Mapping the diffuse interstellar bands λ5780 and λ6284 in the luminous infrared galaxy merger NGC 6240

¹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ European Southern Observatory, Alonso de Cordova, 3107 Vitacura, Santiago, Chile

^⋆ Corresponding author.

Received: 29 January 2025
Accepted: 19 March 2025

Abstract

Context. Diffuse interstellar bands (DIBs) are faint absorption features of a generally unknown origin. Observational constraints on their carriers have been provided in the vast majority of the cases thanks to observations in our Galaxy. Detections in other galaxies are scarce, both in the Local Group and beyond. However, they can further constrain the nature of the carriers by sampling different environments. They can put the ubiquity of the molecules creating these features to the test.

Aims. We aim to map some of the strongest DIBs in an environment that has not been tested thus far: a system harbouring two active galactic nuclei (AGNs). We explore the relation of these DIBs with other components and properties of the interstellar medium, in particular, the dust traced by the attenuation, the sodium absorption doublet, and previously published maps of the atomic and molecular matter.

Methods. We used archival Multi Unit Spectroscopic Explorer (MUSE) data of the luminous infrared galaxy (LIRG) NGC 6240. We spatially binned the data with the Voronoi binning technique and modeled the emission of the underlying stellar population with the pPXF code. We measured the spectral features of interest, both in the emission and absorption, with a self-written algorithm using multiple Gaussians.

Results. We mapped the DIBλ5780 over an almost contiguous area of ∼76.96 kpc² in the center of the system. We also traced the DIBλ6284 over two separate areas toward the north and south of the system, with an extent of ∼21.22 kpc² and ∼31.41 kpc² (with a total detected area of ∼59.78 kpc²). This is the first time that the λ6284 DIB has been mapped outside our Galaxy. Both maps were compared with the attenuation on the overall stellar population and the ionized gas. As expected, both DIBs are detected in locations with high attenuation (E(B − V)_Gas ≳ 0.3 and E(B − V)_Stellar ≳ 0.1), supporting the connection between DIB carriers and dust. Moreover, when compared with other galaxies, DIBs are better correlated with the stellar (rather than the ionized gas) attenuation. In particular, the DIBλ6284 presents a stronger correlation with reddening than the λ5780 DIB, as determined by the Pearson correlation coefficient with value ρ_t, λ6284 = 0.82 and ρ_t, λ5780 = 0.77. This better correlation can be attributed to a different nature of the carriers causing these DIBs or a combined effect of a dependence with the metallicity and the different locations where these DIBs have been measured. We argue that the latter effect can have a more substantial impact as both λ5780 and λ6284 DIBs belong to the σ-DIB family; thus, they are expected to have similar properties. In addition, we show that Na I D is strongly correlated with both DIBs. We advocate for the utilization of DIBs as a first-order tracer of specific amounts of material in cases where Na I D reaches saturation. This saturation effect can be an observational complication in systems with a large amount of gas, such as (U)LIRGs.

Conclusions. The findings presented here show that DIB carriers can exist and survive in an environment as extreme as a galaxy hosting an AGN. These features enable us to envision the possibilities of integral field spectrographs in studying DIBs well beyond our Galaxy.