Molecular line emission in NGC 1068 imaged with ALMA

Abstract

Aims. We present a detailed analysis of Atacama Large Millimeter/submillimeter Array (ALMA) Bands 7 and 9 data of CO, HCO + , HCN, and CS, augmented with Plateau de Bure Interferometer (PdBI) data of the ∼200 pc circumnuclear disc (CND) and the ∼1.3 kpc starburst ring (SB ring) of NGC 1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy. We aim to determine the physical characteristics of the dense gas present in the CND, and to establish whether the different line intensity ratios we find within the CND, as well as between the CND and the SB ring, are due to excitation effects (gas density and temperature differences) or to a different chemistry. Methods. We estimate the column densities of each species in local thermodynamic equilibrium (LTE). We then compute large one-dimensional, non-LTE radiative transfer grids (using RADEX) by using only the CO transitions first, and then all the available molecules to constrain the densities, temperatures, and column densities within the CND. We finally present a preliminary set of chemical models to determine the origin of the gas. Results. We find that, in general, the gas in the CND is very dense (>10 5 cm −3 ) and hot (T > 150 K), with differences especially in the temperature across the CND. The AGN position has the lowest CO/HCO + , CO/HCN, and CO/CS column density ratios. The RADEX analyses seem to indicate that there is chemical differentiation across the CND. We also find differences between the chemistry of the SB ring and some regions of the CND; the SB ring is also much colder and less dense than the CND. Chemical modelling does not succeed in reproducing all the molecular ratios with one model per region, suggesting the presence of multi-gas phase components. Conclusions. The LTE, RADEX, and chemical analyses all indicate that more than one gas-phase component is necessary to uniquely fit all the available molecular ratios within the CND. A higher number of molecular transitions at the ALMA resolution is necessary to determine quantitatively the physical and chemical characteristics of these components.

Multimolecule ALMA observations toward the Seyfert 1 galaxy NGC 1097

“…Similarly, the LVG analysis of a sample of AGN sources with enhanced HCN emission resulted in kinetic temperatures T K > 40 K (Krips et al 2008) at single dish resolution. High resolution multitransition observations toward NGC 1068 show clumps in the CND with temperatures above 100 K Viti et al 2014). Regarding the origin of such heating, mechanical heating is suggested to explain the required high temperature in NGC 1097 (Loenen et al 2008;Izumi et al 2013).…”

“…3.2) would increase by a factor of 5−6 for excitation temperatures of ∼20 K. Figure 5 clearly suggests observable variations in the HCN/HCO + line ratios within the CND even though the central region is only resolved over a few synthesized beams. Molecular variations in the central few hundred parsec CND around the AGN in NGC 1068 have been reported both at very high spatial resolution (35 pc, García-Burillo et al 2014;Viti et al 2014) for HCN and HCO + and at a coarser spatial resolution of 300 × 170 for a wider sample of species (Takano et al 2014). Here we discuss the different morphologies of the observed species toward NGC 1097 and compare it to what is observed in NGC 1068.…”

“…In fact, within the past year and while still in the early science phase, ALMA is already providing extremely high quality data on the chemistry of some of the brightest nearby prototypical galaxies, both AGNs (NGC 1068, García-Burillo et al 2014;Takano et al 2014;Viti et al 2014;NGC 1097, Izumi et al 2013 and SBs (NGC 253, Meier et al 2014). The unprecedented capabilities of these instruments allow us to extend molecular studies to either farther or fainter objects, thus increasing the significance of chemical comparative studies, now limited to a very reduced number of sources .…”

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

“…Extragalactic chemistry is a field that is quickly expanding leading to new, powerful diagnostic tools for the star-forming and active galactic nuclei (AGN) activity in galaxies (e.g., Meier et al 2014;Viti et al 2014;Martín et al 2015). The extreme most of their energy as thermal dust emission in the infrared and constitute the dominant population among the most luminous extragalactic objects.…”

How Dense Is Your Gas? On the Recoverability of LVG Model Parameters

“…is the molecular abundance. While a single molecular abundance is not particularly informative, with reliable estimates of the abundances of several species significant progress can be made (Meijerink & Spaans 2005;Meijerink et al 2007Meijerink et al , 2011Bayet et al 2008;Viti et al 2014). An interesting question therefore is whether LVG models can in fact be used to reliably recover molecular abundances.…”