Deep Fabry-Perot Halpha Observations of NGC 7793: a Very Extended Halpha Disk and a Truly Declining Rotation Curve

TL;DR

Deep Halpha observations of NGC 7793 reveal an extensive HII disk reaching the HI edge and confirm a truly declining rotation curve that still requires dark matter for explanation.

Contribution

First detailed Halpha imaging of NGC 7793's extended disk showing ionization sources and rotation dynamics in a non-AGN galaxy.

Findings

Halpha emission extends to the HI disk edge

Outer rotation curve is declining but not Keplerian

Dark halo remains necessary for mass modeling

Abstract

Deep Halpha observations of the Sculptor Group galaxy NGC 7793 were obtained on the ESO 3.60m and the Marseille 36cm telescopes at La Silla, Chile. Halpha emission is detected all the way to the edge of the HI disk, making of the HII disk of NGC 7793 one of the largest ever observed in a quiet non-AGN late-type system. Even in the very outer parts, the HII ionizing sources are probably mainly internal (massive stars in the disk) with an unlikely contribution from the extragalactic ionizing background. The Halpha kinematics confirms what had already been seen with the HI observations: NGC 7793 has a truly declining rotation curve. However, the decline is not Keplerian and a dark halo is still needed to explain the rotation velocities in the outer parts.