5,686 research outputs found
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
Understanding chemical evolution in resolved galaxies -- I The local star fraction-metallicity relation
This work studies the relation between gas-phase oxygen abundance and
stellar-to-gas fraction in nearby galaxies. We first derive the theoretical
prediction, and argue that this relation is fundamental, in the sense that it
must be verified regardless of the details of the gas accretion and star
formation histories. Moreover, it should hold on "local" scales, i.e. in
regions of the order of 1 kpc. These predictions are then compared with a set
of spectroscopic observations, including both integrated and resolved data.
Although the results depend somewhat on the adopted metallicity calibration,
observed galaxies are consistent with the predicted relation, imposing tight
constraints on the mass-loading factor of (enriched) galactic winds. The
proposed parametrization of the star fraction-metallicity relation is able to
describe the observed dependence of the oxygen abundance on gas mass at fixed
stellar mass. However, the "local" mass-metallicity relation also depends on
the relation between stellar and gas surface densities.Comment: 10 pages, 4 figures. Matches accepted version (significant typo
corrected
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 colour --
with other physical properties (mass, metallicity, environment, morphology, and
the presence of close companions) in a sample of 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) and bluer
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
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