Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...280..451b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 280, no. 2, p. 451-467
Astronomy and Astrophysics
Astrophysics
25
Atomic Spectra, Gas Density, Interstellar Matter, Molecular Spectra, Spiral Galaxies, Carbon Monoxide, Dark Matter, H Lines, Molecular Clouds, Radio Spectra, Velocity Measurement
Scientific paper
(12)CO(1-0), (12)CO(2-1), (13)CO(1-0), (13)CO(2-1), and H I observations with comparable angular resolution (approximately equal 22 arcsecs) are presented for the flocculent Sc-type galaxy NGC 4414. The neutral gas surface density in the disk is one of the highest known, about 50 solar mass, sq pc, comparable to that of the grand design galaxy M51. The large-scale conditions do not appear to change from point to point as the (12/13)CO(1-0) emission ratio is equal to 9 and the (12)CO(2-1)/(1-0) line ratio equal to 0.7 throughout the disk. The variation in the (12/13)CO(1-0) ratio is much lower than has yet been observed. We believe that this is largely due to the quality of the data. For the points near the center where the (13)CO(2-1)/(1-0) line ratio is well determined, it is also equal to 0.7 and the (12/13)CO(2-1) ratio is about 9. We note that despite the huge difference is optical depth (12)CO(2-1)/(1-0) approximately equal (13)CO(2-1)/(1-0) as well in several other galaxies, although at different values. This behavior is not easy to explain. We have determined the (12/13)CO(1-0) ratio as a function of velocity and find that the (12/13)CO ratio is about 8 throughout most of the velocity space for most spectra but that the low-velocity line wing of each side of the galaxy has a higher (12/13)CO ratio. This likely implies a lower optical depth for these clouds. The H-alpha picture reveals a central hole in star-formation, of 8 arcsec diameter. The molecular line data is not conclusive about the presence of a central hole. No compact radio continuum source is present and the 1.4 GHz continuum appears entirely associated with star-formation in the disk. Despite the high gas surface density, NGC 4414 is not a 'starburst' galaxy, a fact that could be related to its isolation. The H I disk extends far beyond the optical outlines of the galaxy, and it is quite asymmetric at low column densities. A combined CO(2-1) and H I rotation curve shows that the rotational velocities start declining outside the optical disk a phenomenon found only in compact galaxies with high rotational velocities. Therefore, no direct evidence for a massive dark halo in NGC 4414 is found from the rotation curve, a rare case for an Sc spiral.
Braine Jonathan
Combes François
Driel Wim van
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