[Paper Review] The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen through Diffuse Interstellar Bands and neutral sodium

J. Th. van Loon, Melanie J. Bailey+12 more|UvA-DARE (University of Amsterdam)|Oct 16, 2012

Astrophysics and Star Formation Studies64 references47 citations

TL;DR

This study uses high-resolution spectroscopy of 800+ early-type stars from the VLT-FLAMES Tarantula Survey to map diffuse interstellar bands (DIBs) and neutral sodium (Na i) absorption across the Tarantula Nebula and surrounding interstellar medium. It reveals that the 4428 Å DIB is resilient to harsh radiation, suggesting a large, neutral carrier, while other DIBs (e.g., 5780, 5797, 6614 Å) are progressively destroyed in exposed regions, indicating smaller, more fragile, possibly charged carriers; the data also reveal a shell-like structure around R 136 and evidence of gas inflow, implying a galactic fountain mechanism.

ABSTRACT

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".

Motivation & Objective

• To probe the physical conditions of the cool–warm interstellar medium (ISM) in the Tarantula Nebula and the Galactic disc using diffuse interstellar bands (DIBs) and neutral sodium (Na i).
• To investigate the relationship between DIB strengths, Na i column densities, visual extinction, and radiation environment in diverse ISM regions.
• To determine the nature of DIB carriers by analyzing their correlation with interstellar extinction and ionization state, especially in harsh environments near massive OB associations.
• To map the kinematic structure of the ISM in the Large Magellanic Cloud (LMC) and Milky Way using DIB and Na i absorption profiles.

Proposed method

• High-resolution spectroscopy of 800+ early-type stars from the VLT-FLAMES Tarantula Survey (VFTS) was used to measure DIB and Na i absorption along multiple sightlines.
• Spectral analysis focused on the 4428 Å, 5780 Å, 5797 Å, and 6614 Å DIBs and the Na i D doublet (5890, 5896 Å), with equivalent width measurements for each feature.
• Spatial maps of DIB and Na i absorption were constructed by combining measurements across the survey field, with velocity-resolved decomposition to identify kinematic components.
• Correlations between DIB strengths, Na i column density, visual extinction (E(B−V)), and local radiation environment were quantified using statistical analysis.
• The data were compared with Hα, [O iii], and far-IR dust emission maps to assess the relationship between DIB carriers and different ISM phases.
• A shell-like structure around R 136 was identified via kinematic decomposition of Na i and DIB profiles, suggesting expansion and possible infall.

Experimental results

Research questions

• RQ1 How do the strengths of different DIBs correlate with interstellar extinction and Na i column density in varying radiation environments?
• RQ2 What do the relative resilience of DIB carriers to radiation imply about their physical size, charge state, and molecular structure?
• RQ3 What kinematic structures, such as shells or inflows, are revealed in the ISM of the Tarantula Nebula through DIB and Na i absorption profiles?
• RQ4 How do the distributions of DIB carriers compare to those of dust and ionized gas in the LMC and Galactic disc?
• RQ5 What insights do DIB ratios (e.g., 5780/5797 Å) provide about metallicity or radiation field differences between the LMC and the Milky Way?

Key findings

• The 4428 Å DIB is detectable at low Na i column densities, indicating its carrier is large, compact, and neutral, and highly resilient to ionizing radiation.
• The 6614, 5780, and 5797 Å DIBs only become detectable at higher Na i column densities, suggesting their carriers are smaller, more fragile, and possibly weakly charged.
• The 5780 and 5797 Å DIBs show a significantly lower relative strength in the Tarantula Nebula and Galactic high-latitude regions, confirming these regions as harsh radiation environments.
• A shell-like structure of expanding gas with a mass of ~10² M⊙ and expansion speed of ~40 km s⁻¹ is detected around the R 136 cluster, likely driven by stellar winds.
• Evidence for gas inflow is observed, suggesting a galactic 'fountain' cycle in the LMC, with material cycling between hot and cool phases.
• DIBs trace cool–warm, neutral or weakly ionized gas more effectively than Na i, revealing small-scale structure down to 0.01 pc in the solar neighborhood and ~1 pc in the LMC.