Integral Field Spectroscopy of the cometary starburst galaxy NGC 4861

Abstract

Using the PMAS Integral Field Unit on the Calar Alto 3.5 m telescope, we observed the southern component (Markarian 59) of the ‘cometary’ starburst galaxy NGC 4861. Mrk 59 is centred on a giant nebula and concentration of stars 1 kpc in diameter. Strong H α emission points to a star-formation rate (SFR) at least 0.47 M⊙ yr−1. Mrk 59 has a very high [O III] λ5007/H β ratio, reaching 7.35 in the central nebula, with a second peak at a star-forming hotspot further north. Fast outflows are not detected but nebular motion and galaxy rotation produce relative velocities up to 40 km s−1. Spectral analysis of different regions with ‘Fitting Analysis using Differential evolution Optimization’ (FADO) finds that the stars in the central and ‘spur’ nebulae are very young, ≤ 125 Myr with a large < 10 Myr contribution. Older stars (∼ 1 Gyr) make up the northern disk component, while the other regions show mixtures of 1 Gyr age with very young stars. This and the high specific SFR ∼ 3.5 Gyr−1 imply a bimodal star formation history, with Mrk 59 formed in ongoing starbursts fuelled by a huge gas inflow, turning the galaxy into an asymmetric ‘green pea’ or blue compact dwarf. We map the He II λ4686 emission, and identify a broad component from the central nebula, consistent with the emission of ∼300 Wolf–Rayet stars. About a third of the He II λ4686 flux is a narrow line emitted from a more extended area covering the central and spur nebulae, and may have a different origin.