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Astron. Astrophys. 351, 10-20 (1999)

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5. Conclusions

  1. Analysis of the J=2-1 12CO distribution and kinematics shows the presence of enhanced molecular emission in a ringlike zone in NGC 7331, peaking at a radial distance of 3.5 kpc with a width of about 2 kpc. At R = 3.5 kpc, the velocity-integrated CO intensity of the ring itself is about 0.6 times that of the underlying more smoothly distributed CO emission that fills the entire bulge of NGC 7331.

  2. The velocity-integrated CO intensities in the center of NGC 7331 decrease strongly with increasing rotational level. The intensities in the J=1-0, J=2-1, J=3-2 transitions are in the ratio of 1.0: 0.55: 0.35 respectively. The observed 12CO/13CO isotopic ratios are 6.7 and 5.6 in the J=1-0 and J=2-1 transitions respectively. Positions at larger radial distances have similar ratios, albeit with somewhat stronger J=3-2 CO emission, and weaker 13CO emission. Weak [CI] emission was detected from the center.

  3. Modelling of the observed line ratios suggest a multi-component molecular medium. Gas with a kinetic temperature of about 10 K appears to be present at both low and high densities. At high densities, a warmer component with a kinetic temperature of 20 K or more is also present within the observing beams. The gas is probably distributed in a clumpy and filamentary form.

  4. Assuming a [C]/[H] abundance ratio of the order of 1-2 [FORMULA], the mean CO-to-H2 conversion factor is [FORMULA] = 4 [FORMULA] 1019 cm-2 in the bulge region, and double that value in the ring and beyond. These values are well below those found in the Solar Neighbourhood, but they are consistent with the high metallicity of NGC 7331 and with submillimeter dust observations.

  5. In the bulge, interstellar gas (HI + H2 + He) mass densities, projected onto the plane of the galaxy, are of the order of 11 [FORMULA] cm-2. In the ring itself, now properly placed at R = 3.1 kpc, the gas mass density is almost twice as high. Within the ring, the interstellar gas mass is dominated by the molecular hydrogen contribution. Gas to total (dynamical) mass ratios are about 1 [FORMULA] in the center and about 1.5 [FORMULA] in the ring.

  6. The molecular ring coincides more or less with the mostly nonthermal radio continuum ring and the 850 µm ring representing emission from cold dust. Emission from warmer dust in the 100 µm wavelength range peaks well inside the molecular ring; dust temperatures appear to be decreasing with radius reaching a mininmum in the ring. The radial distribution of HI reaches it maximum well beyond the molecular ring.

  7. The molecular ring is well inside the radius of peak rotational velocity. Its maximum is just at the edge of the region of solid-body rotation, and just at the radius where disk light becomes dominant over bulge light. The ring is not associated with an inner Lindblad resonance. The molecular gas inside the ring may have originated from mass loss by late type stars in the bulge. If this is the case, the ring is probably the result of wind-driven gas removal from the center.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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