Planetary Nebulae in the Circumnuclear Region of M31: A Spectroscopic Sample

Abstract

Planetary nebulae (PNe) are an important tool for studying the dynamics and chemical evolution of galaxies in the Local Universe, given their characteristic bright emission line spectra. The Andromeda Galaxy (M31) provides a unique laboratory for studying PNe in the circumnuclear region, thanks to its proximity and almost uniformly low line-of-sight extinction that ensures observations with high resolution and sensitivity. Using the WIYN/Hydra multi-fiber spectrograph, we have obtained optical (4119–6882 Å) spectra of 77 PN candidates selected from Hubble Space Telescope narrow-band imaging, which are located within the central ~500 pc region of M31. Among these candidates, 49 (64%) are spectroscopically observed for the first time. The spectra of 300 previously known PNe and H ii regions, which primarily reside in the disk, are also taken for comparison. All 77 circumnuclear PN candidates exhibit prominent emission lines, including [O iii] λ5007, [N ii] λ6583, Hα and Hβ, strongly suggesting that they are genuine PNe. We measured the line fluxes, radial velocities and line widths for all objects, and found that the radial velocities of the circumnuclear PNe generally trace rotation of the inner bulge. We also estimated a dynamical mass of ~6.4 ± 0.5 × 109 M ⊙ enclosed within an effective galactocentric radius of 340 pc, which is compatible with the previously estimated total stellar mass in this region.