Star-formation rates from young-star counts and the structure of the ISM across the NGC346/N66 complex in the SMC

Abstract

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 (Sigma_*) and dust surface density (Sigma_d) across NGC346 (N66) in 115 independent pixels of 6x6 pc^2. We find a correlation between Sigma_* and Sigma_d with a considerable scatter. A power law fit to the data yields a steep relation with an exponent of 2.6+-0.2. We convert Sigma_d to gas surface density (Sigma_g) and Sigma_* to star formation rate (SFR) surface densities (Sigma_SFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4+-1 10^-3 M_sun/yr) is consistent with SFR estimated from the Ha emission integrated over the Ha nebula. On small scales the Sigma_SFR derived using Ha systematically underestimates the count-based Sigma_SFR, 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 pc^2 pixels fall systematically above integrated disc-galaxies in the Schmidt-Kennicutt diagram by on average a factor of ~7. The NGC346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ~3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (~10 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.

Accepted for publication in MNRAS

References in corpus (17)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Slow Star Formation in Dense Gas: Evidence and Implications
• COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The non-universality of the Molecular Gas Depletion Timescale
• Before the first supernova: combined effects of HII regions and winds on molecular clouds
• Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. I. Dust Properties and Insights into the Origin of the Submm Excess Emission
• Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
• The VLT-FLAMES survey of massive stars: Mass loss and rotation of early-type stars in the SMC
• Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66
• G0.253+0.016: A centrally condensed, high-mass protocluster
• The Stellar Mass Distribution in the Giant Star Forming Region NGC 346
• The Discovery of a Population of Pre-Main Sequence Stars in N66/NGC346 from Deep HST/ACS Image
• The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HII Region NGC 346
• Spitzer View of Young Massive Stars in the LMC HII Complexes. II. N159
• The Star-Forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations I. Photometry
• Star Formation Rates in Resolved Galaxies: Calibrations with Near and Far Infrared Data for NGC5055 and NGC6946
• Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge
• Low-mass pre--main-sequence stars in the Magellanic Clouds

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• SEDIGISM-ATLASGAL: Dense Gas Fraction and Star Formation Efficiency Across the Galactic Disk
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