Explorations in Hubble space: A quantitative tuning fork

In order to establish an objective framework for studying galaxy morphology , we have developed a quantitative two-parameter description of galactic st ructure that maps closely onto Hubble's original tuning fork. Any galaxy ca n be placed in this "Hubble space," where the x-coordinate measures positio n along the early-to-late sequence while the y-coordinate measures in a qua ntitative way the degree to which the galaxy is barred. The parameters defi ning Hubble space are sufficiently robust to allow the formation of Hubble' s tuning fork to be mapped out to high redshifts. In the present paper, we describe a preliminary investigation of the distribution of local galaxies in Hubble space based on the CCD imaging atlas published by Frei et al. in 1996. We find that barred, weakly barred, and unbarred galaxies are remarka bly well separated on this diagnostic diagram. The spiral sequence is clear ly bimodal and indeed approximates a tuning fork: strongly barred and unbar red spirals do not simply constitute the extrema of a smooth unimodal distr ibution of bar strength but, rather, populate two parallel sequences. Stron gly barred galaxies lie on a remarkably tight sequence, strongly suggesting the presence of an underlying unifying physical process. Rather surprising ly, weakly barred systems do not seem to correspond to objects bridging the parameter space between unbarred and strongly barred galaxies but, instead , form an extension of the regular spiral sequence. This relation lends sup port to models in which the bulges of late-type spirals originate from secu lar processes driven by bars.