1997A&A...325...81P

The molecular gas properties and circumnuclear star formation in the nearby nuclear starburst galaxies NGC 2903, NGC 3351 and NGC 3504 are investigated in detail. The circumnuclear HII regions in the star-forming rings of these galaxies are characterised by an ionized gas mass in the 10⁴-10⁵M_☉ range, and by an ionizing flux in the 10⁵¹-10⁵²ph/s range, typical of giant HII regions in external galaxies. The CO 2->1 emission in NGC 3504 indicates the presence of two emitting regions separated by 115km/s in velocity and 5" (500pc) spatially, what approximately corresponds to the location of the the inner inner Lindblad resonance. The measured (CO 2->1)/(CO 1->0) integrated intensity ratio for the inner kpc of NGC 3351 is 0.8, in agreement with the empirical average value found for spiral galaxies with a normal metallicity regardless of the presence or the absence of a starburst. Molecular gas masses in the range of M_H2≃10⁸-10⁹M_☉ are measured in regions of a few hundred parsecs in size. On average, the circumnuclear region of the three galaxies is characterised by an average molecular gas surface density {SIGMA}_H2=280M_☉/pc², and average L_IR/M_H2=21L_☉/M_☉. The L_IR/M_H2-{SIGMA}_H2 relationship covering the {SIGMA}_H2 range from normal spiral galaxies and giant molecular clouds in the Milky Way with {SIGMA}_H2∼10¹-10²M_☉/pc², to nearby starbursts, AGNs with starbursts, and luminous IRAS galaxies with {SIGMA}_H2∼2x10²-10⁵M_☉/pc², is investigated. Nearby starburst galaxies have an average molecular gas surface density {SIGMA}_H2 of 400M_☉/pc² and an average L_IR/M_H2 of 23L_☉/M_☉. These two properties are explained by the existence of giant molecular clouds with associated HII regions where the star formation process is characterised by being short lived (≤3x10⁷years), biased towards a high lower mass limit M_l∼3M_☉, and with an overall gas to stars conversion fraction of ≤10% of the gas mass. The star formation efficiency (SFE) in nearby starbursts and luminous IRAS galaxies (as measured by the L_IR/M_H2 ratio) does not show any indication of a linear correlation with increasing {SIGMA}_H2. On the contrary, the star formation efficiency is restricted to the 10<L_IR/M_H2<100L_☉/M_☉ range, and reaches a maximum value of L_IR/M_H2=100L_☉/M_☉ for {SIGMA}_H2 larger than 10³M_☉/pc². The upper limit found for the L_IR/M_H2 ratio, independent of {SIGMA}_H2, implies the existence of an upper limit in the fraction of gas converted into stars independent of the density. The SFE range observed in nearby starbursts, Seyferts with starbursts, and luminous IRAS galaxies is explained as a combination of massive star formation in the circumnuclear regions of these galaxies plus the additional contribution of an AGN at the center of the galaxy. Under the starburst+AGN scenario, most of the molecular gas mass is directly involved in the star forming process while the AGN contribution to the global IR luminosity is in the 10% to 75% range, depending on the galaxy. In this scenario, the infrared luminosity of galaxies with observed L_IR/M_H2 ratio in the 10 to 30L_☉/M_☉ range is dominated by the circumnuclear star formation alone. The infrared luminosity in those galaxies with L_IR/M_H2 in the 30-100L_☉/M_☉ range is increasingly dominated by the central AGN. Examples of this situation are NGC 1068, NGC 7469 and Mrk 231.