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What Are Astronomers Doing? Week of August 30
NASA Astronomer of the Week:
Shardha Jogee
The barred spriral galaxy NGC 1300 as seen by Hubble Space Telescope. |  |
Until two years ago, only a tiny fraction of the galaxies present over the last 13 billion years had been imaged at the superb resolution provided by the Hubble Space Telescope (HST). The situation changed dramatically in 2004 when three ground-breaking surveys known as GEMS, GOODS, and the Hubble Ultra Deep Field (HUDF) were conducted with the Hubble's latest camera known as the Advanced Camera for Surveys. University of Texas astronomer Shardha Jogee had the rare privilege of being a team member of all three surveys, due to her expertise and research, which had already been recognized by NASA through the award of a prestigious long-term space astrophysics (LSTA) grant.
The GEMS, GOODS, and HUDF surveys were designed to help solve a cosmic puzzle: How do galaxies form and evolve over time into present-day mature galaxies, such as our own galaxy, the Milky Way? Each survey revealed a different missing part of the puzzle. GEMS, the largest-area survey ever conducted with HST using two filters, mapped an area 120 times larger than that of the famous Hubble Deep field completed in 1996! GEMS acts as a cosmic time probe by imaging 10,000 galaxies that are present out to early epochs, when the universe was merely 40 percent of its present age.
The GOODS and HUDF surveys cover a much smaller area than GEMS, but their strengths lie in the extremely deep exposures they provide, thereby enabling astronomers to push the envelope of time even further. As part of the HUDF home team, Shardha helped to define the observing strategy and was in charge of the quantitative calculations for the exposures, imaging depth, and telescope pointings. Observations distributed over 410 HST orbits eventually produced the HUDF, making it the deepest visible-light image ever made of the universe.
The HUDF chronicles a period when the universe was only 700 million years old — a mere five percent of its present age. At such early times, the very first galaxies were forming, chaos reigned, and violent interactions between galaxies were the norm.
As an expert on barred galaxies, Shardha has led the GEMS research on these fascinating systems. Barred galaxies refer to spiral galaxies, such as our on Milky Way , whose disks harbor a majestic elongated or oval-shaped feature called a stellar bar. Bars can profoundly influence the evolution of a spiral galaxy by funneling large amounts of gas from the outer parts of the galaxy into the central regions, where the gas “ignites” extremely powerful episodes of star formation called starbursts.
As reported in the Astrophysical Journal in 2004, Shardha's research has demonstrated that barred spiral galaxies, analogous to ancestors of the Milky Way, have been around for a very long time — nearly nine billion years, or since the universe was 30 percent of the age that it is today. In fact, Shardha's findings categorically refute the earlier controversial claim made by other smaller studies that barred galaxies like our own Milky Way were practically absent at early epochs and only came into existence very recently.
Shardha's results have important implications for the lifetime of bars and support a picture where bars are long-lived over many billions of years. Pushing these investigations further using HST images and detailed computer simulations, Shardha and her colleagues have used the abundance of bars at early epochs to draw important conclusions on the shape of the mysterious dark matter halo that surrounds young galaxies.
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