Astronomy

How many Supernovae have been Imaged in the Hubble Ultra-Deep Field?

How many Supernovae have been Imaged in the Hubble Ultra-Deep Field?

The Hubble Ultra-Deep Field is an image showing about 10.000 galaxies:

2 Image Credit: NASA

Consider Victor Buso's spectacular discovery of an early supernova in NGC 613 (named SN 2016gkg):

4 Image Credit: Carnegie Institution for Science/Las Campanas Observatory/UC Santa Cruz

This got me wondering about how many supernovae the Hubble Ultra-Deep Field may contain without us knowing because we would need a light curve or reference image to identify one with certainty. Some supernovae also would be only a pixel or less and impossible to identify.

Is it possible to calculate a meaningful lower bound?


Supernovae hunters report finding about 1 supernova per 5000 images. Hunters will, naturally, focus on larger galaxies, as these are more likely to contain a supernova, whereas the deep field survey contains galaxies of all sizes, so perhaps about 1 supernova per 10000 galaxies imaged would be a reasonable estimate. Thus you might expect there to be about 1 supernova visible in this image, but as you note, if there is one, it may be too dim to be visible.

Supernovae are all one pixel or less (they are so small and distant that they appear as point-like sources of light), they only appear to be bigger due to atmospheric blurring or imperfections in optics and/or electronics.


Supernovae probably occur about once per century in our own (quite large) galaxy.

If we were to assume similar star formation rates and a similar stellar birth-mass distribution in other galaxies and that the galaxies in the HST-UDF were similar to our own, then we could also assume that the supernova rate was similar.

The UDF was taken over about 4 months, or 1/300 of a century. So one might expect a supernova to occur in about 1/300 galaxies - so about 300 of the 10,000 galaxies observed will have hosted a supernova during the period that the image was built up.

Of course, even if this figure is correct (see below), many of these may have been missed because the galaxies were too far away or the supernovae were obscured by dust, or because they brightened and faded between observations (there was a 6 week gap in the observations). On the other hand, the UDF was imaged later by HST with other instruments, leading to further opportunities to identify supernova candidates.

The calculated number could be awry in many ways. Most galaxies are smaller than the Milky Way, so one might expect fewer supernovae. On the other hand, most of these galaxies are far away and the star formation rate was likely higher in the past, leading to more supernovae. On the other hand, the supernova rate will be higher in some types of galaxies (Starbursts), but lower in others (ellipticals). Without a very detailed demography of the types, metallicity, masses and ages of the galaxies, and a detailed understanding of how the supernova rate varies with these parameters, your question is difficult to answer.


How Much Better Than Hubble Would A Roman Deep Field Image Be?

The scale of the Hubble Ultra Deep Field (blue box) versus the field of view of the Nancy Roman . [+] Telescope (orange boxes). Each of Roman's 18 independent viewing instruments is more than 10 times the field of view of the most famous deep Hubble image.

NASA, ESA, and A. Koekemoer (STScI) Acknowledgement: Digitized Sky Survey

NASA’s Hubble Space Telescope, launched in 1990, revealed the previously unseen Universe.

The most distant galaxy ever found: GN-z11, in the GOODS-N field as imaged deeply by Hubble. The . [+] same observations that Hubble made to obtain this image will give WFIRST/Nancy Roman more than 100 times the number of ultra-distant galaxies with the same exposure time`.

NASA, ESA, and P. Oesch (Yale University)

Its large aperture, excellent instrumentation, and location in space enabled ultra-distant views.

The original Hubble Deep Field, which discovered thousands of new galaxies in the abyss of deep . [+] space. This region was previously thought to be entirely devoid of galaxies with only a few faint Milky Way stars present. The original Deep Field found thousands of galaxies lurking in this region of sky.

R. Williams (STScI), the Hubble Deep Field Team and NASA

Iconically, Hubble’s deep field images best showcase its capabilities.

The full UV-visible-IR composite of the Hubble eXtreme Deep Field the greatest image ever released . [+] of the distant Universe. There are 5,500 galaxies identified in this tiny region of sky it would take 32 million of them to cover the

40,000 square degrees of what's present in the entire sky.

NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

By repeatedly pointing its “eye” on a single region, it compiles photons one-at-a-time from the distant Universe.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the . [+] total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal.

HUDF09 AND HXDF12 TEAMS / E. SIEGEL (PROCESSING)

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Through multi-wavelength observations, Hubble uncovered thousands of the Universe’s most distant objects.

Galaxies identified in the eXtreme Deep Field image can be broken up into nearby, distant, and . [+] ultra-distant components, with Hubble only revealing the galaxies it's capable of seeing in its wavelength ranges and at its optical limits. The dropoff in the number of galaxies seen at very great distances may indicate the limitations of our observatories, rather than the non-existence of faint, small, low-brightness galaxies at great distances.

NASA, ESA, AND Z. LEVAY, F. SUMMERS (STSCI)

Viewing larger regions, Hubble’s Frontier Fields campaign was also revolutionary.

The streaks and arcs present in Abell 370, a distant galaxy cluster some 5-6 billion light-years . [+] away, are some of the strongest evidence for gravitational lensing and dark matter that we have. The lensed galaxies are even more distant, with some of them making up the most distant galaxies ever seen. This imagery was part of the Hubble Frontier Fields program.

NASA, ESA/Hubble, HST Frontier Fields

Gravitation from distant, massive galaxy clusters magnifies and distorts light from background galaxies.

The galaxy cluster MACS 0416 from the Hubble Frontier Fields, with the mass shown in cyan and the . [+] magnification from lensing shown in magenta. That magenta-colored area is where the lensing magnification will be maximized. Mapping out the cluster mass allows us to identify which locations should be probed for the greatest magnifications and ultra-distant candidates of all.

STSCI/NASA/CATS TEAM/R. LIVERMORE (UT AUSTIN)

Even today, Hubble remains astronomy’s best space-based optical observatory.

HE0435-1223, located in the centre of this wide-field image, is among the five best lensed quasars . [+] discovered to date. The foreground galaxy creates four almost evenly distributed images of the distant quasar around it. Quasars are the most distant objects found in the observable Universe.

ESA/Hubble, NASA, Suyu et al.

Many often ask, “why don’t we just build another Hubble?”

The Hubble Space Telescope, as imaged during its last and final servicing mission. Although it . [+] hasn't been serviced in over a decade, Hubble continues to be humanity's flagship ultraviolet, optical, and near-infrared telescope in space, and has taken us beyond the limits of any other space-based or ground-based observatory.

For the same price, current technology enables superior options.

This is an illustration of the different elements in NASA’s exoplanet program, including . [+] ground-based observatories, like the W. M. Keck Observatory, and space-based observatories, like Hubble, Spitzer, Kepler, Transiting Exoplanet Survey Satellite, James Webb Space Telescope, Wide Field Infrared Survey Telescope (now the Nancy Roman telescope) and future missions. The power of TESS and James Webb combined will reveal the most Moon-like exomoons to date, possibly even in their star's habitable zone, while ground-based 30 meter telescopes, the Nancy Roman telescope (formerly WFIRST), and possibly a next-generation space-based observatory like LUVOIR or HabEx is required to truly find what humanity has been dreaming of for so long: an inhabited world outside of our Solar System.

NASA’s post-James Webb flagship will be the Nancy Grace Roman Telescope.

American astronomer Dr. Nancy Grace Roman, who was one of the first female executives at NASA, . [+] attends the Earth Day March for Science Rally on April 22, 2017. Nancy Grace Roman was NASA’s first chief astronomer, who paved the way for space telescopes focused on the broader universe. (Paul Morigi/Getty Images)

Formerly known as WFIRST, it’s similarly Hubble-sized, but with much wider fields-of-view.

NASA’s Wide Field Infrared Survey Telescope (WFIRST) is now named the Nancy Grace Roman Space . [+] Telescope, after NASA’s first Chief of Astronomy. It is designed to perform wide-field imaging and spectroscopy of the infrared sky. One of the Roman Space Telescope's objectives will be looking for clues about dark energy — the mysterious force that is accelerating the expansion of the universe. Another objective of the mission will be finding and studying exoplanets.

Roman could create images with Hubble-like depth, but spanning over 100 times Hubble’s viewing area.

The Hubble Ultra-Deep Field, shown in blue, is currently the largest, deepest long-exposure campaign . [+] undertaken by humanity. For the same amount of observing time, the Nancy Grace Roman Telescope will be able to image the orange area to the exact same depth, revealing over 100 times as many objects as are present in the comparable Hubble image.

NASA, ESA, and A. Koekemoer (STScI) Acknowledgement: Digitized Sky Survey

Instead of thousands of ultra-distant galaxies, a single deep-field campaign will uncover millions.

A small section of the original Hubble Deep Field, featuring hundreds of easily distinguishable . [+] galaxies. The original Hubble Deep Field may have only covered a tiny region of the sky, but taught us that there were at least hundreds of billions of galaxies contained within the observable Universe. Today, superior data and analysis has placed that figure closer to

2 trillion. The Nancy Roman telescope's field of view will be approximately

1000 times the area of this portion of a deep Hubble image.

R. Williams (STScI), the Hubble Deep Field Team and NASA

They will include the faintest, most distant, most active galaxies ever discovered.

Its Wide-Field Instrument might, upon launch, become astronomy’s greatest imager in history.

The main imaging camera for the Nancy Roman Telescope, the Wide Field Instrument (WFI), may . [+] immediately become the most advanced imaging instrument in history when Roman launches and deploys. Its 300-megapixel infrared camera will image, during its primary 5 year mission, 50 times the amount of sky that Hubble imaged during its entire 31 year lifetime.

NASA's Goddard Space Flight Center

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less smile more.


One Image, One Million Galaxies

One of the Hubble Space Telescope&rsquos most iconic images is the Hubble Ultra Deep Field, which unveiled myriad galaxies across the universe, stretching back to within a few hundred million years of the Big Bang. Hubble peered at a single patch of seemingly empty sky for hundreds of hours beginning in September 2003, and astronomers first unveiled this galaxy tapestry in 2004, with more observations in subsequent years.

NASA&rsquos upcoming Nancy Grace Roman Space Telescope will be able to photograph an area of the sky at least 100 times larger than Hubble with the same crisp sharpness. Among the many observations that will be enabled by this wide view on the cosmos, astronomers are considering the possibility and scientific potential of a Roman Space Telescope &ldquoultra-deep field.&rdquo Such an observation could reveal new insights into subjects ranging from star formation during the universe&rsquos youth to the way galaxies cluster together in space.

Roman will enable new science in all areas of astrophysics, from the solar system to the edge of the observable universe. Much of Roman&rsquos observing time will be dedicated to surveys over wide swaths of the sky. However, some observing time will also be available for the general astronomical community to request other projects. A Roman ultra deep field could greatly benefit the scientific community, say astronomers.

As an example, a Roman ultra-deep field could be similar to the Hubble Ultra Deep Field &ndash looking in a single direction for a few hundred hours to build up an extremely detailed image of very faint, distant objects. Yet while Hubble snagged thousands of galaxies this way, Roman would collect millions. As a result, it would enable new science and vastly improve our understanding of the universe.

One of the greatest challenges of the Roman mission will be learning how to analyze the abundance of scientific information in the public datasets that it will produce. In a sense, Roman will create new opportunities not only in terms of sky coverage but also in data mining.

A Roman ultra-deep field would contain information on millions of galaxies &ndash far too many to be studied by researchers one at a time. Machine learning &mdash a form of artificial intelligence &mdash will be needed to process the massive database. While this is a challenge, it also offers an opportunity.


Hubble Ultra Deep Field 2014

Astronomers using NASA's Hubble Space Telescope have assembled a comprehensive picture of the evolving universe – among the most colorful deep space images ever captured by the 24-year-old telescope.

Researchers say the image, from a new study called the Ultraviolet Coverage of the Hubble Ultra Deep Field, provides the missing link in star formation. The Hubble Ultra Deep Field 2014 image is a composite of separate exposures taken in 2002 to 2012 with Hubble's Advanced Camera for Surveys and Wide Field Camera 3.

Astronomers previously studied the Hubble Ultra Deep Field (HUDF) in visible and near-infrared light in a series of images captured from 2003 to 2009. The HUDF shows a small section of space in the southern-hemisphere constellation Fornax. Now, using ultraviolet light, astronomers have combined the full range of colors available to Hubble, stretching all the way from ultraviolet to near-infrared light. The resulting image – made from 841 orbits of telescope viewing time – contains approximately 10,000 galaxies, extending back in time to within a few hundred million years of the big bang.

Prior to the Ultraviolet Coverage of the Hubble Ultra Deep Field study of the universe, astronomers were in a curious position. Missions such as NASA's Galaxy Evolution Explorer (GALEX) observatory, which operated from 2003 to 2013, provided significant knowledge of star formation in nearby galaxies. Using Hubble's near-infrared capability, researchers also studied star birth in the most distant galaxies, which appear to us in their most primitive stages due to the significant amount of time required for the light of distant stars to travel into a visible range. But for the period in between, when most of the stars in the universe were born – a distance extending from about 5 billion to 10 billion light-years – they did not have enough data.

"The lack of information from ultraviolet light made studying galaxies in the HUDF like trying to understand the history of families without knowing about the grade-school children," said principal investigator Harry Teplitz of Caltech in Pasadena, California. "The addition of the ultraviolet fills in this missing range."

Ultraviolet light comes from the hottest, largest, and youngest stars. By observing at these wavelengths, researchers get a direct look at which galaxies are forming stars and where the stars are forming within those galaxies.

Studying the ultraviolet images of galaxies in this intermediate time period enables astronomers to understand how galaxies grew in size by forming small collections of very hot stars. Because Earth's atmosphere filters most ultraviolet light, this work can only be accomplished with a space-based telescope.

"Ultraviolet surveys like this one using the unique capability of Hubble are incredibly important in planning for NASA's James Webb Space Telescope," said team member Rogier Windhorst of Arizona State University in Tempe. "Hubble provides an invaluable ultraviolet-light dataset that researchers will need to combine with infrared data from Webb. This is the first really deep ultraviolet image to show the power of that combination."

Credits:NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)


Hubble finds most distant primeval galaxies

The NASA/ESA Hubble Space Telescope has broken the distance limit for galaxies and uncovered a primordial population of compact and ultra-blue galaxies that have never been seen before. The data from the Hubble’s new infrared camera, the Wide Field Camera 3 (WFC3), on the Ultra Deep Field (taken in August 2009) have been analysed by five international teams of astronomers. Some of these early results are being presented by various team members on 6 January 2010, at the 215th meeting of the American Astronomical Society in Washington DC, USA.

The deeper Hubble looks into space, the farther back in time it looks, because light takes billions of years to cross the observable Universe. This makes Hubble a powerful "time machine" that allows astronomers to see galaxies as they were 13 billion years ago, just 600 million to 800 million years after the Big Bang.

"With the rejuvenated Hubble and its new instruments, we are now entering unchartered territory that is ripe for new discoveries," says Garth Illingworth of the University of California, Santa Cruz, leader of the survey team that was awarded the time to take the new WFC3 infrared data on the Hubble Ultra Deep Field (imaged in visible light by the Advanced Camera for Surveys in 2004). "The deepest-ever near-infrared view of the Universe — the HUDF09 image — has now been combined with the deepest-ever optical image — the original HUDF (taken in 2004) — to push back the frontiers of the searches for the first galaxies and to explore their nature," Illingworth says.

Ross McLure of the Institute for Astronomy at Edinburgh University and his team detected 29 galaxy candidates, of which twelve lie beyond redshift 6.3 and four lie beyond redshift 7 (where the redshifts correspond to 890 million years and 780 million years after the Big Bang respectively). He notes that "the unique infrared sensitivity of Wide Field Camera 3 means that these are the best images yet for providing detailed information about the first galaxies as they formed in the early Universe".

James Dunlop of the University of Edinburgh agrees. "These galaxies could have roots stretching into an earlier population of stars. There must be a substantial component of galaxies beyond Hubble's detection limit."

Three teams worked hard to find these new galaxies, announced in a burst of papers immediately after the data were released in September, and were soon joined by a fourth team, and later a fifth. A total of 15 papers have been submitted to date by astronomers worldwide. The existence of these newly found galaxies pushes back the time when galaxies began to form to before 500-600 million years after the Big Bang. This is good news for astronomers building the much more powerful James Webb Space Telescope (JWST planned for launch in 2014), which will allow astronomers to study the detailed nature of primordial galaxies and discover many more even farther away. There should be plenty for JWST to hunt for.

The deep observations also demonstrate the progressive buildup of galaxies and provide further support for the hierarchical model of galaxy assembly where small objects accrete mass, or merge, to form bigger objects over a smooth and steady, but still dramatic, process of collision and agglomeration, as these small building blocks fuse into the larger galaxies known today.

"These ancient galaxies are only one twentieth of the Milky Way's diameter," reports HUDF09 team member Pascal Oesch of the Swiss Federal Institute of Technology in Zurich. "Yet they must be the seeds from which the great galaxies of today were formed," adds HUDF09 team member Marcella Carollo of the Swiss Federal Institute of Technology.

These newly found objects are crucial to understanding the evolutionary link between the birth of the first stars, the formation of the first galaxies and the sequence of evolutionary events that resulted in the assembly of our Milky Way and the other "mature" elliptical and majestic spiral galaxies in today's Universe.

The HUDF09 team also combined the new Hubble data with observations from NASA's Spitzer Space Telescope to estimate the ages and masses of these primordial galaxies. "The masses are just 1 percent of those of the Milky Way," explains team member Ivo Labbe of the Carnegie Institute of Washington, lead author of two papers on the data from the combined NASA Great Observatories. He further noted that "to our surprise, the results show that these galaxies at 700 million years after the Big Bang must have started forming stars hundreds of millions of years earlier, pushing back the time of the earliest star formation in the Universe."

The results are gleaned from the HUDF09 observations, which are deep enough at near-infrared wavelengths to reveal galaxies at redshifts from 7 to beyond redshift 8. [1] The clear detection of galaxies between redshifts 7 and 8.5 corresponds to lookback times of approximately 12.9 billion years to 13.1 billion years.

"This is about as far as we can go to do detailed science with the new HUDF09 image. It shows just how much the James Webb Space Telescope is needed to unearth the secrets of the first galaxies," says Illingworth. The challenge is that spectroscopy is needed to provide definitive redshift values, but the objects are too faint for spectroscopic observations (until JWST is launched), and the redshifts have to be inferred from the apparent colours of the galaxies.

The teams are finding that the number of galaxies per unit of volume of space drops off smoothly with increasing distance, and the HUDF09 team has also found that the galaxies become surprisingly blue intrinsically. The ultra-blue galaxies are extreme examples of objects that appear so blue because they may be deficient in the heavier elements, and as a result, are quite free of the dust that reddens light through scattering.

A longstanding problem with these findings is that it still appears that these early galaxies did not emit enough radiation to "reionise" the early Universe by stripping electrons from the neutral hydrogen that cooled after the Big Bang. This "reionisation" event occurred between about 400 million and 900 million years after the Big Bang, but astronomers still don't know which light sources caused it to happen. These newly discovered galaxies date from this important epoch in the evolution of the Universe.

Perhaps the density of very faint galaxies below the current detection limit is so high that there may be enough of them to support reionisation. Or there was an earlier wave of galaxy formation that decayed and then was "rebooted" by a second wave of galaxy formation. Or, possibly the early galaxies were extraordinarily efficient at reionising the Universe.

Due to these uncertainties it is not clear which type of object or evolutionary process did the "heavy lifting" by ionising the young Universe. The calculations are inconclusive, and so galaxies may do more than currently expected, or astronomers may need to invoke other phenomena such as mini-quasars (active supermassive black holes in the cores of galaxies) — current estimates suggest that quasars are even less likely than galaxies to be the cause of reionisation. This is an enigma that still challenges astronomers and the very best telescopes.

"We know the gas between galaxies in the Universe was ionised early in history, but the total light from these new galaxies may not be sufficient to achieve this." said Andrew Bunker of the University of Oxford, a researcher on one of the European teams.

Hubble's WFC3 infrared camera was able to make deep exposures to uncover new galaxies at roughly 40 times greater efficiency than its earlier infrared camera that was installed in 1997. The WFC3 brought new infrared technology to Hubble and accomplished in four days of observing what would have previously taken almost half a year for Hubble to complete.

Notes

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

[1] The redshift value is a measure of the stretching of the wavelength or “reddening” of starlight due to the expansion of space.

Image credit: NASA, ESA, G. Illingworth (UCO/Lick Observatory and University of California, Santa Cruz) and the HUDF09 Team


Data processing [ edit | edit source ]

A section of the HDF about 14 arcseconds across in each of the four wavelengths used to construct the final version: 300 nm (top left), 450 nm (top right), 606 nm (bottom left) and 814 nm (bottom right)

The production of a final combined image at each wavelength was a complex process. Bright pixels caused by cosmic ray impacts during exposures were removed by comparing exposures of equal length taken one after the other, and identifying pixels that were affected by cosmic rays in one exposure but not the other. Trails of space debris and artificial satellites were present in the original images, and were carefully removed. Δ]

Scattered light from the Earth was evident in about a quarter of the data frames, creating a visible "X" pattern on the images. This was removed by taking an image affected by scattered light, aligning it with an unaffected image, and subtracting the unaffected image from the affected one. The resulting image was smoothed, and could then be subtracted from the bright frame. This procedure removed almost all of the scattered light from the affected images. Δ]

Once the 342 individual images were cleaned of cosmic-ray hits and corrected for scattered light, they had to be combined. Scientists involved in the HDF observations pioneered a technique called 'drizzling', in which the pointing of the telescope was varied minutely between sets of exposures. Each pixel on the WFPC2 CCD chips recorded an area of sky 0.09 arcseconds across, but by changing the direction in which the telescope was pointing by less than that between exposures, the resulting images were combined using sophisticated image-processing techniques to yield a final angular resolution better than this value. The HDF images produced at each wavelength had final pixel sizes of 0.03985 arcseconds. Δ]

The data processing yielded four monochrome images (at 300 nm, 450 nm, 606 nm and 814 nm), one at each wavelength. ⎖] One image was designated as red (814 nm), the second as green (606 nm) and the third as blue (450 nm), and the three images were combined to give a colour image. Α] Because the wavelengths at which the images were taken do not correspond to the wavelengths of red, green and blue light, the colours in the final image only give an approximate representation of the actual colours of the galaxies in the image the choice of filters for the HDF (and the majority of Hubble images) was primarily designed to maximize the scientific utility of the observations rather than to create colours corresponding to what the human eye would actually perceive. ⎖]


XDF PROJECT

The XDF team has published a paper titled "The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever" discussing the methods used to produce the XDF dataset.

The paper can be downloaded from ADS or arXiv.org

The XDF Data Release v1.0

The XDF team has release the XDF v1.0 data products on the MAST HST data archive. These data include all optical and infrared data taken by Hubble covering the original Hubble Ultra-Deep Field (HUDF) program.

The data can be downloaded from the MAST High Level Science Products page XDF Data Release 1.0

XDF Press Release

Hubble Goes to the eXtreme to Assemble Farthest Ever View of the Universe

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind's deepest-ever view of the universe. Read more.

What is the eXtreme Deep Field (XDF)

The XDF is the deepest image of the sky taken with Hubble for searching for the earliest galaxies. It includes ALL images taken by Hubble on the small patch of sky first imaged as the Hubble Ultra-Deep Field (HUDF) and subsequently as the HUDF09 and HUDF12. The XDF also adds images that overlapped the HUDF from many other programs including CANDELS, supernova searches and many others (19 in total). These images were taken over a decade from mid-2002 through to early 2013. The XDF is an exposure of 2 million seconds total from Hubble's two premier cameras, the Advanced Camera (ACS) and the Wide Field Camera 3 (WFC3). It consists of 2963 separate images from the ACS and WFC3/IR. ACS flew on the Shuttle to Hubble in 2002 on servicing mission SM3B, while the Wide Field Camera 3 (WFC3) flew to Hubble in 2009 on the final Hubble Shuttle mission (SM4).

Garth Illingworth, Dan Magee, Pascal Oesch, Rychard Bouwens and the XDF Team

The Making of the XDF

The XDF includes ALL data taken by Hubble on the small patch of sky first imaged as the Hubble Ultra-Deep Field (HUDF). These images were taken over a decade from mid-2002 through to early 2013. The XDF is an exposure of 2 million seconds total from Hubble's two premier cameras, the Advanced Camera (ACS) and the Wide Field Camera 3 (WFC3). It consists of 2963 separate exposures from the ACS and WFC3/IR. ACS flew on the Shuttle to Hubble in 2002 on servicing mission SM3B, while the Wide Field Camera 3 (WFC3) flew to Hubble in 2009 on the final Hubble Shuttle mission (SM4).

The original HUDF data demonstrated the power of Hubble’s new ACS camera in 2004. The original HUDF contributes, by time, a little more that half to the XDF, but only contains data in the optical ("visible") region of the spectrum. In 2009 and 2010, the HUDF09 project took images towards the red end of the spectrum in the near-infrared with the new WFC3/ IR camera. These new data doubled the waveband coverage and enabled exploration of a new realm of the most distant galaxies for the first time. The HUDF09 field with WFC3/IR and ACS contributed around 20% of the data by time to XDF. A subsequent study HUDF12 added more WFC3/IR data that enhanced the overall dataset by about another 10% by time, though both the HUDF09 and HUDF12 images contributed unique IR data that is essential for finding the earliest galaxies. The HUDF and HUDF09/12 fields are shown in the first and second panels of the “Hubble Dataset Used to Build Up the XDF” slide shown below.

The XDF/HUDF09 team then took ALL the other data on this region taken by numerous programs (see "Programs Used to Make the XDF" below) and combined it through a very laborious and careful series of steps into one incredibly deep image, the eXtreme Deep Field (XDF). These data fall at many locations and orientations and much careful checking was needed to make sure all the Hubble ACS and WFC3 images could be properly aligned and added together. The contributions from these other programs comprised nearly a quarter of the time, of which the largest was the CANDELS images from ACS and WFC3. The numerous images are shown in third panel of the “Hubble Dataset Used to Build Up the XDF” slide shown below.

The XDF was then assembled using data from every image over the last decade from ACS and the WFC3 (the orange region in the fourth panel of the “Hubble Dataset Used to Build Up the XDF” slide shown below). XDF reaches back around 13.2 billion years, to just 450 million years after the Big Bang. The history of galaxies — from soon after the first galaxies were born to the appearance of the great galaxies of today, like our Milky Way — is laid out in this one remarkable image.

The XDF combined image goes incredibly faint. The combined image XDF reaches to approximately one ten-billionth of what the eye can see. In astronomer-speak, XDF reaches to a 5 sigma limit is 31.2 AB mag (the limit for a 5 sigma detection of a point source, like a star, corrected to total magnitude, is ∼30.7 AB magnitude). This is equivalent to a 1 sigma noise fluctuation of 32.9 AB mag, as measured directly in a 0.33" aperture.

The XDF is such an important field that it will be imaged in the future and more data will be added, but future gains will be slow until JWST is launched.. The first deep data in the ultra-violet from WFC3/UVIS has already been obtained. The ultra-violet imaging complements the redder optical/IR XDF and is being used by astronomers to study galaxies at later times when they are in transition to galaxies like today’s galaxies (including our Milky Way and Andromeda). The ultraviolet data has not been included since the UV data is relatively noisy and also cannot reveal galaxies in the first 2 billion years due to the absorbing effects of hydrogen in the universe.

The details of how the XDF was generated from the decade-long source data from the HUDF region can be found in the paper that was written about the XDF ("The HST eXtreme Deep Field (XDF): Combining All ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever", Illingworth, G. D. Magee, D. Oesch, P. A. Bouwens, R. J. Labbé, I. Stiavelli, M. van Dokkum, P. G. Franx, M. Trenti, M. Carollo, C. M. Gonzalez, V., 2013, ApJS, 209, 6). The paper was published in ApJS in 2013 and can be found at: http://adsabs.harvard.edu/abs/2013ApJS..209. 6I.

XDF Facts

2 million seconds taken over

Hubble Datasets Used to Build-up the XDF

This sequence shows how the XDF was built up from the numerous exposures that have been made over the HUDF field.


Hubble's deep field images of the early universe are postcards from billions of years ago

The Hubble Extreme Deep Field, looking back towards the birth of the universe. Credit: NASA/ESA

This insignificant patch of sky in the fairly obscure constellation of Fornax is the setting for one of the most remarkable images ever captured. Although only a fraction of the full moon in size, this image traces thousands of distant galaxies to the edge of the observable universe.

The Hubble Space Telescope began observing "deep fields" in 1995. The idea was not new – astronomers have always tried to take longer photographic captures that draw in more light to reveal ever more faint and distant objects. Observing more distant galaxies sheds light on how they form, and how their shapes and sizes change over time. Hubble's key advantage is that, floating in orbit, it's unaffected by the blurring effect of the atmosphere and so can provide images of far superior resolution than ground-based telescopes.

Careful planning was required for the deep field images. An "empty" piece of sky was needed that contained no bright sources of visible light that might drown out fainter objects. There could also be no bright sources at other wavelengths, such as X-ray or radio waves where complementary supporting observations might be made. The direction chosen was away from the millions of faint stars and dust of the plane of our own galaxy, the Milky Way.

This was not without risk, however. Hubble was, and still is, a hugely popular, world-class research facility. It is typical that demand for Hubble's instruments outstrips availability by six or seven times. There was always the possibility that the 10 days observation required might reveal the carefully-chosen blank part of the sky to be just that: blank.

Even dark skies are filled with stars

Instead the results of the first Hubble Deep Field image were breathtaking. After stitching together the composite images and careful processing, the final image revealed around 3,000 galaxies, most of which would otherwise have never been seen. This success triggered plans for further "deep field" images.

A southern-hemisphere counterpart followed in 1998, and after a powerful instrument upgrade in 2002, the Hubble Ultra Deep Field images were taken in 2004.

The Hubble Ultra Deep Field, as seen from the ground. Credit: Digitised Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO

Finally, advances in data processing techniques, some new infrared data and more images of Ultra Deep Field area were in 2012 combined to create the Hubble eXtreme Deep Field (HXDF), the main image above, and the same image with ultraviolet included, below – humanity's most sensitive images of the cosmos ever taken.

Looking out at the cosmos

The HXDF image is the result of 2m seconds (more than 23 days) of exposure time taken over the course of 2,963 images. The captures that make up the composite image are sensitive to light from the ultraviolet to the near-infrared spectrum, which are used in the processing to create the colours visible. The faintest objects in the HXDF, barely visible on screen, are a remarkable ten-billionth the brightness of the faintest star visible to the naked eye.

How Hubble’s astronomy looks back in time. Credit: NASA/ESA/Z. Levay/F. Summers

A handful of the objects in the image are comparatively "nearby" foreground stars in our own galaxy located perhaps a few tens or hundreds of light-years away. Although the field was chosen to have few of these, some faint stars are present. The stars are quite easy to identify due to the diffraction spikes caused by Hubble's optics – the tell-tale cross of light across their centre. All the other objects in the image are galaxies.

The image acts a little like a time machine. The further away a galaxy is, the longer it has taken light to reach us – and the earlier in the universe we are looking. Since that light was emitted the universe has continued to expand. Some galaxies in the picture lie close to the edge of the observable universe. This is the furthest point in space to which we could, in principle, see. This is about 45 billion light years from the Earth: light from more distant objects has not yet had time to reach us.

Far away and long, long ago

With the addition of IR and UV, this becomes the most colourful view of the universe. NASAESAIPACCaltechSTScIArizona State University

Some of these galaxies are comparatively near by, perhaps a few hundreds of millions of light years away. These are the bigger objects in the picture: blue or white galaxies with sharply-focused spiral arms, or large red and orange blurs. Galaxies like this are very similar to galaxies we see near the Milky Way.

The most distant objects look very different to galaxies nearer to us, tracing how rapidly galaxies change in the early universe. They shine brightly with the light of young stars, revealing that more stars are formed in the early universe than previously thought.

One of these objects, dubbed 39546284 in the zoomed picture above, is thought to be the most distant: it has taken light about 13.3 billion years to reach us (the universe itself is thought to be about 13.7 billion years old). Many of these very young galaxies will eventually evolve into galaxies that look more like the Milky Way.

Twinkle, twinkle, lots of stars (and their diffraction spikes). Credit: NASA/ESA/H. Richer

The image also contains a few supernovae, exploding stars detected more than halfway across the universe. SN Primo is one of these, nearly ten billion light years away. Supernovae like these are being used to map out the expansion history of the universe.

Perhaps the most remarkable fact is that this image represents just a tiny fraction of our universe. All told, there are estimated to be some 100 to 200 billion galaxies in the universe, of which only just over 5,000 appear in the HXDF. Put another way, you'd need about 30m images like the HXDF to map the entire sky.

This remarkable image will be one of Hubble's lasting legacies. With the Space Shuttle now retired from service, there are no future servicing missions planned. This means no future instrument upgrades, so it's unlikely that Hubble will ever be able to improve significantly on the depth of this image. That honour may await the James Webb Space Telescope, scheduled for launch in 2018, and the beginning of another astronomical legacy.

A zoom on the HXDF. Credit: NASA, ESA, and Z. Levay (STScI)

This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).


Looking out at the cosmos

The HXDF image is the result of 2m seconds (more than 23 days) of exposure time taken over the course of 2,963 images. The captures that make up the composite image are sensitive to light from the ultraviolet to the near-infrared spectrum, which are used in the processing to create the colours visible. The faintest objects in the HXDF, barely visible on screen, are a remarkable ten-billionth the brightness of the faintest star visible to the naked eye.

Twinkle, twinkle, lots of stars (and their diffraction spikes). NASA/ESA/H. Richer

A handful of the objects in the image are comparatively “nearby” foreground stars in our own galaxy located perhaps a few tens or hundreds of light-years away. Although the field was chosen to have few of these, some faint stars are present. The stars are quite easy to identify due to the diffraction spikes caused by Hubble’s optics – the tell-tale cross of light across their centre. All the other objects in the image are galaxies.

How Hubble’s astronomy looks back in time. NASA/ESA/Z. Levay/F. Summers

The image acts a little like a time machine. The further away a galaxy is, the longer it has taken light to reach us – and the earlier in the universe we are looking. Since that light was emitted the universe has continued to expand. Some galaxies in the picture lie close to the edge of the observable universe. This is the furthest point in space to which we could, in principle, see. This is about 45 billion light years from the Earth: light from more distant objects has not yet had time to reach us.


Research Box Title

NASA's Hubble Space Telescope has looked deep into the distant universe and detected the feeble glow from a star that exploded more than 9 billion years ago.

This isn't just any dying star. It belongs to a special class called Type Ia supernovae, which are bright beacons used as distance markers for studying the expansion rate of the universe. Type Ia supernovae most likely arise when white dwarf stars – the burned-out cores of normal stars – siphon too much material from their companion stars and explode.

The stellar explosion, given the nickname SN Primo, will help astronomers place better constraints on the nature of dark energy – a mysterious repulsive force that is causing the universe to fly apart ever faster.

SN Primo is the farthest Type Ia supernova whose distance has been confirmed through spectroscopic observations. Spectroscopy is the "gold standard" for measuring supernova distances. A spectrum splits the light from a supernova into its constituent colors. By analyzing those colors, astronomers can confirm its distance by measuring how much the supernova's light has been stretched, or reddened, into near-infrared wavelengths due to the expansion of the universe.

The sighting is the first result from a three-year Hubble program to survey faraway Type Ia supernovae, opening a new distance realm for searching for this special class of stellar explosion. The remote supernovae will help astronomers determine whether the exploding stars remain dependable cosmic yardsticks across vast distances of space in an epoch when the cosmos was only one-third its current age of 13.7 billion years.

Called the CANDELS+CLASH Supernova Project, the census is using the sharpness and versatility of Hubble's Wide Field Camera 3 (WFC3) to help astronomers search for supernovae in near-infrared light and verify their distance with spectroscopy. WFC3 is looking in regions targeted by two large Hubble programs called the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and the Cluster Lensing and Supernova Survey with Hubble (CLASH).

"In our search for supernovae, we had gone as far as we could go in optical light," said the project's lead investigator, Adam Riess of the Space Telescope Science Institute and The Johns Hopkins University in Baltimore, Md. "But it's only the beginning of what we can do in infrared light. This discovery demonstrates that we can use the Wide Field Camera 3 to search for supernovae in the distant universe."

The new results are being presented today at the American Astronomical Society meeting in Austin, Texas. A paper describing the study has been accepted for publication in The Astrophysical Journal.

The supernova team's search technique involved taking multiple near-infrared images over several months, looking for a supernova's faint glow. Once the team spotted the stellar blast in October 2010, they used WFC3's spectrograph to verify SN Primo's distance and to decode its light, finding the unique signature of a Type Ia supernova. The team then re- imaged SN Primo periodically for eight months, measuring the slow dimming of its light.

By taking the census, the astronomers hope to determine the frequency of Type Ia supernovae during the early universe and glean insights into the mechanisms that detonated them.

"If we look into the early universe and measure a drop in the number of supernovae, then it could be that it takes a long time to make a Type Ia supernova," said Steve Rodney of The Johns Hopkins University, the science paper's first author. "Like corn kernels in a pan waiting for the oil to heat up, the stars haven't had enough time at that epoch to evolve to the point of explosion. However, if supernovae form very quickly, like microwave popcorn, then they will be immediately visible, and we'll find many of them, even when the universe was very young. But each supernova is unique. It's possible that there are multiple ways to make a supernova."

If astronomers discover that Type Ia supernovae begin to depart from how they expect them to look, they might be able to gauge those changes and make the measurements of dark energy more precise, Riess explained. Riess and two other astronomers shared the 2011 Nobel Prize in Physics for discovering dark energy 13 years ago, using Type Ia supernovae to plot the universe's expansion rate.

After extending the frontier for supernova discoveries with Hubble, a full scrutiny of this new territory will have to wait for the James Webb Space Telescope (JWST). Scheduled to launch later this decade, JWST will probe exploding stars at much farther distances than Hubble can reach.

JWST will be able to see farther into the infrared than Hubble does. This capability will push back the frontier by probing more than 11 billion years back in time, when the universe was only 2 billion years old.

Credits:NASA, ESA, A. Riess (STScI and JHU), and S. Rodney (JHU)


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