SuperCAM CO(3–2) APEX survey at a 6 pc resolution in the Small Magellanic Clouds^★

¹ Instituto de Investigaciones en Energía no Convencional, Universidad Nacional de Salta, C.P. 4400, Salta, Argentina
e-mail: hpablohugo@gmail.com
² Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2290, CABA, CPC 1425FQB, Argentina
³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁴ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁵ Center for Astrophysics & Space Sciences, Department of Physics, University of California, San Diego, 9500 Gilman Drive, San Diego, CA 92093, USA
⁶ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85719, USA
⁷ Joint ALMA Observatory (JAO), Alonso de Córdova 3107, Vitacura, Santiago de Chile
⁸ Owens Valley Radio Observatory, California Institute of Technology, Big Pine, CA 93513, USA
⁹ Department of Physics and Astronomy and George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA
¹⁰ Onsala Space Observatory, Department of Space, Earth and Environment, Chalmers University of Technology, 439 92 Onsala, Sweden

Received: 27 October 2023
Accepted: 20 March 2024

Abstract

Context. The Small Magellanic Cloud (SMC) is an ideal laboratory for studying the properties of star-forming regions thanks to its low metallicity, which has an impact on the molecular gas abundance. However, a small number of molecular gas surveys of the entire galaxy have been carried out in the last few years, limiting the measurements of interstellar medium (ISM) properties in a homogeneous manner.

Aims. We present the CO(3-2) APEX survey at a 6 pc resolution of the bar of the SMC, observed with the SuperCAM receiver attached to the APEX telescope. This high-resolution survey has allowed us to study certain properties of the ISM and to identify CO clouds in the innermost parts of the H₂ envelopes.

Methods. We adopted the CO analysis in the SMC bar comparing the CO(3–2) survey with that of the CO(2–1) of a similar resolution. We studied the CO(3–2)-to-CO(2–1) ratio (R₃₂), which is very sensitive to the environment properties (e.g., star-forming regions). We analyzed the correlation of this ratio with observational quantities that trace the star formation such as the local CO emission, the Spitzer color [70/160], and the total IR surface brightness measured from the Spitzer and Herschel bands. For the identification of the CO(3–2) clouds, we used the CPROPS algorithm, which allowed us to measure the physical properties of the clouds. We analyzed the scaling relationships of such physical properties.

Results. We obtained R₃₂ = 0.65 ± 0.02 for the SW bar and a slightly higher ratio, R₃₂ = 0.7 ± 0.1, for N66 in the SMC. We found that R₃₂ varies from region to region, depending on the star formation activity. In regions dominated by HII and photo-dissociated regions (e.g., N22, N66) R₃₂ tends to be higher than the median values. Meanwhile, lower values were found toward quiescent clouds. We also found that R₃₂ is correlated with the IR color [70/160] and the total IR surface brightness. This finding indicates that R₃₂ increases with environmental properties, such as the dust temperature, total gas density, and radiation field. We identified 225 molecular clouds with sizes of R > 1.5 pc and signal-to-noise ratios (S/N) of >3, of which only 17 are well resolved CO(3–2) clouds with S/N ≳ 5. These 17 clouds follow consistent scaling relationships to the inner Milky Way clouds but with some departures. For instance, CO(3–2) tends to be less turbulent and less luminous than the inner Milky Way clouds of similar sizes. Finally, we estimated a median virial-based CO(3–2)-to-H₂ conversion factor of 12.6₋₇⁺¹⁰ M_⊙ (K km s⁻¹ pc²)⁻¹ for the total sample.