Introduction: We present the results from the observations of the galaxy NGC 628 with the Planetary Nebulae Spectrograph (PN.S) equipped with the Hα arm. With the third PN.S arm, the Hα arm, we measure the Hα fluxes, in addition to fluxes and line-of-sight velocities (LOSV) of monochromatic, spatially unresolved [OIII] 5007Å sources, in the nearly face-on disc of NGC 628.

Methods: The narrow band color ([OIII] 5007Å–Hα) vs m5007 magnitude diagram does separate planetary nebulae (PNe) from single compact ionized HII regions and supernovae remnants (SNRs), which also emit in [OIII]5007 Å. The goals are to detect bona fide PNe in the face-on spiral galaxy NGC 628 (M74) so that we can measure the velocity dispersion of the stars perpendicular to the main plane of the disc. By associating the velocity dispersion orthogonal to the disc of the evolved stars whose scale height is measured for edge-on discs in the near-infrared wavelengths, we can measure the surface mass density directly and break the disc-halo degeneracy.

Results: This study motivates and validates the empirical selection criteria for PNe with the PN.S in star forming discs, while the modelling of the disc dynamics, based on the selected PN samples, is done in companion papers. We classified 442 PNe and 251 spatially isolated, unresolved HII regions: the PN.S with the Hα arm increased the number of known PNe in NGC 628 by a factor 4. In this study, we find evidence for two kinematically distinct PN populations in the NGC 628 disc. The kinematically ‘cold’ PN population dominates the PN luminosity function (LF) close to the bright cut-off magnitude, indicating that the PN massive, short-lived progenitors dominate the PNLF bright cut-off in NGC 628. The ‘warmer’ PN component increasingly dominates at fainter magnitudes. The velocity dispersion orthogonal to the disc plane of these two populations are σz,cold = 8.8 km–1 and σz,warm = 26.1 km–1 respectively, over a range of radii 80’’ to 425’’.

Discussion: The cold and warmer components contribute to the total sample of the PN population in NGC 628 with the ratio 46% (cold) and 54% (warm). Once the velocity dispersion of the old component is matched with the population’s scale height, the decomposition of the rotation curve for NGC 628 leads to a maximal disc, with the rotation of the baryonic component accounting for 78% of the total rotational velocity in NGC 628.