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Astron. Astrophys. 325, 81-97

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Circumnuclear molecular gas and star formation
in starburst galaxies

P. Planesas1, L. Colina2,*, and D. Pérez-Olea2,3

1Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800 Alcalá de Henares, Spain
2Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3Departamento de Física Teórica, Módulo C-XI, Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain

Received 18 April 1996 / Accepted 10 March 1997


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 FORMULA range, and by an ionizing flux in the 1051 1052 ph s-1 range, typical of giant HII regions in external galaxies.

The CO 21 emission in NGC 3504 indicates the presence of two emitting regions separated by 115 km s-1 km s-1 in velocity and 5FORMULA (500 pc) spatially, what approximately corresponds to the location of the the inner inner Lindblad resonance. The measured (CO 21)/(CO 10) 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 FORMULA FORMULA 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 FORMULA FORMULA , and average FORMULA FORMULA .

The FORMULA FORMULA relationship covering the FORMULA range from normal spiral galaxies and giant molecular clouds in the Milky Way with FORMULA pc-2, to nearby starbursts, AGNs with starbursts, and luminous IRAS galaxies with FORMULA pc-2, is investigated. Nearby starburst galaxies have an average molecular gas surface density FORMULA of 400 FORMULA pc-2 and an average FORMULA of 23 L FORMULA FORMULA . 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 ( FORMULA years), biased towards a high lower mass limit M FORMULA , and with an overall gas to stars conversion fraction of FORMULA % of the gas mass.

The star formation efficiency (SFE) in nearby starbursts and luminous IRAS galaxies (as measured by the FORMULA ratio) does not show any indication of a linear correlation with increasing FORMULA . On the contrary, the star formation efficiency is restricted to the 10 < FORMULA < 100 L FORMULA FORMULA range, and reaches a maximum value of FORMULA = 100 L FORMULA FORMULA for FORMULA larger than FORMULA pc-2. The upper limit found for the FORMULA ratio, independent of FORMULA , 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 FORMULA ratio in the 10 to 30 L FORMULA FORMULA range is dominated by the circumnuclear star formation alone. The infrared luminosity in those galaxies with FORMULA in the 30-100 LFORMULA FORMULA range is increasingly dominated by the central AGN. Examples of this situation are NGC 1068, NGC 7469 and Mrk 231.

Key words: galaxies: general - galaxies: ISM - galaxies: Seyfert - galaxies: starburst

*On assignment from the Space Science Department of ESA

© European Southern Observatory (ESO) 1997