# Spatial structure of several diffuse interstellar band carriers

## TL;DR

This study uses advanced statistical techniques to measure the spatial correlation scales of various diffuse interstellar bands, revealing differences in their clumpiness and distribution across the Milky Way.

## Contribution

It introduces a Gaussian process and Bayesian MCMC methodology to accurately determine the spatial correlation scales of DIB carriers in high-resolution spectra.

## Key findings

- Correlation scales vary from 0.23° to 3.5° on the sky.
- Different DIBs have different clumpiness and occupy similar space.
- Correlation scales correspond to ISM cloud sizes of 5 to 100 parsecs.

## Abstract

Diffuse interstellar bands (DIBs) hold a lot of information about the state and the structure of the ISM. Structure can most directly be observed by extensive spectroscopic surveys, including surveys of stars where DIBs are especially important, as they are conveniently found in all observed bands. Large surveys lack the quality of spectra to detect weak DIBs, so many spectra from small regions on the sky have to be combined before a sufficient signal-to-noise ratio (SNR) is achieved. However, the clumpiness of the DIB clouds is unknown, which poses a problem, as the measured properties can end up being averaged over a too large area. We use a technique called Gaussian processes to accurately measure profiles of interstellar absorption lines in 145 high SNR and high resolution spectra of hot stars. Together with Bayesian MCMC approach we also get reliable estimates of the uncertainties. We derive scales at which column densities of 18 DIBs, CH, CH$^+$, Ca I, and Ca II show some spatial correlation. This correlation scale is associated with the size of the ISM clouds. Scales expressed as the angle on the sky vary significantly from DIB to DIB between $\sim0.23^\circ$ for the DIB at 5512 {\AA} and 3.5$^\circ$ for the DIB at 6196 {\AA}, suggesting that different DIB carriers have different clumpiness but occupy the same general space. Our study includes lines-of-sight all over the northern Milky Way, as well as out of the Galactic plane, covering regions with different physical conditions. The derived correlation scales therefore represent a general image of the Galactic ISM on the scales of $\sim5$ pc to $100$ pc.