Probing the Physics of Molecular Clouds in Spiral Galaxies NGC 5055 and NGC 3627
Abstract
Although galaxies can be grouped in a few categories in terms of morphology, they have remarkably different intrinsic properties. Spiral galaxies host substantial amounts of molecular gas and have ongoing star formation activity with respect to elliptical galaxies lacking star formation. Molecular emission lines are used to probe the internal properties of molecular gas clouds where stars are born and die. Carbon monoxide (CO) is easily detectable in the interstellar medium (ISM) of galaxies. In this research, we probe the physics of the gas clouds at multiple positions in disc galaxies NGC 5055 (M63) and NGC 3627 (M66) using four CO transitions and their line ratios. 12CO(J=1-0) is the brightest across the disc of both galaxies compared to the other lines, i.e., 12CO(J=2-1, J=3-2) and 13CO(J=1-0). The CO intensities show a decrease from the center of the galaxies to the outskirts. However, NGC 3627 shows a rather irregular decrease pattern compared to NGC 5055. The CO line ratios show an increase up to a distance from the center and start to decrease. Although NGC 5055 shows a similar variation in the line ratios on each side of the disc, NGC 3627 has an opposite trend on either side. Therefore, the ISM could have different temperatures, opacity, densities, and levels of star formation in different regions of the galaxy’s disc. Our results indicate that the line ratios found at the center of both galaxies are different. The difference could be the result of the bar-driven gas accumulation in the center of NGC 3627. The line ratios in the center of NGC 5055 are within the range found for the centers of other spiral and active galaxies in the literature, but the ratios in the center of NGC 3627 are relatively lower.
Keywords
Galaxies, Molecular clouds, Spiral galaxies, Star formation
Thanks
The authors thank the anonymous referees for their useful comments and suggestions. This research has used the NASA/IPAC Infrared Science Archive, operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan; Academia Sinica Institute of Astronomy and Astrophysics; the Korea Astronomy and Space Science Institute; the National Astronomical Research Institute of Thailand; Center for Astronomical Mega-Science (as well as the National Key R&D Program of China with No. 2017YFA0402700). Additional funding support is provided by the Science and Technology Facilities Council of the United Kingdom and participating universities and organizations in the United Kingdom and Canada. This publication made use of data from COMING, CO Multi-line Imaging of Nearby Galaxies, a legacy project of the Nobeyama 45-m radio telescope.
References
- Cappellari, M., Emsellem, E., Krajnović, D., McDermid, R. M., Serra, P., Alatalo, K., ..., & Young, L. M. (2011). The ATLAS3D project–VII. A new look at the morphology of nearby galaxies: the kinematic morphology–density relation. Monthly Notices of the Royal Astronomical Society, 416(3), 1680-1696. doi:10.1111/j.1365-2966.2011.18600.x
- de Vaucouleurs, G. (1974). Structure, dynamics and statistical properties of galaxies. Symposium - International Astronomical Union, 58, 1-53. doi:10.1017/S007418090002430X
- de Vaucouleurs, G., de Vaucouleurs, A., Corwin, Jr. H. G., Buta, R. J., Paturel, G., Fouque, P. (1991). Third Reference Catalogue of Bright Galaxies. Volume I: Explanations and references. Volume II: Data for galaxies between 0h and 12h. Volume III: Data for galaxies between 12h and 24h. New York, USA: Springer.
- Endres, C. P., Schlemmer, S., Schilke, P., Stutzki, J., & Müller, H. S. (2016). The cologne database for molecular spectroscopy, CDMS, in the virtual atomic and molecular data centre, VAMDC. Journal of Molecular Spectroscopy, 327, 95-104. doi:10.1016/j.jms.2016.03.005
- Fukui, Y., Kawamura, A., Wong, T., Murai, M., Iritani, H., Mizuno, N., ..., & Kim, S. (2009). Molecular and atomic gas in the large magellanic cloud. II. Three-dimensional correlation between CO and H I. The Astrophysical Journal, 705(1), 144. doi:10.1088/0004-637X/705/1/144
- Hollenbach, D. J., & Tielens, A. G. G. M. (1999). Photodissociation regions in the interstellar medium of galaxies. Reviews of Modern Physics, 71(1), 173. doi:10.1103/RevModPhys.71.173
- Hubble, E. P. (1936). Realm of the Nebulae. USA: Yale University Press.
- Israel, F. P. (2020). Central molecular zones in galaxies: 12CO-to-13CO ratios, carbon budget, and X factors. Astronomy & Astrophysics, 635, A131. doi:10.1051/0004-6361/201834198
- Kormendy, J., & Bender, R. (2012). A Revised parallel-sequence morphological classification of galaxies: structure and formation of S0 and spheroidal galaxies. The Astrophysical Journal Supplement Series, 198, 2-41. doi:10.1088/0067-0049/198/1/2
- Lamperti, I., Saintonge, A., Koss, M., Viti, S., Wilson, C. D., He, H., ..., & Tacconi, L. J. (2020). The CO (3–2)/CO (1–0) luminosity line ratio in nearby star-forming galaxies and active galactic nuclei from xCOLD GASS, BASS, and SLUGS. The Astrophysical Journal, 889(2), 103. doi:10.3847/1538-4357/ab6221