摘要：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 affected by the environmental conditions where the dust is embedded.
Aims. We study here the dust properties of a set of H ii regions in the Local Group galaxy M 33 presenting different spatial configurations between the stars, gas, and dust to understand the dust evolution in different 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 H ii regions in M 33, 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 H ii 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-1. The gas-to-dust ratio derived for the H ii regions in our sample exhibits two regimes related to the H i−H2 transition of the interstellar medium (ISM). Regions corresponding to the H i 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 ~50–90 km s-1 existing in the interior of the most luminous H ii 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 H ii regions in M 33.
关键词：galaxies: individual: M 33;galaxies: ISM;Hiiregions;ISM: bubbles;dust, extinction;infrared: ISM
