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
Studying the violent collisions of black holes and neutron stars may
soon provide a new measurement of the Universe’s expansion rate, helping
to resolve a long-standing dispute, suggests a new simulation study led
by researchers at UCL.
Our two current best ways of estimating the
Universe’s rate of expansion – measuring the brightness and speed of
pulsating and exploding stars, and looking at fluctuations in radiation
from the early Universe – give very different answers, suggesting our
theory of the Universe may be wrong.
A
third type of measurement, looking at the explosions of light and
ripples in the fabric of space caused by black hole-neutron star
collisions, should help to resolve this disagreement and clarify whether
our theory of the Universe needs rewriting.
The new study, published in Physical Review Letters,
simulated 25,000 scenarios of black holes and neutron stars colliding,
aiming to see how many would likely be detected by instruments on Earth
in the mid- to late-2020s.
The
researchers found that, by 2030, instruments on Earth could sense
ripples in space-time caused by up to 3,000 such collisions, and that
for around 100 of these events, telescopes would also see accompanying
explosions of light.
They
concluded that this would be enough data to provide a new, completely
independent measurement of the Universe’s rate of expansion, precise and
reliable enough to confirm or deny the need for new physics.
Lead
author Dr Stephen Feeney (UCL Physics & Astronomy) said: “A neutron
star is a dead star, created when a very large star explodes and then
collapses, and it is incredibly dense – typically 10 miles across but
with a mass up to twice that of our Sun. Its collision with a black hole
is a cataclysmic event, causing ripples of space-time, known as
gravitational waves, that we can now detect on Earth with observatories
like LIGO and Virgo.
“We
have not yet detected light from these collisions. But advances in the
sensitivity of equipment detecting gravitational waves, together with
new detectors in India and Japan, will lead to a huge leap forward in
terms of how many of these types of events we can detect. It is
incredibly exciting and should open up a new era for astrophysics.”
To
calculate the Universe’s rate of expansion, known as the Hubble
constant, astrophysicists need to know the distance of astronomical
objects from Earth as well as the speed at which they are moving away.
Analysing gravitational waves tells us how far away a collision is,
leaving only the speed to be determined.
To
tell how fast the galaxy hosting a collision is moving away, we look at
the “redshift” of light – that is, how the wavelength of light produced
by a source has been stretched by its motion. Explosions of light that
may accompany these collisions would help us pinpoint the galaxy where
the collision happened, allowing researchers to combine measurements of
distance and measurements of redshift in that galaxy.
Dr
Feeney said: “Computer models of these cataclysmic events are
incomplete and this study should provide extra motivation to improve
them. If our assumptions are correct, many of these collisions will not
produce explosions that we can detect – the black hole will swallow the
star without leaving a trace. But in some cases a smaller black hole may
first rip apart a neutron star before swallowing it, potentially
leaving matter outside the hole that emits electromagnetic radiation.”
Co-author
Professor Hiranya Peiris (UCL Physics & Astronomy and Stockholm
University) said: “The disagreement over the Hubble constant is one of
the biggest mysteries in cosmology. In addition to helping us unravel
this puzzle, the spacetime ripples from these cataclysmic events open a
new window on the universe. We can anticipate many exciting discoveries
in the coming decade.”
Gravitational
waves are detected at two observatories in the United States (the LIGO
Labs), one in Italy (Virgo), and one in Japan (KAGRA). A fifth
observatory, LIGO-India, is now under construction.
Our
two best current estimates of the Universe’s expansion are 67
kilometres per second per megaparsec (3.26 million light years) and 74
kilometres per second per megaparsec. The first is derived from
analysing the cosmic microwave background, the radiation left over from
the Big Bang, while the second comes from comparing stars at different
distances from Earth – specifically Cepheids, which have variable
brightness, and exploding stars called type Ia supernovae.
Dr
Feeney explained: “As the microwave background measurement needs a
complete theory of the Universe to be made but the stellar method does
not, the disagreement offers tantalising evidence of new physics beyond
our current understanding. Before we can make such claims, however, we
need confirmation of the disagreement from completely independent
observations – we believe these can be provided through black
hole-neutron star collisions.”
The
study was carried out by researchers at UCL, Imperial College London,
Stockholm University and the University of Amsterdam. It was supported
by the Royal Society, the Swedish Research Council (VR), the Knut and
Alice Wallenberg Foundation, and the Netherlands Organisation for
Scientific Research (NWO).
This illustration of the newly forming exoplanet
PDS 70b shows how material may be falling onto the giant world as it
builds up mass. By employing Hubble’s ultraviolet light (UV)
sensitivity, researchers got a unique look at radiation from extremely
hot gas falling onto the planet, allowing them to directly measure the
planet’s mass growth rate for the first time. The planet PDS 70b is
encircled by its own gas-and-dust disk that’s siphoning material from
the vastly larger circumstellar disk in this solar system. The
researchers hypothesize that magnetic field lines extend from its
circumplanetary disk down to the exoplanet’s atmosphere and are
funneling material onto the planet’s surface. The illustration shows one
possible magnetospheric accretion configuration, but the magnetic
field’s detailed geometry requires future work to probe. The remote
world has already bulked up to five times the mass of Jupiter over a
period of about five million years, but is anticipated to be in the tail
end of its formation process. PDS 70b orbits the orange dwarf star PDS
70 approximately 370 light-years from Earth in the constellation
Centaurus.Credits: NASA, ESA, STScI, Joseph Olmsted (STScI).Hi-res image
NASA’s Hubble Space Telescope is giving astronomers a rare look at a
Jupiter-sized, still-forming planet that is feeding off material
surrounding a young star.
“We just don’t know very much about how giant planets grow,” said
Brendan Bowler of the University of Texas at Austin. “This planetary
system gives us the first opportunity to witness material falling onto a
planet. Our results open up a new area for this research.”
Though over 4,000 exoplanets have
been cataloged so far, only about 15 have been directly imaged to date
by telescopes. And the planets are so far away and small, they are
simply dots in the best photos. The team’s fresh technique for using
Hubble to directly image this planet paves a new route for further
exoplanet research, especially during a planet’s formative years.
This huge exoplanet, designated PDS 70b, orbits the orange dwarf star
PDS 70, which is already known to have two actively forming planets
inside a huge disk of dust and gas encircling the star. The system is
located 370 light-years from Earth in the constellation Centaurus.
“This system is so exciting because we can witness the formation of a
planet,” said Yifan Zhou, also of the University of Texas at Austin.
“This is the youngest bona fide planet Hubble has ever directly imaged.”
At a youthful five million years, the planet is still gathering
material and building up mass.
Hubble’s ultraviolet light (UV) sensitivity offers a unique look at
radiation from extremely hot gas falling onto the planet. “Hubble’s
observations allowed us to estimate how fast the planet is gaining
mass,” added Zhou.
The European Southern Observatory’s Very Large
Telescope caught the first clear image of a forming planet, PDS 70b,
around a dwarf star in 2018. The planet stands out as a bright point to
the right of the center of the image, which is blacked out by the
coronagraph mask used to block the light of the central star. Credits: ESO, VLT, André B. Müller (ESO).Hi-res image
The UV observations, which add to the body of research about this
planet, allowed the team to directly measure the planet’s mass growth
rate for the first time. The remote world has already bulked up to five
times the mass of Jupiter over a period of about five million years. The
present measured accretion rate has dwindled to the point where, if the
rate remained steady for another million years, the planet would only
increase by approximately an additional 1/100th of a Jupiter-mass.
Zhou and Bowler emphasize that these observations are a single
snapshot in time – more data are required to determine if the rate at
which the planet is adding mass is increasing or decreasing. “Our
measurements suggest that the planet is in the tail end of its formation
process.”
The youthful PDS 70 system is filled with a primordial gas-and-dust
disk that provides fuel to feed the growth of planets throughout the
entire system. The planet PDS 70b is encircled by its own gas-and-dust
disk that’s siphoning material from the vastly larger circumstellar
disk. The researchers hypothesize that magnetic field lines extend from
its circumplanetary disk down to the exoplanet’s atmosphere and are
funneling material onto the planet’s surface.
“If this material follows columns from the disk onto the planet, it
would cause local hot spots,” Zhou explained. “These hot spots could be
at least 10 times hotter than the temperature of the planet.” These hot
patches were found to glow fiercely in UV light.
Hubble
observations pinpoint planet PDS 70b. A coronagraph on Hubble’s camera
blocks out the glare of the central star for the planet to be directly
observed. Though over 4,000 exoplanets have been cataloged so far, only
about 15 have been directly imaged to date by telescopes. The team’s
fresh technique for using Hubble to directly image this planet paves a
new route for further exoplanet research, especially during a planet’s
formative years. Credits: Joseph DePasquale (STScI).Hi-res image
These observations offer insights into how gas giant planets formed
around our Sun 4.6 billion years ago. Jupiter may have bulked up on a
surrounding disk of infalling material. Its major moons would have also
formed from leftovers in that disk.
A challenge to the team was overcoming the glare of the parent star.
PDS 70b orbits at approximately the same distance as Uranus does from
the Sun, but its star is more than 3,000 times brighter than the planet
at UV wavelengths. As Zhou processed the images, he very carefully
removed the star’s glare to leave behind only light emitted by the
planet. In doing so, he improved the limit of how close a planet can be
to its star in Hubble observations by a factor of five.
“Thirty-one years after launch, we’re still finding new ways to use
Hubble,” Bowler added. “Yifan’s observing strategy and post-processing
technique will open new windows into studying similar systems, or even
the same system, repeatedly with Hubble. With future observations, we
could potentially discover when the majority of the gas and dust falls
onto their planets and if it does so at a constant rate.”
The
Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington, D.C.
Artist's conception illustrates process seen
in forming stars much more massive than the Sun. At top left, material
is being drawn into the young star through an orbiting disk which
generates a fast-moving jet of material outward. At top right, material
begins coming in from another direction, and at bottom left, begins
deforming the original disk until, at bottom right, the disk orientation
-- and the jet orientation -- have changed. Credit: Bill Saxton, NRAO/AUI/NSF.Hi-res image
ALMA image of the chaotic scene around a
massive young protostar, in this case one called W51e2e. Grey shows dust
close to the star, while the red and blue indicate material in the jets
moving rapidly outward from the star. Red shows material moving away
from Earth and blue material moving toward Earth.Credit: Goddi, Ginsburg, et al., Sophia Dagnello, NRAO/AUI/NSF.Hi-res image
ALMA image of the chaotic scene around a
massive young protostar, in this case one called W51e8 . Grey shows
dust close to the star, while the red and blue indicate material in the
jets moving rapidly outward from the star. Red shows material moving
away from Earth and blue material moving toward Earth. Credit: Goddi, Ginsburg, et al., Sophia Dagnello, NRAO/AUI/NSF.Hi-Res image
ALMA image of the chaotic scene around a
massive young protostar, in this case one called W51north. Grey shows
dust close to the star, while the red and blue indicate material in the
jets moving rapidly outward from the star. Red shows material moving
away from Earth and blue material moving toward Earth. Credit: Goddi, Ginsburg, et al., Sophia Dagnello, NRAO/AUI/NSF. Hi-Res image
ALMA Shows Massive Young Stars Forming in "Chaotic Mess"
Credit: Goddi, Ginsburg, et al., S. Dagnello, B. Saxton, NRAO/AUI/NSF
A eam of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA)
has taken a big step toward answering a longstanding question — do
stars much more massive than the Sun form in the same way as their
smaller siblings?
Young, still-forming stars similar in mass to the Sun are observed
gaining material from their surrounding clouds of gas and dust in a
relatively orderly manner. The incoming material forms a disk orbiting
the young star and that disk feeds the star at a pace it can digest.
Condensations of material within the disk form planets that will remain
after the star’s growth process is complete.
The disks are commonly seen around young low-mass stars, but have not
been found around much more massive stars in their forming stages.
Astronomers questioned whether the process for the larger stars is
simply a scaled-up version of that for the smaller ones.
“Our ALMA observations now provide compelling evidence that the
answer is no,” said Ciriaco Goddi, of Radboud University Nijmegen in the
Netherlands.
Goddi led a team that used ALMA to study three high-mass, very young
stars in a star-forming region called W51, about 17,000 light-years from
Earth. They used ALMA when its antennas were spread apart to their
farthest extent, providing resolving power capable of making images 10
times sharper than previous studies of such objects.
They were looking for evidence of the large, stable disks seen
orbiting smaller young stars. Such disks propel fast-moving jets of
material outward perpendicular to the plane of the disk.
“With ALMA’s great resolving power, we expected to finally see a
disk. Instead, we found that the feeding zone of these objects looks
like a chaotic mess,” said Adam Ginsburg of the University of Florida.
The observations showed streamers of gas falling toward the young
stars from many different directions. Jets indicated that there must be
small disks that are yet unseen. In one case, it appears that some event
actually flipped a disk about 100 years ago.
The researchers concluded that these massive young stars form, at
least in their very early stages, by drawing in material from multiple
directions and at unsteady rates, in sharp contrast to the stable
inflows seen in smaller stars. The multiple channels of incoming
material, the astronomers said, probably prevent the formation of the
large, steady disks seen around smaller stars.
“Such a ‘disordered infall’ model was first proposed based on
computer simulations, and we now have the first observational evidence
supporting that model,” Goddi said.
Goddi, Ginsburg and their colleagues from the U.S., Mexico, and Europe reported their findings in the Astrophysical Journal.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
* * *
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO),
the U.S. National Science Foundation (NSF) and the National Institutes
of Natural Sciences (NINS) of Japan in cooperation with the Republic of
Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in
cooperation with the National Research Council of Canada (NRC) and the
Ministry of Science and Technology (MOST) and by NINS in cooperation
with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and
Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its
Member States; by the National Radio Astronomy Observatory (NRAO),
managed by Associated Universities, Inc. (AUI), on behalf of North
America; and by the National Astronomical Observatory of Japan (NAOJ) on
behalf of East Asia. The Joint ALMA Observatory (JAO) provides the
unified leadership and management of the construction, commissioning and
operation of ALMA.
* * *
Media Contact:
Dave Finley, Public Information Officer
(505) 241-9210 dfinley@nrao.edu
In celebration of the 31st anniversary of the launching of NASA's
Hubble Space Telescope, astronomers aimed the renowned observatory at a
brilliant "celebrity star," one of the brightest stars seen in our
galaxy, surrounded by a glowing halo of gas and dust.
The price for the monster star's opulence is "living on the edge."
The star, called AG Carinae, is waging a tug-of-war between gravity and
radiation to avoid self-destruction.
The expanding shell of gas and dust that surrounds the star is about
five light-years wide, which equals the distance from here to the
nearest star beyond the Sun, Proxima Centauri.
The huge structure was created from one or more giant eruptions about
10,000 years ago. The star's outer layers were blown into space—like a
boiling teapot popping off its lid. The expelled material amounts to
roughly 10 times our Sun's mass.
These outbursts are the typical life of a rare breed of star called a
luminous blue variable, a brief convulsive phase in the short life of
an ultra-bright, glamorous star that lives fast and dies young. These
stars are among the most massive and brightest stars known. They live
for only a few million years, compared to the roughly 10-billion-year
lifetime of our Sun. AG Carinae is a few million years old and resides
20,000 light-years away inside our Milky Way galaxy.
Luminous blue variables exhibit a dual personality: They appear to
spend years in quiescent bliss and then they erupt in a petulant
outburst. These behemoths are stars in the extreme, far different from
normal stars like our Sun. In fact, AG Carinae is estimated to be up to
70 times more massive than our Sun and shines with the blinding
brilliance of 1 million suns.
"I like studying these kinds of stars because I am fascinated by
their instability. They are doing something weird," said Kerstin Weis, a
luminous blue variable expert at Ruhr University in Bochum, Germany.
Major outbursts such as the one that produced the nebula occur once
or twice during a luminous blue variable's lifetime. A luminous blue
variable star only casts off material when it is in danger of
self-destruction as a supernova. Because of their massive forms and
super-hot temperatures, luminous blue variable stars like AG Carinae are
in a constant battle to maintain stability.
It's an arm wrestling contest between radiation pressure from within
the star pushing outward and gravity pressing inward. This cosmic match
results in the star expanding and contracting. The outward pressure
occasionally wins the battle, and the star expands to such an immense
size that it blows off its outer layers, like a volcano erupting. But
this outburst only happens when the star is on the verge of coming
apart. After the star ejects the material, it contracts to its normal
size, settles back down, and becomes quiescent for a while.
Like many other luminous blue variables, AG Carinae remains unstable.
It has experienced lesser outbursts that have not been as powerful as
the one that created the present nebula.
Although AG Carinae is quiescent now, as a super-hot star it
continues pouring out searing radiation and powerful stellar wind
(streams of charged particles). This outflow continues shaping the
ancient nebula, sculpting intricate structures as outflowing gas slams
into the slower-moving outer nebula. The wind is traveling at up to
670,000 miles per hour (1 million km/hr), about 10 times faster than the
expanding nebula. Over time, the hot wind catches up with the cooler
expelled material, plows into it, and pushes it farther away from the
star. This "snowplow" effect has cleared a cavity around the star.
The red material is glowing hydrogen gas laced with nitrogen gas. The
diffuse red material at upper left pinpoints where the wind has broken
through a tenuous region of material and swept it into space.
The most prominent features, highlighted in blue, are filamentary
structures shaped like tadpoles and lopsided bubbles. These structures
are dust clumps illuminated by the star's reflected light. The
tadpole-shaped features, most prominent at left and bottom, are denser
dust clumps that have been sculpted by the stellar wind. Hubble's sharp
vision reveals these delicate-looking structures in great detail.
The image was taken in visible and ultraviolet light. Ultraviolet
light offers a slightly clearer view of the filamentary dust structures
that extend all the way down toward the star. Hubble is ideally suited
for ultraviolet-light observations because this wavelength range can
only be viewed from space.
Massive stars, like AG Carinae, are important to astronomers because
of their far-reaching effects on their environment. The largest program
in Hubble's history—the Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES)—is studying the ultraviolet light of young stars and the way they shape their surroundings.
Luminous blue variable stars are rare: less than 50 are known among
the galaxies in our local group of neighboring galaxies. These stars
spend tens of thousands of years in this phase, a blink of an eye in
cosmic time. Many are expected to end their lives in titanic supernova
blasts, which enrich the universe with heavier elements beyond iron.
Hubble Trivia
Launched on April 24, 1990, NASA's Hubble Space Telescope has made
more than 1.5 million observations of about 48,000 celestial objects.
In its 31-year lifetime, the telescope has racked up more than
181,000 orbits around our planet, totaling over 4.5 billion miles.
Hubble observations have produced more than 169 terabytes of data,
which are available for present and future generations of researchers.
Astronomers using Hubble data have published more than 18,000
scientific papers, with more than 900 of those papers published in 2020.
The Hubble Space Telescope is a project of international
cooperation between NASA and ESA (European Space Agency). NASA's Goddard
Space Flight Center in Greenbelt, Maryland, manages the telescope. The
Space Telescope Science Institute (STScI) in Baltimore, Maryland,
conducts Hubble science operations. STScI is operated for NASA by the
Association of Universities for Research in Astronomy in Washington,
D.C.
Credits:
Media Contact:
Donna
Weaver
Space Telescope Science Institute, Baltimore,
Maryland
Ray
Villard
Space Telescope Science Institute, Baltimore,
Maryland
Publication Partners: NASA, ESA, STScI
Image of the Milky Way and the Large Magellanic Cloud (LMC) are
overlaid on a map of the surrounding galactic halo. The smaller
structure is a wake created by the LMC’s motion through this region. The
larger light-blue feature corresponds to a high density of stars
observed in the northern hemisphere of our galaxy. Credit:
NASA/ESA/JPL-Caltech/Conroy et. al. 2021
The highlight of the new chart is a wake of stars, stirred up by a small
galaxy set to collide with the Milky Way. The map could also offer a
new test of dark matter theories.
Astronomers
using data from NASA and ESA (European Space Agency) telescopes have
released a new all-sky map of the outermost region of our galaxy. Known
as the galactic halo, this area lies outside the swirling spiral arms
that form the Milky Way’s recognizable central disk and is sparsely
populated with stars. Though the halo may appear mostly empty, it is
also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all the mass in the universe.
The data for the new map comes from ESA’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer, or NEOWISE,
which operated from 2009 to 2013 under the moniker WISE. The study
makes use of data collected by the spacecraft between 2009 and 2018.
The
new map reveals how a small galaxy called the Large Magellanic Cloud
(LMC) – so named because it is the larger of two dwarf galaxies orbiting
the Milky Way – has sailed through the Milky Way’s galactic halo like a
ship through water, its gravity creating a wake in the stars behind it.
The LMC is located about 160,000 light-years from Earth and is less
than one-quarter the mass of the Milky Way.
A
simulation of dark matter surrounding the Milky Way galaxy (small ring
at center) and the Large Magellanic Cloud (LMC) reveals two areas of
high density: the smaller of the two light blue areas is a wake created
by the LMC’s motion through this region. The larger corresponds to an
excess of stars in the Milky Way’s northern hemisphere. Credit: NASA/JPL-Caltech/NSF/R. Hurt/N. Garavito-Camargo & G. Besla
Though the inner portions of the halo have been mapped with a high level
of accuracy, this is the first map to provide a similar picture of the
halo’s outer regions, where the wake is found – about 200,000
light-years to 325,000 light-years from the galactic center. Previous
studies have hinted at the wake’s existence, but the all-sky map
confirms its presence and offers a detailed view of its shape, size, and
location.
This disturbance in the halo also provides astronomers
with an opportunity to study something they can’t observe directly: dark
matter. While it doesn’t emit, reflect, or absorb light, the
gravitational influence of dark matter has been observed across the
universe. It is thought to create a scaffolding on which galaxies are
built, such that without it, galaxies would fly apart as they spin. Dark
matter is estimated to be five times more common in the universe than
all the matter that emits and/or interacts with light, from stars to
planets to gas clouds.
Although there are multiple theories about the nature of dark matter,
all of them indicate that it should be present in the Milky Way’s halo.
If that’s the case, then as the LMC sails through this region, it
should leave a wake in the dark matter as well. The wake observed in the
new star map is thought to be the outline of this dark matter wake; the
stars are like leaves on the surface of this invisible ocean, their
position shifting with the dark matter.
The interaction between
the dark matter and the Large Magellanic Cloud has big implications for
our galaxy. As the LMC orbits the Milky Way, the dark matter’s gravity
drags on the LMC and slows it down. This will cause the dwarf galaxy’s
orbit to get smaller and smaller, until the galaxy finally collides with
the Milky Way in about 2 billion years. These types of mergers might be
a key driver in the growth of massive galaxies across the universe. In
fact, astronomers think the Milky Way merged with another small galaxy
about 10 billion years ago.
“This robbing of a smaller galaxy’s energy is not only why the LMC is merging with the Milky Way, but also why all
galaxy mergers happen,” said Rohan Naidu, a doctoral student in
astronomy at Harvard University and a co-author of the new paper. “The
wake in our map is a really neat confirmation that our basic picture for
how galaxies merge is on point!”
A Rare Opportunity
The authors of the paper also think the new map – along with
additional data and theoretical analyses – may provide a test for
different theories about the nature of dark matter, such as whether it
consists of particles, like regular matter, and what the properties of
those particles are.
“You can imagine that the wake behind a boat
will be different if the boat is sailing through water or through
honey,” said Charlie Conroy, a professor at Harvard University and an
astronomer at the Center for Astrophysics | Harvard & Smithsonian,
who coauthored the study. “In this case, the properties of the wake are
determined by which dark matter theory we apply.”
Conroy led the
team that mapped the positions of over 1,300 stars in the halo. The
challenge arose in trying to measure the exact distance from Earth to a
large portion of those stars: It’s often impossible to figure out
whether a star is faint and close by or bright and far away. The team
used data from ESA’s Gaia mission, which provides the location of many
stars in the sky but cannot measure distances to the stars in the Milky
Way’s outer regions.
After identifying stars most likely located
in the halo (because they were not obviously inside our galaxy or the
LMC), the team looked for stars belonging to a class of giant stars with
a specific light “signature” detectable by NEOWISE.
Knowing the basic properties of the selected stars enabled the team to
figure out their distance from Earth and create the new map. It charts a
region starting about 200,000 light-years from the Milky Way’s center,
or about where the LMC’s wake was predicted to begin, and extends about
125,000 light-years beyond that.
Conroy and his colleagues were
inspired to hunt for LMC’s wake after learning about a team of
astrophysicists at the University of Arizona in Tucson that makes
computer models predicting what dark matter in the galactic halo should
look like. The two groups worked together on the new study.
One
model by the Arizona team, included in the new study, predicted the
general structure and specific location of the star wake revealed in the
new map. Once the data had confirmed that the model was correct, the
team could confirm what other investigations have also hinted at: that
the LMC is likely on its first orbit around the Milky Way. If the
smaller galaxy had already made multiple orbits, the shape and location
of the wake would be significantly different from what has been
observed. Astronomers think the LMC formed in the same environment as
the Milky Way and another nearby galaxy, M31, and that it is close to
completing a long first orbit around our galaxy (about 13 billion
years). Its next orbit will be much shorter due to its interaction with
the Milky Way.
“Confirming our theoretical prediction with
observational data tells us that our understanding of the interaction
between these two galaxies, including the dark matter, is on the right
track,” said University of Arizona doctoral student in astronomy Nicolás
Garavito-Camargo, who led work on the model used in the paper.
The
new map also provides astronomers with a rare opportunity to test the
properties of the dark matter (the notional water or honey) in our own
galaxy. In the new study, Garavito-Camargo and colleagues used a popular
dark matter theory called cold dark matter that fits the observed star
map relatively well. Now the University of Arizona team is running
simulations that use different dark matter theories to see which one
best matches the wake observed in the stars.
“It’s a really
special set of circumstances that came together to create this scenario
that lets us test our dark matter theories,” said Gurtina Besla, a
co-author of the study and an associate professor at the University of
Arizona. “But we can only realize that test with the combination of this
new map and the dark matter simulations that we built.”
Launched
in 2009, the WISE spacecraft was placed into hibernation in 2011 after
completing its primary mission. In September 2013, NASA reactivated the
spacecraft with the primary goal of scanning for near-Earth objects, or
NEOs, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet
Propulsion Laboratory in Southern California managed and operated WISE
for NASA’s Science Mission Directorate. The mission was selected
competitively under NASA’s Explorers Program managed by the agency’s
Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project
of JPL, a division of Caltech, and the University of Arizona, supported
by NASA’s Planetary Defense Coordination Office.
Left: This is an image of the star HR 8799 taken by
Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) in
1998. A mask within the camera (coronagraph) blocks most of the light
from the star. Astronomers also used software to digitally subtract more
starlight. Nevertheless, scattered light from HR 8799 dominates the
image, obscuring four faint planets later discovered from ground-based
observations. Right: A re-analysis of NICMOS data in 2011 uncovered
three of the exoplanets, which were not seen in the 1998 images. Webb
will probe the planets’ atmospheres at infrared wavelengths astronomers
have rarely used to image distant worlds. Credits: NASA, ESA, and R. Soummer (STScI).Hi-res image
Before planets around other stars were first discovered in the 1990s,
these far-flung exotic worlds lived only in the imagination of science
fiction writers.
But even their creative minds could not have conceived of the variety
of worlds astronomers have uncovered. Many of these worlds, called exoplanets,
are vastly different from our solar system’s family of planets. They
range from star-hugging “hot Jupiters” to oversized rocky planets dubbed
“super Earths.” Our universe apparently is stranger than fiction.
Seeing these distant worlds isn’t easy because they get lost in the
glare of their host stars. Trying to detect them is like straining to
see a firefly hovering next to a lighthouse’s brilliant beacon.
That’s why astronomers have identified most of the more than 4,000
exoplanets found so far using indirect techniques, such as through a
star’s slight wobble or its unexpected dimming as a planet passes in
front of it, blocking some of the starlight.
These techniques work best, however, for planets orbiting close to
their stars, where astronomers can detect changes over weeks or even
days as the planet completes its racetrack orbit. But finding only
star-skimming planets doesn’t provide astronomers with a comprehensive
picture of all the possible worlds in star systems.
Another technique researchers use in the hunt for exoplanets, which
are planets orbiting other stars, is one that focuses on planets that
are farther away from a star’s blinding glare. Scientists have uncovered
young exoplanets that are so hot they glow in infrared light using
specialized imaging techniques that block out the glare from the star.
In this way, some exoplanets can be directly seen and studied.
NASA’s upcoming James Webb Space Telescope will help astronomers
probe farther into this bold new frontier. Webb, like some ground-based
telescopes, is equipped with special optical systems called
coronagraphs, which use masks designed to block out as much starlight as
possible to study faint exoplanets and to uncover new worlds.
This
schematic shows the positions of the four exoplanets orbiting far away
from the nearby star HR 8799. The orbits appear elongated because of a
slight tilt of the plane of the orbits relative to our line of sight.
The size of the HR 8799 planetary system is comparable to our solar
system, as indicated by the orbit of Neptune, shown to scale. Credits: NASA, ESA, and R. Soummer (STScI)
Two targets early in Webb’s mission are the planetary systems 51
Eridani and HR 8799. Out of the few dozen directly imaged planets,
astronomers plan to use Webb to analyze in detail the systems that are
closest to Earth and have planets at the widest separations from their
stars. This means that they appear far enough away from a star’s glare
to be directly observed. The HR 8799 system resides 133 light-years and
51 Eridani 96 light-years from Earth.
Webb's Planetary Targets
Two observing programs early in Webb’s mission combine the spectroscopic capabilities of the Near Infrared Spectrograph (NIRSpec) and the imaging of the Near Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI)
to study the four giant planets in the HR 8799 system. In a third
program, researchers will use NIRCam to analyze the giant planet in 51
Eridani.
The four giant planets in the HR 8799 system are each roughly 10
Jupiter masses. They orbit more than 14 billion miles from a star that
is slightly more massive than the Sun. The giant planet in 51 Eridani is
twice the mass of Jupiter and orbits about 11 billion miles from a
Sun-like star. Both planetary systems have orbits oriented face-on
toward Earth. This orientation gives astronomers a unique opportunity to
get a bird's-eye view down on top of the systems, like looking at the
concentric rings on an archery target.
Many exoplanets found in the outer orbits of their stars are vastly
different from our solar system planets. Most of the exoplanets
discovered in this outer region, including those in HR 8799, are between
5 and 10 Jupiter masses, making them the most massive planets ever
found to date.
These outer exoplanets are relatively young, from tens of millions to
hundreds of millions of years old—much younger than our solar system’s
4.5 billion years. So they’re still glowing with heat from their
formation. The images of these exoplanets are essentially baby pictures,
revealing planets in their youth.
Webb will probe into the mid-infrared, a wavelength range astronomers
have rarely used before to image distant worlds. This infrared “window”
is difficult to observe from the ground because of thermal emission
from and absorption in Earth’s atmosphere.
“Webb’s strong point is the uninhibited light coming through space in
the mid-infrared range,” said Klaus Hodapp of the University of Hawaii
in Hilo, lead investigator of the NIRSpec observations of the HR 8799
system. “Earth’s atmosphere is pretty difficult to work through. The
major absorption molecules in our own atmosphere prevent us from seeing
interesting features in planets.”
This
discovery image of a Jupiter-sized extrasolar planet orbiting the
nearby star 51 Eridani was taken in near-infrared light in 2014 by the
Gemini Planet Imager. The bright central star is hidden behind a mask in
the center of the image to enable the detection of the exoplanet, which
is 1 million times fainter than 51 Eridani. The exoplanet is on the
outskirts of the planetary system 11 billion miles from its star. Webb
will probe the planet’s atmosphere at infrared wavelengths astronomers
have rarely used to image distant worlds. Credits: International Gemini
Observatory/NOIRLab/NSF/AURA, J. Rameau (University of Montreal), and C.
Marois (National Research Council of Canada Herzberg).Hi-res image
The mid-infrared “is the region where Webb really will make seminal
contributions to understanding what are the particular molecules, what
are the properties of the atmosphere that we hope to find which we don’t
really get just from the shorter, near-infrared wavelengths,” said
Charles Beichman of NASA’s Jet Propulsion Laboratory in Pasadena,
California, lead investigator of the NIRCam and MIRI observations of the
HR 8799 system. “We’ll build on what the ground-based observatories
have done, but the goal is to expand on that in a way that would be
impossible without Webb.”
How Do Planets Form?
One of the researchers’ main goals in both systems is to use Webb to
help determine how the exoplanets formed. Were they created through a
buildup of material in the disk surrounding the star, enriched in heavy
elements such as carbon, just as Jupiter probably did? Or, did they form
from the collapse of a hydrogen cloud, like a star, and become smaller
under the relentless pull of gravity?
Atmospheric makeup can provide clues to a planet’s birth. “One of the
things we’d like to understand is the ratio of the elements that have
gone into the formation of these planets,” Beichman said. “In
particular, carbon versus oxygen tells you quite a lot about where the
gas that formed the planet comes from. Did it come from a disk that
accreted a lot of the heavier elements or did it come from the interstellar medium? So it’s what we call the carbon-to-oxygen ratio that is quite indicative of formation mechanisms.”
To answer these questions, the researchers will use Webb to probe
deeper into the exoplanets’ atmospheres. NIRCam, for example, will
measure the atmospheric fingerprints of elements like methane. It also
will look at cloud features and the temperatures of these planets. “We
already have a lot of information at these near-infrared wavelengths
from ground-based facilities,” said Marshall Perrin of the Space
Telescope Science Institute in Baltimore, Maryland, lead investigator of
NIRCam observations of 51 Eridani b. “But the data from Webb will be
much more precise, much more sensitive. We’ll have a more complete set
of wavelengths, including filling in gaps where you can’t get those
wavelengths from the ground.
This
video shows four Jupiter-sized exoplanets orbiting billions of miles
away from their star in the nearby HR 8799 system. The planetary system
is oriented face-on toward Earth, giving astronomers a unique bird’s-eye
view of the planets’ motion. The exoplanets are orbiting so far away
from their star that they take anywhere from decades to centuries to
complete an orbit. The video consists of seven images of the system
taken over a seven-year period with the W.M. Keck Observatory on Mauna
Kea, Hawaii. Keck’s coronagraph blocks out most of the starlight so that
the much fainter and smaller exoplanets can be seen. Credits: Jason Wang (Caltech) and Christian Marois (NRC Herzberg)
The astronomers will also use Webb and its superb sensitivity to hunt
for less-massive planets far from their star. “From ground-based
observations, we know that these massive planets are relatively rare,”
Perrin said. “But we also know that for the inner parts of systems,
lower-mass planets are dramatically more common than larger-mass
planets. So the question is, does it also hold true for these further
separations out?” Beichman added, “Webb’s operation in the cold
environment of space allows a search for fainter, smaller planets,
impossible to detect from the ground.”
Another goal is understanding how the myriad planetary systems discovered so far were created.
“I think what we are finding is that there is a huge diversity in
solar systems,” Perrin said. “You have systems where you have these hot
Jupiter planets in very close orbits. You have systems where you don’t.
You have systems where you have a 10-Jupiter-mass planet and ones in
which you have nothing more massive than several Earths. We ultimately
want to understand how the diversity of planetary system formation
depends on the environment of the star, the mass of the star, all sorts
of other things and eventually through these population-level studies,
we hope to place our own solar system in context.
This
video shows a Jupiter-sized exoplanet orbiting far away—roughly 11
billion miles—from a nearby, Sun-like star, 51 Eridani. The planetary
system is oriented face-on toward Earth, giving astronomers a unique
bird’s-eye view of the planet’s motion. The video consists of five
images taken over four years with the Gemini South Telescope’s Gemini
Planet Imager, in Chile. Gemini’s coronagraph blocks out most of the
starlight so that the much fainter and smaller exoplanet can be seen. Credits: Jason Wang (Caltech)/Gemini Planet Imager Exoplanet Survey
The NIRSpec spectroscopic observations of HR 8799 and the NIRCam observations of 51 Eridani are part of the Guaranteed Time Observations
programs that will be conducted shortly after Webb’s launch later this
year. The NIRCam and MIRI observations of HR 8799 is a collaboration of
two instrument teams and is also part of the Guaranteed Time
Observations program.
The James Webb Space Telescope will be the world's premier space
science observatory when it launches in 2021. Webb will solve mysteries
in our solar system, look beyond to distant worlds around other stars,
and probe the mysterious structures and origins of our universe and our
place in it. Webb is an international program led by NASA with its
partners, ESA (European Space Agency) and the Canadian Space Agency.
By Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
Tile 107, or “the Outlier” as it is known, is one of 256 tiles of the
MWA located 1.5km from the core of the telescope. The MWA is a precursor
instrument to the SKA. Photographed by Pete Wheeler, ICRAR
Astronomers have discovered a pulsar—a dense and rapidly spinning
neutron star sending radio waves into the cosmos—using a low-frequency
radio telescope in outback Australia.
The pulsar was detected with the Murchison Widefield Array (MWA) telescope, in Western Australia’s remote Mid West region.
It’s the first time scientists have discovered a pulsar with the MWA but they believe it will be the first of many.
The finding is a sign of things to come from the multi-billion-dollar
Square Kilometre Array (SKA) telescope. The MWA is a precursor
telescope for the SKA.
Nick Swainston, a PhD student at the Curtin University node of the
International Centre for Radio Astronomy Research (ICRAR), made the
discovery while processing data collected as part of an ongoing pulsar
survey.
“Pulsars are born as a result of supernovae—when a massive star
explodes and dies, it can leave behind a collapsed core known as a
neutron star,” he said.
“They’re about one and a half times the mass of the Sun, but all
squeezed within only 20 kilometres, and they have ultra-strong magnetic
fields.”
Mr Swainston said pulsars spin rapidly and emit electromagnetic radiation from their magnetic poles.
“Every time that emission sweeps across our line of sight, we see a
pulse—that’s why we call them pulsars,” he said. “You can imagine it
like a giant cosmic lighthouse.”
ICRAR-Curtin astronomer Dr Ramesh Bhat said the newly discovered
pulsar is located more than 3000 light-years from Earth and spins about
once every second.
“That’s incredibly fast compared to regular stars and planets,” he said. “But in the world of pulsars, it’s pretty normal.”
Dr Bhat said the finding was made using about one per cent of the large volume of data collected for the pulsar survey.
“We’ve only scratched the surface,” he said. “When we do this project
at full-scale, we should find hundreds of pulsars in the coming years.”
Pulsars are used by astronomers for several applications including testing the laws of physics under extreme conditions.
“A spoonful of material from a neutron star would weigh millions of tonnes,” Dr Bhat said.
“Their magnetic fields are some of the strongest in the Universe—about 1000 billion times stronger than that we have on Earth.”
“So we can use them to do physics that we can’t do in any of the Earth-based laboratories.”
An
artist’s impression of Pulsar — a dense and rapidly spinning neutron
star sending radio waves into the cosmos. Credit: ICRAR / Curtin
University.
Finding pulsars and using them for extreme physics is also a key science driver for the SKA telescope.
MWA Director Professor Steven Tingay said the discovery hints at a
large population of pulsars awaiting discovery in the Southern
Hemisphere.
“This finding is really exciting because the data processing is
incredibly challenging, and the results show the potential for us to
discover many more pulsars with the MWA and the low-frequency part of
the SKA.”
“The study of pulsars is one of the headline areas of science for the
multi-billion-dollar SKA, so it is great that our team is at the
forefront of this work,” he said.
An artist’s impression of one of 256 tiles of the Murchison Widefield
Array radio telescope observing a pulsar — a dense and rapidly spinning
neutron star sending radio waves into the cosmos. Credit: Dilpreet Kaur /
ICRAR / Curtin University
‘Discovery of a steep-spectrum low-luminosity pulsar with the Murchison Widefield Array’, published in The Astrophysical Journal Letters on April 21, 2021. Click here for the paper
More Info
ICRAR
The International Centre for Radio Astronomy Research (ICRAR) is a joint venture between Curtin University and The University of Western Australia with support and funding from the State Government of Western Australia.
Murchison Widefield Array
The Murchison Widefield Array (MWA) is a low-frequency radio telescope and is the first of four Square Kilometre Array (SKA) precursors to be completed. A consortium of partner institutions from seven countries (Australia, USA, India, New Zealand, Canada, Japan, and China) financed the development, construction, commissioning, and operations of the facility. The MWA consortium is led by Curtin University.
This stunning composite image (click for the full view!) reveals the
radio emission (shown in red) from a bent jet that was launched from the
galaxy NGC 1272, the bright source just to the right of the image
center. The 12’ x 12’ image of the Perseus galaxy cluster is captured by
the Sloan Digital Sky Survey; the brightest central galaxy of the
cluster, NGC 1275, can be seen to NGC 1272’s left. A new publication led
by Marie-Lou Gendron-Marsolais (European Southern Observatory) presents
high-resolution Very Large Array images of the detailed radio
structures in the Perseus cluster. The authors use these new data to
study how the galaxy’s movement as it falls into the cluster, as well as
the bulk motions of the intracluster gas, shape the powerful radio jet
into the dramatic shapes we see here. For more information, check out
the original article below.Hi-res Image
Citation
“VLA Resolves Unexpected Radio Structures in the Perseus Cluster of Galaxies,” M.-L. Gendron-Marsolais et al 2021 ApJ911 56. doi:10.3847/1538-4357/abddbb
Direct image of exoplanet YSES 2b (bottom right) and its star (centre). c) ESO/SPHERE/VLT/Bohn et al.
A team of astronomers led by Dutch scientists have directly imaged a
giant planet orbiting at a large distance around a sun-like star. Why
this planet is so massive, and how it got to be there, is still a
mystery. The researchers will publish their findings in the journal Astronomy & Astrophysics.
The planet in question is YSES 2b, located 360
light years from Earth in the direction of the southern constellation of
Musca (Latin for The Fly). The gaseous planet is six times heavier than
Jupiter, the largest planet in our solar system. The newly discovered
planet orbits 110 times more distant from its star than the Earth does
from the Sun (or 20 times the distance between the Sun and Jupiter). The
accompanying star is only 14 million years old and resembles our Sun in
its childhood.
The large distance from the planet to the star
presents a puzzle to astronomers, because it does not seem to fit either
of the two most well-known models for the formation of large gaseous
planets. If the planet had grown in its current location far from the
star by means of core accretion, it would be too heavy because there is
not enough material to make a huge planet at this large distance from
the star. If the planet was created by so-called gravitational
instability in the planetary disk, it appears to be not heavy enough. A
third possibility is that the planet formed close to the star by core
accretion and then migrated outwards. Such a migration, however, would
require the gravitational influence of a second planet, which the
researchers have not yet found.
Young Suns Exoplanet Survey (YSES)
The astronomers will continue to investigate the surroundings of this
unusual planet and its star in the near future and hope to learn more
about the system, and they will continue to search for other gaseous
planets around young, sun-like stars. Current telescopes are not yet
large enough to carry out direct imaging of earth-like planets around
sun-like stars.
Lead researcher Alexander Bohn
(Leiden University): "By investigating more Jupiter-like exoplanets in
the near future, we will learn more about the formation processes of gas
giants around sun-like stars."
The planet YSES 2b was discovered with the Young Suns Exoplanet Survey (YSES). This survey already provided the first direct image of a multi-planet system around a Sun-like
star in 2020. The researchers made their observations in 2018 and 2020
using the Very Large Telescope of the European Southern Observatory
(ESO) in Chile. They used the telescope's SPHERE instrument for this.
This instrument was co-developed by the Netherlands and can capture
direct and indirect light from exoplanets.
Scientific paper
Discovery of a directly imaged planet to the young solar analog YSES 2.
By: Alexander J. Bohn et al. Accepted for publication in Astronomy &
Astrophysics [original | free preprint (pdf)].
Artist’s impression of a galaxy with an active nucleus, a
supermassive black hole in the centre. When the black hole swallows
matter, two powerful jets can form at the edges of the black hole. These
jets form gigantic 'radio clouds' that can be detected by radio
telescopes. (c) ESA/C. Carreau
All supermassive black holes in the centres of galaxies appear to have
periods when they swallow matter from their close surroundings. But that
is about as far as the similarities go. That's the conclusion reached
by British and Dutch astronomers from their research with
ultra-sensitive radio telescopes in a well-studied region of the
universe. They publish their findings in two articles in the
international journal Astronomy & Astrophysics.
Astronomers have studied active galaxies since the
1950s. Active galaxies have a super-massive black hole at their centre
that is swallowing matter. During these active phases the objects often
emit extremely strong radio, infrared, ultraviolet and X-ray radiation.
In two new publications, an international team of astronomers focused on all the active galaxies in the well-studied GOODS-North region
in the constellation of Ursa Major. Until now that region had been
studied mainly by space telescopes collecting visible light, infrared
light and UV light. The new observations add data from sensitive
networks of radio telescopes, including the UK’s e-MERLIN national
facility and the European VLBI Network (EVN).
Three conclusions
Thanks to this systematic study, three things become clear. Firstly, it
turns out that the nuclei of many different types of galaxies can be
active, in different ways. Some are extremely greedy, gobbling up as
much material as they can, others digest their 'food' more slowly, and
others are nearly starving of hunger.
Secondly, occasionally an accretion phase occurs
simultaneous with a star-formation phase and sometimes not. If star
formation is ongoing, activity in the nucleus is difficult to detect.
Thirdly, the nuclear accretion process may, or may
not, generate radio jets – regardless the speed at which the black hole
swallows its food.
Good news
According to principal investigator Jack Radcliffe (formerly University
of Groningen and ASTRON in the Netherlands and University of Manchester
in the United Kingdom, now University of Pretoria, South Africa), the
observations also show that radio telescopes are optimally useful to
study the eating habits of black holes in the distant universe. "That's
good news, because the SKA radio telescopes are coming and they will
allow us to look deeper into the universe with even more detail."
Co-author Peter Barthel (University of Groningen,
the Netherlands) adds: "We are getting more and more indications that
all galaxies have enormously massive black holes in their centres. Of
course, these must have grown to their current mass. It seems that,
thanks to our observations, we now have these growth processes in view
and are slowly but surely starting to understand them."
Co-author Michael Garrett (University of
Manchester, United Kingdom) adds: "These beautiful results demonstrate
the unique capacities of radio astronomy. Telescopes such as the VLA,
e-MERLIN and the EVN are transforming our view of how galaxies evolve in
the early universe."
Scientific papers
Nowhere to Hide: Radio-faint AGN in the GOODS-N field. By: J.F.
Radcliffe et al. Accepted for publication in Astronomy &
Astrophysics [original | free preprint].
The radio emission from Active Galactic Nuclei. By: J.F. Radcliffe et
al. Accepted for publication in Astronomy & Astrophysics [original | free preprint]
Artist’s impression of the collision of two black holes that produced the gravitational-wave signal GW190521.
[LIGO/Caltech/MIT/R. Hurt (IPAC)]
Could the biggest — literally — gravitational-wave discovery yet be something other than what it initially seemed? A new study suggests that the most massive merger of black holes detected by LIGO/Virgo may have included a surprising lightweight.
The rapidly expanding “stellar graveyard”, a plot of the masses of the different components of observed compact binary mergers. GW190521, top center, is more massive than any other binary merger we’ve observed. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]
Echoes of a Surprising Merger
In May 2019, a collision of two black holes shook spacetime, registering in the LIGO and Virgo gravitational-wave detectors as the heaviest black-hole merger discovered yet. Initial analysis of GW190521 suggested that the participants in this cosmic collision were ~85 and ~66 times the mass of the Sun, and that they formed a final black hole of ~142 solar masses — an unexpectedly heavy outcome that lands in the elusive category of intermediate-mass black holes.
But GW190521 raised eyebrows for another reason as well: the estimated masses of the two merging black holes fell between 65 and 120 solar masses, a region known as the pair-instability mass gap. This range of masses should be inherently off-limits for black holes born from collapsed stars, based on our current understanding of stellar evolution processes.
While there are many hypotheses about how mass-gap black holes could potentially form, two scientists have focused on an alternative angle: what if we were simply wrong in our estimate of GW190521’s component masses?
Checking Our Assumptions
How do we measure component masses from a gravitational-wave signal? Decades of theoretical research have produced a vast array of model signals for mergers with different parameters. By comparing the observed gravitational-wave signal to the various models, we can calculate which ones fit best. But this comparison relies on what are called priors — a set of assumptions that go into the analysis and affect the outcome.
In a recent publication, scientists Alexander Nitz and Collin Capano (Max Planck Institute for Gravitational Physics and Leibniz University Hannover, Germany) reanalyze the gravitational-wave signal for GW190521 using a different set of priors and constraints than the original analysis completed by the LIGO collaboration.
Nitz and Capano find that their analysis admits two possible solutions for GW190521: one similar to that found by the LIGO collaboration — and another, in which the component black holes are ~16 and ~170 solar masses. This second option becomes even more heavily favored when the authors analyze the gravitational-wave signal simultaneously with an electromagnetic flare that may have been associated with the merger.
The observed gravitational-wave signal of GW190521 in each of the three detectors (black), plotted with two best-fit models: one for when the component mass ratio is between 1 and 2 (blue) and one for a mass ratio between 2 and 25 (orange). [Nitz & Capano 2021]
An Uneven Pair?
What does this outcome tell us? The masses in Nitz and Capano’s second solution both lie outside of the pair-instability mass gap, neatly resolving the paradox previously created by this merger.
If the authors’ interpretation is correct, then GW190521 would represent the first detected intermediate-mass-ratio inspiral — a type of merger in which one component is substantially larger than the other. This signal then provides an exciting milestone and an opportunity to learn more about the different types of dramatic collisions that occur in our galaxy.
Citation
“GW190521 May Be an Intermediate-mass Ratio Inspiral,” Alexander H. Nitz and Collin D. Capano 2021 ApJL 907 L9.doi:10.3847/2041-8213/abccc5
