Dust properties in H II regions in M 33^⋆

¹ Dept. Física Teórica y del Cosmos, Universidad de Granada, 18010 Granada, Spain
e-mail: mrelano@ugr.es
² Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁴ Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
⁵ Unidad de Astronomía, Fac. Cs. Básicas, Universidad de Antofagasta, 02800 Avda. U. de Antofagasta, Antofagasta, Chile
⁶ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Univ. Bordeaux, Laboratoire d’Astrophysique de Bordeaux, CNRS, LAB, UMR 5804, 33270 Floirac, France
⁸ Instituto de Astrofísica de Andalucía – CSIC. Apdo. 3004, 18008 Granada, Spain
⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁰ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹¹ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, 15236 Athens, Greece
¹² Department of Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55414, USA
¹³ NASA Goddard Space Flight Center, Code 665, Greenbelt, MD 20771, USA
¹⁴ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, 21218, USA

Received: 15 January 2016
Accepted: 10 June 2016

Abstract

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, Y_VSG), polycyclic aromatic hydrocarbons (Y_PAH), and big grains (BG, Y_BG). 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 (Y_VSG/Y_TOTAL) changes with the morphology of the region: Y_VSG/Y_TOTAL 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.