Neural network ensembles: Evaluation of aggregation algorithms

Ensembles of artificial neural networks show improved generalization capabilities that outperform those of single networks. However, for aggregation to be effective, the individual networks must be as accurate and diverse as possible. An important problem is, then, how to tune the aggregate members in order to have an optimal compromise between these two conflicting conditions. We present here an extensive evaluation of several algorithms for ensemble construction, including new proposals and comparing them with standard methods in the literature. We also discuss a potential problem with sequential aggregation algorithms: the non-frequent but damaging selection through their heuristics of particularly bad ensemble members. We introduce modified algorithms that cope with this problem by allowing individual weighting of aggregate members. Our algorithms and their weighted modifications are favorably tested against other methods in the literature, producing a sensible improvement in performance on most of the standard statistical databases used as benchmarks.Comment: 35 pages, 2 figures, In press AI Journa

Evidence for azimuthal variations of the oxygen abundance gradient tracing the spiral structure of the galaxy HCG91c

Context. The distribution of elements in galaxies forms an important diagnostic tool to characterize the system's formation and evolution. This tool is however complex to use in practice, as galaxies are subject to a range of simultaneous physical processes active from pc to kpc scales. This renders observations of the full optical extent of galaxies down to sub-kpc scales essential. Aims. Using the WiFeS integral field spectrograph, we previously detected abrupt and localized variations in the gas-phase oxygen abundance of the spiral galaxy HCG91c. Here, we follow-up on these observations to map HCG91c's disk out to ~2Re at a resolution of 600pc, and characterize the non-radial variations of the gas-phase oxygen abundance in the system. Methods. We obtained deep MUSE observations of the target under ~0.6 arcsec seeing conditions. We perform both a spaxel-based and aperture-based analysis of the data to map the spatial variations of 12+log(O/H) across the disk of the galaxy. Results. We confirm the presence of rapid variations of the oxygen abundance across the entire extent of the galaxy previously detected with WiFeS, for all azimuths and radii. The variations can be separated in two categories: a) localized and associated with individual HII regions, and b) extended over kpc scales, and occurring at the boundaries of the spiral structures in the galaxy. Conclusions. Our MUSE observations suggest that the enrichment of the interstellar medium in HGC91c has proceeded preferentially along spiral structures, and less efficiently across them. Our dataset highlights the importance of distinguishing individual star-forming regions down to scales of a few 100pc when using integral field spectrographs to spatially resolve the distribution of oxygen abundances in a given system, and accurately characterize azimuthal variations and intrinsic scatter.Comment: 12 pages, 11 figures, accepted for publication in A&A. Supplementary movie assocociated with Fig. 8 is available (until publication) at: http://www.sc.eso.org/~fvogt/supp_mat/HCG91c/O_gradient.mp

Dynamical effects of interactions and the Tully-Fisher relation for Hickson compact groups

We investigate the properties of the B-band Tully-Fisher (T-F) relation for 25 compact group galaxies, using Vmax derived from 2-D velocity maps. Our main result is that the majority of the Hickson Compact Group galaxies lie on the T-F relation. However, about 20% of the galaxies, including the lowest-mass systems, have higher B luminosities for a given mass, or alternatively, a mass which is too low for their luminosities. We favour a scenario in which outliers have been brightened due to either enhanced star formation or merging. Alternatively, the T-F outliers may have undergone truncation of their dark halo due to interactions. It is possible that in some cases, both effects contribute. The fact that the B-band T-F relation is similar for compact group and field galaxies tells us that these galaxies show common mass-to-size relations and that the halos of compact group galaxies have not been significantly stripped inside R25. We find that 75% of the compact group galaxies studied (22 out of 29) have highly peculiar velocity fields. Nevertheless, a careful choice of inclination, position angle and center, obtained from the velocity field, and an average of the velocities over a large sector of the galaxy enabled the determination of fairly well-behaved rotation curves for the galaxies. However, two of the compact group galaxies which are the most massive members in M51--like pairs, HCG 91a and HCG 96a, have very asymmetric rotation curves, with one arm rising and the other one falling, indicating, most probably, a recent perturbation by the small close companions.Comment: 15 pages, 4 figures, accepted for publication in the Astronomical Journa

Past and Future of CG J1720-67.8: Constraints from Observations and Models

We discuss the evolution of the peculiar, nearby (z = 0.045), compact galaxy group CG J1720-67.8, by interpreting a large amount of observational information on the basis of our recent results from spectrophotometric evolutionary synthesis models and new N-body/SPH simulations. The group, that is composed of two spiral galaxies with a mass ratio approximately 4:1 and an S0 galaxy in a particularly compact configuration, is undergoing an active pre-merging phase. Several tidal features are signposts of the complex dynamics of the system. We suggest that the observed structure of the tidal features can be explained only if all three galaxies are involved in a strong interaction process.Comment: 5 pages, 3 (degraded) figures. Proc. ESO Workshop "Groups of galaxies in the nearby Universe", Santiago, Chile, 5-9 Dec. 2005, ESO Astrophysics Symposia, eds. I. Saviane, V. Ivanov & J. Borissova, Springer-Verla

Galaxy Interactions in Compact Groups II: abundance and kinematic anomalies in HCG 91c

Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS integral field spectrograph as part of our ongoing campaign targeting the ionized gas physics and kinematics inside star forming members of compact groups. Here, we report the discovery of HII regions with abundance and kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c. The optical emission line analysis of this galaxy reveals that at least three HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their immediate surroundings and from the abundance gradient present in the inner regions of HCG 91c. The same star forming regions are also associated with a small kinematic offset in the form of a lag of 5-10 km/s with respect to the local circular rotation of the gas. HI observations of HCG 91 from the Very Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c is caught early in its interaction with the other members of HCG 91. We discuss different scenarios to explain the origin of the peculiar star forming regions detected with WiFeS, and show that evidence point towards infalling and collapsing extra-planar gas clouds at the disk-halo interface, possibly as a consequence of long-range gravitational perturbations of HCG 91c from the other group members. As such, HCG 91c provides evidence that some of the perturbations possibly associated with the early phase of galaxy evolution in compact groups impact the star forming disk locally, and on sub-kpc scales.Comment: 25 pages, 21 figures, MNRAS accepted. Until publication of the article, the interactive component of Figure 4 is available at this URL: http://www.mso.anu.edu.au/~fvogt/website/misc.htm

Choroidal thickness measured using swept-source optical coherence tomography is reduced in patients with type 2 diabetes

Objective To compare choroidal thickness between patients with type 2 diabetes (T2D) and healthy controls measured using swept-source optical coherence tomography (SS-OCT). Methods The sample comprised 157 eyes of 94 T2D patients, 48 eyes of which had diabetic macular edema (DME), and 71 normal eyes of 38 healthy patients. Subfoveal (SF) choroidal thickness, and choroidal thickness at 500-µm intervals up to 2500 µm nasal and temporal from the fovea were measured using the SS-OCT. Choroidal thicknesses were compared between groups using Student’s t-test. Additionally, Pearson correlations were calculated between diabetes duration, glycosylated hemoglobin (HbA1c) levels, and choroidal thickness. Results Mean diabetes duration was 16.6±9.5 years, while mean glycosylated hemoglobin was 7.7 ±1.3%. Overall, the choroid was significantly thinner in T2D patients. Individuals with DME had reduced choroidal thickness in all measurements, except at 2000 and 2500-µm nasal positions, compared to healthy controls. There was a moderate correlation between choroidal thickness and HbA1c levels in DME patients (SF: r = 0.342; p = 0.017). Diabetes duration did not correlate significantly with choroidal thickness. Conclusion SS-OCT measurements revealed that the choroid was significantly thinner in T2D patients, moderate non-proliferative diabetic retinopathy patients, and DME patients than in healthy individuals. Further studies are needed to clarify the effect of diabetes on this layer and the relationship between choroidal thickness and DME