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SPIDER - III. Environmental Dependence of the Fundamental Plane of Early-type Galaxies

We analyse the Fundamental Plane (FP) relation of $39,993$ early-type galaxies (ETGs) in the optical (griz) and $5,080$ ETGs in the Near-Infrared (YJHK) wavebands, forming an optical$+$NIR sample of $4,589$ galaxies. We focus on the analysis of the FP as a function of the environment where galaxies reside. We characterise the environment using the largest group catalogue, based on 3D data, generated from SDSS at low redshift ($z < 0.1$). We find that the intercept $``c''$ of the FP decreases smoothly from high to low density regions, implying that galaxies at low density have on average lower mass-to-light ratios than their high-density counterparts. The $``c''$ also decreases as a function of the mean characteristic mass of the parent galaxy group. However, this trend is weak and completely accounted for by the variation of $``c''$ with local density. The variation of the FP offset is the same in all wavebands, implying that ETGs at low density have younger luminosity-weighted ages than cluster galaxies, consistent with the expectations of semi-analytical models of galaxy formation. We measure an age variation of $\sim 0.048$~dex ($\sim 11\%$) per decade of local galaxy density. This implies an age difference of about $32 \%$ ($\sim 3 \, Gyr$) between galaxies in the regions of highest density and the field. We find the metallicity decreasing, at $\sim 2$~$σ$, from low to high density. We also find $2.5 \, σ$ evidence that the variation in age per decade of local density augments, up to a factor of two, for galaxies residing in massive relative to poor groups. (abridged)