Molecular gas properties of the giant molecular cloud complexes in the arms and inter-arms of the spiral galaxy NGC 6946

Abstract

Combining observations of multiple CO lines with radiative transfer modeling is a very powerful tool to investigate the physical properties of the molecular gas in galaxies. Using new observations as well as literature data, we provide the most complete CO ladders ever generated for eight star-forming regions in the spiral arms and inter-arms of the spiral galaxy NGC 6946, with observations of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5), 13 CO(1-0) and 13 CO(2-1) transitions. For each region, we use the large velocity gradient assumption to derive beam-averaged molecular gas physical properties, namely the gas kinetic temperature (T K ), H 2 number volume density (n(H 2 )) and CO number column density (N (CO)). Two complementary approaches are used to compare the observations with the model predictions: χ 2 minimisation and likelihood. The physical conditions derived vary greatly from one region to the next: T K = 10-250 K, n(H 2 )= 10 2.3 -10 7.0 cm −3 and N (CO)= 10 15.0 -10 19.3 cm −2 . The spectral line energy distribution (SLED) of some of these extranuclear regions indicates a star-formation activity that is more intense than that at the centre of our own Milky Way. The molecular gas in regions with a large SLED turnover transition (J max > 4) is hot but tenuous with a high CO column density, while that in regions with a low SLED turnover transition (J max 4) is cold but dense with a low CO column density. We finally discuss and find some correlations between the physical properties of the molecular gas in each region and the presence of young stellar population indicators (supernova remnants, H II regions, H I holes, etc).

Molecular line ratio diagnostics and gas kinematics in the AGN host Seyfert galaxy NGC 5033

“…As seen from Figure 7, the ∆χ 2 r contour maps indicate that TK increases sharply at lower n(H2), and it gradually decreases as n(H2) increases (i.e. the banana-shaped degeneracy, see van der Tak et al 2007;Topal et al 2014;Topal 2020). Single Gaussian profiles are seen in the PDFs (see Section 4.2), except that for TK (Fig.…”

“…The data for Seyfert galaxies in panel c were taken from Papadopoulos & Seaquist (1998). Black circles with a cross in panels a and d represent the GMCs in NGC 6946 (Topal et al 2014) and in M31 (Brouillet et al 2005), respectively. The light grey shaded areas in panels a and c show the range for the R 11 ratios in spirals (Paglione et al 2001), while the dark grey shaded area in panels a, b and d show the range for R DC ratios in spirals (Gao & Solomon 2004).…”

Multiple-line study of molecular gas in spiral galaxy NGC 2903

“…As revealed from Figure 5, there is degeneracy in T K due to its characteristic 'banana'-shaped ∆χ 2 r contours or inverse n (H 2 ) − T K relationship ( [25,37]; see also appendix C of [15]). The PDF of the kinetic temperature T K also shows a single-peaked distribution peaked at 20 K where the χ 2 result meets with the peak, followed by a steady decrease.…”

“…We compare our best-fitting results (listed in Table 5) to that of literature studies done for the center and along the disc of one spiral and one ETG using similar lines and the same methods. Molecular gas properties along the disc of the spiral galaxy NGC 6946 and ETG NGC 4710 [25][26][27]37] were also studied using the LVG approximation. We found that n (H 2 ) is to be higher in the center of NGC 2903 compared to that found in most of the regions studied in the arms of NGC 6946 [25] and in the center of NGC 6946 [26,27].…”

“…For 13 CO(1-0) and 12 CO(3-2) spectra, we applied a beam correction to their integrated intensities by following the expression below [25][26][27]:…”