually high merger fraction — but projection coincidences or lensing are far less likely scenarios to account for all three pairs. If the three pairs are indeed all merging galaxies, it could indicate that this Hubble field corresponds to a local overdensity at a redshift of z ~ 8.

Looking Ahead

The best way to improve on these measurements is to repeat this study with more advanced telescopes. Chaikin and collaborators demonstrate the superiority of the observations that the upcoming James Webb Space Telescope (JWST) will provide. They also point out the potential power of the Wide Field Infrared Survey Telescope (WFIRST) — currently under threat under the proposed 2019 federal budget — to extend the observational horizon well into the epoch of reionization.

Continued studies backed by the power of these future telescopes are sure to discover a wealth of additional distant galactic duos, helping us to characterize the universe in its early stages.

Citation

Evgenii A. Chaikin et al 2018 ApJ 853 81. doi:10.3847/1538-4357/aaa196

12 February 2018

Are We Really Missing Small Galaxies?

One long-standing astrophysical puzzle is that of so-called “missing” dwarf galaxies: the number of small dwarf galaxies that we observe is far fewer than that predicted by theory. New simulations, however, suggest that perhaps there’s no mystery after all.

Missing Dwarfs

Dark-matter cosmological simulations predict many small galaxy halos for every large halo that forms. [The Via Lactea project]

Models of a lambda-cold-dark-matter (ΛCDM) universe predict the distribution of galaxy halo sizes throughout the universe, suggesting there should be many more small galaxies than large ones. In what has become known as “the missing dwarf problem”, however, we find that while we observe the expected numbers of galaxies at the larger end of the scale, we don’t see nearly enough small galaxies to match the predictions.

Are these galaxies actually missing? Are our predictions wrong? Or are the galaxies there and we’re just not spotting them? A recent study led by Alyson Brooks (Rutgers University) uses new simulations to explore what’s causing the difference between theory and observation.

$detectable fraction of galaxies$

The fraction of detectable halos as a function of velocity, according to the authors’ simulations. Below ~35 km/s, the detectability of the galaxies drops precipitously. [Brooks et al. 2017]

Simulating Galactic Velocities

Because we can’t weigh a galaxy directly, one proxy used for galaxy mass is its circular velocity; the more massive a galaxy, the faster gas and stars rotate around its center. The discrepancy between models and observations lies in what’s known as the “galaxy velocity function”, which describes the number density of galaxies for a given circular velocity. While theory and observations agree for galaxies with circular velocities above ~100 km/s, theory predicts far more dwarfs below this velocity than we observe.

To investigate this problem, Brooks and collaborators ran a series of cosmological simulations based on our understanding of a ΛCDM universe. Instead of exploring the result using only dark matter, however, the team included baryons in their simulations. They then produced mock observations of the resulting galaxy velocities to see what an observed velocity function would look like for their simulated galaxies.

No Problem After All?

Comparison of theoretical velocity functions to observations. The black dashed line shows the original, dark-matter-only model predictions; the black solid line includes the effects of detectability. Blue lines show the authors’ new model, including the effects of detectability and inclusion of baryons. The red and teal data points from observations match this corrected model well. [Brooks et al. 2017]

Based on their baryon-inclusive simulations, Brooks and collaborators argue that there are two main factors that have contributed to the seeming theory/observation mismatch of the missing dwarf problem:

Galaxies with low velocities aren’t detectable by our current surveys.
The authors found that the detectable fraction of their simulated galaxies plunges as soon as galaxy velocity drops below ~35 km/s. They conclude that we’re probably unable to see a large fraction of the smallest galaxies.
We’re not correctly inferring the circular velocity of the galaxies.
Circular velocity is usually measured by looking at the line width of a gas tracer like HI. The authors find that this doesn’t trace the full potential wells of the dwarf galaxies, however, resulting in an incorrect interpretation of their velocities.

The authors show that the inclusion of these effects in the theoretical model significantly changes the predicted shape of the galaxy velocity function. This new function beautifully matches observations, neatly eliminating the missing dwarf problem. Perhaps this long-standing mystery has been a problem of interpretation all along!

Citation

Alyson M. Brooks et al 2017 ApJ 850 97. doi:10.3847/1538-4357/aa9576

9 February 2018

Probing the Structure of Our Solar System’s Edge

The boundary between the solar wind and the interstellar medium (ISM) at the distant edge of our solar system has been probed remotely and directly by spacecraft, but questions about its properties persist. What can models tell us about the structure of this region?

The Heliopause: A Dynamic Boundary

Schematic illustrating different boundaries of our solar system and the locations of the Voyager spacecraft. [Walt Feimer/NASA GSFC’s Conceptual Image Lab]

As our solar system travels through interstellar space, the magnetized solar wind flows outward and pushes back on the oncoming ISM, forming a bubble called the heliosphere. The clash of plasmas generates a boundary region called the heliopause, the shape of which depends strongly on the properties of the solar wind and the local ISM.

Much of our understanding of the outer heliosphere and the local ISM comes from observations made by the International Boundary Explorer (IBEX) and the Voyager 1 and Voyager 2 spacecraft. IBEX makes global maps of the flux of neutral atoms, while Voyagers 1 and 2 record the plasma density and magnetic field parameters along their trajectories as they exit the solar system. In order to interpret the IBEX and Voyager observations, astronomers rely on complex models that must capture both global and local effects.

Simulations of the plasma density in the meridional plane of the heliosphere due to the interaction of the solar wind with the ISM for the case of a relatively dense ISM with a weak magnetic field. [Adapted from Pogorelov et al. 2017]

Modeling the Edge of the Solar System

In this study, Nikolai Pogorelov (University of Alabama in Huntsville) and collaborators use a hybrid magneto-hydrodynamical (MHD) and kinetic simulation to capture fully the physical processes happening in the outer heliosphere.

MHD models have been used to understand many aspects of plasma flow in the heliosphere. However, they struggle to capture processes that are better described kinetically, like charge exchange or plasma instabilities. Fully kinetic models, on the other hand, are too computationally expensive to be used for global time-dependent simulations.

In order to combine the strengths of MHD and kinetic models, the authors also use adaptive mesh refinement — a technique in which the grid size is whittled down at key locations where small-scale physics can have a large effect — to resolve the important kinetic processes taking place at the heliopause while lowering the overall computational cost.

Physics of the Border

Top: Simulation results for the plasma density observed by Voyager 1 along its trajectory. Bottom: Voyager 1 observations of plasma waves. An increase in the plasma wave frequency corresponds to an increase in the ambient plasma density. Click for a closer look. [Adapted from Pogorelov et al. 2017]

The authors varied the ISM’s density and magnetic field, exploring how this changed the interaction between the ISM and the solar wind. Among their many results, the authors found:

There exists a plasma density drop and magnetic field strength increase in the ISM, just beyond the heliopause. This narrow boundary region is similar to a plasma depletion layer formed upstream from the Earth’s magnetopause as the solar wind streams around it.
The authors’ model for the plasma density along the trajectory of Voyager 1 is consistent with the actual plasma density inferred from Voyager 1’s measurements.
The heliospheric magnetic field likely dissipates in the region between the termination shock — the point at which the solar wind speed drops below the speed of sound — and the heliopause.

While this work by Pogorelov and collaborators has brought to light new aspects of the boundary between the solar wind and the ISM, the challenge of linking data and models continues. Future simulations will help us further interpret observations by IBEX and the Voyager spacecraft and advance our understanding of how our solar system interacts with the surrounding ISM.

Citation

N. V. Pogorelov et al 2017 ApJ 845 9. doi:10.3847/1538-4357/aa7d4f

7 February 2018

A New Look at Speeding Outflows

The compact centers of active galaxies — known as active galactic nuclei, or AGN — are known for the dynamic behavior they exhibit as the supermassive black holes at their centers accrete matter. New observations of outflows from a nearby AGN provide a more detailed look at what happens in these extreme environments.

Outflows from Giants

The powerful radio jets of Cygnus A, which extend far beyond the galaxy. [NRAO/AUI]

AGN consist of a supermassive black hole of millions to tens of billions of solar masses surrounded by an accretion disk of in-falling matter. But not all the material falling toward the black hole accretes! Some of it is flung from the AGN via various types of outflows.

The most well-known of these outflows are powerful radio jets — collimated and incredibly fast-moving streams of particles that blast their way out of the host galaxy and into space. Only around 10% of AGN are observed to host such jets, however — and there’s another outflow that’s more ubiquitous.

Fast-Moving Absorbers

Perhaps 30% of AGN — both those with and without observed radio jets — host wider-angle, highly ionized gaseous outflows known as ultra-fast outflows (UFOs). Ultraviolet and X-ray radiation emitted from the AGN is absorbed by the UFO, revealing the outflow’s presence: absorption lines appear in the ultraviolet and X-ray spectra of the AGN, blue-shifted due to the high speeds of the absorbing gas in the outflow.

Quasar PG 1211+143, indicated by the crosshairs at the center of the image, in the color context of its surroundings. [SDSS/S. Karge]

But what is the nature of UFOs? Are they disk winds? Or are they somehow related to the radio jets? And what impact do they have on the AGN’s host galaxy?

X-ray and Ultraviolet Cooperation

New observations are now providing fresh information about one particular UFO. A team of scientists led by Ashkbiz Danehkar (Harvard-Smithsonian Center for Astrophysics) recently used the Chandra and Hubble space telescopes to make the first simultaneous observations of the same outflow — a UFO in quasar PG 1211+143 — in both X-rays and in ultraviolet.

Danehkar and collaborators found absorption lines in both sets of data revealing an outflow moving at ~17,000 km/s (for reference, that’s ~5.6% of the speed of light, and more than 1,500 times faster than Elon Musk’s roadster will be traveling at its maximum speed in the orbit it was launched onto yesterday by the Falcon Heavy). Having the information both from the X-ray and the ultraviolet data provides the opportunity to better asses the UFO’s physical characteristics.

The X-ray spectrum for PG 1211+143 was obtained by Chandra HETGS (top); the ultraviolet spectrum was obtained by HST-COS G130M (bottom). [Adapted from Danehkar et al. 2018]

A Link Between Black Holes and Galaxies?

The authors use models of the data to demonstrate the plausibility of a scenario in which a shock driven by the radio jet gives rise to the fast bulk outflows detected in the X-ray and ultraviolet spectra.

They also estimate the impact that the outflows might have on the AGN’s host galaxy, demonstrating that the energy injected into the galaxy could be somewhere between 0.02% and 0.6% of the AGN’s total luminosity. At the higher end of this range, this could have an evolutionary impact on the host galaxy, suggesting a possible link between the black hole’s behavior and how its host galaxy evolves.

In order to draw definitive conclusions, we will need higher-resolution observations that can determine the total size and extent of these outflows. For that, we may need to wait for 2023, when a proposed X-ray spectrometer that might fit the bill, Arcus, may be launched.

Citation

Ashkbiz Danehkar et al 2018 ApJ 853 165. doi:10.3847/1538-4357/aaa427

2 February 2018

Hubble’s View of Little Blue Dots

The recent discovery of a new type of tiny, star-forming galaxy is the latest in a zoo of detections shedding light on our early universe. What can we learn from the unique “little blue dots” found in archival Hubble data?

Peas, Berries, and Dots

Green pea galaxies identified by citizen scientists with Galaxy Zoo. [Richard Nowell – Carolin Cardamone]

As telescope capabilities improve and we develop increasingly deeper large-scale surveys of our universe, we continue to learn more about small, faraway galaxies. In recent years, increasing sensitivity first enabled the detection of “green peas” — luminous, compact, low-mass (<10 billion solar masses; compare this to the Milky Way’s ~1 trillion solar masses!) galaxies with high rates of star formation.

Not long thereafter, we discovered galaxies that form stars similarly rapidly, but are even smaller — only ~3–30 million solar masses, spanning less than ~3,000 light-years in size. These tiny powerhouses were termed “blueberries” for their distinctive color.

Now, scientists Debra and Bruce Elmegreen (of Vassar College and IBM Research Division, respectively) report the discovery of galaxies that have even higher star formation rates and even lower masses: “little blue dots”.

Exploring Tiny Star Factories

The Elmegreens discovered these unique galaxies by exploring archival Hubble data. The Hubble Frontier Fields data consist of deep images of six distant galaxy clusters and the parallel fields next to them. It was in the archival data for two Frontier Field Parallels, those for clusters Abell 2744 and MAS J0416.1-2403, that the authors noticed several galaxies that stand out as tiny, bright, blue objects that are nearly point sources.

Top: a few examples of the little blue dots recently identified in two Hubble Frontier Field Parallels. Bottom: stacked images for three different groups of little blue dots. [Elmegreen & Elmegreen 2017]

The authors performed a search through the two Frontier Field Parallels, discovering a total of 55 little blue dots with masses spanning 10^5.8–10^7.4solar masses, specific star formation rates of >10^-7.4, and redshifts of 0.5 < z < 5.4.

Exploring these little blue dots, the Elmegreens find that the galaxies’ sizes tend to be just a few hundred light-years across. They are gas-dominated; gas currently outweighs stars in these galaxies by perhaps a factor of five. Impressively, based on the incredibly high specific star formation rates observed in these little blue dots, they appear to have formed all of their stars in the last 1% of the age of the universe for them.

An Origin for Globulars?

Log-log plot of star formation rate vs. mass for the three main groups of little blue dots (red, green, and blue markers), a fourth group of candidates with different properties (brown markers), and previously discovered local blueberry galaxies. The three main groups of little blue dots appear to be low-mass analogs of blueberries. [Elmegreen & Elmegreen 2017]

Intriguingly, this rapid star formation might be the key to answering a long-standing question: where do globular clusters come from? The Elmegreens propose that little blue dots might actually be an explanation for the origin of these orbiting, spherical, low-metallicity clusters of stars.

The authors demonstrate that, if the current star formation rates observed in little blue dots were to persist for another ~50 Myr before feedback or gas exhaustion halted star production, the little blue dots could form enough stars to create clusters of roughly a million solar masses — which is large enough to explain the globular clusters we observe today.

If little blue dots indeed rapidly produced such star clusters in the past, the clusters could later be absorbed into the halos of today’s spiral and elliptical galaxies, appearing to us as the low-metallicity globular clusters that orbit large galaxies today.

Citation

Debra Meloy Elmegreen and Bruce G. Elmegreen 2017 ApJL 851 L44. doi:10.3847/2041-8213/aaa0ce

31 January 2018

A Nine-Year Hunt for Neutrinos

How do we hunt for elusive neutrinos emitted by distant astrophysical sources? Submerge a huge observatory under ice or water … and then wait patiently.

Sneaky Messengers

Neutrinos — tiny, nearly massless particles that only weakly interact with other matter — are thought to be produced as a constant background originating from throughout our universe. In contrast to known point sources of neutrinos (for instance, nearby supernovae), the diffuse flux of cosmic neutrinos could be emitted from unresolved astrophysical sources too faint to be individually detected, or from the interactions of high-energy cosmic rays propagating across the universe.

Observations of this diffuse flux of cosmic neutrinos would be a huge step toward understanding cosmic-ray production, acceleration, and interaction properties. Unfortunately, these observations aren’t easy to make!

Diagram showing the path of a neutrino from a distant astrophysical source (accelerator) through the Earth. It is eventually converted into an upward-traveling muon that registers in the ANTARES detector under the sea. [ANTARES]

Looking for What Doesn’t Want to Be Found

Because neutrinos so rarely interact with matter, most pass right through us, eluding detection. The most common means of spotting the rare interacting neutrino is to look for Cherenkov radiation in a medium like ice or water, produced when a neutrino has interacted with matter to produce a charged particle (for instance, a muon) moving faster than the speed of light in the medium.

Muons produced in our atmosphere can also register in such detectors, however, so we need a way of filtering out these non-cosmic background events. The solution is a clever trick: search for particles traveling upward, not downward. Atmospheric muons will come only from above, whereas muons produced by neutrinos should travel through the detectors in all directions, since cosmic neutrinos arrive from all directions — including from below, after passing through the Earth.

Observatories on the Hunt

Neutrino observatories are often built to take advantage of pre-existing deep bodies of ice or water for their detectors. One of the most well-known neutrino observatories is IceCube, an array of detectors located far beneath the Antarctic ice. A few years ago, IceCube announced the observation of an excess of events over the expected atmospheric background — the first detection of a diffuse flux of cosmic neutrinos. The next step: confirmation from another observatory.

ANTARES detections across different energy bins, for both track-like (top) and shower-like (bottom) events. Plot includes data (black), model for atmospheric events (blue), and two different models for cosmic events (red). Above an energy cutoff of 20 TeV (grey line), nine excess neutrinos are detected relative to the atmospheric model. [Albert et al. 2018]

Enter ANTARES, short for “Astronomy with a Neutrino Telescope and Abyss Environmental Research.” Completed in 2008, this neutrino telescope was built 1.5 miles beneath the surface of the Mediterranean Sea. Now the collaboration is presenting the results of their nine-year search for a diffuse cosmic neutrino flux.

A Mild Excess

The outcome? Success! …sort of.

The very nature of neutrinos’ elusiveness means that we have to draw conclusions with very small numbers of detections. Over nine years, ANTARES detected a total of 33 events above an energy cutoff of 20 TeV, whereas models predict it should have seen only 24 such events due to atmospheric particles. This detection of nine extra neutrinos may sound insubstantial — but statistically, it allows the team to reject the hypothesis that there is no diffuse cosmic flux at an 85% confidence level.

The “mild excess” of neutrinos detected by ANTARES is by no means a smoking gun, but the properties of this cosmic neutrino flux are consistent with those detected by IceCube, which is a very promising outcome. At the moment, it would seem that a diffuse flux of cosmic neutrinos is present — and the next generation of neutrino observatories may be what we need to properly characterize it.

Citation

A. Albert et al 2018 ApJL 853 L7. doi:10.3847/2041-8213/aaa4f6

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