The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen
  through Diffuse Interstellar Bands and neutral sodium
  The Tarantula Nebula (30 Dor) is a spectacular star-forming region in the
Large Magellanic Cloud, seen through gas in the Galactic Disc and Halo. Diffuse
Interstellar Bands offer a unique probe of the diffuse, cool-warm gas in these
regions. The aim is to use DIBs as diagnostics of the local interstellar
conditions, whilst at the same time deriving properties of the yet-unknown
carriers. Spectra of over 800 early-type stars from the VLT Flames Tarantula
Survey (VFTS) were analysed. Maps were created, separately, for the Galactic
and LMC absorption in the DIBs at 4428 and 6614 Ang and - in a smaller region
near the central cluster R136 - neutral sodium (Na I D); we also measured the
DIBs at 5780 and 5797 Ang. The maps show strong 4428 and 6614 Ang DIBs in the
quiescent cloud complex to the south of 30 Dor but weak absorption in the
harsher environments to the north (bubbles) and near the OB associations. The
Na maps show at least five kinematic components in the LMC and a shell-like
structure surrounding R136, and small-scale structure in the Milky Way. The
strengths of the 4428, 5780, 5797 and 6614 Ang DIBs are correlated, also with
Na absorption and visual extinction. The strong 4428 Ang DIB is present already
at low Na column density but the 6614, 5780 and 5797 Ang DIBs start to be
detectable at subsequently larger Na column densities. The relative strength of
the 5780 and 5797 Ang DIBs clearly confirm the Tarantula Nebula and Galactic
high-latitude gas to represent a harsh radiation environment. The resilience of
the 4428 Ang DIB suggests its carrier is large, compact and neutral. Structure
is detected in the distribution of cool-warm gas on scales between one and >100
pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds
from the central cluster R136 have created an expanding shell; some infalling
gas is also detected, reminiscent of a galactic "fountain".
