A dust-penetrated classification scheme for bars as inferred from their gravitational force fields

The division of galaxies into "barred" (SB) and "normal" (S) spirals is a fundamental aspect of the Hubble galaxy classification system. This "tuning fork" view was revised by de Vaucouleurs, whose classification volume recognized apparent "bar strength" (SA, SAB, SB) as a continuous property of galaxies called the "family." However, the SA, SAB, and SB families are purely visual judgments that can have little bearing on the actual bar strength in a given galaxy. Until very recently, published bar judgments were based exclusively on blue light images, where internal extinction or star formation can either mask a bar completely or give the false impression of a bar in a nonbarred galaxy. Near-infrared camera arrays, which principally trace the old stellar population in both normal and barred galaxies, now facilitate a quantification of bar strength in terms of their gravitational potentials and force fields. In this paper, we show that the maximum value, Q(b), of the ratio of the tangential force to the mean axisymmetric radial force in a barred disk galaxy is a quantitative measure of the strength of a bar. Q(b) does not measure bar ellipticity or bar shape but rather depends on the actual forcing due to the bar embedded in its disk. We show that a wide range of true bar strengths characterizes the category "SB," while the de Vaucouleurs category "SAB" corresponds to a narrower range of bar strengths. We present Q(b) values for 36 galaxies, and we incorporate our bar classes into a dust-penetrated classification system for spiral galaxies.