Dirichlet process mixture models for non-stationary data streams

In recent years, we have seen a handful of work on inference algorithms over non-stationary data streams. Given their flexibility, Bayesian non-parametric models are a good candidate for these scenarios. However, reliable streaming inference under the concept drift phenomenon is still an open problem for these models. In this work, we propose a variational inference algorithm for Dirichlet process mixture models. Our proposal deals with the concept drift by including an exponential forgetting over the prior global parameters. Our algorithm allows adapting the learned model to the concept drifts automatically. We perform experiments in both synthetic and real data, showing that the proposed model outperforms state-of-the-art variational methods in density estimation, clustering and parameter tracking

Nonlinear Stochastic Resonance with subthreshold rectangular pulses

We analyze the phenomenon of nonlinear stochastic resonance (SR) in noisy bistable systems driven by pulsed time periodic forces. The driving force contains, within each period, two pulses of equal constant amplitude and duration but opposite signs. Each pulse starts every half-period and its duration is varied. For subthreshold amplitudes, we study the dependence of the output signal-to-noise ratio (SNR) and the SR gain on the noise strength and the relative duration of the pulses. We find that the SR gains can reach values larger than unity, with maximum values showing a nonmonotonic dependence on the duration of the pulses.Comment: 7 pages, 2 figure

Nature or nurture? Clues from the distribution of specific star formation rates in SDSS galaxies

This work investigates the main mechanism(s) that regulate the specific star formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this quantity -- the equivalent width of the \Ha\ line and the $(u-r)$ colour -- with other physical properties (mass, metallicity, environment, morphology, and the presence of close companions) in a sample of $\sim82500$ galaxies extracted from the Sloan Digital Sky Survey (SDSS). The existence of a relatively tight `ageing sequence' in the colour-equivalent width plane favours a scenario where the secular conversion of gas into stars (i.e. `nature') is the main physical driver of the instantaneous SSFR and the gradual transition from a `chemically primitive' (metal-poor and intensely star-forming) state to a `chemically evolved' (metal-rich and passively evolving) system. Nevertheless, environmental factors (i.e. `nurture') are also important. In the field, galaxies may be temporarily affected by discrete `quenching' and `rejuvenation' episodes, but such events show little statistical significance in a probabilistic sense, and we find no evidence that galaxy interactions are, on average, a dominant driver of star formation. Although visually classified mergers tend to display systematically higher EW(H$\alpha$) and bluer $(u-r)$ colours for a given luminosity, most galaxies with high SSFR have uncertain morphologies, which could be due to either internal or external processes. Field galaxies of early and late morphological types are consistent with the gradual `ageing' scenario, with no obvious signatures of a sudden decrease in their SSFR. In contrast, star formation is significantly reduced and sometimes completely quenched on a short time scale in dense environments, where many objects are found on a `quenched sequence' in the colour-equivalent width plane.Comment: 18 pages, 9 figure

Galaxies going MAD: The Galaxy-Finder Comparison Project

With the ever increasing size and complexity of fully self-consistent simulations of galaxy formation within the framework of the cosmic web, the demands upon object finders for these simulations has simultaneously grown. To this extent we initiated the Halo Finder Comparison Project that gathered together all the experts in the field and has so far led to two comparison papers, one for dark matter field haloes (Knebe et al. 2011), and one for dark matter subhaloes (Onions et al. 2012). However, as state-of-the-art simulation codes are perfectly capable of not only following the formation and evolution of dark matter but also account for baryonic physics (e.g. hydrodynamics, star formation, feedback) object finders should also be capable of taking these additional processes into consideration. Here we report on a comparison of codes as applied to the Constrained Local UniversE Simulation (CLUES) of the formation of the Local Group which incorporates much of the physics relevant for galaxy formation. We compare both the properties of the three main galaxies in the simulation (representing the MW, M31, and M33) as well as their satellite populations for a variety of halo finders ranging from phase-space to velocity-space to spherical overdensity based codes, including also a mere baryonic object finder. We obtain agreement amongst codes comparable to (if not better than) our previous comparisons, at least for the total, dark, and stellar components of the objects. However, the diffuse gas content of the haloes shows great disparity, especially for low-mass satellite galaxies. This is primarily due to differences in the treatment of the thermal energy during the unbinding procedure. We acknowledge that the handling of gas in halo finders is something that needs to be dealt with carefully, and the precise treatment may depend sensitively upon the scientific problem being studied.Comment: 14 interesting pages, 17 beautiful figures, and 2 informative tables accepted for publication in MNRAS (matches published version