Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...325...81p&link_type=abstract
Astronomy and Astrophysics, v.325, p.81-97
Astronomy and Astrophysics
Astrophysics
43
Galaxies: General, Galaxies: Ism, Galaxies: Seyfert, Galaxies: Starburst
Scientific paper
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 10^4^-10^5^Msun_ range, and by an ionizing flux in the 10^51^-10^52^ph/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_H_2__=~10^8^-10^9^Msun_ 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}_H_2__=280Msun_/pc^2^, and average L_IR_/M_H_2__=21Lsun_/Msun_. The L_IR_/M_H_2__-{SIGMA}_H_2__ relationship covering the {SIGMA}_H_2__ range from normal spiral galaxies and giant molecular clouds in the Milky Way with {SIGMA}_H_2__~10^1^-10^2^Msun_/pc^2^, to nearby starbursts, AGNs with starbursts, and luminous IRAS galaxies with {SIGMA}_H_2__~2x10^2^-10^5^Msun_/pc^2^, is investigated. Nearby starburst galaxies have an average molecular gas surface density {SIGMA}_H_2__ of 400Msun_/pc^2^ and an average L_IR_/M_H_2__ of 23Lsun_/Msun_. 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 (<=3x10^7^years), biased towards a high lower mass limit M_l_~3Msun_, 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_H_2__ ratio) does not show any indication of a linear correlation with increasing {SIGMA}_H_2__. On the contrary, the star formation efficiency is restricted to the 10
Colina Luis
Perez-Olea Diego
Planesas Pere
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