Other
Scientific paper
Jul 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002hst..prop.9983n&link_type=abstract
HST Proposal ID #9983
Other
Hst Proposal Id #9983
Scientific paper
Ring galaxies are an aesthetically captivating galactic/inter-galactic phenomenon which HST is exceptionally qualified to observe. AM0644-741, the Lindsay-Shapley ring galaxy, will be observed as part of the Hubble Heritage Project. The Cartwheel galaxy {AM0035-335} is similar in shape and morphology to AM0644-741, but the Cartwheel's ring has a smaller angular scale on the sky and is about 50% further away. With a larger apparent size not completely capturable in a single WFPC2 pointing, about 30% better absolute spatial resolution should be achievable with AM0644-741 compared with the Cartwheel. AM0644-741 and two companion galaxies {including a candidate impactor} fits within a single rough ACS pointing. The ring itself represents a relatively brief but important transient evolutionary stage in only a small fraction of galaxies in the universe. It is believed that rings such as found in AM0035-335 and AM0644-741 are likely produced after a collision between two or more galaxies with a relatively narrow range of encounter velocities, relative masses, and impact parameters. If the encounter is outside of this parameter space, other unique tidal features can be produced, such as tails and mergers. The Cartwheel and AM0644-741 are at roughly similar evolutionary stages. But rings are not uniquely formed from collisions of the type producing the Cartwheel. For example, Hoag's Object represents an apparent ring galaxy likely formed by a different mechanism: complete tidal disruption of a closely orbiting small galaxy around a giant elliptical. In the case of the Cartwheel and the southern ring galaxy AM0644-741, the rings are not entire disrupted galaxies, but probably material from within one of the galaxies which traces the result of a pass-through collision of an intruder galaxy. Indeed, both objects have nearby collision candidates. After passage of the intruder, the remanant velocity of the material is sufficient to continue expansion after closest approach shortly following pass-through, thereby forming a coherent, expanding ring structure. Local conditions in the ring are not the same as before the encounter, so gas and dust resident in the ring can collapse to produce starbursts. If AM0644-741 is so similar to the Cartwheel, then why image it with HST? The aim of the Heritage Project is to provide spectacular HST color images for public release, outreach, and education. AM0644-741 is nearly ideally sized to fit into the ACS/WFC. The Cycle 4 GO 5410 Cartwheel { http://www.stsci.edu/cgi-bin/get-proposal-info?5410 } program with WFPC2 was a 3-orbit observation set in only two colors: B {3200 seconds} & I {1800 seconds}. Nevertheless, that composite picture has had enduring, wide appeal with the public, and has provided an aesthetic benchmark motivating this target selection. Five orbits in B, V, R, and I with ACS/WFC are to be used to produce a higher signal-to-noise product and provide richer color information at the high angular resolution inherent with HST.
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