Taft Armandroff

Tucson Nighttime Scientific Staff

Areas of Interest

Stellar Populations in the Galaxy and Nearby Galaxies; Dwarf Spheroidal Galaxies; Globular Clusters

Recent Research Results

Armandroff has been studying the dwarf spheroidal satellite galaxies of M31, in collaboration with Da Costa (Mt. Stromlo), Caldwell (SAO) and Seitzer (Michigan). This was motivated by the opportunity to increase the number of galaxies defining the properties of dwarf spheroidals, and by the fact that the somewhat different environment of the M31 dwarfs compared to those of the Galaxy allows a first look at how dwarf spheroidal properties change with environment. Images were obtained with the HST WFPC2 camera of And I & II, dwarf spheroidal (dSph) galaxies located in the outer halo of M31. The resulting color-magnitude diagram reveals for the first time the morphology of the horizontal branch in these galaxies. They find that, in a similar fashion to many of the Galactic dSph companions, the horizontal branch (HB) of And I & II is predominantly red. Combined with the metal abundances of the dSphs, this red HB morphology indicates that And I & II can be classified as a "second parameter" systems in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy. In addition to the red HB stars, blue HB and RR Lyrae variable stars are also found in the And I & II color-magnitude diagrams. The presence of these stars indicates that And I & II contain a minority population whose age is comparable to that of the Galactic globular clusters. Thus, again like many of the Galactic dSphs, there is clear evidence for an extended epoch of star formation in And I & II. This most recent of this research is published in the Astronomical Journal (2000, volume 119, page 705).

Future Research Plans

Pursuing the theme described above of studying the properties of dwarf spheroidal galaxies as a function of environment represents a significant portion of Armandroff's research agenda for the coming year. WFPC2 observations of three other M31 dwarf spheroidals, And III, V & VI have been obtained. From these data, he hopes to learn whether these three galaxies resemble their Galactic counterparts as much as And I & II do.


Armandroff serves as U.S. Gemini Project Manager. In this capacity, he is responsible for the U.S. contributions to instrumentation for the Gemini telescopes. Armandroff is a member of the Committee of Gemini Offices, participating in the Gemini Instrument Forum and Operations Forum. Armandroff also chairs NOAO's Instrument Projects Advisory Committee (IPAC). Finally, Armandroff serves as a WIYN Consortium Board member and the Board's Secretary.

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Posted: 22Jun2000