Dust properties in H II regions in M 33

Context. The infrared emission (IR) of the interstellar dust has been claimed to be a tracer of the star formation rate. However, the conversion of the IR emission into star formation rate can be strongly dependent on the physical properties of the dust, which are a ected by the environmental conditions where the dust is embedded. Aims. We study here the dust properties of a set of Hii regions in the Local Group galaxy M33 presenting different spatial configurations between the stars, gas, and dust to understand the dust evolution indierent environments. Methods. We modelled the spectral energy distribution (SED) of each region using the DustEM tool and obtained the mass relative to hydrogen for very small grains (VSG, YVSG), polycyclic aromatic hydrocarbons (YPAH), and big grains (BG, YBG). We furthermore performed a pixel-by-pixel SED modelling and derived maps of the relative mass of each grain type for the whole surface of the two most luminous Hii regions in M33, NGC 604 and NGC 595. Results. The relative mass of the VSGs (YVSG/YTOTAL) changes with the morphology of the region: YVSG/YTOTAL is a factor of ∼1.7 higher for Hii regions classified as filled and mixed than for regions presenting a shell structure. The enhancement in VSGs within NGC 604 and NGC 595 is correlated to expansive gas structures with velocities ≥50 km s. The gas-to-dust ratio derived for the Hii regions in our sample exhibits two regimes related to the Hi.H2 transition of the interstellar medium (ISM). Regions corresponding to the Hi diffuse regime present a gas-to-dust ratio compatible with the expected value if we assume that the gas-to-dust ratio scales linearly with metallicity, while regions corresponding to a H2 molecular phase present a flatter dust-gas surface density distribution. Conclusions. The fraction of VSGs can be affected by the conditions of the interstellar environment: strong shocks of ∼5090km s existing in the interior of the most luminous Hii regions can lead to fragmentation of BGs into smaller ones, while the more evolved shell and clear shell objects provide a more quiescent environment where reformation of dust BGs might occur. The gas-to-dust variations found in this analysis might imply that grain coagulation and/or gas-phase metal incorporation into the dust mass is occurring in the interior of the Hii regions in M33. © 2016 ESO.
Part of this research has been supported by the PERG08-GA-2010-276813 from the EC. This work was partially supported by the Junta de Andalucia Grant FQM108 and Spanish MEC Grants, AYA-2011-24728 and AYA-2014-53506-P. EPM thanks Spanish MINECO grant AYA-2013-47742-C4-1-P of the Spanish Plan for Astronomy and Astrophysics. This work was in part supported through NSF ATI grant 1106284.