VLT-SINFONI integral field spectroscopy of low-z luminous and ultraluminous infrared galaxies^⋆

II. 2D extinction structure and distance effects

¹ Centro de Astrobiología (INTA–CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: piqueraslj@cab.inta-csic.es
² Instituto de Física de Cantabria, CSIC–UC, Avenida de los Castros S/N, 39005 Santander, Spain

Received: 22 December 2012
Accepted: 14 March 2013

Abstract

We present a 2D study of the internal extinction on (sub)kiloparsec scales of a sample of local (z < 0.1) LIRGs (10) and ULIRGs (7), based on near-infrared Paα, Brδ, and Brγ line ratios, obtained with VLT-SINFONI integral-field spectroscopy (IFS). The 2D extinction (A_V) distributions of the objects, map regions of ~ 3 × 3 kpc (LIRGs) and ~ 12 × 12 kpc (ULIRGs), with average angular resolutions (FWHM) of ~0.2 kpc and ~0.9 kpc, respectively. The individual A_V galaxy distributions indicate a very clumpy dust structure already on sub-kiloparsec scales, with values (per spaxel) ranging from A_V ~ 1 to 20 mag in LIRGs, and from A_V ~ 2 to 15 mag in ULIRGs. As a class, the median values of the distributions are A_V = 5.3 mag and A_V = 6.5 mag for the LIRG and ULIRG subsamples, respectively. In ~70% of the objects, the extinction peaks at the nucleus with values ranging from A_V ~ 3 to 17 mag. Within each galaxy, the A_V radial profile shows a mild decrement in LIRGs within the inner 2 kpc radius, while the same radial variation is not detected in ULIRGs, likely because of the lower linear scale resolution of the observations at the distance of ULIRGs. We evaluated the effects of the galaxy distance in the measurements of the extinction as a function of the linear scale (in kpc) of the spaxel (i.e. due to the limited angular resolution of the observations). If the distribution of the gas/dust and star-forming regions in local LIRGs (63 Mpc, 40 pc/spaxel on average) is the same for galaxies at greater distances, the observed median A_V values based on emission line ratios would be a factor ~0.8 lower at the average distance of our ULIRG sample (328 Mpc, 0.2 kpc/spaxel), and a factor ~0.67 for galaxies located at distances of more than 800 Mpc (0.4 kpc/spaxel). This distance effect would have implications for deriving the intrinsic extinction in high-z star-forming galaxies and for subsequent properties such as star formation rate, star formation surface density, and KS- law, based on Hα line fluxes. If local LIRGs are analogues of the main-sequence star-forming galaxies at cosmological distances, the extinction values (A_V) derived from the observed emission lines in these high-z sources would need to be increased by a factor 1.4 on average.