Evolution of dwarf galaxies: a dynamical perspective^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
e-mail: [email protected]
² Department of Physics and Astronomy, University of Bologna, via Berti Pichat 6/2, 40127 Bologna, Italy

Received: 12 September 2013
Accepted: 15 November 2013

Abstract

For a rotating galaxy, the inner circular-velocity gradient d_RV(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 H I and/or stellar rotation curves (including starbursting dwarfs, irregulars, and spheroidals), and estimate d_RV(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 μ₀, the mean atomic gas surface density Σ_gas, and the star formation rate surface density Σ_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 μ₀ 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 Σ_SFR = ϵ   Σ_gas/τ_orb, where τ_orb is the orbital time and ϵ ≃ 0.02.