We investigate the physical significance of a new spectral parameter, η. This parameter was defined based on a Principal Component Analysis of the 2dF Galaxy Redshift Survey, to retain astrophysical information while minimising the effect of measurement uncertainties. We find that while η is correlated with morphological type, there is a large scatter in this relationship. A tighter empirical relationship is found between η and the equivalent width of the Hα line, suggesting a connection with the star formation rate. We pursue this connection using spectral synthesis models. Using models in which the star formation history is parameterised in terms of an exponentially decreasing function of time, we find that there is a tight correlation between η and the ratio of the present to the past-averaged rate of star-formation, often known as the “birthrate ” parameter b. This correlation also holds in models with much more complicated star formation histories, generated by a semi-analytic model of galaxy formation based upon the hierarchical formation scenario. There are two possible causes for the tight correlations we find, between η and b, in those galaxies with the most complex star formation histories; Firstly, the spectra themselves may be degenerate to the actual long-term star formatio
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