Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. II. 30 Doradus

With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions, we performed Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of 30 Doradus in the Large Magellanic Cloud. In our FTS observations, important far-infrared (FIR) cooling lines in the interstellar medium, including CO J = 4-3 to J = 13-12, [C I] 370 μm, and [N II] 205 μm, were clearly detected. In combination with ground-based CO J = 1-0 and J = 3-2 data, we then constructed CO spectral line energy distributions (SLEDs) on 10 pc scales over a 60 pc × 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus. For example, the peak transition J_p varies from J = 6-5 to J = 10-9, while the slope characterized by the high-to-intermediate J ratio α ranges from 0.4 to 1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, H₂ 0-0 S(3), and FIR luminosity data, using state-of-the-art models of photodissociation regions and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly compressed (thermal pressure P/k_B 10⁷-10⁹ K cm^-3) clumps on 0.7-2 pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field G_UV 10³-10⁵ Mathis fields) or low-velocity C-type shocks (pre-shock medium density n_pre 10⁴-10⁶ cm^-3 and shock velocity v_s 5-10 km s^-1). Considering the stellar content in 30 Doradus, however, we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by kiloparsec-scale processes (e.g., interaction between the Milky Way and the Magellanic Clouds) are likely the dominant source of heating for CO. The shocked CO-bright medium was then found to be warm (temperature T 100-500 K) and surrounded by a UV-regulated low-pressure component (P/k_B a few (10⁴ -10⁵) K cm^-3) that is bright in [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, and FIR dust continuum emission.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.