STScI Annual Report 2002: A Living Mission

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Science with ACS: A New View — Adam G. Riess EVERYONE APPRECIATES A NICE VIEW, AND ASTRONOMERS ARE NO EXCEPTION. The new Advanced Camera for Surveys (ACS) offers them twice the field of view and five times the depth of the Wide Field and Planetary Camera 2, its predecessor as the primary Hubble imaging instrument. They have used this increased grasp of light for many investigations in its first observing cycle. Although astronomers will need a year or more to analyze and draw conclusions from these early ACS observations, a number of exciting discoveries, confirmations, and controversies are already emerging. One issue is whether galaxies at redshift z > 6 are less numerous than expected from the 1995 Hubble Deep Field North (HDFN) investigation. The team that developed ACS, led by Principal Investigator (PI) Holland Ford and deputy PI Garth Illingworth, reported an abundance of high-redshift galaxies in new images of the HDFN and in the background of their images of low redshift galaxies, like the Tadpole and the Mice, which were early-release observations. Nevertheless, the ACS team found a surprising lack of galaxies at z > 6—fewer by factor five to ten than expected from extrapolating the z ~ 5 sample in the original HDFN study. Meanwhile, another group has claimed the detection of ~ 30 galaxies at z ~ 6 using the ‘drop-out’ search technique, with the criterion that the object is present in the z-band image but absent in the I-band image. The ACS team invested 20 Hubble orbits to obtain a stunning deep image of a massive cluster of galaxies, Abell 1689. The image shows red elliptical galaxies framed by spidery blue arcs of strongly lensed background sources. By analyzing the separations of pairs of background sources—including one as distant as z = 4.9— they measured the cluster mass within the Einstein radius of its lens to be 1.5 x 1014 solar masses, a value in good agreement with the estimate from X-ray studies.
Although these are still early days with ACS, the expanded view offered by this new camera has already provided a welcomed boost in the scientific capabilities of Hubble. In their re-imaging of the HDFN, the ACS team discovered two type Ia supernovae at maximum light. They identified SN 2002dc (z = 0.47) and SN 2002dd (z = 0.96) by the ‘wiggly’ appearance of their spectra in a grism image. From quick follow-up observations of the supernova light curves, astronomers found distances to the host galaxies that are consistent with the current paradigm of an accelerating universe. The ACS team obtained coronagraphic images of bright objects to find any faint sources nearby. Images of the nearby quasar 3C273 show multiple, complex structures around the unresolved central source, which may indicate a recent merger. Images of young stars show dust disks. Observations of HD 141569A, a Herbig AE star, discovered a spiral debris disk showing possible tidal distortion by a nearby binary system. Led by PI Mauro Giavalisco, the Great Observatories Origins Deep Survey (GOODS) program obtained deep, wide, four-color images of two fields centered on the HDFN and the Chandra Deep Field South to study galaxy evolution. The data are shallower by ~ 0.5 to 0.7 magnitude than the original HDFN, but the area covered is 30 times larger, providing a more significant sample statistically for studying the development of galactic complexity. The largest fraction of the GOODS observing time has gone into the z-band observations, a valuable wavelength range not available for the original HDF observations using WFPC2. With STScI AR 02 | A Living Mission careful mosaicing, cataloging, and culling, these images are yielding large samples of ‘extremely red objects’, ‘drop-outs’ (candidates for very high-redshift galaxies), Lymanbreak galaxies (intermediate redshift), and optical counterparts to bright X-ray sources. Interestingly, most ‘extremely red objects’ appear to be relatively clean elliptical galaxies, not heavily extincted, poststarburst galaxies. The GOODS team optimized the sequencing of their z-band images for discovery of type Ia supernovae at higher redshift than can be discovered from the ground. They are taking exposures every 45 days for 5 epochs of 15 pointings. A so-called ‘piggyback program’ (PI Adam Riess) works with the GOODS team to identify transients rapidly and to analyze theirs colors and photometric redshifts. This is producing a sample of type Ia supernovae at z > 1.2, which Riess is studying for evidence of an earlier epoch of decelerated expansion during the matter-dominated phase of cosmic expansion. Other ACS programs in Cycle 11 with good potential for interesting results include a deep imaging campaign on Andromeda (PI Tom Brown) to study stellar evolution near the main-sequence turn-off, a light echo from a nova that provides a rare, geometric distance within the galaxy (PI Howard Bond), and z ~ 5.6 to 6.0 Lyman-α emitters from grism imaging of parallel fields (PI James Rhoads). � news news 9
Page 1 and 2: Space Telescope Science Institute A
Page 3 and 4: STScI AR 02 | A Living Mission fore
Page 5: We at the Space Telescope Science I
Page 8 and 9: profiles Melissa McGrath Melissa Mc
Page 12 and 13: profiles Marty Durkin Marty Durkin
Page 14 and 15: Riccardo Giacconi receiving his Nob
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Page 18 and 19: profiles Lisa Wolff Lisa Wolff join
Page 20 and 21: 18 news IDTL personnel and detector
Page 22 and 23: 20 science essays The Lessons of De
Page 24 and 25: 22 science essays Bar-Driven Evolut
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Page 30 and 31: 28 organization Steven Beckwith Dep
Page 32 and 33: 30 organization Mario Livio Antonel
Page 34 and 35: 32 organization Chris Blades Bill S
Page 36 and 37: 34 teams Teams Outside The TREASURY
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Page 42 and 43: goals 40 goals Goal 1. Hubble is ou
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Page 54 and 55: 52 achievements We completed a majo
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Page 58 and 59: Melissa McGrath (cont’d) Project
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science publications 58 publication
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60 publications Griffin, R. G. and
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62 publications O’Dea, C. P., de
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64 publications Suchkov, A. A., Sch
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STScI Annual Report 2002: A Living Mission

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