Towards a census of super-compact massive galaxies in the Kilo Degree Survey

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 (VST), 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 square degrees of KiDS. Our analysis relies on g-, r-, and i-band effective radii ($R_{rm e}$), derived by fitting galaxy images with PSF-convolved Sersic models, high-quality photometric redshifts, $z_{rm phot}$, estimated from machine learning techniques, and stellar masses, $M_{rm star}$, calculated from KiDS aperture photometry. After massiveness ($M_{rm star} > 8 times 10^{10}, rm M_{odot}$) and compactness ($R_{rm e} < 1.5 , rm kpc$ 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 catalog, to be spectroscopically confirmed, consists of 92 systems in the redshift range $z sim 0.2-0.7$. This sample, which we expect to increase by a factor of ten over the total survey area, represents the first attempt to select massive super-compact 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 $z < 0.2$.

Evolution of central dark matter of early-type galaxies up to z ~ 0.8

We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies (ETGs) up to z ~ 0.8. We use a spectroscopically selected sample of 154 cluster and field galaxies from the EDisCS survey, covering a wide range in redshifts (z ~ 0.4-0.8), stellar masses ($log M_{star}/ M_{odot}$ ~ 10.5-11.5 dex) and velocity dispersions ($sigma_{star}$ ~ 100-300 , km/s). We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the $M_{star}$ 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, $R_{rm e}$), $M_{star}$ and $sigma_{star}$ 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 IMF on our DM inferences and argue that this can be non-universal with the lookback time. In particular, we find 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-$sigma_{star}$ systems).

Systematic variations of central mass density slopes in early-type galaxies

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 $rm ATLAS^{3D}$. Our analysis extends the range of galaxy stellar mass ($M_{star}$) probed by gravitational lensing, down to ~ $10^{10}, rm M_{odot}$. We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter (DM) halo profile (i.e. NFW, NFW-contracted, and Burkert profiles), and leaving stellar mass-to-light ($M_{star}/L$) ratios as free fitting parameters to the data. For all plausible halo models, the best-fitting $M_{star}/L$, normalized to that for a Chabrier IMF, 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 modeling (e.g., halo contraction, anisotropy, and rotation velocity in ETGs). For the most massive ($M_{star}$ ~ $10^{11.5} , M_{odot}$) or largest ($R_{rm e}$ ~ $15 , rm kpc$) ETGs, the profile is isothermal in the central regions (~$R_{rm e}/2$), while for the low-mass ($M_{star}$ ~ $10^{10.2} , M_odot$) or smallest ($R_{rm e}$ ~ $0.5 , rm kpc$) 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 $R_{rm e}$ 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.

MOND and IMF variations in early-type galaxies from $ATLAS^{3D}$

MOdified Newtonian dynamics (MOND) represents a phenomenological alternative to dark matter (DM) for the missing mass problem in galaxies and clusters of galaxies. We analyze the central regions of a local sample of $sim 220$ early-type galaxies from the $rm ATLAS^{3D}$ 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 $rm ATLAS^{3D}$ 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 $a_{rm 0}$. Turning from the space of observables to model space, a) fixing the IMF, a universal value for $a_{rm 0}$ cannot be fitted, while, b) fixing $a_{rm 0}$ 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 detailed analysis of spectral absorption lines, adding to the converging lines of evidence for a systematically-varying IMF.

Colour gradients of high-redshift Early-Type Galaxies from hydrodynamical monolithic models

We analyze the evolution of colour gradients predicted by the hydrodynamical models of early type galaxies (ETGs) in Pipino et al. (2008), 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 eps_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 eps_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 eps_SF systems with the shallower colour gradients observed at lower z we suggest intervention of 1-2 dry mergers. We suggest that future studies should explore the impact of wet galaxy mergings, interactions with environment, dust content and a variation of the Initial Mass Function from the galactic centers to the peripheries.

Stellar mass-to-light ratio gradients in galaxies: correlations with mass

We analyze the stellar mass-to-light ratio (M/L) gradients in a large sample of local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar masses and morphological types. As suggested by the well known relationship between M/L ratios and colors, we show that M/L gradients are strongly correlated with colour gradients, which we trace to the effects of age variations. Stellar M/L gradients generally follow patterns of variation with stellar mass and galaxy type that were previous found for colour and metallicty gradients. In late-type galaxies M/L gradients are negative, steepening with increasing mass. In early-type galaxies M/L gradients are shallower while presenting a two-fold trend: they decrease with mass up to a characteristic mass of M* sim 10^10.3 M_sun and increase at larger masses. We compare our findings with other analyses and discuss some implications for galaxy formation and for dark matter estimates.

Central dark matter trends in early-type galaxies from strong lensing, dynamics and stellar populations

We analyze the correlations between central dark matter (DM) content of early-type galaxies and their sizes and ages, using a sample of intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and by comparing them to a larger sample of z ~ 0 galaxies. We decompose the deprojected galaxy masses into DM and stellar components using combinations of strong lensing, stellar dynamics, and stellar populations modeling. For a given stellar mass, we find that for galaxies with larger sizes, the DM fraction increases and the mean DM density decreases, consistently with the cuspy halos expected in cosmological formation scenarios. The DM fraction also decreases with stellar age, which can be partially explained by the inverse correlation between size and age. The residual trend may point to systematic dependencies on formation epoch of halo contraction or stellar initial mass functions. These results are in agreement with recent findings based on local galaxies by Napolitano, Romanowsky & Tortora (2010) and suggest negligible evidence of galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.

The global mass - to - light ratio of SLACS lenses

The dark matter content of early,- type galaxies (ETGs) is a hotly debated topic with contrasting results arguing in favour or against the presence of significant dark mass within the effective radius and the change with luminosity and mass. In order to address this question, we investigate here the global mass - to - light ratio $Upsilon(r) = M(r)/L(r)$ of a sample of 21 lenses observed within the Sloan Lens ACS (SLACS) survey. We follow the usual approach of modeling the galaxy as a two component systems, but we use a phenomenological ansatz for $Upsilon(r)$, proposed by some of us in Tortora et al. (2007), able to smoothly interpolate between constant $M/L$ models and a wide class of dark matter haloes. The resulting galaxy model is then fitted to the data on the Einstein radius and velocity dispersion. Our phenomenological model turns out to be in well agreement with the data suggesting the presence of massive dark matter haloes in order to explain the lensing and dynamics properties of the SLACS lenses. According to the values of the dark matter mass fraction, we argue that the halo may play a significant role in the inner regions probed by the data, but such a conclusion strongly depends on the adopted initial mass function of the stellar population. Finally, we find that the dark matter mass fraction within $R_{eff}$ scales with both the total luminosity and stellar mass in such a way that more luminous (and hence more massive) galaxies have a larger dark matter content.

Central mass-to-light ratios and dark matter fractions in early-type galaxies

Dynamical studies of local ETGs and the Fundamental Plane point to a strong dependence of M/L ratio on luminosity (and stellar mass) with a relation of the form $M/L propto L^{gamma}$. The tilt $gamma$ may be caused by various factors, including stellar population properties, IMF, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform dataset of local ETGs from Prugniel & Simien (1997). We take particular care in estimating the stellar masses, using a general star formation history, and comparing different population synthesis models. We find that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple Jeans dynamical models, and find that adopting accurate luminosity profiles is important but does not remove the need for an additional tilt component, which we ascribe to DM. We survey trends of the DM fraction within one effective radius, finding it to be roughly constant for galaxies fainter than $M_B sim -20.5$, and increasing with luminosity for the brighter galaxies; we detect no significant differences among S0s and fast- and slow-rotating ellipticals. We construct simplified cosmological mass models and find general consistency, where the DM transition point is caused by a change in the relation between luminosity and effective radius. A more refined model with varying galaxy star formation efficiency suggests a transition from total mass profiles (including DM) of faint galaxies distributed similarly to the light, to near-isothermal profiles for the bright galaxies. These conclusions are sensitive to various systematic uncertainties which we investigate in detail, but are consistent with the results of dynamics studies at larger radii.