The Star Formation Histories of Dwarf Galaxies in the Sculptor and M81 Groups

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Session 35 -- Dwarf Galaxies
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[35.03D] The Star Formation Histories of Dwarf Galaxies in the Sculptor and M81 Groups

B. W. Miller (DTM/STScI)

Because they are numerous and have a wide range of properties, dwarf galaxies are useful for studying the process of star formation, the evolution of galaxies, and the relative importance of environment and intrinsic characteristics on the evolution of dwarf galaxies. The Sculptor Group is diffuse while the M81 Group is more compact and there is evidence that several of its galaxies are interacting. The observational program consists of narrow band imaging to detect H~II regions, optical nebular spectroscopy to determine oxygen abundances, and $BVI$ imaging to investigate the stellar populations.

Present star formation activity ranges from quiescent for M81dA and six of the eight Sculptor galaxies, to very active for M81dB. The oxygen abundances are about 10\% of the solar value and these galaxies follow the known trend that more luminous galaxies have higher abundances. While only two of the Sculptor galaxies have current star formation, the stellar populations show a variety of recent star formation histories. The two dE galaxies have color gradients and there is evidence for star formation within the last billion years. E407-G18 has the color of a dE galaxy but has significant H~I emission and a concentration of blue supergiants at its center. Three other dI galaxies show evidence of star formation within the last 100~Myr. Distance estimates of the two galaxies in the Sculptor sample that have current star formation indicate that both are too far away to be group members. This implies that intrinsic properties, especially mass, are of primary importance in determining a dwarf galaxy's evolution. However, the two dwarf galaxies with the shortest star formation timescales are in the M81 Group and there is evidence that Ho~IX may be being formed due to the tidal interactions between M81, M82, and NGC~3077. Thus, environment can also be important for driving the evolution of dwarf galaxies; in denser environments there is a higher probability of galaxies having close interactions which may trigger star formation.

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