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NGC 1892

Much of today’s astronomy happens via methodical searches, but sometimes serendipitous discoveries still surprise us. Such is the case with the transient CGS2004A, a possible supernova recently detected in a galaxy nearly 50 million light-years away.

Observing Explosions

Supernovae — some of the brightest phenomena in our universe — are vast explosions thought to mark the destruction of stars in the end stages of their evolution.

The history of supernova observations is long: the first recorded supernova was seen in China in 185 AD! Because supernovae are scarce (there are perhaps 1–3 per century in the Milky Way) and their brightest stages of are short-lived (lasting just a few months), only a handful of supernova were spotted by naked eye through the ages. The invention of the telescope, however, changed this: as technology improved, astronomers became able to observe bright supernovae in galaxies beyond the Milky Way.

NGC 1892

Chronological observations of NGC 1892. From the top, a Hubble image from 2001, the CGS image from 2004, Stockler de Moraes’s image from 2017, and a Magellan image from 2018. The transient is visible only in the 2004 CGS image. [Guillochon et al. 2018]

Today, around 50,000 supernovae have been observed. The field has been vastly expanded by recent automated sky surveys that methodically hunt for transients. Nonetheless, intrepid individual astronomers still contribute to this scene — as evidenced by the recent discovery by Brazilian amateur astronomer Jorge Stockler de Moraes.

An Unexpected Find

In January of 2017, Stockler de Moraes imaged the distant galaxy NGC 1892 using a 12-inch diameter telescope. When he later compared his image to an archival image from 2004 of the same galaxy, taken as part of the Carnegie-Irvine Galaxy Survey (CGS), he discovered a distinct difference between the two photos: a bright source was present in the archival image that wasn’t visible in his recent photo.

Stockler de Moraes next contacted astronomer James Guillochon (Harvard Center for Astrophysics), who first eliminated possible alternate explanations for the source — such as minor planets in our solar system that might have coincided with NGC 1892 at the time. Guillochon then worked with a team of collaborators to explore other images of the galaxy and conduct follow-up imaging, as well as analyze the transient in the CGS image.

Core Collapse

The transient — labeled CGS2004A — was found to be absent in all additional images the authors explored, both in years before and after the CGS observation. Guillochon and collaborators’ photometric analysis of the transient and our knowledge of the nature of NGC 1892, a massive, star-forming galaxy, further suggest that this transient was likely a Type IIP supernova, caused when the core of a massive star (perhaps 8–50 solar masses) suddenly collapses.

Based on the authors’ analysis, it would seem that Stockler de Moraes serendipitously discovered a stellar explosion that went unnoticed 14 years ago. Discoveries such as these help us to continue to expand our understanding of how stars evolve throughout the universe.

Bonus

For a cool way to experience the history of supernova detections over time, check out the video by astronomer Greg Salvesen below. Chronological discoveries of supernovae are displayed both visually and using sound, for the time period of 1950 to the start of 2018. You can skip ahead to ~1990 to see detection rates pick up as more surveys come online! Data is from the Open Supernova Catalog by Guillochon et al.

Citation

“Serendipitous Discovery of a 14-year-old Supernova at 16 Mpc,” James Guillochon et al 2018 Res. Notes AAS 2 165. doi:10.3847/2515-5172/aade89

A pair of black holes

The advent of gravitational-wave astrophysics has made possible the study of elusive cosmic phenomena — like the mysterious merging of stellar-mass black holes.

When Black Holes Meet

Black hole mass chart

As of November 15, 2017, six black-hole mergers have been discovered via gravitational waves. [LSC/LIGO/Caltech/Sonoma State (Aurore Simonnet)]

The cataclysmic inspiraling of a pair of black holes doomed to merge sends ripples through space-time. Thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO), we have now detected a handful of instances of these ripples — enough to take a closer look at the broader population of binary black-hole mergers.

Beyond just collecting individual merger events, we can now explore whether or not the rate at which black hole mergers occur has evolved over the course of cosmic time. The merger rate reflects the underlying star formation rate as well as the particulars of stellar evolution. Ultimately, understanding how the merger rate has changed can help us learn how black-hole binaries form.

How can we tell whether or not the rate of binary black-hole mergers has evolved with redshift? A team led by Maya Fishbach (University of Chicago) aimed to extract this information from the first six binary black-hole detections from LIGO/Virgo.

Fishbach et al. Figure 1

The cumulative probability distribution of detected black-hole binaries depends on black hole mass, detector sensitivity (with the dashed lines indicating a more sensitive detector), and the underlying redshift distribution. Evolution of the merger rate with redshift would shift these curves — which demonstrate the case of a uniform redshift distribution — to the left or right. [Fishbach et al. 2018]

LIGO Provides a Listening Ear

One challenge is that the redshift distribution of black-hole binaries that we observe from LIGO/Virgo isn’t just a function of the underlying redshift distribution — it’s also a function of the mass distribution. Since mergers of more massive black holes generate “louder” signals and are more likely to be detected, a binary black-hole population with more massive members will generate more detections at high redshift than a population with fewer massive members.

To remedy this, Fishbach and collaborators used models of realistic redshift distributions to fit the redshift and the two component masses simultaneously. Based on the six available binary black-hole detections, Fishbach and collaborators find that the observations are consistent with a merger rate that is constant in redshift.

There does appear to be a slight decrease in the merger rate density with increasing redshift, but the authors caution that this could arise if the detections of the “quieter” mergers are published later; an artificially large proportion of “louder” events could skew the redshift distribution toward low-redshift events.

Looking Ahead to Future Detections

Fishbach et al. Figure 4

Merger rate density as a function of redshift for two redshift parameterizations. Both redshift models are consistent with a constant merger rate, which is indicated by the dotted line. Click to enlarge. [Fishbach et al. 2018]

What does the future hold for estimating the black hole merger rate as a function of redshift? To explore this question, Fishbach and collaborators generated synthetic black hole populations and modeled the likely detections by LIGO/Virgo.

They find that with a few hundred binary black-hole detections per year — an estimate based off of the expected improvements to LIGO/Virgo sensitivity — any deviations from a constant merger rate should be detectable within a few years. Exciting developments to come!

Citation

Maya Fishbach et al 2018 ApJL 863 L41. doi:10.3847/2041-8213/aad800

Illustration of a rocky body next to a gas giant planet, with a distant star and another small body in the background.

One of the primary goals of exoplanet-hunting missions like Kepler is to discover Earth-like planets in their hosts’ habitable zones. But could there be other relevant worlds to look for? A new study has explored the possibility of habitable moons around giant planets.

Seeking Rocky Worlds

Since its launch, the Kepler mission has found hundreds of planet candidates within their hosts’ habitable zones — the regions where liquid water can exist on a planet surface. In the search for livable worlds beyond our solar system, it stands to reason that terrestrial, Earth-like planets are the best targets. But stand-alone planets aren’t the only type of rocky world out there!

Many of the Kepler planet candidates found to lie in their hosts’ habitable zones are larger than three Earth radii. These giant planets, while unlikely to be good targets themselves in the search for habitable worlds, are potential hosts to large terrestrial satellites that would also exist in the habitable zone. In a new study led by Michelle Hill (University of Southern Queensland and University of New England, Australia; San Francisco State University), a team of scientists explores the occurrence rate of such moons.

habitable-zone gas giants

Kepler has found more than 70 gas giants in their hosts’ habitable zones. These are shown in the plot above (green), binned according to the temperature distribution of their hosts and compared to the broader sample of Kepler planet candidates (grey). [Hill et al. 2018]

A Giant-Planet Tally

Hill and collaborators combine the known Kepler detections of giant planets located within their hosts’ optimistic habitable zones with calculated detection efficiencies that measure the likelihood that there are additional, similar planets that we’re missing. From this, the authors estimate the frequency with which we expect giant planets to occur in the habitable zones of different types of stars.

The result: a frequency of 6.5 ± 1.9%, 11.5 ± 3.1%, and 6 ± 6% for giant planets lying in the habitable zones of G, K, and M stars, respectively. This is lower than the equivalent occurrence rate of habitable-zone terrestrial planets — which means that if the giant planets all host an average of one moon, habitable-zone rocky moons are less likely to exist than habitable-zone rocky planets. However, if each giant planet hosts more than one moon, the occurrence rates of moons in the habitable zone could quickly become larger than the rates of habitable-zone planets.

Lessons from Our Solar System

planet–moon angular separation

Distribution of the estimated planet–moon angular separation for known Kepler habitable-zone giant planets. Future missions would need to be able to resolve a separation between 1 and 90 microarcsec to detect potential moons. [Hill et al. 2018]

What can we learn from our own solar system? Of the ~185 moons known to orbit planets within our solar system, all but a few are in orbit around the gas giants. Jupiter, in particular, recently upped its tally to a whopping 79 moons! Gas giants therefore seem quite capable of hosting many moons.

Could habitable-zone moons reasonably support life? Jupiter’s moon Io provides a good example of how radiative and tidal heating by the giant planet can warm a moon above the temperature of its surroundings. And Jupiter’s satellite Ganymede demonstrates that large moons can even have their own magnetic fields, potentially shielding the moons’ atmospheres from their host planets.

Overall, it seems that the terrestrial satellites of habitable-zone gas giants are a valuable target to consider in the ongoing search for habitable worlds. Hill and collaborators’ work goes on to discuss observational strategies for detecting such objects, providing hope that future observations will bring us closer to detecting habitable moons beyond our solar system.

Citation

“Exploring Kepler Giant Planets in the Habitable Zone,” Michelle L. Hill et al 2018 ApJ 860 67. doi:10.3847/1538-4357/aac384

Gravitational microlensing illustration

The gravity of massive galaxies can warp the light from distant background sources into dramatic shapes. Gravitational lensing by foreground stars, planets, and other low-mass objects may be less visually stunning, but these tiny lensing events hold big promise.

Spitzer Space Telescope

The Spitzer Space Telescope, shown in this artist’s impression, trails Earth at a distance of over 1 AU. The large distance between Spitzer and ground-based observatories makes it a good choice for carrying out microlensing measurements. [NASA/JPL-Caltech]

Multifaceted Microlensing

When light from a distance source is bent by an object passing through our line of sight, we observe a temporary brightening of the source — a gravitational microlensing event.

While microlensing may be best known as a method of discovering exoplanets, it has the potential to detect other types of astrophysical objects as well. Most notably, microlensing can reveal faint, isolated objects that wander into our line of sight: rogue planets that have been ejected from stellar systems, brown dwarfs, and compact stellar remnants like neutron stars. Microlensing may also help us learn about the enigmatic population of stellar-mass black holes, which seem to be curiously rare between two and five solar masses.

Like many promising astrophysical techniques, microlensing comes at a cost: lots of telescope time, be it ground-based, space-based, or a combination of the two. Is it possible to take advantage of this powerful technique in a less costly way?

Shin et al. 2018 Fig. 2

Light curves and best-fit model curves for actual, realistic, and idealized data sets. The grey symbols are the ground-based observations. Click to enlarge. [Adapted from Shin et al. 2018]

Putting It to the Test

In order to determine the properties of the lensing object, you need to first determine the microlens parallax by comparing observations taken at different times of the year (annual parallax), at different ground-based observatories (terrestrial parallax), or simultaneously at ground- and space-based observatories (space-based parallax). A team led by In-Gu Shin (Harvard-Smithsonian Center for Astrophysics) tested the idea that the space-based microlens parallax can be determined accurately from just a handful of observations.

In order to test this theory, Shin and collaborators paired ground-based observations of OGLE-2016-BLG-1045, a microlensing event discovered by the Optical Gravitational Lensing Experiment (OGLE), with space-based observations from the Spitzer Space Telescope.

The authors considered three cases derived from the Spitzer observations:

  1. “Actual” case: the existing Spitzer measurements, which span roughly 15 days
  2. “Realistic” case: two space-based observations near the time of the ground-based peak and one baseline observation
  3. “Ideal” case: one space-based observation at the moment the ground-based peak occurs — a moment that’s nearly impossible to capture under normal circumstances — and one baseline observation taken well after the microlensing event
Shin et al. 2018 Fig. 4

The lens mass and distance derived from the realistic, actual, and idealized data. The two rows represent the results for the two degenerate solutions. For both parameters, the realistic result is within 1σ of the actual result. Click to enlarge. [Shin et al. 2018]

Getting to Know OGLE-2016-BLG-1045

By modeling the light curves for each of the three cases, the authors find that OGLE-2016-BLG-1045 is a low-mass stellar object (0.08 solar masses) located 16,000 light-years away. The results from the “realistic” and “actual” cases agreed to within 1σ, confirming that just two or three observations can adequately constrain the properties of the lensing object.

This promising result hints that it may be possible to study a large sample of isolated objects with just a few measurements each, helping us to understand better the populations of brown dwarfs, stellar remnants, and black holes in our galaxy. Expect to see more exciting results from microlensing in the future!

Citation

I.-G. Shin et al 2018 ApJ 863 23. doi:10.3847/1538-4357/aacdf4

transitional disk

HL Tau

This ALMA image of the protoplanetary disk surrounding the star HL Tauri reveals the detailed substructure of the disk, including gaps that may have been cleared by planets. [ALMA (ESO/NAOJ/NRAO)]

The gas giant PDS 70b made headlines last month as the first newly forming planet to ever be directly imaged. Now a team of scientists has gone a step further: they’ve captured evidence that this planet is actively accreting material, and they’ve measured the rate at which it’s growing.

The Search for Evidence

In the recent era of high-resolution observations, indirect evidence of planet formation abounds. In particular, we’ve captured a number of spectacular images of gapped disks surrounding young stars — disks in which we think the first planets of those systems are being born. According to models, planets will grow as they accrete matter from the surrounding protostellar disk, simultaneously clearing a gap in the disk as they orbit.

PDS 70

This image of PDS 70 was taken in the infrared by the SPHERE instrument on ESO’s Very Large Telescope. This is the first clear image of a planet — PDS 70b, visible as the bright spot to the right of the masked-out star — caught in the act of forming. [ESO/A. Müller et al.]

In spite of the accumulation of indirect evidence, direct evidence was long lacking — until recently. The young (10 million years old) dwarf star PDS 70, located just 370 light-years from Earth, is surrounded by a disk with a distinctive gap. And just last month, scientists announced that they’ve directly imaged, and confirmed, the presence of a newborn planet orbiting within the gap.

But just demonstrating that a planet lies within the gap isn’t yet enough — the next step is to prove that this planet, PDS 70b, is actively accreting material. This is where high-contrast observations from the Magellan Adaptive Optics system come in.

A Sign of Accretion

In a new study led by Kevin Wagner (University of Arizona, Amherst College, and NASA NExSS Earths in Other Solar Systems Team), a team of scientists used the adaptive optics system on the 6.5-m Magellan Clay Telescope in Chile to image the PDS 70 system in Hα (656 nm) and nearby continuum wavelengths. The presence of Hα emission at the location of the planet PDS 70b would indicate shocked, hot, infalling hydrogen gas — a smoking gun demonstrating that this planet is still accreting matter.

PDS 70 Halpha

New MagAO Hα observations of PDS 70 reveal the planet as a bright source in the top panel. The bottom panel is a schematic false-color diagram of PDS 70 assembled from the Hα image of the planet (red) and the infrared image (blue) of the thermal emission of the planet and starlight scattered by the disk. [Wagner et al. 2018]

Sure enough, Wagner and collaborators detected the presence of an Hα signal from PDS 70b on two sequential nights this past May — a signal that has less than a 0.1% probability of being a false positive. It seems fairly safe to say this baby planet is still growing.

Nearing Full Size

But how fast is it growing, and how far along is it? Wagner and collaborators use their Hα luminosity measurements to calculate a mass accretion rate for PDS 70b, finding that the gas giant is growing at a rate of 10-8±1 Jupiter masses per year. At this rate, and based on the age of the system, the authors estimate that PDS 70b likely accreted mass at a much higher rate in the past, and it has already acquired more than 90% of its final mass.

This nearby planet caught in the act of forming will make for an excellent study target in the future, as we continue to piece together our understanding of how planets are born and grow in protoplanetary disks.

Citation

“Magellan Adaptive Optics Imaging of PDS 70: Measuring the Mass Accretion Rate of a Young Giant Planet within a Gapped Disk,” Kevin Wagner et al 2018 ApJL 863 L8. doi:10.3847/2041-8213/aad695

solar flare

The Sun is a rather well-studied star, so it’s always exciting when we get the opportunity to observe it in a new way. One such opportunity is upcoming, via the Parker Solar Probe that just launched last week. But while we wait for that new view of the Sun, we have another one to examine: the Sun in microwaves.

Peering into Flares

solar flare spectrum over time

The spectrum over time for the first ~1 hr of the solar flare SL2017-09-10, shown at different wavelengths. [Gary et al. 2018]

In our efforts to better understand solar flares — sudden eruptions that occur when magnetic energy is abruptly released from the Sun, sending a burst of particles and radiation into space — we’ve observed these phenomena across a wide range of wavelengths. One wavelength regime known to be valuable for understanding the physics of solar flares is that of microwaves, which are emitted by high-energy electrons that are accelerated as energy is released in the flare.

But before now, the vast majority of microwave studies of solar flares have relied on data from the Nobeyama Radioheliograph in Japan, which observes the Sun at just two fixed frequencies that lie well above the peak of the microwave spectrum. This spectral regime explores only regions of high magnetic field strength.

What could we learn about solar flares from the lower-frequency microwaves emitted from more weakly magnetized regions? A newly upgraded array, the Expanded Owens Valley Solar Array (EOVSA) in California, is now helping us to answer this question.

Owens Valley Solar Array

One of the antennas in the Owens Valley Solar Array. [Dale E. Gary]

An Upgraded Array

After its recent upgrade, which concluded in April 2017, EOVSA now consists of 15 antennas — which produce imaging and spectroscopy data that span the microwave spectrum, including lower microwave frequencies. In a recent study led by Dale Gary (New Jersey Institute of Technology), a team of scientists has presented the first example of microwave imaging spectroscopy from EOVSA, demonstrating the powerful new observations capable with this technology.

As a target to test the array’s capabilities, Gary and collaborators selected a solar flare that occurred on the limb of the Sun — i.e., the edge of its disk, as seen from Earth — in September of 2017: SOL2017-09-10.

High-Energy Electrons Everywhere!

EOVSA microwave flare

EOVSA microwave data plotted in color over a 5’ x 5’ AIA image of the Sun during SOL2017-09-10. RHESSI hard X-ray data is shown in contours. The EOVSA data reveals the presence of high-energy electrons in multiple locations: in small reconnecting loops, well above these bright loops, and, at the north and sourth, associated with the legs of a much larger loop. [Gary et al. 2018]

High-frequency microwave observations of flares like SOL2017-09-10 had already demonstrated the presence of high-energy electrons in regions of high magnetic fields, like the small closed magnetic loops anchored in the Sun’s surface. But EOVSA’s view of the whole microwave spectrum has revealed that the spatial extent of high-energy electrons is much larger than we thought — these energetic electrons also lie well above the small reconnected loops, in the space between the loops and an erupting rope of magnetic flux associated with the flare.

This discovery indicates the necessity of some amendments to our standard model for the physics of solar flares. Though these early results from EOVSA may be preliminary, they clearly demonstrate the powerful capabilities of this new technology. We can look forward to more new observations of the Sun in the future, continuing to advance our understanding of how energy is released from our nearest star.

Citation

“Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare,” Dale E. Gary et al 2018 ApJ 863 83. doi:10.3847/1538-4357/aad0ef

How do we find extraterrestrial intelligence (ETI) — not just life beyond Earth, but advanced extraterrestrial civilizations? One approach is to seek signals from ETI that may be attempting to communicate with us — but the problem of where, when, and how to look for such communications is a complex one. A new study explores one way we could optimize this hunt: by searching for communication signals that are synchronized with the merger of two neutron stars.

“Hey, Look Over Here!”

transmission geometry

Schematic of how ETI in a distant galaxy could send a communication that would arrive at Earth (receiver) at the same time as the signal from the merger of two neutron stars in the ETI’s galaxy. The distance l between the ETI and the merger is much shorter than the distance between their galaxy and Earth. [Nishino & Seto 2018]

Transient, flaring astronomical phenomena — like supernovae and gamma-ray bursts — draw our eye and cause us to point our telescopes in the direction of the transient. A clever advanced civilization might take advantage of this, knowing that the moment when we on Earth observe such a transient astronomical source is the perfect time for us to receive communication from ETI near the transient.

But this strategy requires the ETI to predict the instant such an explosion near them — i.e., within the same galaxy — would occur, far enough in advance that they can send a communication signal that will arrive here on Earth at the same time as the transient signal.

Conveniently, there’s a bright transient that can be predicted in advance: the electromagnetic and gravitational-wave radiation from the merger of two neutron stars. In a new study from Kyoto University in Japan, researchers Yuki Nishino and Naoki Seto explore a scenario in which extragalactic intelligent life synchronizes a signal to us with a binary-neutron-star merger in their own galaxy.

Hulse-Taylor binary

The orbital decay of the binary neutron star PSR B1913+16 has been precisely measured over decades using the timing of its radio pulses. With this approach, ETI could estimate when a binary in their galaxy will eventually merge. [Inductiveload]

A Merger Predicted

Binary-neutron-star systems — which are prolific across the universe — are often discoverable and measurable due to pulses of light from one or both components of the system. From the exact timing of pulsars in a compact binary system in their own galaxy, ETI could measure the orbit and decay of the binary, allowing them to calculate when the system will merge.

Nishino and Seto argue that the ETI could then target a distant galaxy like ours for a communication signal that would arrive at roughly the same time as the gravitational-wave burst from the merger.

Powering Up

What kind of technology is needed for such a signal? The authors estimate that, for a civilization in a galaxy 130 million light-years away, ten megabytes of data could be sent to a receiver like the Square Kilometer Array on Earth using a ~1 terawatt radio transmitter. A terawatt is a lot of power — it’s 10% of the current energy consumption on Earth — but Nishino and Seto argue that such outputs are not out of reach for advanced civilizations.

SKA

Artist’s impression of the antennas in the 5-km central core of the future Square Kilometer Array. [SKA/Swinburne Astronomy Productions]

This feasibility indicates that, in the future, we might consider narrowing the search for ETI by focusing on the host galaxies of the many neutron-star mergers we hope to detect in the future with gravitational-wave observatories like LIGO, Virgo, and LISA. By doing so, there’s a chance we could spot a signal from a civilization that was hoping we’d look their way!

Citation

“The Search for Extra-Galactic Intelligence Signals Synchronized with Binary Neutron Star Mergers,” Yuki Nishino and Naoki Seto 2018 ApJL 862 L21. doi:10.3847/2041-8213/aad33d

Blanco Telescope Dome

Over billions of years, globular clusters and dwarf galaxies orbiting the Milky Way have been torn apart and stretched out by tidal forces. The disruption of these ancient stellar populations results in narrow trails of stars called stellar streams. These stellar streams can help us understand how the Milky Way halo was constructed and what our galaxy’s dark matter distribution is like — but how do we find them? 

Millennium Simulation

Along with cosmological simulations, like the Millennium Simulation pictured here, stellar streams can help us understand how dark matter is distributed in galaxies like the Milky Way. [Max Planck Institute for Astrophysics]

On the Trail of Tidal Streams

Understanding how our galaxy came to look the way it does is no easy task. Trying to discern the structure and formation history of the outer reaches of the Milky Way from our vantage point on Earth is a bit like trying to see the forest for the trees — while also trying to learn how old the forest is and where the trees came from!

One way to do so is to search for the stellar streams that form when globular clusters and dwarf galaxies are disrupted and torn apart by our galaxy. Stellar streams tend to be faint, diffuse, and obscured by foreground stars, which makes them tricky to observe. Luckily, recent data releases from the Dark Energy Survey are perfectly suited to the task.

Shipp et al. 2018 Fig. 4

Top: Density map of stars with a distance modulus of 16.7 for a given matched-filter isochrone. Bottom: Stellar streams identified in this work, including those previously known. Click to enlarge. [Shipp et al. 2018]

Dark Energy Survey Brings Faint Stars to Light

Nora Shipp (University of Chicago) and collaborators analyzed three years of data from the Dark Energy Survey in search of these stellar streams. The Dark Energy Survey is well-suited for stellar-stream hunts since it covers a wide area (5,000 square degrees of the southern sky) and can observe objects as faint as 26th magnitude.

Shipp and collaborators use a matched-filter technique to pinpoint the old, low-metallicity stars that belong to stellar streams. This method uses the modeled properties of stars of a certain age — synthetic isochrones — to identify stars within a background stellar stream with minimal contamination from foreground stars.

Using their matched filters, the authors found 15 stellar streams, 11 of which had never been seen before. They then estimated the age, metallicity, and distance modulus for each stream — all critical to understanding how the individual streams fit into the larger picture of galactic structure. 

Shipp et al. 2018 Fig. 5

A closer look at the stellar streams in the first quadrant of the surveyed area. Top: Density map of stars with a distance modulus of 15.4. Bottom: Stars with a distance modulus of 17.5. [Adapted from Shipp et al. 2018]

Reconstructing the Galactic Halo

These 11 newly discovered stellar streams will greatly enhance our understanding of the history of the galactic halo. Spectroscopy can help clarify the ages of these structures, while kinematic studies can help us understand if and how these structures are associated.

Future work may also help us discern the origin of the streams; the stark dichotomy in the mass-to-light ratios of the stellar streams discovered in this work hints that it may be possible to link some streams to globular clusters and others to dwarf galaxies. Look for this and more exciting results from galactic archaeologists in the future!

Citation

N. Shipp et al 2018 ApJ 862 114. doi:10.3847/1538-4357/aacdab

Segue 1

The mysterious object Segue 1 has intrigued astronomers since it was first discovered in 2007. A new study has now measured the first proper motions for this tiny neighboring galaxy, providing clues as to how it came to be in orbit around the Milky Way.

An Unusual Neighbor

NGC 147

NGC 147 is another example of a dwarf spheroidal galaxy in the Local Group. [Ole Nielsen]

Until recently, the distinction between globular clusters and dwarf galaxies was fairly clear-cut. In the last decade, however, new objects have been discovered that have muddied the waters: compact, faint dwarf galaxies that are so small and dim as to be nearly indistinguishable from globular clusters.

Segue 1, a Milky Way satellite just ~75,000 light-years away (that’s half the distance to the Large Magellanic Cloud!), was the first of these ultra-faint dwarf spheroidal galaxies to be discovered — and scientists have been debating its nature ever since.

It’s not just Segue 1’s nature that’s debated, however; we also want to understand where this tiny galaxy came from, and how it ended up in orbit around the Milky Way. To address these questions, a team of scientists led by Tobias Fritz (University of Virginia; IAC and University of La Laguna, Spain) have made the first proper-motion observations of this unusual object.

Segue 1 stars

Color-magnitude diagram of stars in Segue 1; spectroscopically identified members are shown in blue and non-members in red. [Fritz et al. 2018]

Pinning Down Proper Motion

Fritz and collaborators measured Segue 1’s proper motion using data from the Sloan Digital Sky Survey and the Large Binocular Camera over a baseline of 10 years. Their measurements put Segue 1 on an orbit that circles the Milky Way once every ~600 million years — which suggests it’s quite tightly bound to our galaxy.

The authors point out that the orbit inferred for Segue 1 support its classification as a galaxy rather than a disrupted star cluster: Segue 1 doesn’t pass close enough to the Milky Way to have been tidally disrupted. But how did this tiny galaxy arrive in orbit around the Milky Way in the first place?

Simulated Origins

There are two main options: Segue 1 was either accreted on its own, or it was the satellite of a larger, classical satellite that was accreted by the Milky Way. The parameters of its orbit rule out the possibility that it once orbited one of the known massive classical satellites of the Milky Way, but it’s still possible that it could have initially orbited a long-since destroyed massive satellite.

Infall times

Infall times for Segue 1 analogs in the cosmological simulations. The median first infall time is about 8 billion years ago. [Adapted from Fritz et al. 2018]

The authors explore cosmological zoom-in simulations of Milky-Way-mass galaxies to statistically determine which of these scenarios is statistically the most likely for Segue-1-like satellites, based on the tiny galaxy’s measured properties.

Fritz and collaborators find a 25% likelihood that Segue 1 first arrived in orbit around a long-since-destroyed satellite, with both it and its host accreted by the Milky Way ~12 billion years ago. Even more likely, at 75% probability, Segue 1 accreted on its own perhaps 8 billion years ago.

More detailed future measurements will likely give us an even clearer picture of Segue 1’s orbit and potential origins. In the meantime, there’s a whole wealth of other ultra-faint dwarfs like Segue 1 to explore!

Citation

T. K. Fritz et al 2018 ApJ 860 164. doi:10.3847/1538-4357/aac516

Sun magnetic fields

Solar flares, coronal mass ejections, prominences, sunspots — the most exciting features of the Sun are all driven by complex magnetic activity within the Sun’s interior and at its surface. Indeed, observations of the Sun have long revealed regions of magnetic flux of various magnitudes and polarities across the Sun’s disk.

full-disk solar images

Sample full-disk image of the Sun, with various features contoured in different colors, including plages (red), enhanced networks (blue); sunspots (green), and active networks (yellow). Panel F shows all contours overplotted on a magnetogram, with the grey regions corresponding to the background field. [Bose & Nagaraju 2018]

But it wasn’t until 1970 that scientists started measuring a different aspect of the Sun’s magnetization: its mean magnetic field. By treating the Sun like a distant star and measuring its net magnetic field by integrating across its whole disk, scientists have been able to effectively measure the imbalance in the magnetic flux of opposite polarities across its visible disk. A new study explores what might create this overall imbalance.

Learning from a Mean Field

The Sun’s mean magnetic field can reveal information about global behavior of our home star, helping us to better understand how magnetic fields form and evolve in stars and providing a deeper understanding of the interplanetary magnetic field that threads the space throughout our solar system.

Scientists have now regularly monitored the Sun’s mean magnetic field variations for more than two full solar cycles, on timescales ranging from a few days to several years, and we’ve learned that the mean field doesn’t stay constant — it can vary from ± 0.2 G to ± 2 G (for reference, a typical refrigerator magnet has a strength of ~50 G).

What we haven’t yet determined is the origin of the Sun’s mean magnetic field: does it come from the Sun’s large-scale, background magnetic field structure? Or is it driven by the magnetic fields of smaller active regions, like sunspots? A new study by scientists Souvik Bose (University of Oslo, Norway) and K. Nagaraju (Indian Institute of Astrophysics) explores these relative contributions.

Active Regions and Backgrounds

By decomposing Solar Dynamics Observatory observations of the Sun’s full disk into different regions, Bose and Nagaraju track the magnetic flux resulting from three categories:

  1. sunspots, dark spots that appear in the Sun’s photosphere as magnetic flux emerges;
  2. plages, enhanced networks, and active networks, surface features created by emergence and dispersion of weaker magnetic fields; and
  3. the Sun’s large-scale magnetic field — i.e., background regions that don’t fall into categories (1) and (2).

They then explore the variability of these fluxes over the span of several years.

magnetic field variation

Magnetic field variability vs. time, for A) the solar mean magnetic field; B) the background field; C) plages, enhanced networks, and active networks; and D) sunspots. The largest contributor to solar mean magnetic field variability is clearly the large-scale background field. [Bose & Nagaraju 2018]

Bose and Nagaraju’s calculations show that the variation in the solar mean magnetic field most closely tracks that of the background field, and it shows very little correlation with active regions. In particular, about 89% of the variability in the mean solar field is contributed by the background field, with only ~10% contributed by plages and the network field.

The authors point out that their work only indicates the origin of the solar mean magnetic field’s variability; its amplitude may yet be governed by the presence of sunspot activity on the surface of the Sun. Nonetheless, this study brings us a little closer to understanding the complex magnetic activity of our nearest star. 

Citation

Souvik Bose and K. Nagaraju 2018 ApJ 862 35. doi:10.3847/1538-4357/aaccf1

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