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Molecular line images of 13CO, C18O, CN, CS, CH3OH, and HNCO are obtained toward the spiral arm of M51 at a 7 times 6 resolution with the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Distributions of the molecules averaged over a 300 pc scale are found to be almost similar to one another and to essentially trace the spiral arm. However, the principal component analysis shows a slight difference of distributions among molecular species particularly for CH3OH and HNCO. These two species do not correlate well with star-formation rate, implying that they are not enhanced by local star-formation activities but by galactic-scale phenomena such as spiral shocks. Furthermore, the distribution of HNCO and CH3OH are found to be slightly different, whose origin deserves further investigation. The present results provide us with an important clue to understanding the 300 pc scale chemical composition in the spiral arm and its relation to galactic-scale dynamics.
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Observations of various molecular lines toward a disk region of a nearby galaxy are now feasible, and they are being employed as diagnostic tools to study star-formation activities there. However, the spatial resolution attainable for a nearby galaxy with currently available radio telescopes is $10-1000$ pc, which is much larger than the scales of individual star-forming regions and molecular cloud cores. Hence, it is of fundamental importance to elucidate which part of an interstellar medium such spatially-unresolved observations are tracing. Here we present sensitive measurements of the H$_2$CO ($1_{01}-0_{00}$) line at 72 GHz toward giant molecular clouds (GMCs) in the spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction with the previously observed H$_2$CO ($2_{02}-1_{01}$) and CS ($2-1$ and $3-2$) lines, we derive the H$_2$ density of the emitting regions to be $(0.6-2.6)times10^4$ cm$^{-3}$ and $(2.9-12)times10^4$ cm$^{-3}$ for H$_2$CO and CS, respectively, by the non-LTE analyses, where we assume the source size of $0.8-1$ kpc and the gas kinetic temperature of $10-20$ K. The derived H$_2$ density indicates that the emission of H$_2$CO and CS is not localized to star-forming cores, but is likely distributed over an entire region of GMCs. Such widespread distributions of H$_2$CO and CS are also supported by models assuming lognormal density distributions over the 1 kpc region. Thus, contributions from the widespread less-dense components should be taken into account for interpretation of these molecular emission observed with a GMC-scale resolution. The different H$_2$ densities derived for H$_2$CO and CS imply their different distributions. We discuss this differences in terms of the formation processes of H$_2$CO and CS.
We present simulations of a 500 pc$^2$ region, containing gas of mass 4 $times$ 10$^6$ M$_odot$, extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionising feedback from stars of mass > 18 M$_odot$. Our region is evolved for 10 Myr and shows clustered star formation along the arm generating $approx$ 5000 cluster sink particles $approx$ 5% of which contain at least one of the $approx$ 4000 stars of mass > 18 M$_odot$. Photoionisation has a noticeable effect on the gas in the region, producing ionised cavities and leading to dense features at the edge of the HII regions. Compared to the no-feedback case, photoionisation produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long range effects of ionisation, with some already-dense gas becoming compressed from multiple sides by neighbouring HII regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30% of our cluster sink particle mass forming at distances > 5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.
We have conducted a spectral line survey in the 3 mm and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the IRAM 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N2H+, HNCO, and CH3OH. Furthermore, 6 isotopologues of the major species have been detected. On the other hand, SiO, HC3N, CH3CN, and the deuterated species such as DCN and DCO+ are not detected. The deuterium fractionation ratios are evaluated to be less than 0.8 % and 1.2 % for DCN/HCN and DCO+/HCO+, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1-kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH3OH are found to be abundant. High abundances of CN, and CCH are consistent with the above picture of a wide spread distribution of molecules, because they can be produced by photodissociation. On the other hand, it seems likely that CH3OH is liberated into the gas phase by shocks associated with large scale phenomena such as cloud-cloud collisions and/or by non-thermal desorption processes such as photoevaporation due to cosmic-ray induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in AGNs and starbursts.
We have investigated the dynamics of the molecular gas and the evolution of GMAs in the spiral galaxy M51 with the NRO 45-m telescope. The velocity components of the molecular gas perpendicular and parallel to the spiral arms are derived at each spiral phase from the distribution of the line-of-sight velocity of the CO gas. In addition, the shear motion in the galactic disk is determined from the velocity vectors at each spiral phase. It is revealed that the distributions of the shear strength and of GMAs are anti-correlated. GMAs exist only in the area of the weak shear strength and further on the upstream side of the high shear strength. GMAs and most of GMCs exist in the regions where the shear critical surface density is smaller than the gravitational critical surface density, indicating that they can stably grow by self-gravity and the collisional agglomeration of small clouds without being destroyed by shear motion. These indicate that the shear motion is an important factor in evolution of GMCs and GMAs.
We report systematic variations in the CO(2-1)/CO(1-0) line ratio (R) in M51. The ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions, passage into the spiral arms and back into the downstream interarm regions. In the interarm regions, R is typically low <0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far-IR luminosities. However, the ratio rises to >0.7 (often 0.8-1.0) in the spiral arms, particularly at their leading (downstream) edge. R is also high, 0.8-1.0, in the central region. An LVG calculation provides insight into the changes in the gas physical conditions between the arm and interarm regions: cold and low density gas (~10K, ~300cm-3) is required for the interarm GMCs, but this gas must become warmer and/or denser in the more active star forming spiral arms. R is higher in areas of high 24micron brightness (an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2-1) line relative to CO(1-0) in luminous star forming regions suggests that some caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass.
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