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flare ribbons

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Nature of CME-Flare-Associated Coronal Dimming

Published June 2016

 

Main takeaway:

The Solar Dynamics Observatory (SDO) observed a large solar eruption at the end of December 2011. Scientists Jianxia Cheng (Shanghai Astronomical Observatory and the Chinese Academy of Sciences) and Jiong Qiu (Montana State University) studied this coronal mass ejection and the associated flaring on the Sun’s surface. They found that this activity was accompanied by dimming in the Sun’s corona near the ends of the flare ribbons.

Why it’s interesting:

The process of coronal dimming isn’t fully understood, but Cheng and Qiu’s observations provide a clear link between coronal dimming and eruptions of plasma and energy from the Sun. The locations of the dimming — the footpoints of the two flare ribbons — and the timing relative to the eruption suggests that coronal dimming is caused by the ejection of hot plasma from the Sun’s surface.

How this process was studied:

There are a number of satellites dedicated to observing the Sun, and several of them were used to study this explosion. Data from SDO’s Atmospheric Imaging Assembly (which images in extreme ultraviolet) and its Helioseismic and Magnetic Imager (which measures magnetic fields) were used — as well as observations from STEREO, the pair of satellites orbiting the Sun at ±90° from SDO.

Citation

J. X. Cheng and J. Qiu 2016 ApJ 825 37. doi:10.3847/0004-637X/825/1/37

Dragonfly 44

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra-Diffuse Galaxy Dragonfly 44

Published August 2016

 

Main takeaway:

Using the Keck Observatory and the Gemini North telescope in Hawaii, a team led by Pieter van Dokkum (Yale University) discovered the very dim galaxy Dragonfly 44, located in the Coma cluster. The team estimated the center of this galaxy’s disk to be a whopping 98% dark matter.

Why it’s interesting:

Dragonfly 44, though dim, was discovered to host around 100 globular clusters. Measuring the dynamics of these clusters allowed van Dokkum and collaborators to estimate the mass of Dragonfly 44: roughly a trillion times the mass of the Sun. This is similar to the mass of the Milky Way, and yet the Milky Way has over a hundred times more stars than this intriguing galaxy. It’s very unexpected to find a galaxy this massive that has a dark-matter fraction this high.

What we can learn from this:

How do ultra-faint galaxies like these form? One theory is that they’re “failed” normal galaxies: they have the sizes, dark-matter content, and globular cluster systems of much more luminous galaxies, but they were prevented from building up a normal stellar population. So far, Dragonfly 44’s properties seem consistent with this picture.

Citation

Pieter van Dokkum et al 2016 ApJL 828 L6. doi:10.3847/2041-8205/828/1/L6

IR background

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

LIGO Gravitational Wave Detection, Primordial Black Holes, and the Near-IR Cosmic Infrared Background Anisotropies

Published May 2016

 

Main takeaway:

A study by Alexander Kashlinsky (NASA Goddard SFC) proposes that the cold dark matter that makes up the majority of the universe’s matter may be made of black holes. These black holes, Kashlinsky suggests, are primordial: they collapsed directly from dense regions of the universe soon after the Big Bang.

Why it’s interesting:

This model would simultaneously explain several observations. In particular, we see similarities in patterns between the cosmic infrared and X-ray backgrounds. This would make sense if accretion onto primordial black holes in halos produced the X-ray background in the same regions where the first stars also formed, producing the infrared background.

What this means for current events:

In Kashlinsky’s model, primordial black holes would occasionally form binary pairs and eventually spiral in and merge. The release of energy from such an event would then be observable by gravitational-wave detectors. Could the gravitational-wave signal that LIGO detected last year have been two primordial black holes merging? More observations will be needed to find out.

Citation

A. Kashlinsky 2016 ApJL 823 L25. doi:10.3847/2041-8205/823/2/L25

triple-imaged galaxy

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Detection of Lyman-Alpha Emission from a Triply Imaged z = 6.85 Galaxy Behind MACS J2129.4−0741

Published May 2016

 

Main takeaway:

A team led by Kuang-Han Huang (University of Caliornia, Davis) discovered a faint galaxy at z = 6.846 located behind the galaxy cluster MACS J2129.4–0741. This galaxy contains only one ten-thousandth the stellar mass of the Milky Way, and it’s the faintest galaxy we’ve found at this great distance.

Why it’s interesting:

This galaxy is roughly 13 billion years old, placing it near the end of the reionization epoch (in which the first stars formed and caused our universe to transition from neutral gas to ionized gas). Examining such a small galaxy at this distance provides valuable information about how the process of reionization may have occurred.

About the discovery:

The newly discovered galaxy was found due to a fortunate alignment with a foreground galaxy cluster. Gravitational lensing by the foreground cluster produced three images of the distant galaxy, which were identified as being the same galaxy due to their similar spectra.

Citation

Kuang-Han Huang et al 2016 ApJL 823 L14. doi:10.3847/2041-8205/823/1/L14

IMBH?

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy

Published December 2015

 

Main takeaway:

Tomoharu Oka (Keio University, Japan) and collaborators discovered what they believe is the signature of a 100,000-solar-mass black hole lurking just 200 light-years from the center of our galaxy. If this interpretation is correct, this black hole would be the second-largest in our galaxy (after the supermassive black hole Sgr A* in the Milky Way’s center).

CMZ

Observations of the Milky Way center (a), with a zoom-in view of the gas cloud CO–0.40–0.22 (b). [Oka et al. 2015]

Why it’s interesting:

Intermediate-mass black holes — those with mass between 100 and 100,000 solar masses — are a bridge between the commonly observed stellar-mass black holes and supermassive black holes. Theory says that they should exist, but we haven’t yet found any definitive evidence of them.

About the discovery:

Oka and collaborators’ detection was actually not of the black hole itself — no compact source is visible in the area — but of the strange gas cloud CO–0.40–0.22. This cloud has an unusually broad velocity dispersion without a visible cause. The authors model the cloud to demonstrate that its properties could be reproduced if it were flung by a strong gravitational source of 100,000 solar masses, which must be both compact and invisible.

Citation

Tomoharu Oka et al 2016 ApJL 816 L7. doi:10.3847/2041-8205/816/1/L7

HUDF

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Evolution of Galaxy Number Density at z < 8 and Its Implications

Published October 2016

 

Main takeaway:

How many galaxies are there in the observable universe? The latest estimate is approximately 2 trillion, according to a study led by Christopher Conselice (University of Nottingham, UK). The authors produced this estimate by using observations of the number of galaxies in recent deep-field surveys by Hubble and other telescopes, and then extrapolating this number to account for small and faint galaxies that we aren’t able to see.

Why it’s interesting:

The original Hubble Deep Field study from the mid-1990s provided the basis for our previous working estimate of the number of galaxies the universe contains, which was around 120 billion. The new estimate from Conselice and collaborators therefore suggests that there are a factor of ten more galaxies in the universe than we previously thought!

What to expect from observations:

Right now we only have the capability to see roughly 10% of these 2 trillion galaxies. But future observatories like the James Webb Space Telescope will be able to pick out many more distant galaxies than what we’ve found so far, helping us to understand how these galaxies formed in the early universe.

Citation

Christopher J. Conselice et al 2016 ApJ 830 83. doi:10.3847/0004-637X/830/2/83

HL Tau

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Gas Gaps in the Protoplanetary Disk Around the Young Protostar HL Tau

Published March 2016

 

HL Tau

The dust (left) and gas (right) emission from HL Tau show that the gaps in its disk match up. [Yen et al. 2016]

Main takeaway:

At the end of last year, the Atacama Large Millimeter/Submillimeter Array released some of its first data — including a spectacular observation of a dusty protoplanetary disk around the young star HL Tau. In this follow-up study, a team led by Hsi-Wei Yen (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan) analyzed the ALMA data and confirmed the presence of two gaps in the gas of HL Tau’s disk, at radii of ~28 and ~69 AU.

Why it’s interesting:

The original ALMA image of HL Tau’s disk suggests the presence of gaps in disk, but scientists weren’t sure if they were caused by effects like gravitational instabilities or dust clumping, or if the gaps were created by the presence of young planets. Yen and collaborators showed that gaps in the disk’s gas line up with gaps in its dust, supporting the model in which these gaps have been carved out by newly formed planets.

Added intrigue:

The evidence for planets in this disk came as a bit of a surprise, since it was originally believed that it takes tens of millions of years to form planets from the dust of protoplanetary disks — but HL Tau is only a million years old. These observations therefore suggest that planets start to form much earlier than we thought.

Citation

Hsi-Wei Yen et al 2016 ApJL 820 L25. doi:10.3847/2041-8205/820/2/L25

solar fragment path

falling fragment path

The path taken by the falling fragment in the June 2011 event. [Adapted from Petralia et al. 2016]

Sometimes plasma emitted from the Sun doesn’t escape into space, but instead comes crashing back down to the solar surface. What can observations and models of this process tell us about how the plasma falls and the local conditions on the Sun?

Fallback from a Flare

On 7 June 2011, an M-class flare erupted from the solar surface. As the Solar Dynamics Observatory’s Atmospheric Imaging Assembly looked on, plasma fragments from the flare arced away from the Sun and then fell back to the surface.

Some fragments fell back where the Sun’s magnetic field was weak, returning directly to the surface. But others fell within active regions, where they crashed into the Sun’s magnetic field lines, brightening the channels and funneling along them through the dense corona and back to the Sun’s surface.

fragment model

The authors’ model of the falling blobs at several different times in their simulation. The blobs get disrupted when they encounter the field lines, and are then funneled along the channels to the solar surface. [Adapted from Petralia et al. 2016]

This sort of flare and fall-back event is a common occurrence with the Sun, and SDO’s observations of the June 2011 event present an excellent opportunity to understand the process better. A team of scientists led by Antonino Petralia (University of Palermo, Italy and INAF-OAPA) modeled this event in an effort to learn more about how the falling plasma interacts with strong magnetic fields above the solar surface.

Magnetic Fields as Guides

Petralia and collaborators used three-dimensional magnetohydrodynamical modeling to attempt to reproduce the observations of this event. They simulated blobs of plasma as they fall back to the solar surface and interact with magnetic field lines over a range of different conditions.

The team found that only simulations that assume a relatively strong magnetic field resulted in the blobs funneling along a channel to the Sun’s surface; with weaker fields the blobs to simply broke through the field lines.

The observations were best reproduced by downfall channeled in a million-Kelvin coronal loop confined by a magnetic field of ~10–20 Gauss. In this scenario, a falling fragment is deviated from its path by the field and disrupted. It’s then channeled along the magnetic flux tube, driving a shock and heating in the tube ahead of it — which, the authors find, is the cause the observed brightening that occurs ahead of the actual plasma passage.

Petralia and collaborators point out that this new mechanism for brightening downflows channeled by the magnetic field is applicable not only in our Sun, but also in young, accreting stars. Events like these can therefore work as probes of the ambient atmosphere of such stars, providing information about the local plasma density and magnetic field.

Bonus

Check out the two awesome videos below! In the first one, you can see the SDO/AIA observations of the plasma fragment falling back down and hitting a magnetic channel, which lights up as the shock propagates. In the second one, you can see one of the authors’ models of this process; this video renders the density of blobs of plasma as they fall and strike magnetic field lines.

Citation

A. Petralia et al 2016 ApJ 832 2. doi:10.3847/0004-637X/832/1/2

Milky Way

Though we don’t notice it from our point of view, we’re hurtling through space at breakneck speed — and one of the contributors to our overall motion through the universe is the Sun’s revolution around the center of our galaxy. A recent study uses an unusual approach to measure the speed of this rotation.

Moving While Sitting Still

We know that the Sun zips rapidly around the center of the Milky Way — our orbital speed is somewhere around 250 km/s, or ~560,000 mph! Getting a precise measurement of this velocity is useful because we can combine it with the observed proper motion of Sgr A*, the black hole at the center of our galaxy, to determine the distance from us to the center of the Milky Way. This is an important baseline for lots of other measurements.

modeled orbits

Example particle orbits modeled within the galactic potential. The top panel represents a star with zero angular momentum, which is scattered into a chaotic orbit after interacting with the galactic nucleus. [Hunt et al. 2016]

But how can we measure the Sun’s revolution speed accurately? A team of scientists led by Jason Hunt (Dunlap Institute at University of Toronto, Canada) have suggested a unique approach to pin down this value: look for missing stars in the solar neighborhood.

Missing Stars

The stars around us should exhibit a distribution of velocities describing their orbits about the galactic center — but those stars with zero angular momentum should have plunged directly into the galactic center long ago. These stars would have been scattered onto chaotic halo orbits after their plunge, resulting in a dearth of stars with zero angular momentum around us today.

By looking at the relative speeds of the stars moving around us, then, we should see a dip in the velocity distribution corresponding to the missing zero-angular-momentum stars. By noting the relative velocity at which that dip occurs, we cleverly reveal the negative of our motion around the galactic center.

velocity distribution

Velocity distribution for stars within 700 pc of the Sun. A dip in the distribution (marked with an arrow) is noticeable between –210 and –270 km/s. [Hunt et al. 2016]

Where Are We and How Fast Are We Going?

Hunt and collaborators use a combination of the first data release from ESA’s Gaia mission and a star catalog from the Radial Velocity Experiment to examine the motions of a total of over 200,000 stars in the solar neighborhood. They find that there is indeed a lack of disk stars with velocities close to zero angular momentum. They then compare modeled stellar orbits to the data to estimate the relative speed at which the dip in the velocity distribution occurs.

From this information, the authors obtain a measurement of 239±9 km/s for the Sun’s revolution velocity around the galactic center. They combine this value with a proper motion measurement of Sgr A* to calculate the distance to the galactic center: 7.9±0.3 kpc (or about 26,000 light-years).

Both of these measurements can be improved with future Gaia data releases, which will contain many orders of magnitude more stars. This clever technique, therefore, proves a useful way of better constraining our position and motion through the Milky Way.

Citation

Jason A. S. Hunt et al 2016 ApJL 832 L25. doi:10.3847/2041-8205/832/2/L25

SMBH

Where does the angular momentum come from that causes supermassive black holes (SMBHs) to spin on their axes and launch powerful jets? A new study of nearby SMBHs may help to answer this question.

High-mass SMBHs are thought to form when two galaxies collide and the SMBHs at their centers merge. [NASA/Hubble Heritage Team (STScI)]

High-mass SMBHs are thought to form when two galaxies collide and the SMBHs at their centers merge. [NASA/Hubble Heritage Team (STScI)]

High- vs. Low-Mass Monsters

Observational evidence suggests a dichotomy between low-mass SMBHs (those with 106-7 M☉) and high-mass ones (those with 108-10 M☉). High-mass SMBHs are thought to form via the merger of two smaller black holes, and the final black hole is likely spun up by the rotational dynamics of the merger. But what spins up low-mass SMBHs, which are thought to build up very gradually via accretion?

A team of scientists led by Jing Wang (National Astronomical Observatories, Chinese Academy of Sciences) have attempted to address this puzzle by examining the properties of the galaxies hosting low-mass SMBHs.

A Sample of Neighboring SMBHs

Wang and collaborators began by constructing a sample of radio-selected nearby Seyfert 2 galaxies: those galaxies in which the stellar population and morphology of the host galaxy are visible to us, instead of being overwhelmed by continuum emission from the galaxy’s active nucleus.

sersic index

An example of a galaxy with a concentrated, classical bulge (M87; top) and a one with a disk-like pseudo bulge (Triangulum Galaxy; bottom). The authors find that for galaxies hosting low-mass SMBHs, those with more disk-like bulges appear to have more powerful radio jets. [Top: NASA/Hubble Heritage Team (STScI), Bottom: Hewholooks]

From this sample, the authors then selected 31 galaxies that have low-mass SMBHs at their centers, as measured using the surrounding stellar dynamics. Wang and collaborators cataloged radio information revealing properties of the powerful jets launched by the SMBHs, and they analyzed the host galaxies’ properties by modeling their brightness profiles.

Spin-Up From Accreting Gas

By examining this sample, the authors discovered an intriguing relationship: the radio power of jets launched by an SMBH appears to be dependent upon its host galaxy’s bulge surface brightness. Specifically, Wang and collaborators found that more powerful radio emission comes from SMBHs associated with less-concentrated bulges, i.e. those that are more disk-like.

The authors’ findings allow them to rule out many common explanations for the radio-loudness of such galaxies with small SMBH masses. Instead, they argue that the tendency for galaxies with more disk-like bulges to host SMBHs with more powerful jets is evidence that low-mass SMBHs are spun up by the accretion of surrounding gas.

In this scenario, the angular momentum of gas with significant disk-like rotational dynamics provides the spin to the SMBH, and this rotational energy can then be extracted to launch the powerful jets. If this explanation is correct, it strengthens the dichotomy between low-mass and high-mass SMBHs, supporting the idea that the two categories of black holes are indeed formed and spun up via completely different mechanisms.

Citation

J. Wang et al 2016 ApJL 833 L2. doi:10.3847/2041-8205/833/1/L2

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