The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418

TL;DR

This study analyzes the 9.7-micron silicate absorption profile in the heavily obscured nucleus of NGC 4418, revealing it closely resembles the diffuse interstellar medium in the Milky Way and providing insights into interstellar dust composition.

Contribution

It presents the first detailed comparison of silicate absorption profiles in NGC 4418 with those in the Milky Way, highlighting similarities with diffuse ISM dust.

Findings

Silicate profile in NGC 4418's nucleus resembles the diffuse ISM in the Milky Way.

No evidence of crystalline silicates or silicon carbide grains in the profile.

The nuclear spectrum isolates the core emission, excluding circumnuclear diffuse emission.

Abstract

The 9.7-micron silicate absorption profile in the interstellar medium provides important information on the physical and chemical composition of interstellar dust grains. Measurements in the Milky Way have shown that the profile in the diffuse interstellar medium is very similar to the amorphous silicate profiles found in circumstellar dust shells around late M stars, and narrower than the silicate profile in denser star-forming regions. Here, we investigate the silicate absorption profile towards the very heavily obscured nucleus of NGC 4418, the galaxy with the deepest known silicate absorption feature, and compare it to the profiles seen in the Milky Way. Comparison between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger aperture spectrum obtained from the Spitzer archive indicates that the former isolates the nuclear emission, while Spitzer detects low surface brightness circumnuclear diffuse emission in addition. The silicate absorption profile towards the nucleus is very similar to that in the diffuse ISM in the Milky Way with no evidence of spectral structure from crystalline silicates or silicon carbide grains.