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

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 H 2 O, which we combine with the ALMA H 2 O 4 23 − 3 30 448 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 H 2 O and continuum data. An envelope, with radius R ∼ 130 − 150 pc, dust temperature T dust ≈ 50 K, and N H2 ∼ 2 × 10 23 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 × 10 24 cm −2 ). In addition, an extremely compact ( R ∼ 12 pc), warm (≈100 K), and buried ( τ 100 μm > 5, N H2 ≳ 5 × 10 24 cm −2 ) core component is required to account for the very high-lying H 2 O 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 H 2 O model to the Herschel far-infrared spectroscopic observations of H 2 18 O, OH, 18 OH, OH + , H 2 O + , H 3 O + , NH, NH 2 , NH 3 , CH, CH + , 13 CH + , HF, SH, and C 3 , and we estimate their abundances.