18 research outputs found
Searching for globular cluster chemical anomalies on the main sequence of a young massive cluster
The spectroscopic and photometric signals of the star-to-star abundance variations found in globular clusters seem to be correlated with global parameters like the cluster’s metallicity, mass, and age. Understanding this behaviour could bring us closer to the origin of these intriguing abundance spreads. In this work we use deep HST photometry to look for evidence of abundance variations in the main sequence of a young massive cluster NGC 419 (∼105 M⊙, ∼1.4 Gyr). Unlike previous studies, here we focus on stars in the same mass range found in old globulars (∼0.75–1 M⊙), where light elements variations are detected. We find no evidence for N abundance variations among these stars in the Un − B and U − B colour–magnitude diagrams of NGC 419. This is at odds with the N variations found in old globulars like 47 Tuc, NGC 6352, and NGC 6637 with similar metallicity to NGC 419. Although the signature of the abundance variations characteristic of old globulars appears to be significantly smaller or absent in this young cluster, we cannot conclude if this effect is mainly driven by its age or its mass
The Interstellar Medium In Galaxies Seen A Billion Years After The Big Bang
Evolution in the measured rest frame ultraviolet spectral slope and
ultraviolet to optical flux ratios indicate a rapid evolution in the dust
obscuration of galaxies during the first 3 billion years of cosmic time (z>4).
This evolution implies a change in the average interstellar medium properties,
but the measurements are systematically uncertain due to untested assumptions,
and the inability to measure heavily obscured regions of the galaxies. Previous
attempts to directly measure the interstellar medium in normal galaxies at
these redshifts have failed for a number of reasons with one notable exception.
Here we report measurements of the [CII] gas and dust emission in 9 typical
(~1-4L*) star-forming galaxies ~1 billon years after the big bang (z~5-6). We
find these galaxies have >12x less thermal emission compared with similar
systems ~2 billion years later, and enhanced [CII] emission relative to the
far-infrared continuum, confirming a strong evolution in the interstellar
medium properties in the early universe. The gas is distributed over scales of
1-8 kpc, and shows diverse dynamics within the sample. These results are
consistent with early galaxies having significantly less dust than typical
galaxies seen at z<3 and being comparable to local low-metallicity systems.Comment: Submitted to Nature, under review after referee report. 22 pages, 4
figures, 4 Extended Data Figures, 5 Extended Data table
Lensing without borders. I. A blind comparison of the amplitude of galaxy-galaxy lensing between independent imaging surveys
Lensing Without Borders is a cross-survey collaboration created to assess the consistency of galaxy-galaxy lensing signals (ΔΣ) across different data-sets and to carry out end-to-end tests of systematic errors. We perform a blind comparison of the amplitude of ΔΣ using lens samples from BOSS and six independent lensing surveys. We find good agreement between empirically estimated and reported systematic errors which agree to better than 2.3σ in four lens bins and three radial ranges. For lenses with zL &gt; 0.43 and considering statistical errors, we detect a 3-4σ correlation between lensing amplitude and survey depth. This correlation could arise from the increasing impact at higher redshift of unrecognised galaxy blends on shear calibration and imperfections in photometric redshift calibration. At zL &gt; 0.54 amplitudes may additionally correlate with foreground stellar density. The amplitude of these trends is within survey-defined systematic error budgets which are designed to include known shear and redshift calibration uncertainty. Using a fully empirical and conservative method, we do not find evidence for large unknown systematics. Systematic errors greater than 15 per cent (25 per cent) ruled out in three lens bins at 68 per cent (95 per cent) confidence at z &lt; 0.54. Differences with respect to predictions based on clustering are observed to be at the 20-30 per cent level. Our results therefore suggest that lensing systematics alone are unlikely to fully explain the ‘lensing is low’ effect at z &lt; 0.54. This analysis demonstrates the power of cross-survey comparisons and provides a promising path for identifying and reducing systematics in future lensing analyses
Euclid preparation: X. The Euclid photometric-redshift challenge
Forthcoming large photometric surveys for cosmology require precise and accurate photometric redshift (photo-z) measurements for the success of
their main science objectives. However, to date, no method has been able to produce photo-zs at the required accuracy using only the broad-band
photometry that those surveys will provide. An assessment of the strengths and weaknesses of current methods is a crucial step in the eventual
development of an approach to meet this challenge. We report on the performance of 13 photometric redshift code single value redshift estimates
and redshift probability distributions (PDZs) on a common set of data, focusing particularly on the 0.2−2.6 redshift range that the Euclid mission
will probe. We designed a challenge using emulated Euclid data drawn from three photometric surveys of the COSMOS field. The data was
divided into two samples: one calibration sample for which photometry and redshifts were provided to the participants; and the validation sample,
containing only the photometry to ensure a blinded test of the methods. Participants were invited to provide a redshift single value estimate and
a PDZ for each source in the validation sample, along with a rejection flag that indicates the sources they consider unfit for use in cosmological
analyses. The performance of each method was assessed through a set of informative metrics, using cross-matched spectroscopic and highlyaccurate photometric redshifts as the ground truth. We show that the rejection criteria set by participants are efficient in removing strong outliers,
that is to say sources for which the photo-z deviates by more than 0.15(1 + z) from the spectroscopic-redshift (spec-z). We also show that, while
all methods are able to provide reliable single value estimates, several machine-learning methods do not manage to produce useful PDZs. We find
that no machine-learning method provides good results in the regions of galaxy color-space that are sparsely populated by spectroscopic-redshifts,
for example z > 1. However they generally perform better than template-fitting methods at low redshift (z < 0.7), indicating that template-fitting
methods do not use all of the information contained in the photometry. We introduce metrics that quantify both photo-z precision and completeness
of the samples (post-rejection), since both contribute to the final figure of merit of the science goals of the survey (e.g., cosmic shear from Euclid).
Template-fitting methods provide the best results in these metrics, but we show that a combination of template-fitting results and machine-learning
results with rejection criteria can outperform any individual method. On this basis, we argue that further work in identifying how to best select
between machine-learning and template-fitting approaches for each individual galaxy should be pursued as a priority
Observations of Ly Emitters at High Redshift
In this series of lectures, I review our observational understanding of
high- Ly emitters (LAEs) and relevant scientific topics. Since the
discovery of LAEs in the late 1990s, more than ten (one) thousand(s) of LAEs
have been identified photometrically (spectroscopically) at to . These large samples of LAEs are useful to address two major astrophysical
issues, galaxy formation and cosmic reionization. Statistical studies have
revealed the general picture of LAEs' physical properties: young stellar
populations, remarkable luminosity function evolutions, compact morphologies,
highly ionized inter-stellar media (ISM) with low metal/dust contents, low
masses of dark-matter halos. Typical LAEs represent low-mass high- galaxies,
high- analogs of dwarf galaxies, some of which are thought to be candidates
of population III galaxies. These observational studies have also pinpointed
rare bright Ly sources extended over kpc, dubbed
Ly blobs, whose physical origins are under debate. LAEs are used as
probes of cosmic reionization history through the Ly damping wing
absorption given by the neutral hydrogen of the inter-galactic medium (IGM),
which complement the cosmic microwave background radiation and 21cm
observations. The low-mass and highly-ionized population of LAEs can be major
sources of cosmic reionization. The budget of ionizing photons for cosmic
reionization has been constrained, although there remain large observational
uncertainties in the parameters. Beyond galaxy formation and cosmic
reionization, several new usages of LAEs for science frontiers have been
suggested such as the distribution of {\sc Hi} gas in the circum-galactic
medium and filaments of large-scale structures. On-going programs and future
telescope projects, such as JWST, ELTs, and SKA, will push the horizons of the
science frontiers.Comment: Lecture notes for `Lyman-alpha as an Astrophysical and Cosmological
Tool', Saas-Fee Advanced Course 46. Verhamme, A., North, P., Cantalupo, S., &
Atek, H. (eds.) --- 147 pages, 103 figures. Abstract abridged. Link to the
lecture program including the video recording and ppt files :
https://obswww.unige.ch/Courses/saas-fee-2016/program.cg
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Stochastic prior for non-parametric star-formation histories
ABSTRACT
The amount of power contained in the variations in galaxy star-formation histories (SFHs) across a range of time-scales encodes key information about the physical processes which modulate star formation. Modelling the SFHs of galaxies as stochastic processes allows the relative importance of different time-scales to be quantified via the power spectral density (PSD). In this paper, we build upon the PSD framework and develop a physically motivated, ‘stochastic’ prior for non-parametric SFHs in the spectral energy distribution (SED)-modelling code prospector. We test this prior in two different regimes: (1) massive, galaxies with both photometry and spectra, analogous to those observed with the LEGA-C survey, and (2) galaxies with photometry only, analogous to those observed with NIRCam on JWST. We find that it is able to recover key galaxy parameters (e.g. stellar mass, stellar metallicity) to the same level of fidelity as the commonly used continuity prior. Furthermore, the realistic variability information incorporated by the stochastic SFH model allows it to fit the SFHs of galaxies more accurately and precisely than traditional non-parametric models. In fact, the stochastic prior is more accurate than the continuity prior in measuring the recent star-formation rates (log SFR and log SFR) of both the and mock systems. While the PSD parameters of individual galaxies are difficult to constrain, the stochastic prior implementation presented in this work allows for the development of hierarchical models in the future, i.e. simultaneous SED-modelling of an ensemble of galaxies to measure their underlying PSD.</jats:p
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The outer stellar mass of massive galaxies: a simple tracer of halo mass with scatter comparable to richness and reduced projection effects
Using the weak gravitational lensing data from the Hyper Suprime-Cam Subaru Strategic Program (HSC survey), we study the potential of different stellar mass estimates in tracing halo mass. We consider galaxies with log10(M∗/M⊙) > 11.5 at 0.2 < z < 0.5 with carefully measured light profiles, and clusters from the redMaPPer and CAMIRA richness-based algorithms. We devise a method (the 'Top-N test') to evaluate the scatter in the halo mass-observable relation for different tracers, and to inter-compare halo mass proxies in four number density bins using stacked galaxy-galaxy lensing profiles. This test reveals three key findings. Stellar masses based on CModel photometry and aperture luminosity within R <30 kpc are poor proxies of halo mass. In contrast, the stellar mass of the outer envelope is an excellent halo mass proxy. The stellar mass within R = [50, 100] kpc, M∗, [50, 100], has performance comparable to the state-of-the-art richness-based cluster finders at log10Mvir ⪎ 14.0 and could be a better halo mass tracer at lower halo masses. Finally, using N-body simulations, we find that the lensing profiles of massive haloes selected by M∗, [50, 100] are consistent with the expectation for a sample without projection or mis-centring effects. Richness-selected clusters, on the other hand, display an excess at R ∼1 Mpc in their lensing profiles, which may suggest a more significant impact from selection biases. These results suggest that M∗-based tracers have distinct advantages in identifying massive haloes, which could open up new avenues for cluster cosmology. The codes and data used in this work can be found here
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As Simple as Possible but No Simpler: Optimizing the Performance of Neural Net Emulators for Galaxy SED Fitting
Abstract
Artificial neural network emulators have been demonstrated to be a very computationally efficient method to rapidly generate galaxy spectral energy distributions, for parameter inference or otherwise. Using a highly flexible and fast mathematical structure, they can learn the nontrivial relationship between input galaxy parameters and output observables. However, they do so imperfectly, and small errors in flux prediction can yield large differences in recovered parameters. In this work, we investigate the relationship between an emulator’s execution time, uncertainties, correlated errors, and ability to recover accurate posteriors. We show that emulators can recover consistent results to traditional fits, with a precision of 25%–40% in posterior medians for stellar mass, stellar metallicity, star formation rate, and stellar age. We find that emulation uncertainties scale with an emulator’s width N as ∝N
−1, while execution time scales as ∝N
2, resulting in an inherent tradeoff between execution time and emulation uncertainties. We also find that emulators with uncertainties smaller than observational uncertainties are able to recover accurate posteriors for most parameters without a significant increase in catastrophic outliers. Furthermore, we demonstrate that small architectures can produce flux residuals that have significant correlations, which can create dangerous systematic errors in colors. Finally, we show that the distributions chosen for generating training sets can have a large effect on an emulator’s ability to accurately fit rare objects. Selecting the optimal architecture and training set for an emulator will minimize the computational requirements for fitting near-future large-scale galaxy surveys. We release our emulators on GitHub (http://github.com/elijahmathews/MathewsEtAl2023).</jats:p
Nested sampling for physical scientists
We review Skilling's nested sampling (NS) algorithm for Bayesian inference
and more broadly multi-dimensional integration. After recapitulating the
principles of NS, we survey developments in implementing efficient NS
algorithms in practice in high-dimensions, including methods for sampling from
the so-called constrained prior. We outline the ways in which NS may be applied
and describe the application of NS in three scientific fields in which the
algorithm has proved to be useful: cosmology, gravitational-wave astronomy, and
materials science. We close by making recommendations for best practice when
using NS and by summarizing potential limitations and optimizations of NS