Small and Complex I: The Three Component Structure of $z \sim 0$ Massive Compact Quiescent Galaxies

Abstract

We investigate the morphology and structural properties of 246 massive compact quiescent galaxies (MCGs; $\log M_{\star} \sim 10$-$11$, $\sigma_{\mathrm{e}} \sim 150$-$350\,$km\,s$^{-1}$, $R_{\mathrm{e}} \sim 0.7$-$2.5\,$kpc) at $z \sim 0$, selected as outliers in the stellar mass-velocity dispersion and velocity dispersion-size relations, using $g$-, $r$-, and $i$-band Hyper Suprime-Cam images. We compare them to a control sample of average-sized quiescent galaxies (CSGs) matched in stellar mass, star formation rate, redshift, and $g-i$ color. Both samples are dominated by S0 galaxies, comprising $93\%$ of MCGs and $71\%$ of CSGs, while ellipticals account for $4\%$ and $11\%$, respectively. The fraction of interacting or morphologically disturbed systems is low in both samples ($13\%$ for MCGs and $16\%$ for CSGs). Multi-component decompositions of the $g$- and $r$-band images show that $75\%$ of MCGs require a three-component model (bulge, disk, and envelope), while $21\%$ are best fit by two components and $4\%$ by a single S\'ersic profile. Two-component MCGs are preferentially low-inclination systems, suggesting that the three-component fraction represents a lower limit. In contrast, only $7\%$ of CSGs exhibit a comparable three-component structure. Bars are present in $29\%$ of CSGs but are absent in MCGs. For three-component systems, MCGs and CSGs have similar bulge ($R_\mathrm{e}=0.39$ vs.\ $0.45$\,kpc) and envelope ($R_\mathrm{e}=6.4$ vs.\ $5.8$\,kpc) sizes, while MCG disks are significantly more compact ($R_\mathrm{e}=1.9$ vs.\ $3.3$\,kpc). The envelope component shows a broad ellipticity distribution ($\epsilon_\mathrm{Envelope} \sim 0.0$-$0.6$), which we interpret as corresponding to either a stellar halo or a thick disk.