Hubble Deep Field

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It's an incredible feeling to realize that every one of those lights is a galaxy with tens or hundreds of billions of stars each. Our Sun is but one of a hundred billion stars in the Milky Way, which itself is but one of a hundred billion galaxies. "Tiny" does not seem adequate to describe how small we are in this vast and amazing Universe.

The completed Hubble Deep Field images were revealed at a American Astronomical Society meeting in in January 1996. Approximately 3,000 distinct galaxies of various shapes (elliptical, spiral and irregular ones) were identified by scientists, some of them being only a few pixels in width. The image also contains about 50 blue point-like objects that are thought to be either regions of intense star formation associated with nearby galaxies or distant quasars. There's also a small possibility that some of these objects are old white dwarf stars.

Light from the galaxies in the HDF images took billions of years to reach Earth, meaning we now see them as they looked billions of years in the past. One of the biggest findings of the Deep Field was a large number of galaxies distanced as far as 12 billion light-years away; since the age of the Universe is thought to be between 13.5 and 14 billion years, those are young galaxies, having existed "only" 1.5 to 2 billion of years after the Big Bang. This provided cosmologists with a lot of rich material concerning the evolution of galaxies and the rate of star formation in the early Universe.

The Deep Field wasn't the end of Hubble's achievements though...

NASA astronauts visited the Hubble for the fourth time in March 2002 aboard a space shuttle Columbia. Servicing Mission 3B installed a new optical Advanced Camera for Surveys (ACS), revived an unusable infrared camera that had previously ran out of coolant, and gave the space telescope new - smaller and more powerful - solar wings.

Thanks to these upgrades performed in orbit, Hubble became a much more powerful machine than it was on it's launch back in 1990. The ACS in particular increased HST's discovery efficiency by a factor of ten and quickly became the primary imaging instrument of the Hubble.

This allowed astronomers to see even farther into the depths of the Universe and produce the deepest, most sensitive astronomical image ever made at visible wavelengths: the Hubble Ultra Deep Field.

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The Hubble Ultra Deep Field too is an image of a very small section of space, equal to roughly one thirteen-millionth of the total area of the sky, or just one-tenth the diameter of the full moon as viewed from Earth. Indeed, NASA themselves compared HUDF to "peering through an eight-foot-long soda straw".

The target region is located in the southern hemisphere constellation Fornax, chosen like in HDF's case due to a low density of bright stars nearby (HUDF is thought to contain as little as 7 foreground stars).

The image was accumulated over a 4 month period from September 24, 2003 to January 16, 2004 (that's 400 orbits and over 800 exposures 21 minutes each!). Such a long observation was needed because photons of light from the very faint objects arrive at a trickle of one photon per minute, compared with millions of photons per minute from nearby galaxies. The Hubble Ultra Deep Field remains to this day the deepest portrait of the visible Universe ever achieved by humankind.

The Hubble Ultra Deep Field image is estimated to contain 10,000 galaxies. It looks back approximately 13 billion years in the past and is used to search for young galaxies that existed "merely" 400-800 million years after the Big Bang.

The high resolution HUDF image includes galaxies of different ages, sizes, shapes, and colors various distances away from Earth. Unlike current galaxies which usually assume elliptical or spiral shapes, these young ones are more chaotic: some look like toothpicks, links on a bracelet, or are merged together. The smallest and reddest galaxies of which there are around 100 in the picture are the most distant galaxies to ever have been imaged by an optical telescope, existing at the time "shortly" (~800 million years) after the Big Bang.

The HUDF build on the work of HDF and provided further material for cosmologists to analyze. The most important scientific results are:

• High rates of new star formation were observed during the very early stages of galaxy formation under a billion years after the Big Bang (that's how youngest galaxies were able to grow big in a relatively short time).
• The characterization of the distribution of galaxies, their numbers, sizes and luminosities throughout different epochs of the Universe was improved, providing insights into their birth and evolution.
• The HUDF confirmed that galaxies at high redshifts (farther from us) are smaller and less symmetrical than ones at lower redshifts (closer to us), clearly showing the rapid evolution of galaxies in the first couple of billion years after the Big Bang.

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You can take a closer look at one area of the Hubble Ultra Deep Field in this galactic snapshot. A number of galaxies of various shapes and colors are visible. You can also see one bright foreground star from our own galaxy in this picture recognizable by it's distinct, cross-like appearance.