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