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Diffuse interstellar bands in the Antennae Galaxy as seen by MUSE

Author/s: A. Monreal-Ibero, P. M. Weilbacher, M. Wendt

Reference: 2018 A&A 615 A33 | Link

Map of equivalent width (in mÅ) of the 5780 Å Diffuse Interstellar Band recorded in the Antennae Galaxies (NGC 4038/NGC 4039) by the MUSE instrument on the Very Large Telescope (VLT).
Map of equivalent width (in mÅ) of the 5780 Å Diffuse Interstellar Band recorded in the Antennae Galaxies (NGC 4038/NGC 4039) by the MUSE instrument on the Very Large Telescope (VLT).

Diffuse interstellar bands (DIBs) are spectral features associated to the interstellar medium. Their existence is known since almost one century ago. However, the precise nature of the molecules causing these absorptions is still a mystery. In general, carbon seems to be involved and in that sense, DIBs have been proposed as the largest reservoir of organic matter in the Universe. DIBs are intrinsically faint features. As a consequence, most studies related to the DIBs have been confined to objects in the Milky Way. Outside our Galaxy there are some detections of DIBs, but only very rarely have they been detected well beyond the confines of the Local Group. However, looking for them well beyond the Milky Way it is of interest to learn about their behaviour in e.g. highly energetic interstellar medium conditions, such as those found in a starburst galaxy, where stars are being formed at a much higher rate than in our Galaxy. This can give additional clues about the possible nature of the molecules that cause DIBs, but they can also provide tools for astronomers to characterize the interstellar medium to which they belong. The work explores the potential of using integral field spectrographs, such as HARMONI (an instrument designed for the future 39m telescope, the E-ELT), in whose construction the IAC participates. We used what constitutes, today, the crème de la crème in this type of instrumentation, MUSE on the VLT, to obtain data from the closest merging spiral galaxy system: The Antennae Galaxy. MUSE obtains a huge number of spectra from a relatively large area on the sky from a single exposure. Based on adding the signal of neighbouring spectra and carefully modelling and separating the emission due to the stars and the ionized gas in the system, we managed to detect the signal from two of the best-known DIBs and, in fact, the first two to be identified as such, along more than 200 and 100 independent lines of sight, respectively. Besides, the study compares our detections with other properties and components of the interstellar medium in this system, in particular: the attenuation (directly related to the amount of dust) and the distribution of atomic hydrogen, molecular gas and some bands in emission in the middle infrared that also appear to be associated with organic compounds.

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