A Formalization of The Natural Gradient Method for General Similarity Measures

In optimization, the natural gradient method is well-known for likelihood maximization. The method uses the Kullback-Leibler divergence, corresponding infinitesimally to the Fisher-Rao metric, which is pulled back to the parameter space of a family of probability distributions. This way, gradients with respect to the parameters respect the Fisher-Rao geometry of the space of distributions, which might differ vastly from the standard Euclidean geometry of the parameter space, often leading to faster convergence. However, when minimizing an arbitrary similarity measure between distributions, it is generally unclear which metric to use. We provide a general framework that, given a similarity measure, derives a metric for the natural gradient. We then discuss connections between the natural gradient method and multiple other optimization techniques in the literature. Finally, we provide computations of the formal natural gradient to show overlap with well-known cases and to compute natural gradients in novel frameworks

High-Dimensional Feature Selection by Feature-Wise Kernelized Lasso

The goal of supervised feature selection is to find a subset of input features that are responsible for predicting output values. The least absolute shrinkage and selection operator (Lasso) allows computationally efficient feature selection based on linear dependency between input features and output values. In this paper, we consider a feature-wise kernelized Lasso for capturing non-linear input-output dependency. We first show that, with particular choices of kernel functions, non-redundant features with strong statistical dependence on output values can be found in terms of kernel-based independence measures. We then show that the globally optimal solution can be efficiently computed; this makes the approach scalable to high-dimensional problems. The effectiveness of the proposed method is demonstrated through feature selection experiments with thousands of features.Comment: 18 page

Improved measurements of the intergalactic medium temperature around quasars: possible evidence for the initial stages of He II reionization at z ≃ 6

We present measurements of the intergalactic medium (IGM) temperature within ∼5 proper Mpc of seven luminous quasars at z ≃ 6. The constraints are obtained from the Doppler widths of Lyα absorption lines in the quasar near zones and build upon our previous measurement for the z = 6.02 quasar SDSS J0818+1722. The expanded data set, combined with an improved treatment of systematic uncertainties, yields an average temperature at the mean density of log(T_0/K) = 4.21±^(0.03)_(0.03)(±^(0.06)_(0.07)) at 68 (95) per cent confidence for a flat prior distribution over 3.2 ≤ log (T_0/K) ≤ 4.8. In comparison, temperatures measured from the general IGM at z ≃ 5 are ∼0.3 dex cooler, implying an additional source of heating around these quasars which is not yet present in the general IGM at slightly lower redshift. This heating is most likely due to the recent reionization of He ii in vicinity of these quasars, which have hard and non-thermal ionizing spectra. The elevated temperatures may therefore represent evidence for the earliest stages of He ii reionization in the most biased regions of the high-redshift Universe. The temperature as a function of distance from the quasars is consistent with being constant, log (T_0/K) ≃ 4.2, with no evidence for a line-of-sight thermal proximity effect. However, the limited extent of the quasar near zones prevents the detection of He III regions larger than ∼5 proper Mpc. Under the assumption that the quasars have reionized the He II in their vicinity, we infer that the data are consistent with an average optically bright phase of duration in excess of 10^(6.5) yr. These measurements represent the highest redshift IGM temperature constraints to date, and thus provide a valuable data set for confronting models of H I reionization

A consistent determination of the temperature of the intergalactic medium at redshift z=2.4

We present new measurements of the thermal state of the intergalactic medium (IGM) at z∼2.4 derived from absorption line profiles in the Lyα forest. We use a large set of high-resolution hydrodynamical simulations to calibrate the relationship between the temperature-density (T--Δ) relation in the IGM and the distribution of HI column densities, NHI, and velocity widths, bHI, of discrete Lyα forest absorbers. This calibration is then applied to the measurement of the lower cut-off of the bHI--NHI distribution recently presented by Rudie et al. (2012). We infer a power-law T--Δ relation, T=T0Δγ−1, with a temperature at mean density, T0=[1.00+0.32−0.21]×104K and slope (γ−1)=0.54±0.11. The slope is fully consistent with that advocated by the analysis of Rudie et al (2012); however, the temperature at mean density is lower by almost a factor of two, primarily due to an adjustment in the relationship between column density and physical density assumed by these authors. These new results are in excellent agreement with the recent temperature measurements of Becker et al. (2011), based on the curvature of the transmitted flux in the Lyα forest. This suggests that the thermal state of the IGM at this redshift is reasonably well characterised over the range of densities probed by these methods

Heating and ionization of the primordial intergalactic medium by high mass x-ray binaries

We investigate the influence of high-mass X-ray binaries (HMXBs) on their high-redshift environments. Using a one-dimensional radiative transfer code, we predict the ionization and temperature profiles surrounding a coeval stellar population, composed of main-sequence stars and HMXBs, at various times after its formation. We consider both uniform density surroundings, and a cluster embedded in a 108 M⊙ Navarro–Frenk–White (NFW) halo. HMXBs in a constant density environment produce negligible enhanced ionization because of their high-energy spectral energy distributions and short lifetimes. In this case, HMXBs only marginally contribute to the local heating rate. For NFW profiles, radiation from main-sequence stars cannot prevent the initially ionized volume from recombining since it is unable to penetrate the high-density galactic core. However, HMXB photons stall recombinations behind the front, keeping it partially ionized for longer. The increased electron density in these partially ionized regions promotes further cooling, resulting in lower intergalactic medium (IGM) temperatures. In the context of this starburst model, we have shown that HMXBs do not make a major contribution to reionization or IGM heating. However, X-ray escape fractions are high in both density profile cases. Continuous star formation may result in the build up of X-rays over time, reducing the ionization time-scale and potentially leading to low level ionization of the distant IGM

Studying reionization with the next generation of Lyα emitter surveys

We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at z > 6 with the next generation of large Lyα emitter surveys. We make predictions for the upcoming Subaru Hyper Suprime-Cam (HSC) Lyα survey and a hypothetical spectroscopic survey performed with the James Webb Space Telescope (JWST). Considering various scenarios where the observed evolution of the Lyα luminosity function of Lyα emitters at z > 6 is explained partly by an increasingly neutral IGM and partly by intrinsic galaxy evolution, we show how clustering measurements will be able to distinguish between these scenarios. We find that the HSC survey should be able to detect the additional clustering induced by a neutral IGM if the global IGM neutral fraction is greater than ∼20 per cent at z = 6.5. If measurements of the Lyα equivalent widths (EWs) are also available, neutral fractions as small as 10 per cent may be detectable by looking for correlation between the EW and the local number density of objects. In this case, if it should turn out that the IGM is significantly neutral at z = 6.5 and the intrinsic EW distribution is relatively narrow, the observed EWs can also be used to construct a map of the locations and approximate sizes of the largest ionized regions. For the JWST survey, the results appear a bit less optimistic. Since such surveys probe a large range of redshifts, the effects of the IGM will be mixed up with any intrinsic galaxy evolution that is present, making it difficult to disentangle the effects. However, we show that a survey with the JWST will have a possibility of observing a large group of galaxies at z ∼ 7, which would be a strong indication of a partially neutral IGM

A new mechanical stellar wind feedback model for the Rosette Nebula

The famous Rosette Nebula has an evacuated central cavity formed from the stellar winds ejected from the 2–6 Myr old codistant and comoving central star cluster NGC 2244. However, with upper age estimates of less than 110 000 yr, the central cavity is too young compared to NGC 2244 and existing models do not reproduce its properties. A new proper motion study herein using Gaia data reveals the ejection of the most massive star in the Rosette, HD 46223, from NGC 2244 occurred 1.73 (+0.34, −0.25) Myr (1σ uncertainty) in the past. Assuming this ejection was at the birth of the most massive stars in NGC 2244, including the dominant centrally positioned HD 46150, the age is set for the famous ionized region at more than 10 times that derived for the cavity. Here, we are able to reproduce the structure of the Rosette Nebula, through simulation of mechanical stellar feedback from a 40 Mₒ star in a thin sheet-like molecular cloud. We form the 135 000 Mₒ cloud from thermally unstable diffuse interstellar medium (ISM) under the influence of a realistic background magnetic field with thermal/magnetic pressure equilibrium. Properties derived from a snapshot of the simulation at 1.5 Myr, including cavity size, stellar age, magnetic field, and resulting inclination to the line of sight, match those derived from observations. An elegant explanation is thus provided for the stark contrast in age estimates based on realistic diffuse ISM properties, molecular cloud formation and stellar wind feedback

Abundance ratios and IMF slopes in the dwarf elliptical galaxy NGC 1396 with MUSE

Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (MV = −16.60, Mass ~4 × 10^8 M�), located in the Fornax cluster, have been performed with the VLT/ MUSE spectrograph in the wavelength region from 4750 − 9350 Å. In this paper we present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galacto-centric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of ∼ 6 Gyr and a metallicity [Fe/H]∼ −0.4, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] ∼ +0.1, and under-abundant Sodium, with [Na/Fe] values around −0.1, while [Mg/Fe] is overabundant at all radii, increasing from ∼ +0.1 in the centre to ∼ +0.2 dex. We notice a significant metallicity and age gradient within this dwarf galaxy. To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out. An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended star formation history. This would be the case if the galaxy originated from a LMC-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster

Kesiapsiagaan Individu dan RumahTangga dalam Menghadapi Bencana Banjir di Kota Suakarta Tahun 2017

Abstract This research case aims to determine (1) The spread of floods in the city of Surakarta in 2016; (2) Uncertainty factors that causing floods based on community perception in Surakarta City; (3) Individual and household preparedness levels in dealing with flood disaster in Surakarta City; (4) Implementation of science as a teaching material of geography in grade XI Senior High School on basic material of natural disaster mitigation. The research was conducted in Western City of Surakarta. The area assumed in Western City of Surakarta are Banjarsari district, Laweyan District, and Serengan District Eligible samples consisted of 11 villages affected by flood in 2016, sampling of administrative unit using purposive sampling technique. The sample has been taken is the number of individuals / households in each ‘RW’ affected by floods in each villages, the number of samples is using snowball sampling technique. Data collection was done by documentation study, interview, questionnaire, and interview. Data validity test is done by data triangulation method. Data analysis is using Likert approach and LIPI preparedness measurement framework - UNESCO / ISDR. The results of the research are as follows: (1) flooding spread in eleven sub-districts in West Surakarta City which is divided into 3 regions based on administrative unit of analysis. Banjarsari District having local flood characteristics and submissions with elevation and duration of time falling into the low category. Laweyan District areas have local flood characteristics and postings with elevation and duration of time that falling into the low category. Serengan District has local flood characteristics and post with elevation and duration falling into the medium category; (2). Factors causing high flooding, garbage disposal, and flood control building conditions; (3) Individual and household preparedness studies in all villages are in a ready category; (4) The result of the research becomes the material in the form of geography module on the material of natural disaster mitigation class XI even semester high school. Keywords: Preparedness, Disaster, Flood, Perception Abstrak Penelitian ini bertujuan untuk mengetahui (1) persebaran banjir di Kota Surakarta pada tahun 2016; (2) persepsi masyarakat terhadap faktor – faktor yang menyebabkan banjir di Kota Surakarta; (3) tingkat kesiapsiagaan individu dan rumah tangga dalam menghadapi bencana banjir di Kota Surakarta; (4) implementasi tema kajian sebagai bahan ajar geografi di kelas XI SMA pada materi pokok mitigasi bencana alam. Penelitian ini dilakukan di Kota Surakarta bagian barat. Wilayah yang diasumsikan berada di bagian barat adalah Kecamatan Banjarsari, Kecamatan Laweyan, dan Kecamatan Serengan. Sampel satuan administratif yang diambil meliputi 11 kelurahan terdampak banjir pada tahun 2016, menggunakan teknik purposive sampling. Sampel responden yang diambil adalah perwakilan individu/rumah tangga pada tiap RW terdampak banjir di masing – masing kelurahan, menggunakan teknik snowball sampling. Pengumpulan data dilakukan dengan cara studi dokumentasi, observasi lapangan, angket, dan wawancara. Uji validitas data dilakukan dengan triangulasi. Analisis data menggunakan pendekatan likert dan framework pengukuran kesiapsiagaan LIPI – UNESCO/ISDR. Hasil penelitian adalah : (1) banjir tersebar di sebelas kelurahan yang terbagi menjadi 3 wilayah berdasarkan unit analisis satuan administratif. Wilayah Kecamatan Banjarsari memiliki karakteristik banjir lokal dan kiriman dengan elevasi dan durasi waktu yang masuk dalam kategori rendah. Wilayah Kecamatan Laweyan memiliki karakteristik banjir lokal dan kiriman dengan elevasi dan durasi waktu yang masuk dalam kategori rendah. Wilayah Kecamatan Serengan memiliki karakteristik banjir lokal dan kiriman dengan elevasi dan durasi waktu yang masuk dalam kategori sedang; (2) Faktor penyebab banjir paling dominan berdasarkan persepsi masyarakat meliputi intensitas curah hujan yang tinggi, pembuangan sampah, dan kondisi bangunan pengendali banjir yang telah mengalami penurunan kapasitas; (3) Kajian kesiapsiagaan individu dan rumah tangga di seluruh kelurahan masuk dalam kategori siap; (4) Hasil penelitian menjadi suplemen materi pembelajaran dalam bentuk modul geografi pada materi pokok mitigasi bencana alam kelas XI SMA semester genap. Kata Kunci : Kesiapsiagaan, Bencana, Banjir, Perseps