Evolution of dwarf galaxies: a dynamical perspective

For a rotating galaxy, the inner circular-velocity gradient d(R)V(0) provides a direct estimate of the central dynamical mass density, including gas, stars, and dark matter. We consider 60 low-mass galaxies with high-quality HI and/or stellar rotation curves (including starbursting dwarfs, irregulars, and spheroidals), and estimate d(R)V(0) as V-Rd/R-d, where R-d is the galaxy scale length. For gas-rich dwarfs, we find that V-Rd/R-d correlates with the central surface brightness mu(0), the mean atomic gas surface density Sigma(gas), and the star formation rate surface density Sigma(SFR). Starbursting galaxies, such as blue compact dwarfs (BCDs), generally have higher values of V-Rd/R-d than dwarf irregulars, suggesting that the starburst is closely related to the inner shape of the potential well. There are, however, some compact irregulars with values of V-Rd/R-d similar to BCDs. Unless a redistribution of mass takes place, BCDs must evolve into compact irregulars. Rotating spheroidals in the Virgo cluster follow the same correlation between V-Rd/R-d and mu(0) as gas-rich dwarfs. They have values of V-Rd/R-d comparable to those of BCDs and compact irregulars, pointing to evolutionary links between these types of dwarfs. Finally, we find that, as for spiral galaxies and massive starbursts, the star-formation activity in dwarfs can be parametrized as Sigma(SFR) = epsilon Sigma(gas)/tau(orb), where tau(orb) is the orbital time and epsilon similar or equal to 0.02.