Galaxy spectral parametrization in the 2dF Galaxy Redshift Survey as a diagnostic of star formation history

We investigate the physical significance of a new spectral parameter, eta. This parameter was defined using a principal component analysis of the 2dF Galaxy Redshift Survey (2dFGRS), to retain astrophysical information while minimizing the effect of measurement uncertainties. We find that although eta is correlated with morphological type, there is a large scatter in this relationship. A tighter empirical relationship is found between eta and the equivalent width of the Halpha 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 parametrized in terms of an exponentially decreasing function of time, we find that there is a tight correlation between eta and the ratio of the present- to the past-averaged rate of star formation, often known as the 'birth rate' 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 eta and b in those galaxies with the most complex star formation histories as follows. First, the spectra themselves may be degenerate to the actual long-term star formation history of each galaxy in the optical wavelength range probed by the 2dFGRS. Secondly, b may represent a physically fundamental quality of galaxy haloes - their overdensity relative to the background density - such that small-b galaxies form in high peaks (which collapse early), whereas large-b galaxies represent lower peaks (which collapse later). We conclude that the tight connection with b makes eta a physically meaningful - as well as convenient and robust - statistic for galaxy parametrization and classification.