Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC

Hony, S., Gouliermis, D. A., Galliano, F., Galametz, M., Cormier, D., Chen, C.-H. R., Dib, S., Hughes, A., Klessen, R. S., Roman-Duval, J., Smith, L., Bernard, J.-P., Bot, C., Carlson, L., Gordon, K., Indebetouw, R., Lebouteiller, V., Lee, M.-Y., Madden, S. C., Meixner, M., Oliveira, J., Rubio, M., Sauvage, M., & Wu, R.
2015, Monthly Notices of the Royal Astronomical Society, 448, 1847

The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (нёБ‹†) and dust surface density (нёdust) across NGC 346 (N66) in 115 independent pixels of 6 ц— 6 pc2. We find a correlation between нёБ‹† and нёdust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 б± 0.2. We convert нёdust to gas surface density (нёgas) and нёБ‹† to star formation rate (SFR) surface densities (нёSFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 б± 1ц—10-3 MБЉ™ yr-1) is consistent with SFR estimated from the Hн± emission integrated over the Hн± nebula. On small scales the нёSFR derived using Hн± systematically underestimates the count-based нёSFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc2 pixels fall systematically above integrated disc galaxies in the Schmidt-Kennicutt diagram by on average a factor of књ7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of књ3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (књ10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Therefore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation.

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