Systematic Variations of CO J = 2−1/1–0 Ratio and Their Implications in The Nearby Barred Spiral Galaxy M83

Koda, Jin; Sawada, Tsuyoshi; Sakamoto, Kazushi; Hirota, Akihiko; Egusa, Fumi; Boissier, S.; Calzetti, Daniela; Meyer, Jennifer Donovan; Elmegreen, Bruce G.; Paz, A. Gil de; Harada, Nanase; Ho, Luis C.; Kobayashi, Masato I. N.; Kuno, Nario; Martín, S.; Muraoka, Kazuyuki; Nakanishi, Koichiro; Scoville, N. Z.; Seibert, Mark; Vlahakis, C.; Watanabe, Yoshimasa

doi:10.3847/2041-8213/ab70b7

Cited by 31 publications

(46 citation statements)

References 42 publications

(51 reference statements)

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“…We target the J = 2 → 1 rotational transition of CO, CO(2−1). This line is readily excited at temperatures and densities that characterize molecular clouds, and its spatial distribution in nearby galaxies correlates well with the fundamental CO(1−0) transition (e.g., see Braine & Combes 1992;Leroy et al 2009Leroy et al , 2013bKoda et al 2020;den Brok et al 2021). This choice represents a key practical element of our observing strategy.…”

Section: Observationsmentioning

confidence: 94%

PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

Leroy

Schinnerer

Hughes

et al. 2021

ApJS

253

262

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We present PHANGS–ALMA, the first survey to map CO J = 2 → 1 line emission at ∼1″ ∼100 pc spatial resolution from a representative sample of 90 nearby (d ≲ 20 Mpc) galaxies that lie on or near the z = 0 “main sequence” of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS–ALMA, each beam reaches the size of a typical individual giant molecular cloud, so that these data can be used to measure the demographics, life cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z = 0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, Atacama Large Millimeter/submillimeter Array (ALMA) observations, and characteristics of the delivered data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with MUSE on the Very Large Telescope, the Hubble Space Telescope, AstroSat, the Very Large Array, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle 5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1″ resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS–ALMA public data release.

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Section: Observationsmentioning

confidence: 94%

PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

Leroy

Schinnerer

Hughes

et al. 2021

ApJS

253

262

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“…But the number and quality of CO maps of galaxies have grown significantly compared to any study currently in the literature, particularly with the release of PHANGS-ALMA. Quite a few studies have examined these ratios in individual galaxies and noted local variations in individual ratios (e.g., Crosthwaite & Turner 2007;Koda et al 2012;Vlahakis et al 2013;Ueda et al 2012;Druard et al 2014;Law et al 2018;Koda et al 2020), but so far there has been relatively little attempt to synthesize these mapping measurements (though see the beam-matched, single-pointing measurements by Saintonge et al 2017;Lamperti et al 2020).…”

mentioning

confidence: 99%

“…These general trends are largely born out by detailed studies of individual galaxies (e.g., Koda et al 2012Koda et al , 2020, though there remains disagreement in the literature about the behavior of the ratios, e.g., relative to spiral arms or within individual targets. Some of this may reflect that at high resolution, line ratios can show detailed variations that track the location of individual heating sources or vary across spiral arms and bars (e.g., Ueda et al 2012;Law et al 2018, and T. Saito et al in preparation).…”

mentioning

confidence: 99%

Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

Leroy,

Rosolowsky,

Usero

et al. 2021

Preprint

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We measure the low-J CO line ratio R 21 ≡ CO (2−1)/CO (1−0), R 32 ≡ CO (3−2)/CO (2−1), and R 31 ≡ CO (3−2)/CO (1−0) using whole-disk CO maps of nearby galaxies. We draw CO (2−1) from PHANGS-ALMA, HERACLES, and follow-up IRAM surveys; CO (1−0) from COMING and the Nobeyama CO Atlas of Nearby Spiral Galaxies; and CO (3−2) from the JCMT NGLS and APEX LASMA mapping. Altogether this yields 76, 47, and 29 maps of R 21 , R 32 , and R 31 at 20 ∼ 1.3 kpc resolution, covering 43, 34, and 20 galaxies. Disk galaxies with high stellar mass, log(M /M ) = 10.25−11 and star formation rate, SFR = 1−5 M yr −1 , dominate the sample. We find galaxy-integrated mean values and 16%−84% range of R 21 = 0.65 (0.50−0.83), R 32 = 0.50 (0.23−0.59), and R 31 = 0.31 (0.20−0.42). We identify weak trends relating galaxy-integrated line ratios to properties expected to correlate with excitation, including SFR/M and SFR/L CO . Within galaxies, we measure central enhancements with respect to the galaxy-averaged value of ∼0.18 +0.09 −0.14 dex for R 21 , 0.27 +0.13 −0.15 dex for R 31 , and 0.08 +0.11 −0.09 dex for R 32 . All three line ratios anti-correlate with galactocentric radius and positively correlate with the local star formation rate surface density and specific star formation rate, and we provide approximate fits to these relations. The observed ratios can be reasonably reproduced by models with low temperature, moderate opacity, and moderate densities, in good agree-

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“…In spiral arm structures, changes in R 21 from low in upstream interarm regions to high in the downstream side of the spiral arms have been reported in some disk galaxies; the Milky Way (Sakamoto et al 1997), M51 (Koda et al 2012), and M83 (Koda et al 2020). These results show the evolution in physical conditions of the gas as it passes through the spiral arms.…”

Section: Evolution In Physical Conditions Of the Molecular Gasmentioning

confidence: 72%

“…In the Milky Way, R 21 varies from ∼ 0.75 at 4 kpc to ∼ 0.6 at 8 kpc in Galactocentric distance (Sakamoto et al 1995(Sakamoto et al , 1997. Furthermore, R 21 in inter-arm regions and bar regions, where star formation activity is low, tends to be lower than that in spiral arms and bar-end regions (e.g., Koda et al 2012Koda et al , 2020Muraoka et al 2016;Maeda et al 2020b). According to the non-LTE analysis, gas in the interarm region should be colder and less dense than in the spiral arms (Koda et al 2012).…”

Section: Introductionmentioning

confidence: 97%

CO(2-1)/CO(1-0) line ratio on $\sim$100 parsec scale in the nearby barred galaxy NGC1300

Maeda,

Egusa,

Ohta

et al. 2021

Preprint

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CO(2-1) emission is often used as a tracer of the giant molecular clouds (GMCs) as an alternative to CO(1-0) emission in recent years. Therefore, understanding the environmental dependence of the line ratio of CO(2-1)/CO(1-0), R 21 , on GMC scale is important to accurately estimate the mass of the GMCs. We thus measured the R 21 in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on ∼ 100 pc scale. CO images were obtained from ALMA and Nobeyama 45-m telescope. The resultant typical R 21 in NGC 1300 is 0.57 ± 0.06. We find environmental variations in R 21 ; it is the highest in the bar-end region (0.72 ± 0.08), followed by arm (0.60 ± 0.07) and bar regions (0.50 ± 0.06). GMCs with Hα emission show a systematically higher ratio (0.67±0.07) than those without Hα (0.47±0.05). In the bar region, where massive star formation is suppressed, Hα emission is not associated with most GMCs, resulting in the lowest R 21 . These results raise a possibility that properties of GMCs derived from CO(2-1) observations with the assumption of a constant R 21 are different from those derived from CO(1-0) observations. Furthermore, we find the R 21 measured on kpc scale tends to be lower than that of the GMCs probably due to the presence of an extended diffuse molecular gas in NGC 1300.

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Systematic Variations of CO J = 2−1/1–0 Ratio and Their Implications in The Nearby Barred Spiral Galaxy M83

Cited by 31 publications

References 42 publications

PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA

CO(2-1)/CO(1-0) line ratio on $\sim$100 parsec scale in the nearby barred galaxy NGC1300

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