Releases from NASA, HubbleSite, Spitzer, ESO, ESA, NASA’s Chandra X-ray Observatory, Royal Astronomical Society, Harvard-Smithsonian Center For Astrophysics, Max Planck Institute, Gemini Observatory, Subaru Telescope, W. M. Keck Observatory, JPL-Caltech, ICRAR, Webb Space Telescope, etc
This spectacular image from the NASA/ESA
Hubble Space Telescope shows the trailing arms of NGC 2276, a spiral
galaxy 120 million light-years away in the constellation of Cepheus. At
first glance, the delicate tracery of bright spiral arms and dark dust
lanes resembles countless other spiral galaxies. A closer look reveals a
strangely lopsided galaxy shaped by gravitational interaction and
intense star formation.
This striking image showcases the unusually contorted
appearance of NGC 2276, an appearance caused by two different
astrophysical interactions — one with the superheated gas pervading
galaxy clusters, and one with a nearby galactic neighbour.
The interaction of NGC 2276 with the intracluster medium —
the superheated gas lying between the galaxies in galaxy clusters — has
ignited a burst of star formation along one edge of the galaxy. This
wave of star formation is visible as the bright, blue-tinged glow of
newly formed massive stars towards the left side of this image, and
gives the galaxy a strangely lopsided appearance. NGC 2276’s recent
burst of star formation is also related to the appearance of more exotic
inhabitants — black holes and neutron stars in binary systems.
On the other side of the galaxy from this burst of new
stars, the gravitational attraction of a smaller companion is pulling
the outer edges of NGC 2276 out of shape. This interaction with the
small lens-shaped galaxy NGC 2300 has distorted the outermost spiral
arms of NGC 2276, giving the false impression that the larger galaxy is
orientated face-on to Earth [1].
NGC 2276 and its disruptive companion NGC 2300 can both be seen in the
accompanying image, which shows a wider view of the interacting
galaxies.
NGC 2276 is by no means the only galaxy with a strange
appearance. The Atlas of Peculiar Galaxies — a catalogue of unusual
galaxies published in 1966 — contains a menagerie of weird and wonderful
galaxies, including spectacular galaxy mergers, ring-shaped galaxies,
and other galactic oddities. As befits an unusually contorted galaxy,
NGC 2276 has the distinction of being listed in the Atlas of Peculiar
Galaxies twice — once for its lopsided spiral arms and once for its
interaction with its smaller neighbour NGC 2300.
Notes
[1] The actual alignment of NGC 2276 can be inferred from the position
of its brightly glowing galactic core, which is offset from the
distorted spiral arms.
An investigation carried out by the
astrophysicists of the Instituto de Astrofísica de Canarias (IAC) Žofia
Chrobáková, a doctoral student at the IAC and the University of La
Laguna (ULL), and Martín López Corredoira, questions one of the most
interesting findings about the dynamics of the Milky Way in recent
years: the precession, or the wobble in the axis of rotation of the disc
warp is incorrect. The results have just been published in The Astrophysical Journal.
The Milky Way is a spiral galaxy, which means that
it is composed, among other components, of a disc of stars, gas and
dust, in which the spiral arms are contained. At first, it was thought
that the disc was completely flat, but for some decades now it is known
that the outermost part of the disc is distorted into what is called a
“warp”: in one direction it is twisted upwards, and in the opposite
direction downwards. The stars, the gas, and the dust are all warped,
and so are not in the same plane as the extended inner part of the disc,
and an axis perpendicular to the planes of the warp defines their
rotation.
In 2020, an investigation announced the detection
of the precession of the warp of the Milky Way disc, which means that
the deformation in this outer region is not static, but that just like a
spinning top the orientation of its axis is itself rotating with time.
Furthermore, these researchers found that it was quicker than the
theories predicted, a cycle every 600-700 million years, some three
times the time it takes the Sun to travel once round the centre of the
Galaxy.
Precession is not a phenomenon which occurs only in
galaxies, it also happens to our planet. As well as its annual
revolution around the Sun, and its rotation period of 24 hours, the axis
of the Earth precesses, which implies that the celestial pole is not
always close to the present pole star, but that (as an example) 14,000
years ago it was close to the star Vega.
Now, a new study by Žofia Chrobáková and Martín López Corredoira
has taken into account the variation of the amplitude of the warp with
the ages of the stars. The study concludes that, using the warp of the
old stars whose velocities have been measured, it is possible that the
precession can disappear, or at least become slower than what is
presently believed. To arrive at this result the researchers have used
data from the Gaia Mission of the European Space Agency (ESA), analysing
the positions and velocities of hundreds of millions of stars in the
outer disc.
“In previous studies it had not been noticed”, explains Žofia Chrobáková,
a predoctoral researcher at the IAC and the first author of the
article, “that the stars which are a few tens of millions of years old,
such as the Cepheids, have a much larger warp than that of the stars
visible with the Gaia mission, which are thousands of millions of years
old”.
“This does not necessarily mean that the warp does
not precess at all, it could do so, but much more slowly, and we are
probably unable to measure this motion until we obtain better data”,
concludes Martín López Corredoira, and IAC researcher and co-author of the article.
The first image of a black hole shows the core of
galaxy Messier 87 as resolved by radio waves by the Event Horizon
Telescope in 2019. Credits: National Science Foundation/Event Horizon Telescope Consortium
Astronomers continue to develop computer simulations to help future
observatories better home in on black holes, the most elusive
inhabitants of the universe.
Though black holes likely exist abundantly in the universe, they are
notoriously hard to see. Scientists did not capture the first radio
image of a black hole until 2019, and only about four dozen black hole
mergers have been detected through their signature gravitational ripples
since the first detection in 2015.
That is not a lot of data to work with. So scientists look to black
hole simulations to gain crucial insight that will help find more
mergers with future missions. Some of these simulations, created by
scientists like astrophysicist Scott Noble, track supermassive black
hole binary systems. That is where two monster black holes like those
found in the centers of galaxies orbit closely around each other until
they eventually merge.
The simulations, created by computers working through sets of
equations too complicated to solve by hand, illustrate how matter
interacts in merger environments. Scientists can use what they learn
about black hole mergers to identify some telltale characteristics that
let them distinguish black hole mergers from stellar events. Astronomers
can then look for these telltale signs and spot real-life black hole
mergers.
Noble, who works at NASA’s Goddard Space Flight Center in Greenbelt,
Maryland, said these binary systems emit gravitational waves and
influence surrounding gases, leading to unique light shows
detectable with conventional telescopes. This allows scientists to
learn about different aspects of the same system. Multimessenger
observations that combine different forms of light or gravitational
waves could allow scientists to refine their models of black hole binary
systems.
“We’ve been relying on light to see everything out there,” Noble
said. “But not everything emits light, so the only way to directly ‘see’
two black holes is through the gravitational waves they generate.
Gravitational waves and the light from surrounding gas are independent
ways of learning about the system, and the hope is that they will meet
up at the same point.”
Binary black hole simulations can also help the Laser Interferometer
Space Antenna (LISA) mission. This space-based gravitational wave
observatory, led by the European Space Agency with significant
contributions from NASA, is expected to launch in 2034. If simulations
determine what electromagnetic characteristics distinguish a binary
black hole system from other events, scientists could detect these
systems before LISA flies, Noble said. These observations could then be
confirmed through additional detections once LISA launches.
That would allow scientists to verify that LISA is working, observe
systems for a longer period before they merge, predict what is going to
happen, and test those predictions.
“We’ve never been able to do that before,” Noble said. “That’s really exciting.”
Noble is working with Goddard and university partners, including
Bernard Kelly at the University of Maryland, Manuela Campanelli leading a
team of researchers at the Rochester Institute of Technology, and
Julian Krolik leading a Johns Hopkins University research team.
Kelly creates simulations using a special approach called a moving puncture simulation.
These simulations allow scientists to avoid representing a
singularity inside the event horizon — the part of the black hole from
which nothing can escape, Kelly said. Everything outside of that event
horizon evolves, while the objects inside remain frozen from earlier in
the simulation. This allows scientists to overlook the fact that they do
not know what happens within an event horizon.
To mimic real-life situations, where black holes accumulate accretion
disks of gas, dust, and diffuse matter, scientists have to incorporate
additional code to track how the ionized material interacts with
magnetic fields.
“We’re trying to seamlessly and correctly glue together different
codes and simulation methods to produce one coherent picture,” Kelly
said.
In 2018, the team published an analysis of a new simulation in The Astrophysical Journal
that fully incorporated the physical effects of Einstein’s general
theory of relativity to show a merger’s effects on the environment
around it. The simulation established that the gas in binary black hole
systems will glow predominantly in ultraviolet and X-ray light.
Simulations also showed that accretion disks in these systems are not
completely smooth. A dense clump forms orbiting the binary, and every
time a black hole sweeps close, it pulls off matter from the clump. That
collision heats up the matter, producing a bright signal and creating
an observable fluctuation of light.
In addition to improving their confidence in the accuracy of the
simulations, Goddard astrophysicist Jeremy Schnittman said they also
need to be able to apply the same simulation code to a single black hole
or a binary and show the similarities and also the differences between
the two systems.
“The simulation are going to tell us what the systems should look
like,” Schnittman said. “LISA works more like a radio antenna as opposed
to an optical telescope. We’re going to hear something in the universe
and get its basic direction, but nothing very precise. What we have to
do is take other telescopes and look in that part of the sky, and the
simulations are going to tell us what to look for to find a merging
black hole.”
Kelly said LISA will be more sensitive to lower gravitational wave
frequencies than the current ground-based gravitational wave observer,
the Laser Interferometer Gravitational-Wave Observatory (LIGO). That
means LISA will be able to sense smaller-mass binary systems much
earlier and will likely detect merging systems in time to alert
electromagnetic telescopes.
For Schnittman, these simulations are key to understanding the
real-life data LISA and other spacecraft collect. The case for models
may be even stronger for binary black holes, Schnittman said, because
the scientific community has little data.
“We probably will never find a binary black hole with a telescope
until we simulate them to the point we know exactly what we’re looking
for, because they’re so far away, they’re so tiny, you’re going to see
just one speck of light,” Schnittman said. “We need to be able to look
for that smoking gun.”
NASA’s upcoming Nancy Grace Roman Space Telescope
will see thousands of exploding stars called supernovae across vast
stretches of time and space. Using these observations, astronomers aim
to shine a light on several cosmic mysteries, providing a window onto
the universe’s distant past and hazy present.Credits: NASA's Goddard Space Flight Center/CI Labs.Download high-resolution video and images from NASA’s Scientific Visualization Studio
NASA’s upcoming Nancy Grace Roman Space Telescope will see thousands
of exploding stars called supernovae across vast stretches of time and
space. Using these observations, astronomers aim to shine a light on
several cosmic mysteries, providing a window onto the universe’s distant
past and hazy present.
Roman’s supernova survey will help clear up clashing measurements of
how fast the universe is currently expanding, and even provide a new way
to probe the distribution of dark matter, which is detectable only
through its gravitational effects. One of the mission’s primary science
goals involves using supernovae to help pin down the nature of dark energy – the unexplained cosmic pressure that’s speeding up the expansion of the universe.
Space’s biggest mystery
“Dark energy makes up the majority of the cosmos, but we don’t
actually know what it is,” said Jason Rhodes, a senior research
scientist at NASA’s Jet Propulsion Laboratory in Southern California.
“By narrowing down possible explanations, Roman could revolutionize our
understanding of the universe – and dark energy is just one of the many
topics the mission will explore!”
Roman will use multiple methods to investigate dark energy. One
involves surveying the sky for a special type of exploding star, called a
type Ia supernova.
Many supernovae occur when massive stars run out of fuel, rapidly
collapse under their own weight, and then explode because of strong
shock waves that propel out of their interiors. These supernovae occur
about once every 50 years in our Milky Way galaxy. But evidence shows
that type Ia supernovae originate from some binary star systems that
contain at least one white dwarf – the small, hot core remnant of a
Sun-like star. Type Ia supernovae are much rarer, happening roughly once
every 500 years in the Milky Way.
In some cases, the dwarf may siphon material from its companion. This
ultimately triggers a runaway reaction that detonates the thief once it
reaches a specific point where it has gained so much mass that it
becomes unstable. Astronomers have also found evidence supporting
another scenario, involving two white dwarfs that spiral toward each
other until they merge. If their combined mass is high enough that it
leads to instability, they, too, may produce a type Ia supernova.
These explosions peak at a similar, known intrinsic brightness,
making type Ia supernovae so-called standard candles – objects or events
that emit a specific amount of light, allowing scientists to find their
distance with a straightforward formula. Because of this, astronomers
can determine how far away the supernovae are by simply measuring how
bright they appear.
Astronomers will also use Roman to study the light of these
supernovae to find out how quickly they appear to be moving away from
us. By comparing how fast they’re receding at different distances,
scientists will trace cosmic expansion over time. This will help us
understand whether and how dark energy has changed throughout the
history of the universe.
“In the late 1990s, scientists discovered that the expansion of the
universe was speeding up using dozens of type Ia supernovae,” said
Daniel Scolnic, an assistant professor of physics at Duke University in
Durham, North Carolina, who is helping design Roman’s supernova survey.
“Roman will find them by the thousands, and much farther away than the
majority of those we’ve seen so far.”
Previous type Ia supernova surveys have concentrated on the
relatively nearby universe, largely due to instrument limitations.
Roman’s infrared vision, gigantic field of view, and exquisite
sensitivity will dramatically extend the search, pulling the cosmic
curtains far enough aside to allow astronomers to spot thousands of
distant type Ia supernovae.
The mission will study dark energy’s influence in detail over more
than half of the universe’s history, when it was between about four and
12 billion years old. Exploring this relatively unprobed region will
help scientists add crucial pieces to the dark energy puzzle.
“Type Ia supernovae are among the most important cosmological probes
we have, but they’re hard to see when they’re far away,” Scolnic said.
“We need extremely precise measurements and an incredibly stable
instrument, which is exactly what Roman will provide.”
Hubble constant hubbub
In addition to providing a cross-check with the mission’s other dark
energy surveys, Roman’s type Ia supernova observations could help
astronomers examine another mystery. Discrepancies keep popping up in measurements of the Hubble constant, which describes how fast the universe is currently expanding.
Predictions based on early universe data, from about 380,000 years
after the big bang, indicate that the cosmos should currently expand at
about 42 miles per second (67 kilometers per second) for every
megaparsec of distance (a megaparsec is about 3.26 million light-years).
But measurements of the modern universe indicate faster expansion,
between roughly 43 to 47 miles per second (70 to 76 kilometers per
second) per megaparsec.
Roman will help by exploring different potential sources of these
discrepancies. Some methods to determine how fast the universe is now
expanding rely on type Ia supernovae. While these explosions are
remarkably similar, which is why they’re valuable tools for gauging
distances, small variations do exist. Roman’s extensive survey could
improve their use as standard candles by helping us understand what causes the variations.
The mission should reveal how the properties of type Ia supernovae
change with age, since it will view them across such a vast sweep of
cosmic history. Roman will also spot these explosions in various
locations in their host galaxies, which could offer clues to how a
supernova’s environment alters its explosion.
Illuminating dark matter
In a 2020 paper,
a team led by Zhongxu Zhai, a postdoctoral research associate at
Caltech/IPAC in Pasadena, California, showed that astronomers will be
able to glean even more cosmic information from Roman’s supernova
observations.
“Roman will have to look through enormous stretches of the universe
to see distant supernovae,” said Yun Wang, a senior research scientist
at Caltech/IPAC and a co-author of the study. “A lot can happen to light
on such long journeys across space. We’ve shown that we can learn a lot
about the structure of the universe by analyzing how light from type Ia
supernovae has been bent as it traveled past intervening matter.”
Anything with mass warps the fabric of space-time. Light travels in a
straight line, but if space-time is bent – which happens near massive
objects – light follows the curve. When we look at distant type Ia
supernovae, the warped space-time around intervening matter – such as
individual galaxies or clumps of dark matter – can magnify the light
from the more distant explosion.
By studying this magnified light, scientists will have a new way to
probe how dark matter is clustered throughout the universe. Learning
more about the matter that makes up the cosmos will help scientists
refine their theoretical model of how the universe evolves.
By charting dark energy’s behavior across cosmic history, homing in
on how the universe is expanding today, and providing more information
on mysterious dark matter, the Roman mission will deliver an avalanche
of data to astronomers seeking to solve these and other longstanding
problems. With its ability to help solve so many cosmic mysteries, Roman
will be one of the most important tools for studying the universe we’ve
ever built.
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard
Space Flight Center in Greenbelt, Maryland, with participation by NASA's
Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the
Space Telescope Science Institute in Baltimore, and science teams
comprising scientists from various research institutions.
This composite image reveals electric blue ram pressure stripping streaks seen emanating from ESO 137-001, as well as a giant gas stream that can be seen extending towards the bottom of the frame in X-rays. Credit: NASA/ESA/CXC
Dwarf galaxies
are thought to be incredibly suggestible; there has been a range of
diverse dwarf galaxies observed in our universe, indicating that they
are extremely sensitive to their surroundings. The observed differences
in sizes, shapes, and colours of dwarf galaxies is believed to be at
least in part due to differences in the environment they inhabit. All
galaxies are thought to be surrounded by a halo of dark matter (see this
astrobite
for more details). Many dwarf galaxies are satellite galaxies, meaning
that they are found in orbit within a larger host dark matter halo that
also typically contains a larger central galaxy (for example, the Small
and Large Magellanic Clouds are satellite galaxies, both in orbit of our own Milky Way).
Satellite galaxies are subject to many different interactions with
their host dark matter halo. These interactions between a satellite
galaxy and its host can have devastating effects on the satellite galaxy
itself. For example, their gas content can become extremely disturbed
(and sometimes completely removed) by ram pressure stripping, which can eventually bring star formation in the satellite to a halt (see this astrobite for a summary of the seminal paper on ram pressure stripping). Similarly, their stars are subject to tidal stripping, which arises due to differences in the gravitational potential of the satellite galaxy and its host.
Figure
1: Examples of dwarfs visually classified as early-type (ETG) and
late-type (LTG). Late-type dwarfs are irregular, with apparent active
star formation throughout the galaxy while early-types are smooth and
featureless without any star-forming clumps.Credit: Carlsten et al. 2021
Despite
the observed diversity of dwarf galaxies, they can broadly be
classified into two morphological types: late-type and early-type (see
Figure 1 for examples). Late-type galaxies are typically star-forming,
whereas early-type galaxies lack star-forming regions and appear
smoother than late-types. Today’s paper uses the ongoing Exploration of
Local VolumE Satellites (ELVES) Survey to investigate how the structural
properties of dwarf galaxies can change depending on the environment
and morphology of the galaxy. The galaxies in the ELVES sample are all
within the Local Volume (D < 12 Mpc), and are satellite galaxies in
orbit of Milky Way-like halos.
Going from a Late-type to an Early-type?
The current picture of dwarf galaxy evolution suggests that
early-type dwarfs are formed from late-type dwarfs interacting with a
host halo. If this is the case, then early-type dwarfs can be thought of
as dwarf galaxies in the last throes of their evolution, and any
differences in characteristics of late-type and early-type galaxies
could provide insights into the physical mechanisms behind this
evolution (such as the removal of star-forming gas through ram pressure
stripping).
Figure
2: Log effective radius vs. log stellar mass for the dwarf galaxies in
the Local Volume sample. The upper panel displays points for each dwarf
galaxy in the sample, with red indicating early-type and blue indicating
late-type. The bottom panel shows average trends binned by stellar
mass. The dashed lines show the mass-size relations for early-type (red)
and late-type (blue) dwarf galaxies of higher stellar mass from the
GAMA Survey. [Adapted from Carlsten et al. 2021]
To investigate
whether there are any structural differences between early- and
late-types, the authors plot the effective radius of the dwarf galaxies
in their sample (essentially the galaxy’s size) by their stellar mass. It
can be seen from Figure 2 that there is no significant difference
between the early- and late-type galaxies at fixed stellar mass. This
similarity between late-types and early-types suggests that the
physical processes relevant in forming early-type galaxies (such as ram
pressure stripping) do not necessarily induce any change in the galaxy’s
size. These results indicate that the transformation process from
late-type to early-type requires only the removal of the galaxy’s
star-forming gas — significant structural change to the galaxy is not
necessarily required. Also of note is the difference between the
author’s results, where the sample is limited to dwarf galaxies with M* < 108.5 M⊙ and
results for satellite galaxies with higher masses (indicated by the
blue and red lines in the bottom panel of Figure 2). The authors suggest
that this difference hints that there is a characteristic stellar mass
scale, above which additional physical processes may be required to
explain the sudden difference in sizes between early- and late-types.
Environmental Effects
The next question the authors aim to answer is: how does the mass of
the dwarf galaxy’s host dark matter halo affect the evolution of the
dwarf galaxy? To consider this, the authors again compare the sizes of
dwarf galaxies. This time, a comparison is made between dwarf galaxies
that are orbiting within larger cluster environments and the dwarf galaxies in their Local Volume environment.
Figure
3: The mass–size relations of the cluster (grey) and field (cyan) dwarf
samples normalized to the full Local Volume sample (green). At fixed
stellar mass, the cluster sample is offset to larger sizes, whereas the
isolated field sample is offset to smaller sizes. Field galaxies are
isolated dwarf galaxies that have been taken from an auxiliary sample,
using additional observational data. Credit: Adapted from Carlsten et al. 2021
As
can be seen in Figure 3, dwarf galaxies in cluster environments tend to
be slightly larger than dwarf galaxies in the Local Volume at a fixed
stellar mass. The authors argue that the observed increase in size is
down to more intense tidal stripping and heating of galaxies in extreme
cluster environments, which aligns with theoretical expectations. While
an ~8% increase in sizes for the dwarfs in cluster environments is
observed, the authors note the mass–size relation is strikingly similar
between the two environments, especially since the mass of the host dark
matter halos differ by a factor of 10. This is perhaps indicative
that the exact environment plays a fairly small role in dwarf galaxy
evolution — a somewhat surprising result!
In conclusion, today’s authors are able to gain insights into the
physics of dwarf galaxy transformation from late-types to early-types,
and how these processes vary between the Milky Way-like and cluster
environments. The authors comment that a comparison with simulations
will be useful in constraining the physics of how dwarf galaxies evolve.Their
observational results have quantified the start and end points of the
transformation, and simulations may now be able to tie them together to
tell the middle part of the story!
Editor’s note:Astrobitesis a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of thepartnership between the AAS and astrobites, we occasionally repost astrobites
content here at AAS Nova. We hope you enjoy this post from astrobites;
the original can be viewed atastrobites.org.
A dense neutron star (right) pulling matter off a nearby star (left)
Credit: Colby Earles, ORNLHi-res image
At the heart of some of the smallest and densest stars in the
universe lies nuclear matter that might exist in never-before-observed
exotic phases. Neutron stars, which form when the cores of massive stars
collapse in a luminous supernova explosion, are thought to contain
matter at energies greater than what can be achieved in particle
accelerator experiments, such as the ones at the Large Hadron Collider
and the Relativistic Heavy Ion Collider.
Although
scientists cannot recreate these extreme conditions on Earth, they can
use neutron stars as ready-made laboratories to better understand exotic
matter. Simulating neutron stars, many of which are only 12.5 miles in
diameter but boast around 1.4 to two times the mass of our sun, can
provide insight into the matter that might exist in their interiors and
give clues as to how it behaves at such densities.
A team of nuclear astrophysicists led by Michael Zingale at Stony
Brook University is using the Oak Ridge Leadership Computing Facility's
(OLCF's) IBM AC922 Summit, the nation's fastest supercomputer, to model a
neutron star phenomenon called an X-ray burst—a thermonuclear explosion
that occurs on the surface of a neutron star when its gravitational
field pulls a sufficiently large amount of matter off a nearby star.
Now, the team has modeled a 2D X-ray burst flame moving across the
surface of a neutron star to determine how the flame acts under
different conditions. Simulating this astrophysical phenomenon provides
scientists with data that can help them better measure the radii of
neutron stars, a value that is crucial to studying the physics in the interior of neutron stars. The results were published in the Astrophysical Journal.
"Astronomers can use X-ray bursts to measure the radius of a neutron
star, which is a challenge because it's so small," Zingale said. "If we
know the radius, we can determine a neutron star's properties and
understand the matter that lives at its center. Our simulations will
help connect the physics of the X-ray burst flame burning to
observations."
The group found that different initial models and physics led to
different results. In the next phase of the project, the team plans to
run one large 3D simulation based on the results from the study to
obtain a more accurate picture of the X-ray burst phenomenon.
Switching physics
Neutron star simulations require a massive amount of physics input
and therefore a massive amount of computing power. Even on Summit,
researchers can only afford to model a small portion of the neutron star
surface.
To accurately understand the flame's behavior, Zingale's team used
Summit to model the flame for various features of the underlying neutron
star. The team's simulations were completed under an allocation of
computing time under the Innovative and Novel Computational Impact on
Theory and Experiment (INCITE) program. The team varied surface
temperatures and rotation rates, using these as proxies for different
accretion rates—or how quickly the star increases in mass as it
accumulates additional matter from a nearby star.
Alice Harpole, a postdoctoral researcher at Stony Brook University
and lead author on the paper, suggested that the team model a hotter
crust, leading to unexpected results.
"One of the most exciting results from this project was what we saw
when we varied the temperature of the crust in our simulations," Harpole
said. "In our previous work, we used a cooler crust. I thought it might
make a difference to use a hotter crust, but actually seeing the
difference that the increased temperature produced was very
interesting."
Massive computing, more complexity
The team modeled the X-ray burst flame phenomenon on the OLCF's
Summit at the US Department of Energy's (DOE's) Oak Ridge National
Laboratory (ORNL). Nicole Ford, an intern in the Science Undergraduate
Laboratory Internship Program at Lawrence Berkeley National Laboratory
(LBNL), ran complementary simulations on the Cori supercomputer at the
National Energy Research Scientific Computing Center (NERSC). The OLCF
and NERSC are a DOE Office of Science user facilities located at ORNL
and LBNL, respectively.
With simulations of 9,216 grid cells in the horizontal direction and
1,536 cells in the vertical direction, the effort required a massive
amount of computing power. After the team completed the simulations,
team members tapped the OLCF's Rhea system to analyze and plot their
results.
On Summit, the team used the Castro code—which is capable of modeling
explosive astrophysical phenomena—in the adaptive mesh refinement for
the exascale (AMReX) library, which allowed team members to achieve
varying resolutions at different parts of the grid. AMReX is one of the
libraries being developed by the Exascale Computing Project, an effort
to adapt scientific applications to run on DOE's upcoming exascale
systems, including the OLCF's Frontier. Exascale systems will be capable
of computing in the exaflops range, or 1018 calculations per second.
AMReX provides a framework for parallelization on supercomputers, but
Castro wasn't always capable of taking advantage of the GPUs that make
Summit so attractive for scientific research. The team attended
OLCF-hosted hackathons at Brookhaven National Laboratory and ORNL to get
help with porting the code to Summit's GPUs.
"The hackathons were incredibly useful to us in understanding how we
could leverage Summit's GPUs for this effort," Zingale said. "When we
transitioned from CPUs to GPUs, our code ran 10 times faster. This
allowed us to make less approximations and perform more physically
realistic and longer simulations."
The team said that the upcoming 3D simulation they plan to run will
not only require GPUs—it will eat up nearly all of the team's INCITE
time for the entire year.
"We need to get every ounce of performance we can," Zingale said.
"Luckily, we have learned from these 2D simulations what we need to do
for our 3D simulation, so we are prepared for our next big endeavor."
Schematic representation of cosmic rays propagating through magnetic clouds
Credit: Salvatore Buonocore
WASHINGTON, May 25, 2021 — Cosmic rays are high-energy atomic
particles continually bombarding Earth’s surface at nearly the speed of
light. Our planet’s magnetic field shields the surface from most of the
radiation generated by these particles. Still, cosmic rays can cause
electronic malfunctions and are the leading concern in planning for
space missions.
Researchers know cosmic rays originate from the multitude of stars in
the Milky Way, including our sun, and other galaxies. The difficulty is
tracing the particles to specific sources, because the turbulence of
interstellar gas, plasma, and dust causes them to scatter and rescatter
in different directions.
In
AIP Advances, by AIP Publishing, University of Notre Dame researchers
developed a simulation model to better understand these and other cosmic
ray transport characteristics, with the goal of developing algorithms
to enhance existing detection techniques.
Brownian motion theory is generally employed to study cosmic ray
trajectories. Much like the random motion of pollen particles in a pond,
collisions between cosmic rays within fluctuating magnetic fields cause
the particles to propel in different directions.
But this classic diffusion approach does not adequately address the
different propagation rates affected by diverse interstellar
environments and long spells of cosmic voids. Particles can become
trapped for a time in magnetic fields, which slow them down, while
others are thrust into higher speeds through star explosions.
To address the complex nature of cosmic ray travel, the researchers
use a stochastic scattering model, a collection of random variables that
evolve over time. The model is based on geometric Brownian motion, a
classic diffusion theory combined with a slight trajectory drift in one
direction.
In their first experiment, they simulated cosmic rays moving through
interstellar space and interacting with localized magnetized clouds,
represented as tubes. The rays travel undisturbed over a long period of
time. They are interrupted by chaotic interaction with the magnetized
clouds, resulting in some rays reemitting in random directions and
others remaining trapped.
Monte Carlo numerical analysis, based on repeated random sampling,
revealed ranges of density and reemission strengths of the interstellar
magnetic clouds, leading to skewed, or heavy-tailed, distributions of
the propagating cosmic rays.
The analysis denotes marked superdiffusive behavior. The model’s
predictions agree well with known transport properties in complex
interstellar media.
“Our model provides valuable insights on the nature of complex
environments crossed by cosmic rays and could help advance current
detection techniques,” author Salvatore Buonocore said.
AIP Advances is a fully open
access, online-only, peer-reviewed journal. It covers all areas of
applied physical sciences. With its advanced web 2.0 functionality, the
journal puts relevant content and discussion tools in the hands of the
community to shape the direction of the physical sciences.
The Milky Way-like galaxy NGC 1232 (center) shows the Milky Way's
location and relative size. Images of dwarf galaxies are centered close
to their true locations but have been magnified for visibility. Credit:
Charlotte Olsen.Hi-res image
Three dozen dwarf galaxies far
from each other had a simultaneous “baby boom” of new stars, an
unexpected discovery that challenges current theories on how galaxies
grow and may enhance our understanding of the universe.
Galaxies more than 1 million light-years apart should have completely
independent lives in terms of when they give birth to new stars. But
galaxies separated by up to 13 million light-years slowed down and then
simultaneously accelerated their birth rate of stars, according to a
Rutgers-led study published in the Astrophysical Journal.
“We found that regardless of whether these galaxies were next-door
neighbors or not, they stopped and then started forming new stars at the
same time, as if they’d all influenced each other through some
extra-galactic social network,” said co-author Eric Gawiser, a professor in the Department of Physics and Astronomy.
The simultaneous decrease in the stellar birth rate in the 36 dwarf
galaxies began 6 billion years ago, and the increase began 3 billion
years ago. Understanding how galaxies evolve requires untangling the
many processes that affect them over their lifetimes (billions of
years). Star formation is one of the most fundamental processes. The
stellar birth rate can increase when galaxies collide or interact, and
galaxies can stop making new stars if the gas (mostly hydrogen) that
makes stars is lost.
Star formation histories can paint a rich record of environmental
conditions as a galaxy “grew up.” Dwarf galaxies are the most common but
least massive type of galaxies in the universe, and they are especially
sensitive to the effects of their surrounding environment.
The 36 dwarf galaxies included a diverse array of environments at
distances as far as 13 million light-years from the Milky Way. The
environmental change the galaxies apparently responded to must be
something that distributes fuel for galaxies very far apart. That could
mean encountering a huge cloud of gas, for example, or a phenomenon in
the universe we don’t yet know about, according to Olsen.
The scientists used two methods to compare star formation histories. One
uses light from individual stars within galaxies; the other uses the
light of a whole galaxy, including a broad range of colors.
“The full impact of the discovery is not yet known as it remains to be
seen how much our current models of galaxy growth need to be modified to
understand this surprise,” Gawiser said. “If the result cannot be
explained within our current understanding of cosmology, that would be a
huge implication, but we have to give the theorists a chance to read
our paper and respond with their own research advances.”
“The James Webb Space Telescope, scheduled to be launched by NASA this
October, will be the ideal way to add that new data to find out just how
far outwards from the Milky Way this ‘baby boom’ extended,” Olsen
added.
Rutgers co-authors include Professor Kristen B. W. McQuinn; Grace Telford, a postdoctoral associate; and Adam Broussard, a doctoral student. Scientists at the University of Toronto, the Harvard-Smithsonian Center for Astrophysics, Johns Hopkins University and NASA’s Goddard Space Flight Center contributed to the study.
FRB 190714 - FRB 191001 - FRB 180924 - FRB 190608
Credit: NASA, ESA, A. Mannings (UC Santa Cruz), W. Fong (Northwestern), A. Pagan (STScI)
Hunting for the neighborhoods of enigmatic, fast radio bursts (FRBs), astronomers using the NASA/ESA Hubble Space Telescope tracked four of them to the spiral arms of the four distant galaxies shown in the image. The bursts are catalogued as FRB 190714, at top left; FRB 191001, at top right; FRB 180924, at bottom left; and FRB 190608, at bottom right.
Because these radio pulses disappear in much less than the blink of an eye, researchers have had a hard time tracking down where they come from.
The galaxies are far from Earth, appearing as they looked billions of years ago. With the help of Hubble's sharp vision, astronomers pinpointed the fast radio bursts' location (denoted by the dotted oval lines) to the galaxies' spiral arms.
These galaxies are part of a survey to determine the origin of these brilliant flares, which can release as much energy in a thousandth of a second as the Sun does in a year.
Identifying the radio bursts' location helped researchers narrow the list of possible FRB sources that can generate such prodigious tsunamis of energy. One of the leading possible explanations is a torrential blast from a young magnetar. Magnetars are a type of neutron star with extraordinarily powerful magnetic fields.
The observations were made in ultraviolet and near-infrared light with Hubble's Wide Field Camera 3. The images were taken between November 2019 and April 2020.
Henize 2-10 is an example of a dwarf galaxy that hosts an active galactic nucleus. A new technique may help us to discover similarly low-mass galaxies hosting the relics of supermassive black hole seeds. [X-ray (NASA/CXC/Virginia/A.Reines et al); Radio (NRAO/AUI/NSF); Optical (NASA/STScI)]
Using a new technique, scientists have identified a supermassive black hole lurking in a low-mass, low-metallicity galaxy. Could this discovery be just the tip of the iceberg?
Artist’s illustration of a primordial galaxy dominated by the supermassive black hole in its center.
Credit: [NASA/ESA/ESO/Wolfram Freudling et al. (STECF)]
Hunting for Seeds
How did the first supermassive black holes — black holes of millions or billions of solar masses — form?
Today, we know that giant black holes lie at the heart of most galaxies. Many of them have grown substantially since they first formed, via galaxy mergers and accretion of mass around them. But did they start out as large stars? Or collapse directly from molecular clouds? Or build up rapidly from the merger of smaller black holes?
To identify the seeds of supermassive black holes and address these questions, we need to explore the least-disturbed supermassive black holes that we can find today. Small, low-metallicity galaxies — those that have had a peaceful cosmic history, devoid of the mergers that drive significant black-hole growth — are thus the perfect targets to search for the relics of supermassive black hole seeds.
The catch? These are precisely the environments in which it’s difficult to spot black holes!
Artist’s depiction of the active nucleus of a galaxy, including an accretion disk spiraling around the supermassive black hole and jets of material flung out from both poles. Credit: [NASA/Dana Berry/SkyWorks Digital]
A New Approach
The easiest black holes to detect are those that are actively feeding, known as active galactic nuclei (AGNs). But the typical method for identifying an AGN — which relies on specific signatures in the source’s optical spectrum — is biased against low-metallicity and relatively merger-free galaxies, missing the precise population we want to find! Only a handful of AGNs have been identified in dwarf galaxies, and most of these lie in high-metallicity environments. So how do we find our seed relics?
According to a team of scientists led by Jenna Cann (George Mason University), it’s time for a different approach. Instead of relying on optical signatures, Cann and collaborators focus on finding coronal lines — near-infrared emission lines produced by ions that are excited by high-energy radiation. The presence of these lines can reveal a hidden AGN, even when a galaxy shows no sign of an AGN in optical emission.
The near-infrared spectrum of J1601+3113, captured using the GNIRS instrument at Gemini North, shows the presence of the [Si VI] coronal line, the tiny orange bump on the right of the spectrum. This provides evidence of an AGN. Credit: [Adapted from Cann et al. 2021]
Discovery of a Relic
In a recent study, Cann and collaborators demonstrate that their unique method works: they detected a coronal line in J1601+3113: a nearby, low-metallicity galaxy that’s only a tenth of the mass of the Large Magellanic Cloud! The authors’ detection is consistent with the presence of a supermassive black hole of roughly 100,000 solar masses, opening a window onto precisely the relic black hole seeds we’re hoping to find.
Cann and collaborators’ discovery marks the first time that an AGN has been identified in a low-mass, low-metallicity galaxy with no optical signs of AGN activity, underscoring how the coronal-line technique can help us find AGNs that might otherwise go undetected.
And with the James Webb Space Telescope scheduled to launch this year, we’ll (hopefully!) soon be collecting infrared spectra with unprecedented sensitivity. With any luck, we’re about to have access to a remarkable new population of lightweight AGNs hiding in small, low-metallicity galaxies — and with it, valuable insight into how these objects were born.
Citation
“Relics of Supermassive Black Hole Seeds: The Discovery of an Accreting Black Hole in an Optically Normal, Low Metallicity Dwarf Galaxy,” Jenna M. Cann et al 2021 ApJL 912 L2.doi:10.3847/2041-8213/abf56d
Image of the 2I/Borisov interstellar comet captured with the VLT
A new study by a Belgian team using data
from the European Southern Observatory’s Very Large Telescope (ESO’s
VLT) has shown that iron and nickel exist in the atmospheres of comets
throughout our Solar System, even those far from the Sun. A separate
study by a Polish team, who also used ESO data, reported that nickel
vapour is also present in the icy interstellar comet 2I/Borisov. This is
the first time heavy metals, usually associated with hot environments,
have been found in the cold atmospheres of distant comets.
“It
was a big surprise to detect iron and nickel atoms in the atmosphere of
all the comets we have observed in the last two decades, about 20 of
them, and even in ones far from the Sun in the cold space environment," says Jean Manfroid from the University of Liège, Belgium, who lead the new study on Solar System comets published today in Nature.
Astronomers know that heavy metals exist in comets’ dusty
and rocky interiors. But, because solid metals don’t usually “sublimate”
(become gaseous) at low temperatures, they did not expect to find them
in the atmospheres of cold comets that travel far from the Sun. Nickel
and iron vapours have now even been detected in comets observed at more
than 480 million kilometres from the Sun, more than three times the
Earth-Sun distance.
The Belgian team found iron and nickel in comets’
atmospheres in approximately equal amounts. Material in our Solar
System, for example that found in the Sun and in meteorites, usually
contains about ten times more iron than nickel. This new result
therefore has implications for astronomers’ understanding of the early
Solar System, though the team is still decoding what these are.
“Comets formed around 4.6 billion years ago, in the
very young Solar System, and haven’t changed since that time. In that
sense, they’re like fossils for astronomers,” says study co-author Emmanuel Jehin, also from the University of Liège.
While the Belgian team has been studying these “fossil” objects with ESO’s VLT for nearly 20 years, they had not spotted the presence of nickel and iron in their atmospheres until now. “This discovery went under the radar for many years,” Jehin says.
The team used data from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO’s VLT, which uses a technique called spectroscopy,
to analyse the atmospheres of comets at different distances from the
Sun. This technique allows astronomers to reveal the chemical makeup of
cosmic objects: each chemical element leaves a unique signature — a set
of lines — in the spectrum of the light from the objects.
The Belgian team had spotted weak, unidentified spectral
lines in their UVES data and on closer inspection noticed that they were
signalling the presence of neutral atoms of iron and nickel. A reason
why the heavy elements were difficult to identify is that they exist in
very small amounts: the team estimates that for each 100 kg of water in
the comets’ atmospheres there is only 1 g of iron, and about the same
amount of nickel.
“Usually there is 10 times more iron than nickel, and
in those comet atmospheres we found about the same quantity for both
elements. We came to the conclusion they might come from a special kind
of material on the surface of the comet nucleus, sublimating at a rather
low temperature and releasing iron and nickel in about the same
proportions,” explains Damien Hutsemékers, also a member of the Belgian team from the University of Liège.
Although the team aren’t sure yet what material this might
be, advances in astronomy — such as the Mid-infrared ELT Imager and
Spectrograph (METIS) on ESO’s upcoming Extremely Large Telescope (ELT) — will allow researchers to confirm the source of the iron and nickel atoms found in the atmospheres of these comets.
The Belgian team hope their study will pave the way for future research. “Now people will search for those lines in their archival data from other telescopes,” Jehin says. “We think this will also trigger new work on the subject.”
Interstellar heavy metals
Another remarkable study published today in Nature shows that heavy metals are also present in the atmosphere of the interstellar comet 2I/Borisov. A team in Poland observed this object, the first alien comet to visit our Solar System, using the X-shooter
spectrograph on ESO’s VLT when the comet flew by about a year and a
half ago. They found that 2I/Borisov’s cold atmosphere contains gaseous
nickel.
“At first we had a hard time believing that atomic
nickel could really be present in 2I/Borisov that far from the Sun. It
took numerous tests and checks before we could finally convince
ourselves,” says study author Piotr Guzik from the Jagiellonian
University in Poland. The finding is surprising because, before the two
studies published today, gases with heavy metal atoms had only been
observed in hot environments, such as in the atmospheres of ultra-hot
exoplanets or evaporating comets that passed too close to the Sun.
2I/Borisov was observed when it was some 300 million kilometres away
from the Sun, or about twice the Earth-Sun distance.
Studying interstellar bodies in detail is fundamental to
science because they carry invaluable information about the alien
planetary systems they originate from. “All of a sudden we understood that gaseous nickel is present in cometary atmospheres in other corners of the Galaxy,” says co-author Michał Drahus, also from the Jagiellonian University.
The Polish and Belgian studies show that 2I/Borisov and Solar System comets have even more in common than previously thought. “Now imagine that our Solar System's comets have their true analogues in other planetary systems — how cool is that?,” Drahus concludes.
More Information
This research was presented in two papers to appear in Nature.
The team that carried out the study “Iron and nickel atoms in cometary atmospheres even far from the Sun“ (https://doi.org/10.1038/s41586-021-03435-0) is composed of J. Manfroid, D. Hutsemékers & E. Jehin (STAR Institute, University of Liège, Belgium).
The team that carried out the study “Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov” (https://doi.org/10.1038/s41586-021-03485-4) is composed of Piotr Guzik and Michał Drahus (Astronomical Observatory, Jagiellonian University, Kraków, Poland).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a Strategic Partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.
