We report the results of spectroscopic observations , obtained with the GEMINI Multi-Object Spectrograph , of 8 planetary nebulae ( PNe ) in the dwarf spheroidal ( dSph ) galaxy NGC 147 , a companion of M 31 .
The physico-chemical properties of the six brightest PNe ( 6 ) were derived using both the empirical icf method and photoionization modelling with cloudy .
Different aspects of the evolution of low and intermediate mass stars in a low-metallicity environment are analysed using relationships between chemical abundances .
In addition , certain features of the chemical evolution of NGC 147 were examined .
In particular , the mean metallicity of PNe , O/H=8.06 _ { -0.12 } ^ { +0.09 } ( corresponding to [ Fe/H ] _ { PNe } \sim -0.97 ) , is close to the metallicity of the old stellar population , [ Fe/H ] = -1.0 ( i.e . 3 ) , suggesting a negligible chemical enrichment during a substantial amount of time .
Finally , the luminosity-metallicity relationship for the dwarf galaxies of the Local Group is discussed .
The location in the luminosity-metallicity diagram of dSphs does not exclude their formation from old dwarf irregular ( dIrs ) galaxies , but it does exclude their formation from the present time dIrs , since the differences between their metallicities are already present in their older populations .
The offset in the luminosity-metallicity relationship indicates a faster enrichment of dSphs , and together with the different average abundance ratio [ O/Fe ] demonstrates the different star formation histories for these two types of galaxies .