Filamentary Switching: Synaptic Plasticity through Device Volatility

Replicating the computational functionalities and performances of the brain remains one of the biggest challenges for the future of information and communication technologies. Such an ambitious goal requires research efforts from the architecture level to the basic device level (i.e., investigating the opportunities offered by emerging nanotechnologies to build such systems). Nanodevices, or, more precisely, memory or memristive devices, have been proposed for the implementation of synaptic functions, offering the required features and integration in a single component. In this paper, we demonstrate that the basic physics involved in the filamentary switching of electrochemical metallization cells can reproduce important biological synaptic functions that are key mechanisms for information processing and storage. The transition from short- to long-term plasticity has been reported as a direct consequence of filament growth (i.e., increased conductance) in filamentary memory devices. In this paper, we show that a more complex filament shape, such as dendritic paths of variable density and width, can permit the short- and long-term processes to be controlled independently. Our solid-state device is strongly analogous to biological synapses, as indicated by the interpretation of the results from the framework of a phenomenological model developed for biological synapses. We describe a single memristive element containing a rich panel of features, which will be of benefit to future neuromorphic hardware systems

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

Global Properties of the Rich Cluster ABCG 209 at z~0.2. Spectroscopic and Photometric Catalogue

This paper is aimed at giving an overview of the global properties of the rich cluster of galaxies ABCG 209. This is achieved by complementing the already available data with new medium resolution spectroscopy and NIR photometry which allow us to i) analyse in detail the cluster dynamics, distinguishing among galaxies belonging to different substructures and deriving their individual velocity distributions, using a total sample of 148 galaxies in the cluster region, of which 134 belonging to the cluster; ii) derive the cluster NIR luminosity function; iii) study the Kormendy relation and the photometric plane of cluster early-type galaxies (ETGs). Finally we provide an extensive photometric (optical and NIR) and spectroscopic dataset for such a complex system to be used in further analyses investigating the nature, formation and evolution of rich clusters of galaxies. The observational scenario confirms that ABCG 209 is presently undergoing strong dynamical evolution with the merging of two or more subclumps. This interpretation is also supported by the detection of a radio halo (Giovannini et al. 2006) suggesting that there is a recent or ongoing merging. Cluster ETGs follow a Kormendy relation whose slope is consistent with previous studies both at optical and NIR wavelengths. We investigate the origin of the intrinsic scatter of the photometric plane due to trends of stellar populations, using line indices as indicators of age, metallicity and alpha/Fe enhancement. We find that the chemical evolution of galaxies could be responsible for the intrinsic dispersion of the Photometric Plane.Comment: 39 pages, 17 figures, MNRAS in pres

SPIDER - IV. Optical and NIR color gradients in Early-type galaxies: New Insights into Correlations with Galaxy Properties

We present an analysis of stellar population gradients in 4,546 Early-Type Galaxies with photometry in $grizYHJK$ along with optical spectroscopy. A new approach is described which utilizes color information to constrain age and metallicity gradients. Defining an effective color gradient, $\nabla_{\star}$, which incorporates all of the available color indices, we investigate how $\nabla_{\star}$ varies with galaxy mass proxies, i.e. velocity dispersion, stellar (M_star) and dynamical (M_dyn) masses, as well as age, metallicity, and alpha/Fe. ETGs with M_dyn larger than 8.5 x 10^10, M_odot have increasing age gradients and decreasing metallicity gradients wrt mass, metallicity, and enhancement. We find that velocity dispersion and alpha/Fe are the main drivers of these correlations. ETGs with 2.5 x 10^10 M_odot =< M_dyn =< 8.5 x 10^10 M_odot, show no correlation of age, metallicity, and color gradients wrt mass, although color gradients still correlate with stellar population parameters, and these correlations are independent of each other. In both mass regimes, the striking anti-correlation between color gradient and alpha-enhancement is significant at \sim 4sigma, and results from the fact that metallicity gradient decreases with alpha/Fe. This anti-correlation may reflect the fact that star formation and metallicity enrichment are regulated by the interplay between the energy input from supernovae, and the temperature and pressure of the hot X-ray gas in ETGs. For all mass ranges, positive age gradients are associated with old galaxies (>5-7 Gyr). For galaxies younger than \sim 5 Gyr, mostly at low-mass, the age gradient tends to be anti-correlated with the Age parameter, with more positive gradients at younger ages.Comment: Accepted for Publication in the Astronomical Journa