As early as 10 Gyr ago, galaxies with more than 10 11 M ⊙ in stars already existed. While most of these massive galaxies must have subsequently transformed through on-going star formation and mergers with other galaxies, a small fraction ( 0.1%) may have survived untouched till today. Searches for such relic galaxies, useful windows to explore the early Universe, have been inconclusive to date: galaxies with masses and sizes like those observed at high redshift (M ⋆ 10 11 M ⊙ ; R e 1.5 kpc) have been found in the local Universe, but their stars are far too young for the galaxy to be a relic galaxy. This paper explores the first case of a nearby galaxy, NGC1277 (at a distance of 73 Mpc in the Perseus galaxy cluster), which fulfills many criteria to be considered a relic galaxy. Using deep optical spectroscopy, we derive the star formation history along the structure of the galaxy: the stellar populations are uniformly old (>10 Gyr) with no evidence for more recent star formation episodes. The metallicity of their stars is super-solar ([Fe/H]=0.20±0.04 with a smooth decline towards the outer regions) and alpha enriched ([α/Fe]=0.4±0.1). This suggests a very short formation time scale for the bulk of stars of this galaxy. This object also rotates very fast (V rot ∼300 km/s) and has a large central velocity dispersion (σ>300 km/s). NGC1277 allows the explorations in full detail of properties such as the structure, internal dynamics, metallicity and initial mass function at ∼10-12 Gyr back in time when the first massive galaxies were built.

In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar populations of seven spectroscopically confirmed ultra-diffuse galaxies (UDGs) in the Coma cluster. We find intermediate to old ages (∼ 7 Gyr), low metallicities ([Z/H]∼ -0.7 dex) and mostly super-solar abundance patterns ([Mg/Fe] ∼ 0.13 dex). These properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same area in the cluster and are mostly consistent with being the continuity of the stellar mass scaling relations of more massive galaxies. These UDGs' star formation histories imply a relatively recent infall into the Coma cluster, consistent with the theoretical predictions for a dwarf-like origin. However, considering the scatter in the resulting properties and including other UDGs in Coma, together with the results from the velocity phase-space study of the Paper I in this series, a mixed-bag of origins is needed to explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs are field dwarfs that become quenched through their later infall onto cluster environments, whereas some UDGs could be be genuine primordial galaxies that failed to develop due to an early quenching phase. The unknown proportion of dwarf-like to primordial-like UDGs leaves the enigma of the nature of UDGs still open.

Massive relic galaxies formed the bulk of their stellar component before z ∼ 2 and have remained unaltered since then. Therefore, they represent a unique opportunity to study in great detail the frozen stellar population properties of those galaxies that populated the primitive Universe. We have combined optical to near-infrared linestrength indices in order to infer, out to 1.5 R e , the IMF of the nearby relic massive galaxy NGC 1277. The IMF of this galaxy is bottom-heavy at all radii, with the fraction of low-mass stars being at least a factor of two larger than that found in the Milky Way. The excess of low-mass stars is present throughout the galaxy, while the velocity dispersion profile shows a strong decrease with radius. This behaviour suggests that local velocity dispersion is not the only driver of the observed IMF variations seen among nearby early-type galaxies. In addition, the excess of low-mass stars shown in NGC 1277 could reflect the effect on the IMF of dramatically different and intense star formation processes at z ∼ 2, compared to the less extreme conditions observed in the local Universe.

We use globular cluster kinematics data, primarily from the SLUGGS survey, to measure the dark matter fraction ( f DM ) and the average dark matter density ( ρ DM ) within the inner 5 effective radii (R e ) for 32 nearby early-type galaxies (ETGs) with stellar mass log (M * /M ) ranging from 10.1 to 11.8. We compare our results with a simple galaxy model based on scaling relations as well as with cosmological hydrodynamical simulations where the dark matter profile has been modified through various physical processes.We find a high f DM (≥ 0.6) within 5 R e in most of our sample, which we interpret as a signature of a late mass assembly history that is largely devoid of gas-rich major mergers. However, around log (M * /M ) ∼ 11, there is a wide range of f DM which may be challenging to explain with any single cosmological model. We find tentative evidence that lenticulars (S0s), unlike ellipticals, have mass distributions that are similar to spiral galaxies, with decreasing f DM within 5 R e as galaxy luminosity increases. However, we do not find any difference between the ρ DM of S0s and ellipticals in our sample, despite the differences in their stellar populations. We have also used ρ DM to infer the epoch of halo assembly (z∼2 − 4). By comparing the age of their central stars with the inferred epoch of halo formation, we are able to gain more insight into their mass assembly histories. Our results suggest a fundamental difference in the dominant late-phase mass assembly channel between lenticulars and elliptical galaxies.

We study a sample of eight massive galaxies that are extreme outliers (3-5σ) in the M • − M bulge local scaling relation. Two of these galaxies are confirmed to host extremely large super-massive black holes (SMBHs), whereas the virial mass estimates for the other six are also consistent with having abnormally large SMBHs. From the analysis of their star formation histories and their structural properties, we find that all of these extreme outliers can be considered to be relic galaxies from the early (z ∼ 2) universe: i.e., they are compact (R 2 e < kpc) and have purely old stellar populations (t 10  Gyr). In order to explain the nature of such deviations from the local relations, we propose a scenario in which the hosts of these über-massive SMBHs are galaxies that have followed a different evolutionary path than the two-phase growth channel assumed for massive galaxies. Once the SMBH and the core of the galaxy are formed at z ∼ 2, the galaxy skips the second phase, remaining structurally untouched and without further mass and size increase. We show that if the outliers had followed the normal evolutionary path by growing in size via merger activity, the expected (mild) growth in mass would place them closer to the observed local relations. Our results suggest that the SMBH growth epoch for the most massive galaxies stopped ∼10 Gyr ago.