My Favorite Photo in the Whole Wide Universe: Welcome to the “Deep Field” (Hubble, 2014)

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Hubble Telescope’s “Deep Filed” Photograph. (News release ID: STScI-2014-27/Release Date: Jun 3, 2014, NASA)

What is the “Deep Field”?

‘The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is equivalent in angular size to a tennis ball at a distance of 100 meters. The image was assembled from 342 separate exposures taken with the Space Telescope’s Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.

The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe…

Click on the thumbnail image below for the full-sized 6200×6200 resolution image of Hubble’s Deep Field photograph…

‘Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the Hubble Deep Field South. The similarities between the two regions strengthened the belief that the universe is uniform over large scales and that the Earth occupies a typical region in the Universe (the cosmological principle). A wider but shallower survey was also made as part of the Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the Hubble Ultra-Deep Field (HUDF), was constructed from a few months of light exposure. The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, and it remained so until the Hubble eXtreme Deep Field (XDF) was released in 2012.’ (Wikipedia)


The dramatic improvement in Hubble’s imaging capabilities after corrective optics were installed encouraged attempts to obtain very deep images of distant galaxies.

Conceptually Speaking


The HDF was located in Hubble’s northern Continuous Viewing Zone, as shown by this diagram.Enter a caption.

‘One of the key aims of the astronomers who designed the Hubble Space Telescope was to use its high optical resolution to study distant galaxies to a level of detail that was not possible from the ground. Positioned above the atmosphere, Hubble avoids atmospheric airglow allowing it to take more sensitive visible and ultraviolet light images than can be obtained with seeing-limited ground-based telescopes (when good adaptive optics correction at visible wavelengths becomes possible, 10 m ground-based telescopes may become competitive). Although the telescope’s mirror suffered from spherical aberration when the telescope was launched in 1990, it could still be used to take images of more distant galaxies than had previously been obtainable. Because light takes billions of years to reach Earth from very distant galaxies, we see them as they were billions of years ago; thus, extending the scope of such research to increasingly distant galaxies allows a better understanding of how they evolve.

After the spherical aberration was corrected during Space Shuttle mission STS-61 in 1993, the improved imaging capabilities of the telescope were used to study increasingly distant and faint galaxies. The Medium Deep Survey (MDS) used the Wide Field and Planetary Camera 2 (WFPC2) to take deep images of random fields while other instruments were being used for scheduled observations. At the same time, other dedicated programs focused on galaxies that were already known through ground-based observation. All of these studies revealed substantial differences between the properties of galaxies today and those that existed several billion years ago.


Diagram illustrating comparative sampling distance of the HDF and the 2004 Hubble Ultra-Deep Field.

‘Up to 10% of the HST’s observation time is designated as Director’s Discretionary (DD) Time, and is typically awarded to astronomers who wish to study unexpected transient phenomena, such as supernovae. Once Hubble’s corrective optics were shown to be performing well, Robert Williams, the then-director of the Space Telescope Science Institute, decided to devote a substantial fraction of his DD time during 1995 to the study of distant galaxies. A special Institute Advisory Committee recommended that the WFPC2 be used to image a “typical” patch of sky at a high galactic latitude, using several optical filters. A working group was set up to develop and implement the project.’


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New Map Sheds Light on Dark Matter That Binds Universe Together…


‘Dark matter is the most common stuff in the universe. A billion sub-atomic particles of dark matter pass through your outstretched hand every second, yet few if any of these ethereal particles might actually touch and rebound from your hand in your lifetime. Now, studies are beginning to shed some light on this mysterious substance.

Astronomers have known since the 1930s that there is more than just the visible universe. The Milky Way, the galaxy we live in, is spinning too fast to be held together by the gravity between its stars. If stars were all there is, we should have long ago been flung off our cosmic roundabout. Instead, our galaxy contains about six times more material of some kind than is accounted for by every atom of all the elements in the periodic table: material known as dark matter.

Dark matter is invisible and can be detected only though the effect its gravity has on things we can see such as passing rays of light – an effect known as gravitational lensing. This is like looking through an uneven pane of glass which, while transparent, is obvious because of the distortions it produces on objects seen through it. By calculating the extent of the distortion, it’s possible to work out the thickness of the glass.

The recently released Dark Energy Survey used gravitational lensing to generate a huge map of dark matter. As seen from the Blanco telescope in Chile, they saw a crisscrossing web of thick, dark matter filaments.

The recently released Dark Energy Survey used gravitational lensing to generate a huge map of dark matter. As seen from the Blanco telescope in Chile, they saw a crisscrossing web of thick, dark matter filaments.


A map of cosmic dark matter, as seen from the Earth. Colour represents dark matter, increasing towards red. Circles mark galaxies and galaxy clusters.(Source: Dark Energy Survey)

The dark matter web is the invisible scaffolding that underpins the entire visible universe. The scaffold formed very soon after the big bang, and its gravity began to pull in all the ordinary material of which stars, planets and people are then built. Indeed, the galaxies are all found along dark matter filaments, with clusters of up to a thousand galaxies located wherever filaments cross each other.

So now we know where dark matter is. To find out what it is, we need the Hubble Space Telescope to zoom right in to the dark matter map.

Light vs Dark

On Earth we use particle accelerators to find out what matter is made of, firing particles at each other with enormous energy and seeing what the collision produces. This has been the principle behind experiments from Lord Rutherford’s discovery of atoms in 1908, to mankind’s biggest experiment, the Large Hadron Collider at CERN. But we can’t capture dark matter, nor interact with it at all. Nature, however, provides the experiment for us: we can watch what happens when the dark matter around galaxies or clusters of galaxies smash into each other by chance.

Galaxies are made from three ingredients: stars, dark matter and swirling clouds of gas. When two galaxies collide – an event involving expanses of time and space and size that makes it hard to comprehend – individual stars almost always pass straight unscathed. They are pinpoints of matter separated by vast regions of empty void. Conversely, the clouds of gas smash into each other and are pulled by friction to a stop. Dark matter is expected to behave somewhere in between, and its trajectory out of a collision should reveal its properties.


Chart: Points representing visible stars and galaxies (left) as seen through effects of gravitational lensing (right). (Source: TallJimbo, CC BY-SA)

Astronomical Particle Colliders

In one well-studied collision known as the Bullet Cluster, dark matter appeared to whizz straight through the collision. As close as we can tell, dark matter kept pace with the stars, not measurably slowed by its ordeal. But interpreting only a single collision is difficult. We must reconstruct the 3D scene from just one viewpoint and one freeze-frame from a movie that lasts 100m years.

To crowd-source the full movie, we recently observed 72 high-speed collisions of the dark matter around galaxy clusters. We view some from the side, others head-on and each at a different stage in the crash. Reconstructing the statistical properties of a dark matter collision, we confirmed robustly that dark matter interacts very, very little with anything else.


The Cerro Tololo observatory, home of the Victor Blanco telescope used. (Source:David Walker, CC BY-SA)

The Dark Matter Drag Factor…

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