Misleading results from low-resolution spectroscopy: from galaxy interstellar medium chemistry to cosmic star formation density
Abstract
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 Hα emission line, but no Hβ emission line and barely detected [O II]3727 and [O III]5007 lines. We call these objects "CFRS Hα-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 Hα-single" galaxies and most of the star forming spirals have high extinctions (AV>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.
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