| Issue |
A&A
Volume 417, Number 3, April III 2004
|
|
|---|---|---|
| Page(s) | 905 - 918 | |
| Section | Extragalactic astronomy | |
| DOI | https://doi.org/10.1051/0004-6361:20040007 | |
| Published online | 26 March 2004 | |
Misleading results from low-resolution spectroscopy: From galaxy interstellar medium chemistry to cosmic star formation density*
1
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, yanchun.liang@obspm.fr
Received:
21
February
2003
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
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