Modeling the Molecular Gas in NGC 6240

Abstract

We present the first observations of H 13 CN 1 0 ( ) -, H 13 CO + 1 0 ( ) -, and SiO 2 1 ( ) -in NGC 6240, obtained with the IRAM Plateau de Bure Interferometer. Combining a Markov Chain Monte Carlo code with Large Velocity Gradient (LVG) modeling, and with additional data from the literature, we simultaneously fit three gas phases and six molecular species to constrain the physical condition of the molecular gas, including mass−luminosity conversion factors. We find M 10 10  of dense molecular gas in cold, dense clouds (T 10 k~K , n 10 ) which we attribute to isotope fractionation in the cold, dense clouds.

The evolution of CNO isotopes: a new window on cosmic star formation history and the stellar IMF in the age of ALMA

“…13 C + + CN ⇋ C + + 13 CN + 31 K (4) (Röllig & Ossenkopf 2013;Tunnard et al 2015, and references therein). The effects of isotope fractionation have recently been invoked to explain the isotopologue 13 CO, 12 CO and H 13 CN, H 12 CN line ratios observed in the (U)LIRG, NGC 6240 (Tunnard et al 2015).…”

“…13 C + + CN ⇋ C + + 13 CN + 31 K (4) (Röllig & Ossenkopf 2013;Tunnard et al 2015, and references therein). The effects of isotope fractionation have recently been invoked to explain the isotopologue 13 CO, 12 CO and H 13 CN, H 12 CN line ratios observed in the (U)LIRG, NGC 6240 (Tunnard et al 2015). However, this work did not consider the minimum T kin set by the CR heating that is expected to be ubiquitous in the environments of starbursts, and argued that dense (n = 10 6 cm −3 ) and very cold (T kin = 10 K) gas comprises the most massive phase of H 2 in NGC 6240.…”

Imaging the molecular outflows of the prototypical ULIRG NGC 6240 with ALMA

“…This system has a double nucleus detected in a variety of wavelengths from X-ray to radio (e.g., Komossa et al 2003;Hagiwara et al 2011;Stierwalt et al 2014;Ilha et al 2016), indicating the presence of two separated AGNs. The cold molecular ISM in NGC 6240 has been observed through many rotational transitions of CO, HCN, and HCO + , showing a strong gas concentration between the nuclei (e.g., Nakanishi et al 2005;Iono et al 2007;Papadopoulos et al 2014;Scoville et al 2015;Tunnard et al 2015;Sliwa & Downes 2017). Although the properties shown above are indeed suitable for representing nearby ULIRGs, past observations have revealed a different side of NGC 6240, which makes it rather unique.…”

Incorporating astrochemistry into molecular line modelling via emulation

“…As we set both prior distributions to cover the full emulated-range, the forward model could hypothetically fit the observations with two hot gas phases or two cold gas phases. This is in contrast with what has sometimes been done, such as in Tunnard et al (2015), where they have constrained the two phases to non-overlapping parameter ranges thus artificially enforcing the gas to exist in two very distinct phases. In this analysis we have forced the two phases of gas to share the same metallicity and line-width.…”