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We investigated the stellar population properties of a sample of 1858 massive compact galaxies (MCGs) extracted from the SDSS survey. 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 $\sigma_\mathrm{e}$ vs. $R_\mathrm{e}$ and $M_\star$ vs. $\sigma_\mathrm{e}$ relations as selection criteria. We found that MCGs are old ($\gtrsim 10$ Gyr), $\alpha$-enhanced ([$\alpha/\mathrm{Fe}] \sim 0.2$) and have solar to super-solar stellar metallicities. Metallicity increases with $\sigma_\mathrm{e}$, while age and [$\alpha$/Fe] do not vary significantly. Moreover, at fixed $\sigma_\mathrm{e}$, metallicity and stellar mass are correlated. Compared to a control sample of typical quiescent galaxies, MCGs have, on average, lower metallicities than control sample galaxies (CSGs) of similar $\sigma_\mathrm{e}$. For $\sigma_\mathrm{e} \lesssim 225$ km/s, MCGs are older and more $\alpha$-enhanced than CSGs, while for higher $\sigma_\mathrm{e}$ ages and $\alpha$-enhancement are similar. The differences in age and $\alpha$-enhancement can be explained by lower-$\sigma_\mathrm{e}$ 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 fiber finding that, at fixed fiber 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 $\sim 1-2$ kpc., Comment: Accepted for publication in MNRAS

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 $\sim 10.8$ Gyr), have super-solar metallicities (median $\log Z/Z_{\odot}\sim0.35$) and are $\alpha$-enhanced (median $[\alpha/Fe]\sim0.25$). The age distribution extends to younger ages, however, and a few MCGs are as young as $\sim7$ 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% or more. The accretion of low angular momentum gas leads to enhanced SMBH growth, and MCGs reach the threshold SMBH mass of $\log M_\mathrm{BH}\sim10^{8.5} M_\odot$ - when kinetic AGN 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% of MCGs do not merge after quenching compared to 37% 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., Comment: 15 pages, 15 figures (not including appendices). Accepted for publication in the Monthly Notices of the Royal Astronomical Society

We analysed the nuclear region of all 56 early-type galaxies from the DIVING$^\mathrm{3D}$ Project, which is a statistically complete sample of objects that contains all 170 galaxies of the Southern Hemisphere with B < 12.0 mag and galactic latitude |b| < 15$^{\circ}$. Observations were performed with the Integral Field Unit of the Gemini Multi-Object Spectrograph. Emission lines were detected in the nucleus of 86$\pm$5% of the objects. Diagnostic diagrams were used to classify 52$\pm$7% of the objects as LINERs or Seyferts, while the other 34$\pm$6% galaxies without H$\beta$ or [O III] lines in their spectra were classified as weak emission line objects. Transition Objects are not seen in the sample, possibly because the seeing-limited data cubes of the objects allow one to isolate the nuclei of the galaxies from their circumnuclear regions, avoiding contamination from H II regions. A broad line region is seen in 29$\pm$6% of the galaxies. Of the 48 galaxies with emission-line nuclei, 41 have signs of AGNs. Some objects have also indications of shocks in their nuclei. Lenticular galaxies are more likely to have emission lines than ellipticals. Also, more luminous objects have higher [N II]/H$\alpha$ ratios, which may be associated with the mass-metalicity relation of galaxies. A direct comparison of our results with the Palomar Survey indicates that the detection rates of emission lines and also of type 1 AGNs are higher in the DIVING$^\mathrm{3D}$ objects. This is a consequence of using a more modern instrument with a better spatial resolution than the Palomar Survey observations., Comment: 16 pages (plus appendices), 6 figures, 9 tables, accepted for publication in MNRAS