Regularities in galaxy spectra and the nature of the Hubble sequence

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We have investigated some statistical properties of integrated spectra of galaxies with principal component analysis. The projection of the spectra onto the plane defined by the first two principal components shows that normal galaxies are in a quasi-linear sequence that we call spectral sequence and is closely related to the Hubble morphological sequence. We verify that the spectral sequence is also an evolutive sequence, with galaxy spectra evolving from later to earlier spectral types. Considering the close correspondence between the spectral and morphological sequences, we speculate that galaxies may evolve morphologically along the Hubble sequence, from Sm/Im to E. If this is the case, the first galaxies were mainly gas rich irregular objects, which evolved later along the morphological sequence as long as mergers and interactions increased their masses and developed their spheroidal components.