The Hunt for Jellyfish Galaxies in the Frontier Fields

Jellyfish galaxies, exotic galaxies with “tentacles” made of stars and gas, appear as though they are swimming through space. So far, astronomers studying the Frontier Fields have found several of these strange galaxies, and they are currently combing through the mountains of data to find even more.

Sometimes also known as “parachute galaxies” or “comet galaxies,” jellyfish galaxies form when spiral galaxies collide with galaxy clusters. When the cold gas from an approaching spiral hits the hot gas from a galaxy cluster, the stars continue on, but the collision blasts the cold gas out of the galaxy in trailing tails, or “tentacles.” Bursts of stars form in these streamers, sparked by the shock of cold gas hitting hot gas. The tentacles, with their knots of newborn stars, trace the path of the colliding, compressed gas. Eventually, these jellyfish galaxies are thought to settle into elliptical galaxies.

Three examples of jellyfish galaxies in the Frontier Fields. In each image, the telltale, trailing “tentacles” of stars and gas are present. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Some examples of jellyfish galaxies in the Frontier Fields. In each image, note the telltale, trailing “tentacles” of stars and gas. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Jellyfish galaxies are sometimes also seen in less massive groups of galaxies. Their characteristic shape is, however, usually much more pronounced for spirals falling into massive galaxy clusters, because the gas they encounter there is denser, and because they move faster due to the stronger gravitational pull of the cluster. The higher speed results in a more energetic collision that, in turn, increases the pressure that strips the infalling galaxy of its cold gas and triggers widespread star formation.

Astronomers have studied similar interactions in detail in nearby galaxy clusters but do not fully understand the much more violent process that creates jellyfish galaxies in very massive clusters. If the cold galactic gas is stripped very quickly these collisions could be the primary way by which spiral galaxies are transformed into ellipticals. Unfortunately, because the phenomenon is over so quickly, it is very difficult to observe. One expert on jellyfish galaxies—Dr. Harald Ebeling of the Institute for Astronomy at the University of Hawaii—explains that this is why astronomers are looking at extremely massive clusters, such as those in the Frontier Fields, in their search for a large sample of these galaxies.

Aside from helping to explain why elliptical galaxies are so common in the universe, jellyfish galaxies capture the process of galaxy/gas collisions in action. Their trailing, star-forming tentacles may also explain the presence of “orphan” stars that do not belong to any galaxy.

The work to uncover the secrets of the Frontier Fields goes on. Stay tuned for more exciting news on jellyfish galaxies and other oddities as scientists continue to study the vast amount of data collected in the Frontier Fields.

A Deep View Down Broadway

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

[Note: this blog post also appears on the Hubble’s Universe Unfiltered blog.]

One of the more philosophical concepts that astronomers have to deal with on an everyday basis is the commingling of space and time in astronomical images.

The underlying idea is straightforward. The speed of light is finite. Light from a star or nebula or galaxy takes a measurable amount of time to cross the space between it and us. Hence, the light we see now left that object at some previous time. We view astronomical objects as they were in the past. As I like to say, looking out in space is also looking back in time.

The implications of this maxim are considerable, especially in dealing with the deep field images from Hubble (see the accompanying image of the Abell 2744 Parallel deep field). Such images contain a wonderful assortment of galaxies, with a few stars here and there. Each object is at a different position in space, both in the two-dimensional sense of a different position within the image and in the three-dimensional sense of being at a different distance from Earth. Further, objects at different distances are seen at different times in the past. Hence, astronomers must examine these deep field images in four-dimensional space-time.

Tackling the expanse of space and time in these images can be mind-boggling. We’ll start with the stars, which are easier to understand. All the stars are local, within our Milky Way galaxy. These stars are generally hundreds to thousands of light-years away. The light we observe today might have left the star while the pyramids of Egypt were being built. Because stars don’t change appreciably on scales of thousands of years, stars in deep fields are just like stars in other astronomical images.

The galaxies, however, stretch much farther into space. The nearest are many millions of light-years away, while the most distant are around ten billion light-years away. Galaxies don’t change much on million-year timescales. For example, it takes over 200 million years for our Sun to orbit once within our galaxy. Even though the light may have left a galaxy when dinosaurs first started to dominate our planet, the same galaxy would look similar today. Thus, the nearby galaxies in these images are comparable to local galaxies.

Given billions of years, however, galaxies do change, and these deep field images provide compelling evidence. Distant galaxies do not have the standard spiral and elliptical shapes. They are often elongated, have bright spots of star formation, and are much smaller in size. We see galaxies as they were before the Sun, Earth, and the solar system formed. We study the development of galaxies over time to see how they form and grow. The perplexing point is that, for any given galaxy in the image, there is no distinct visual indicator of its distance in space or time. The layers of the universe are jumbled together across the image, and it is a grand puzzle of cosmology to sort them out.

The usual method to determine distances, and therefore times, is to measure the cosmological redshift of each galaxy. That concept has been discussed in a Frontier Fields blog post by Dr. Brandon Lawton: “Light Detectives: Using Color to Estimate Distance”. Thus, I’d like to take this essay in a different direction.

The Manhattan Deep Field

When discussing the cosmic mixture of space-time with an artist visiting from Spain, I happened upon a novel idea for a human-centric analogy.

Imagine you are in New York City, specifically Times Square in Manhattan. You look down Broadway to the southern end of the island about 4 miles away. If the speed of light were extremely slow, traveling only one mile per century, what would you see?

Each mile down Broadway would represent one hundred years of New York’s history. Each block would be 5 to 10 years earlier in the development of the metropolis.

A quarter of a mile away, the southern end of the theater district would appear as it did in the early 1990s when “Miss Saigon” came to Broadway. Only a few blocks farther would be the disco era and the civil unrest of the 1960s, then the World War II years and the Great Depression.

The Empire State Building, about a mile away, would vanish, as it was not built until 1931. At a similar distance, Madison Square Garden would be seen hosting heavyweight boxing matches in its original building, before the demolition and re-construction in the late 1920s.

Progressing another mile down Broadway to Union Square would travel back past the Civil War, Tammany Hall politics, economic growth fostered by the Erie Canal, and Alexander Hamilton’s original run on the New York stage.

The mile beyond to the SoHo district progresses through the times of New York as the capital of the United States, the Revolutionary War, the founding of Columbia University, and the importation of slaves by the Dutch West Indies Company.

The final mile to Battery Park leads through the colonial era alternately dominated by Dutch or English foreign powers, past the garrison of Fort Amsterdam, to the island’s Native American roots and the initial explorations by Henry Hudson.

A “slow speed of light” view from Times Square would lay out the entire history of the city of New York in a single view. The commingling of space and time would make it the historian’s exceptional equivalent of the astronomer’s standard observation: a deep view down Broadway.

This idea of a time-warped view of New York provides an analogy to what Hubble uncovers: the history of galaxies compressed and jumbled within each deep field. Perhaps it can help you to look at these images from that requisite four-dimensional perspective. These deep field images are truly a trip down memory lane.

Beyond the Frontier Fields: How JWST Will Push the Science to a New Frontier

The Frontier Fields Project has been an ambitious campaign to see deep into our universe. Gravitational lensing, as used by the Frontier Fields Project, enables Hubble to see fainter and more-distant galaxies than would otherwise be possible. These images push to the very limits of how deeply Hubble can see out into space.

Hubble, Spitzer, Chandra, and other observatories are doing cutting-edge science through the Frontier Fields Project, but there’s a challenge. Even though leveraging gravitational lensing has allowed astronomers to see objects that otherwise could not be detected with today’s telescopes, the technique still isn’t enough to see the most distant galaxies. As the universe expands, light gets stretched into longer and longer wavelengths, beyond the visible and near-infrared wavelengths Hubble can detect. To see the most distant galaxies, one needs a space telescope with Hubble’s keen resolution, but at infrared wavelengths.

That infrared telescope is the James Webb Space Telescope, slated to launch in October 2018. It has a mirror 6.5 meters (21 feet) across, can observe wavelengths up to 10 times longer than Hubble can observe, and is the mission that will detect and study the first appearances of galaxies in the universe.full_jwst_hst_mirror_comparison

Figure 1: Webb will have a 6.5-meter-diameter primary mirror, which would give it a significant larger collecting area than the mirrors available on the current generation of space telescopes. Hubble’s mirror is a much smaller 2.4 meters in diameter, and its corresponding collecting area is 4.5 square meters, giving Webb around seven times more collecting area! Webb’s field of view is more than 15 times larger than the NICMOS near-infrared camera on Hubble. It also will have significantly better spatial resolution than is available with the infrared Spitzer Space Telescope. Credit: NASA. http://webbtelescope.org/gallery

Observations of the early universe are still incomplete. To build the full cosmological history of our universe, we need to see how the first stars and galaxies formed, and how those galaxies evolved into the grand structures we see today.

 

Looking back in time to the first light in the universe:

Astronomers use light to explore the universe, but there are pieces of our universe’s early history where there wasn’t much light. The era of the universe called the “Dark Ages” is as mysterious as its name implies. Shortly after the Big Bang, our universe was filled with glowing plasma, or ionized gas. As the universe cooled and expanded, electrons and protons began to bind together to form neutral hydrogen atoms (one proton and one electron each). The last of the light from the Big Bang escaped (becoming what we now detect as the Cosmic Microwave Background). The universe would have been a dark place, with no sources of light to reveal this cooling, neutral hydrogen gas.

Some of that gas would have begun coalescing into dense clumps, pulled together by gravity. As these clumps grew larger, they would become stars and eventually galaxies. Slowly, starlight would begin to shine in the universe. Eventually, as the early stars grew in numbers and brightness, they would have emitted enough ultraviolet light to “reionize” the universe by stripping electrons off neutral hydrogen atoms, leaving behind individual protons. This process created a hot plasma of free electrons and protons. At this point, the light from star and galaxy formation could travel freely across space and illuminate the universe. It is important to note here, astronomers are currently unsure whether the energy responsible for reionization came from stars in the early-forming galaxies; rather, it might have come from hot gas surrounding massive black holes or some even more exotic source such as decaying dark matter.

The universe’s first stars, believed to be 30 to 300 times as massive as our Sun and millions of times as bright, would have burned for only a few million years before dying in tremendous explosions, or “supernovae.” These explosions spewed the recently manufactured chemical elements of stars outward into the universe before the expiring stars collapsed into black holes.

Astronomers know the universe became reionized because when they look back at quasars — incredibly bright objects thought to be powered by supermassive black holes — in the distant universe, they don’t see the dimming of their light that would occur if the light passed through a fog of neutral hydrogen gas. While they find clouds of neutral hydrogen gas, they see almost no signs of neutral hydrogen gas in the matter located in the space between galaxies. This means that at some point the matter was reionized. Exactly when this occurred is one of the questions Webb will help answer, by looking for glimpses of very distant objects still dimmed by neutral hydrogen gas.

Much remains to be uncovered about the time of reionization. The universe right after the Big Bang would have consisted of hydrogen, helium, and a small amount of lithium. But the stars we see today also contain heavier elements — elements that are created inside stars. So how did those first stars form from such limited ingredients? Webb may not be able to see the very first stars of the Dark Ages, but it’ll witness the generation of stars immediately following, and analyze the kinds of materials they contain.

Webb’s ability to see the infrared light from the most distant objects in the universe will allow it to truly identify the sources that gave rise to reionization. For the first time, we will be able to see the stars and quasars that unleashed enough energy to illuminate the universe again.

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Figure 2: JWST will be able to see back to when the first bright objects (stars and galaxies) were forming in the early universe. Credit: STScI. http://jwst.nasa.gov/firstlight.html

 

Early Galaxies:

Webb will also show us how early galaxies formed from those first clumps of stars. Scientists suspect the black holes born from the explosions of the earliest stars (supernovae) devoured gas and stars around them, becoming the extremely bright objects called “mini-quasars.” The mini-quasars, in turn, may have grown and merged to become the huge black holes found in the centers of present-day galaxies. Webb will try to find and understand these supernovae and mini-quasars to put theories of early galaxy formation to the test. Do all early galaxies have these mini-quasars or only some? These regions give off infrared light as the gas around them cools, allowing Webb to glean information about how mini-quasars in the early universe work — how hot they are, for instance, and how dense.

Webb will show us whether the first galaxies formed along lines and webs of dark matter, as expected, and when. Right now we know the first galaxies formed anywhere from 378,000 years to 1 billion years after the Big Bang. Many models have been created to explain which era gave rise to galaxies, but Webb will pinpoint the precise time period.

Hubble is known for its deep-field images, which capture slices of the universe throughout time. But these images stop at the point beyond which Hubble’s vision cannot reach. Webb will fill in the gaps in these images, extending them back to the Dark Ages. Working together, Hubble and Webb will help us visualize much more of the universe than we ever have before, creating for us an unprecedented picture that stretches from the current day to the beginning of the recognizable universe.

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Figure 3: This illustration shows the cold side of the Webb telescope, where the mirrors and instruments are positioned. Credit: Northrop Grumman. http://webbtelescope.org/gallery

 

Resources:

https://frontierfields.org/2016/07/21/the-final-frontier-of-the-universe/

http://hubble25th.org/science/8

http://webbtelescope.org/article/13

The Final Frontier of the Universe

[Note: this article is cross-posted on the Hubble’s Universe Unfiltered blog.]

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Gravitational lensing in galaxy cluster Abell S1063 Credit: NASA, ESA, and J. Lotz (STScI)

Fifty years ago, in 1966, the Star Trek television series debuted. Given the incredible longevity of the franchise, it seems remarkable that the original television series only lasted three seasons.

Each episode famously began with the words “Space: the final frontier.” To me, those thoughts evoke an idea of staring into the night sky and yearning to know what is out there. They succinctly capture an innate desire for exploration, adventure, and understanding. Such passions are the same ones that drive astronomers to decipher the universe through science.

While Captain Kirk and company could make a physical voyage into interstellar space, our technology has (so far) only taken humans to the Moon and sent our probes across the solar system. For the rest of the cosmos, we must embark on an intellectual journey. Telescopes like Hubble are the vehicles that bring the universe to us.

To explore remote destinations, the Enterprise relied upon a faster-than-light warp drive. Astronomy, in turn, can take advantage of gravitational warps in space-time to boost the light of distant galaxies. Large clusters of galaxies are so massive that, under the dictates of general relativity, they warp the space around them. Light that travels through that warped space is redirected, distorted, and amplified by this “gravitational lensing.”

Gravitational lensing enables Hubble to see fainter and more-distant galaxies than would otherwise be possible. It is the essential “warp factor” that motivates the Frontier Fields project, one of the largest Hubble observation programs ever. The “frontier” in the name of the project reflects that these images will push to the very limits of how deeply Hubble can see out into space.

But is this the “final frontier” of astronomy? Not yet.

Abell S1063 Parallel Field - This deep galaxy image is of a random field located near the galaxy cluster Abell S1063. As part of the Frontier Fields Project, while one of Hubble's instruments was observing the cluster, another instrument observed this field in parallel. These deep fields provide invaluable images and statistics about galaxies stretching toward the edge of the observable universe.

Abell S1063 Parallel Field – This deep galaxy image is of a random field located near the galaxy cluster Abell S1063. As part of the Frontier Fields Project, while one of Hubble’s instruments was observing the cluster, another instrument observed this field in parallel. These deep fields provide invaluable images and statistics about galaxies stretching toward the edge of the observable universe.

The expanding universe stretches the light that travels across it. Light from very distant galaxies travels across the expanding universe for so long that it becomes stretched beyond the visible and near-infrared wavelengths Hubble can detect. To see the most distant galaxies, one needs a space telescope with Hubble’s keen resolution, but at infrared wavelengths.

In what may have been an homage to the Star Trek television series with Captain Picard, the project for such a telescope was originally called the “Next Generation Space Telescope.” Today we know it as the James Webb Space Telescope, and it is slated to launch in October 2018. Webb has a mirror 6.5 meters (21 feet) across, can observe wavelengths up to ten times longer than Hubble can observe, and is the mission that will detect and study the first appearances of galaxies in the universe.

In the Star Trek adventures, Federation starships explore our galaxy, and much of that only within our local quadrant. Astronomical observatories do the same for scientific studies of planets, stars, and nebulae in our Milky Way; and go beyond to galaxies across millions and billions of light-years of space. Telescopes like Hubble and Webb carry that investigation yet further, past giant clusters of galaxies, and out to the deepest reaches of the cosmos. With deference to Gene Roddenberry, one might say “Space telescopes: the final frontier of the universe.”

Telescopes Team up to View Cosmic Collisions

NASA’s Frontier Fields is a program to capture new deep-field images across the electromagnetic spectrum, from X-rays to infrared light.  NASA’s Great Observatories — the Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope — are taking the lead on this ambitious effort. Other observatories around the world, including the Jansky Very Large Array (JVLA) in New Mexico, which observes radio light, also contribute considerable time to observing the new deep fields.

A new Frontier Fields release from the Chandra X-ray Center highlights the energetic chaos that occurs when massive galaxy clusters collide. The two new images combine data from the Chandra X-ray Observatory, the Hubble Space Telescope, and the JVLA radio dishes. Astronomers are interested in understanding how merging galaxy clusters grow with time and what happens to the galaxies, their gas, and the associated, enigmatic dark matter.

The images of these galaxy clusters (MACS J0416 and MACS J0717) are described below. Read the full release from the Chandra X-ray Center here.

MACS J0416

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The galaxy cluster MACS J0416 seen in X-rays (blue), visible light (red, green, and blue), and radio light (pink), taken by the Chandra X-ray Observatory, Hubble Space Telescope, and Jansky Very Large Array, respectively.  Credit: X-ray: NASA/CXC/SAO/G. Ogrean et al.; Optical: NASA/STScI; Radio: NRAO/AUI/NSF.

The object known as MACS J0416 is actually composed of two clusters of galaxies that will eventually merge to create a single larger massive galaxy cluster.  The image of MACS J0416 contains Chandra X-ray data (blue), Hubble Space Telescope data (red, green, and blue), and a halo of radio light imaged by the JVLA (pink).

According to a paper published in The Astrophysical Journal, the cores of the two galaxy clusters have likely not passed through each other yet, indicating an early phase of their merger.

Astronomers discovered this by studying the cluster’s appearance in visible and X-ray light.  Hubble’s visible-light images show both the galaxies themselves and their gravitational lensing effects, helping us pinpoint the location of dark matter in the cluster. X-ray observations from Chandra show us the location of the heated gas. In MACS J0416, the galaxies and their dark matter are still lined up with the heated gas, meaning their merger has not progressed very far yet. In other observations of merging galaxy clusters, such as the Bullet Cluster, gas heated by the shock of collision eventually separates from the dark matter.

MACS J0717

Galaxy cluster MACS J0717.5+3745 with dark matter map

The galaxy cluster MACS J0717 seen in X-rays (blue), visible light (red, green, and blue), and radio light (pink), taken by the Chandra X-ray Observatory, Hubble Space Telescope, and Jansky Very Large Array, respectively.  Credit: X-ray: NASA/CXC/SAO/G. Ogrean et al.; Optical: NASA/STScI; Radio: NRAO/AUI/NSF.

The massive galaxy cluster MACS J0717 results from a merger of four clusters of galaxies. The image of MACS J0717 contains Chandra X-ray data (blue), Hubble Space Telescope data (red, green, and blue), and JVLA radio data (pink).  Unlike MACS J0416, MACS J0717 appears to have been merging for quite some time. The evidence of merging includes the separated knots of X-rays (blue) formed by the collision of high concentrations of gas, and the giant arcs of radio emission (pink) stretched and distorted by the merger.

MACS J0717 is also the largest known cosmic lens, and thus a prime candidate for observing distant objects magnified by gravitational lensing. The galaxy clusters in MACS J0717 are still merging and are not yet confined to a smaller area — leaving a large total mass over a relatively large area of the sky. This large gravitational lens can magnify and uncover galaxies of the early universe, a key goal of the Frontier Fields project.

Often, observations of these distant, young galaxies only capture the brightest objects. But observations of MACS J0717 demonstrate how Frontier Fields can be used to view some of the universe’s more ordinary early galaxies. In a paper published in The Astrophysical Journal, astronomers discovered seven gravitationally lensed radio sources in MACS J0717. Many of these galaxies would not be observable without the benefit of magnification due to gravitational lensing. The gravitational lensing of massive clusters in radio waves provides a new view of these radio sources, which are thought to be common — but not well-studied — star-forming galaxies in the early universe.

Hubble has also observed distant galaxies using gravitational lensing.  An example is noted here. By using a combination of telescopes, and a combination of different wavelength observations, the Frontier Fields project is providing a broader and deeper view into the galaxies of the early universe.

The Whirlpool Galaxy Seen Through a Cosmic Lens

The Frontier Fields images, while beautiful, aren’t all that easy to comprehend to eyes outside the astronomy community. Look at them and you see streaks of light and blurry smudges mixed into a field of obvious galaxies. It can be difficult to interpret the distortions that occur as light from distant galaxies becomes magnified and bent by the vast mass of the Frontier Fields’ galactic clusters.

So here’s an interesting thought experiment. What if we could take a well-known galaxy and put it behind one of our Frontier Fields galaxy clusters? What would that look like?

Thanks to Dr. Rachael Livermore of the University of Texas at Austin and Dr. Frank Summers of the Space Telescope Science Institute, you can see for yourself. In this video simulation, the Whirlpool Galaxy, also known as M51, sweeps behind the Frontier Fields galaxy cluster Abell 2744. As it moves, the gravity of the galaxy cluster distorts the light of the Whirlpool, warping and magnifying and even multiplying its image.

Obviously, this isn’t a realistic video — galaxies don’t just take jaunts through the cosmos. But it illustrates how our image of the Whirlpool would change depending on where it was placed behind the galaxy cluster. Livermore used the Whirlpool Galaxy for this video because it’s a well-known, popular Hubble image, easily recognizable through the distortions that happen at different locations in the lensing cluster.

Take a look. After the intro, the image on the left of the dotted line shows the location of the Whirlpool behind the cluster, while the image on the right shows the lensing distortion underway.

In this simulation, we’ve moved the Whirlpool to a distance astronomers refer to as redshift 2. That far back, it would be so distant that the light we’re seeing from it would have started traveling away from the galaxy when the universe was just a quarter of its current age. If the Whirlpool were that far away in real life, its light would take 10 billion years to reach Earth.

Note that this isn’t how the Whirlpool would really appear at that distance. At such a distance, all we would be able to make out is the vivid central bulge of stars. But for the purpose of this illustration, the whole galaxy has been kept artificially bright.

The most impressive distortions occur as the Whirlpool passes behind the center of the galaxy cluster, with multiple, stretched, distorted images of the galaxy appearing. At this point, the light of the Whirlpool beaming toward Earth bends to go around the cluster, but can go either left or right. There’s no preference, so some of it goes one way, and some goes another, and we get many images of the same galaxy.

This location is ideal for astronomers, because as you can see in this illustration, the images become both stretched and magnified, allowing the galaxy structure to be seen in greater detail. Furthermore, because a gravitational lens acts much as a telescope lens, more light is focused our way, making the galaxies brighter.

This, Livermore notes, is a primary reason why astronomers are interested in these galaxy clusters – the chance to see the distant background galaxies in so much greater detail than Hubble would be able to produce on its own.

 

 

 

 

 

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A Century Later, General Relativity is Still Making Waves

[Note: this article is cross-posted on the Hubble’s Universe Unfiltered blog.]

In November 1915, Albert Einstein published a series of papers that laid out the ideas, equations, and some astronomical applications of the general theory of relativity. While Isaac Newton described gravity as a force between two massive bodies, Einstein’s general relativity re-interprets gravity as a geometric distortion of space and time (see my previous blog post “Einstein’s Crazy Idea” ).

One example cited in those papers was that general relativity can explain the extra precession of Mercury’s orbit that Newton’s formulation does not explain. Another prediction, the bending of light as it passes a massive object, was tested and shown accurate less than four years later. This effect, called gravitational lensing has been shown in tremendous detail by the Hubble Space Telescope (see my previous blog post “Visual “Proof” of General Relativity“), and is one of the prime motivations behind the Frontier Fields project.

Last year, scientists celebrated the centennial of general relativity. The theory has been a resounding success in diverse astronomical situations. However, there was one major prediction that had not yet been tested: gravitational waves.

General relativity predicts that mass not only can create distortions in space-time, but also can create waves of those distortions propagating across space-time. In cosmology, the global expansion of space over time is a familiar concept. For a gravitational wave, space also stretches / shrinks, but that localized distortion moves across space at the speed of light.

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The Laser Interferometer Gravitational-Wave Observatory (LIGO) is one of the projects designed to observe the minute distortions of gravitational waves. It consists of two detectors, one in Hanford, WA, and one in Livingston, LA. Each detector has two perpendicular arms, consisting of ultra-high-vacuum chambers four kilometers (two and a half miles) in length.

For the experiment, a laser light source is split and sent down and back each arm. By measuring how the laser light signals interfere with each other when recombined, extremely precise measurements of any change in distances can be made. The idea is that when a gravitational wave passes by, the minuscule stretch of one arm and shrink of the other will be observable.

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The signal observed in the LIGO event GW150914

On September 14, 2015, both LIGO detectors observed an event (see the accompanying image). The pattern in the signal indicates that a series of gravitational waves passed through the detectors in about two-tenths of a second. It is extremely important that multiple detectors saw the same event so that local disturbances can be ruled out. Plus, the time delay between the detectors helps measure the speed of the waves.

To analyze the event, the LIGO team used computer simulations. The shape and duration of the event waveform matched that expected for the merger of two black holes. The amplitude of the detection helped determine how far away the black-hole merger took place. The best fit is a merger of a 36-solar-mass black hole with a 29-solar-mass black hole to form a 62-solar-mass black hole, about 1.3 billion light-years away.

The energetics of the merger are simply astounding.Recognizing that 36 + 29 = 65, one can see that three solar masses of material did not end up in the resulting black hole. Instead, it was converted in the energy that created the gravitational wave. Released in less than half a second, the peak wattage of the event was greater than the visible light wattage from all the stars in the observable universe.

And yet, when detected on Earth, the measured space distortion was smaller than the size of a proton. The reason it took a century to find gravitational waves is because one has to measure subatomic displacements. Gravity is demonstrably the weakest of the four fundamental forces. It takes a tremendous amount of energy to produces a gravitational wave that can be seen at cosmic distances.

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There are several major results from this observation. The detection shows, for the first time, that both black-hole mergers and gravitational waves exist. The time delay between detectors, and analysis of the signal at different frequencies, demonstrates that gravitational waves travel at the speed of light. All the results are consistent with the predictions of general relativity.

This event marks the beginning of gravitational-wave astronomy. With more detectors coming online and planned improvements to current detectors, the field is burgeoning. Dozens to thousands of black-hole or neutron-star mergers, with more detail about each event, should be found in the next decade.

More than a billion years ago, two black holes merged in a distant galaxy, emitted a tremendous amount of energy, and created a gravitational ripple moving across space. Recently, the LIGO project detected this almost infinitesimal motion of space; a deviation much smaller than the size of an atom. With that amazing observation, the last major prediction of general relativity was verified. A century later, Einstein still rules.

How Hubble “Sees” Gravity

[Note: this post is cross-posted on the Hubble’s Universe Unfiltered blog.]

Gravity is the familiar force of nature responsible for the diverse motions of a baseball thrown high into the air, a planet orbiting a star, or a star orbiting within a galaxy. Astronomers have long observed such motions and deduced the amount of gravity, and therefore the amount of matter, present in the planet, star, or galaxy. When taken to the extreme, gravity can also create some intriguing visual effects that are well suited to Hubble’s high-resolution observations.

Einstein’s general theory of relativity expresses how very large mass concentrations distort the space around them. Light passing through that distorted space is re-directed, and can produce a variety of interesting imagery. The bending of light by gravity is similar to the bending of light by a glass lens, hence we call this effect “gravitational lensing”.

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An “Einstein Cross” gravitational lens.

The simplest type of gravitational lensing is called “point source” lensing. There is a single concentration of matter at the center, such as the dense core of a galaxy. The light of a distant galaxy is re-directed around this core, often producing multiple images of the background galaxy (see the image above for an example). When the lensing approaches perfect symmetry, a complete or almost complete circle of light is produced, called an “Einstein ring”. Hubble observations have helped to greatly increase the number of Einstein rings known to astronomers.

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Gravitational lensing in galaxy cluster Abell 2218

More complex gravitational lensing arises in observations of massive clusters of galaxies. While the distribution of matter in a galaxy cluster generally does have a center, it is never perfectly circularly symmetric and is usually significantly lumpy. Background galaxies are lensed by the cluster with their images often appearing as short thin “lensed arcs” around the outskirts of the cluster. Hubble’s images of galaxy clusters, such as Abell 2218 (above) and Abell 1689, showed the large number and detailed distribution of these lensed images throughout massive galaxy clusters.

These lensed images also act as probes of the matter distribution in the galaxy cluster. Astronomers can measure the motions of the galaxies within a cluster to determine the total amount of matter in the cluster. The result indicates that the most of the matter in a galaxy cluster is not in the visible galaxies, does not emit light, and is thus called “dark matter”. The distribution of lensed images reflects the distribution of all matter, both visible and dark. Hence, Hubble’s images of gravitational lensing have been used to create maps of dark matter in galaxy clusters.

In turn, a map of the matter in a galaxy cluster helps provide better understanding and analysis of the gravitational lensed images. A model of the matter distribution can help identify multiple images of the same galaxy or be used to predict where the most distant galaxies are likely to appear in a galaxy cluster image. Astronomers work back and forth between the gravitational lenses and the cluster matter distribution to improve our understanding of both.

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Three lensed images of a distant galaxy seen through a cluster of galaxies.

On top of it all, gravitational lenses extend Hubble’s view deeper into the universe. Very distant galaxies are very faint. Gravitational lensing not only distorts the image of a background galaxy, it can also amplify its light. Looking through a lensing galaxy cluster, Hubble can see fainter and more distant galaxies than otherwise possible. The Frontier Fields project has examined multiple galaxy clusters, measured their lensing and matter distribution, and identified a collection of these most distant galaxies.

While the effects of normal gravity are measurable in the motions of objects, the effects of extreme gravity are visible in images of gravitational lensing. The diverse lensed images of crosses, rings, arcs, and more are both intriguing and informative. Gravitational lensing probes the distribution of matter in galaxies and clusters of galaxies, as well as enables observations of the distant universe. Hubble’s data will also provide a basis and guide for the future James Webb Space Telescope, whose infrared observations will push yet farther into the cosmos.

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A “smiley face” gravitational lens in a galaxy cluster.

The distorted imagery of gravitational lensing often is likened to the distorted reflections of funhouse mirrors, but don’t take that comparison too far. Hubble’s images of gravitational lensing provide a wide range of serious science.

‘Hubble’s Views of the Deep Universe’ – Public Lecture

On November 3, 2015, I gave a presentation called “Hubble’s Views of the Deep Universe”.  This presentation was to commemorate some of Hubble’s most influential observing campaigns during this 25th anniversary year.  Of course, I could not get to all of Hubble’s programs that observed the deep universe in just an hour.  For additional information, check out the science articles on the Hubble 25th website and, of course, keep checking back to this blog.

Dr. Brandon Lawton
“Hubble’s Views of the Deep Universe”

November 3, 2015

For two decades, the Hubble Space Telescope has pioneered the exploration of the distant universe with images known as the “deep fields”. These deep fields have given astronomers unprecedented access to understanding how galaxies form and develop over billions of years in the history of our universe, from shortly after the Big Bang to today. Join us for a retrospective view of Hubble’s contributions to the investigation of the deep reaches of the cosmos and some fresh glimpses of what Hubble is currently doing to further our understanding of the most distant parts of the universe.

This lecture is part of the monthly public lecture series at the Space Telescope Science Institute in Baltimore, Maryland. Each month addresses a different cosmic topic, usually related to Hubble, but always venturing to some fascinating part of the universe. For more information, check out the web page on HubbleSite:
http://hubblesite.org/about_us/public_talks/

Galaxy Shapes in the Frontier Fields Observations

We can learn a lot about galaxies by analyzing their light, through computer modeling, and using other complex techniques. But at the most basic level, we can learn about galaxies by studying their shapes.

Galaxy appearance immediately reveals certain characteristics. Elliptical galaxies contain a wealth of old stars. Spiral galaxies are full of gas and dust. Distorted galaxies have likely experienced a gravitational interaction with another galaxy that warped their structure.

The Mice, as these colliding galaxies are called, are a pair of spiral galaxies seen about 160 million years after their closest encounter. Gravity has drawn stars and gas out of the galaxies into long tails.  Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

The Mice, as these distorted colliding galaxies are called, are a pair of spiral galaxies seen about 160 million years after their closest encounter. Gravity has drawn stars and gas out of the galaxies into long tails. Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

The Frontier Fields project adds another dimension to this simple analysis. When we look at extremely distant galaxies with the magnification of gravitational lensing, we see new detail that was previously obscured by distance. Their shapes are clues to what occurred within those galaxies when they were very young.

Galaxies viewed through the gravitational lenses of the Frontier Fields clusters can be seen at a resolution 10 times greater than non-lensed galaxies. That means those tiny red dots that so thrill astronomers in normal Hubble images actually have some structure in Frontier Fields imagery.

Previous studies, such as the Hubble Ultra Deep Field, The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, or even adaptive optics-enhanced studies by ground telescopes have shown that young, star-forming galaxies at about a redshift of 2 (existing when the universe was about 3.3 billion years old) appear to have a certain lumpiness. But without gravitational lensing, we lack the resolution to say for sure whether those lumps were massive clusters of newly forming stars, or whether some other factor was causing those galaxies to have a clumpy appearance.

Frontier Fields has revealed that yes, many of those galaxies have star-forming knots that really are quite large, implying that star formation occurred in a very different way in the early universe, perhaps involving greater quantities of gas in those young galaxies than previously expected.

Frontier Fields has also given us a better grasp of the physical size of gravitationally lensed young galaxies even farther away, at a redshift of 9 (when the universe was around 500 million years old). Observations show that these galaxies are actually quite small – perhaps 200 parsecs across, while a typical galaxy you see today is closer to 10,000 parsecs across. These observations help plan future observations with the Webb Space Telescope, picking out what will hopefully be the best targets for study.

This composite image shows examples of galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. The galaxies are arranged according to time. Those on the left reside nearby; those at far right existed when the cosmos was about 2 billion years old. The Frontier Fields project is collecting galaxies from the earliest epochs of the universe to add to such comparisons. Credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team

This composite image shows examples of galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. The galaxies are arranged according to time. Those on the left reside nearby; those at far right existed when the cosmos was about 2 billion years old. The Frontier Fields project is collecting galaxies from the earliest epochs of the universe to add to such comparisons. Credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team

Galaxy shape also plays a role in discoveries in the Frontier Fields’ six parallel fields, which are unaffected by gravitational lensing but provide a view into space almost as deep as Hubble’s famous Ultra Deep Field, with three times the area.

It’s well known that galaxies collide and interact, drawn to one another by gravity. Most galaxies in the universe are thought to have gone through the merger process in the early universe, but the importance of this process is an open question. The transitional period during which galaxies are interacting and merging is relatively short, making it difficult to capture. A distant galaxy may appear clumpy and distorted, but is its appearance due to a merger – or is it just a clumpy galaxy?

Collision-related features — such as tails of stars and gas drawn out into space by gravity, or shells around elliptical galaxies that occur when stars get locked into certain orbits – are excellent indicators of merging galaxies but are hard to detect in distant galaxies with ordinary observations. Frontier Fields’ parallel fields are providing astronomers with a collection of faraway galaxies with these collision-related features, allowing astronomers to learn more about how these mergers affected the galaxies we see today.

As time goes on and the cluster and parallel Frontier Fields are explored in depth by astronomers, we expect to to learn much more about how galaxy evolution and galaxy shapes intertwine. New results are on the way.