  Low resolution spectroscopy (R=150) from the Canada-France-Redshift Survey
(CFRS) had revealed intriguing properties for low redshift galaxies (z<=0.3):
nearly half of their spectra show prominent H\alpha emission line, but no
H\beta emission line and barely detected [O II]3727 and [O III]5007 lines. We
call these objects "CFRS H\alpha-single" galaxies and have re-observed a
subsample of them at higher spectral resolution, associated with a subsample of
more normal emission line galaxies. Good S/N spectroscopy at the VLT and the
CFHT, with moderate spectral resolution (R>600), reveals that the "CFRS
H\alpha-single" galaxies and most of the star forming spirals have high
extinctions (A_V>2), high stellar masses and over-solar oxygen abundances. The
present study shows that it is hard to derive the detailed properties of
galaxies (gas chemical abundances, interstellar extinction, stellar population,
star formation rates and history) using spectra with resolution below 600. One
major drawback is indeed that the underlying Balmer absorption cannot be
estimated properly, which could cause the SFRs be either underestimated or
overestimated by factors reaching 10 (average 3.1) for the sample galaxies.
These effects are prominent for a large fraction of evolved massive galaxies
especially those experiencing successive bursts (A and F stars dominating their
absorption spectra). Further estimates of the cosmic star formation density at
all redshifts mandatorily requires moderate resolution spectroscopy to avoid
severe biases.
