The gas content and kinematics of nearby blue compact galaxies: Implications for studies at intermediate and high redshift

Abstract

We present Arecibo 21 cm spectroscopy, Keck HIRES Hβ spectroscopy, and WIYN R-band imaging of 11 nearby blue compact galaxies with effective B-band surface brightnesses of SBe = 19.4–21.2 mag arcsec^2 and effective radii of R_ eff = 0.6–1.9 kpc. This sample was selected to test the reliability of mass estimates derived using optical emission line widths, particularly for the blue compact star-forming galaxies observed at intermediate redshifts (0.1 < z < 1). In addition, we also measure the H I content and gas depletion timescales for the nearby blue compact galaxies in an attempt to infer the present nature and possible future evolution of their intermediate-redshift analogs. We detected H I in 10 of 11 sample galaxies. They have H I masses of 0.3–4 × 10^9 M_⊙, H I line widths, W_20, of 133–249 km s^-1, dynamical masses of 0.5 to 5 × 10^10 M_⊙, gas depletion timescales, τgas, of 0.3–7 Gyr, H I mass fractions of 0.01–0.58, and mass-to-light ratios of 0.1–0.8. These values span the range of values typical of nearby H II galaxies, irregulars, and spirals. Despite the restricted morphological selection, our sample of galaxies is quite heterogeneous in terms of H I content, dynamical mass, and gas depletion timescale. Therefore, these galaxies have a variety of evolutionary paths and should look very different from each other in 5 Gyr. Those with high masses and gas depletion timescales are likely to retain their interstellar medium for future star formation, while the lower mass objects with small gas depletion timescales may be undergoing their last major event of star formation. Hence, the fading of intermediate-redshift, luminous, blue compact galaxies into NGC 205-type spheroidals is a viable evolutionary scenario, but only for the least massive, most gas-poor objects in this sample. The most consistent characteristic of our morphologically selected sample is that the ratios of H II line widths to H I 21 cm line widths, Script Script R = W20(H )/W20(H ), are systematically less than unity, with an average value of Script R = 0.66 ± 0.16, similar to findings for local H II galaxies. The simplest explanation for this result is that the ionized gas is more centrally concentrated than the neutral gas within the gravitational potential. We find that Script R is a function of line width, such that smaller line width galaxies have smaller values of Script R. Correcting optical line widths by this factor not only raises the derived masses of these galaxies, but also makes them consistent with the local luminosity–line width (Tully-Fisher) relation as well. If this ratio applies to intermediate-redshift galaxies, then the masses of intermediate-redshift, blue compact galaxies can be obtained from optical line widths after applying a small correction factor, and the proposed luminosity evolution of the Tully-Fisher relation is much smaller and more gradual than suggested by studies using optical emission line width measurements.