We investigate the origin of the evolution of the population-averaged size of quenched galaxies (QGs) through a spectroscopic analysis of their stellar ages. The two most favoured scenarios for this evolution are either the size growth of individual galaxies through a sequence of dry minor merger events, or the addition of larger, newly quenched galaxies to the pre-existing population (i.e., a progenitor bias effect). We use the 20k zCOSMOS-bright spectroscopic survey to select bona fide quiescent galaxies at 0.2 < z < 0.8. We stack their spectra in bins of redshift, stellar mass and size to compute stellar population parameters in these bins through fits to the rest-frame optical spectra and through Lick spectral indices. We confirm a change of behaviour in the size-age relation below and above the ∼ 10 11 M stellar mass scale: In our 10.5 < log M * /M < 11 mass bin, over the entire redshift window, the stellar populations of the largest galaxies are systematically younger than those of the smaller counterparts, pointing at progenitor bias as the main driver of the observed average size evolution at sub-10 11 M masses. In contrast, at higher masses, there is no clear trend in age as a function of galaxy size, supporting a substantial role of dry mergers in increasing the sizes of these most massive QGs with cosmic time. Within the errors, the [α/Fe] abundance ratios of QGs are (i) above-solar over the entire redshift range of our analysis, hinting at universally short timescales for the buildup of the stellar populations of QGs, and (ii) similar at all masses and sizes, suggesting similar (short) timescales for the whole QG population and strengthening the role of mergers in the buildup of the most massive QGs in the Universe.

Narrow-band imaging surveys allow the study of the spectral characteristics of galaxies without the need of performing their spectroscopic follow-up. In this work, we forward-model the Physics of the Accelerating Universe Survey (PAUS) narrow-band data. The aim is to improve the constraints on the spectral coefficients used to create the galaxy spectral energy distributions (SED) of the galaxy population model in In that work, the model parameters were inferred from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) data using Approximate Bayesian Computation (ABC). This led to stringent constraints on the B-band galaxy luminosity function parameters, but left the spectral coefficients only broadly constrained. To address that, we perform an ABC inference using CFHTLS and PAUS data. This is the first time our approach combining forwardmodelling and ABC is applied simultaneously to multiple datasets. We test the results of the ABC inference by comparing the narrow-band magnitudes of the observed and simulated galaxies using Principal Component Analysis, finding a very good agreement. Furthermore, we prove the scientific potential of the constrained galaxy population model to provide realistic stellar population properties by measuring them with the CIGALE SED fitting code. We use CFHTLS broad-band and PAUS narrow-band photometry for a flux-limited (i < 22.5) sample of galaxies spanning the redshift range 0 < z < 1.0. We find that properties like stellar masses, star-formation rates, mass-weighted stellar ages and metallicities are in agreement within errors between observations and simulations. Overall, this work shows the ability of our galaxy population model to correctly forward-model a complex dataset such as PAUS and the ability to reproduce the diversity of galaxy properties at the redshift range spanned by CFHTLS and PAUS.

Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we identify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to ω Cen, the most luminous Milky Way globular cluster, which has long been suspected to be the stripped remnant of an accreted dwarf galaxy. The object has a magnitude of i = 19.3 mag (L i = 1.4 × 10 6 L ). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of ∼ 10 pc. We measure an integrated brightness for the stellar stream of i = 13.1 mag, with (g − i) = 1.0. This would correspond to an accreted dwarf galaxy with an approximate luminosity of L i ∼ 4.1 × 10 8 L . Spectral analysis reveals that UCD1 has an age of 6.6 Gyr , [Z/H] = −0.75, and [α/Fe] = −0.10. We propose that UCD1 is an example of an ω Cen-like star cluster possibly forming from the nucleus of an infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population may be created through tidal stripping.

Here we present new Keck ESI high-resolution spectroscopy and deep archival HST/ACS imaging for S999, an ultra-compact dwarf in the vicinity of M87, which was claimed to have an extremely high dynamical-to-stellar mass ratio. Our data increase the total integration times by a factor of 5 and 60 for spectroscopy and imaging, respectively. This allows us to constrain the stellar population parameters for the first time (simple stellar population equivalent age = 7.6 +2.0 −1.6 Gyr; [Z/H] = −0.95 +0.12 −0.10 ; [α/Fe] = 0.34 +0.10 −0.12 ). Assuming a Kroupa stellar initial mass function, the stellar population parameters and luminosity (M F 814W = −12.13±0.06 mag) yield a stellar mass of M * = 3.9 +0.9 −0.6 × 10 6 M , which we also find to be consistent with near-infrared data. Via mass modelling, with our new measurements of velocity dispersion (σ ap = 27 ± 2 km s −1 ) and size (R e = 20.9±1.0 pc), we obtain an elevated dynamical-to-stellar mass ratio M dyn /M * = 8.2 (with a range 5.6 M dyn /M * 11.2). Furthermore, we analyse the surface brightness profile of S999, finding only a small excess of light in the outer parts with respect to the fitted Sérsic profile, and a positive colour gradient. Taken together these observations suggest that S999 is the remnant of a much larger galaxy that has been tidally stripped. If so, the observed elevated mass ratio may be caused by mechanisms related to the stripping process: the existence of an massive central black hole or internal kinematics that are out of equilibrium due to the stripping event. Given the observed dynamical-to-stellar mass ratio we suggest that S999 is an ideal candidate to search for the presence of an overly massive central black hole.