The Deepest Picture Ever Taken

To learn how galaxies formed, astronomers took their best facility and pushed it to the limit. The result was the deepest picture of the sky ever made. The Hubble Ultra Deep Field had its genesis in an earlier project called the Hubble Deep Field, and a bold decision by the second director of the Space Telescope Science Institute, Bob Williams. In 1995, Williams devoted the 10 percent of the observ­ing time that he had at his discretion as director to a very deep multi-color image of a single patch of sky. To see why this was bold, let’s take a brief excursion into the culture and sociology of research astronomy.

For astronomers, the Hubble Space Telescope is the best game in town. The Hubble has made more discoveries and generated more papers than any other research facility. Every year, astronomers craft proposals for time, and there are many times more proposals than can get on the telescope. There’s a natural tendency to spread the bets. Directors had used their discretionary time similarly, giv­ing most of it to small proposals to ease the over-subscription. Williams decided to put all his eggs in one basket by devoting 150 orbits—a huge allocation of time—to a single deep image. His de­cision changed the culture of astronomy. He let the research com­munity decide where the telescope should be pointed for those 140 hours, and what color of filters should be used, but he insisted that the data be processed and made public immediately for any astron­omer to use. The tiny region chosen in Ursa Major—1/28,000,000 of the sky—contained over three thousand galaxies, and the data paper for the Hubble Deep Field has been cited more than eight hundred times by other research papers.46 Also, this large invest­ment of a scarce resource in one field persuaded infrared, radio, and X-ray astronomers to follow suit. Other leading telescopes put copious time into complementing the optical images with data across the electromagnetic spectrum, and in many cases those data

were also made available quickly. An intriguing mix of competi­tion and altruism spurred the research forward.

But what if the one field you pick isn’t typical for some rea­son? A premise of cosmology is that our location isn’t special or unusual. This assumption is called the cosmological principal; in practice it means showing that the universe is homogeneous and isotropic. Homogeneous means roughly the same at all locations, which is hard to prove since we can’t travel beyond the galaxy in a space ship. Isotropic means the same in all directions. While there’s been no indication that we see different numbers and types of galaxies looking in one direction in the universe compared to any other, astronomers were nervous, so Bob Williams committed additional Hubble time to a small, deep field in the southern sky in 2000. Since then, deep fields have sprouted like mushrooms. When a new sensitive camera was installed during the fourth servicing mission in 2002, the Space Telescope Institute director at the time, Steve Beckwith, upped the ante by putting four hundred orbits, a million seconds of observing time distributed in four colors, into a tiny patch of sky in the direction of the Fornax constellation. That’s the Hubble Ultra Deep Field.47 The most distant light in this image has taken 95 percent of the age of the universe to reach us, so it comes from close to the “dawn” of light. To get a sense of this incredible image, hold a pin out at arm’s length; the head of the pin covers as much sky as the image produced by Hubble’s CCD camera. Astronomers harvested 10,000 galaxies from this minuscule bit of the sky. The faintest are five billion times fainter than the eye can see, and Hubble can only collect one photon per minute from them—think of trying to see a firefly on the Moon. Surveying the entire sky to this depth would take a million years of uninterrupted observing.

The numbers are staggering, and they can be used to derive some important information about the contents of the universe. Since the Ultra Deep Field covers 1/13,000,000 of the sky, the projected total number of galaxies in all directions is 130 billion. Each galaxy will on average contain 400 billion stars, so there are about 1023 stars in the visible universe, or a hundred thousand bil­lion billion. That’s a mind-bending number, but the real excitement comes from the implications for life. We’ve learned that planets are ubiquitous around Sun-like stars and expect to know soon about the abundance of habitable and Earth-like planets. The number of potential biological experiments in the universe may not be very different from the number of stars. What odds would you put on us being alone? When you look at the faint galaxies littering these deep fields, mere smudges of ancient light, it’s irresistible to imag­ine that in many of them or even all of them someone or something is looking across the canyons of time and space back at you.