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
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
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 5 (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 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
The relationship covering the
normal spiral galaxies and giant molecular clouds in the Milky Way with
pc-2, to nearby starbursts,
AGNs with starbursts, and luminous IRAS galaxies with
pc-2, is investigated.
Nearby starburst galaxies have an average molecular gas surface density
of 400 pc-2 and an average
of 23 L . 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
( years), biased towards a high lower mass limit
and with an overall gas to stars conversion fraction of
% of the gas mass.
The star formation efficiency (SFE)
in nearby starbursts and luminous IRAS galaxies
(as measured by the ratio)
does not show any indication of a linear correlation with increasing
. On the contrary, the star formation efficiency
is restricted to the 10 < < 100 L
range, and reaches a maximum value of = 100 L
for larger than pc-2.
The upper limit found for the ratio, independent of ,
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 ratio in the
10 to 30 L range is dominated by the
circumnuclear star formation alone.
The infrared luminosity in those galaxies with in the
30-100 L 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 -
*On assignment from the Space Science Department of ESA
© European Southern Observatory (ESO) 1997