SIMBAD references

1997A&A...325...81P - Astronomy and Astrophysics, volume 325, 81-97 (1997/9-1)

Circumnuclear molecular gas and star formation in starburst galaxies.


Abstract (from CDS):

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 MH2≃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 LIR/MH2=21L/M. The LIR/MH2-{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 LIR/MH2 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 Ml∼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 LIR/MH2 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<LIR/MH2<100L/M range, and reaches a maximum value of LIR/MH2=100L/M for {SIGMA}H2 larger than 103M/pc2. The upper limit found for the LIR/MH2 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 LIR/MH2 ratio in the 10 to 30L/M range is dominated by the circumnuclear star formation alone. The infrared luminosity in those galaxies with LIR/MH2 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.

Abstract Copyright:

Journal keyword(s): galaxies: general - galaxies: ISM - galaxies: Seyfert - galaxies: starburst

Nomenclature: Fig.7, Table 3a: [PCP97] NGC 2903 RN (Nos R1-R8). Fig.8, Table 3b: [PCP97] NGC 3351 RN (Nos R1-R8). Fig.9, Table 3c: [PCP97] NGC 3504 RN (Nos R1-R4).

Status at CDS : All or part of tables of objects will not be ingested in SIMBAD.

Simbad objects: 54

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