ALBERT

Description: We analyse the evolution of colour gradients predicted by the hydrodynamical models of early-type galaxies (ETGs) in Pipino et al., which reproduce fairly well the chemical abundance pattern and the metallicity gradients of local ETGs. We convert the star formation (SF) and metal content into colours by means of stellar population synthetic model and investigate the role of different physical ingredients, as the initial gas distribution and content, and SF , i.e. the normalization of SF rate. From the comparison with high-redshift data, a full agreement with optical rest-frame observations at z   1 is found, for models with low SF , whereas some discrepancies emerge at 1 〈  z  〈 2, despite our models reproduce quite well the data scatter at these redshifts. To reconcile the prediction of these high SF systems with the shallower colour gradients observed at lower z we suggest intervention of one to two dry mergers. We suggest that future studies should explore the impact of wet galaxy merging, interactions with environment, dust content and a variation of the initial mass function from the galactic centres to the peripheries.

Description: We perform in-depth dynamical modelling of the luminous and dark matter (DM) content of the elliptical galaxy NGC 1407. Our strategy consists of solving the spherical Jeans equations for three independent dynamical tracers: stars, blue globular clusters (GCs) and red GCs in a self-consistent manner. We adopt a maximum-likelihood Markov Chain Monte Carlo fitting technique in the attempt to constrain the inner slope of the DM density profile (the cusp/core problem), and the stellar initial mass function (IMF) of the galaxy. We find the inner logarithmic slope of the DM density profiles to be  = 0.6 ± 0.4, which is consistent with either a DM cusp ( = 1) or with a DM core ( = 0). Our findings are consistent with a Salpeter IMF, and marginally consistent with a Kroupa IMF. We infer tangential orbits for the blue GCs, and radial anisotropy for red GCs and stars. The modelling results are consistent with the virial mass–concentration relation predicted by cold dark matter (CDM) simulations. The virial mass of NGC 1407 is log M vir  = 13.3 ± 0.2 M , whereas the stellar mass is log M *  = 11.8 ± 0.1 M . The overall uncertainties on the mass of NGC 1407 are only 5 per cent at the projected stellar effective radius. We attribute the disagreement between our results and previous X-ray results to the gas not being in hydrostatic equilibrium in the central regions of the galaxy. The halo of NGC 1407 is found be DM-dominated, with a dynamical mass-to-light ratio of ${\rm M/L}=260_{-100} ^{+174} \,\mathrm{M}_{\odot }/\mathrm{L}_{\odot , B}$ . However, this value can be larger up to a factor of 3 depending on the assumed prior on the DM scale radius.

Description: We construct a suite of discrete chemo-dynamical models of the giant elliptical galaxy NGC 5846. These models are a powerful tool to constrain both the mass distribution and internal dynamics of multiple tracer populations. We use Jeans models to simultaneously fit stellar kinematics within the effective radius R e , planetary nebula (PN) radial velocities out to 3 R e , and globular cluster (GC) radial velocities and colours out to 6 R e . The best-fitting model is a cored dark matter halo which contributes ~10 per cent of the total mass within 1 R e , and 67 per cent ± 10 per cent within 6 R e , although a cusped dark matter halo is also acceptable. The red GCs exhibit mild rotation with v max / 0  ~ 0.3 in the region R 〉 R e , aligned with but counter-rotating to the stars in the inner parts, while the blue GCs and PNe kinematics are consistent with no rotation. The red GCs are tangentially anisotropic, the blue GCs are mildly radially anisotropic, and the PNe vary from radially to tangentially anisotropic from the inner to the outer region. This is confirmed by general made-to-measure models. The tangential anisotropy of the red GCs in the inner regions could stem from the preferential destruction of red GCs on more radial orbits, while their outer tangential anisotropy – similar to the PNe in this region – has no good explanation. The mild radial anisotropy of the blue GCs is consistent with an accretion scenario.

Description: We present an integral-field study of the internal structure, kinematics and stellar population of the almost edge-on, intermediate-luminosity ( L * ) elliptical galaxy NGC 4697. We build extended two-dimensional (2D) maps of the stellar kinematics and line strengths of the galaxy up to ~0.7 effective radii ( R eff ) using a mosaic of eight VIMOS (VIsible Multi-Objects Spectrograph, on the Very Large Telescope) integral-field unit pointings. We find clear evidence for a rotation-supported structure along the major axis from the 2D kinematical maps, confirming the previous classification of this system as a ‘fast rotator’. We study the correlations between the third and fourth Gauss–Hermite moments of the line-of-sight velocity distribution (LOSVD) h 3 and h 4 with the rotation parameter ( V /), and compare our findings to hydrodynamical simulations. We find remarkable similarities to predictions from gas-rich mergers. Based on photometry, we perform a bulge/disc decomposition and study the stellar population properties of the two components. The bulge and the disc show different stellar populations, with the stars in the bulge being older (age $_{\rm bulge}=13.5^{+1.4}_{-1.4}$ Gyr, age $_{\rm disc}=10.5^{+1.6}_{-2.0}$ Gyr) and more metal poor ( ${[M/{\rm H}]_{{\rm bulge}}} = -0.17^{+0.12}_{-0.1}$ , ${[M/{\rm H}]_{{\rm disc}}} = -0.03^{+0.02}_{-0.1}$ ). The evidence of a later-formed, more metal-rich disc embedded in an older, more metal poor bulge, together with the LOSVD structure, supports a mass assembly scenario dominated by gas-rich minor mergers and possibly with a late gas-rich major merger that left a previously rapidly rotating system unchanged. The bulge and the disc do not show signs of different stellar initial mass function (IMF) slopes, and both match well with a Milky Way-like IMF.

Description: The abundance of compact, massive, early-type galaxies (ETGs) provides important constraints to galaxy formation scenarios. Thanks to the area covered, depth, excellent spatial resolution and seeing, the ESO Public optical Kilo Degree Survey (KiDS), carried out with the VLT Survey Telescope, offers a unique opportunity to conduct a complete census of the most compact galaxies in the Universe. This paper presents a first census of such systems from the first 156 deg 2 of KiDS. Our analysis relies on g -, r - and i -band effective radii ( R e ), derived by fitting galaxy images with point spread function (PSF)-convolved Sérsic models, high-quality photometric redshifts, z phot , estimated from machine learning techniques, and stellar masses, M * , calculated from KiDS aperture photometry. After massiveness ( ${M_{\star }}\gtrsim 8 \times 10^{10}\, \rm {\text{M}_{\odot }}$ ) and compactness ( ${R_{\rm e}}\lesssim 1.5 \, \rm kpc$ in g , r and i bands) criteria are applied, a visual inspection of the candidates plus near-infrared photometry from VIKING-DR1 are used to refine our sample. The final catalogue, to be spectroscopically confirmed, consists of 92 systems in the redshift range z ~ 0.2–0.7. This sample, which we expect to increase by a factor of 10 over the total survey area, represents the first attempt to select massive supercompact ETGs ( MSCGs ) in KiDS. We investigate the impact of redshift systematics in the selection, finding that this seems to be a major source of contamination in our sample. A preliminary analysis shows that MSCGs exhibit negative internal colour gradients, consistent with a passive evolution of these systems. We find that the number density of MSCGs is only mildly consistent with predictions from simulations at z  〉 0.2, while no such system is found at z  〈 0.2.

Description: Modified Newtonian Dynamics (MOND) represents a phenomenological alternative to dark matter (DM) for the missing mass problem in galaxies and clusters of galaxies. We analyse the central regions of a local sample of ~220 early-type galaxies from the ATLAS 3D survey, to see if the data can be reproduced without recourse to DM. We estimate dynamical masses in the MOND context through Jeans analysis and compare to ATLAS 3D stellar masses from stellar population synthesis. We find that the observed stellar mass–velocity dispersion relation is steeper than expected assuming MOND with a fixed stellar initial mass function (IMF) and a standard value for the acceleration parameter a 0 . Turning from the space of observables to model space (a) fixing the IMF, a universal value for a 0 cannot be fitted, while, (b) fixing a 0 and leaving the IMF free to vary, we find that it is ‘lighter’ (Chabrier like) for low-dispersion galaxies and ‘heavier’ (Salpeter like) for high dispersions. This MOND-based trend matches inferences from Newtonian dynamics with DM and from the detailed analysis of spectral absorption lines, adding to the converging lines of evidence for a systematically varying IMF.

Description: We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies up to z  ~ 0.8. We use a spectroscopically selected sample of 154 cluster and field galaxies from the ESO Distant Clusters Survey (EDisCS), covering a wide range in redshifts ( z  ~ 0.4–0.8), stellar masses (log M /M  ~ 10.5–11.5 dex) and velocity dispersions ($ * $ ~ 100–300 km s –1 ). We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the M * from galaxy colours, and compare the results with local samples. We discuss how the correlations of central DM with galaxy size (i.e. the effective radius, R e ), M * and * evolve as a function of redshift, finding clear indications that local galaxies are, on average, more DM dominated than their counterparts at larger redshift. This DM fraction evolution with z can be only partially interpreted as a consequence of the size–redshift evolution. We discuss our results within galaxy formation scenarios, and conclude that the growth in size and DM content which we measure within the last 7 Gyr is incompatible with passive evolution, while it is well reproduced in the multiple minor merger scenario. We also discuss the impact of the initial mass function (IMF) on our DM inferences and argue that this can be non-universal with the look-back time. In particular, we find that the Salpeter IMF can be better accommodated by low-redshift systems, while producing stellar masses at high z which are unphysically larger than the estimated dynamical masses (particularly for lower * systems).

Description: STREGA (STRucture and Evolution of the GAlaxy) is a guaranteed time survey being performed at the VST (the ESO Very Large Telescope Survey Telescope) to map about 150 square degrees in the Galactic halo, in order to constrain the mechanisms of galactic formation and evolution. The survey is built as a 5 yr project, organized in two parts: a core programme to explore the surrounding regions of selected stellar systems and a second complementary part to map the southern portion of the Fornax orbit and extend the observations of the core programme. The adopted stellar tracers are mainly variable stars (RR Lyraes and long-period variables) and main-sequence turn-off stars for which observations in the g , r , i bands are obtained. We present an overview of the survey and some preliminary results for three observing runs that have been completed. For the region centred on Cen (37 deg 2 ), covering about three tidal radii, we also discuss the detected stellar density radial profile and angular distribution, leading to the identification of extratidal cluster stars. We also conclude that the cluster tidal radius is about 1.2 deg, in agreement with values in the literature based on the Wilson model.

Description: We study the total density distribution in the central regions (1 effective radius, R e ) of early-type galaxies (ETGs), using data from SPIDER and ATLAS 3D . Our analysis extends the range of galaxy stellar mass ( M * ) probed by gravitational lensing, down to ~ 10 10 M . We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter halo profile (i.e. NFW, NFW-contracted, and Burkert profiles), and leaving stellar mass-to-light ( M * / L ) ratios as free fitting parameters to the data. For all plausible halo models, the best-fitting M * / L , normalized to that for a Chabrier initial mass function, increases systematically with galaxy size and mass. For an NFW profile, the slope of the total mass profile is non-universal, independently of several ingredients in the modelling (e.g. halo contraction, anisotropy, and rotation velocity in ETGs). For the most massive ( M *  ~ 10 11.5 M ) or largest ( R e ~ 15 kpc) ETGs, the profile is isothermal in the central regions (~ R e /2), while for the low-mass ( M *  ~ 10 10.2 M ) or smallest ( R e ~ 0.5 kpc) systems, the profile is steeper than isothermal, with slopes similar to those for a constant- M / L profile. For a steeper concentration–mass relation than that expected from simulations, the correlation of density slope with galaxy mass tends to flatten, while correlations with R e and velocity dispersions are more robust. Our results clearly point to a ‘non-homology’ in the total mass distribution of ETGs, which simulations of galaxy formation suggest may be related to a varying role of dissipation with galaxy mass.