Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models

By R. Meijerink, M. Spaans, F. Israel. 2006

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Aims. The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to t he sources of radiation, numerical models are constructed. Methods. These models iteratively determine the thermal and chemical balance of molecular gas that is exposed to X-rays (1-100 keV) and far-ultraviolet radiation (6-13.6 eV), as a functi on of depth. Results. We present a grid of XDR and PDR models that span ranges in density (10 2 10 6.5 cm 3 ), irradiation (10 0.5 10 5 G0 and FX = 1.6 × 10 2 160 erg cm 2 s 1 ) and column density (3 × 10 21 1 × 10 25 cm 2 ). Predictions are made for the most important atomic fine-structure lines, e.g., [CII], [OI], [ CI], [SiII], and for molecular species like HCO + , HCN, HNC, CS and SiO up to J = 4, CO and 13 CO up to J = 16, and column densities for CN, CH, CH + , HCO, HOC + , NO and N2H + . We find that surface temperatures are higher (lower) in PDRs compared to XDRs for densities > 10 4 ( 1) for XDRs (PDRs) if the density exceeds 10 5 cm 3 and if the column density is larger than 10 23 cm 2 . For columns less than 10 22.5 cm 2 the XDR HCN/HCO + 1-0 ratio becomes larger than one, although the individual HCN 1-0 and HCO + 1-0 line intensities are weaker. For modest densities, n = 10 4 10 5 cm 3 , and strong radiation fields ( > 100 erg s 1 cm 2 ), HCN/HCO + ratios can become larger in XDRs than PDRs as well. Also, the HCN/CO 1-0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for high (column) density gas, with columns in excess of 10 23 cm 2 and densities larger than 10 4 cm 3 . Furthermore, CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO J=N+1-N/CO 1-0, N � 1, line ratios in XDRs. In particular, lines with N � 10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. This is crucial since the XDR/AGN contribution will typically be of a much smaller (possibly beam diluted) angular scale and a 10-25% PDR contribution can already suppress XDR distinguishing features involving HCN/HCO+ and HNC/HCN. For possible future observations, column density ratios indicate that CH, CH + , NO, HOC + and HCO

Subfields: Astronomy and Astrophysics

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The fast molecular outflow in the Seyfert galaxy IC 5063 as seen by ALMA (2015, background): “…McKee 1989), cosmic rays (e.g. Sternberg et al. 1997; Ferland et al. 2009; Meijerink et al. 2011; Bayet et al. 2011), mechanical heating (Loenen et al. 2008; Meijerink et al. 2011) and photodissociation (including X-rays; e.g. Tielens & Hollenbach 1985; Maloney et al. 1996; Meijerink et al. 2007).”
Cosmic evolution of the atomic and molecular gas contents of galaxies (2011, background): “…calculationspredict that low metallicities, characteristic of dwarf galaxies, or hig densities and large optical depths of molecular clumps in starburst galaxies, should change X by a factor of up to5 − 10 in either direction (e.g. Bell et al. 2007; Meijerink et al. 2007; Bayet et al. 2009).”
A Multi-Transition HCN and HCO+ Study of 12 Nearby Active Galaxies: Active Galactic Nucleus versus Starburst Environments (2007, background): “Comparison to theoretical results The lower gas densities in our AGN sources may actually resolve a ’conflict’ introduced by recent theoretical studies (Meijerink & Spaans 2005; Meijerink et al. 2006, 2007) on XDR and PDR environments.”
Molecular line emission in NGC 1068 imaged with ALMA: I. An AGN-driven outflow in the dense molecular gas (2014, background): “Although theoretical models have tried to explain these differences, the underlying reasons for the apparent AGN specificity are still debated (Lepp & Dalgarno 1996; Maloney et al. 1996; Meijerink & Spaans 2005; Meijerink et al. 2007; Yamada et al. 2007; Harada et al. 2013).”
Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions (2010, background): “…models of molecular clouds (e.g., Hasegawa et al., 1992), protoplanetary disks (e.g., Aikawa et al., 2002), shells around late-type stars (e.g., Agúndez & Cernicharo, 2006), AGN tori (e.g., Meijerink et al., 2007), and planetary atmospheres (e.g., Gladstone et al., 1996) have been developed.”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “…HCN is a minor species subject to the abundance of Nitrogen and the details of the chemical network (e.g. Sternberg et al. 1994; Usero et al. 2004; Meijerink et al. 2007), and its (1-0) transition is sensitive to the specific physical excitation conditions with mid-infrared pumping possibly…”
The properties of the interstellar medium within a star-forming galaxy at z= 2.3 (2010, methodology): “These account for variations in density, temperature, clumpiness and time-dependent chemistry (e.g. Meijerink et al. 2007; Kaufman et al. 1999) and using their predictions of the ratios of [Cii], [Ci] and 12CO we can investigate the typical far-UV intensity and ISM density within the star-forming…”
A NEW NETWORK FOR HIGHER-TEMPERATURE GAS-PHASE CHEMISTRY. I. A PRELIMINARY STUDY OF ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI (2010, methodology): “…models for both cases, Meijerink (2006) analyzed different CO transitions and the HCO+/HCN intensity ratio in different types of galaxies to see whether UV photons or X-rays have the dominant effect, and a larger set of observational results was compared with grid models by Meijerink et al. (2007).”