Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...309...43c&link_type=abstract
Astronomy and Astrophysics, v.309, p.43-58
Astronomy and Astrophysics
Astrophysics
15
Galaxies: Evolution, Stars: Formation, Galaxies: Ism, Radio Lines: Galaxies
Scientific paper
We present a study of the relationships between the gas content and star formation activity in a sample of 101 disc galaxies located in the region of the Coma supercluster. We use as star formation indicators the optical color indices, the Hα equivalent width, the radio continuum and far-infrared emissions, while the gas content of the galaxies is measured by the H I line emission at 21cm for atomic hydrogen and indicated by the CO(1-0) line at 2.6mm for molecular hydrogen. The CO(1-0) data come from previous observations using the IRAM 30m, the Onsala 20m, and the NRAO 12m telescopes. We have found that molecular gas constitutes only one fifth of the gas content of the sample galaxies which are not strongly H I-deficient. Late-type galaxies (Sc) are slightly richer in both atomic and molecular gas than early-type ones (Sa), but the molecular fraction does not vary significantly between Sa and Sc. The average total gas surface density, including upper limits but excluding H I-deficient objects, amounts to 10^(6.88 +/-0.04)^Msun_/kpc^2^. In this sample, the H I-deficient galaxies are similar in their CO emission to the H I-normal galaxies belonging to the periphery of the cluster and globally gas-deficient. It is thus likely that the missing H I has not been converted into H_2_, but rather removed from the galaxy. The above conclusions do not depend on the quantity chosen for the normalization of the observed quantities, optical area or H-band luminosity. On the other hand, using one or the other scaling gives a fairly different picture of the relationships between the gas contents and the star formation indicators. The H_2_ surface density is well correlated with the far-infrared and radio continuum surface brightness, but shows no trend with either the color indices U-B and B-V or the Hα equivalent width. On the other hand, the H I surface density is well correlated with these optical star formation tracers, but neither with the radiocontinuum and far-infrared surface brightnesses, nor with the H_2_ surface density. The various star formation tracers show a better correlation with the H_2_ content when it is normalized to L_H_. They are however still correlated with the H I and total gas contents, except for the radio continuum emission. This discrepancy with what is found when scaling with the optical area suggests that the lack of correlation of the H_2_ surface density with the optical star formation tracers cannot be entirely attributed to the uncertainties on the computation of H_2_ masses, or to extinction effects in the optical data.
Baumgardt K.
Boselli Alessandro
Casoli Fabienne
Dickey John
Gavazzi P.
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