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Session 73 - Spiral Galaxies.
Oral session, Thursday, June 11
We present the analysis of HST \tt WFPC2 photometry in the F606W filter for a sample of 75 spiral galaxies. The survey shows that: (i) In only a fraction (\approx 40%) of the sample there is morphological/photometric evidence for a smooth, ``classical'' R^1/4-law bulge. In a similar fraction of galaxies, the inner regions either show an irregular, star forming morphology (which did not allow us to perform an isophotal fit), or are well fitted by an exponential profile. (ii) Resolved, regular-shaped central compact sources (CCSs), photometrically-distinct from the surrounding regions, are detected in most of the exponential bulges, and in galaxies with an irregular, star forming bulge-like structure. (iii) The R^1/4-law bulges lie on the relation traced by early-type galaxies between the nuclear stellar cusp slope and the absolute bulge magnitude (with the nuclear stellar cusp slope quantified by the average logarithmic slope within 0.1''-0.5''): the fainter the bulge, the steeper is its nuclear stellar cusp slope. (iv) Classical R^1/4-law bulges, dominant exponential structures embedded in faint disks, and small exponential structures hosted by normal dominant disks, occupy distinct regions of parameter space on the \mu_e-R_e plane, with \mu_e the mean surface brightness within the half-light radius R_e. While classical R^1/4-law bulges form the known continuum with the dynamically-hot, giant early-type galaxies, the inner bulge-like exponential structures have systematically fainter mean surface brightness for any constant radius, and \mu_e-R_e values comparable to those of the inner disks often embedded in elliptical galaxies. (v) The large, dominant, exponential bulges, whose morphologies resemble that of bright dwarf ellipticals, lie on the \mu_e-R_e plane in the same region of parameter space that these systems occupy, which suggests that dynamically-hot, bright dwarf ellipticals may form within disks, and possibly by disk-related mechanisms. These, and the preliminary results from our HST/NICMOS follow-up survey of the same sample, challenge the orthodox view about ``what is a bulge'', and generally ask for a change of its definition.
Program listing for Thursday