EVIDENCE FOR SECULAR EVOLUTION IN LATE-TYPE SPIRALS

We combine deep optical and IR photometry for 326 spiral galaxies from two recent galaxy samples and report that the surface brightness prof iles of late-type spirals are best fitted by two exponentials. Moreove r, the ratio of bulge and disk scale lengths takes on a restricted ran ge of values and is uncorrelated with Hubble type. This suggests a sca le-free Hubble sequence for late-type spirals. Careful numerical simul ations ensure that our results are not affected by seeing or resolutio n effects. Many of these galaxies show spiral structure continuing int o the central regions with a previously undetected small bar and slowl y changing colors between the inner disk and the bulge. We invoke secu lar dynamical evolution and interpret the nature of disk central regio ns in the context of gas inflow via angular momentum transfer and visc ous transport. In this scenario, galaxy morphologies in late-type spir als are not imprinted at birth but are the result of evolution.