Molecular line ratio diagnostics along the radial cut and dusty ultraviolet-bright clumps in a spiral galaxy NGC 0628

Abstract

Molecular emission lines are essential tools to shed light on many questions regarding star formation in galaxies. Multiple molecular lines are particularly useful to probe different phases of star-forming molecular clouds. In this study, we investigate the physical properties of giant molecular clouds (GMCs) using multiple lines of CO, i.e. CO(1–0, 2–1, 3–2) and 13CO(1–0), obtained at selected 20 positions in the disc of NGC 0628. A total of 11 positions were selected over the radial cut, including the centre, and remaining 9 positions were selected across the southern and northern arms of the galaxy. A total of 13 out of 20 positions are brighter at $24\, \mu {\rm m}$ and ultraviolet (UV) emission and hosting significantly more H ii regions compared to the rest of the positions indicating opposite characteristics. Our line ratio analysis shows that the gas gets warmer and thinner as a function of radius from the galaxy centre up to 1.7 kpc, and then the ratios start to fluctuate. Our empirical and model results suggest that the UV-bright positions have colder and thinner CO gas with higher hydrogen and CO column densities. However, the UV-dim positions have relatively warmer CO gas with lower densities bathed in GMCs surrounded by less number of H ii regions. Analysis of multiwavelength infrared and UV data indicates that the UV-bright positions have higher star formation efficiency than that of the UV-dim positions.

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

“…A CO survey of about 80 nearby spiral galaxies indicates an average ratio of R21 = 0.89 ± 0.06 (Braine et al 1993). Similarly, the range for R21 ratios in nearby galaxies (including our own Milky Way) is from 0.6 to 1 (Hasegawa 1997;Leroy et al 2009;Topal 2020), lower than the ratio of 1.1 ± 0.1 we find at the centre of NGC 5033. It is also seen in panels b and c of Figure 5 is that the R21 ratio at the cen-tre of NGC 5033 is higher than in most of the galaxies, indicating a relatively warm and/or dense gas component.…”

“…It is also seen in panels b and c of Figure 5 is that the R21 ratio at the cen-tre of NGC 5033 is higher than in most of the galaxies, indicating a relatively warm and/or dense gas component. The panel c of the same figure also indicates that NGC 5033 has a higher R21 compared to the centre of the spiral galaxies NGC 0628 (Topal 2020) and NGC 2903 (Topal 2018) showing no AGN activity, again indicating relatively highly excited gas in the AGN host centre of the spiral 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.…”

“…TK = 20 K and log(n(H2)) = 4.2 cm −3 ) with the same N (CO) compared to the centre of the Seyfert galaxy NGC 5033. Additionally, various positions over the disc of the spiral galaxy NGC 0628 were studied using an identical method to ours (Topal 2020). They found that the central gas complex in NGC 0628 has the following physical properties; TK = 30 K, log(n(H2)) = 2.2 cm −3 , and log(N (CO)) = 17.7 cm −2 .…”

“…In galaxies, centre, arms and inter-arm regions could, therefore, have different R11 ratios, causing fluctuations in the R11 ratio, generally seen across the disc of a galaxy (e.g. Topal 2020).…”