Panchromatic Hubble Andromeda Treasury IX: A Photometric Survey of Planetary Nebulae in M31

TL;DR

This study utilizes HST imaging data to identify, refine, and analyze planetary nebulae in M31, improving astrometric accuracy, confirming true PNe, and developing a photometric excitation measurement method.

Contribution

It refines the catalog of PNe in M31, confirms the nature of candidates, and introduces a new photometric method to assess excitation levels in PNe.

Findings

Refined astrometry of PNe to 0.05 arcseconds.

Confirmed 152 objects are not PNe.

Identified a potential new PN candidate.

Abstract

We search Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) broadband imaging data from the Panchromatic Hubble Andromeda Treasury (PHAT) survey to identify detections of cataloged planetary nebulae (PNe). Of the 711 PNe currently in the literature within the PHAT footprint, we find 467 detected in the broadband. For these 467 we are able to refine their astrometric accuracy from ~0."3 to 0."05. Using the resolution of HST, we are able to show that 152 objects currently in the catalogs are definitively not PNe, and we show that 32 objects thought to be extended in ground-based images are actually point-like and therefore good PN candidates. We also find one PN candidate that is marginally resolved. If this is a PN, it is up to 0.7 pc in diameter. With our new photometric data, we develop a method of measuring the level of excitation in individual PNe by comparing broadband and narrowband imaging and describe the effects of excitation on a PN's photometric signature. Using the photometric properties of the known PNe in the PHAT catalogs, we search for more PN, but do not find any new candidates, suggesting that ground-based emission-line surveys are complete in the PHAT footprint to F475W $\simeq$ 24.