☆ 4.7 Article

Deriving photometric redshifts using fuzzy archetypes and self-organizing maps - I. Methodology

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY (2017)

Journal

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

Volume 469, Issue 1, Pages 1186-1204

Publisher

OXFORD UNIV PRESS

DOI: 10.1093/mnras/stw1485

Keywords

methods: statistical; techniques: photometric; galaxies: distances and redshifts

Categories

Astronomy & Astrophysics

Funding

CREST programme
FAS Division of Science's Research Computing Group
Japan Science and Technology (JST)

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Abstract

We propose a method to substantially increase the flexibility and power of template fitting-based photometric redshifts by transforming a large number of galaxy spectral templates into a corresponding collection of 'fuzzy archetypes' using a suitable set of perturbative priors designed to account for empirical variation in dust attenuation and emission-line strengths. To bypass widely separated degeneracies in parameter space (e.g. the redshift-reddening degeneracy), we train self-organizing maps (SOMs) on large 'model catalogues' generated from Monte Carlo sampling of our fuzzy archetypes to cluster the predicted observables in a topologically smooth fashion. Subsequent sampling over the SOM then allows full reconstruction of the relevant probability distribution functions (PDFs). This combined approach enables the multimodal exploration of known variation among galaxy spectral energy distributions with minimal modelling assumptions. We demonstrate the power of this approach to recover full redshift PDFs using discrete Markov chain Monte Carlo sampling methods combined with SOMs constructed from Large Synoptic Survey Telescope ugrizY and Euclid YJH mock photometry.

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The Stellar Halo of the Galaxy is Tilted and Doubly Broken

Jiwon Jesse Han, Charlie Conroy, Benjamin D. Johnson, Joshua S. Speagle, Ana Bonaca, Vedant Chandra, Rohan P. Naidu, Yuan-Sen Ting, Turner Woody, Dennis Zaritsky

Summary: Modern Galactic surveys have identified an ancient merger, known as Gaia-Sausage-Enceladus (GSE), that dominates the stellar halo of our galaxy. Through the use of chemical abundances and kinematics, researchers have inferred the density profile of the accreted component of the stellar halo and discovered a triaxial ellipsoid tilted 25 degrees above the Galactic plane. This finding provides new insights into the initial conditions and evolution of the GSE merger, as well as implications for the dynamical mass modeling of the galaxy and dark matter detection experiments.

ASTRONOMICAL JOURNAL (2022)

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Article Astronomy & Astrophysics

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II. Spectroscopic and Environmental Analysis of Thorne-Zytkow Object and Super-AGB Star Candidates

Anna J. G. O'Grady, Maria R. Drout, B. M. Gaensler, C. S. Kochanek, Kathryn F. Neugent, Carolyn L. Doherty, Joshua S. Speagle, B. J. Shappee, Michael Rauch, Ylva Gotberg, Bethany Ludwig, Todd A. Thompson

Summary: In this study, a population of 38 cool and luminous variable stars in the Magellanic Clouds was investigated, and 11 of them were examined in detail to classify them as either Thorne-Zytkow objects or super-asymptotic giant branch stars. These stars are not associated with recent star formation and there is no evidence of past supernovae at their locations.

ASTROPHYSICAL JOURNAL (2023)

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Article Astronomy & Astrophysics

A Ghost in Bootes: The Least-Luminous Disrupted Dwarf Galaxy

Vedant Chandra, Charlie Conroy, Nelson Caldwell, Ana Bonaca, Rohan P. P. Naidu, Dennis Zaritsky, Phillip A. A. Cargile, Jiwon Jesse Han, Benjamin D. D. Johnson, Joshua S. S. Speagle, Yuan-Sen Ting, Turner Woody

Summary: We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy. It is a stream of stars that can only be detected through strict kinematic cuts. Spectroscopic analysis suggests that it is the remnant of an ancient dwarf galaxy. Specter has an extremely low luminosity and there may be similar streams yet to be discovered.

ASTROPHYSICAL JOURNAL (2022)

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Article Astronomy & Astrophysics

The HERBAL Model: A Hierarchical Errors-in-variables Bayesian Lognormal Hurdle Model for Galactic Globular Cluster Populations

Samantha C. Berek, Gwendolyn M. Eadie, Joshua S. Speagle, William E. Harris

Summary: The relationship between the stellar mass of dwarf galaxies and the size of their globular cluster populations becomes ambiguous for smaller dwarfs, as there is evidence for a downturn in population size. The HERBAL model accurately represents this relationship and accounts for all uncertainties. The hierarchical Bayesian model also allows for estimation of galaxy masses and mass-to-light ratios.

ASTROPHYSICAL JOURNAL (2023)

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Article Astronomy & Astrophysics

SBI++: Flexible, Ultra-fast Likelihood-free Inference Customized for Astronomical Applications

Bingjie Wang, Joel Leja, V. Ashley Villar, Joshua S. Speagle

Summary: Flagship surveys in the near future will provide unprecedented resolution on galaxy assembly processes, but this poses a computational challenge. A simulation-based inference method, SBI (++), is proposed to handle out-of-distribution errors and missing data in astronomical surveys. SBI (++) can accurately infer photometric redshifts and significantly improve inference speed.

ASTROPHYSICAL JOURNAL LETTERS (2023)

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