Herschelspectroscopic observations of the compact obscured nucleus in Zw 049.057

Abstract

Context. The luminous infrared galaxy Zw 049.057 contains a compact obscured nucleus where a considerable amount of the galaxy's luminosity is generated. This nucleus contains a dusty environment that is rich in molecular gas. One approach to probing this kind of environment and to revealing what is hidden behind the dust is to study the rotational lines of molecules that couple well with the infrared radiation emitted by the dust. Aims. We probe the physical conditions in the core of Zw 049.057 and establish the nature of its nuclear power source (starburst or active galactic nucleus). . We modeled the unresolved core of the galaxy using a spherically symmetric radiative transfer code. To account for the different excitation requirements of the various molecular transitions, we use multiple components and different physical conditions. Results. We present the full high-resolution SPIRE FTS spectrum of Zw 049.057, along with relevant spectral scans in the PACS range. We find that a minimum of two different components (nuclear and extended) are required in order to account for the rich molecular line spectrum of Zw 049.057. The nuclear component has a radius of 10−30 pc, a very high infrared surface brightness (∼10 14 L kpc −2 ), warm dust (T d > 100 K), and a very large H 2 column density (N H 2 = 10 24 −10 25 cm −2 ). The modeling also indicates high nuclear H 2 O (∼5 × 10 −6 ) and OH (∼4 × 10 −6 ) abundances relative to H 2 as well as a low 16 O/ 18 O-ratio of 50−100. We also find a prominent infall signature in the [O I] line. We tentatively detect a 500 km s −1 outflow in the H 2 O 3 13 → 2 02 line. Conclusions. The high surface brightness of the core indicates the presence of either a buried active galactic nucleus or a very dense nuclear starburst. The estimated column density towards the core of Zw 049.057 indicates that it is Compton-thick, making a buried X-ray source difficult to detect even in hard X-rays. We discuss the elevated H 2 O abundance in the nucleus in the context of warm grain and gas-phase chemistry. The H 2 O abundance is comparable to that of other compact (ultra-)luminous infrared galaxies such as NGC 4418 and Arp 220 -and also to hot cores in the Milky Way. The enhancement of 18 O is a possible indicator that the nucleus of Zw 049.057 is in a similar evolutionary stage as the nuclei of Arp 220 -and more advanced than NGC 4418. We discuss the origin of the extreme nuclear gas concentration and note that the infalling gas detected in [O I] implies that the gas reservoir in the central region of Zw 049.057 is being replenished. If confirmed, the H 2 O outflow suggests that the nucleus is in a stage of rapid evolution.

Molecular Outflows in Local ULIRGs: Energetics from Multitransition OH Analysis

“…(1) in González-Alfonso et al (2014b) with an OH abundance relative to H nuclei of X OH = 2.5 × 10 −6 . The adopted abundance is, within a factor of ∼ 3, consistent with the value inferred from multi-transition observations of OH in the Galactic Sgr B2 and Orion KL outflow (Goicoechea & Cernicharo 2002;Goicoechea et al 2006) and in buried galaxy nuclei (GA12, Falstad et al 2015), and with chemical models of dense photodissociation regions (the peak value, Sternberg & Dalgarno 1995), and of cosmic-rays and X-rays dominated regions (Meijerink et al 2011). The source structure includes expansion with constant velocity v out to describe the lowvelocity blueshifts of the peak absorption of OH65 that indicate outflowing gas in the nuclear region ( §4.5), and is divided into shells to account for the varying excitation with depth into the source.…”

“…In addition to the previously studied case of NGC 4418 (González-Alfonso et al 2012), where the inflow has also been inferred from highangular resolution observations Costagliola et al 2013), the case of Zw 049 where an inverse P-Cygni profile is also observed in the [O i]63 µm line (Falstad et al 2015), the inverse P-Cygni profile of OH119 in Circinus (Stone et al 2016), and the inflow observed in OH119 around Arp 299a (Falstad et al 2017), the clearest examples are the LIRG IRAS 11506−3851 and the ULIRG IRAS 15250+3609. Their OH spec- tra are shown in Fig.…”

“…a The line was also targeted in instrument verification observations with OBSIDs 1342184688, 1342184689, 1342184690, 1342184691, 1342184692, and the averaged spectrum is used. sible contamination by lines of H 2 O, dozens of which appear in the far-IR spectra of some (U)LIRGs (e.g., Falstad et al 2015;González-Alfonso et al 2012, hereafter GA12). The strongest of these lines is the 4 23 − 3 12 (E lower = 250 K) centered at ≈ −1400 km s −1 in the OH79 spectra.…”

A precessing molecular jet signaling an obscured, growing supermassive black hole in NGC 1377?

“…This would imply that the nucleus of NGC 1377 is Compton thick and similar to the nuclei of other extremely obscured early type disk galaxies, such as NGC 4418 Costagliola et al 2013), IC 860 and Zw049.057 (Falstad et al 2015;Aalto et al 2015a), but more studies are required to confirm this high N(H 2 ) for NGC 1377.…”

Feedback and Feeding in the Context of Galaxy Evolution withSPICA: Direct Characterisation of Molecular Outflows and Inflows

“…The technique has been applied to local (U)LIRGs, enabling the characterization of the SEDs associated with buried galaxy nuclei (e.g. Falstad et al 2015, and would be applied to high-z sources with the advent of SPICA.…”