Gravitational Forensics: Astronomers Discover a Distant Galaxy in the Frontier Fields

The first Hubble Frontier Fields observations of a galaxy cluster and adjacent parallel field are complete, and interesting results are starting to arrive from astronomers. In this post, we explore how astronomers used the tools available to them to piece together the discovery of a very distant galaxy.

The Discovery

A team of international astronomers, led by Adi Zitrin of the California Institute of Technology in Pasadena, Calif., have discovered a very distant galaxy observed to be multiply lensed by the foreground Abell 2744 galaxy cluster. The light from this distant galaxy was distorted into three images and magnified via gravitational lensing of Abell 2744. This magnification provided the astronomers with a means to detect the incredibly faint galaxy with Hubble.

Astronomers are interested in finding these very distant galaxies because they represent an early stage of galaxy formation that occurred just after the Big Bang. Light from this galaxy has been traveling for quite some time. We are seeing this galaxy as it existed when the universe was only about 500 million years old. For context, the current age of the universe is around 13.8 billion years old.

Like visitors to a nursery, astronomers can see this baby galaxy is much smaller than present-day adult galaxies. In fact, they measure it to be about 500 times smaller than our own Milky Way galaxy. This baby galaxy is estimated to be forming new stars at a rate of one star every three years. That is about 1/3 the current rate of star formation of our own Milky Way, but keep in mind that this infant galaxy is much smaller than the present-day Milky Way. This baby galaxy is not just small but also a lightweight. It has the mass, in stars, of only about 40 million suns. Compare that to the Milky Way, which has a mass of several hundred billion suns. It is also one of the intrinsically faintest distant galaxies ever discovered.

The three lensed images of the baby galaxy are highlighted in the composite image below.

Credit: NASA, ESA, A. Zitrin (California Institute of Technology, Pasadena), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (Space Telescope Science Institute, Baltimore, Md.) Shown is the discovery of a high redshift galaxy candidate, triply lensed by Abell 2744. The high redshift galaxy candidate's lensed images are labeled as a, b, and c.

Credit: NASA, ESA, A. Zitrin (California Institute of Technology, Pasadena), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (Space Telescope Science Institute, Baltimore, Md.) Shown is the discovery of a very distant galaxy, triply lensed by the foreground galaxy cluster Abell 2744. The distant galaxy’s lensed images are labeled as a, b, and c.

This is now one of only a small handful — about 10 — of galaxies we have discovered at such great distances. The way the team discovered this distant galaxy is, perhaps, as interesting as the galaxy itself. The team of astronomers used a traditional color-based method for determining that the galaxy is a candidate for being a distant, baby galaxy. They then followed up with a pioneering new technique to confirm the distance via the geometry of gravitational lensing.

Using Colors to Find Candidate Distant Galaxies

Why do we think that the galaxy is very far away? Astronomers used Hubble’s filters to capture the light from this baby galaxy in several different colors. The intensity of light coming from the galaxy at different colors can give an estimate of the galaxy’s cosmological redshift. Cosmological redshift, commonly denoted by the letter “z,” is a number that signifies how reddened a galaxy is due to the expansion of space. A distance can be estimated once a cosmological redshift is measured. Larger cosmological redshifts correspond to larger distances.

Adi Zitrin and his collaborators initially found the distant galaxy (labeled “a” in the figure above) by noticing that it remained when they were looking for only the reddest galaxies. Remember, a galaxy may appear red if its light is redshifted due to the expansion of the universe. The farther the galaxy, the longer its light has to traverse the expanding universe, getting more and more stretched (redshifted) along the way. Astronomers are particularly interested in finding a population of galaxies with large cosmological redshifts — values of z around 10 or greater — because they represent some of the earliest galaxies to form after the Big Bang.

From the colors of the galaxy found in box ‘a,’ the team estimated that the galaxy has a redshift greater than 4, with 95% confidence. In fact, the colors of the galaxy in box ‘a’ highly favored a galaxy around z=10, but they could not discount that what they were measuring was an intrinsically red galaxy at a lower redshift, around a z=2. How do we sort this out?

Deciphering the Geometry of Abell 2744′s Gravitational Lens

Astronomers can do better, and these astronomers have shown that with knowledge of how mass is distributed in the foreground galaxy cluster, it is possible to distinguish between higher redshift and lower redshift background galaxies. Thus, with updated maps of the mass distribution of the Abell 2744 galaxy cluster, astronomers created more precise mathematical models of how light from a more distant galaxy behaves as it passes around the galaxy cluster’s warped space.

The geometry of a gravitational lens is such that the more distant a background galaxy behind the galaxy cluster, the farther from the center of the galaxy cluster we observe the distorted and magnified, lensed versions of the galaxy. This is portrayed in the graphic below, where two lensed versions of the more distant, highly redshifted, red galaxy appears on the sky at larger apparent distances from the central, foreground, lensing galaxy cluster.

Credit: Courtesy of Dr. Dan Coe (STScI). Shown here is an illustration of how the multiple lensing of a background galaxy will show its maximum magnification depending on its distance to the foreground galaxy cluster. More distant galaxies will be lensed such that we observe them further from the center of the galaxy cluster.

Credit: Dan Coe (STScI). Shown here is an illustration of how the multiple lensing of a background galaxy will show its maximum magnification depending on its distance to the foreground galaxy cluster. More distant galaxies will be lensed such that we observe them farther from the center of the galaxy cluster.

Astronomers can use the computed geometry of gravitational lensing to ascertain the cosmological redshift of the lensed galaxy based on its observed positions relative to the foreground galaxy cluster. If multiple images of the lensed galaxy appear nearby the cluster, it is at a lower redshift. If the multiple images of the lensed galaxy appear more separated from the cluster, it is at a larger redshift.

Finding the Multiple Images of a Distant Lensed Galaxy

With the updated mathematical models of the gravitational lensing by Abell 2744, Adi Zitrin and his team could follow up and look for multiply lensed images of the one potentially distant galaxy they had found, labeled “a”  in the image at top. The mathematical models give them positions on the sky to look for the lensed siblings of galaxy ‘a’ for various redshifts. If the distant galaxy is at a relatively low redshift, multiply lensed images will appear nearer the cluster. If the distant galaxy is at a high redshift, multiply lensed images will appear farther from the cluster.

With the computational tools and mathematical knowledge available to them, the team discovered the lensed versions of galaxy “a” at positions that match a high-redshift solution. In the figure below, they marked the locations of the lensed images, labeled “B” and “C”, along with their best mathematical estimates of redshift for each of them (labeled along the blue- and green-colored redshift lines). What is labeled as the initially discovered candidate galaxy “a” in the image at top is now labeled as “A” in the image below.

Credit: Adi Zitrin et al. 2014. Shown here are the expected positions of the three lensed versions of the newly discovered high redshift galaxy candidate, based on mathematical models of the gravitational lensing from Abell 2744. Galaxy lens A, B, and C are all in positions that match high redshift solutions in the models, i.e. redshifts of around 8 or greater.

Credit: Modified from Adi Zitrin et al., ApJ, 793 (2014). Shown here are the expected positions of the three lensed versions of the newly discovered high-redshift galaxy candidate, based on mathematical models of the gravitational lensing from Abell 2744. The multiply-lensed positions of the galaxy, labeled “A”, “B”, and “C,” match the high-redshift solution in the models, i.e., redshifts of around 8 or greater.

This is but a taste of how astronomers will use the Frontier Fields to combine exquisite imaging with updated mathematical models to detect and study some of the first galaxies to form after the Big Bang. We are just at the beginning of collecting the baby pictures of galaxies in our universe. Stay tuned as we detect more baby galaxies from the dawn of time!

Looking to the Future

The galaxy presented here is one of the least luminous high-redshift galaxies ever detected. This bodes very well for finding future baby galaxies in the Frontier Fields. We also expect that studies of the galaxy clusters themselves, via the new data in the Frontier Fields, will lead to more accurate mass distribution maps and more accurate mathematical models of how light from distant galaxies are gravitationally lensed and magnified.

This really is a new age in using humankind’s most sophisticated telescopes with nature’s lenses to probe deeper into our cosmic past than ever before. Stay tuned for more results from the Frontier Fields.

 

Light Detectives: Using Color to Estimate Distance

Distances are notoriously difficult to measure in astronomy. Astronomers use many methods for estimating distances, but the farther away an object is, the more uncertain the results. Cosmological distances, distances on the largest scales of our universe, are the most difficult to estimate. To measure the distances to the farthest galaxies, those gravitationally lensed by massive foreground galaxy clusters, astronomers really have their work cut out for them.

If a massive stellar explosion, known as a supernova, happens to go off in a galaxy and we catch it, then we can use the “standard candle” method of computing the distance to the galaxy. Supernovae are expected to be discovered in the Frontier Fields, but not at the numbers that will help us find distances to most of the galaxies in the images. Without these standard candles, astronomers must use other means to estimate distances.

A Spectrum is Worth a Thousand Pictures

One of the more accurate methods for measuring the distance to a distant galaxy involves obtaining a spectrum of the galaxy. Getting a galaxy’s spectrum basically means taking the light from that galaxy and breaking it up into its component colors, much like a prism breaks up white light into the rainbow of visible colors. By comparing the brightness of light at each component color, a spectrum can give us a wealth of information. This can include detailed information about a galaxy’s composition, temperature, and how fast it is moving relative to us. Because the universe is expanding, we observe most galaxies, and all distant galaxies, to be moving away from us.

When looking at a distant galaxy’s spectrum, the expansion of the universe causes the component colors in the spectrum to be stretched to longer wavelengths. For visible light, red has the longest wavelengths, which leads to the term ‘redshift’. This cosmological redshift can be accurately measured from a spectrum. Astronomers then use mathematical models of the expansion rate of our universe to convert the measured redshift into an estimate of distance. Larger values of redshift correspond to larger distances.

This video, developed by the Office of Public Outreach at the Space Telescope Science Institute, gives a demonstration of how light is redshifted as it travels through the expanding universe. Here, the lightbulb stands in place of a galaxy. As the universe expands, it stretches the light traveling through the universe, increasing the light’s wavelength. As the wavelength increases, it becomes more red. Light traveling longer distances through the universe will be stretched/reddened more than light traveling short distances. This is why astronomers use instruments sensitive to redder light, including infrared light, when they attempt to observe the light from very distant galaxies. Watch this video on Youtube.

Larger redshifts not only correspond to larger distances, but they also correspond to earlier times in our universe’s history. This is because light takes time to travel to us from these distant galaxies. The more distant the galaxy, the longer the light has been traveling before we intercept it with sensitive telescopes, like Hubble.

Assuming typical contemporary mathematical models, the universe is about 13.8 billion years old. Galaxies at a redshift of 1 are seen as they existed when the universe was about 6 billion years old.  Galaxies at a redshift of 3 are seen as they existed when the universe was about 2 billion years old. Galaxies at a redshift of 6 are seen as they existed when the universe was about 1 billion years old.  Galaxies at a redshift of 10 are seen as they existed when the universe was only about 500 million years old.

It is notoriously difficult to obtain a spectrum of a very distant galaxy.  They are very faint, and an accurate spectrum relies on obtaining a lot of light.  One is, after all, taking what little light you get and breaking it up further into the component colors, meaning that you start with little light and get out even less light at each component color.  Getting enough light to take an accurate spectrum of a distant galaxy requires very lengthy observations with sensitive telescopes.  This is not always feasible.

Redshifts measured via spectra are called spectroscopic redshifts. Many of the nearer galaxies in Abell 2744 have measured spectroscopic redshifts. There will likely be many follow-up observations from ground- and space-based observatories to obtain spectra of many of the fainter and more distant galaxies in the Frontier Fields. So stay tuned!

I Can’t Obtain a Spectrum!  What to do?

If you do not have a spectrum, are there other ways to estimate the redshift and distance to a galaxy?  Yes!  Just take a look at the galaxy’s colors.

All Hubble images are taken with filters. Blue filters allow Hubble’s instruments to capture only blue light, red filters allow Hubble’s instruments to capture only red light, and so on. By comparing a galaxy’s brightnesses in these different colors, astronomers can estimate the distance to the galaxy. The redder the color, the more likely the galaxy is to be redshifted, and thus, farther away.

This technique of using color to estimate redshift is called photometric redshift. The following two primary methods are used for estimating a photometric redshift:

  1. compare the colors of your high-redshift galaxy candidate to a set of typical galaxy color templates at various redshifts, or
  2. compare the colors of your high-redshift galaxy candidate to a set of galaxies with measured spectroscopic redshifts and, utilizing specialized software, compute the most likely redshift for your galaxy.

In the first case, the photometric redshift comes from the best match between the observed high-redshift candidate colors and the colors of the template galaxies. The template galaxy colors stem from observations of galaxies that tend to be relatively close but are then mathematically reddened over a range of redshift values.

In the second case, astronomers use a set of observed galaxies whose redshifts have been measured spectroscopically, as explained in the prior section. This set contains galaxies at various redshifts. They then use machine-learning algorithms to compare the colors of this set of galaxies with the colors of the target high-redshift galaxy candidate. The software selects the most likely redshift.

Whichever method is used, astronomers are careful to give confidence levels in their calculations. For the computation of photometric redshift, there is typically an uncertainty of around a few percent for high-quality data. In addition, there is the lingering issue of whether the high-redshift galaxy candidate is truly redshifted, or if it is a nearer galaxy that is intrinsically redder. It is not uncommon to read results where astronomers find a galaxy with a probable high photometric redshift and a less probable low photometric redshift, or vice versa.

Zitrin_etal_2014_Abell2744_Models_3_v2

Credit: Adapted from Adi Zitrin, et al., ApJ, 793 (2014). Shown is a high-redshift galaxy candidate in Hubble’s observations of Abell 2744, discovered using filters. Dark regions represent light in these images. Notice how the galaxy drops out of the image in the bluest filters. This is a hint that the galaxy may be significantly redshifted.

Many of the first results for the Frontier Fields utilize photometric redshifts. In the absence of spectra, photometric redshifts are the next best thing to obtaining estimates of distances for large samples of galaxies. They are readily computed from the current Frontier Fields data.

First Galaxy Field Complete: Abell 2744

This past summer, the Hubble Frontier Fields team completed observations of the first cluster on its list: Abell 2744!  The second set of observations — astronomers call them epochs — consisted of 70 orbits and marks the completion of the first Frontier Fields galaxy cluster. During this set, Hubble’s Advanced Camera for Surveys (ACS) was pointed at the main galaxy cluster and studied the visible-light portions of the spectrum, while the Wide Field Camera 3 (WFC3) looked at the parallel field in the infrared.

Remember that Hubble will visit each field multiple times, with Hubble oriented such that one set of observations will point WFC3 at the cluster and ACS at a parallel field adjacent to the cluster (that’s one epoch).   The telescope will then come back and do another set of observations with the cameras switched: ACS pointing at the cluster and WFC3 pointing to the parallel field (that’s the second one).

The Frontier Fields team does this to allow for complete wavelength coverage in both infrared and visible light for the galaxy cluster and the parallel field.

The first epoch, completed in November 2013, consisted of  87 orbits.  This brings the total amount of time Hubble looked at this cluster to 157 orbits.

Here’s the result.  This is the galaxy cluster Abell 2744:

Final mosaic of the Frontier Fields galaxy cluster Abell 2744.  This image is the culmination of both epochs totaling 157 Hubble orbits. The numbers prefixed with "F" are the Hubble filters used by the ACS and WFC3 cameras to take the image.  The scale bar of 30" is approximately 2% the angular size of the full moon as seen from Earth - very small! Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Final mosaic of the Frontier Fields galaxy cluster Abell 2744. This image is the culmination of both epochs totaling 157 Hubble orbits. The numbers prefixed with “F” are the Hubble filters used by the ACS and WFC3 cameras to take the image. The scale bar of 30″ is approximately 2% the angular size of the full moon as seen from Earth – very small!
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

And here is the parallel field:

Parallel field of Frontier Field Abell 2744

This is the completed composite mosaic of the Parallel Fields observed with galaxy cluster Abell 2744.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

See? Epic! Er, I mean epoch.

Once the second epoch was completed, some of the faintest galaxies ever seen were measured for the first time.  Astronomers have been working on these images since their release, and we are anxiously awaiting to hear what they find.

What is Dark Energy?

There is a dark side to the universe; in fact, most of what makes up our cosmos is dark. In our post, entitled “What Is Dark Matter?” we introduced this pie chart that shows the relative composition of everything in the universe.

121236_NewPieCharts720

Composition of matter in the universe. These numbers have been revised by results from the Planck mission. More info here. Credit: NASA/ESA

This deceptively simple diagram shows the percentages of everything the universe is made of. Embedded in this uncomplicated, straightforward pie chart is a story full of surprises and anxiety.

Measuring the Universe

With the exception of Einstein’s “biggest blunder,” few prior to the 1990′s had any expectation that a cosmological force, such as dark energy, even existed. It was thought that the universe was solely comprised of normal matter and dark matter. There was much debate on the nature of dark matter.  How much is there? How much is made of exotic undiscovered particles versus the more mundane but visibly dark stuff like planets, small stars, etc.?  Much has been learned, but dark matter is still largely a mystery today.  Theories and experiments abound to find all constituents of the missing dark matter, particularly the exotic variety that does not contain normal matter, i.e., those particles that do not interact with normal matter other than via gravitational force.

Dark matter and normal matter both have one thing in common: gravity. Thus, the expectation for astronomers was that they would observe some decrease in the expansion rate of the universe over time due to the pull of gravity from all of the matter in the universe. In the 1990′s, two groups of astronomers attempted to measure the deceleration rate of the universe independently by looking at a whole bunch of Type 1a supernovae.  Type 1a supernovae are the explosions of a certain type of star, where the explosions themselves all have the same intrinsic brightness. You can determine how far away the star is by how bright it appears to us; the dimmer a Type 1a supernova appears, the farther away it must be.  Just like the equivalent of a standard 60–watt lightbulb, finding these “standard candles” allows astronomers to accurately measure the distances, and thus the time in the past, where these explosions took place.

Click here for more information on how Type 1a supernovae were used to measure distances.

What the astronomers actually discovered was far more surprising, and it was important that two different groups did this because, if only one had done it, no one would have believed what actually happened. These teams of astronomers noticed that distant supernovae, whose light from the early epochs of the universe was just now reaching our telescopes, were fainter and thus farther away than expected. In 1998, these two groups both declared that the universe wasn’t decelerating at all – it was accelerating!

This was a completely unexpected result — no one saw it coming. I mean, the universe is full of normal matter and dark matter, all gravitationally pulling on each other as the universe expands. Shouldn’t that mean the universe is slowing down its expansion? One could hear hyperventilating cosmologists from across the globe.

After everyone started to calm down, astronomers began to ask, “OK, so what does it take to have an accelerating universe?”

The answer is, you need something else besides matter. Whatever that is, we call it dark energy.

But What Does Dark Energy Mean?

After the initial surprise of finding an accelerating universe wore off and people started thinking about it, astronomers did something they rarely do — they accepted the idea rather quickly. Usually, an unexpected result like this generates huge debates among scientists, and this did too. The thing is, the notion of a cosmological force like dark energy now solved a lot more problems than it created. In an uncharacteristically short period of time, people started warming to the idea of dark energy.

As a function of time, galaxies are moving away from us at a faster and faster rate, and that is what is meant by an accelerating universe. The discovery of dark energy has brought the ultimate fate of our universe back into question. Will dark energy continue to increase its dominance over gravity and cause our universe to rip apart — a potential fate known as the Big Rip? Or will the repulsive force of dark energy and the attractive force of gravity balance out so that the universe expands forever at a constant, non-accelerating rate? With the current understanding of dark energy, it seems improbable that gravity will reverse the expansion and collapse the universe back in on itself. However, the nature of dark energy is not well understood yet.

 

What’s Next for Dark Energy?

Right now, astronomers are making observations designed to constrain some of the many dark energy models that are out there.  The nature of this research is often done from the ground so that wide areas of the sky can be observed for a very long time. This kind of campaign is not well-suited to a high-demand telescope like the Hubble Space Telescope. The idea is to “constrain,” or better understand, the expansion rate of the universe, and measure the growth of large–scale structure (like galaxy clusters).

Past surveys like the Sloan Digital Sky Survey have made some progress, and current projects like the Dark Energy Survey (DES) has started its observing runs. DES will observe 5,000 square degrees of the night sky over 525 nights, making measurements that should help us whittle down some of the many dark–energy models presently being considered. Currently being built is the Large Synoptic Survey Telescope, an 8.4–meter ground-based telescope in Chile, which will image the entire sky every few nights at several wavelengths, and will no doubt play a large role in helping us understand dark energy.

Space-based telescopes do have an essential role to play in characterizing dark energy. For example, Hubble has played a key role in getting data on distant supernovae — hence the discovery of dark energy. It is the combination of ground-based large surveys with space-based pointed deep follow-ups that give us our breakthroughs. Future missions are being envisioned to build on the best of both ground-based surveys and space-based observations. The Wide-Field Infrared Survey Telescope (WFIRST) will use a Hubble-class, space-based telescope to survey a large portion of the sky in an effort to better constrain the nature of dark energy through the history of the universe.

Frontier Fields and Dark Energy

While the Frontier Fields were not designed to capture the large numbers of supernovae needed to explore dark energy through cosmic time, the observations of strong galaxy cluster lensing will be used in combination with cosmological measurements from other missions to help constrain the nature of dark energy.  Stay tuned for more!

 

James Edwin Webb: Turning Imagination into Reality

by Holly Ryer and Ann Jenkins

The Frontier Fields program peers into the universe’s distant past, yet it also offers a glimpse of the future work that the powerful James Webb Space Telescope will conduct. Webb, known as Hubble’s successor, will use infrared vision to detect galaxies beyond even Hubble’s reach.

But the man for whom the Webb telescope is named is not commonly linked to space science. James Edwin Webb (1906–1992) wasn’t a scientist or engineer; he was a businessman, attorney, and manager. Still, many believe that this second administrator of NASA, who ran the fledgling agency from 1961 to 1968, did more to advance science and space exploration than perhaps any other government official. He laid the foundations at NASA for one of the most successful periods of astronomical discovery, one that continues today.

James Edwin Webb, the second administrator of NASA, was a staunch champion of space exploration. Photo credit: NASA.

James Webb was born in Granville County, N.C. He completed his college education at the University of North Carolina at Chapel Hill, where he received a degree in education. Webb then became a second lieutenant in the United States Marine Corps and served as a Marine Corps pilot. Afterward, he studied law at the George Washington University Law School in Washington, D.C. and was admitted to the Bar of the District of Columbia in 1936.

Webb’s long career in public service included serving as director of the Bureau of Budget and Under Secretary of State under President Harry Truman. In 1961, when he was selected by President John Kennedy to serve as the NASA administrator, Webb was reluctant to take the job. He assumed that it might be better handled by someone with a firmer grasp of science or technology. However, Kennedy wanted a leader with keen political insight and management skills for the position.

Webb oversaw great progress in the Space Program while serving as NASA’s administrator. During his tenure, NASA developed robotic spacecraft, which explored the lunar environment so that astronauts could do so later. On his watch, NASA also sent scientific probes to Mars and Venus. By the time Webb retired, NASA had launched more than 75 space science missions to study the stars and galaxies, our own Sun and the as-yet-unknown environment of space above the Earth’s atmosphere.

Webb also weathered the turmoil of the 1967 Apollo 1 tragedy, in which three astronauts—“Gus” Grissom, Edward White, and Roger Chaffee—died in a flash fire during a simulation test on the launch pad at Kennedy Space Center in Florida. Firmly committed to getting NASA back on its feet after this terrible setback, Webb strove to maintain support for the program. His success helped to pave the way to future NASA triumphs, such as the historic Apollo moon landing, which took place shortly after his retirement from NASA in 1968.

Webb remained in Washington, D.C., where he served on several advisory boards and as a regent of the Smithsonian Institution. In 1981, he was awarded the Sylvanus Thayer Award by the United States Military Academy at West Point for his dedication to his country. Former NASA Administrator Sean O’Keefe said of Webb: “He took our nation on its first voyages of exploration, turning our imagination into reality.”

Edwin Hubble Expands Our View of the Universe

by Donna Weaver and Ann Jenkins

American astronomer Edwin Powell Hubble (1889–1953) never lived to see the development or launch of his namesake, the Hubble Space Telescope. But like the telescope that bears his name, Dr. Hubble played a crucial role in advancing the field of astronomy and changing the way we view the universe. As Hubble’s namesake is breaking new ground in the exploration of the distant universe via the Frontier Fields, let us take a step back and learn more about Hubble, the man.

This is an illustration of Dr. Edwin Powell Hubble.

Edwin Hubble is regarded as one of the most important observational cosmologists of the 20th century. Illustration credit: Kathy Cordes of STScI.

As a young boy, Edwin Hubble read tales of traveling to undersea cities, journeying to the center of the Earth, and trekking to the remote mountains of South Africa. These stories by adventure novelists Jules Verne and H. Rider Haggard stoked young Hubble’s imagination of faraway places. He fulfilled those childhood dreams as an astronomer, exploring distant galaxies with a telescope and developing celestial theories that revolutionized astronomy.

But Hubble didn’t settle immediately on the astronomy profession. He studied law as a Rhodes Scholar at Queens College in Oxford, England. A year after passing the bar exam, Hubble realized that his love of exploring the stars was greater than his attraction to law, so he abandoned law for astronomy. “I chucked the law for astronomy and I knew that, even if I were second rate or third rate, it was astronomy that mattered,” Hubble said. (1)

Our Galaxy Is Not Alone

He studied astronomy at the University of Chicago and completed his doctoral thesis in 1917. After serving in World War I, he began working at the Mount Wilson Observatory near Pasadena, Calif., studying the faint patches of luminous “fog” or nebulae — the Latin word for clouds — in the night sky. Hubble and other astronomers were puzzled by these gas clouds and wanted to know what they were.

Using the 100-inch reflecting Hooker Telescope — the largest telescope of its day — Hubble peered beyond our Milky Way Galaxy to study an object known then as the Andromeda Nebula. He discovered special, “variable stars” on the outskirts of the nebula that changed in brightness over time. These stars brightened and dimmed in a predictable way that allowed Hubble to determine their distances from Earth. Hubble showed that the distance to the nebula was so great that it had to be outside the Milky Way Galaxy. Hubble realized that the Andromeda Nebula was a separate galaxy much like our own. The discovery of the Andromeda Galaxy helped change our understanding of the universe by proving the existence of other galaxies.

Hubble also devised the classification system for galaxies, grouping them by sizes and shapes, that astronomers still use today. He also obtained extensive evidence that the laws of physics outside our galaxy are the same as on Earth, verifying the principle of the uniformity of nature.

The Expanding Universe

As Hubble continued his study, he made another startling discovery: The universe is expanding. In 1929 he determined that the more distant the galaxy is from Earth, the faster it appears to move away. Known as Hubble’s Law, this discovery is the foundation of the Big Bang theory. The theory says that the universe began after a huge cosmic explosion and has been expanding ever since. Hubble’s discovery is considered one of the greatest triumphs of 20th-century astronomy.

Albert Einstein could have foretold Hubble’s discovery in 1917 when he applied his newly developed General Theory of Relativity to the universe. His theory — that space is curved by gravity — predicted that the universe could not be static but had to expand or contract. Einstein found this prediction so unbelievable that he modified his original theory to avoid the problem. Upon learning of Hubble’s discovery, Einstein immediately regretted revising his theory.

For his many contributions to astronomy, Hubble is regarded as one of the most important observational cosmologists of the 20th century.

(1) As quoted by Nicholas U. Mayall (1970). Biographical memoir. Volume 41, Memoirs of the National Academy of Sciences, National Academy of Sciences (U.S.). National Academy of Sciences. p. 179.

Frontier Fields Public Lecture

Want to hear about the Frontier Fields project straight from the scientist? On August 5, 2014, principal investigator Dr. Jennifer Lotz gave a public lecture entitled “The Frontier Fields: a Sneak Peek at the First Billion Years of the Universe” and the recorded webcast is available at the link below.

Dr. Jennifer Lotz
The Frontier Fields: a Sneak Peek at the First Billion Years of the Universe

August 5, 2014

How we far can we go? What are the faintest objects the Hubble Space Telescope can possibly see? Can we get a sneak peek at the early universe before the James Webb Space Telescope is launched? These are the key questions we hope to answer with the Frontier Fields campaign. Over this three year program, astronomers at the Space Telescope Science Institute will attempt to push the Hubble Space Telescope’s capabilities to its limits. This ambitious effort will combine the power of Hubble with the natural gravitational telescopes of massive clusters of galaxies that magnify more distant galaxies. Hubble will obtain the deepest ever optical and infrared images of six massive clusters, in parallel with the deep images of six neighboring “blank” fields. These observations will reveal galaxies about 10-20 times fainter than any previously seen, allowing astronomers to study the birth of galaxies like our own Milky Way.

https://webcast.stsci.edu/webcast/detail.xhtml?talkid=4287

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/

 

Seeing Double (or More!) in Frontier Fields Images

The immense gravity in this foreground galaxy cluster, Abell 2744, warps space to brighten and magnify images of far-more-distant background galaxies as they looked over 12 billion years ago, not long after the big bang.  This is the first of the Frontier Fields to be imaged.

Galaxy cluster Abell 2744, the first of the Frontier Fields to be imaged.

Take a long look at this image. You’re seeing a lot of distant galaxies magnified by the natural “gravitational lens” of galaxy cluster Abell 2744. But you aren’t seeing as many as you think.

Gravitational lenses, natural magnifiers created in space when light is bent by the enormous mass of galaxy clusters, distort and enlarge the images of distant galaxies behind the cluster. But they do more than that: sometimes they replicate them, like multiple images in a funhouse mirror.

abell multiple

Galaxy cluster Abell 2744, with multiple images of individual galaxies marked. These multiple images are produced by the cluster’s gravitational lens.

In the above image, we’ve marked the galaxies that are actually images of the same galaxy by overlaying them with numbered triangles. Each galaxy has a number. The multiple images are identified by letters. The galaxies labeled 1a, 1b, and 1 c, for instance, are one galaxy, its image repeated three times. (Only numbers and letters are significant. The colors don’t represent anything, but are used to make it easier to distinguish the various numbered galaxies.)

In previous posts, we explained that mass distorts space. Light from a distant galaxy follows space’s curve like a ball rolling along a putting green. (Think of space as a miniature golf course with fewer animatronic dinosaurs.)

Sometimes, the level of distortion sends the light to multiple places. If you’ve ever seen a single candle reflected multiple times in the bottom of a wineglass, you’ve seen this distorting effect occur in lenses. In fact, gravitational lensing is so similar to glass lensing that you could replicate the distortions of a gravitational lens by grinding a glass lens to the same proportions and bumps.

And cosmic lenses are quite lumpy. The galaxies of the cluster, embedded in halos of dark matter, create bumps of mass. Light can take multiple paths around the galaxy cluster as it encounters the distortions in space-time created by the cluster’s mass. The closer the light of more-distant galaxies passes to the lens, the stronger the deflection. If the light passes close enough to the lens, these multiple images are likely to appear. The individual galaxies in the cluster also add small deflections, and occasionally help produce multiple paths for the light to reach us.

When astronomers look at a lensed image, they’re looking at a giant puzzle. They need to figure out where all the mass is in the image – most of it, being dark matter, is invisible. Pinpointing the multiple images of identical galaxies helps accomplish this because they’re a good indicator of how dramatically the light is being deflected.

Abell2744-multilens-1+markers Abell2744-multilens-3+markers

Some of the multiple images are obvious. Galaxy images 1a, 1b and 1c (left image) are good examples – they’re blue galaxies with red centers, and they look very like one another. The green-hued galaxy identified by 3a, 3b and 3c (right image) is another good example. Astronomers seek out those obvious candidates to start with, then try to build a model of how the mass in the cluster is distributed. Based on that model, they start identifying the multiple images that aren’t so obvious: Does this reddish galaxy to the side have a counterpart where the model says it should be? Analysis of attributes like color, and especially distance, also play an important role in determining which galaxies are multiples — a technique that comes in handy in many situations.

Thanks to reader Judy Schmidt for the idea for this post.

 

How Hubble Observations Are Scheduled

This is the third in a three-part series.

After observing time is awarded, the Institute creates a long-range plan. This plan ensures that the diverse collection of observations are scheduled as efficiently as possible. This task is complicated because the telescope cannot be pointed too close to bright objects like the Sun, the Moon, and the sunlit side of Earth. Adding to the difficulty, most astronomical targets can only be seen during certain months of the year; some instruments cannot operate in the high space-radiation areas of Hubble ’s orbit; and the instruments regularly need to be calibrated. These diverse constraints on observations make telescope scheduling a complex optimization problem that Institute staff are continually solving, revising, and improving.”

Preparing for an observation also involves selecting guide stars to stabilize.the telescope’s pointing and center the target in the instrument’s field of view. The selection is done automatically by the Institute’s computers, which choose two stars per pointing from a catalog of almost a billion stars. These guide stars will be precisely positioned within the telescope’s fine guidance sensors, ensuring that the target region and orientation of the sky is observed by the desired instrument.”]

A weekly, short-term schedule is created from the long-range plan. This schedule is translated into detailed instructions for both the telescope and its instruments to perform the observations and calibrations for the week. From this information, daily command loads are then sent from the Institute to NASA’s Goddard Space Flight Center to be uplinked to Hubble.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md.  In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md. In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

The journey from proposal through selection and scheduling culminates in the email informing astronomers that their data is ready to be accessed. Usually, the process takes more than a year from idea to data—sometimes even longer. Of course, that’s when the real work begins—the analysis of the data and the hard work of uncovering another breakthrough Hubble discovery!