Herschel far-infrared ( FIR ) continuum data obtained as part of the Hi-GAL survey have been used , together with the GLIMPSE 8 \mu m and MIPSGAL 24 \mu m data , to attempt the first 3D-decomposition of dust emission associated with atomic , molecular and ionized gas at 15 arcmin angular resolution .
Our initial test case is a 2 \times 2 square degrees region centred on ( l , b ) = ( 30 ^ { \circ } ,0 ^ { \circ } ) , a direction that encompasses the origin point of the Scutum-Crux Arm at the tip of the Galactic Bar .
Coupling the IR maps with velocity maps specific for different gas phases ( HI 21cm , ^ { 12 } CO and ^ { 13 } CO , and Radio Recombination Lines , RRLs ) , we estimate the properties of dust blended with each of the gas components and at different Galactocentric distances along the Line of Sight ( LOS ) .

A statistical Pearson ’ s coefficients analysis is used to study the correlation between the column densities estimated for each gas component and the intensity of the IR emission .
This analysis provides evidence that the 2 \times 2 square degree field under consideration is characterized by the presence of a gas component not accounted for by the standard tracers , possibly associated with warm H _ { 2 } and cold HI .

We demonstrate that the IR radiation in the range 8 \mu m < \lambda < 500 \mu m is systematically dominated by emission originating within the Scutum-Crux Arm .
By applying an inversion method , we recover the dust emissivities associated with atomic , molecular and ionized gas .
Using the DustEM model , we fit the Spectral Energy Distributions ( SEDs ) for each gas phase , and find average dust temperatures of T _ { d, \mathrm { HI } } =18.82 \pm 0.47 K , T _ { d, \mathrm { H } _ { 2 } } =18.84 \pm 1.06 K and T _ { d, \mathrm { HII } } =22.56 \pm 0.64 K , respectively .
We also obtain an indication for Polycyclic Aromatic Hydrocarbons ( PAHs ) depletion in the diffuse ionized gas .

We demonstrate the importance of including the ionized component in 3D-decompositions of the total IR emission .

However , the main goal of this work is to discuss the impact of the missing column density associated with the dark gas component on the accurate evaluation of the dust properties , and to shed light on the limitations of the inversion method approach when this is applied to a small section of the Galactic Plane and when the working resolution allows sufficient de-blending of the gas components along the LOS .