Infrared imaging and spectroscopy of NGC 7469

As part of an extensive study of the infrared properties of luminous galactic nuclei we report here subarcsecond-resolution near-infrared imaging and spectroscopy of the nucleus of the Seyfert 1 galaxy NGC 7469. Our measurements elucidate for the first time the spatial distribution of the near-infrared continuum and a number of bright near-infrared lines (Brγ, [Fe II], [Si vI], H₂, He 1, and CO) on a scale of less than a few hundred parsecs. We find that these tracers originate from three distinct components: the nucleus, a 1″.5 (480 pc) radius circumnuclear ring/spiral-arm system with embedded knots, and a 1″ ridge of blueshifted, radially streaming gas emanating southward from the nucleus. The circumnuclear ring is the location of a powerful starburst deeply embedded in a large concentration of molecular gas and dust and has a luminosity of ≈ 3 × 10¹¹ L⊙, two-thirds of the bolometric luminosity of the entire galaxy. It contains a number of supergiant star formation regions with a few 10⁴ OB stars each. Our 0″.4 resolution near-infrared images do not show evidence for a stellar bar. Gas influx into the nucleus and starburst ring were most likely triggered > 10⁸ yr ago by the interaction of NGC 7469 with its neighbor, IC 5283. A comparison of the mass derived from gas and stellar dynamics and from the CO 1-0 millimeter line emission suggests that the conversion factor from molecular hydrogen column density to integrated CO flux [X = N(H₂)/I(CO)] in the central 800 pc is significantly smaller than in the Galactic disk, presumably as the result of the impact of the intense star formation activity on the surrounding interstellar medium. We have carried out a quantitative analysis of the ring's visible, infrared, and radio emission using star cluster models. The data are well fit by two types of solutions. One possibility is that the star formation rate has been constant (≈30 M⊙ yr^-1) over the last several 10⁷ yr and stars more massive than 30-40 M⊙ have not been forming during this period. An alternative model that does not require an upper mass cutoff is a decaying burst with a decay time significantly less than its present age (≈1.5 × 10⁷ yr). For both star formation histories and for the small N(H₂)/I(CO) conversion factor mentioned above, the star formation efficiency in NGC 7469 is high and the burst is soon going to end because the gas reservoir is exhausted. Our 2.1 μm H₂ S(1) line maps show that there is a large concentration of molecular gas close to the Seyfert 1 nucleus of NGC 7469. The central 10² pc of NGC 7469 contains 10^8±1 M⊙ of neutral interstellar gas, consistent with current unifying models for Seyfert 1 and 2 nuclei. Broad (FWHM 1400 km s^-1) 1.96 μm [Si Vi] line emission arises in moderately dense (n_e ≥ 10² cm^-3) gas within a few 10² pc from the nucleus. Our work clearly shows that NGC 7469 is a luminous Seyfert galaxy whose total energy output is predominantly powered by star formation.