Spectral energy distributions of HII regions in M33 (HerM33es)

Abstract

Aims. Within the framework of the Herschel M 33 extended survey HerM33es and in combination with multi-wavelength data we study the spectral energy distribution (SED) of a set of H ii regions in the Local Group galaxy M 33 as a function of the morphology. We analyse the emission distribution in regions with different morphologies and present models to infer the Hα emission measure observed for H ii regions with well defined morphology. Methods. We present a catalogue of 119 H ii regions morphologically classified: 9 filled, 47 mixed, 36 shell, and 27 clear shell H ii regions. For each object we extracted the photometry at twelve available wavelength bands, covering a wide wavelength range from FUV-1516 Å (GALEX) to IR-250 μm (Herschel), and we obtained the SED for each object. We also obtained emission line profiles in vertical and horizontal directions across the regions to study the location of the stellar, ionised gas, and dust components. We constructed a simple geometrical model for the clear shell regions, whose properties allowed us to infer the electron density of these regions. Results. We find trends for the SEDs related to the morphology of the regions, showing that the star and gas-dust configuration affects the ratios of the emission in different bands. The mixed and filled regions show higher emission at 24 μm, corresponding to warm dust, than the shells and clear shells. This could be due to the proximity of the dust to the stellar clusters in the case of filled and mixed regions. The far-IR peak for shells and clear shells seems to be located towards longer wavelengths, indicating that the dust is colder for this type of object. The logarithmic 100 μm/70 μm ratio for filled and mixed regions remains constant over one order of magnitude in Hα and FUV surface brightness, while the shells and clear shells exhibit a wider range of values of almost two orders of magnitude. We derive dust masses and dust temperatures for each H ii region by fitting the individual SEDs with dust models proposed in the literature. The derived dust mass range is between 10^2−10^4 M_⊙ and the cold dust temperature spans T_(cold) ~ 12−27 K. The spherical geometrical model proposed for the Hα clear shells is confirmed by the emission profile obtained from the observations and is used to infer the electron density within the envelope: the typical electron density is 0.7 ± 0.3 cm^(-3), while filled regions can reach values that are two to five times higher.

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