arXiv subjects

Siri Chongchitnan

Publications and source records attributed to Siri Chongchitnan.

11 recordsLinked to original sources

arXiv2026

Extreme Values of Black Hole to Stellar Mass Ratio for High-Redshift Galaxies

With recent data from the \emph{James Webb Space Telescope} (JWST), it is possible to calculate the mass of the supermassive black holes at the center of galaxies, and the stellar mass of the host galaxies at redshift $z \gtrsim 5$. In this work, we apply extreme-value statistics to calculate the distributions of extreme black-hole and stellar masses for galaxies in the redshift range $3 \lesssim z \lesssim 8$. We show that under certain assumptions about the stellar and black-hole mass functions, a high ratio of $M_\text{BH}/M_*\sim0.3-0.5$ can be obtained without invoking additional black-hole growth physics. Nevertheless, surveying a range of extreme-value methodologies, we find predictions of the extreme ratio $M_\text{BH}/M_*$ to still be in slight tension with the high values observed by JWST.

astro-ph.GA↗

arXiv2025

The brightest X-ray AGNs at redshift $3\lesssim z \lesssim 6$

Given recent X-ray observations of high-redshift active galactic nuclei (AGNs), we consider whether the extreme luminosities of these AGNs are consistent with current semi-analytical models. In particular, we apply extreme-value statistics (EVS) to obtain predictions of extreme X-ray luminosities of AGNs in the redshift range $3\lesssim z\lesssim 6$. We apply this formalism using different X-ray luminosity functions and compare the predicted extreme luminosities to AGNs in the Stripe 82 X-ray catalogue. We find a general consistency between data and the EVS predictions although there is some tension with certain luminosity functions. We discuss possible extensions to this model, including extrapolating our results to even higher redshifts ($z\gtrsim10$) where AGNs have recently been observed.

astro-ph.GA↗

arXiv2024

Extreme-value modelling of the brightest galaxies at $z\gtrsim9$

Data from the James Webb Space Telescope have revealed an intriguing population of bright galaxies at high redshifts. In this work, we use extreme-value statistics to calculate the distribution (in UV magnitude) of the brightest galaxies in the redshift range $9 \lesssim z \lesssim 16$. We combine the Generalised Extreme Value (GEV) approach with modelling of the galaxy luminosity function. We obtain predictions of the brightest galaxies for a suite of luminosity functions, including the Schechter and double power law functions, as well as a model parametrised by the stellar formation efficiency $f_*$. We find that the JWST data is broadly consistent with $f_*$ of $5\%-10\%$, and that the brightest galaxy at $z\sim16$ will have $M_{\rm UV}\approx -23.5^{0.8}_{0.4}$. If $f_*$ is dependent on halo mass, we predict $M_{\rm UV}\approx -22.5^{0.5}_{1.5}$ for such an object. We show that extreme-value statistics not only predicts the magnitude of the brightest galaxies at high redshifts, but may also be able to distinguish between models of star formation in high-redshift galaxies.

astro-ph.GA↗

arXiv2023

The most massive Population III stars

Recent data from the James Webb Space Telescope suggest that there are realistic prospects for detecting the earliest generation of stars at redshift ~20. These metal-poor, gaseous Population III (Pop III) stars are likely in the mass range 10-1000 solar masses. We develop a framework for calculating the abundances of Pop III stars as well as the distribution of the most massive Pop III stars based on an application of extreme-value statistics. Our calculations use the star formation rate density from a recent simulation to calibrate the star-formation efficiency from which the Pop III stellar abundances are derived. Our extreme-value modelling suggests that the most massive Pop III stars at redshifts 10-20 are likely to be $\gtrsim10^3-10^4\,{\rm M}_\odot$. Such extreme Pop III stars were sufficiently numerous to be the seeds of supermassive black holes at high redshifts and possibly source detectable gravitational waves. We conclude that the extreme-value formalism provides an effective way to constrain the stellar initial mass function.

astro-ph.SR↗

arXiv2021

Extreme-Value Statistics of the Spin of Primordial Black Holes

How rare are extreme-spin primordial black holes? We show how, from an underlying distribution of PBH spin, extreme-value statistics can be used to quantify the rarity of spinning PBHs with Kerr parameter close to 1. Using the Peaks-Over-Threshold method, we show how the probability that a PBH forms with spin exceeding a sufficiently high threshold can be calculated using the Generalised Pareto Distribution. This allows us to estimate the average number of PBHs amongst which we can find a single PBH which formed with spin exceeding a high threshold. We found that the primordial spin distribution gives rise to exceedingly rare near-extremal spin PBHs at formation time: for typical parameter values, roughly up to one in a hundred million PBHs would be formed with spin exceeding the Thorne limit. We discuss conditions under which even more extreme-spin PBHs may be produced, including modifying the skewness and kurtosis of the spin distribution via a smooth transformation. We deduce from our calculations that, if indeed asteroid-mass PBHs above the current observational limit on evaporating PBHs of mass ~10^{17} g contribute significantly to the dark matter, it is likely that some of them could be near-extremal PBHs.

astro-ph.CO↗

arXiv2021

Extreme Primordial Black Holes

We present a formalism for calculating the probability distribution of the most massive primordial black holes (PBHs) expected within an observational volume. We show how current observational upper bounds on the fraction of PBHs in dark matter translate to constraints on extreme masses of primordial black holes. We demonstrate the power of our formalism via a case study, and argue that our formalism can be used to produce extreme-value distributions for a wide range of PBH formation theories.

astro-ph.CO↗

arXiv2021

Project ThaiPASS: International Outreach Blending Astronomy and Python

$\textit{Thailand-UK Python+Astronomy Summer School}$ (ThaiPASS), a collaborative project comprising UK and Thai institutions and assess its impact and possible application to schools in the United Kingdom. Since its inception in 2018, the annual ThaiPASS have trained around 60 Thai high-school students in basic data handling skills using Python in the context of various astronomy topics, using current research from the teaching team. Our impact assessment of the 5 day summer schools show an overwhelmingly positive response from students in both years, with over $80\%$ of students scoring the activities above average in all activities but one. We use this data to suggest possible future improvements. We also discuss how ThaiPASS may inspire further outreach and engagement activities within the UK and beyond.

physics.ed-ph↗

arXiv2019

Electronic Preparatory Test for Mathematics Undergraduates: Implementation, Results and Correlations

We present a study of the implementation of the Electronic Preparatory Test for beginning undergraduates reading mathematics. The Test comprises two elements: diagnostic and self-learning. The diagnostic element identifies gaps in the background knowledge, whilst the self-learning element guides students through an upcoming material. The Test lends itself to an early identification of weak and strong students coming from a wide range of background, allowing follow-ups to be made on a topic-specific basis. The results from the Tests, collected over three years, correlate positively with end-of-year examination results. We show that such a Preparatory Test can be a better predictor of success in the first-year examination in comparison with university entry qualifications alone.

math.HO↗

arXiv2018

Primordial Gravitational Waves and Reheating in a New Class of Plateau-like Inflationary Potentials

We study a new class of inflation model parametrized by the Hubble radius, such that $aH\propto \exp(-kϕ^n)$. These potentials are plateau-like, and reduce to the power-law potentials in the simplest case $n=2$. We investigate the range of model parameters that is consistent with current observational constraints on the scalar spectral index and the tensor-to-scalar ratio. The amplitude of primordial gravitational waves in these models is shown to be accessible by future laser interferometers such as DECIGO. We also demonstrate how these observables are affected by the temperature and equation of state during reheating. We find that a large subset of this model can support instantaneous reheating, as well as very low reheating temperatures of order a few MeV, giving rise to interesting consequences for dark-matter production.

astro-ph.CO↗

arXiv2017

Inflationary e-folding and the implications for gravitational-wave detection

We demonstrate that the approximation for the number of inflationary e-folds commonly used in the literature can lead to highly inaccurate predictions for the amplitude of primordial gravitational waves. We show that such an approximation can lead to perfectly viable inflation models being falsely ruled out by direct or indirect gravitational-wave measurements. We illustrate this point using a new class of inflation models which include the power-law potential as the simplest limit. These models are simple to construct without using the slow-roll approximation, and are consistent with constraints from Planck. Crucially, these models may suffer from an order-of-magnitude error in the prediction for the gravitational-wave amplitude if the common definition of e-folding is used. Our findings have strong implications for the classes of inflation models that can be ruled out by future space-based laser interferometers such as BBO and DECIGO.

astro-ph.CO↗

arXiv2017

On the Abundance of Extreme Voids II: A Survey of Void Mass Functions

The abundance of cosmic voids can be described by an analogue of halo mass functions for galaxy clusters. In this work, we explore a number of void mass functions: from those based on excursion-set theory to new mass functions obtained by modifying halo mass functions. We show how different void mass functions vary in their predictions for the largest void expected in an observational volume, and compare those predictions to observational data. Our extreme-value formalism is shown to be a new practical tool for testing void theories against simulation and observation.

astro-ph.CO↗