Molecular distribution and kinematics in nearby galaxies. II. Sub-mm observations of NGC4945.

Details Molecular distribution and kinematics in nearby galaxies. II. Sub-mm observations of NGC4945. Molecular distribution and kinematics in nearby galaxies. II. Sub-mm observations of NGC4945.

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...309..705m&link_type=abstract

Astronomy and Astrophysics, v.309, p.705-714

Astronomy and Astrophysics

Astrophysics

42

Galaxies: Individual: Ngc4945, Galaxies: Starburst, Galaxies: Ism, Galaxies: Nuclei, Ism: Clouds

Scientific paper

The central 80"x80" of the nearby edge-on starburst galaxy NGC4945 has been mapped in the J=3-2 line of ^12^CO with a resolution of 15". Spectra of the J=3-2 transition of ^13^CO and tentative detections of the J=4-3 line of HCN and the J_K_a_K_c__=3_12_-2_11_ line of H_2_CO are also presented. The CO J=3-2 emission is concentrated toward the kinematical center of the galaxy with a deconvolved full width at half power (FWHP) size of (11.5+/-3)" corresponding to (200+/-50)pc at a distance D=3.7Mpc. This is less than the extent of the nuclear molecular complex as derived from the distributions of the lower rotational CO transitions. Position-velocity maps reveal three condensations located at the center and at offsets of 5" toward either side of the nucleus. The dynamical mass in the inner R<=600pc is ~3x10^9^Msun_. Applying a "standard" N(H_2_)/I(CO) conversion factor of 2x10^20^cm^-2^/(K.km/s), the central molecular gas mass is ~9x10^8^Msun_. Like in the case of the central regions of the Milky Way and NGC253, integrated ^12^CO line intensities combined with a standard conversion factor yield a gas mass which is 6 to 14 times higher than that obtained from multi-transition CO models and from the dust continuum emission. If the latter results are correct, we find a conversion factor of only 0.15-0.35x10^20^cm^-2^/(K.km/s) for the central region of NGC4945. An embedded active nucleus may contribute to the very high "star forming efficiency" L_IR_/M_gas_~140Lsun_/Msun_. The contribution of the CO lines to the "continuum" flux density in a 50GHz 1.3mm band is comparable to that of the dust. From the strength of the CO J=3-2 emission and our tentative detection of high density tracing molecules, the bulk of the CO emitting gas should have a high density (n(H_2_)=10^3.5^-10^4^cm^-3^) and is clumped with a beam filling factor of roughly 20%.

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