arXiv subjects

Sahil Hegde

Publications and source records attributed to Sahil Hegde.

8 recordsLinked to original sources

arXiv2026

Searching for Population III stars with line intensity mapping cross-correlations

Decades of searches for Population III stars in individual galaxies have yielded a few potential candidates, but a statistically robust characterization of the demographics of the first stars in the lowest-mass systems remains elusive. Line intensity mapping (LIM), an observational technique that measures fluctuations in the aggregate emission from the entire galaxy population --- including the faintest sources --- offers an alternative strategy that is especially well-suited for the Pop III era. With the recent launch of SPHEREx and the rapid development of a number of complementary LIM studies, we are poised to place some of the first LIM constraints on sites of star formation at high-redshift. In this work, we expand an analytical model for LIM power spectra, Zeus21/oLIMpus, to include Pop III stars and the emission lines identified as diagnostic signatures of star formation with a low-metallicity, top-heavy IMF, such as H$\alpha$ and HeII. We introduce a flexible framework to estimate the measurement uncertainties associated with instrument and survey configurations, and apply these to study LIM signatures of the first stars in mock surveys carried out with SPHEREx and potential next-generation instruments. We quantify the sensitivity of the LIM signal to variations in Pop II and III parameters and forecast joint limits that can be placed on the Pop III star formation efficiency (SFE) and IMF shape with SPHEREx. We find that next-generation instruments will be necessary to comprehensively survey the Pop III theoretical landscape --- both with respect to `classical' and `exotic' Pop III models --- and identify design improvements that will enable such studies. Finally, we carry out a Fisher analysis to characterize synergies between SPHEREx and JWST, suggesting that joint constraints on the Pop III SFE and extensions to conventional Pop III models may be within reach.

astro-ph.GA↗

arXiv2026

Do we understand the star formation history of the universe?

The evolving relationship between a galaxy's mass and star formation rate -- the so-called `star-forming main sequence' (MS) -- provides a critical benchmark for understanding star formation across time. Despite its fundamental importance, the observed main sequence remains subject to substantial systematic uncertainties in normalization, shape, and redshift evolution, and a longstanding discrepancy persists between the main sequence and its integral, the stellar mass function. We revisit the star-forming MS in the era of the James Webb Space Telescope by asking what star formation rates are required by the stellar mass function to create a self-consistent picture of galaxies across time. We fit compiled ground- and space-based measurements of star-forming and quiescent mass functions from $z=0.1-9$. By tracing galaxy growth histories through these mass functions, we present a statistically-robust inference of the main sequence over $10^8 M_\odot \leq m_\star \leq 10^{11} M_\odot$, from the local universe to the first 500 Myr of cosmic history. Our procedure implies a main sequence that agrees with independent spectroscopic measurements of star formation rates from $z\sim 2-7$, is consistent with SED fitting-based analyses of photometric samples at $z\lesssim 3$, and aligns with theoretical models of galaxy evolution. However, we find that our MS differs from commonly-used `concordance' relations and thus caution against applications of these compilations without appropriately characterizing the underlying uncertainties. Finally, we explore the implications of our inferred main sequence for the galaxy-halo connection and star formation rate density, highlighting the need for further theoretical work to comprehensively understand the star formation history of the universe.

astro-ph.GA↗

arXiv2025

Efficient semi-analytic modelling of Pop III star formation from Cosmic Dawn to Reionization

The quest to find the first stars has driven astronomers across cosmic time, from hopes to identify their signatures in their heyday at cosmic dawn to deep searches for their remnants in our local neighborhood. Such work crucially relies on robust theoretical modelling to understand when and where we expect pristine star formation to have occurred and survived. To that end, here we introduce an analytic bathtub for cosmic dawn, the abcd model, to efficiently trace the formation of the first stars from their birth through the first billion years of our universe's history, jointly following star formation out of pristine and metal-enriched gas over time. Informed by the latest theoretical developments in our understanding of star formation in molecular cooling halos, metal mixing, and early galaxies, we expand pre-existing minimal models for galaxy formation to include Population III stars and many of the processes - both internal and environmental - affecting their evolution, while remaining fast and interpretable. With this framework, we can bridge the gap between numerical simulations and previous semi-analytic models, as we self-consistently follow star formation in dark matter halos from the minihalo era through the epoch of reionization, finding that, under plausible physical conditions, pristine star formation can persist at a high level in the presence of Pop II star formation down to $z\sim 5$, but is limited to the most massive halos. We highlight areas of theoretical uncertainty in the physics underpinning Pop III star formation and demonstrate the effects of this uncertainty first on individual star formation histories and subsequently bracketing the range of global star formation levels we expect. Finally, we leverage this model to make preliminary observable predictions, generating forecasts for high-$z$ luminosity functions, transient rates, and the 21-cm global signal.

astro-ph.GA↗

arXiv2024

A hidden population of active galactic nuclei can explain the overabundance of luminous $z>10$ objects observed by JWST

The first wave of observations with JWST has revealed a striking overabundance of luminous galaxies at early times ($z>10$) compared to models of galaxies calibrated to pre-JWST data. Early observations have also uncovered a large population of supermassive black holes (SMBHs) at $z>6$. Because many of the high-$z$ objects appear extended, the contribution of active galactic nuclei (AGNs) to the total luminosity has been assumed to be negligible. In this work, we use a semi-empirical model for assigning AGNs to galaxies to show that active galaxies can boost the stellar luminosity function (LF) enough to solve the overabundance problem while simultaneously remaining consistent with the observed morphologies of high-$z$ sources. We construct a model for the composite AGN+galaxy LF by connecting dark matter halo masses to galaxy and SMBH masses and luminosities, accounting for dispersion in the mapping between host galaxy and SMBH mass and luminosity. By calibrating the model parameters -- which characterize the $M_\bullet-M_\star$ relation -- to a compilation of $z>10$ JWST UVLF data, we show that AGN emission can account for the excess luminosity under a variety of scenarios, including one where 10\% of galaxies host BHs of comparable luminosities to their stellar components. Using a sample of simulated objects and real observations, we demonstrate that such low-luminosity AGNs can be `hidden' in their host galaxies and be missed in common morphological analyses. We find that for this explanation to be viable, our model requires a population of BHs that are overmassive ($M_\bullet/M_\star\sim10^{-2}$) with respect to their host galaxies compared to the local relation and are more consistent with the observed relation at $z=4-8$. We explore the implications of this model for BH seed properties and comment on observational diagnostics necessary to further investigate this explanation.

astro-ph.GA↗

arXiv2023

Improving Undergraduate Astronomy Students' Skills with Research Literature via Accessible Summaries: An Exploratory Case Study with Astrobites-based Reading Assignments

Undergraduate physics and astronomy students are expected to engage with scientific literature as they begin their research careers, but reading comprehension skills are rarely explicitly taught in major courses. We seek to determine the efficacy of a reading assignment designed to improve undergraduate astronomy (or related) majors' perceived ability to engage with research literature by using accessible summaries of current research written by experts in the field. During the 2022-2023 academic year, faculty members from six institutions incorporated reading assignments using accessible summaries from Astrobites into their undergraduate astronomy major courses, surveyed their students before and after the activities, and participated in follow-up interviews with our research team. Quantitative and qualitative survey data from 52 students show that students' perceptions of their abilities with jargon and identifying main takeaways of a paper significantly improved with use of the tested assignment template. Additionally, students report increased confidence of their abilities within astronomy after exposure to these assignments, and instructors valued a ready-to-use resource to incorporate reading comprehension in their pedagogy. This exploratory case study with Astrobites-based assignments suggests that incorporating current research in the undergraduate classroom through accessible literature summaries may increase students' confidence and ability to engage with research literature, assisting in their preparation for participation in research careers.

physics.ed-ph↗

arXiv2023

A self-consistent semi-analytic model for Population III star formation in minihalos

The formation of the first stars marks a watershed moment in the history of our universe. As the first luminous structures, these stars (also known as Population III, or Pop III stars) seed the first galaxies and begin the process of reionization. We construct an analytic model to self-consistently trace the formation of Pop III stars inside minihalos in the presence of the fluctuating ultraviolet background, relic dark matter-baryon relative velocities from the early universe, and an X-ray background, which largely work to suppress cooling of gas and delay the formation of this first generation of stars. We demonstrate the utility of this framework in a semi-analytic model for early star formation that also follows the transition between Pop III and Pop II star formation inside these halos. Using our new prescription for the criteria allowing Pop III star formation, we follow a population of dark matter halos from $z=50$ through $z=6$ and examine the global star formation history, finding that each process defines its own key epoch: (i) the stream velocity dominates at the highest redshifts ($z\gtrsim30$), (ii) the UV background sets the tone at intermediate times ($30\gtrsim z\gtrsim15$), and (iii) X-rays control the end of Pop III star formation at the latest times ($z\lesssim 15$). In all of our models, Pop III stars continue to form down to $z\sim 7-10$, when their supernovae will be potentially observable with forthcoming instruments. Finally, we identify the signatures of variations in the Pop III physics in the global 21-cm spin-flip signal of atomic hydrogen.

astro-ph.CO↗

arXiv2022

Reorientation Rates of Structural and Kinematic Axes in Simulated Massive Galaxies and the Origins of Prolate Rotation

In this work, we analyze a sample of $\sim$4000 massive ($M_*\geq 10^{11} M_\odot$ at $z=0$) galaxies in TNG300, the $(300 \mathrm{Mpc})^3$ box of the IllustrisTNG simulation suite. We characterize the shape and kinematics of these galaxies with a focus on the kinematic misalignment ($Ψ_\mathrm{int}$) between the angular momentum (AM) and morphological major axis. We find that the traditional purely shape- or kinematics-based classifications are insufficient to characterize the diversity of our sample and define a new set of classes based on the rates of change of the galaxies' morphological and kinematic axes. We show that these classes are mostly stable over time and correspond to six distinct populations of galaxies: the rapid AM reorienters (58% of our sample), unsettled galaxies (10%), spinning disks (10%), twirling cigars (16%), misaligned slow reorienters (3%), and regular prolate rotators (galaxies that display major axis rotation; 2%). We demonstrate that the most-recent significant (mass-ratio $μ>1/10$) mergers of these galaxies are the primary cause for their present-day properties and find that these mergers are best characterized at the point of the satellite's final infall -- that is, much closer to the final coalescence than has been previously thought. We show that regular prolate rotators evolve from spinning disk progenitors that experience a radial merger along their internal AM direction. Finally, we argue that these regular prolate rotators are distinct from the similarly-sized population of rapid AM reorienters with large $Ψ_\mathrm{int}$, implying that a large $Ψ_\mathrm{int}$ is not a sufficient condition for major axis rotation.

astro-ph.GA↗

arXiv2018

Classifying Signatures of Sudden Ionospheric Disturbances

Solar activity, such as flares, produce bursts of high-energy radiation that temporarily enhance the D-region of the ionosphere and attenuate low-frequency radio waves. To track these Sudden Ionospheric Disturbances (SIDs), which disrupt communication signals and perturb satellite orbits, Scherrer et al. (2008) developed an international, ground-based network of around 500 SID monitors that measure the signal strength of low-frequency radio waves. However, these monitors suffer from a host of noise contamination issues that preclude their use for rigorous scientific analysis. As such, we attempt to create an algorithm to automatically identify noisy, contaminated SID data sets from clean ones. To do so, we develop a set of features to characterize times series measurements from SID monitors and use these features, along with a binary classifer called a support vector machine, to automatically assess the quality of the SID data. We compute the True Skill Score, a metric that measures the performance of our classifier, and find that it is ~0.75+/-0.06. We find features characterizing the difference between the daytime and nighttime signal strength of low-frequency radio waves most effectively discern noisy data sets from clean ones.

astro-ph.SR↗