Looking back in time Searching for the most distant galaxies

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ESTHER HU, University of Hawaii

Esther Hu was born and raised in New York City. She is a second generation Chinese-American whose parents came to the US as students at the end of the Second World War. Like her sister Evelyn, Esther decided to be a scientist before attending college. Esther was educated in physics at MIT and earned her PhD in astrophysics at Princeton. She then became a research associate with the X-ray group at NASA's Goddard Space Flight Center, and then a postdoctoral fellow at the Space Telescope Science Institute. She is now a professor of astronomy at the Institute for Astronomy at the University of Hawaii in Honolulu. In the course of her career, Esther has studied successively more distant objects across the Universe using more and more sensitive telescopes and instruments. Despite her friendly and easy-going nature, Esther is as competitive as they come; she presently holds the record for distant object detection. Esther enjoys reading, classical music, and ''living in a place as beautiful as Hawaii.''

The past is a foreign country: they do things differently there.

L.P. Hartley

When I was seven, at my first school book fair, I came away with a title, Insight into Astronomy. The ''pull'' behind the choice came from the quotation by Ralph Waldo Emerson in the preface: ''If the stars should appear one night in a thousand years, how would men believe and adore, and preserve for many generations the remembrance . . .'' The idea of vast cosmic distances, measured in light travel time, so that celestial objects are viewed through a kind of time machine, captured my imagination. What would an early snapshot of our own galaxy look like?

By the time I finished high school, the subject of astronomy had expanded to include studies of quasars, black holes, pulsars, relic

radiation from the primordial fireball of the Big Bang, and other exotic phenomena, and now encompassed results from a growing space exploration program. Our observable Universe had become larger both in kind and extent. The most distant galaxies made up a frontier with a moving boundary—in more senses than one. Not only did the position of this boundary reflect distance limits continually being pushed back by new scientific observations, but the individual galaxies at these boundaries were themselves moving away from us.

The discovery that the recession speed of distant galaxies increases proportionately with their distance was made by the American astronomer, Edwin P. Hubble, in 1929, and is a result of the expansion of the Universe. Hubble used observed shifts in the frequency of light from galaxies to deduce their motions. Determining a source's motion by shifts in the frequency of its emitted light or sound is most familiar to us when we use the rising or falling pitch of an ambulance siren to judge whether the vehicle is approaching or moving away from us.

We perceive higher or lower frequency light as bluer or redder colors. Light from a receding galaxy is spoken of as ''redshifted,'' and features, such as a pattern of emission or absorption lines, appear displaced to longer wavelengths. A galaxy's redshift, z, is defined as this increase in the wavelength of a feature expressed as a fraction of its wavelength when at rest. For nearby galaxies, the recession velocity is simply z multiplied by the speed of light.1

The expansion of the Universe causes recession velocity to increase with an object's distance, so the higher the redshift the farther away the galaxy. Distances to the highest redshift galaxies can also be translated through light travel times into ''look-back times'' in the early Universe.2 The most distant galaxies discussed in Hubble's original paper had redshifts z < 0.004, or look-back times to when the Universe was 99.5% of its age—not very far back in time at all! Sixty years later, when a space

1 This is approximately correct for redshifts much less than 1 (z « 1). For high-redshift (distant) galaxies, there is a relativistic correction, and the recession velocity, v, and redshift, z, are related by the equation:

(1 + z) = 7(c + v)/(c - v), where c is the speed of light.

2 The detailed transformation of redshift, z, into distance and look-back time also depends on how much the current expansion of the Universe has slowed. This deceleration term depends on the density of the Universe.

telescope named in Hubble's honor was launched, the highest measured galaxy redshifts were typically z ~ 1—or a look-back to when the Universe was about a third of its present age. Viewed in terms of a person's lifetime, these redshifts show us galaxies as adults, not infants. The faintness of distant galaxies makes it difficult to determine their redshifts and to identify a high-redshift population without some way of making these objects stand out from the crowd. And the redshift-distance relation means that our view of distant galaxies is not only filtered through a time machine, it is also translated in wavelength.

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