The Planetary Nebulae Population in the Nuclear Regions of M31: the SAURON view

TL;DR

This study uses integral-field spectroscopy to analyze the planetary nebulae population in the nuclear regions of M31, revealing a deficiency of bright PNe likely due to stellar population characteristics and metallicity effects.

Contribution

It provides new insights into the PNe luminosity function in M31's nucleus and links stellar population properties to PNe visibility and brightness.

Findings

Nuclear PNe in M31 are fewer and less bright than expected from standard PNLF models.

The lack of bright PNe is linked to a stellar population with fewer massive, cool stars and more UV-bright stars.

Results support the idea that metallicity influences horizontal-branch star populations and PNe formation.

Abstract

Following a first study of the central regions of M32 that illustrated the power of integral-field spectroscopy (IFS) in detecting and measuring the [O III]{\lambda}5007 emission of PNe against a strong stellar background, we turn to the very nuclear PN population of M31, within 80 pc of its centre. We show that PNe can also be found in the presence of emission from diffuse gas and further illustrate the excellent sensitivity of IFS in detecting extragalactic PNe through a comparison with narrowband images obtained with the Hubble Space Telescope. We find the nuclear PNe population of M31 is only marginally consistent with the generally adopted form of the PNe luminosity function (PNLF). In particular, this is due to a lack of PNe with absolute magnitude M5007 brighter than -3, which would only result from a rather unfortunate draw from such a model PNLF. We suggest that the observed lack of bright PNe in the nuclear regions of M31 is due to a horizontal-branch population that is more tilted toward less massive and hotter He-burning stars, so that its progeny consists mostly of UV-bright stars that fail to climb back up the asymptotic giant branch (AGB) and only of few, if any, bright PNe powered by central post-AGB stars. These results are also consistent with recent reports on a dearth of bright post-AGB stars towards the nucleus of M31, and lend further support to the idea that the metallicity of a stellar population has an impact on the way the horizontal branch is populated and to the loose anticorrelation between the strength of the UV-upturn and the specific number of PNe that is observed in early-type galaxies. Finally, our investigation also serves to stress the importance of considering the same spatial scales when comparing the PNe population of galaxies with the properties of their stellar populations.