Misleading results from low-resolution spectroscopy:: From galaxy interstellar medium chemistry to cosmic star formation density

Low resolution spectroscopy (R=150) from the Canada-France-Redshift Survey (CFRS) revealed intriguing properties for low redshift galaxies (zless than or equal to0.3): nearly half of their spectra show prominent Halpha emission lines, but no Hbeta emission line and barely detected [O II] lambda3727, and [O III] lambda5007 lines. We call these objects CFRS Halpha-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), have allowed us to perform a full diagnostic of their interstellar medium chemistry. CFRS Halpha-single galaxies and most of the star forming spirals have high extinctions (A(V)>2), high stellar masses and over-solar oxygen abundances. From the present study, we find it hazardous to derive the detailed properties of galaxies (gas chemical abundances, interstellar extinction, stellar population, star formation rates and history) using spectra with resolutions below 600. One major drawback is indeed the estimated extinction which requires a proper analysis of the underlying Balmer absorption lines. We find that, with low resolution spectroscopy, star formation rates (SFRs) can be either underestimated or overestimated by factors reaching 10 (average 3.1), even if one accounts for ad hoc extinction corrections. 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 and uncontrolled biases.