The Flocculent Structure of the Inner Milky Way Disk

TL;DR

Problem: does the inner Milky Way disk show Grand-design spiral structure or a more flocculent morphology? Method: the authors test two HI-based, distance-independent signatures of interarm structure using HI4PI data, applying Gaussian fits to high-velocity shoulders and integrating HI over relevant velocity ranges. Findings: no broad shoulders or dips indicating interarm or Grand-design structure; inner disk appears disordered and flocculent; results hold across Northern and Southern quadrants and do not depend on distance. Significance: revises the conventional view of the Milky Way's inner morphology and informs models of Galactic dynamics and star formation by favoring a flocculent description.

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.

Figures (3)

• Figure 1: Representative HI4PI H i spectra for $\ell = 45$$\,^\circ$ (left) and $\ell = 315$$\,^\circ$ (right). The green curve is the Gaussian fit of the high-velocity shoulder and the red curve shows the portion of the H i data that was used to constrain the fit. The shape of the extreme-velocity shoulder is measured by the dispersion, $\sigma$, of the Gaussian fit.
• Figure 2: Apparent velocity dispersions as a function of the Galactocentric radius of the subcentral point for the Northern data (left) and Southern data (right). The red curve is a linear fit to the data, yielding $\sigma = -0.43\,R_{\min} + 11.67$$\,{\rm km\, s^{-1}}$ for the Northern data and $\sigma = -0.16\,R_{\min} + 9.71$$\,{\rm km\, s^{-1}}$ for the Southern data. The units of $R_{\rm min} = R_{\rm o}\,{\rm sin}(\ell)$ are in kpc and we assume $R_{\rm o} = 10$$\,{\rm kpc}$ (see text). The measurements show no high $\sigma$ excursions that would be expected from relatively empty interarm regions.
• Figure 3: Integrated brightness temperatures as a function of Galactic longitude for the Northern data (left) and Southern data (right). Upper: The brightness temperature integrated over all LSR velocities: the positive and negative LSR velocities are shown independently, representing the inner and outer parts of the Galactic disk. The hills and valleys that would be expected if a dominant spiral pattern pertained in the inner Galaxy are absent. The North-South differences are no doubt due to the slightly lopsided nature of the Milky Way, a property not uncommon. Lower: The brightness temperature integrated only over the high-velocity wings, pertaining to gas near the subcentral region. The red curve is a linear fit to the data, yielding $\int{T_{B}\,dV} = 6.48\,\ell + 1672\,{\rm K}\,{\rm km}\,{\rm s}^{-1}$ for the Northern data and $\int{T_{B}\,dV} = -5.19\,\ell + 1524\,{\rm K}\,{\rm km}\,{\rm s}^{-1}$ for the Southern data. There are no broad dips that would be expected for the interarm region of a Grand-design. We comment in the text on the apparently "noisy" nature of the lower plots.