A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar

¹ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
e-mail: eduardo.gonzalez@uah.es
² Center for Astrophysics Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive Fairfax, VA 22030, USA
⁵ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁷ Centro de Astrobiología (CAB, CSIC-INTA), ESAC Campus, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ IAA – Instituto de Astrofísica de Andalucía (CSIC), Apdo. 3004, 18008 Granada, Spain

Received: 28 July 2020
Accepted: 27 October 2020

Abstract

Galaxies with nuclear bars are believed to efficiently drive gas inward, generating a nuclear starburst and possibly an active galactic nucleus. We confirm this scenario for the isolated, double-barred, luminous infrared galaxy ESO 320-G030 based on an analysis of Herschel and ALMA spectroscopic observations. Herschel/PACS and SPIRE observations of ESO 320-G030 show absorption or emission in 18 lines of H₂O, which we combine with the ALMA H₂O 4₂₃ − 3₃₀ 448 GHz line (E_upper ∼ 400 K) and continuum images to study the physical properties of the nuclear region. Radiative transfer models indicate that three nuclear components are required to account for the multi-transition H₂O and continuum data. An envelope, with radius R ∼ 130 − 150 pc, dust temperature T_dust ≈ 50 K, and N_H2 ∼ 2 × 10²³ cm⁻², surrounds a nuclear disk with R ∼ 40 pc that is optically thick in the far-infrared (τ_100 μm ∼ 1.5 − 3, N_H2 ∼ 2 × 10²⁴ cm⁻²). In addition, an extremely compact (R ∼ 12 pc), warm (≈100 K), and buried (τ_100 μm >  5, N_H2 ≳ 5 × 10²⁴ cm⁻²) core component is required to account for the very high-lying H₂O absorption lines. The three nuclear components account for 70% of the galaxy luminosity (SFR ∼ 16 − 18 M_⊙ yr⁻¹). The nucleus is fed by a molecular inflow observed in CO 2-1 with ALMA, which is associated with the nuclear bar. With decreasing radius (r = 450 − 225 pc), the mass inflow rate increases up to Ṁ_inf ∼ 20 Ṁ yr⁻¹, which is similar to the nuclear star formation rate (SFR), indicating that the starburst is sustained by the inflow. At lower r, ∼100 − 150 pc, the inflow is best probed by the far-infrared OH ground-state doublets, with an estimated Ṁ_inf ∼ 30 Ṁ yr⁻¹. The inferred short timescale of ∼20 Myr for nuclear gas replenishment indicates quick secular evolution, and indicates that we are witnessing an intermediate stage (< 100 Myr) proto-pseudobulge fed by a massive inflow that is driven by a strong nuclear bar. We also apply the H₂O model to the Herschel far-infrared spectroscopic observations of H₂¹⁸O, OH, ¹⁸OH, OH⁺, H₂O⁺, H₃O⁺, NH, NH₂, NH₃, CH, CH⁺, ¹³CH⁺, HF, SH, and C₃, and we estimate their abundances.