This event not only confirmed a century-old prediction made by Einstein’s Theory of General Relativity, it also led to a revolution in astronomy. It also stoked the hopes of some scientists who believed that black holes could account for the Universe’s “missing mass”. Unfortunately, a new study by a team of UC Berkeley physicists has shown that black holes are not the long-sought-after source of Dark Matter.
When looking to study the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.
This technique has allowed for the study of individual stars in distant galaxies. In a recent study, an international team of astronomers used a galaxy cluster to study the farthest individual star ever seen in the Universe. Although it normally to faint to observe, the presence of a foreground galaxy cluster allowed the team to study the star in order to test a theory about dark matter.
For the sake of their study, Prof. Kelly and his associates used the galaxy cluster known as MACS J1149+2223 as their lens. Located about 5 billion light-years from Earth, this galaxy cluster sits between the Solar System and the galaxy that contains Icarus. By combining Hubble’s resolution and sensitivity with the strength of this gravitational lens, the team was able to see and study Icarus, a blue giant.
Icarus, named after the Greek mythological figure who flew too close to the Sun, has had a rather interesting history. At a distance of roughly 9 billion light-years from Earth, the star appears to us as it did when the Universe was just 4.4 billion years old. In April of 2016, the star temporarily brightened to 2,000 times its normal luminosity thanks to the gravitational amplification of a star in MACS J1149+2223.
As Prof. Kelly explained in a recent UCLA press release, this temporarily allowed Icarus to become visible for the first time to astronomers:
“You can see individual galaxies out there, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions.”
Kelly and a team of astronomers had been using Hubble and MACS J1149+2223 to magnify and monitor a supernova in the distant spiral galaxy at the time when they spotted the new point of light not far away. Given the position of the new source, they determined that it should be much more highly magnified than the supernova. What’s more, previous studies of this galaxy had not shown the light source, indicating that it was being lensed.
As Tommaso Treu, a professor of physics and astronomy in the UCLA College and a co-author of the study, indicated:
“The star is so compact that it acts as a pinhole and provides a very sharp beam of light. The beam shines through the foreground cluster of galaxies, acting as a cosmic magnifying glass… Finding more such events is very important to make progress in our understanding of the fundamental composition of the universe.
In this case, the star’s light provided a unique opportunity to test a theory about the invisible mass (aka. “dark matter”) that permeates the Universe. Basically, the team used the pinpoint light source provided by the background star to probe the intervening galaxy cluster and see if it contained huge numbers of primordial black holes, which are considered to be a potential candidate for dark matter.
These black holes are believed to have formed during the birth of the Universe and have masses tens of times larger than the Sun. However, the results of this test showed that light fluctuations from the background star, which had been monitored by Hubble for thirteen years, disfavor this theory. If dark matter were indeed made up of tiny black holes, the light coming from Icarus would have looked much different.
Since it was discovered in 2016 using the gravitational lensing method, Icarus has provided a new way for astronomers to observe and study individual stars in distant galaxies. In so doing, astronomers are able to get a rare and detailed look at individual stars in the early Universe and see how they (and not just galaxies and clusters) evolved over time.
When the James Webb Space Telescope (JWST) is deployed in 2020, astronomers expect to get an even better look and learn so much more about this mysterious period in cosmic history.
For the sake of studying the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.
The study which details their findings, titled “Predicting gravitational lensing by stellar remnants” appeared in the Monthly Noticed of the Royal Astronomical Society. The study was led by Alexander J. Harding of the CfA and included Rosanne Di Stefano, and Claire Baker (also from the CfA), as well as members from the University of Southampton, Georgia State University, the University of Nigeria, and Cornell University.
To put it simply, determining the mass of an astronomical object is one the greatest challenges for astronomers. Until now, the most successful method relied on binary systems because the orbital parameters of these systems depend on the masses of the two objects. Unfortunately, objects that are at the end states of stellar evolution – like black holes, neutron stars or white dwarfs – are often too faint or isolated to be detectable.
This is unfortunate, since these objects are responsible for a lot of dramatic astronomical events. These include the accretion of material, the emission of energetic radiation, gravitational waves, gamma-ray bursts, or supernovae. Many of these events are still a mystery to astronomers or the study of them is still in its infancy – i.e. gravitational waves. As they state in their study:
“Gravitational lensing provides an alternative approach to mass measurement. It has the advantage of only relying on the light from a background source, and can therefore be employed even for dark lenses. In fact, since light from the lens can interfere with the detection of lensing effects, compact objects are ideal lenses.”
As they go on to state, of the 18,000 lensing events that have been detected to date, roughly 10 to 15% are believed to have been caused by compact objects. However, scientists are unable to tell which of the detected events were due to compact lenses. For the sake of their study then, the team sought to circumvent this problem by identifying local compact objects and predicting when they might produce a lensing event so they could be studied.
“By focusing on pre-selected compact objects in the near vicinity of the Sun, we ensure that the lensing event will be caused by a white dwarf, neutron star, or black hole,” they state. “Furthermore, the distance and proper motion of the lens can be accurately measured prior to the event, or else afterwards. Armed with this information, the lensing light curve allows one to accurately measure the mass of the lens.”
In the end, the team determined that lensing events could be predicted from thousands of local objects. These include 250 neutron stars, 5 black holes, and roughly 35,000 white dwarfs. Neutron stars and black holes present a challenge since the known populations are too small and their proper motions and/or distances are not generally known.
But in the case of white dwarfs, the authors anticipate that they will provide for many lensing opportunities in the future. Based on the general motions of the white dwarfs across the sky, they obtained a statistical estimate that about 30-50 lensing events will take place per decade that could be spotted by the Hubble Space Telescope, the ESA’s Gaia mission, or NASA’s James Webb Space Telescope (JWST). As they state in their conclusions:
“We find that the detection of lensing events due to white dwarfs can certainly be observed during the next decade by both Gaia and HST. Photometric events will occur, but to detect them will require observations of the positions of hundreds to thousands of far-flung white dwarfs. As we learn the positions, distances to, and proper motions of larger numbers of white dwarfs through the completion of surveys such as Gaia and through ongoing and new wide-field surveys, the situation will continue to improve.”
The future of astronomy does indeed seem bright. Between improvements in technology, methodology, and the deployment of next-generation space and ground-based telescopes, there is no shortage of opportunities to see and learn more.
When it comes to studying some of the most distant and oldest galaxies in the Universe, a number of challenges present themselves. In addition to being billions of light years away, these galaxies are often too faint to see clearly. Luckily, astronomers have come to rely on a technique known as Gravitational Lensing, where the gravitational force of a large object (like a galactic cluster) is used to enhance the light of these fainter galaxies.
Using this technique, an international team of astronomers recently discovered a distant and quiet galaxy that would have otherwise gone unnoticed. Led by researchers from the University of Hawaii at Manoa, the team used the Hubble Space Telescope to conduct the most extreme case of gravitational lensing to date, which allowed them to observe the faint galaxy known as eMACSJ1341-QG-1.
For the sake of their study, the team relied on the massive galaxy cluster known as eMACSJ1341.9-2441 to magnify the light coming from eMACSJ1341-QG-1, a distant and fainter galaxy. In astronomical terms, this galaxy is an example of a “quiescent galaxy”, which are basically older galaxies that have largely depleted their supplies of dust and gas and therefore do not form new stars.
The team began by taking images of the faint galaxy with the Hubble and then conducting follow-up spectroscopic observations using the ESO/X-Shooter spectrograph – which is part of the Very Large Telescope (VLT) at the Paranal Observatory in Chile. Based on their estimates, the team determined that they were able to amplify the background galaxy by a factor of 30 for the primary image, and a factor of six for the two remaining images.
This makes eMACSJ1341-QG-1 the most strongly amplified quiescent galaxy discovered to date, and by a rather large margin! As Johan Richard – an assistant astronomer at the University of Lyon who performed the lensing calculations, and a co-author on the study – indicated in a University of Hawaii News release:
“The very high magnification of this image provides us with a rare opportunity to investigate the stellar populations of this distant object and, ultimately, to reconstruct its undistorted shape and properties.”
Although other extreme magnifications have been conducted before, this discovery has set a new record for the magnification of a rare quiescent background galaxy. These older galaxies are not only very difficult to detect because of their lower luminosity; the study of them can reveal some very interesting things about the formation and evolution of galaxies in our Universe.
As Ebeling, an astronomer with the UH’s Institute of Astronomy and the lead author on the study, explained:
“We specialize in finding extremely massive clusters that act as natural telescopes and have already discovered many exciting cases of gravitational lensing. This discovery stands out, though, as the huge magnification provided by eMACSJ1341 allows us to study in detail a very rare type of galaxy.”
Quiescent galaxies are common in the local Universe, representing the end-point of galactic evolution. As such, this record-breaking find could provide some unique opportunities for studying these older galaxies and determining why star-formation ended in them. As Mikkel Stockmann, a team member from the University of Copenhagen and an expert in galaxy evolution, explained:
“[A]s we look at more distant galaxies, we are also looking back in time, so we are seeing objects that are younger and should not yet have used up their gas supply. Understanding why this galaxy has already stopped forming stars may give us critical clues about the processes that govern how galaxies evolve.”
In a similar vein, recent studies have been conducted that suggest that the presence of a Supermassive Black Hole (SMBH) could be what is responsible for galaxies becoming quiescent. As the powerful jets these black holes create begin to drain the core of galaxies of their dust and gas, potential stars find themselves starved of the material they would need to undergo gravitational collapse.
In the meantime, follow-up observations of eMACSJ1341-QG1 are being conducted using telescopes at the Paranal Observatory in Chile and the Maunakea Observatories in Hawaii. What these observations reveal is sure to tell us much about what will become of our own Milky Way Galaxy someday, when the last of the dust and gas is depleted and all its stars become red giants and long-lived red dwarfs.
Welcome back to our series on Exoplanet-Hunting methods! Today, we look at the curious and unique method known as Gravitational Microlensing.
The hunt for extra-solar planets sure has heated up in the past decade. Thanks to improvements made in technology and methodology, the number of exoplanets that have been observed (as of December 1st, 2017) has reached 3,710 planets in 2,780 star systems, with 621 system boasting multiple planets. Unfortunately, due to various limits astronomers are forced to contend with, the vast majority have been discovered using indirect methods.
One of the more commonly-used methods for indirectly detecting exoplanets is known as Gravitational Microlensing. Essentially, this method relies on the gravitational force of distant objects to bend and focus light coming from a star. As a planet passes in front of the star relative to the observer (i.e. makes a transit), the light dips measurably, which can then be used to determine the presence of a planet.
In this respect, Gravitational Microlensing is a scaled-down version of Gravitational Lensing, where an intervening object (like a galaxy cluster) is used to focus light coming from a galaxy or other object located beyond it. It also incorporates a key element of the highly-effective Transit Method, where stars are monitored for dips in brightness to indicate the presence of an exoplanet.
In accordance with Einstein’s Theory of General Relativity, gravity causes the fabric of spacetime to bend. This effect can cause light affected by an object’s gravity to become distorted or bent. It can also act as a lens, causing light to become more focused and making distant objects (like stars) appear brighter to an observer. This effect occurs only when the two stars are almost exactly aligned relative to the observer (i.e. one positioned in front of the other).
These “lensing events” are brief, but plentiful, as Earth and stars in our galaxy are always moving relative to each other. In the past decade, over one thousand such events have been observed, and typically lasted for a few days or weeks at a time. In fact, this effect was used by Sir Arthur Eddington in 1919 to provide the first empirical evidence for General Relativity.
This took place during the solar eclipse of May 29th, 1919, where Eddington and a scientific expedition traveled to the island of Principe off the coast of West Africa to take pictures of the stars that were now visible in the region around the Sun. The pictures confirmed Einstein’s prediction by showing how light from these stars was shifted slightly in response to the Sun’s gravitational field.
The technique was originally proposed by astronomers Shude Mao and Bohdan Paczynski in 1991 as a means of looking for binary companions to stars. Their proposal was refined by Andy Gould and Abraham Loeb in 1992 as a method of detecting exoplanets. This method is most effective when looking for planets towards the center of the galaxy, as the galactic bulge provides a large number of background stars.
Microlensing is the only known method capable of discovering planets at truly great distances from the Earth and is capable of finding the smallest of exoplanets. Whereas the Radial Velocity Method is effective when looking for planets up to 100 light years from Earth and Transit Photometry can detect planets hundreds of light-years away, microlensing can find planets that are thousands of light-years away.
While most other methods have a detection bias towards smaller planets, the microlensing method is the most sensitive means of detecting planets that are around 1-10 astronomical units (AU) away from Sun-like stars. This makes it especially effective when paired with the Radial Velocity and Transit Methods, which can confirm the existence of exoplanets as well as yield accurate estimates of a planet’s radius and mass.
Taken together, these benefits make microlensing the most effective method for finding Earth-like planets around Sun-like stars (alone or in combination with other methods). In addition, microlensing surveys can be effectively mounted using ground-based facilities. Like Transit Photometry, the Microlensing Method benefits from the fact that it can be used to survey tens of thousands of stars simultaneously.
Because microlensing events are unique and not subject to repeat, any planets detected using this method will not be observable again. In addition, those planets that are detected tend to be very far way, which makes follow-up investigations virtually impossible. This makes microlensing a good means for detecting exoplanet candidates, but a very poor means for confirming candidates.
Another problem with microlensing is that it is subject to a considerable margin of error when placing constraints on a planet’s characteristics. For example, microlensing surveys can only produce rough estimations of a planet’s distance, leaving large margins for error. This means that planets that are tens of thousands of light-years from Earth would produce distance estimates with a margin of several thousand light-years.
Microlensing is also unable to yield accurate estimates of a planet’s size, and mass estimates are subject to loose constraints. Orbital properties are also difficult to determine, since the only orbital characteristic that can be directly determined with this method is the planet’s current semi-major axis. As such, planet’s with an eccentric orbit will only be detectable for a tiny portion of its orbit (when it is far away from its star).
The gravitational microlensing effect increases as a result of the planet-to-star mass ratio, which means that it is easiest to detect planets around low-mass stars. This makes microlensing effective in the search for rocky planets around low mass, M-type (red dwarf) stars, but limits its effectiveness with more massive stars. Finally, microlensing is dependent on rare and random events – the passage of one star precisely in front of another, as seen from Earth – which makes detections both rare and unpredictable.
Examples of Gravitational Microlensing Surveys:
Surveys that rely on the Microlensing Method include the Optical Gravitational Lensing Experiment (OGLE) at the University of Warsaw. Led by Andrzej Udalski, the director of the University’s Astronomical Observatory, this international project uses the 1.3 meter “Warsaw” telescope at Las Campanas, Chile, to search for microlensing events in a field of 100 stars around the galactic bulge.
There is also the Microlensing Observations in Astrophysics (MOA) group, a collaborative effort between researchers in New Zealand and Japan. Led by Professor Yasushi Muraki of Nagoya University, this group uses the Microlensing Method to conduct surveys for dark matter, extra-solar planets, and stellar atmospheres from the southern hemisphere.
And then there’s the Probing Lensing Anomalies NETwork (PLANET), which consists of five 1-meter telescopes distributedaroundthe southernhemisphere. In collaboration with RoboNet, this project is able to provide near-continuous observations for microlensing events caused by planets with masses as low as Earth’s.
In 2012, the Hubble Space Telescope Frontier Fields program (aka. Hubble Deep Fields Initiative 2012) officially kicked off. The purpose of this project was to study the faintest and most distant galaxies in the Universe using the gravitational lensing technique, thus advancing our knowledge of early galaxy formation. By 2017, the Frontier Field program wrapped up, and the hard work of analyzing all the data it collected began.
One of the more interesting finds within the Frontier Fields data has been the discovery of low mass galaxies with high star formation rates. After examining the “parallel fields” for Abell 2744 and MACS J0416.1-2403 – two galaxy clusters studied by the program – a pair of astronomers noted the presence of what they refer to as “Little Blue Dots” (LBDs), a finding which has implications for galaxy formation and globular clusters.
To put it simply, the Frontier Fields program used the Hubble Space Telescope to observe six massive galaxy clusters at optical and near-infrared wavelengths – with its Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3), respectively. These massive galaxies were used to magnify and stretch images of remote galaxies located behind them which were otherwise too faint for Hubble to see directly (aka. gravitational lensing).
While one of these Hubble cameras would look at a galaxy cluster, the other would simultaneously view an adjacent patch of sky. These adjacent patches are known as “parallel fields”, otherwise faint regions that provide some of the deepest looks into the early Universe. As Dr. Bruce Elmegreen told Universe Today via email:
“The purpose of the HFF program is to take deep images of 6 regions of the sky where there are clusters of galaxies, because these clusters magnify background galaxies through the gravitational lens effect. In this way, we can see further than just with direct imaging of the sky alone. Many galaxies have been studied using this magnification technique. The clusters of galaxies are important because they are big mass concentrations which make strong gravitational lenses.”
This six galaxy clusters used for the sake of the project included Abell 2744, MACS J0416.1-2403 and their parallel fields, the latter of which were the focal point in this study. These and the other clusters were used to find galaxies that existed just 600 to 900 million years after the Big Bang. These galaxies and their respective parallels had already been cataloged using computer algorithms that automatically found galaxies in the images and determined their properties.
As the research duo go on to explain in their study, recent large-scale deep surveys have enabled studies of smaller galaxies at higher redshifts. These include “green peas” – luminous, compact and low mass galaxies with high specific star formation rates – and even lower-mass “blueberries”, small starburst galaxies that are a faint extension of the green peas that also show intense rates of star formation.
Using the aforementioned catalogues, and examining the parallel fields for Abell 2744 and MACS J0416.1-2403, the team went looking for other examples of low-mass galaxies with high star formation rates. The purpose of this was to measure the properties of these dwarf galaxies, and to see if any of their positions accorded with where globular clusters are known to have formed.
What they found was what they referred to as “Little Blue Dots” (LBSs), which are even lower-mass versions of “blueberries”. As Dr. Debra Elmegreen told Universe Today via email:
“When I was examining the images (there are about 3400 galaxies detected in each field), I noticed occasional galaxies that appeared as little blue dots, which was very intriguing because of Bruce’s previous theoretical work on dwarf galaxies. The published catalogs included redshifts and star formation rates and masses for each galaxy, and it turns out the little blue dots are low mass galaxies with very high star formation rates for their mass.”
These galaxies didn’t show structure, so Debra and Bruce stacked the images of galaxies into 3 different ranges of redshift (which worked out to about 20 galaxies each) to create deeper images. “Still they showed no structure or faint extended outer disk,” said Debra, “so they are at the limit of resolution, with average sizes of 100-200 parsecs (about 300-600 light years) and masses of a few million times the mass of our sun.”
In the end, they determined that within these LBDs, star formation rates were very high. They also noted that these dwarf galaxies were very young, being less than 1% the age of the Universe at the time that they were observed. “So the tiny galaxies just formed,: said Bruce, “and their star formation rates are high enough to account for the globular clusters, maybe one in each LBD, when the star bursts in them wind down after a few tens of million years.”
Debra and Bruce Elmegreen are no strangers to high redshift galaxies. Back in 2012, Bruce published a paper that suggested that the globular clusters that orbit the Milky Way (and most other galaxies) formed in dwarf galaxies during the early Universe. These dwarf galaxies would have since been acquired by larger galaxies like our own, and the clusters are essentially their remnants.
Globular clusters are essentially massive star clusters that orbit around the Milky Way Halo. They are typically around 1 million Solar masses and are made up of stars that are very old – somewhere on the order of 10 to 13 billion years. Beyond the Milky Way, many appear in common orbits and in the Andromeda Galaxy, some even appear connected by a stream of stars.
As Bruce explained, his is a compelling argument for the theory that globular clusters formed from dwarf galaxies in the early Universe:
“This suggests that the metal-poor globular clusters are the dense remnants of little galaxies that got captured by bigger galaxies, like the Milky Way, and ripped apart by tidal forces. This idea for the origin of halo globular clusters goes back several decades… It would be only the metal-poor one that are like this, which are about half the total, because dwarf galaxies are metal poor compared to big galaxies, and they were also more metal poor in the early universe.”
This study has many implications for our understanding of how the Universe evolved, which was the chief aim of the Hubble Frontier Fields program. By examining objects in the early Universe, and determining their properties, scientists are able to determine how the structures that we are familiar with today – i.e. stars, galaxies, clusters, etc. – truly came from.
These same studies also allow scientists to make educated guesses about where the Universe is going and what will become of those same structures millions or even billions of years from now. In short, knowing where we’ve been lets us predict where we are headed!
Since the deployment of the Hubble Space Telescope, astronomers have been able to look deeper into the cosmic web than ever before. The farther they’ve looked, the deeper back in time they are able to see, and thus learn what the Universe looked like billions of years ago. With the deployment of other cutting-edge telescopes and observatories, scientists have been able to learn a great deal more about the history and evolution of the cosmos.
Most recently, an international team of astronomers using the Gemini North Telescope in Hawaii were able to spot a spiral galaxy located 11 billion light years away. Thanks to a new technique that combined gravitational lensing and spectrography, they were able to see an object that existed just 2.6 billion years after the Big Bang. This makes this spiral galaxy, known as A1689B11, the oldest and most distant spiral galaxy spotted to date.
Together, the team relied on the gravitational lensing technique to spot A1689B11. This technique has become a mainstay for astronomers, and involves using a large object (like a galaxy cluster) to bend and magnify the light of a galaxy located behind it. As Dr. Tiantian Yuan, a Swinburne astronomer and the lead author on the research study, explained in a Swinburne press statement:
“This technique allows us to study ancient galaxies in high resolution with unprecedented detail. We are able to look 11 billion years back in time and directly witness the formation of the first, primitive spiral arms of a galaxy.”
They then used the Near-infrared Integral Field Spectrograph (NIFS) on the Gemini North telescope to verify the structure and nature of this spiral galaxy. This instrument was built Peter McGregor of The Australian National University (ANU), which now is responsible for maintaining it. Thanks to this latest discovery, astronomers now have some additional clues as to how galaxies took on the forms that we are familiar with today.
Based on the classification scheme developed by famed astronomer Edwin Hubble (the “Hubble Sequence“), galaxies are divides into 3 broad classes based on their shapes – ellipticals, lenticulars and spirals – with a fourth category reserved for “irregularly-shaped” galaxies. In accordance with this scheme, galaxies start out as elliptical structures before branching off to become spiraled, lenticular, or irregular.
As such, the discovery of such an ancient spiral galaxy is crucial to determining when and how the earliest galaxies began changing from being elliptical to taking on their modern forms. As Dr Renyue Cen, an astronomer from Princeton University and a co-author on the study, says:
“Studying ancient spirals like A1689B11 is a key to unlocking the mystery of how and when the Hubble sequence emerges. Spiral galaxies are exceptionally rare in the early Universe, and this discovery opens the door to investigating how galaxies transition from highly chaotic, turbulent discs to tranquil, thin discs like those of our own Milky Way galaxy.”
On top of that, this study showed that the A1689B11 spiral galaxy has some surprising features which could also help inform (and challenge) our understanding of this period in cosmic history. As Dr. Yuan explained, these features are in stark contrast to galaxies as they exist today. But equally interesting is the fact that it also differentiates this spiral galaxy from other galaxies that are similar in age.
“This galaxy is forming stars 20 times faster than galaxies today – as fast as other young galaxies of similar masses in the early Universe,” said Dr. Yuan. “However, unlike other galaxies of the same epoch, A1689B11 has a very cool and thin disc, rotating calmly with surprisingly little turbulence. This type of spiral galaxy has never been seen before at this early epoch of the Universe!”
In the future, the team hopes to conduct further studies of this galaxy to further resolve its structure and nature, and to compare it to other spiral galaxies from this epoch. Of particular interest to them is when the onset of spiral arms takes place, which should serve as a sort of boundary marker between ancient elliptical galaxies and modern spiral, lenticular and irregular shapes.
They will continue to rely on the NIFS to conduct these studies, but the team also hopes to rely on data collected by the James Webb Space Telescope (which will be launched in 2019). These and other surveys in the coming years are expected to reveal vital information about the earliest galaxies in the Universe, and reveal further clues as to how it changed over time.
Gravitational lenses are an important tool for astronomers seeking to study the most distant objects in the Universe. This technique involves using a massive cluster of matter (usually a galaxy or cluster) between a distant light source and an observer to better see light coming from that source. In an effect that was predicted by Einstein’s Theory of General Relativity, this allows astronomers to see objects that might otherwise be obscured.
Recently, a group of European astronomers developed a method for finding gravitational lenses in enormous piles of data. Using the same artificial intelligence algorithms that Google, Facebook and Tesla have used for their purposes, they were able to find 56 new gravitational lensing candidates from a massive astronomical survey. This method could eliminate the need for astronomers to conduct visual inspections of astronomical images.
While useful to astronomers, gravitational lenses are a pain to find. Ordinarily, this would consist of astronomers sorting through thousands of images snapped by telescopes and observatories. While academic institutions are able to rely on amateur astronomers and citizen astronomers like never before, there is imply no way to keep up with millions of images that are being regularly captured by instruments around the world.
To address this, Dr. Petrillo and his colleagues turned to what are known as “Convulutional Neural Networks” (CNN), a type of machine-learning algorithm that mines data for specific patterns. While Google used these same neural networks to win a match of Go against the world champion, Facebook uses them to recognize things in images posted on its site, and Tesla has been using them to develop self-driving cars.
“This is the first time a convolutional neural network has been used to find peculiar objects in an astronomical survey. I think it will become the norm since future astronomical surveys will produce an enormous quantity of data which will be necessary to inspect. We don’t have enough astronomers to cope with this.”
The team then applied these neural networks to data derived from the Kilo-Degree Survey (KiDS). This project relies on the VLT Survey Telescope (VST) at the ESO’s Paranal Observatory in Chile to map 1500 square degrees of the southern night sky. This data set consisted of 21,789 color images collected by the VST’s OmegaCAM, a multiband instrument developed by a consortium of European scientist in conjunction with the ESO.
These images all contained examples of Luminous Red Galaxies (LRGs), three of which wee known to be gravitational lenses. Initially, the neural network found 761 gravitational lens candidates within this sample. After inspecting these candidates visually, the team was able to narrow the list down to 56 lenses. These still need to be confirmed by space telescopes in the future, but the results were quite positive.
As they indicate in their study, such a neural network, when applied to larger data sets, could reveal hundreds or even thousands of new lenses:
“A conservative estimate based on our results shows that with our proposed method it should be possible to find ?100 massive LRG-galaxy lenses at z ~> 0.4 in KiDS when completed. In the most optimistic scenario this number can grow considerably (to maximally ? 2400 lenses), when widening the colour-magnitude selection and training the CNN to recognize smaller image-separation lens systems.”
In addition, the neural network rediscovered two of the known lenses in the data set, but missed the third one. However, this was due to the fact that this lens was particularly small and the neural network was not trained to detect lenses of this size. In the future, the researchers hope to correct for this by training their neural network to notice smaller lenses and rejects false positives.
But of course, the ultimate goal here is to remove the need for visual inspection entirely. In so doing, astronomers would be freed up from having to do grunt work, and could dedicate more time towards the process of discovery. In much the same way, machine learning algorithms could be used to search through astronomical data for signals of gravitational waves and exoplanets.
Much like how other industries are seeking to make sense out of terabytes of consumer or other types of “big data”, the field astrophysics and cosmology could come to rely on artificial intelligence to find the patterns in a Universe of raw data. And the payoff is likely to be nothing less than an accelerated process of discovery.
The observable Universe is an extremely big place, measuring an estimated 91 billion light-years in diameter. As a result, astronomers are forced to rely on powerful instruments to see faraway objects. But even these are sometimes limited, and must be paired with a technique known as gravitational lensing. This involves relying on a large distribution of matter (a galaxy or star) to magnify the light coming from a distant object.
Using this technique, an international team led by researchers from the California Institute of Technology’s (Caltech) Owens Valley Radio Observatory (OVRO) were able to observe jets of hot gas spewing from a supermassive black hole in a distant galaxy (known as PKS 1413 + 135). The discovery provided the best view to date of the types of hot gas that are often detected coming from the centers of supermassive black holes (SMBH).
The research findings were described in twostudies that were published in the August 15th issue of The Astrophysical Journal. Both were led by Harish Vedantham, a Caltech Millikan Postdoctoral Scholar, and were part of an international project led by Anthony Readhead – the Robinson Professor of Astronomy, Emeritus, and director of the OVRO.
This OVRO project has been active since 2008, conducting twice-weekly observations of some 1,800 active SMBHs and their respective galaxies using its 40-meter telescope. These observations have been conducted in support of NASA’s Fermi Gamma-ray Space Telescope, which has be conducting similar studies of these galaxies and their SMBHs during the same period.
As the team indicated in their two studies, these observations have provided new insight into the clumps of matter that are periodically ejected from supermassive black holes, as well as opening up new possibilities for gravitational lensing research. As Dr. Vedantham indicated in a recent Caltech press statement:
“We have known about the existence of these clumps of material streaming along black hole jets, and that they move close to the speed of light, but not much is known about their internal structure or how they are launched. With lensing systems like this one, we can see the clumps closer to the central engine of the black hole and in much more detail than before.”
While all large galaxies are believed to have an SMBH at the center of their galaxy, not all have jets of hot gas accompanying them. The presence of such jets are associated with what is known as an Active Galactic Nucleus (AGN), a compact region at the center of a galaxy that is especially bright in many wavelengths – including radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray radiation.
These jets are the result of material that is being pulled towards an SMBH, some of which ends up being ejected in the form of hot gas. Material in these streams travels at close to the speed of light, and the streams are active for periods ranging from 1 to 10 million years. Whereas most of the time, the jets are relatively consistent, every few years, they spit out additional clumps of hot matter.
Back in 2010, the OVRO researchers noticed that PKS 1413 + 135’s radio emissions had brightened, faded and then brightened again over the course of a year. In 2015, they noticed the same behavior and conducted a detailed analysis. After ruling out other possible explanations, they concluded that the overall brightening was likely caused by two high-speed clumps of material being ejected from the black hole.
These clumps traveled along the jet and became magnified when they passed behind the gravitational lens they were using for their observations. This discovery was quite fortuitous, and was the result of many years of astronomical study. As Timothy Pearson, a senior research scientist at Caltech and a co-author on the paper, explained:
“It has taken observations of a huge number of galaxies to find this one object with the symmetrical dips in brightness that point to the presence of a gravitational lens. We are now looking hard at all our other data to try to find similar objects that can give a magnified view of galactic nuclei.”
What was also exciting about the international team’s observations was the nature of the “lens” they used. In the past, scientists have relied on massive lenses (i.e. entire galaxies) or micro lenses that consisted of single stars. However, the team led by Dr. Vedantham and Dr. Readhead relied on an what they describe as a “milli-lens” of about 10,000 solar masses.
This could be the first study in history that relied on an intermediate-sized lens, which they believe is most likely a star cluster. One of the advantages of a milli-sized lens is that it is not large enough to block out the entire source of light, making it easier to spot smaller objects. With this new gravitational lensing system, it is estimated that astronomers will be able to observe clumps at scales about 100 times smaller than before. As Readhead explained:
“The clumps we’re seeing are very close to the central black hole and are tiny – only a few light-days across. We think these tiny components moving at close to the speed of light are being magnified by a gravitational lens in the foreground spiral galaxy. This provides exquisite resolution of a millionth of a second of arc, which is equivalent to viewing a grain of salt on the moon from Earth.”
What’s more, the researchers indicate that the lens itself is of scientific interest, for the simple reason that not much is known about objects in this mass range. This potential star cluster could therefore act as a sort of laboratory, giving researchers a chance to study gravitational milli-lensing while also providing a clear view of the nuclear jets streaming from active galactic nuclei.
Looking ahead, the team hopes to confirm the results of their studies using another technique known as Very-Long Baseline Interferometry (VLBI). This will involve radio telescopes from around the world taking detailed images of PKS 1413 + 135 and the SMBH at its center. Given what they have observed so far, it is likely that this SMBH will spit out another clump of matter in a few years time (by 2020).
Vedantham, Readhead and their colleagues plan to be ready for this event. Spotting this next clump would not only validate their recent studies, it would also validate the milli-lens technique they used to conduct their observations. As Readhead indicated, “We couldn’t do studies like these without a university observatory like the Owens Valley Radio Observatory, where we have the time to dedicate a large telescope exclusively to a single program.”
Everybody knows that galaxies are enormous collections of stars. A single galaxy can contain hundreds of billions of them. But there is a type of galaxy that has no stars. That’s right: zero stars.
These galaxies are called Dark Galaxies, or Dark Matter Galaxies. And rather than consisting of stars, they consist mostly of Dark Matter. Theory predicts that there should be many of these Dwarf Dark Galaxies in the halo around ‘regular’ galaxies, but finding them has been difficult.
Now, in a new paper to be published in the Astrophysical Journal, Yashar Hezaveh at Stanford University in California, and his team of colleagues, announce the discovery of one such object. The team used enhanced capabilities of the Atacamas Large Millimeter Array to examine an Einstein ring, so named because Einstein’s Theory of General Relativity predicted the phenomenon long before one was observed.
An Einstein Ring is when the massive gravity of a close object distorts the light from a much more distant object. They operate much like the lens in a telescope, or even a pair of eye-glasses. The mass of the glass in the lens directs incoming light in such a way that distant objects are enlarged.
Einstein Rings and gravitational lensing allow astronomers to study extremely distant objects, by looking at them through a lens of gravity. But they also allow astronomers to learn more about the galaxy that is acting as the lens, which is what happened in this case.
If a glass lens had tiny water spots on it, those spots would add a tiny amount of distortion to the image. That’s what happened in this case, except rather than microscopic water drops on a lens, the distortions were caused by tiny Dwarf Galaxies consisting of Dark Matter. “We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects,” explained Hezaveh. The difference is that water distorts light by refraction, whereas matter distorts light by gravity.
As the ALMA facility increased its resolution, astronomers studied different astronomical objects to test its capabilities. One of these objects was SDP81, the gravitational lens in the above image. As they examined the more distant galaxy being lensed by SDP81, they discovered smaller distortions in the ring of the distant galaxy. Hezaveh and his team conclude that these distortions signal the presence of a Dwarf Dark Galaxy.
But why does this all matter? Because there is a problem in the Universe, or at least in our understanding of it; a problem of missing mass.
Our understanding of the formation of the structure of the Universe is pretty solid, at least in the larger scale. Predictions based on this model agree with observations of the Cosmic Microwave Background (CMB) and galaxy clustering. But our understanding breaks down somewhat when it comes to the smaller scale structure of the Universe.
One example of our lack of understanding in this area is what’s known as the Missing Satellite Problem. Theory predicts that there should be a large population of what are called sub-halo objects in the halo of dark matter surrounding galaxies. These objects can range from things as large as the Magellanic Clouds down to much smaller objects. In observations of the Local Group, there is a pronounced deficit of these objects, to the tune of a factor of 10, when compared to theoretical predictions.
Because we haven’t found them, one of two things needs to happen: either we get better at finding them, or we modify our theory. But it seems a little too soon to modify our theories of the structure of the Universe because we haven’t found something that, by its very nature, is hard to find. That’s why this announcement is so important.
The observation and identification of one of these Dwarf Dark Galaxies should open the door to more. Once more are found, we can start to build a model of their population and distribution. So if in the future more of these Dwarf Dark Galaxies are found, it will gradually confirm our over-arching understanding of the formation and structure of the Universe. And it’ll mean we’re on the right track when it comes to understanding Dark Matter’s role in the Universe. If we can’t find them, and the one bound to the halo of SDP81 turns out to be an anomaly, then it’s back to the drawing board, theoretically.
It took a lot of horsepower to detect the Dwarf Dark Galaxy bound to SDP81. Einstein Rings like SDP81 have to have enormous mass in order to exert a gravitational lensing effect, while Dwarf Dark Galaxies are tiny in comparison. It’s a classic ‘needle in a haystack’ problem, and Hezaveh and his team needed massive computing power to analyze the data from ALMA.
ALMA, and the methodology developed by Hezaveh and team will hopefully shed more light on Dwarf Dark Galaxies in the future. The team thinks that ALMA has great potential to discover more of these halo objects, which should in turn improve our understanding of the structure of the Universe. As they say in the conclusion of their paper, “… ALMA observations have the potential to significantly advance our understanding of the abundance of dark matter substructure.”