Misleading results from low-resolution spectroscopy: From galaxy interstellar medium chemistry to cosmic star formation density*
GEPI, Observatoire de Paris-Meudon, 92195 Meudon, France e-mail: [yanchun.liang,francois.hammer]@obspm.fr
2 Department of Astronomy, University of Florida, 216 Bryant Space Science Center, PO Box 112055, Gainesville, FL 32611-2055, USA
Corresponding author: Y. C. Liang, firstname.lastname@example.org
Accepted: 19 December 2003
Low resolution spectroscopy () from the Canada-France-Redshift Survey (CFRS) revealed intriguing properties for low redshift galaxies (): nearly half of their spectra show prominent Hα emission lines, but no Hβ emission line and barely detected  λ3727, and  λ5007 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 (), have allowed us to perform a full diagnostic of their interstellar medium chemistry. “CFRS Hα-single" galaxies and most of the star forming spirals have high extinctions (), 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.
Key words: galaxies: abundances / galaxies: photometry / galaxies: evolution / galaxies: spiral / galaxies: starburst
Based on observations collected with the ESO Very Large Telescope at the Paranal Observatory (under programmes 66.A-0599(A) and 67.A-0218(A)) and with the Canada-France-Hawaii Telescope (under programmes 96IIF25, 98IIF16, 98IF65A and 98IIC14), which is operated by CNRS of France, NRC of Canada, and the University of Hawaii.
© ESO, 2004