INFRARED NARROWBAND TOMOGRAPHY OF THE LOCAL STARBURST NGC 1569 WITH THE LARGE BINOCULAR TELESCOPE/LUCIFER

We used the near-IR imager/spectrograph LUCIFER mounted on the Large Binocular Telescope to image, with subarcsecond seeing, the local dwarf starburst NGC 1569 in the JHK bands and He i 1.08 μm, [Fe ii] 1.64 μm, and Brγ narrowband filters. We obtained high-quality spatial maps of He i 1.08 μm, [Fe ii] 1.64 μm, and Brγ emission across the galaxy, and used them together with Hubble Space Telescope/Advanced Camera for Surveys images of NGC 1569 in the Hα filter to derive the two-dimensional spatial map of the dust extinction and surface star formation rate (SFR) density. We show that dust extinction (as derived from the Hα/Brγ flux ratio) is rather patchy and, on average, higher in the northwest (NW) portion of the galaxy (Eg(B − V) ≃ 0.71 mag) than in the southeast (Eg(B − V) ≃ 0.57 mag). Similarly, the surface density of SFR (computed from either the dereddened Hα or dereddened Brγ image) peaks in the NW region of NGC 1569, reaching a value of about 4 × 10−6 M☉ yr−1 pc−2. The total SFR as estimated from the integrated, dereddened Hα (or, alternatively, Brγ) luminosity is about 0.4 M☉ yr−1, and the total supernova rate from the integrated, dereddened [Fe ii] 1.64 μm luminosity is about 0.005 yr−1 (assuming a distance of 3.36 Mpc). The azimuthally averaged [Fe ii] 1.64 μm/Brγ flux ratio is larger at the edges of the central, gas-deficient cavities (encompassing the superstar clusters A and B) and in the galaxy outskirts. If we interpret this line ratio as the ratio between the average past star formation (as traced by supernovae) and ongoing activity (represented by OB stars able to ionize the interstellar medium), it would then indicate that star formation has been quenched within the central cavities and lately triggered in a ring around them. The number of ionizing hydrogen and helium photons as computed from the integrated, dereddened Hα and He i 1.08 μm luminosities suggests that the latest burst of star formation occurred about 4 Myr ago and produced new stars with a total mass of ≃1.8 × 106 M☉.