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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 noH\beta emission line and barely detected [O II]3727 and [O III]5007 lines. Wecall these objects "CFRS H\alpha-single" galaxies and have re-observed asubsample of them at higher spectral resolution, associated with a subsample ofmore normal emission line galaxies. Good S/N spectroscopy at the VLT and theCFHT, with moderate spectral resolution (R>600), reveals that the "CFRSH\alpha-single" galaxies and most of the star forming spirals have highextinctions (A_V>2), high stellar masses and over-solar oxygen abundances. Thepresent study shows that it is hard to derive the detailed properties ofgalaxies (gas chemical abundances, interstellar extinction, stellar population,star formation rates and history) using spectra with resolution below 600. Onemajor drawback is indeed that the underlying Balmer absorption cannot beestimated properly, which could cause the SFRs be either underestimated oroverestimated by factors reaching 10 (average 3.1) for the sample galaxies.These effects are prominent for a large fraction of evolved massive galaxiesespecially those experiencing successive bursts (A and F stars dominating theirabsorption spectra). Further estimates of the cosmic star formation density atall redshifts mandatorily requires moderate resolution spectroscopy to avoidsevere biases.Comment: 14 pages, 12 PS figures, Accepted for publication in A&

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