Authors: Dexter S. H. Hon, Alister W. Graham, Nandini Sahu
Abstract: We analyse the bulge/spheroid size-(stellar mass), Re,Sph—M∗,Sph, relation and spheroid structural parameters for 202 local (predominantly <110 Mpc) galaxies spanning M∗∼3×109−1012 Msolar and 0.1≤Re,Sph≤32 kpc from multicomponent decomposition. The correlations between the spheroid Sérsic index (nSph), central surface brightness (μ0,Sph), effective half-light radius (Re,Sph), absolute magnitude (MSph) and stellar mass (M∗,Sph) are explored. We also investigate the consequences of using different scale radii, Rz,Sph, encapsulating a different fraction (z, from 0 to 1) of the total luminosity. The correlation strengths for projected mass densities, Σz and ⟨Σ⟩z , vary significantly with the choice of z . Spheroid size (Rz,Sph ) and mass (M∗,Sph) are strongly correlated for all light fractions z . We find: log(Re,Sph/kpc)=0.88log(M∗,Sph/Msolar)−9.15 with a small scatter of Δrms=0.24 dex . This result is discussed relative to the curved size-mass relation for early-type galaxies due to their discs yielding larger galaxy radii at lower masses. Moreover, the slope of our spheroid size-mass relation is a factor of ~3 , steeper than reported bulge size-mass relations, and with bulge sizes at M∗,Sph~3×109 Msolar which are 2 to 3 times smaller. Finally, we show that the local spheroids align well with quiescent galaxies at z~1.25–2.25 . In essence, local spheroids and high-z quiescent galaxies appear structurally similar, likely dictated by the virial theorem.