Circumnuclear star formation in Mrk 42 mapped with Gemini Near-infrared Integral Field Spectrograph

Abstract We present Gemini Near-infrared Integral Field Spectrograph (NIFS) observations of the inner 1.5 × 1.5 kpc2 of the narrow-line Seyfert 1 galaxy Mrk 42 at a spatial resolution of 60 pc and spectral resolution of 40 $\rm km\,s^{-1}$. The emission-line flux and equivalent width maps clearly show a ring of circumnuclear star formation regions surrounding the nucleus with radius of ∼500 pc. The spectra of some of these regions show molecular absorption features which are probably of CN, TiO, or VO, indicating the presence of massive evolved stars in the thermally pulsing asymptotic giant branch phase. The gas kinematics of the ring is dominated by rotation in the plane of the galaxy, following the large-scale disc geometry, while at the nucleus an additional outflowing component is detected blueshifted by 300–500 $\rm km\,s^{-1}$, relative to the systemic velocity of the galaxy. Based on the equivalent width of Br γ we find pieces of evidence of gradients in the age of H ii regions along the ring of Mrk 42, favouring the pearls on a string scenario of star formation. The broad component of Pa β emission line presents a Full Width at Half Maximum of ∼1480 $\rm km\,s^{-1}$, implying in a mass of ∼2.5 × 106 M⊙ for the central supermassive black hole. Based on emission-line ratios we conclude that besides the active galactic nucleus, Mrk 42 presents nuclear Starburst activity.