Dynamical condition of neutral hydrogen envelopes of dwarf galaxies and their possible morphological evolution

Department of Astronomy, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan e-mail: kamaya@kusastro.kyoto-u.ac.jp

Corresponding author: Y. Y. Tajiri, tajiri@kusastro.kyoto-u.ac.jp

Received: 29 August 2001
Accepted: 15 April 2002

Abstract

We investigate the star-formation history of gas-rich dwarf galaxies, taking account of the dynamical evolution of their neutral hydrogen (Hi) envelope. Gas-rich dwarfs are classified into blue compact dwarfs (BCDs) and dwarf irregulars (dIrrs). In this paper, their Hi envelope is clearly shown not to be blown away by their stellar feedback. This is concluded since the observed star-formation rate (SFR) of gas-rich dwarfs is generally smaller than a critical SFR, , at which stellar feedback accelerates the Hi envelope to the escape velocity. From this standpoint and the chemical property of sample BCDs, we suggest two possibilities; (1) the Hi gas in the envelope of BCDs is consumed to fuel their star-formation; and (2) BCDs have a similar star-formation history. We also discuss morphological evolution among dwarf galaxies. As long as gas-rich dwarfs are isolated, it is difficult for them to evolve into dwarf ellipticals (dEs). When the Hi envelope in gas-rich dwarfs is consumed in subsequent star-formation, a morphological exchange between BCDs and dIrrs is still expected, consistent with previous studies. If the SFR of gas-rich dwarfs was much higher than in the past, interestingly, an evolutionary scenario from dEs to gas-rich dwarfs is possible.