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Arp 274

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?

epoch of reionization

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:

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

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

simulated galaxies

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

dwarf galaxy

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

Via Lactea

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?

galaxy velocity function

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:

  1. 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.
  2. 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

Heliosphere

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

heliosphere

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.

heliosphere model

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

Voyager 1 observations

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:

  1. 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.
  2. 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.
  3. 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

ultra-fast 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

Cygnus A

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.

PG 1211+143

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.

PG 1211+143 spectra

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

Hubble Frontier Fields

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

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.

Little blue dots

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 105.8–107.4 solar 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?

star formation rate vs. mass

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

ANTARES

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!

neutrino detection schematic

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

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

TRAPPIST-1 and planets

TRAPPIST-1, a nearby ultracool dwarf star, was catapulted into the public eye roughly a year ago when it was determined to host seven transiting, Earth-sized planets — three of which are located in its habitable zone. But how correct are the properties we’ve measured for this system?

Sun vs. TRAPPIST-1

TRAPPIST-1 is a very small, dim star — it’s only 11% the diameter of the Sun — which makes it easier for us to learn about its planets from transit data. [ESO]

Intrigue of TRAPPIST-1

One reason the TRAPPIST-1 system is of particular interest to scientists is that its small star (roughly the size of Jupiter) means that the system has a very favorable planet-to-star ratio. This makes it possible to learn a great deal about the properties of the planets using current and next-generation telescopes.

The observations we expect to be able to make of TRAPPIST-1 exoplanets — of the planet atmospheres, surface conditions, and internal compositions, for example — will allow us to test planet formation and evolution theories and assess the prospects of habitability for Earth-sized planets orbiting cool M dwarfs.

Why Stellar Measurements Matter

parallax of TRAPPIST-1

The parallax motion of TRAPPIST-1 in dec (top) and R.A. (bottom) as a function of day. Observations were made between 2013 and 2016 and then folded over a year. [Van Grootel et al. 2018]

In order to make these measurements, however, we first need very precise measurements of the host star’s parameters. This is because transiting exoplanet parameters are generally determined relative to those of the host. A few examples:

  • Determining how much irradiation a planet receives requires knowing the luminosity of the host star and planet’s orbit size. The latter is calculated based on the host star’s mass.
  • Determining the planet’s radius requires knowing the host star’s radius, as the planet’s transit depth tells us only the star-to-planet radius ratio.
  • Determining whether or not the planet is able to retain an atmosphere — and therefore whether it has exhibited long-term habitability — requires knowing the time the host star takes to contract onto the main sequence, which depends on the star’s mass.

When the TRAPPIST-1 planetary system was discovered, measurements of TRAPPIST-1’s properties were made to the best of our abilities at the time. Now, in a new study led by Valérie Van Grootel (University of Liège, Belgium), a team of scientists has used new observations and analysis techniques to refine our measurements of the star.

luminosity vs. age

Stellar luminosity for evolution models for various masses and metallicities. The green dashed horizontal lines bracket the authors’ observed value for TRAPPIST-1’s luminosity. A stellar mass of ~0.09 M⊙ is needed to account for the old age and luminosity of the star. [Van Grootel et al. 2018]

New Estimates

Using 188 epochs of observations of TRAPPIST-1 from multiple telescopes between 2013 and 2016, Van Grootel and collaborators obtained a very precise measurement for TRAPPIST-1’s parallax. This allowed them to refine the estimate of its luminosity — now measured at (5.22 ± 0.19) x 10-4 that of the Sun — to twice the precision of the previous estimate.

The team then produced a new estimate for TRAPPIST-1’s mass using new stellar evolution modeling and analysis, combined with empirical mass derived for similar ultracool dwarfs in astrometric binaries. This approach produces a final mass for TRAPPIST-1 of 0.089 ± 0.006 M⊙ — which is nearly 10% higher than the previous estimate and significantly more precise. Finally, the authors use these values to obtain new estimates of TRAPPIST-1’s radius (0.121 ± 0.003 R⊙) and effective temperature (2516 ± 41 K).

These new, refined measurements will ensure that our future observations of the TRAPPIST-1 planets are being interpreted correctly — which is critical for a system that will be so thoroughly scrutinized in coming years. Keep an eye out for new results about TRAPPIST-1 in the future!

Citation

Valérie Van Grootel et al 2018 ApJ 853 30. doi:10.3847/1538-4357/aaa023

double neutron star

More than forty years after the first discovery of a double neutron star, we still haven’t found many others — but a new survey is working to change that.

The Hunt for Pairs

Hulse-Taylor binary

The observed shift in the Hulse-Taylor binary’s orbital period over time as it loses energy to gravitational-wave emission. [Weisberg & Taylor, 2004]

In 1974, Russell Hulse and Joseph Taylor discovered the first double neutron star: two compact objects locked in a close orbit about each other. Hulse and Taylor’s measurements of this binary’s decaying orbit over subsequent years led to a Nobel prize — and the first clear evidence of gravitational waves carrying energy and angular momentum away from massive binaries.

Forty years later, we have since confirmed the existence of gravitational waves directly with the Laser Interferometer Gravitational-Wave Observatory (LIGO). Nonetheless, finding and studying pre-merger neutron-star binaries remains a top priority. Observing such systems before they merge reveals crucial information about late-stage stellar evolution, binary interactions, and the types of gravitational-wave signals we expect to find with current and future observatories.

Since the Hulse-Taylor binary, we’ve found a total of 16 additional double neutron-star systems — which represents only a tiny fraction of the more than 2,600 pulsars currently known. Recently, however, a large number of pulsar surveys are turning their eyes toward the sky, with a focus on finding more double neutron stars — and at least one of them has had success.

pulse profile

The pulse profile for PSR J1411+2551 at 327 MHz. [Martinez et al. 2017]

A Low-Mass Double

Conducted with the 1,000-foot Arecibo radio telescope in Puerto Rico, the Arecibo 327 MHz Drift Pulsar Survey has enabled the recent discovery of dozens of pulsars and transients. Among them, as reported by Jose Martinez (Max Planck Institute for Radio Astronomy) and coauthors in a recent publication, is PSR J1411+2551: a new double neutron star with one of the lowest masses ever measured for such a system.

Through meticulous observations over the span of 2.5 years, Martinez and collaborators were able to obtain a number of useful measurements for the system, including the pulsar’s period (62 ms), the period of the binary (2.62 days), and the system’s eccentricity (e = 0.17).

In addition, the team measured the rate of advance of periastron of the system, allowing them to estimate the total mass of the system: M = ~2.54 solar masses. This mass, combined with the eccentricity of the orbit, demonstrate that the companion of the pulsar in PSR J1411+2551 is almost certainly a neutron star — and the system is one of the lightest known to date, even including the double neutron-star merger that was observed by LIGO in August this past year.

Constraining Stellar Physics

recycled pulsar

Based on its measured properties, PSR J1411+2551 is most likely a recycled pulsar in a double neutron-star system. [Martinez et al. 2017]

The intriguing orbital properties and low mass of PSR J1411+2551 have already allowed the authors to explore a number of constraints to stellar evolution models, including narrowing the possible equations of state for neutron stars that could produce such a system. These constraints will be interesting to compare to constraints from LIGO and Virgo in the future, as more merging neutron-star systems are observed.

Meanwhile, our best bet for obtaining further constraints is to continue searching for more pre-merger double neutron-star systems like the Hulse-Taylor binary and PSR J1411+2551. Let the hunt continue!

Citation

J. G. Martinez et al 2017 ApJL 851 L29. doi:10.3847/2041-8213/aa9d87

star formation near Sgr A*

Is it possible to form stars in the immediate vicinity of the hostile supermassive black hole at the center of our galaxy? New evidence suggests that nature has found a way.

infrared galactic center

Infrared view of the central 300 light-years of our galaxy. [Hubble: NASA/ESA/Q.D. Wang; Spitzer: NASA/JPL/S. Stolovy]

Too Hostile for Stellar Birth?

Around Sgr A*, the supermassive black hole lurking at the Milky Way’s center, lies a population of ~200 massive, young, bright stars. Their very tight orbits around the black hole pose a mystery: did these intrepid stars somehow manage to form in situ, or did they instead migrate to their current locations from further out?

For a star to be born out of a molecular cloud, the self-gravity of the cloud clump must be stronger than the other forces it’s subject to. Close to a supermassive black hole, the brutal tidal forces of the black hole dominate over all else. For this reason, it was thought that stars couldn’t form in the hostile environment near a supermassive black hole — until clues came along suggesting otherwise.

Science as an Iterative Process

proplyds

Very Large Array observations of candidate photoevaporative protoplanetary disks discovered in 2015. [Yusef-Zadeh et al. 2015]

Longtime AAS Nova readers might recall that one of our very first highlights on the site, back in August of 2015, was of a study led by Farhad Yusef-Zadeh of Northwestern University. In this study, the authors presented observations of candidate “proplyds” — photoevaporative protoplanetary disks suggestive of star formation — within a few light-years of the galactic center.

While these observations seemed to indicate that stars might, even now, be actively forming near Sgr A*, they weren’t conclusive evidence. Follow-up observations of these and other signs of possible star formation were hindered by the challenges of observing the distant and crowded galactic center.

Two and a half years later, Yusef-Zadeh and collaborators are back — now aided by high-resolution and high-sensitivity observations of the galactic center made with the Atacama Large Millimeter-Submillimeter Array (ALMA). And this time, they consider what they found to be conclusive.

bipolar outflow in galactic center

ALMA observations of BP1, one of 11 bipolar outflows — signatures of star formation — discovered within the central few light-years of our galaxy. BP1 is shown in context at left and zoomed in at right; click for a closer look. [Yusef-Zadeh et al. 2017]

Unambiguous Signatures

The authors’ deep ALMA observations of the galactic center revealed the presence of 11 bipolar outflows within a few light-years of Sgr A*. These outflows appear as approaching and receding lobes of dense gas that were likely swept up by the jets created as stars were formed within the last ~10,000 years. Yusef-Zadeh and collaborators argue that the bipolar outflows are “unambiguous signatures of young protostars.”

Based on these sources, the authors calculate an approximate rate of star formation of ~5 x 10-4 solar masses per year in this region. This is large enough that such low-mass star formation over the past few billion years could be a significant contributor to the stellar mass budget in the galactic center.

bipolar outflow locations

Locations and orientations of the 11 bipolar outflows found. [Yusef-Zadeh et al. 2017]

The question of how these stars were able to form so near the black hole remains open. Yusef-Zadeh and collaborators suggest the possibility of events that compress the host cloud, creating star-forming condensations with enough self-gravity to resist tidal disruption by Sgr A*’s strong gravitational forces.

To verify this picture, the next step is to build a detailed census of low-mass star formation at the galactic center. We’re looking forward to seeing how this field has progressed by the next time we report on it!

Citation

F. Yusef-Zadeh et al 2017 ApJL 850 L30. doi:10.3847/2041-8213/aa96a2

'Oumuamua

What’s the news coming from the research world on the interstellar asteroid visitor, asteroid 1I/’Oumuamua? Read on for an update from a few of the latest studies.

What is ‘Oumuamua?

In late October 2017, the discovery of minor planet 1I/’Oumuamua was announced. This body — which researchers first labeled as a comet and later revised to an asteroid — had just zipped around the Sun and was already in the process of speeding away when we trained our telescopes on it. Its trajectory, however, marked it as being a visitor from outside our solar system: the first known visitor of its kind.

Since ‘Oumuamua’s discovery, scientists have been gathering as many observations of this body as possible before it vanishes into the distance. Simultaneously, theorists have leapt at the opportunity to explain its presence and the implications its passage has on our understanding of our surroundings. Here we present just a few of the latest studies that have been published on this first detected interstellar asteroid — including several timely studies published in our new journal, Research Notes of the AAS.

'Oumuamua velocity

The galactic velocity of ‘Oumuamua does not coincide with any of the nearest stars to us. [Mamajek 2018]

Where Did ‘Oumuamua Come From?

Are we sure ‘Oumuamua didn’t originate in our solar system and get scattered into a weird orbit? Jason Wright (The Pennsylvania State University) demonstrates via a series of calculations that no known solar system body could have scattered ‘Oumuamua onto its current orbit — nor could any still unknown object bound to our solar system.

Eric Mamajek (Caltech and University of Rochester) shows that the kinematics of ‘Oumuamua are consistent with what we might expect of interstellar field objects, though he argues that its kinematics suggest it’s unlikely to have originated from many of the nearest stellar systems.

What Are ‘Oumuamua’s Properties?

'Oumuamua light curve

‘Oumuamua’s light curve. [Bannister et al. 2017]

A team of University of Maryland scientists led by Matthew Knight captured a light curve of ‘Oumuamua using Lowell Observatory’s 4.3-m Discovery Channel Telescope. The data indicate that the asteroid’s period is at least 3 hours in length, and most likely more than 5 hours. Assuming the light curve’s variation is caused by the tumbling asteroid’s changing cross-section, ‘Oumuamua must be a minimum of 3 times as long as it is wide. Knight and collaborators see no signs in their images of a coma or tail emitted from ‘Oumuamua, suggesting there is no volatile material sublimating from its surface under the heat of the Sun.

coma

No coma is visible around ‘Oumuamua. [Knight et al. 2017]

A study of the asteroid’s photometry, led by Michele Bannister (Queen’s University Belfast, UK), used the Gemini-North telescope in Hawaii and the William Herschel Telescope in Spain to explore the asteroid’s shape and color. Bannister and collaborators refined the estimate of the asteroid’s shape to be at least 5.3 times as long as it is wide, which requires this body to have significant internal cohesion to hold together as it tumbles. Their measured color for ‘Oumuamua is largely neutral.

What Does This Visitor Imply?

missing planets

Masses and semimajor axes of known exoplanets. Colors correspond to the ratio of escape velocity to circular velocity. The presence of ‘Oumuamua implies a vast and cool, still undetected population of planets. [Laughlin & Batygin, 2017]

Gregory Laughlin of Yale University and Konstantin Batygin of Caltech (and Planet Nine fame) explore some of the consequences of ‘Oumuamua’s parameters. They argue that its current passage, if it’s not a fluke, suggests the presence of an enormous number (1027) of such objects in our galaxy alone — enough to account for two Earth-masses of material for every star in the galaxy. Flinging asteroids like ‘Oumuamua out into interstellar space isn’t easy, though; the necessary multi-body interaction requires the system to contain a giant and long-period planet like our Neptune or Jupiter. Taken together, this information suggests that every star in the galaxy may host a Neptune-like planet at a Neptune-like distance.

More information on ‘Oumuamua is sure to come in the next few months as scientists continue to process their data from the asteroid’s swift passage. In the meantime, this interstellar visitor continues to challenge our understanding of our nearby surroundings and the broader context of the galaxy around us.

Citation

Jason T. Wright 2017 Res. Notes AAS 1 38. doi:10.3847/2515-5172/aa9f23
Eric Mamajek 2017 Res. Notes AAS 1 21. doi:10.3847/2515-5172/aa9bdc
Matthew M. Knight et al 2017 ApJL 851 L31. doi:10.3847/2041-8213/aa9d81
Michele T. Bannister et al 2017 ApJL 851 L38. doi:10.3847/2041-8213/aaa07c
Gregory Laughlin and Konstantin Batygin 2017 Res. Notes AAS 1 43. doi:10.3847/2515-5172/aaa02b

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