VLT-SINFONI integral field spectroscopy of low-z luminous and ultraluminous infrared galaxies II. 2D extinction structure and distance effects

TL;DR

This study maps the 2D internal dust extinction in local luminous and ultraluminous infrared galaxies using near-infrared spectroscopy, revealing clumpy dust structures and assessing how distance affects extinction measurements relevant for high-redshift galaxy studies.

Contribution

It provides detailed 2D extinction maps of local (U)LIRGs and analyzes the impact of galaxy distance on extinction estimates, aiding interpretation of high-redshift galaxy observations.

Findings

Dust extinction varies significantly on sub-kiloparsec scales.

Distance effects can cause underestimation of extinction in high-z galaxies.

Extinction in local LIRGs may be underestimated by a factor of ~1.4 at high redshift.

Abstract

We present a 2D study of the internal extinction on (sub)kiloparsec scales of a sample of local (z<0.1) (U)LIRGs, based on near-infrared Pa_a, Br_d, and Br_g line ratios, obtained with VLT-SINFONI integral-field spectroscopy. The 2D extinction (Av) distributions of the objects, map regions of ~3x3 kpc (LIRGs) and ~12x12 kpc (ULIRGs), with average angular resolutions (FWHM) of ~0.2 kpc and ~0.9 kpc, respectively. The individual Av galaxy distributions indicate a very clumpy dust structure already on sub-kiloparsec scales, with values (per spaxel) ranging from Av ~ 1 to 20 mag in LIRGs, and from Av ~ 2 to 15 mag in ULIRGs. As a class, the median values of the distributions are Av=5.3 mag and Av=6.5 mag for the LIRG and ULIRG subsamples, respectively. 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 Av 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 H_alpha line fluxes. If local LIRGs are analogues of the main-sequence star-forming galaxies at cosmological distances, the extinction values (Av) derived from the observed emission lines in these high-z sources would need to be increased by a factor 1.4 on average.