Search

STREAMFINDER is a new algorithm that we have built to detect stellar streams in an automated and systematic way in astrophysical datasets that possess any combination of positional and kinematic information. In Paper I, we introduced the methodology and the workings of our algorithm and showed that it is capable of detecting ultra-faint and distant halo stream structures containing as few as ∼15 members (ΣG ∼ 33.6 mag arcsec−2) in the Gaia dataset. Here, we test the method with real proper motion data from the Pan-STARRS1 survey, and by selecting targets down to r 0 = 18.5 mag we show that it is able to detect the GD-1 stellar stream, whereas the structure remains below a useful detection limit when using a Matched Filter technique. The radial velocity solutions provided by STREAMFINDER for GD-1 candidate members are found to be in good agreement with observations. Furthermore, our algorithm detects a $${\sim } 40^{^\circ }$$ long structure approximately parallel to GD-1, and which fans out from it, possibly a sign of stream-fanning due to the triaxiality of the Galactic potential. This analysis shows the promise of this method for detecting and analysing stellar streams in the upcoming Gaia DR2 catalogue. [ABSTRACT FROM AUTHOR]

ABSTRACT A companion paper presents an algorithm for estimating the actions of orbits in axisymmetric potentials. This algorithm is fast enough for it to be feasible to fit automatically a parametrized distribution function to observational data for the solar neighbourhood. We explore the predictive power of these models and the extent to which global models are constrained by data confined to the solar cylinder. We adopt a gravitational potential that is generated by three discs (gas and both thin and thick stellar discs), a bulge and a dark halo, and fit the thin-disc component of the distribution function to the solar-neighbourhood velocity distribution from the Geneva-Copenhagen survey. We find that the disc's vertical density profile is in good agreement with data at z ≲ 500 pc. The thick-disc component of the distribution function is then used to extend the fit to data from Gilmore & Reid for z ≲ 2.5 kpc. The resulting model predicts excellent fits to the profile of the vertical velocity dispersion σ z( z) from the RAVE survey and to the distribution of vφ velocity components at | z| ∼ 1 kpc from the Sloan Digital Sky Survey. The ability of this model to predict successfully data that were not used in the fitting process suggests that the adopted gravitational potential (which is close to a maximum-disc potential) is close to the true one. We show that if another plausible potential is used, the predicted values of σ z are too large. The models imply that in contrast to the thin disc, the thick disc has to be hotter vertically than radially, a prediction that it will be possible to test in the near future. When the model parameters are adjusted in an unconstrained manner, there is a tendency to produce models that predict unexpected radial variations in quantities such as scale height. This finding suggests that to constrain these models adequately one needs data that extends significantly beyond the solar cylinder. The models presented in this paper might prove useful to the interpretation of data for external galaxies that have been taken with an integral field unit. [ABSTRACT FROM AUTHOR]

The dynamical ages and some other physical parameters of the opposite lobes of 20 subjectively selected giant radio galaxies (GRGs), as well as of their environment, are estimated using the algorithm of Machalski et al. These data, supplemented with the relevant data determined for another 10 GRGs in Paper III, are used to analyse the aspects of the apparent length and luminosity asymmetries in the lobes in view of their dynamical evolution. As in Paper III, the fits give slightly different dynamical ages and other model parameters for the opposite lobes modelled independently of each other. Therefore, demanding similar values of the jet power and the radio core density for the same source, a self-consistent age solution for the opposite lobes is considered, which results in different density profiles along the lobes and different initial energy distributions of the emitting particles at their heads. The analysis shows the following. (i) The age of the longer lobe, fitted independently, tends to be younger than that for the shorter lobe. This contradicts with the axial expansion expected in a symmetrical environment (a halo) surrounding the source and strongly supports the self-consistent solution with different ambient density profiles along the opposite lobes. In the frame of the latter solution, the difference between the ages of the opposite lobes almost always diminishes, compared to that found with the independent fits, not exceeding a factor of ∼1.25. (ii) There is a clear correlation between the age ratio and the difference in radio spectra: longer lobes with a steeper spectrum in most cases are older; however, longer lobes having a flatter spectrum are mostly younger. The age difference is always larger than that expected from the unknown inclination of the jets' axis to the observer's line of sight and a pure kinematic effect. (iii) The slope of the injected (initial) radio spectrum, , has a narrow distribution around a value of 0.5. Its value tends to be lower in the longer lobe than in the shorter lobe. The difference between the values of in the opposite lobes is very low; a standard deviation of ∼0.02 from the mean is found. This difference increases about three times in the self-consistent solution, but always within the relatively narrow range of 0.46-0.62. (iv) There is a strong correlation between the difference in values of β (the exponent describing the external density profile) for the opposite lobes and the ratio of their volume. This is much stronger than a dependence of on the ratio of the lobes' length. This likely suggests that the environment of the GRGs is non-uniform or clumpy. [ABSTRACT FROM AUTHOR]

Merging galaxy systems provide observational evidence of the existence of dark matter and constraints on its properties. Therefore, statistically uniform samples of merging systems would be a powerful tool for several studies. In this paper, we present a new methodology for the identification of merging systems and the results of its application to galaxy redshift surveys. We use as a starting point a mock catalogue of galaxy systems, identified using friends-of-friends algorithms, that have experienced a major merger, as indicated by its merger tree. By applying machine learning techniques in this training sample, and using several features computed from the observable properties of galaxy members, it is possible to select galaxy groups that have a high probability of having experienced a major merger. Next, we apply a mixture of Gaussian techniques on galaxy members in order to reconstruct the properties of the haloes involved in such mergers. This methodology provides a highly reliable sample of merging systems with low contamination and precisely recovered properties. We apply our techniques to samples of galaxy systems obtained from the Sloan Digital Sky Survey Data Release 7, the Wide-Field Nearby Galaxy-Cluster Survey (WINGS) and the Hectospec Cluster Survey (HeCS). Our results recover previously known merging systems and provide several new candidates. We present their measured properties and discuss future analysis on current and forthcoming samples. [ABSTRACT FROM AUTHOR]

The detection and characterization of filamentary structures in the cosmic web allows cosmologists to constrain parameters that dictate the evolution of the Universe. While many filament estimators have been proposed, they generally lack estimates of uncertainty, reducing their inferential power. In this paper, we demonstrate how one may apply the subspace constrained mean shift (SCMS) algorithm (Ozertem & Erdogmus 2011; Genovese et al. 2014) to uncover filamentary structure in galaxy data. The SCMS algorithm is a gradient ascent method that models filaments as density ridges, one-dimensional smooth curves that trace high-density regions within the point cloud. We also demonstrate how augmenting the SCMS algorithm with bootstrap-based methods of uncertainty estimation allows one to place uncertainty bands around putative filaments.We apply the SCMS first to the data set generated from the Voronoi model. The density ridges show strong agreement with the filaments from Voronoi method.We then apply the SCMS method data sets sampled from a P3M N-body simulation, with galaxy number densities consistent with SDSS and WFIRST-AFTA, and to LOWZ and CMASS data from the Baryon Oscillation Spectroscopic Survey (BOSS). To further assess the efficacy of SCMS, we compare the relative locations of BOSS filaments with galaxy clusters in the redMaPPer catalogue, and find that redMaPPer clusters are significantly closer (with p-values <10-9) to SCMS-detected filaments than to randomly selected galaxies. [ABSTRACT FROM AUTHOR]

ABSTRACT Machine learning, algorithms designed to extract empirical knowledge from data, can be used to classify data, which is one of the most common tasks in observational astronomy. In this paper, we focus on Bayesian data classification algorithms using the Gaussian mixture model and show two applications in pulsar astronomy. After reviewing the Gaussian mixture model and the related expectation-maximization algorithm, we present a data classification method using the Neyman-Pearson test. To demonstrate the method, we apply the algorithm to two classification problems. First, it is applied to the well-known period-period derivative diagram, where we find that the pulsar distribution can be modelled with six Gaussian clusters, with two clusters for millisecond pulsars (recycled pulsars) and the rest for normal pulsars. From this distribution, we derive an empirical definition for millisecond pulsars as [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, we present a catalogue of dense molecular clumps located within the W51 giant molecular cloud (GMC). This work is based on Heterodyne Array Receiver Programme 13CO J = 3-2 observations of the W51 GMC and uses the automated CLUMPFIND algorithm to decompose the region into a total of 1575 clumps of which 1130 are associated with the W51 GMC. We clearly see the distinct structures of the W51 complex and the high-velocity stream previously reported. We find the clumps have characteristic diameters of 1.4 pc, excitation temperatures of 12 K, densities of 5.6 × 1021 cm−2, surface densities 0.02 g cm−2 and masses of 90 M⊙. We find a total mass of dense clumps within the GMC of 1.5 × 105 M⊙, with only 1 per cent of the clumps detected by number and 4 per cent by mass found to be supercritical. We find a clump-forming efficiency of 14 ± 1 per cent for the W51 GMC and a supercritical clump-forming efficiency of [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, we set up a procedure that aims at improving the orbits of the known near-Earth asteroids (NEAs) by making use of the One Night Stands (ONS) file distributed by the Minor Planet Center. The identification algorithm employed is of the observation attribution type and takes into account the higher apparent motion exhibited by this kind of asteroids. The application of this algorithm together with the ONS file is not straightforward, as in practice such a file presents several problems that must be managed prior to the massive treatment of the information contained in it. We include a three-step protocol to handle these drawbacks. Finally, we present a detailed analysis of the results obtained with this procedure, the main conclusion being that more than 80 NEA orbits have been improved. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a catalogue of about six million unresolved photometric detections in the Sloan Digital Sky Survey (SDSS) Seventh Data Release, classifying them into stars, galaxies and quasars. We use a machine learning classifier trained on a subset of spectroscopically confirmed objects from 14th to 22nd magnitude in the SDSS i band. Our catalogue consists of 2 430 625 quasars, 3 544 036 stars and 63 586 unresolved galaxies from 14th to 24th magnitude in the SDSS i band. Our algorithm recovers 99.96 per cent of spectroscopically confirmed quasars and 99.51 per cent of stars to i ∼ 21.3 in the colour window that we study. The level of contamination due to data artefacts for objects beyond i = 21.3 is highly uncertain and all mention of completeness and contamination in the paper are valid only for objects brighter than this magnitude. However, a comparison of the predicted number of quasars with the theoretical number counts shows reasonable agreement. [ABSTRACT FROM AUTHOR]

ABSTRACT In this third paper of the series, we continue to investigate the transition between atomic and molecular gas in two potential sites of molecule formation at high Galactic latitude, which we call the Spider and Ursa Major. Using Five College Radio Astronomical Observatory 12CO ( J= 1-0) observations and four clump identification algorithms, the clump properties are first determined in both regions, showing similar properties compared to the clumps found in translucent clouds. New higher resolution 12CO ( J= 1-0) and 13CO ( J= 1-0) observations from the Institut de Radioastronomie Millimétrique and new 12CO ( J= 2-1) observations from the James Clerk Maxwell Telescope (resolution ≈20 arcsec) are also presented in five fields for the Spider (560 spectra) and in two fields for Ursa Major (288 spectra). Using a large velocity gradient model, physical parameters (density and column density) are determined in each field. The densities are smaller at the location of the infrared excess peaks (<200 cm−3) than at the locations of the 12CO peaks (≈103 cm−3), confirming that a small density could explain the absence of coincidence between the infrared excess peak and the 12CO peak. Self-shielding is probably efficient given the computed 12CO column densities (≈1015 cm−2). [ABSTRACT FROM AUTHOR]

The recently introduced discrete persistent structure extractor (DisPerSE, Sousbie, Paper I) is implemented on realistic 3D cosmological simulations and observed redshift catalogues; it is found that DisPerSE traces very well the observed filaments, walls and voids seen both in simulations and in observations. In either setting, filaments are shown to connect on to haloes, outskirt walls, which circumvent voids, as is topologically required by the Morse theory. Indeed this algorithm returns the optimal critical set while operating directly on the particles. DisPerSE, as illustrated here, assumes nothing about the geometry of the survey or its homogeneity, and yields a natural (topologically motivated) self-consistent criterion for selecting the significance level of the identified structures. It is shown that this extraction is possible even for very sparsely sampled point processes, as a function of the persistence ratio (a measure of the significance of topological connections between critical points). Hence, astrophysicists should be in a position to trace precisely the locus of filaments, walls and voids from such samples and assess the confidence of the post-processed sets as a function of this threshold, which can be expressed relative to the expected amplitude of shot noise. In a cosmic framework, this criterion is shown to level with the friends-of-friends structure finder for the identification of peaks, while it also identifies the connected filaments and walls, and quantitatively recovers the full set of topological invariants (number of holes, etc.) directly from the particles, and at no extra cost as a function of the persistence threshold. This criterion is found to be sufficient even if one particle out of two is noise, when the persistence ratio is set to or more. The algorithm is also implemented on the SDSS catalogue and used to locate interesting configurations of the filamentary structure. In this context, we carried the identification of an 'optically faint' cluster at the intersection of filaments through the recent observation of its X-ray counterpart by Suzaku. [ABSTRACT FROM AUTHOR]

Radio transients are sporadic signals, requiring that the backends of radio telescopes be equipped with the appropriate hardware and software for their detection. Observational programmes for detecting transients can be dedicated to that purpose or can rely upon observations made by other programmes. It is the single-dish single-transient (non-periodical) mode which is considered in this paper. Because neither the width of a transient nor the time of its arrival is known, a sequential analysis in the form of a cumulative sum (cusum) algorithm is proposed here. Computer simulations and real observation data processing are included to demonstrate the performance of the cusum. The use of the Hough transform is proposed here for the purpose of non-coherent de-dispersion. It is possible that the detected transients could be radio frequency interferences (RFI), and a procedure is proposed here which can distinguish between celestial signals and man-made RFI. This procedure is based on an analysis of the statistical properties of the signals. [ABSTRACT FROM AUTHOR]

With increasingly large data sets, weak lensing measurements are able to measure cosmological parameters with ever-greater precision. However, this increased accuracy also places greater demands on the statistical tools used to extract the available information. To date, the majority of lensing analyses use the two-point statistics of the cosmic shear field. These can be either studied directly using the two-point correlation function or in Fourier space, using the power spectrum. But analysing weak lensing data inevitably involves the masking out of regions, for example to remove bright stars from the field. Masking out the stars is common practice but the gaps in the data need proper handling. In this paper, we show how an inpainting technique allows us to properly fill in these gaps with only operations, leading to a new image from which we can compute straightforwardly and with a very good accuracy both the power spectrum and the bispectrum. We then propose a new method to compute the bispectrum with a polarfft algorithm, which has the main advantage of avoiding any interpolation in the Fourier domain. Finally, we propose a new method for dark matter mass map reconstruction from shear observations, which integrates this new inpainting concept. A range of examples based on 3D N-body simulations illustrates the results. [ABSTRACT FROM AUTHOR]