Features Archives - Page 101 of 122

Features RSS

24 April 2017

Worlds Without Moons

Many of the exoplanets that we’ve discovered lie in compact systems with orbits very close to their host star. These systems are especially interesting in the case of cool stars where planets lie in the star’s habitable zone — as is the case, for instance, for the headline-making TRAPPIST-1 system.

But other factors go into determining potential habitability of a planet beyond the rough location where water can remain liquid. One possible consideration: whether the planets have moons.

Supporting Habitability

Locations of equality between the Hill and Roche radius for five different potential moon densities. The phase space allows for planets of different semi-major axes and stellar host masses. Two example systems are shown, Kepler-80 and TRAPPIST-1, with dots representing the planets within them. [Kane 2017]

Earth’s Moon is thought to have been a critical contributor to our planet’s habitability. The presence of a moon stabilizes its planet’s axial tilt, preventing wild swings in climate as the star’s radiation shifts between the planet’s poles and equator. But what determines if a planet can have a moon?

A planet can retain a moon in a stable orbit anywhere between an outer boundary of the Hill radius (beyond which the planet’s gravity is too weak to retain the moon) and an inner boundary of the Roche radius (inside which the moon would be torn apart by tidal forces). The locations of these boundaries depend on both the planet’s and moon’s properties, and they can be modified by additional perturbative forces from the host star and other planets in the system.

In a new study, San Francisco State University scientist Stephen R. Kane modeled these boundaries for planets specifically in compact systems, to determine whether such planets can host moons to boost their likelihood of habitability.

Allowed moon density as a function of semimajor axis for the TRAPPIST-1 system, for two different scenarios with different levels of perturbations. The vertical dotted lines show the locations of the six innermost TRAPPIST-1 planets. [Kane 2017]

Challenge of Moons in Compact Systems

Kane found that compact systems have a harder time supporting stable moons; the range of radii at which their moons can orbit is greatly reduced relative to spread-out systems like our own. As an example, Kane calculates that if the Earth were in a compact planetary system with a semimajor axis of 0.05 AU, its Hill radius would shrink from being 78.5 times to just 4.5 times its Roche radius — greatly narrowing the region in which our Moon would be able to reside.

Kane applied his models to the TRAPPIST-1 system as an example, demonstrating that it’s very unlikely that many — if any — of the system’s seven planets would be able to retain a stable moon unless that moon were unreasonably dense.

Is TRAPPIST-1 Really Moonless?

Image of the Moon as it transits across the face of the Sun, as viewed from the Stereo-B spacecraft (which is in an Earth-trailing orbit). [NASA]

How do these results fit with other observations of TRAPPIST-1? Kane uses our Moon as an example again: if we were watching a transit of the Earth and Moon in front of the Sun from a distance, the Moon’s transit depth would be 7.4% as deep as Earth’s. A transit of this depth in the TRAPPIST-1 system would have been detectable in Spitzer photometry of the system — so the fact that we didn’t see anything like this supports the idea that the TRAPPIST-1 planets don’t have large moons.

On the other hand, smaller moons (perhaps no more than 200–300 km in radius) would have escaped detection. Future long-term monitoring of TRAPPIST-1 with observatories like the James Webb Space Telescope or 30-meter-class ground-based telescopes will help constrain this possibility, however.

Citation

Stephen R. Kane 2017 ApJL 839 L19. doi:10.3847/2041-8213/aa6bf2

21 April 2017

An Exploration of Dusty Galaxies

Submillimeter galaxies — i.e., galaxies that we detect in the submillimeter wavelength range — are mysterious creatures. It’s only within the last couple decades that we’ve had telescope technology capable of observing them, and we’re only now getting to the point where angular resolution limits allow us to examine them closely. A new study has taken advantage of new capabilities to explore the properties of a sample of 52 of these galaxies.

Dusty Star Formation

Submillimeter galaxies are generally observed in the early universe. Though they’re faint in other wavebands, they’re extremely luminous in infrared and submillimeter — their infrared luminosities are typically trillions of times the Sun’s luminosity. This is thought to be because these galaxies are very actively forming stars at rates of hundreds of times that of the Milky Way!

Example 10” × 10” true-color images of ten submillimeter galaxies in the authors’ ALMA-identified sample. [Simpson et al. 2017]

Submillimeter galaxies are also extremely dusty, so we don’t see their star formation directly in optical wavelengths. Instead, we see the stellar light after it’s been absorbed and reemitted by interstellar dust lanes — we’re indirectly observing heavily obscured star formation.

Why look for submillimeter galaxies? Studying them can help us to learn about galaxy and star formation early in our universe’s history, and help us to understand how the universe has evolved into what we see locally today.

Submillimeter Struggles

Due to angular resolution limitations in the past, we often couldn’t pin down the exact locations of submillimeter galaxies, preventing us from examining them properly. But now a team of scientists has used the Atacama Large Millimeter/submillimeter array (ALMA) to precisely locate 52 submillimeter galaxies identified by the Submillimeter Common-User Bolometer Array (SCUBA-2) in the UKIDSS Ultra Deep Survey field.

The precise locations made possible by ALMA allowed the team — led by James Simpson (University of Edinburgh and Durham University) — to identify the multi-wavelength properties of these galaxies in a pilot study that they hope to extend to many more similar galaxies in the future.

Lessons from Distant Galaxies

What did Simpson and collaborators learn in this study?

Photometric redshift distribution of the ALMA-identified submillimeter galaxies in the authors’ sample (grey). [Simpson et al. 2017]
For the set of galaxies for which the team could measure photometric redshifts, the median redshift was z ~ 2.65 (though redshifts ranged up to z ~ 5).
Submillimeter galaxies are cooler and larger than local far-infrared galaxies (known as ULIRGs). The authors therefore argue that it’s unlikely that ULIRGs are evolved versions of submillimeter galaxies.
Estimates of dust mass in these galaxies suggest that effectively all of the optical-to-near-infrared light from colocated stars is obscured by dust.
Estimates of the future stellar mass of these galaxies suggest that they cannot evolve into lenticular or spiral galaxies. Instead, the authors conclude, submillimeter galaxies must be the progenitors of local elliptical galaxies.

Citation

J. M. Simpson et al 2017 ApJ 839 58. doi:10.3847/1538-4357/aa65d0

19 April 2017

What Shaped Elias 2-27’s Disk?

The young star Elias 2-27 is surrounded by a massive disk with spectacular spiral arms. A team of scientists from University of Cambridge’s Institute of Astronomy has now examined what might cause this disk’s appearance.

Top: ALMA 1.3-mm observations of Elias 2-27’s spiral arms, processed with an unsharp masking filter. Two symmetric spiral arms, a bright inner ellipse, and two dark crescents are clearly visible. Bottom: a deprojection of the top image (i.e., what the system would look like face-on). [Meru et al. 2017]

ALMA-Imaged Spiral Arms

With the dawn of new telescopes such as the Atacama Large Millimeter/submillimeter Array, we’re now able to study the birth of young stars and their newly forming planetary systems in more detail than ever before. But these new images require new models and interpretations!

Case in point: Elias 2-27 is a low-mass star that’s only a million years old and is surrounded by an unusually massive disk of gas and dust. Recent spatially-resolved ALMA observations of Elias 2-27 have revealed the stunning structure of the star’s disk: it contains two enormous, symmetric spiral arms, as well as additional features interior to the spirals.

What caused the disk to develop this structure? Led by Farzana Meru, a group of Institute of Astronomy researchers has run a series of simulations that explore different ways that Elias 2-27’s disk might have evolved into the shape we see today.

Modeling a Disk

Meru and collaborators performed a total of 72 three-dimensional smoothed particle hydrodynamics simulations tracking 250,000 gas particles in a model disk around a star like Elias 2-27. They then modeled the transfer of energy through these simulated disks and produced synthetic ALMA observations based on the outcomes.

Left: Synthetic ALMA observations of disks shaped by an internal companion (top), an external companion (middle), and gravitational instability within the disk (bottom). Right: Deprojections of the images on the left. Scales are the same as in the actual observations above. The external companion and the gravitational instability scenarios match the actual ALMA observations of Elias 2-27 well. [Adapted from Meru et al. 2017]

By comparing these synthetic observations to the true ALMA observations of Elias 2-27, the authors hoped to determine which of three possible scenarios could produce the disk shape we see: 1) a companion (a planet or star) internal to the spiral arms, 2) a companion external to the spirals, or 3) gravitational instabilities operating within the disk.

Gravity or a Companion?

Meru and collaborators find that two scenarios produce observations that are very similar to what ALMA imaged. In the first, the disk is so massive that it becomes gravitationally unstable. Self-gravity of the disk then forms the spiral structures. In the second scenario, the arms are formed by a planetary companion of up to ~10–13 Jupiter masses orbiting Elias 2-27 outside of the spiral arms, at a large distance roughly in the range of 300–700 AU.

Though the possible companion inside the spiral arms is ruled out, the scenarios of a gravitational instability or an external companion remain plausible. If the former is true, then Elias 2-27 would be one of the first examples of an observed self-gravitating disk. If the latter is true, then Elias 2-27’s disk likely fragmented recently, forming the giant planet that shapes the disk. This would be the first evidence for a disk that has fragmented into planetary-mass objects.

Future deep near-infrared imaging may offer the chance to distinguish between these scenarios by allowing us to search for the heat from the possible companion.

Citation

F. Meru et al 2017 ApJL 839 L24. doi:10.3847/2041-8213/aa6837

14 April 2017

Habitability of the TRAPPIST-1 System

The recent discovery of seven Earth-sized, terrestrial planets around an M dwarf star was met with excitement and optimism. But how habitable are these planets actually likely to be? A recent study of these planets’ likely climates may provide an answer to this question.

An Optimistic Outlook

In February of this year, the TRAPPIST-1 system was announced: seven roughly Earth-sized, transiting, terrestrial planets all orbiting their host ultracool dwarf star within a distance the size of Mercury’s orbit. Three of the planets were initially declared to be in the star’s habitable zone — and scientists speculated that even those outside the habitable zone could potentially still harbor liquid water — making the system especially exciting.

In Wolf’s simulations, the surface temperature (solid lines) of TRAPPIST-1d grows to more than 380K in just 40 years. [Adapted from Wolf 2017]

The planets were labeled as “temperate” because all seven have equilibrium temperatures that are under ~400K. Since liquid water requires a surface temperature of 273-373K, this certainly seems promising!

Finding Realistic Temperatures

But there’s a catch: equilibrium temperatures are not actual measurements of the planet’s surface temperature, they’re just very rudimentary estimates based on how much light the planet receives. To get a better estimate of the real temperature of the planet — and therefore assess its habitability — you need advanced climate modeling of the planet that include factors like the greenhouse effect and planetary albedo.

In Wolf’s simulations, the surface temperature of TRAPPIST-1f plummets rapidly even when modeled with dense carbon dioxide atmosphere (purple line). The bottom panel shows the corresponding rapid growth of sea-ice on the surface oceans for the different atmospheric models. [Wolf 2017]

To that end, scientist Eric Wolf (University of Colorado Boulder) has conducted state-of-the-art 3D climate calculations for the three center-most planets — planets d, e, and f — in the TRAPPIST-1 system. Wolf assumed traditional terrestrial-planet atmospheres composed of nitrogen, carbon dioxide, and water, and he examined what would happen if these planets had large water supplies in the form of surface oceans.

Runaway and Snowball Planets

Wolf’s climate model indicates that the closest-in of the three planets, planet d, would undergo thermal runaway even in the best case scenario. In just 40 years of the simulation, the planet’s surface temperature exceeds 380K, suggesting it couldn’t continue to sustain liquid water. Wolf argues that planet d and the two planets interior to it, b and c, all lie inside of the traditional liquid water habitable zone — they are hot, dry, and uninhabitable.

Next, Wolf models the outermost of the three center planets, planet f. Even when planet f is modeled with a dense carbon dioxide atmosphere, it can’t avoid its fate of becoming completely ice-covered within roughly 60 years. Wolf concludes that planets f, g and h all lie outside of the traditional habitable zone defined by the maximum carbon dioxide greenhouse limit.

Equilibrium solutions for TRAPPIST-1e with various atmospheric conditions. Top panel: mean surface temperature. Middle panel: sea-ice coverage. Bottom panel: habitable surface area. [Wolf 2017]

Goldilocks?

Lastly, Wolf turns to planet e, the central planet in the system. This planet, he finds, is the most viable candidate for a robustly habitable world. The simulations show that planet e can maintain habitable surface conditions for a variety of atmospheric compositions.

While astrobiologists eyeing TRAPPIST-1 may be disappointed that at second glance the planets are not quite as inhabitable as they first seemed, it is promising to see that the habitability of the central planet holds up reasonably well to some more realistic testing. Either way, future examinations of all seven of these planets should help us learn more about terrestrial, Earth-sized planets.

Citation

Eric T. Wolf 2017 ApJL 839 L1. doi:10.3847/2041-8213/aa693a

12 April 2017

New Discoveries Fill the Quasar Gap

Quasars — active and luminous galactic centers — can be difficult to find at some high redshifts due to their camouflaging color. A team of scientists has now come up with a way to detect these distant monsters in spite of their disguise.

Quasar Camouflage

The color track of quasars between 5 ≤ z ≤ 6 in the commonly used i – z and r – i bands. Each dot on the red line marks a 0.1 difference in redshift. The contours show the colors of M dwarfs, from early type to late type. Quasars at a redshift of 5.3 ≤ z ≤ 5.7 are clearly contaminated by M dwarfs, making them difficult to identify. [Adapted from Yang et al. 2017]

One of the key ways we can study the early universe is by building a large sample of high-redshift quasars. In particular, we believe that reionization of the universe is just completing around z ~ 6. Quasars near this redshift are crucial tools for probing the post-reionization epoch and exploring the evolution of the intergalactic medium, quasar evolution, and early supermassive black hole growth.

But quasars at this redshift are difficult to detect! The problem is contamination: quasars at this distance are the same color in commonly used optical bands as cool M-dwarf stars. As a result, surveys searching for quasars have often just cut out that entire section of the color space in order to avoid this contamination.

This means that there’s a huge gap in our sample of quasars around z ~ 5.5: of the more than 300,000 quasars known, only ~30 have been found in the redshift range of 5.3 ≤ z ≤ 5.7.

The addition of new color–color selection criteria using infrared bands (bottom two plots) allows the authors to differentiate quasars (blue) from M dwarfs (grey), which isn’t possible when only the traditional optical color–color selection criteria are used (top plot). [Adapted from Yang et al. 2017]

A New Approach

In a recent publication led by Jinyi Yang (Peking University, China and Steward Observatory, University of Arizona), a team of scientists has demonstrated a new technique for finding these missing quasars. The team uses this technique to perform the first systematic survey of luminous quasars at a redshift near z = 5.5.

Instead of relying only on the conventional color space in broad optical bands, Yang and collaborators selected candidates by also looking at their colors in near-infrared and mid-infrared bands. The team used observations from the Sloan Digital Sky Survey, the UKIRT Infrared Deep Sky Surveys — Large Area Survey, the VISTA Hemisphere Survey, and the Wide Field Survey Explorer.

Examining the quasar candidates in these color spaces allowed the authors to more clearly differentiate between the M dwarfs and the quasars, so that they could select only the candidates that clearly fell in the regions dominated by quasars in all three spaces. Yang and collaborators then performed spectroscopic follow-up on their candidates to confirm them.

Gap Quasars Uncovered

The authors found 21 new high-redshift quasars (red), including 15 in the range of 5.3 ≤ z ≤ 5.7. [Adapted from Yang et al. 2017]

The team found a total of 21 new quasars from their main sample, with 15 new quasars discovered specifically in the redshift range of 5.3 ≤ z ≤ 5.7. This nearly doubles the number of known quasars at z ~ 5.5!

This initial success has more applications in the future; upcoming surveys will provide an even larger sample to examine for z ~ 5.5 quasars. The team demonstrated that their pipeline can be applied to such surveys by testing it on some preliminary data from the UKIRT Hemisphere Survey. In just this initial test they already discovered another z ~ 5.5 quasar, demonstrating that they’ll have little difficulty finding more once the complete data set is released.

Citation

Jinyi Yang et al 2017 AJ 153 184. doi:10.3847/1538-3881/aa6577

Photograph of a space telescope above the earth.

10 April 2017

PACMan to Help Sort Hubble Proposals

Every year, astronomers submit over a thousand proposals requesting time on the Hubble Space Telescope (HST). Currently, humans must sort through each of these proposals by hand before sending them off for review. Could this burden be shifted to computers?

A Problem of Volume

Astronomer Molly Peeples gathered stats on the HST submissions sent in last week for the upcoming HST Cycle 25 (the deadline was Friday night), relative to previous years. This year’s proposal round broke the record, with over 1200 proposals submitted in total for Cycle 25. [Molly Peeples]

Each proposal cycle for HST time attracts on the order of 1100 proposals — accounting for far more HST time than is available. The proposals are therefore carefully reviewed by around 150 international members of the astronomy community during a six-month process to select those with the highest scientific merit.

Ideally, each proposal will be read by reviewers that have scientific expertise relevant to the proposal topic: if a proposal requests HST time to study star formation, for instance, then the reviewers assigned to it should have research expertise in star formation.

How does this matching of proposals to reviewers occur? The current method relies on self-reported categorization of the submitted proposals. This is unreliable, however; proposals are often mis-categorized by submitters due to misunderstanding or ambiguous cases.

As a result, the Science Policies Group at the Space Telescope Science Institute (STScI) — which oversees the review of HST proposals — must go through each of the proposals by hand and re-categorize them. The proposals are then matched to reviewers with self-declared expertise in the same category.

With the number of HST proposals on the rise — and the expectation that the upcoming James Webb Space Telescope (JWST) will elicit even more proposals for time than Hubble — scientists at STScI and NASA are now asking: could the human hours necessary for this task be spared? Could a computer program conceivably do this matching instead?

Comparison of PACMan’s categorization to the manual sorting for HST Cycle 24 proposals. Green: proposals similarly categorized by both. Yellow: proposals whose manual classifications are within the top 60% of sorted PACMan classifications. Red: proposals categorized differently by each. [Strolger et al. 2017]

Introducing PACMan

Led by Louis-Gregory Strolger (STScI), a team of scientists has developed PACMan: the Proposal Auto-Categorizer and Manager. PACMan is what’s known as a Naive Bayesian classifier; it’s essentially a spam-filtering routine that uses word probabilities to sort proposals into multiple scientific categories and identify people to serve on review panels in those same scientific areas.

PACMan works by looking at the words in a proposal and comparing them to those in a training set of proposals — in this case, the previous year’s HST proposals, sorted by humans. By using the training set, PACMan “learns” how to accurately classify proposals.

PACMan then looks up each reviewer on the Astrophysical Data System (ADS) and compiles the abstracts from the reviewer’s past 10 years’ worth of scientific publications. This text is then evaluated in a similar way to the text of the proposals, determining each reviewer’s suitability to evaluate a proposal.

How Did It Do?

Comparison of PACMan sorting to manual sorting, specifically for the HST Cycle 24 proposals that were recategorized by the Science Policies Group (SPG) from what the submitter (PI) selected. Of these swaps, 48% would have been predicted by PACMan. [Strolger et al. 2017]

Strolger and collaborators show that with a training set of one previous cycle’s proposals, PACMan correctly categorizes the next cycle’s proposals roughly 87% of the time. By increasing the size of the training set to include more past cycles, PACMan’s accuracy can be improved up to 95% (though the algorithm will have to be retrained any time the proposal categories change).

PACMan’s results were also consistent for reviewers: it found that nearly all of the reviewers (92%) asked to serve in the last cycle were appropriate reviewers for the subject area based on their ADS publication record.

There are still some hiccups in automating this process — for instance, finding the reviewers on ADS can require human intervention due to names not being unique. As the scientific community moves toward persistent and distinguishable identifiers (like ORCIDs), however, this problem will be mitigated.

Strolger and collaborators believe that PACMan demonstrates a promising means of increasing the efficiency and impartiality of the HST proposal sorting process. This tool will likely be used to assist or replace humans in this process in future HST and JWST cycles.

Citation

Louis-Gregory Strolger et al 2017 AJ 153 181. doi:10.3847/1538-3881/aa6112

7 April 2017

The Search for Ringed Exoplanets

Are planetary rings as common in our galaxy as they are in our solar system? A new study demonstrates how we might search for ringed exoplanets — and then possibly finds one!

Saturns Elsewhere?

Artist’s illustration of the super ring system around exoplanet J1407b. This is the only exoplanet we’ve found with rings, but it’s not at all like Saturn. [Ron Miller]

Our solar system is filled with moons and planetary rings, so it stands to reason that exoplanetary systems should exhibit the same features. But though we’ve been in the planet-hunting game for decades, we’ve only found one exoplanet that’s surrounded by a ring system. What’s more, that system — J1407b — has enormous rings that are vastly different from the modest, Saturn-like rings that we might expect to be more commonplace.

Have we not discovered ringed exoplanets just because they’re hard to identify? Or is it because they’re not out there? A team of scientists led by Masataka Aizawa (University of Tokyo) has set out to answer this question by conducting a systematic search for rings around long-period planet candidates.

The transit light curve of KIC 10403228, shown with three models: the best-fitting planet-only model (blue) and the two best-fitting planet+ring models (green and red). [Aizawa et al. 2017]

The Hunt Begins

Why long-period planets? Rings are expected to be unstable as the planet gets closer to the central star. What’s more, the planet needs to be far enough away from the star’s warmth for the icy rings to exist. The authors therefore select from the collection of candidate transiting planets 89 long-period candidates that might be able to host rings.

Aizawa and collaborators then fit single-planet models (with no rings) to the light curves of these planets and search for anomalies — curves that aren’t fit well by these standard models. Particularly suspicious characteristics include a long ingress/egress as the planet moves across the face of the star, and asymmetry of the transit shape.

After applying a series of checks to eliminate false positives, the authors are left with one candidate: KIC 10403228.

Rings or Not?

Schematics of the two best-fitting ringed-exoplanet models for KIC 10403228, and the possible parameters of the system. The planet crosses the disk of the star from left to right with a grazing transit. [Adapted from Aizawa et al. 2017]

Next, the authors apply a wide range of ringed-exoplanet models to KIC 10403228’s light curve. They find two different scenarios that fit the data well: one in which the ring is significantly tilted with respect to the orbital plane, and another in which it’s only slightly tilted.

The authors conclude by testing a variety of other scenarios that could explain the anomalies in the light curve instead. They find that two other scenarios are plausible: 1) the star is in an eclipsing binary system, with the second star surrounded by a circumstellar disk, and 2) the star is part of a hierarchical triple, and the transits are caused by a binary star system as it orbits KIC 10403228.

Though Aizawa and collaborators aren’t able to rule either of these other two scenarios out, they suggest that follow-up spectroscopy or high-resolution imaging may help distinguish between the different scenarios. In the meantime, their methodology for systematically searching for ringed exoplanets has proven worthwhile, and they plan to extend it now to a larger data set. Perhaps we’ll soon find other Saturn-like planets in our galaxy!

Citation

Masataka Aizawa (逢澤正嵩) et al 2017 AJ 153 193. doi:10.3847/1538-3881/aa6336

5 April 2017

ALMA Examines a Distant Quasar Host

The dust continuum (top) and the [CII] emission (bottom) maps for the region around J1120+0641. [Adapted from Venemans et al. 2017]

A team of scientists has used the Atacama Large Millimeter/submillimeter Array (ALMA) to explore the host galaxy of the most distant quasar known. Their observations may help us to build a picture of how the first supermassive black holes in the universe formed and evolved.

Faraway Monsters and Their Galaxies

We know that quasars — the incredibly luminous and active centers of some distant galaxies — are powered by accreting, supermassive black holes. These monstrous powerhouses have been detected out to redshifts of z ~ 7, when the universe was younger than a billion years old.

Though we’ve observed over a hundred quasars at high redshift, we still don’t understand how these early supermassive black holes formed, or whether the black holes and the galaxies that host them co-evolved. In order to answer questions like these, however, we first need to gather information about the properties and behavior of various supermassive black holes and their host galaxies.

A team of scientists led by Bram Venemans (Max-Planck Institute for Astronomy, Germany) recently used the unprecedented sensitivity and angular resolution of ALMA — as well as the Very Large Array and the IRAM Plateau de Bure Interferometer — to examine the most distant quasar currently known, J1120+0641, located at a redshift of z = 7.1.

A High-Resolution Look

The team’s observations of the dust and gas emission from the quasar’s host galaxy revealed a number of intriguing things:

The red and blue sides of the [CII] emission line are shown here as contours, demonstrating that there’s no ordered rotational motion of the gas on kpc scales. [Adapted from Venemans et al. 2017]
The majority of the galaxy’s emission is very compact. Around 80% of the observed flux came from a region of only 1–1.5 kpc in diameter.
Despite the fact that the 2.4-billion-solar-mass black hole at the galaxy’s center is accreting at a high rate, the heating in the galaxy is dominated not by the black hole’s accretion, but by star formation.
There’s no sign of the expected structure of a rotating disk on kpc scales.
The authors estimate a dynamical mass of the host galaxy of 43 billion solar masses — and the black hole at the galaxy’s center makes up ~6% of that. This ratio is roughly 10x higher than the black-hole-to-bulge mass ratio in local early-type galaxies.
In the very central region, the black hole accounts for around 20% of the galaxy’s dynamical mass, and gas and dust likely accounts for most of the remainder. This doesn’t leave much room for massive stars in the center of the galaxy.

ALMA’s capabilities have enabled these first efforts to spatially resolve the host galaxy of the most distant quasar known, resulting new and unexpected information. The authors now look hopefully to the future, when even longer baselines of ALMA may allow us a still-higher-resolution look at this distant quasar, possibly providing answers to some of the questions it has raised.

Citation

Bram P. Venemans et al 2017 ApJ 837 146. doi:10.3847/1538-4357/aa62ac

31 March 2017

An Exo-Venus in the Solar Neighborhood

A size comparison of Venus and Earth. Though they are nearly the same size and density, these two planets evolved very differently. [NASA]

Earth is great place for life — but Venus definitely isn’t. Both planets have similar masses and densities. So why did one evolve to support life, while the other turned into a barren and inhospitable hothouse? This is a question we might be able to answer if we can gather observations of other planets similar to Earth and Venus. The recent discovery of an exo-Venus in our solar neighborhood brings us one step closer to this goal!

A New Neighbor

A team of scientists led by Isabel Angelo (SETI Institute, NASA Ames Research Center, and UC Berkeley) has announced the discovery of Kepler-1649b, an exoplanet transiting a star located just 219 light-years away from us. Kepler-1649b is unique in being roughly the same size as Earth and Venus and also receiving a similar amount of starlight as Venus does from our Sun.

Phase-folded light curve showing the transit of Kepler-1649b. [Angelo et al. 2017]

Angelo and collaborators conducted follow-up observations after Kepler’s detection of 1649b to verify its planetary nature and pin down its properties. They found that Kepler-1649b has a radius of 1.08 times that of Earth, and it receives an incident flux of 2.3 times Earth’s — which is very similar to the incident flux received by Venus. Kepler-1649b orbits a star that’s only a quarter of our Sun’s radius, however, and it therefore orbits significantly closer to its star in order to receive the same flux, circling its host once every 8.7 days.

Differences Due to a Small Host

It’s worth identifying how this planet might differ from Venus. The authors suggest a few key factors:

Kepler-1649b may be more prone to effects of host-star variability. M-dwarf stars like this one are typically more magnetically active than our Sun, and Kepler-1649b is orbiting very close to its star.
Kepler-1649b receives comparatively low-energy radiation, compared to Venus. This is because its cooler host emits more light at lower frequencies than the Sun.
Kepler-1649b may be subject to larger tidal effects from its host star. Because it orbits so close in, it might experience tidal heating, synchronous rotation, and tidal locking — all of which can influence its seasons and geologic activity.

Target for the Future

The colored contours show the most likely radius and incident flux measured for Kepler-1649b. Earth, Venus, Mars, and several other exoplanets are plotted for comparison. [Angelo et al. 2017]

In spite of these differences, Kepler-1649b still qualifies as the most similar exoplanet we’ve found to Venus in terms of its size and incident radiation. This marks our first opportunity to study such a target to understand how it differs from Earth-like planets and what conditions might lead to habitability on a planet.

We will be able to gain more information on Kepler-1649b with upcoming missions. The Transiting Exoplanet Survey Satellite (TESS) will observe more transits, and Gaia’s improved-accuracy distance measurements should also improve our measurements of the star’s and planet’s properties. What’s more, Kepler-1649b will make an excellent target for the James Webb Space Telescope (launching in 2018) to examine in the hopes of learning about its atmosphere.

Citation

Isabel Angelo et al 2017 AJ 153 162. doi:10.3847/1538-3881/aa615f

29 March 2017

Counting Rogue Asteroids and Comets

How many comets, asteroids, and planetary bodies are floating through interstellar space, not gravitationally bound to any solar system? A new study explores this question and what its answer means for our understanding of how solar systems form.

Rough Beginnings

Do all solar systems form with gas-giant migration causing a large fraction of the system’s material to be ejected? [ESO/L. Calçada]

Our solar system was not a great place to be when it was first born. Simulations of its formation suggest that early orbital migration of the gas and ice giants caused up to 99% of the original planetesimals in our solar system to be ejected out into space, gravitationally unbound from our star.

If this model is correct, and if other solar systems formed in the same way, then it stands to reason that interstellar space should be littered with free-floating, rogue planets, asteroids, and comets. So why have we never yet observed a sizable interstellar object? Do these non-detections mean that our models are wrong? Or just that our ability to spot interstellar objects isn’t good enough yet?

Trajectories of eight synthetic interstellar objects modeled by the authors as they pass within 50 AU of the Sun. [Engelhardt et al. 2017]

Predicting Detections

To answer these questions, a team of scientists led by Toni Engelhardt (University of Hawaii’s Institute for Astronomy and Technical University of Munich, Germany) and Robert Jedicke (University of Hawaii’s Institute for Astronomy) set out to find an observational upper limit on the spatial number density of interstellar objects using data from three different contemporary wide-field solar system surveys.

The team first simulated a synthetic population of interstellar objects out to a distance of 750 AU from the Sun. The authors then propagated the positions of this population forward in time to account for the effects of our Sun’s gravitational pull on passing objects, known as “gravitational focusing”.

Using the resulting distribution of interstellar objects within 50 AU of the Sun, the authors then simulated the detection of these objects with three surveys: Pan-STARRS1, the Mt. Lemmon Survey, and the Catalina Sky Survey.

The authors calculate the number of interstellar objects in their synthetic population that the surveys should have been able to detect in their 19 cumulative survey years. From the surveys’ non-detections, they then estimate an upper limit to the number density of interstellar objects in the solar neighborhood.

Not Many Interstellar Objects

Orbital elements for the synthetic interstellar objects that the authors find Pan-STARRS1 should be able to detect. Black lines denote active comets and grey lines denote inactive asteroids. [Engelhardt et al. 2017]

The authors find that the number density of interstellar objects larger than 1 km in radius must be lower than ~1.4 per every 10,000 AU³.

What does this mean? This is a stricter upper limit than the number densities predicted by many currently accepted solar system formation models. There are two explanations for this that the authors deem most likely:

The properties we expect for interstellar objects based on what we observe in our own solar system — in particular, the distribution of their sizes — are wrong.
Other solar systems didn’t form like ours did, ejecting the vast majority of its protoplanetary material into interstellar space.

While this remains an unsatisfying conclusion, observations from powerful future sky surveys like the Large Synoptic Survey Telescope (LSST) may help us to further home in on the answer to this puzzle.

Citation

Toni Engelhardt et al 2017 AJ 153 133. doi:10.3847/1538-3881/aa5c8a

Previous 1 … 99 100 101 102 103 … 122 Next