The Combined Dense Gas and Excitation Atlas of the Milky Way I. Mass Distribution and Ripples in the Molecular Layer

Abstract

The first components of a new meta-survey, the Combined Dense Gas and Excitation Atlas (CODEX) of the Milky Way, are presented here with combined data from five previously published surveys: NANTEN+CHaMP, ThrUMMS, FUGIN, and MWISP. The CODEX is first used to harmonise the combination of parameters used to describe the Galaxy's scale and rotation, including $R_0$ and $\Theta_0$ from recent work, since these are critical components of standard techniques for obtaining kinematic distances. An additional feature is added to these formulae to accommodate local clouds that are otherwise kinematically inconsistent with the Local Standard of Rest. The structure in the combined lV diagram spanning 230 deg of longitude is then analysed, yielding a new Ragged Spiral Arm model that better traces the deprojected (x,y) 12CO structure. The widespread existence of ripples in the molecular layer, first reported in the ThrUMMS data of the Fourth Quadrant (4Q), is confirmed in FUGIN data of the First Quadrant (1Q), averaging amplitudes $\sim$40 and $\sim$23 pc respectively, and wavelength $\sim$4 kpc in both. The (x,y) mass distributions as traced by the molecular line data and the published dust extinction data match well across the 4Q+inner1Q, but poorly across the outer 1Q, probably due to lower sensitivity in the 1Q extinction data. In the 1Q+4Q, however, the two mean-z distributions do not match: the dust ripple amplitude is $\sim$3x smaller than, and opposite in sign to, the gas ripples. This is nevertheless consistent since the dust extinction is insensitive to the densest molecular gas.