Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA

Abstract

NGC 602 is a young, low-metallicity star cluster in the “Wing” of the Small Magellanic Cloud. We reveal the recent evolutionary past of the cluster through analysis of high-resolution (∼0.4 pc) Atacama Large Millimeter/submillimeter Array observations of molecular gas in the associated H ii region N90. We identify 110 molecular clumps (R < 0.8 pc) traced by CO emission, and study the relationship between the clumps and associated young stellar objects (YSOs) and pre-main-sequence (PMS) stars. The clumps have high virial parameters (typical α vir = 4–11) and may retain signatures of a collision in the last ≲8 Myr between H i components of the adjacent supergiant shell SMC-SGS 1. We obtain a CO-bright-to-H2 gas conversion factor of X CO,B = (3.4 ± 0.2) × 1020 cm−2 (K km s−1)−1, and correct observed clump properties for CO-dark H2 gas to derive a total molecular gas mass in N90 of 16,600 ± 2400 M ⊙. We derive a recent (≲1 Myr) star formation rate of 130 ± 30 M ⊙ Myr−1 with an efficiency of 8% ± 3% assessed through comparing total YSO mass to total molecular gas mass. Very few significant radial trends exist between clump properties or PMS star ages and distance from NGC 602. We do not find evidence for a triggered star formation scenario among the youngest (≲2 Myr) stellar generations, and instead conclude that a sequential star formation process in which NGC 602 did not directly cause recent star formation in the region is likely.

An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog

“…Several theoretical studies proposed that the X CO factor depends on metallicity with a power-law index of −(0.5-0.8) (e.g., Feldmann et al 2012), indicating that the sub-solar metallicity condition does not significantly change the X CO factor from that of the MW value. Smaller beam size measurements indeed yielded lower values than the above surveys, although the results are based on only partial observations compared to the entire galaxy (e.g., Bolatto et al 2003;Muraoka et al 2017;Jameson et al 2018;O'Neill et al 2022). The fundamental properties of molecular clouds, such as the mass function and size-linewidth relation, are not necessarily sufficiently obtained by compiling a statistically large sample.…”

Variation in XCO Factor in N55 Region

“…Various methods have been used to estimate the total gas mass, including those employing virial mass [7,8], dust emission or extinction [9][10][11], γ-ray emission [12-14], 13 CO [6,15], and [C II] [16][17][18]. Most studies of the X CO factor consider an average value over a cloud and discuss the effects of cloud temperature, density, metallicity, and velocity dispersion on the X CO factor [19][20][21][22][23][24]. Pineda et al and Luo et al [19,25] found that the X CO factor in low-density regions without CO emission detections is six times higher than the average value in the Milky Way.…”