Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

Abstract

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines including CO J = 4 → 3 to J = 12 → 11, [CI] 609 µm and 370 µm, and [NII] 205 µm are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on ∼10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153-754 K) and moderately dense (H 2 number density of (1.1-4.5) × 10 3 cm −3 ). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modelling Herschel observations of [OI] 145 µm, [CII] 158 µm, and [CI] 370 µm fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to the CO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ∼10 km s −1 appears sufficient enough to reproduce the observed warm CO.

CO excitation in the Seyfert galaxy NGC 7130

“…NGC7130 has an observed LCO/LF IR ratio ∼1.5×10 −4 , a factor typical of XDR/PDR models (e.g. Meijerink & Spaans 2005), while a value of ∼7×10 −4 , as found by Meijerink et al (2013) in NGC6240, seems to be more related to shocks, compressing the gas and heating it to higher temperatures, while not affecting the dust (see also Lee et al 2016). Moreover, X-ray photons, as the primary heating mechanism of the high-J CO lines is supported by (i) the clear detection, thanks mostly to NuSTAR data, of a hard X-ray flux originating from a central accreting engine (see Sec.…”

CO line emission from galaxies in the Epoch of Reionization

“…This is in line with what discussed in the previous sections for the Z = Z runs. In their PDR analysis of metal FIR lines, Lee et al (2016) 6 The area of the map in Fig. 8 of Lee et al (2016) is ≈ 150 × 150 arcsec 2 .…”

“…Observations of the CO SLEDs can be used as a tool to constrain the properties of molecular gas and to link them to the star formation process, both from single GMCs (e.g. Pon et al 2016;Lee et al 2016;Indriolo et al 2017) and on galactic scales (e.g. Rosenberg et al 2015;Mashian et al 2015;Lu et al 2017;Pozzi et al 2017).…”

“…physical conditions have been extensively studied at multiple wavelengths (Lee et al 2016, and references therein). Recently, Lee et al (2016) presented a coherent analysis of the CO and fine structure ([CII] 158µm, [OI] 145µm, [CI] 370µm) line emission to assess the properties of the molecular gas in N159W.…”

“…Note, however, that this must be considered as an upper limit on the area of the region from which the CO emission is observed. In fact, only those pixel for which the S/N > 5 were considered by Lee et al (2016) when computing the total luminosity quoted in their Tab. 3.…”