The emission line sequence of normal spiral galaxies

We have analyzed the emission line properties in the integrated spectra of 15 normal spiral galaxies. We show that very clear trends appear when plott ing relevant emission line ratios or equivalent widths as a function of gal axy spectral types, obtained with a Principal Component Analysis of the con tinua and absorption features of spectra. The equivalent widths of all the lines analyzed correlate extremely well with spectral types, implying that each of them can be considered a good indicator of the spectral type in nor mal galaxies. The position of most galaxies of our sample in classical emis sion line diagnostic diagrams follows that of individual giant HII regions in spiral galaxies, but for the earliest type galaxies, the emission line p attern resembles more that of LINERs. Therefore, the direct interpretation of equivalent widths in terms of star formation rates would be misleading i n such cases. The observed trends in the emission line ratios as a function of galaxy spectral type suggest a decrease of O/H, a decrease of N/O, an i ncrease of the average effective temperature or ionization parameter, and a decrease of the effective internal extinction of galaxies with increasing (early to late) spectral type.