A deep narrowband survey for planetary nebulae at the outskirts of M33

TL;DR

This study conducted a deep narrowband imaging survey of M33's outskirts to search for planetary nebulae, aiming to understand the galaxy's extended stellar structures and halo presence.

Contribution

It provides the first wide-field search for PNe in M33's outskirts, setting constraints on the galaxy's faint stellar halo and substructure.

Findings

No PNe detected outside the galaxy's disc, indicating a lack of a massive classical halo.

Eight new PN candidates identified within the disc, three confirmed spectroscopically.

The upper limit on the underlying stellar population's luminosity suggests absence of a significant halo.

Abstract

Context: Planetary nebulae (PNe) are excellent tracers of stellar populations with low surface brightness, and therefore provide a powerful method to detect and explore the rich system of substructures discovered around the main spiral galaxies of the Local Group. Aims: We searched the outskirts of the Local Group spiral galaxy M33 (the Triangulum) for PNe to gain new insights into the extended stellar substructure on the northern side of the disc and to study the existence of a faint classical halo. Methods: The search is based on wide field imaging covering a 4.5 square degree area out to a maximum projected distance of about 40 kpc from the centre of the galaxy. The PN candidates are detected by the combination of images obtained in narrowband filters selecting the [OIII]$\lambda5007\AA$ and H$\alpha$ + [NII] nebular lines and in the continuum g' and r' broadband filters. Results:Inside the bright optical disc of M33, eight new PN candidates were identified, three of which were spectroscopically confirmed. No PN candidates were found outside the limits of the disc. Fourteen additional sources showing [OIII] excess were also discovered. Conclusions:The absence of bright PN candidates in the area outside the galaxy disc covered by this survey sets an upper limit to the luminosity of the underlying population of $\mathrm{\sim1.6\cdot10^{7}L_{\odot}}$, suggesting the lack of a massive classical halo, which is in agreement with the results obtained using the RGB population.