Dark Matter and IMF normalization in Virgo dwarf early-type galaxies

In this work we analyze the dark matter (DM) fraction, $f_{DM}$, and mass-to-light ratio mismatch parameter, $\delta_{IMF}$ (computed with respect to a Milky-Way-like IMF), for a sample of 39 dwarf early-type galaxies (dEs) in the Virgo cluster. Both $f_{DM}$ and $\delta_{IMF}$ are estimated within the central (one effective radius) galaxy regions, with a Jeans dynamical analysis that relies on galaxy velocity dispersions, structural parameters, and stellar M/L ratios from the SMAKCED survey. In this first attempt to constrain, simultaneously, the IMF normalization and the DM content, we explore the impact of different assumptions on the DM model profile. On average, for a NFW profile, the $\delta_{IMF}$ is consistent with a Chabrier-like normalization ($\delta_{IMF} \sim 1$), with $f_{DM} \sim 0.35$. One of the main results of the present work is that for at least a few systems the $\delta_{IMF}$ is heavier than the MW-like value (i.e. either top- or bottom-heavy). When introducing tangential anisotropy, larger $\delta_{IMF}$ and smaller $f_{DM}$ are derived. Adopting a steeper concentration-mass relation than that from simulations, we find lower $\delta_{IMF}$ ($< 1$) and larger $f_{DM}$. A constant M/L profile with null $f_{DM}$ gives the heaviest $\delta_{IMF}$ ($\sim 2$). In the MONDian framework, we find consistent results to those for our reference NFW model. If confirmed, the large scatter of $\delta_{IMF}$ for dEs would provide (further) evidence for a non-universal IMF in early-type systems. On average, our reference $f_{DM}$ estimates are consistent with those found for low-$\sigma_{e}$ ($\rm \sim 100 \, \rm km s^{-1}$) early-type galaxies (ETGs). Furthermore, we find $f_{DM}$ consistent with values from the SMAKCED survey, and find a double-value behavior of $f_{DM}$ with stellar mass, which mirrors the trend of dynamical M/L and global star formation efficiency with mass.Comment: 11 pages, 3 figures, 1 table, published on MNRAS. Figure 1 has been updated with respect to version 1, including the range of values found if the S\'ersic index, n, is varied from 0.5 to 2 (dark-green curves

Hierarchical bounding structures for efficient virial computations: Towards a realistic molecular description of cholesterics

We detail the application of bounding volume hierarchies to accelerate second-virial evaluations for arbitrary complex particles interacting through hard and soft finite-range potentials. This procedure, based on the construction of neighbour lists through the combined use of recursive atom-decomposition techniques and binary overlap search schemes, is shown to scale sub-logarithmically with particle resolution in the case of molecular systems with high aspect ratios. Its implementation within an efficient numerical and theoretical framework based on classical density functional theory enables us to investigate the cholesteric self-assembly of a wide range of experimentally-relevant particle models. We illustrate the method through the determination of the cholesteric behaviour of hard, structurally-resolved twisted cuboids, and report quantitative evidence of the long-predicted phase handedness inversion with increasing particle thread angles near the phenomenological threshold value of $45^\circ$. Our results further highlight the complex relationship between microscopic structure and helical twisting power in such model systems, which may be attributed to subtle geometric variations of their chiral excluded-volume manifold

Galaxy evolution within the Kilo-Degree Survey

The ESO Public Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried out with the VLT Survey Telescope and the OmegaCAM camera. KiDS will scan 1500 square degrees in four optical filters (u, g, r, i). Designed to be a weak lensing survey, it is ideal for galaxy evolution studies, thanks to the high spatial resolution of VST, the good seeing and the photometric depth. The surface photometry have provided with structural parameters (e.g. size and S\'ersic index), aperture and total magnitudes have been used to derive photometric redshifts from Machine learning methods and stellar masses/luminositites from stellar population synthesis. Our project aimed at investigating the evolution of the colour and structural properties of galaxies with mass and environment up to redshift $z \sim 0.5$ and more, to put constraints on galaxy evolution processes, as galaxy mergers.Comment: 4 pages, 2 figures, to appear on the refereed Proceeding of the "The Universe of Digital Sky Surveys" conference held at the INAF--OAC, Naples, on 25th-28th november 2014, to be published on Astrophysics and Space Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo, Iodic

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.Comment: 13 pages, 7 figures, 1 table, accepted for publication on MNRA

Systematic variation of central mass density slope in early-type galaxies

We study the total density distribution in the central regions ($<\, 1$ effective radius, $R_{\rm e}$) of early-type galaxies (ETGs), using data from the SPIDER survey. We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter (DM) halo profile, and leaving stellar mass-to-light ($M_{\rm \star}/L$) ratios as free fitting parameters to the data. For a Navarro et al. (1996) profile, the slope of the total mass profile is non-universal. For the most massive and largest ETGs, the profile is isothermal in the central regions ($\sim R_{\rm e}/2$), while for the low-mass and smallest systems, the profile is steeper than isothermal, with slopes similar to those for a constant-M/L profile. For a concentration-mass relation steeper than that expected from simulations, the correlation of density slope with mass tends to flatten. 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.Comment: 3 pages, 1 figure, to appear on the refereed Proceeding of the "The Universe of Digital Sky Surveys" conference held at the INAF--OAC, Naples, on 25th-28th november 2014, to be published on Astrophysics and Space Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo, Iodic

Unsupervised Learning via Mixtures of Skewed Distributions with Hypercube Contours

Mixture models whose components have skewed hypercube contours are developed via a generalization of the multivariate shifted asymmetric Laplace density. Specifically, we develop mixtures of multiple scaled shifted asymmetric Laplace distributions. The component densities have two unique features: they include a multivariate weight function, and the marginal distributions are also asymmetric Laplace. We use these mixtures of multiple scaled shifted asymmetric Laplace distributions for clustering applications, but they could equally well be used in the supervised or semi-supervised paradigms. The expectation-maximization algorithm is used for parameter estimation and the Bayesian information criterion is used for model selection. Simulated and real data sets are used to illustrate the approach and, in some cases, to visualize the skewed hypercube structure of the components

Testing Verlinde's emergent gravity in early-type galaxies

Verlinde derived gravity as an emergent force from the information flow, through two-dimensional surfaces and recently, by a priori postulating the entanglement of information in 3D space, he derived the effect of the gravitational potential from dark matter (DM) as the entropy displacement of dark energy by baryonic matter. In Emergent Gravity (EG) this apparent DM depends only on the baryonic mass distribution and the present-day value of the Hubble parameter. In this paper we test the EG proposition, formalized by Verlinde for a spherical and isolated mass distribution, using the central velocity dispersion, $\sigma$ and the light distribution in a sample of 4260 massive and local early-type galaxies (ETGs) from the SPIDER sample. Our results remain unaltered if we consider the sample of 807 roundest field galaxies. We derive the predictions by EG for the stellar mass-to-light ratio (M/L) and the Initial Mass Function (IMF), and compare them with the same inferences derived from a) DM-based models, b) MOND and c) stellar population models. We demonstrate that, consistently with a classical Newtonian framework with a DM halo component, or alternative theories of gravity as MOND, the central dynamics can be fitted if the IMF is assumed non-universal. The results can be interpreted with a IMF lighter than a standard Chabrier at low-$\sigma$, and bottom-heavier IMFs at larger $\sigma$. We find lower, but still acceptable, stellar M/L in EG theory, if compared with the DM-based NFW model and with MOND. The results from EG are comparable to what is found if the DM haloes are adiabatically contracted and with expectations from spectral gravity-sensitive features. If the strain caused by the entropy displacement would be not maximal, as adopted in the current formulation, then the dynamics of ETGs could be reproduced with larger M/L. (abridged)Comment: 12 pages, 2 figures, submitted to MNRAS. The updated manuscript presents significantly altered conclusions, after discovering an internal bug in an older version of the Mathematica package, leading to incorrect numerical results when calculating the derivatives of Gamma function