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Prashin Jethwa

Publications and source records attributed to Prashin Jethwa.

At least 19 recordsLinked to original sources
arXiv2026

Semiresolved Stellar Populations as Distance Indicators

Galaxy distances are central to our understanding of the Universe. Despite the success of existing approaches, independent and complementary methods remain valuable for testing systematic effects and extending the applicability range of different distance metrics. Here we demonstrate that the spectrum of an individual semi-resolved stellar population encodes direct information about its distance and that valuable distance constraints can, in principle, be obtained by combining multiple independent measurements. When applied to optical spectra of bulge of the Andromeda galaxy (M\,31), we derive a stellar population-based distance of 756$\pm$57 kpc ($μ_0 = 24.39 \pm 0.15$), in agreement with state-of-the-art measurements. Crucially, semi-resolved stellar population distances does not require secondary calibrations, although its absolute scale remains conditional on the adopted stellar population models. Our findings provide a first assessment of the feasibility of deriving stellar population-based distance estimates from simple stellar population models, motivating further tests of the broader applicability and precision of the method.

astro-ph.GA↗
arXiv2026

FASTAR -- II. Semi-resolved evolutionary stellar population models

Standard evolutionary synthesis models rely on the assumption of a fully sampled stellar initial mass function (IMF). Under this assumption, the age, chemical composition, and IMF uniquely define the predicted absorption spectra. However, with current instrumentation pushing observations towards higher spatial resolutions and lower surface brightnesses, the assumption of a fully sampled IMF does not always hold true. Here we present the semi-resolved version of the FASTAR models, a comprehensive set of evolutionary synthesis predictions able to reproduce the stochastic behavior of discretely-sampled IMFs. Semi-resolved FASTAR predictions share the same evolutionary principles, ingredients, and features of the integral (fully sampled IMF) version of the FASTAR models, expanding a range of ages from 20 Myr to 14 Gyr, metallicities between -2.5 < [M/H] < +0.3, and several IMF functional forms. Detailed spectroscopic measurements can be carried out within the 3,540-7,400 A wavelength range, and low-resolution spectral energy distributions can also be synthesized over a wider 2,000-to-12,000 A coverage. Semi-resolved FASTAR models also depend on the number of stars contributing to the observed spectra, which determines the effective sampling of the different stellar evolutionary phases along the isochrones. This incomplete sampling implies that semi-resolved FASTAR models are inevitably stochastic. On top of the inherent stochasticity of the models, derived quantities such as equivalent widths, colors, or mass-to-light ratios might present strong deviations compared to standard fully sampled simple stellar population models. This stochasticity dilutes the boundary between model predictions and data, promoting new sampling-based inference approaches. FASTAR semi-resolved models allow for the effective exploration of the parameter space thanks to their optimized, JAX-based computation.

astro-ph.GA↗
arXiv2026

FASTAR -- I. Continuous and differentiable evolutionary stellar population models

The development of evolutionary stellar population models is central to interpreting observations of galaxies in terms of astrophysical quantities. Stellar population models must therefore be both accurate and compatible with inversion algorithms in order to extract meaningful information from the observed data. Here we present FASTAR, a fully differentiable stellar population synthesis code. Contrary to traditional, grid-based single stellar population models, FASTAR can be continuously evaluated at any age (between 20 Myr and 14 Gyr), metallicity (-2.5 < [M/H] < +0.3), and initial mass function (IMF). Changes in the IMF parameterization are straightforward, allowing for consistent conversions of colors, magnitudes, and mass-to-light ratios, as well as the synthesis of models under the assumption of arbitrary IMF functional forms. FASTAR provides detailed spectroscopic predictions over the MILES wavelength range (3,540-7,400 A) as well as more coarsely sampled spectral energy distributions across a wider 2,000-to-12,000 A, which can be directly convolved with any arbitrary set of photometric filters. FASTAR performs at the same level of state-of-the-art simple stellar population models benchmarked against observations of globular clusters and high signal-to-noise spectra of early-type galaxies, but it is faster, lighter, and more flexible. Moreover, its differentiable nature allows for a quantitative understanding of model behavior and uncertainties, as well as a natural framework for gradient descent inference algorithms.

astro-ph.GA↗
arXiv2026

Tracing the dynamical and structural complexity of spiral galaxy centres

The formation of late-type galaxies has traditionally been described via two pathways: one producing pressure-supported classical bulges, the other rotationally supported pseudo-bulges. Early studies relied on photometric decompositions assuming an exponential disk extrapolated inwards. Recent high-resolution observations, however, reveal a far more complex landscape in disk galaxy centres. We investigated the morphology of central stellar components in intermediate-to-massive spiral galaxies, focusing on disentangling cold, warm, and hot orbital contributions, critically reassessing the standard approach of extrapolating the exponential disk profile inwards. We developed GLANCE (Galactic archaeoLogy via chronochemicAl and dyNamiCal modElling), a tool for photometric, chronochemical, and dynamical galaxy analysis, applied to 8 high-resolution MUSE galaxies to derive stellar population properties and decompose orbits into cold, warm, hot, and counter-rotating (CR) components. We uncovered remarkable structural diversity in the dynamically cold central component: one galaxy displays an exponential profile throughout, while the majority exhibit either a pronounced central drop resembling a doughnut-shaped structure or a compact inner disk significantly steeper than the outer disk. Most galaxies hosting nuclear disks are classifiable as classical bulges - hot, old, red, high bulge-to-total ratio - contrasting with galaxies showing a central cold-component deficit. Beyond the bulge, cold plus warm orbit contributions remain below the total, indicating non-negligible hot or CR orbits with Sérsic indexes consistently above unity. These results highlight the composite nature of disk galaxy centres and the need for decomposition methods that avoid extrapolating the outer disk inwards, requiring large IFS samples across a broad mass range, complemented by simulations such as IllustrisTNG50.

astro-ph.GA↗
arXiv2026

Glance: A Comprehensive Framework for Galactic Archaeology

A central topic in extragalactic astronomy is understanding the formation and evolutionary histories of galaxies. These systems often comprise multiple structural components with distinct physical and dynamical properties, making it challenging to disentangle their individual contributions. Aiming at investigating the true structure of the inner stellar disk, we have developed a comprehensive pipeline for the chronochemical and dynamical analysis of galaxies (Glance: Galactic archaeoLogy via chronochemicAl & dyNamiCal modElling). The presented pipeline employs several state-of-the-art techniques by integrating them into a single, automated pipeline, enabling streamlined analysis of integral-field spectroscopy data, by allowing users to easily and directly extract valuable information on stellar populations, kinematics, dynamics, and gas properties. It automates multiple analysis techniques, including stellar population synthesis (Fado, Starlight, post-processing with RemoveYoung, kinematic extraction (pPXF, Bayes-LOSVD), and dynamical modelling (Dynamite). It handles tasks such as Galactic extinction correction, de-redshifting, Voronoi binning, and nebular continuum correction, while offering extensive customization options. Parallel processing significantly reduces computational time. When applied to MUSE data sampling the central region of NGC 1566, this methodology reveals that its stellar disk significantly deviates from the conventional exponential model, challenging the assumption of universality in disk morphology. In summary, this work presents a powerful, publicly available pipeline for conducting galactic archaeology, designed to advance our understanding of the formation and evolution of galaxies.

astro-ph.GA↗
arXiv2026

Orbital decomposition of the nuclear regions in the early-type galaxy FCC 47: Unveiling the nuclear cluster origin

Nuclear star clusters (NSCs) are among the densest stellar systems in the Universe and often coexist with supermassive black holes (SMBHs) at galaxy centres. While SMBH formation histories are essentially lost, NSCs preserve evolutionary imprints through their stellar populations and stellar kinematics, reflecting the cumulative effects of mergers, accretion, and internal dynamical evolution. We aim to investigate the orbital structure of the unusually large NSC in FCC 47 (NGC 1336) by decomposing its stellar orbits into dynamically distinct components. We extract stellar kinematics, and in particular the line-of-sight velocity distributions (LOSVDs), from VLT/MUSE integral-field spectroscopy using the non-parametric Bayes-LOSVD approach, and apply triaxial Schwarzschild orbit-superposition modelling with the DYNAMITE software. We decompose the orbit library into hot, warm, cold, and counter-rotating components. We detect triple-peaked LOSVDs in the nucleus, indicating a complex orbital structure. The NSC forms a counter-rotating, kinematically decoupled component. A hot pressure-supported component, a warm counter-rotating structure and a counter-rotating cold disk in the centre suggest hierarchical assembly via early star cluster accretion and later in situ star formation. Our orbital decomposition of FCC 47 supports a hybrid formation scenario for this NSC. Dynamically distinct substructures reflect the interplay of accretion and in situ star formation during galaxy evolution.

astro-ph.GA↗
arXiv2025

3D Full Spectrum Fitting: Algorithm Comparison

Full spectrum fitting is the prevailing method for extracting stellar kinematic and population measurements from 1D galaxy spectra. 3D methods refer to analysis of Integral Field Spectroscopy (IFS) data where spatial and spectral dimensions are modelled simultaneously. While several 3D methods exist for modelling gas structures there has been less investigation into the more computationally demanding problem of 3D full spectrum fitting for stellar recoveries. This work introduces and compares two algorithms for this task: the Projected Nesterov Kaczmarz Reconstruction method (PNKR) and a version of the Bayes-LOSVD software which has been modified to account for spatial correlations. We aim to understand strengths and weaknesses of both algorithms and assess the impact of 3D methods for stellar inferences. We apply both recovery algorithms to a mock IFS data over a signal-to-noise ratio (SNR) range from 20-200 and evaluate the quality of the recoveries compared to the known ground truth. Accounting for spatial correlations in Bayes-LOSVD significantly improved the accuracy and precision of kinematic recoveries. 3D modelling with PNKR did not provide any significant improvement over 1D fits however, for SNR>40, PNKR did recover the most accurate kinematics overall. Additionally, by modelling the joint distribution over kinematics and populations, PNKR could successfully infer trends between these quantities e.g. inferring local metallicity-velocity trends, albeit with a significant bias on the absolute metallicity. Having demonstrated advantages of (i) 3D modelling with Bayes-LOSVD, and (ii) joint kinematic-population analyses with PNKR, we conclude that both methodological advances will prove useful for detecting and characterising stellar structures from IFS data.

astro-ph.GA↗
arXiv2024

Large-Scale Stellar Age-Velocity Spiral Pattern in NGC 4030

The processes driving the formation and evolution of late-type galaxies (LTGs) continue to be a debated subject in extragalactic astronomy. Investigating stellar kinematics, especially when combined with age estimates, provides crucial insights into the formation and subsequent development of galactic discs. Post-processing of exceptionally high-quality Integral Field Spectroscopy (IFS) data of NGC 4030 acquired with the Multi Unit Spectroscopic Explorer (MUSE), clearly reveals a striking grand design spiral pattern in the velocity dispersion map not previously detected in other galaxies. This pattern spatially correlates with HII regions, suggesting that stars currently being born exhibit lower velocity dispersion as compared to surrounding areas where star formation (SF) is less active. We examine the age-velocity relation (AVR) and propose that its configuration might be shaped by a combination of heating mechanisms, seemingly consistent with findings from recent high-resolution cosmological zoom-in simulations. The complex structure of the uncovered AVR of NGC 4030 support the hypothesis that stellar populations initially inherit the velocity dispersion σ of the progenitor cold molecular gas, which depends on formation time and galactocentric distance, subsequently experiencing kinematic heating by cumulative gravitational interactions during their lifetime. While advancing our understanding of the AVR, these findings offer a new framework for investigating disk heating mechanisms, and their role in the evolution of galactic disks.

astro-ph.GA↗
arXiv2024

Schwarzschild Modeling of Barred S0 Galaxy NGC 4371

We apply the barred Schwarzschild method developed by Tahmasebzadeh et al. (2022) to a barred S0 galaxy, NGC 4371, observed by IFU instruments from the TIMER and ATLAS3D projects. We construct the gravitational potential by combining a fixed black hole mass, a spherical dark matter halo, and stellar mass distribution deprojected from $3.6$ $μ$m S$^4$G image considering an axisymmetric disk and a triaxial bar. We independently modelled kinematic data from TIMER and ATLAS3D. Both models fit the data remarkably well. We find a consistent bar pattern speed from the two sets of models with $Ω_{\rm p} = 23.6 \pm 2.8 \hspace{.08cm} \mathrm{km \hspace{.04cm} s^{-1} \hspace{.04cm} kpc^{-1} }$ and $Ω_{\rm p} = 22.4 \pm 3.5 \hspace{.08cm} \mathrm{km \hspace{.04cm} s^{-1} \hspace{.04cm} kpc^{-1} }$, respectively. The dimensionless bar rotation parameter is determined to be $ 1.88 \pm 0.37$, indicating a likely slow bar in NGC 4371. Additionally, our model predicts a high amount of dark matter within the bar region ($M_{\rm DM}/ M_{\rm total}$ $\sim 0.51 \pm 0.06$), which, aligned with the predictions of cosmological simulations, indicates that fast bars are generally found in baryon-dominated disks. Based on the best-fitting model, we further decompose the galaxy into multiple 3D orbital structures, including a BP/X bar, a classical bulge, a nuclear disk, and a main disk. The BP/X bar is not perfectly included in the input 3D density model, but BP/X-supporting orbits are picked through the fitting to the kinematic data. This is the first time a real barred galaxy has been modelled utilizing the Schwarzschild method including a 3D bar.

astro-ph.GA↗
arXiv2024

Isotopic abundance of carbon in the DLA towards QSO B1331+170

Chemical evolution models predict a gradual build-up of $^{13}$C in the universe, based on empirical nuclear reaction rates and assumptions on the properties of stellar populations. However, old metal-poor stars within the Galaxy contain more $^{13}$C than is predicted, suggesting that further refinements to the models are necessary. Gas at high redshift provides important supplementary information at metallicities $-2\lesssim$ [Fe/H] $\lesssim-1$, for which there are only a few measurements in the Galaxy. We obtained new, high-quality, VLT/ESPRESSO observations of the QSO B1331+170 and used them to measure $^{12}$C/$^{13}$C in the damped Lyman-$α$ system (DLA) at $z_{abs}=1.776$, with [Fe/H]=-1.27. AI-VPFIT, an Artificial Intelligence tool based on genetic algorithms and guided by a spectroscopic information criterion, was used to explore different possible kinematic structures of the carbon gas. Three hundred independent AI-VPFIT models of the absorption system were produced using pre-set $^{12}$C/$^{13}$C values, ranging from 4 to 500. Our results show that $^{12}$C/$^{13}$C$=28.5^{+51.5}_{-10.4}$, suggesting a possibility of $^{13}$C production at low metallicity.

astro-ph.GA↗
arXiv2024

A new method for instrumental profile reconstruction of high resolution spectrographs

Knowledge of the spectrograph's instrumental profile (IP) provides important information needed for wavelength calibration and for the use in scientific analyses. This work develops new methods for IP reconstruction in high resolution spectrographs equipped with Laser Frequency Comb calibration (LFC) systems and assesses the impact that assumptions on IP shape have on achieving accurate spectroscopic measurements. Astronomical LFCs produce $\approx10000$ bright, unresolved emission lines with known wavelengths, making them excellent probes of the IP. New methods based on Gaussian Process regression were developed to extract detailed information on the IP shape from this data. Applying them to HARPS, an extremely stable spectrograph installed on the ESO 3.6m telescope, we reconstructed its IP at 512 locations of the detector, covering 60% of the total detector area. We found that the HARPS IP is asymmetric and that it varies smoothly across the detector. Empirical IP models provide wavelength accuracy better than 10 ms$^{-1}$ (5 ms$^{-1}$) with 92% (64%) probability. In comparison, reaching the same accuracy has a probability of only 29% (8%) when a Gaussian IP shape is assumed. Furthermore, the Gaussian assumption is associated with intra-order and inter-order distortions in the HARPS wavelength scale as large as 60ms$^{-1}$. The spatial distribution of these distortions suggests they may be related to spectrograph optics and therefore may generally appear in cross-dispersed echelle spectrographs when Gaussian IPs are used. Methods presented here can be applied to other instruments equipped with LFCs, such as ESPRESSO, but also ANDES and G-CLEF in the future. The empirical IPs will be crucial for obtaining objective and unbiased measurements of fundamental constants from high resolution spectra, as well as measurements of the redshift drift, isotopic abundances, and other science cases.

astro-ph.IM↗
arXiv2023

Total mass slopes and enclosed mass constrained by globular cluster system dynamics

The goal of this work is to probe the total mass distribution of early-type galaxies with globular clusters (GCs) as kinematic tracers, by constraining the parameters of the profile with a flexible modelling approach. To that end, we leverage the extended spatial distribution of GCs from the SLUGGS survey ($\langle R_{\rm GC,\ max} \rangle \sim 8R_{\rm e}$) in combination with discrete dynamical modelling. We use discrete Jeans anisotropic modelling in cylindrical coordinates to determine the velocity moments at the location of the GCs in our sample. We use a Bayesian framework to determine the best-fit parameters of the total mass density profile and orbital properties of the GC systems. We find that the orbital properties (anisotropy and rotation of the dispersion-dominated GC systems) minimally impact the measurements of the inner slope and enclosed mass, while a strong presence of dynamically-distinct subpopulations or low numbers of kinematic tracers can bias the results. Owing to the large spatial extent of the tracers our method is sensitive to the intrinsic inner slope of the total mass profile and we find $\overlineα = -1.88\pm 0.01$ for 12 galaxies with robust measurements. To compare our results with literature values we fit a single power-law profile to the resulting total mass density. In the radial range 0.1-4~$R_{\rm e}$ our measured slope has a value of $\langle γ_{\rm tot}\rangle = -2.22\pm0.14$ and is in good agreement with the literature.

astro-ph.GA↗
arXiv2023

Dynamical modelling of ATLAS$^{\rm 3D}$ galaxies

Triaxial dynamical models of massive galaxies observed in the ATLAS3D project can provide new insights into the complex evolutionary processes that shape galaxies. The ATLAS3D survey is ideal as the sample comprises a good mix of fast and slow rotators with vastly different mass assembly histories. We present a detailed dynamical study with our triaxial modelling code DYNAMITE, which models galaxies as a superposition of their stellar orbits. The models allow us to constrain the intrinsic shape of the stellar component, the distributions of the visible and invisible matter and the orbit distribution in these nearby early-type galaxies and to relate it with different evolutionary scenarios. Triaxial modelling is essential for these galaxies to understand their complex kinematical features.

astro-ph.GA↗
arXiv2023

Effect of the initial mass function on the dynamical SMBH mass estimate in the nucleated early-type galaxy FCC 47

Supermassive black holes (SMBHs) and nuclear star clusters (NSCs) co-exist in many galaxies. While the formation history of the black hole is essentially lost, NSCs preserve their evolutionary history imprinted onto their stellar populations and kinematics. Studying SMBHs and NSCs in tandem might help us to ultimately reveal the build-up of galaxy centres. In this study, we combine large-scale VLT/MUSE and high-resolution adaptive-optics-assisted VLT/SINFONI observations of the early-type galaxy FCC 47 with the goal being to assess the effect of a spatially (non-)variable initial mass function (IMF) on the determination of the mass of the putative SMBH in this galaxy. We achieve this by performing DYNAMITE Schwarzschild orbit-superposition modelling of the galaxy and its NSC. In order to properly take account of the stellar mass contribution to the galaxy potential, we create mass maps using a varying stellar mass-to-light ratio derived from single stellar population models with fixed and with spatially varying IMFs. Using the two mass maps, we estimate black hole masses of $(7.1^{+0.8}_{-1.1})\times 10^7\,M_{\odot}$ and $(4.4^{+1.2}_{-2.1}) \times 10^7\,M_{\odot}$ at $3σ$ signifance, respectively. Compared to models with constant stellar-mass-to-light ratio, the black hole masses decrease by 15% and 48%, respectively. Therefore, a varying IMF, both in its functional form and spatially across the galaxy, has a non-negligible effect on the SMBH mass estimate. Furthermore, we find that the SMBH in FCC 47 has probably not grown over-massive compared to its very over-massive NSC.

astro-ph.GA↗
arXiv2023

Uncertainty-Aware Blob Detection with an Application to Integrated-Light Stellar Population Recoveries

Context. Blob detection is a common problem in astronomy. One example is in stellar population modelling, where the distribution of stellar ages and metallicities in a galaxy is inferred from observations. In this context, blobs may correspond to stars born in-situ versus those accreted from satellites, and the task of blob detection is to disentangle these components. A difficulty arises when the distributions come with significant uncertainties, as is the case for stellar population recoveries inferred from modelling spectra of unresolved stellar systems. There is currently no satisfactory method for blob detection with uncertainties. Aims. We introduce a method for uncertainty-aware blob detection developed in the context of stellar population modelling of integrated-light spectra of stellar systems. Methods. We develop theory and computational tools for an uncertainty-aware version of the classic Laplacian-of-Gaussians method for blob detection, which we call ULoG. This identifies significant blobs considering a variety of scales. As a prerequisite to apply ULoG to stellar population modelling, we introduce a method for efficient computation of uncertainties for spectral modelling. This method is based on the truncated Singular Value Decomposition and Markov Chain Monte Carlo sampling (SVD-MCMC). Results. We apply the methods to data of the star cluster M54. We show that the SVD-MCMC inferences match those from standard MCMC, but are a factor 5-10 faster to compute. We apply ULoG to the inferred M54 age/metallicity distributions, identifying between 2 or 3 significant, distinct populations amongst its stars.

astro-ph.GA↗
arXiv2022

A projected Nesterov-Kaczmarz approach to stellar population-kinematic distribution reconstruction in Extragalactic Archaeology

In this paper, we consider the problem of reconstructing a galaxy's stellar population-kinematic distribution function from optical integral field unit measurements. These quantities are connected via a high-dimensional integral equation. To solve this problem, we propose a projected Nesterov-Kaczmarz reconstruction (PNKR) method, which efficiently leverages the problem structure and incorporates physical prior information such as smoothness and non-negativity constraints. To test the performance of our reconstruction approach, we apply it to a dataset simulated from a known ground truth density, and validate it by comparing our recoveries to those obtained by the widely used pPXF software.

math.NA↗
arXiv2022

On the robustness of triaxial Schwarzschild modelling: The effects of correcting the orbit mirroring

In the past 15 years, the triaxial Schwarzschild orbit-superposition code by van den Bosch et al. (2008) has been widely applied to study the dynamics of galaxies. Recently, Quenneville et al. (2022) reported a bug in the orbit calculation of this code, specifically in the mirroring procedure that is used to speed up the computation. We have fixed the incorrect mirroring in DYNAMITE, which is the publicly-released successor of the triaxial Schwarzschild code by van den Bosch et al. (2008). In this study, we provide a thorough quantification of how this bug has affected the results of dynamical analyses performed with this code. We compare results obtained with the original and corrected versions of DYNAMITE, and discuss the differences in the phase-space distribution of a single orbit and in the global stellar orbit distribution, in the mass estimate of the central black hole in the highly triaxial galaxy PGC 46832, and in the measurement of intrinsic shape and enclosed mass for more than 50 galaxies. Focusing on the typical scientific applications of a Schwarzschild triaxial code, in all our tests we find that differences are negligible with respect to the statistical and systematic uncertainties. We conclude that previous results with the van den Bosch et al. (2008) triaxial Schwarzschild code are not significantly affected by the incorrect mirroring.

astro-ph.GA↗
arXiv2021

What to expect when using globular clusters as tracers of the total mass distribution in Milky Way-mass galaxies

Dynamical models allow us to connect the motion of a set of tracers to the underlying gravitational potential, and thus to the total (luminous and dark) matter distribution. They are particularly useful for understanding the mass and spatial distribution of dark matter (DM) in a galaxy. Globular clusters (GCs) are an ideal tracer population in dynamical models, since they are bright and can be found far out into the halo of galaxies. We aim to test how well Jeans-Anisotropic-MGE (JAM) models using GCs (positions and line-of-sight velocities) as tracers can constrain the mass and radial distribution of DM halos. For this, we use the E-MOSAICS suite of 25 zoom-in simulations of L* galaxies. We find that the DM halo properties are reasonably well recovered by the JAM models. There is, however, a strong correlation between how well we recover the mass and the radial distribution of the DM and the number of GCs in the galaxy: the constraints get exponentially worse with fewer GCs, and at least 150 GCs are needed in order to guarantee that the JAM model will perform well. We find that while the data quality (uncertainty on the radial velocities) can be important, the number of GCs is the dominant factor in terms of the accuracy and precision of the measurements. This work shows promising results for these models to be used in extragalactic systems with a sample of more than 150 GCs.

astro-ph.GA↗