Astron. Astrophys. 325, 961-971 (1997)

3. Observational data

From the literature we compiled inclination corrected data on H I mass surface density, velocity integrated CO intensity, and H surface brightness, with the latter being considered proportional to the SFR. The H line is produced by recombination in H II region ionized by the Lyman-continuum radiation of hot stars, and can thus be considered a most reliable measure of massive star formation (Kennicutt 1983a). If not all stellar photons are absorbed in the H II region gas, this may lead to an underestimation of the SFR (cf. Patel & Wilson 1995a).

The CO brightnesses are converted into H₂ mass surface densities by the same factor for all the galaxies: in accordance with Gordon & Burton (1976), whose data is used in Smith et al. (1978). Usually CO was measured only in a smaller, interior portion of the disk for which H -data is available. Then we consider only that part of the disk for which both data are available. The mass surface density of all the gas is obtained from that of hydrogen by multiplication of a factor 1.4, to account for a helium fraction of ten per cent (by number). All the data is linearly interpolated onto the grid given by the H data.

We do not wish to make any further selection of the data which might reflect some underlying prejudice. Hence, we include all points, for which all three data are available. In the following we give the references and notes for individual galaxies. The types are from Sandage & Tammann (1981):

• Milky Way galaxy: All data are from Smith et al. (1978) who give for H I and H₂ the masses in annuli. The central region with is neglected here, because the uncertain amount of non-thermal contribution, as well as the outer region , where the data are scarce. We have taken their SFR (derived from the galactic distribution of young radio H II regions as measured at 5 GHz) directly in the place of H brightness. The problem of dust obscuration, which is a severe one in H studies, does not exist for these radio selected sources.
• NGC 2841 (Sb): H I from Bosma (1981), CO from Young & Scoville (1982), H from Kennicutt (1989), H starts at 2.7 kpc.
• NGC 4254 = M 99 (Sc(s)I.3): H I and CO from Kenney & Young (1988a), H from Kennicutt (1989).
• NGC 4303 = M 61 (Sc(s)I.2): H I data from Warmels (1988), CO surface brightness from Kenney & Young (1988b), H from Kennicutt (1989),
• NGC 4321 = M 100 (Sc(s)I): H I from Warmels (1988), CO from Kenney & Young (1988b), H from Kennicutt (1989).
• NGC 4535 (SBc(s)I.3): H I from Warmels (1988), CO from Kenney & Young (1988b), H from Kennicutt (1989).
• NGC 4654 (SBc(rs)II): H I from Warmels (1988), CO from Kenney & Young (1988b), H from Kennicutt (1989).
• NGC 4689 (Sc(s)II.3): H I from Warmels (1988), CO from Kenney & Young (1988b), H from Kennicutt (1989).
• NGC 4736 = M 94 (RSab(s)): H I from Garman & Young (1986), CO from Kenney & Young (1988b), H from Kennicutt (1989).
• NGC 5194 = M 51 (Sbc(s)I-II): H I and CO from Scoville & Young (1983), H from Kennicutt (1989).
• NGC 5457 = M 101 (Sc(s)I): all data from Kenney et al. (1991).
• NGC 6946 (Sc(s)II): H I and CO from Tacconi & Young (1986), H from Kennicutt (1989), except for the point with extremely low H at 330 arcmin (his Fig. 2) which is ignored and the data linearly interpolated from the adjacent points.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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