The VMC survey^{⋆,⋆⋆,⋆⋆⋆}

VII. Reddening map of the 30 Doradus field and the structure of the cold interstellar medium

¹ Astrophysics Group, Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
² University of Hertfordshire, Physics Astronomy and Mathematics, Hatfield AL10 9AB, UK
³ University Observatory Munich, Scheinerstraße 1, 81679 Munchen, Germany
⁴ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Astronomy Unit, School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁶ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian District, 100871 Beijing, PR China
⁸ Royal Observatory of Belgium, Ringlaan 3, 1180 Ukkel, Belgium
⁹ European Southern Observatory, Santiago, Av. Alonso de Córdoba 3107, Casilla 19 Santiago, Chile
¹⁰ INAF, Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Naples, Italy

Received: 31 January 2013
Accepted: 29 March 2013

Abstract

Context. The details of how galaxies have evolved over cosmological times are imprinted in their star formation history, chemical enrichment and morpho-kinematic structure. Parameters behind these, such as effective temperature and metallicity, can be measured by combining photometric techniques with modelling. However, there are uncertainties with this indirect approach from the ambiguity of colour and magnitude and the effects of interstellar reddening.

Aims. In this paper we present a detailed reddening map of the central 30 Doradus region of the Large Magellanic Cloud; for both community use and as a test of the methods used for future use on a wider area. The reddening, a measurement of dust extinction, acts as a tracer of the interstellar medium (ISM).

Methods. Near-infrared (NIR) photometry of the red clump stars is used to measure reddening as their fixed luminosity and intermediate age make extinction the dominant cause of colour and magnitude variance. The star formation history derived previously from these data is used to produce an intrinsic colour to act as a zero point in converting colour to reddening values E(J − K_s) which are subsequently converted to visual extinction A_V.

Results. Presented is a dust map for the 30 Doradus field in both A_V and E(J − K_s). This map samples a region of 1° × 1.°5, containing  ~ 1.5 × 10⁵ red clump stars which probe reddening up to A_V ≃ 6 mag. We compare our map with maps from the literature, including optical extinction maps and radio, mid- and far-infrared maps of atomic hydrogen and dust emission. Through estimation of column density we locate molecular clouds.

Conclusions. This new reddening map shows correlation with equivalent maps in the literature, validating the method of red clump star selection. We make our reddening map available for community use. In terms of ISM the red clump stars appear to be more affected by the cooler dust measured by 70  μm emission because there is stronger correlation between increasing emission and extinction due to red clump stars not being located near hot stars that would heat the dust. The transition from atomic hydrogen to molecular hydrogen occurs between densities of N_H    ≃    4 × 10²¹ cm and N_H    ≃    6 × 10²¹ cm.