tokens: reproduced with a two-component photometric model that consists of a PN-poor inner halo with a de Vaucouleurs profile and a PN-rich exponential outer component. Relying solely on the information from surface photometry, we were unable to constrain the scale radius of the exponential profile. We constrained the shape of the exponential profile using the information from resolved stellar populations. Harris et al. (2007a) and Lee Jang (2016) published number-density profiles of MR and MP RGB stellar populations in the halo of M 105. We combined the information from these studies to infer that the density distribution of the most MP stars ([M/H] −1.0) follows an exponential profile with a central surface brightness of μ 0 = 27.7 ± 0.1 mag arcsec −2 and a scale radius of r h = 258 ± 2′′ (Sect. 5 ). Using the information from the RGB populations, we constructed a two-component photometric model, where the two components were weighted by their α -parameter values. In the PN-poor inner halo, the light distribution is dominated by intermediate-metallicity ( − 1.0 ≤ [M/H] −0.5) and MR ([M/H] ≥ −0.5) stars, while in the PN-rich outer halo the MP stars dominate the light distribution. The α -parameter values of the two components are as follows: for the inner PN-poor component, and for the outer PN-rich component. We find a difference of a factor of seven in the α -parameter values of the inner versus outer components, implying that the luminosity-specific PN number of the stellar population that traces the old and MP halo at large radii is seven times higher. We interpret the high α -parameter value and the steep faint-end PNLF slope as tell-tale signs of an additional old and MP stellar population whose contribution towards the overall light distribution increases at large radii. This interpretation is corroborated by the results of Harris et al. (2007a) and Lee Jang (2016) based on resolved stellar populations. While in earlier works ( Longobardi et al. 2013 ; Hartke et al. 2017 ) this association was made mostly by inference, we are now able to directly establish the link between the high α -parameter value, the steep PNLF slope, and the increase in the old and MP stellar population in the outer regions of M 105 (identified by Harris et al. 2007a ). The data of Harris et al. (2007a) clearly ruled out a single chemical evolution sequence for the entire halo stellar population and instead suggested a multi-stage evolutionary model. For the outer MP halo, two different scenarios were proposed: Harris et al. (2007a) argued that the MP halo was built up from pristine gas and that star-formation therein was truncated early, at the time of cosmological reionisation. In contrast, Lee Jang (2016) proposed a formation dominated by dissipationless mergers and accretion. The scenario by Harris et al. (2007a) implies that the faint MP halo reached the current extension out to 40−50 kpc at high redshift, which is at odds with the observed slow growth of the
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