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

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<SUB>2</SUB>O, which we combine with the ALMA H<SUB>2</SUB>O 4<SUB>23</SUB> - 3<SUB>30</SUB> 448 GHz line (E<SUB>upper</SUB> ∼ 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<SUB>2</SUB>O and continuum data. An envelope, with radius R ∼ 130 - 150 pc, dust temperature T<SUB>dust</SUB> ≈ 50 K, and N<SUB>H2</SUB> ∼ 2 × 10<SUP>23</SUP> cm<SUP>-2</SUP>, surrounds a nuclear disk with R ∼ 40 pc that is optically thick in the far-infrared (τ<SUB>100 μm</SUB> ∼ 1.5 - 3, N<SUB>H2</SUB> ∼ 2 × 10<SUP>24</SUP> cm<SUP>-2</SUP>). In addition, an extremely compact (R ∼ 12 pc), warm (≈100 K), and buried (τ<SUB>100 μm</SUB> &gt; 5, N<SUB>H2</SUB> ≳ 5 × 10<SUP>24</SUP> cm<SUP>-2</SUP>) core component is required to account for the very high-lying H<SUB>2</SUB>O absorption lines. The three nuclear components account for 70% of the galaxy luminosity (SFR ∼ 16 - 18 M<SUB>⊙</SUB> yr<SUP>-1</SUP>). 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<SUB>inf</SUB> ∼ 20 Ṁ yr<SUP>-1</SUP>, 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<SUB>inf</SUB> ∼ 30 Ṁ yr<SUP>-1</SUP>. The inferred short timescale of ∼20 Myr for nuclear gas replenishment indicates quick secular evolution, and indicates that we are witnessing an intermediate stage (&lt; 100 Myr) proto-pseudobulge fed by a massive inflow that is driven by a strong nuclear bar. We also apply the H<SUB>2</SUB>O model to the Herschel far-infrared spectroscopic observations of H<SUB>2</SUB><SUP>18</SUP>O, OH, <SUP>18</SUP>OH, OH<SUP>+</SUP>, H<SUB>2</SUB>O<SUP>+</SUP>, H<SUB>3</SUB>O<SUP>+</SUP>, NH, NH<SUB>2</SUB>, NH<SUB>3</SUB>, CH, CH<SUP>+</SUP>, <SUP>13</SUP>CH<SUP>+</SUP>, HF, SH, and C<SUB>3</SUB>, and we estimate their abundances.