0 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 de Vaucouleurs category " SAB " corresponds to a much 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.