The Flocculent Structure of the Inner Milky Way Disk

TL;DR

This study uses HI4PI survey data to analyze the inner Milky Way's spiral structure, finding no evidence of a grand-design pattern and suggesting it is a flocculent spiral galaxy.

Contribution

It introduces two novel methods to analyze HI data for spiral structure, providing robust evidence against a grand-design pattern in the inner Milky Way.

Findings

No shallow shoulders detected in high-velocity HI wings

No broad dips in HI brightness temperature in the inner Galaxy

Supports classification of the Milky Way as a flocculent spiral

Abstract

Observations of HI published in 1957 by Westerhout and Schmidt were presented as showing a global face-on view of spiral structure in the Milky Way. Since then many studies have attempted to improve on the early map, perhaps presupposing our Galaxy to be characterized by a Grand-design pattern of prominent spiral arms. We consider here two approaches to explore the nature of spiral structure of the inner Milky Way disk using the HI4PI survey. The first is to search for shallow shoulders in the high-velocity wings of HI data along the Galactic equatorial disk of the inner Milky Way that would be expected if the lines of sight swept across interarm regions of low HI density. The second is to look for broad dips in the integrated HI brightness temperature over the high-velocity wings, pertaining to gas near the subcentral region, that would be expected for the interarm region of a Grand-design. We find neither shallow shoulders nor broad dips in either the Northern quadrant I or the Southern quadrant IV indicating that the Milky Way seen interior to the Solar orbit is not characterized by a majestic spiral-structure Grand-design; this conclusion is a robust one, in that it does not depend on measures of distance. Taken together with decades of work on the bits and pieces of the quite disorganized shambles of the inner Galaxy, we suggest that the Milky Way belongs to the category of Flocculent spirals.