High-resolution mm and cm study of the obscured LIRG NGC 4418

Abstract

Context. Understanding the nature of the power-source in luminous infrared galaxies (LIRG) is difficult due to their extreme obscuration. Observations at radio and mm wavelengths can penetrate large columns of dust and gas and provide unique insights into the properties of the compact obscured nuclei of LIRGs. Aims. The aim of this study is to constrain the dynamics, structure, and feeding of the compact nucleus of NGC 4418, and to reveal the nature of the main hidden power-source: starburst or active galactic nucleus (AGN). Methods. We obtained high spatial resolution observations of NGC 4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA in very extended configuration. We used the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate, and the dust temperature. Emission lines were used to study kinematics through position-velocity diagrams. Molecular emission was studied with population diagrams and by fitting a local thermal equilibrium (LTE) synthetic spectrum. Results. We detect bright 1-mm-line emission from CO, HC 3 N, HNC, and C 34 S and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s −1 . We detect vibrationally excited HC 3 N and HNC, with T vib ∼ 300 K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160, and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 10 4 M pc −2 is extremely high and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Conclusions. Our observations confirm the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC 4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in the ULIRG Arp 220, with an age of 3-10 Myr and a star formation rate >10 M yr −1 . If an AGN is present, it must be extremely Compton-thick.

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

“…The combination of a bright, compact IR source and a high molecular column produce an incredibly rich vibrationally excited spectrum. Vibrationally excited HCN and HNC were first detected in this source by Sakamoto et al (2010) and Costagliola et al (2013), and the galaxy shows very bright emission from vibrationally excited HC 3 N (Costagliola & Aalto 2010;Costagliola et al 2013).…”

“…UV-fit of the spectrum Previous interferometric observations by Sakamoto et al (2013) and Costagliola et al (2013) show that 70% of the molecular emission in NGC 4418 emerges from a region of less than 0. 4 in diameter, we therefore expected all the emission to be included in our synthesized beam.…”

“…In two recent papers Costagliola et al 2013) we used the Submillimeter Array (SMA) and the MultiElement Radio Linked Interferometer Network (MERLIN) to directly probe the nucleus of NGC 4418 at mm and radio wavelengths with sub-arcsecond resolution. These studies confirm the existence of a ∼20 pc (0.…”

“…The extreme most of their energy as thermal dust emission in the infrared and constitute the dominant population among the most luminous extragalactic objects. Observations at mid-IR and millimeter wavelengths suggest that they may play a crucial role in galaxy evolution, representing the early obscured stages of starburst galaxies and AGNs (Spoon et al 2007;Aalto et al 2007a;Sakamoto et al 2013;Costagliola et al 2013). Because of the large amounts of gas and dust (N(H 2 ) > 10 24 cm −2 ) and the extreme obscuration (A V > 100 mag) the nature of the activity cannot be revealed by standard optical, IR, or even X-ray observations.…”

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

“…While inflowing motions can also be inferred from skewed profiles of pure emission lines and HI redshifted absorption (Costagliola et al 2013), the far-IR offers a very sensitive probe of inflow due to the increasing continuum optical depth.…”