2016A&A...588A..75M

Aims. We use the Herschel Hi-GAL survey data to study the spatial distribution in Galactic longitude and latitude of the interstellar medium (ISM) and of dense, star-forming clumps in the inner Galaxy.
Methods. We assemble a complete mosaic of the inner Galaxy between l=-70° and +68° in the far-infrared continuum from Hi-GAL. The peak position and width of the latitude distribution of the dust column density is analysed by fitting a polynomial function to the diffuse IR surface brightness in 1° longitude bins, and the result is compared to MIPSGAL 24-µm data. The latitude distribution of the number density of compact sources from the band-merged Hi-GAL photometric catalogues is also analysed as a function of longitude.
Results. The width of the diffuse dust column density traced by the Hi-GAL 500-µm emission varies across the inner Galaxy with a mean value of 1.2-1.3, similar to the distribution of MIPSGAL 24-µm sources and of Hi-GAL sources with a 250-µm counterpart. Hi-GAL sources with a 70-µm counterpart define a much thinner disk with a mean FWHM∼0.75, which is in excess of the result obtained by the ATLASGAL submillimetre survey. The discrepancy with the 250-µm source distribution can be explained by relatively higher confusion in the Herschel data in the midplane region. The peak of the average latitude distribution of Hi-GAL sources is at b~-0.06, coincident with the results from ATLASGAL. The detailed latitude distribution as a function of longitude shows clear modulations both for the diffuse emission and for the compact sources. The displacements are mostly towards negative latitudes with excursions of ∼0.2 below the midplane at l~+40°, +12°, -25°, and -40°. The only positive bend peaks at l~-5°. No such modulations can be found in the MIPSGAL 24µm or WISE 22µm data when the entire source samples are considered. Modulations that are in part similar to the ones exhibited by the Herschel sources appear when the mid-infrared catalogues are filtered according to criteria that primarily select Young Stellar Objects (YSOs).
Conclusions. The distortions of the Galactic inner disk revealed by Herschel confirm previous findings from CO surveys and HII/OB source counts but with much greater statistical significance and are interpreted as large-scale bending modes of the plane. The lack of similar distortions in tracers of more evolved YSOs or stars rules out gravitational instabilities or satellite-induced perturbations, because they should act on both the diffuse and stellar disk components. We propose that the observed bends are caused by incoming flows of extra-planar gas from the Galactic fountain or the Galactic halo interacting with the gaseous disk. With a much lower cross-section, stars decouple from the gaseous ISM and relax into the stellar disk potential. The timescale required for the disappearance of the distortions from the diffuse ISM to the relatively evolved YSO stages are compatible with star formation timescales.