Mass of the dynamically hot inner stellar halo predicts the ancient accreted stellar mass

Galactic dynamical structures are fossil records of the assembly histories galaxies. By analyzing cosmological hydrodynamical simulation TNG50, we find that a structure call "hot inner stellar halo," defined by stars on dynamically hot orbits with circularity $\lambda_z < 0.5$ at $3.5\,{\rm kpc}<r \lesssim 2\,R_e$, is strong indicator mass accreted satellite We correlation between this halo and total ex situ mass. There similarly most massive secondary galaxy ever merged. These TNG50 correlations compatible those predicted other simulations, for example TNG100 across whole range under study (galaxy masses, $M_*$, in $10^{10.3-11.6}$\,\Msun\, range) EAGLE $M_* \gtrsim 10^{10.6} $\,\Msun\, galaxies.\ This shows our predictions robust different formation feedback models hold wide numerical resolution. The product typically ancient mergers, inner-halo exhibiting three main physical origins: stripped from satellites, heated mergers bulge and/or disk progenitor, formed star triggered during mergers. paper quantity can be robustly obtained real galaxies applying population-orbit superposition method to integral-field-unit spectroscopy data, out distance $\sim2\,R_e$, which possible current observations. Hence, observations regions provide way quantitatively determine satellites.