Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274..123g&link_type=abstract
Astronomy and Astrophysics, Vol. 274, NO. 1/JUL(I), P. 123, 1993
Astronomy and Astrophysics
Astrophysics
118
Scientific paper
The "grand-design" spiral Messier 51 has been mapped in the J = 2-1 and J = 1-0 transitions of ^12^CO with the IRAM 30m telescope. The angular resolution in the J = 2-1 line is 12" (HPBW), the highest of any single-dish CO surveys. Both maps have been fully sampled (i.e. with steps of 6" for J = 2-1) in the inner 3'x3' region, and the lines accurately calibrated. 40 positions have been re-observed in ^13^CO(2-1) and (1-0) and 10 positions in ^12^C0(3-2). The CO emission, which nicely delineates the two-armed spiral pattern, is weak, albeit ubiquitous in the inner interarm region. The CO arms have deconvolved widths of 20-30" and are broader and stronger than those observed by Rand & Kulkarni (1990) with the OVRO interferometer. They stretch almost continuously from the nuclear "disk" (a molecular condensation of radius r ~ 1 kpc), to r = 8 kpc (the corotation radius, according to Paper II). The arm-interarm intensity contrast is 3-5 through most of the inner region and increases to 5-6 near the `corotation'. It is larger for the southern inner arm (Arm I) than for the northern inner arm (Arm II). The rotation curve derived from CO is best approximated by a steeply rising line, reaching V_max_ = 200 kms^-1^ (cos 20^deg^/cos i) within 10" (0.5 kpc), followed by a flat section. The CO velocity field shows large departures from circular rotation. Those consist of `streaming' motions near the arms and near the center, and of random motions. The streaming motions in the arms, already reported in previous studies, can now be followed in the interarm region from one arm to the other. These motions have the direction and behaviour predicted by the density-wave theory inside corotation; their line-of-sight component is larger in the southeast and northwest quadrants and changes sign in the inner edge of the CO arms. The peak to peak variation of this component reaches 60 kms^-1^ in the southeast quadrant, most of the variation occuring inside Arm I in less than 10" (460 pc). The streaming motions in and near Arm I could be as large as +/-100 kms^-1^,if they are in the plane of the galaxy and if i = 20^deg^. The streaming motions across Arm II are a factor of 2 smaller. Arm I, which, inside `corotation',is twice stronger than Arm II in CO and IR emission, is likely to be driven by a stronger response to the density wave. The gas in the nuclear region also exhibits non-circular motions. Molecular clouds probably follow elliptical orbits and are trapped into a `bar' aligned with the stellar bar apparent on infrared pictures. Up to `corotation', the CO arm emission is tightly correlated with the non-thermal radio-continuum emission and as found by Vogel et al.(1988), peaks on the dark dust lanes. Although the Hα regions tend to lie further out than the dust lanes and the CO peaks, most lie within the CO arm boundaries. Thermal radio-continuum emission is observed toward the CO peaks showing that visual extinction is important in the arms - as also suggested by the large molecular column densities derived from the ^13^CO intensities and by the large extinctions measured by van der Hulst et al. (1988) toward 39 H II regions. The CO arms exhibit a number of discrete emission peaks, spaced by 1.5-2 kpc and symmetrically located with respect to the dynamical center. A similar pattern is observed in HI and Hα, although the brightest Hα peaks are shifted radially outwards by a few arc seconds. The remarkable symmetry of the CO peaks and the similarity between the CO, H I, and Hα patterns in the inner arms suggest that we are observing complexes of molecular gas, H II regions and HI envelopes, located at resonant positions. The ^12^CO J = 2-1 to 1-0 line intensity ratio, corrected for the difference in beamsize, is ~0.7-0.8 all over the disk and shows no systematic difference between the arms and the interarm region. The corresponding ^13^CO ratio, on the other hand, varies from 0.8 in the arms to ~0.4 in the interarm region; it reaches 1.2 near the nucleus. The ^12^CO J = 3-2 to 1-0 ratio is large near the center and in the arms: ~0.7. These line intensity ratios are used to derive core-halo models of the molecular cloud complexes, using LVG and Monte Carlo radiative transfer calculations. Different cloud models are derived for the center, the arms and the interarm gas. The mass fraction in the cores is found to increase from the interarm (<20%) to the arms (~50%) and from the arms to the nuclear region (~60%). The CO cloud models reproduce also the HCN (1- 0) line intensities observed by Rieu et al. (1992). Radiative trapping, which is taken into account in the Monte Carlo code, plays a major role in the 2-component models by rising substantially the ^12^CO and HCN line brightnesses in the halos. The molecular column densities derived from the cloud models are factors of 3-5 smaller than those calculated with the `standard Galactic' X = N(H_2_)/I(CO) conversion factor of Strong et al. (1988). In particular, X is found smaller for the interarm gas than for the clouds in the arms, which suggests that the H_2_ arm-interarm contrast is larger than the CO intensity contrast.
Cernicharo Jose
Garcia-Burillo Santiago
Guélin Michel
No associations
LandOfFree
CO in M51 - Part One - Molecular Spiral Structure does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with CO in M51 - Part One - Molecular Spiral Structure, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CO in M51 - Part One - Molecular Spiral Structure will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1066436