CHEMICALLY DISTINCT NUCLEI AND OUTFLOWING SHOCKED MOLECULAR GAS IN Arp 220

Abstract

We present the results of interferometric spectral line observations of Arp 220 at 3.5 mm and 1.2 mm from the Plateau de Bure Interferometer (PdBI), imaging the two nuclear disks in H 13 CN(1 − 0) and (3 − 2), H 13 CO + (1 − 0) and (3 − 2), and HN 13 C(3 − 2) as well as SiO(2 − 1) and (6 − 5), HC 15 N(3 − 2), and SO(6 6 − 5 5 ). The gas traced by SiO(6 − 5) has a complex and extended kinematic signature including a prominent P Cygni profile, almost identical to previous observations of HCO + (3 − 2). Spatial offsets 0.1 north and south of the continuum centre in the emission and absorption of the SiO(6 − 5) P Cygni profile in the western nucleus (WN) imply a bipolar outflow, delineating the northern and southern edges of its disk and suggesting a disk radius of ∼ 40 pc, consistent with that found by ALMA observations of Arp 220. We address the blending of SiO(6−5) and H 13 CO + (3−2) by considering two limiting cases with regards to the H 13 CO + emission throughout our analysis. Large velocity gradient (LVG) modelling is used to constrain the physical conditions of the gas and to infer abundance ratios in the two nuclei. Our most conservative lower limit on the [H 13 CN]/[H 13 CO + ] abundance ratio is 11 in the WN, cf. 0.10 in the eastern nucleus (EN). Comparing these ratios to the literature we argue on chemical grounds for an energetically significant AGN in the WN driving either X-ray or shock chemistry, and a dominant starburst in the EN.

The unbearable opaqueness of Arp220

“…This feature was previously observed with the SMA also in HCO + 3−2 (Sakamoto et al 2009) and more recently in SiO (Tunnard et al 2015). This means that a total of three absorption systems are identified at V LSR,radio ∼ 4767, 5202, and 5348 km s −1 .…”

“…The location of the emission of HCN and HCO + (Fig. 4a) differs from what Tunnard et al (2015) observed in SiO. This supports their idea that SiO is emitted in the gas affected by the putative outflow, but higher resolution imaging is required to conclude about this question.…”

“…(a) Peak brightness temperatures as described in Wilson et al (2014) and taking 81, 77, 58, and 56 K/Jy conversion factors for 247.0, 263.1, 336.9, and 347.9 GHz, respectively. (Scoville et al 2015), and even SiO (Tunnard et al 2015). The apparent absorption peaks are highlighted with vertical dashed lines in Fig.…”

“…These nuclei are optically thick outside of the 5-350 GHz frequency range as a result of free-free and dust absorption at low and high frequencies (Barcos-Muñoz et al 2015). Although they have been the target of studies covering all available wavelengths for decades, the origin of the ∼10 12 L stemming from Arp 220 remains an unanswered question, with arguments suggesting an active galactic nucleus (AGN) and/or a starburst origin (see references in Wilson et al 2014;Tunnard et al 2015;Barcos-Muñoz et al 2015). Even the many molecular transitions that have been detected and imaged at high resolution (Martín et al 2011, and references therein) have not been able to provide a definitive answer.…”

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

“…When it was introduced, RADEX was recommended as a tool for interpreting molecular line ratios, primarily through diagnostic plots. However, since then, it and other LVG codes have become extensively used as general purpose tools for extracting gas conditions, especially in extragalactic targets, e.g., Krips et al (2008), Greve et al (2009), Rangwala et al (2011), Kamenetzky et al (2014), Papadopoulos et al (2014), Viti et al (2014), andTunnard et al (2015a, where the plethora of molecular lines now being observed by the state-of-the-art single-dish telescopes and interferometer arrays are allowing for modeling of multiple line ratios, and therefore more complex physical models.…”