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We investigated the stellar population properties of a sample of 1858 massive compact galaxies (MCGs) extracted from the Sloan Digital Sky Survey (SDSS). Motivated by previous results showing that older compact galaxies tend to have larger velocity dispersion at fixed stellar mass, we used the distance to the σe versus R e and M ⋆ versus σe relations as selection criteria. We found that MCGs are old (≳10 Gyr), α-enhanced ([α/Fe] ∼ 0.2), and have solar to supersolar stellar metallicities. Metallicity increases with σe, while age and [α/Fe] do not vary significantly. Moreover, at fixed σe, metallicity and stellar mass are correlated. Compared with a control sample of typical quiescent galaxies, MCGs have, on average, lower metallicities than control sample galaxies (CSGs) of similar σe. For σe ≲ 225 km s−1, MCGs are older and more α-enhanced than CSGs, while for higher σe ages and α-enhancement are similar. The differences in age and α-enhancement can be explained by lower σe CSGs being an amalgam of quiescent galaxies with a variety of ages. The origin of the differences in metallicity, however, is not clear. Lastly, we compared the stellar mass within the region probed by the SDSS fibre finding that, at fixed fibre velocity dispersion, MCGs have lower stellar masses on average. Since the velocity dispersion is a tracer of the dynamical mass, this raises the possibility that MCGs have, on average, a bottom-heavier initial mass function or a larger dark matter fraction within the inner ∼1−2 kpc. [ABSTRACT FROM AUTHOR]

We investigate the formation and evolution of z = 0 massive compact galaxies (MCGs) in the IllustrisTNG cosmological simulation. We found that, as in observations, MCGs are mainly old (median age ∼10.8 Gyr), have supersolar metallicities (median log  Z /Z⊙ ∼ 0.35), and are α-enhanced (median [α/Fe] ∼ 0.25). The age distribution extends to younger ages, however, and a few MCGs are as young as ∼7 Gyr. In general, MCGs assemble their mass early and accrete low angular momentum gas, significantly increasing their mass while growing their size much slower. A small fraction of MCGs follow another evolutionary path, going through a compaction event, with their sizes shrinking by 40 per cent or more. The accretion of low angular momentum gas leads to enhanced supermassive black hole (SMBH) growth, and MCGs reach the threshold SMBH mass of |$\log M_\mathrm{BH}\sim 10^{8.5}\ \mathrm{ M}_\odot$| – when kinetic active galactic nucleus feedback kicks in and quenches the galaxy – earlier than non-compact galaxies. Comparing MCGs to a sample of median-sized quiescent galaxies matched in effective velocity dispersion, we find that their accretion histories are very different. 71 per cent of MCGs do not merge after quenching compared to 37 per cent of median-sized quiescent galaxies. Moreover, tracing these populations back in time, we find that at least a third of median-sized quiescent galaxies do not have a compact progenitor, underscoring that both dry mergers and progenitor bias effects are responsible for the differences in the kinematics and stellar population properties of MCGs and median-sized quiescent galaxies. [ABSTRACT FROM AUTHOR]

We characterized the kinematics, morphology, and stellar population (SP) properties of a sample of massive compact quiescent galaxies (MCGs, |$10\, \lesssim \, \log M_\star$| /M |$_\odot \, \lesssim \, 11$| and r e ∼ 1–3 kpc) in the MaNGA Survey, with the goal of constraining their formation, assembly history, and assessing their relation with non-compact quiescent galaxies. We compared their properties with those of a control sample of median-sized quiescent galaxies (r e ∼ 4–8 kpc) with similar effective velocity dispersions. MCGs have elevated rotational support, as evidenced by a strong anticorrelation between the Gauss–Hermite moment h 3 and V / σ. In contrast, 30 |${{\ \rm per\ cent}}$| of control sample galaxies (CSGs) are slow rotators, and fast-rotating CSGs generally show a weak h 3– V / σ anticorrelation. MCGs and CSGs have similar ages, but MCGs are more metal-rich and α -enhanced. Both MCGs and CSGs have shallow negative metallicity gradients and flat [ α /Fe] gradients. On average, MCGs and CSGs have flat age gradients, but CSGs have a significantly larger dispersion of gradient values. The kinematics and SP properties of MCGs suggest that they experienced highly dissipative gas-rich events, such as mergers, followed by an intense, short, and centrally concentrated burst of star formation, between 4 and 10 Gyr ago (z ∼ 0.4–2), and had a quiet accretion history since then. This sequence of events might be analogous to, although less extreme than, the compaction events that formed compact quiescent galaxies at z  ∼ 2. The small sizes of MCGs, and the high efficiency and short duration of their last star formation episode suggest that they are descendants of compact post-starburst galaxies. [ABSTRACT FROM AUTHOR]