Spatial structure of several diffuse interstellar band carriers

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 $\sim 0.23^{\circ }$ for the DIB at 5512 Å and 3.5${}^{\circ }$ for the DIB at 6196 Å, 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 $\sim 5$ pc to $100$ pc.

Accepted to MNRAS

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• Gaia Data Release 3: Exploring and mapping the diffuse interstellar band at 862 nm
• How proper are Bayesian models in the astronomical literature?
• Diffuse interstellar bands λ5780 and λ5797 in the Antennae Galaxy as seen by MUSE
• The diffuse interstellar band around 8620 Å I. Methods and application to the GIBS data set
• The GALAH survey: New diffuse interstellar bands found in residuals of 872,000 stellar spectra
• The Pristine Inner Galaxy Survey (PIGS) VI: Different vertical distributions between two DIBs at 442.8 nm and 862.1 nm
• Mapping the diffuse interstellar bands $\lambda$5780 and $\lambda$6284 in the luminous infrared galaxy merger NGC 6240