5th International Engineering and Natural Sciences Conference (IENSC 2020), 5 - 06 November 2020, pp.51-52
Purpose: Molecular emission lines are the key observables to understand star formation processes in galaxies, and galaxy evolution better. The integrated line intensity measurements of molecular emission lines are particularly useful. Although hydrogen molecule is the most abundant molecule in the Universe, it is extremely hard to observe directly in cold star-forming regions in galaxies. We use CO instead, i.e. the second most abundant molecule in star-forming molecular clouds and easily excited in such cold regions (~ 10 Kelvin). Since a higher transition of a molecule requires a higher temperature to be excited, the ratio between the same molecular species indicates the change in the temperature, e.g. the higher the 12CO(1-0)/12CO(2-1) ratio, the colder the gas. Since 13CO is generally thinner than its parent molecule 12CO in galaxies, an increase in the 12CO(1-0)/13CO(1-0) ratio indicate the existence of more diffuse gas.
Material and Methods: NGC 3521 is a nearby spiral galaxy located at the distance of 17 Mpc, and a good laboratory to study the nature of star formation. The observations of multiple CO lines [i.e. 12CO(1-0), 12CO(2-1), 12CO(3-2), and 13CO(1-0)] covering the entire disc of the galaxy were obtained from the literature. To study the extend, velocity field and kinematics of the gas, we created integrated line intensity (moment 0) and velocity maps (moment 1), and also position-velocity diagrams (PVDs) as follows. To create moment maps we first defined a region of contiguous emission in each CO data cube and created a 3D mask. We then applied those masks to the original cubes to obtained the moment maps. We created the PVDs by taking a three-pixel slit (one beam) along the major axis of the galaxy in the original (masked) cubes. For the data analysis, we used Multichannel Image Reconstruction, Image Analysis and Display (MIRIAD) and Interactive Data Language (IDL) environments. For line ratio diagnostics across the entire disc of the galaxy, we first created identical data cubes having the same angular resolution, pixel size and the same area in the sky. We then created the CO moment maps from those identical cubes. We finally took the ratio of the moment 0 maps, i.e. the ratio of the integrated line intensities across the disc, namely 12CO(1-0) / 13CO(1-0), 12CO(1-0) / 12CO(2-1) and 12CO(1-0) / 12CO(3-2).
Results and Conclusion: The moment 0 maps indicate that the 12CO(1-0) and 12CO(2-1) show similar distribution over the galaxy while the rest of the CO lines are more centrally concentrated. As indicated by the moment 1 map, the gas shows a regular velocity field, indicating no recent or past interactions and/or mergers with other galaxies nearby. PVDs reveal two kinematics components as a signature of a central bar, i.e. the gas is possibly concentrated in a nuclear disc/ring and an inner ring. As the PVDs indicate, the CO gas is reaching the flat part of the rotation curve, i.e. the maximum rotation velocity of the galaxy’s disc, at a velocity of about 230 km/s. As seen from the PVDs, there is particularly strong emission at the edge of both kinematic components. This edge brightening is simply due to the line of sight effect, i.e. the galaxy has a high inclination. Ratios of the moment 0 maps indicate that the gas tends to be colder in the north of the disc compared to the south, and the 12CO(1-0) / 13CO(1-0) ratios seem to be higher around the center of the galaxy, indicating more tenuous gas.
Keywords: galaxies, spiral galaxies, molecular gas, star formation, gas kinematics