Integrated Magnitudes and Mean Colors of DDO Dwarf Galaxies in the UBV System. II – Distances, Luminosities, and H I Properties

Abstract

An analysis of the properties of the DDO magellanic irregular "dwarf" galaxies for which U, B, V photometry was reported in Paper I leads to the following conclusions: (1) The mean effective surface brightness (m'_e)^0 varies with zenith distance as expected from the dependence on sec z of atmospheric extinction and sky brightness. (2) The mean surface brightness is independent of distance modulus (25 < mu_0 < 33). (3) The mean absolute magnitude M^0_T is brighter than average (and the mean effective linear diameter larger) south of +10° declination (dwarfs fainter than -15 are missing) and fainter near the zenith at Palomar. There is a curious near absence of DDO objects in the zone +20° < delta < +30°. The range of absolute magnitudes (-19 < M^0_T < -12) is largest north of +60° declination. (4) The mean corrected effective color index (U - V )^0_e is independent of absolute magnitude at M^0_T < -15, but fainter systems tend to be bluer. (5) The mean absolute magnitude M^0_T is correlated with distance modulus at mu_0 > 28; we confirm that the DDO sample is not restricted to dwarf systems and includes galaxies as bright as M^0_T <˜ -18 at mu_0 > 30. (6) We confirm also that the luminosity class L and luminosity index Lambda are not valid indicators of absolute magnitude for late-type systems beyond mu_0 ~= (Delta = 4 Mpc). (7) Effective linear diameter and absolute magnitude are highly correlated (i.e., mean surface brightness is not a distance indicator). (8) Apparent magnitude and 21-cm line flux are loosely correlated, but there is a large range of HI index (hydrogen/luminosity ratio) among the DDO objects at a given Hubble type (Sd to Im). (9) The relative excess or deficiency in hydrogen/luminosity ratio is uncorrelated with mean effective surface brightness (except for selection effects). (10) The hydrogen/luminosity ratio residuals (at a given type) are loosely correlated with color residuals: DDO objects richer than average in neutral hydrogen tend to be bluer in (U - V)^0_e. (11) The necessity of correcting H I linewidths for velocity dispersion is once again demonstrated by the conspicuous departure from linearity of the Tully-Fisher relation based on raw linewidths. Correction for turbulent motions restores the linearity of the relation. For want of reliable inclination estimates for individual objects a statistical correction depending on the log axis ratio log R is applied. (12) The derived T - F relation for the DDO sample of late-type systems is in near perfect agreement in slope and zero point with the relation adopted by Bottinelli et al. for normal spiral galaxies of earlier types. The distance moduli derived from the 21-cm linewidths are in good statistical agreement with moduli derived from group membership and redshift but with a larger scatter. (13) The supergalactic anisotropy of the velocity-distance relation is in evidence in the DDO sample, confirming the departures from the ideal Hubble law indicated by earlier studies of brighter spiral galaxies.