arXiv subjects

Rosemary F. G. Wyse

Publications and source records attributed to Rosemary F. G. Wyse.

At least 19 recordsLinked to original sources

arXiv2026

Galactic Archaeology with the Subaru `Ōnohi`ula Prime Focus Spectrograph Strategic Program

The recently commissioned Subaru `Ōnohi`ula Prime Focus Spectrograph (PFS) will obtain spectra from nearly 2,400 fibers that cover 1.24 square degrees. The 360 night Subaru Strategic Program for PFS is dedicating approximately one-third of its allocation (130 nights) to study the structure and evolution of galaxies in the Local Group. This Galactic Archaeological survey has three pillars. (1) We will determine whether the mass density profiles of dwarf galaxies are consistent with cusps, as expected for cold dark matter, or cores, as expected from alternative dark matter theories or baryonic feedback. We will deduce the density profiles as a function of radius from modeling of the full line-of-sight velocity and abundance distributions for six dwarf galaxies. Our total sample will consist of 18,000 member stars to beyond the nominal tidal radius of each system. (2) From measurements of the [alpha/Fe] abundance ratio, we will learn the difference in assembly history of the two most massive galaxies in the Local Group: M31 and the Milky Way. We will observe 30,000 member stars over 45 square degrees of M31's halo and outer disk. (3) We will uncover how the most fragile (outer) part of the Milky Way responded to accretion events both in the distant past (such as Gaia-Sausage Enceladus) and in more recent history (such as the Sagittarius dwarf spheroidal galaxy). To support this study, PFS will provide velocities and metallicities--from which, in combination with photometry, we will deduce ages--for tens of thousands of main-sequence stars out to a Galactocentric distance of ~30 kpc.

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arXiv2026

Spiral Morphology and Radial Migration: Kinematically heating, cooling, and cold

Transient spiral arms are known to drive radial redistribution of stars and thus could play a central role in shaping disk galaxies, including modifying, over time, the age, chemical, and kinematic (chrono-chemo-dynamic) distributions in the Milky Way. However, the physical factors governing the efficiency of such processes remain poorly understood. This paper investigates how the morphology of spiral arms -- the number, pitch angle, lifetime, and radial dependence of the pattern speed -- influences orbital redistribution through 'cold torquing' at the corotation resonance(s). Analytic expressions are derived for the maximum radial excursion of stars trapped at corotation that explicitly account for spiral morphology, predicting that the efficiency of cold torquing for a density-wave like spiral is greater for more open spiral patterns. Tracer-particle simulations confirm the analytic prediction, in both two- and three-dimensional galactic potentials. In contrast, spirals that have a radially dependent pattern speed such that they corotate with the disk at all radii exhibit the opposite behavior, with cold torquing becoming more efficient as the spiral winds to smaller pitch angles over time. This study further finds that resonant interactions from the same transient spiral causing cold torquing naturally also produces both kinematic heating and cooling of orbits away from corotation. These results demonstrate that spiral morphology alone cannot predict the efficiency of cold torquing and suggest that the relationship between spiral pitch angle and radial redistribution provides a potential diagnostic for distinguishing between competing theories of spiral structure.

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arXiv2026

Generalization from Low- to Moderate-Resolution Spectra with Neural Networks for Stellar Parameter Estimation: A Case Study with DESI

Cross-survey generalization is a critical challenge in stellar spectral analysis, particularly in cases such as transferring from low- to moderate-resolution surveys. We investigate this problem using pre-trained models, focusing on simple neural networks such as multilayer perceptrons (MLPs), with a case study transferring from LAMOST low-resolution spectra (LRS) to DESI medium-resolution spectra (MRS). Specifically, we pre-train MLPs on either LRS or their embeddings and fine-tune them for application to DESI stellar spectra. We compare MLPs trained directly on spectra with those trained on embeddings derived from transformer-based models (self-supervised foundation models pre-trained for multiple downstream tasks). We also evaluate different fine-tuning strategies, including residual-head fine-tuning, LoRA, and full fine-tuning. We find that MLPs pre-trained on LAMOST LRS achieve strong performance, even without fine-tuning, and that modest fine-tuning with DESI spectra further improves the results. For iron abundance, embeddings from a transformer-based model yield advantages in the metal-rich ([Fe/H] > -1.0) regime, but underperform in the metal-poor regime compared to MLPs trained directly on LRS. We also show that the optimal fine-tuning strategy depends on the specific stellar parameter under consideration. These results highlight that simple pre-trained MLPs can provide competitive cross-survey generalization, while the role of spectral foundation models for cross-survey stellar parameter estimation requires further exploration.

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arXiv2026

The Milky Way Tomography with Subaru Hyper Suprime-Cam. II. Global halo structure

We investigate the structure of the Milky Way's stellar halo within 70 kpc of the Sun using a wide-field photometric catalog obtained from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (HSC-SSP). We employ a large sample of main-sequence turn-off stars as distance tracers. To robustly derive the structural parameters of the stellar halo, we develop a forward-modeling framework that explicitly accounts for distance uncertainties, the solar position, and the limited sky coverage of the survey. Applying this method to the HSC-SSP catalog, we found that the smooth stellar halo is well described by a double power-law density profile, with inner and outer slope of approximately -3.3 and -4.8, respectively, with a break radius of 17.4 kpc. The outer steep density slope derived in this work supports a picture in which the present-day structure of the Milky Way's stellar halo is influenced by early massive accretion events, consistent with inferences from kinematic substructures such as Gaia Enceladus/Sausage. Ongoing wide-field imaging surveys, including UNIONS and LSST, will provide further constraints on the structure of the stellar halo and key insights into its formation history.

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arXiv2026

The Milky Way Tomography with Subaru Hyper Suprime-Cam: Implications for the past orbit of the Large Magellanic Cloud

We report the discovery of diffuse stellar substructure in the Milky Way's outer halo toward Boötes, unveiled by deep imaging data of the Subaru/Hyper Suprime-Cam. This substructure is detected as an excess of faint main-sequence stars, at heliocentric distances beyond 30 kpc, extending over at least 100 $\mathrm{deg^2}$. To infer its origin, we compare the projected spatial distribution of these stars to that of simulated tidal debris from the Large Magellanic Cloud (LMC), under the assumptions that the LMC is on either its first or second passage of the Milky Way. We found that the observed overdensity lies in a region of the halo where debris from the LMC is expected if it is on its initial pericentric phase 7-8 Gyr ago, which is predicted in the second-passage model, while the first-passage model is unable to explain the observed substructure. Chemo-kinematical data are required to further constrain its past orbit and to understand the origin of this new halo substructure, as should be obtained in the near future with deep photometric surveys such as UNIONS and LSST, and wide-field spectroscopy such as possible with PFS and DESI.

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arXiv2025

Machine Learning Techniques to Distinguish Giant Stars from Dwarf Stars Using Only Photometry -- Pushing Redwards

We present our photometric method, which combines Subaru/HSC $NB515$, g, and i band filters to distinguish giant stars in Local Group galaxies from Milky Way dwarf contamination. The $NB515$ filter is a narrow-band filter that covers the MgI+MgH features at $5150$ Å, and is sensitive to stellar surface gravity. Using synthetic photometry derived from large empirical stellar spectral libraries, we model the $NB515$ filter's sensitivity to stellar atmospheric parameters and chemical abundances. Our results demonstrate that the $NB515$ filter effectively separates dwarfs from giants, even for the reddest and coolest M-type stars. To further enhance this separation, we develop machine learning models that improve the classification on the two-color ($g-i$, $NB515-g$) diagram. We apply these models to photometric data from the Fornax dwarf spheroidal galaxy and two fields of M31, successfully identifying red giant branch stars in these galaxies.

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arXiv2025

The Extended Stellar Distribution in the Outskirts of the Ursa Minor Dwarf Spheroidal Galaxy

We discover an extended distribution of main-sequence (MS) stars along the minor axis of the Ursa Minor (UMi) dwarf spheroidal galaxy (dSph). This study is enabled by deep, wide Subaru/Hyper Suprime-Cam data, reaching photometric uncertainties below 0.1 mag at $g,i \sim 26$ mag. Color-magnitude diagrams along the major and minor axes reveal a clear excess of MS stars beyond the nominal tidal radius along the minor axis. To characterize this structure, we derive radial number density profiles in seven azimuthal directions and fit them with an exponential+power-law function to assess the symmetry of the extended component. The power-law slopes tend to be shallower toward the minor axis, though the symmetry remains inconclusive within 1$σ$ uncertainties. This may indicate that the extended component is preferentially distributed along the minor axis, and could be different from the previously suggested tidal features along the major axis. Comparing with simulations, we find that the fraction of stars beyond five effective radii is consistent with expectations from an intermediate mass ratio merger scenario with a stellar mass ratio around 6:1. While these findings provide new insights into the structural complexity and dynamical history of the UMi dSph, alternative mechanisms such as stellar or supernova feedback have also been proposed for extended stellar halos in dwarfs and cannot be ruled out.

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arXiv2025

The Star Formation and Chemical Evolution Histories of Ursa Minor Dwarf Spheroidal Galaxy

We derive the star formation history (SFH) and chemical evolution history (CEH) of the Ursa Minor (UMi) dwarf spheroidal galaxy (dSph). We detect two distinct stellar populations that exist over 6 times half-light radius from its center. The results are obtained by applying a newly developed algorithm to the deep and wide-field photometric dataset taken with Hyper Suprime-Cam on the Subaru Telescope. The algorithm employs the genetic algorithm and the simulated annealing to minimize a $χ^{2}$ value between the observed color-magnitude diagram (CMD) and synthetic CMD generated from the stellar isochrones. The age and metallicity resolutions are set to $0.5$ Gyr and $0.1$ dex, respectively. The accuracy assessment with mock galaxies shows that it returns the peaks of metallicity distributions and star formation period within 1 $σ$ of input value in the case of a single population. In tests with two populations, two distinct metallicity peaks are identified without an offset from the input values, indicating the robustness of this algorithm. The detected two populations in the UMi dSph have the metallicity peaks of [Fe/H] = $-2.2$ and $-2.5$; the metal-rich population started its star formation about 1 Gyr later than the metal-poor one. The SFH of both metal-rich and metal-poor populations varies with distance from the center of the UMi dSph, without any age-gradients. These results suggest that the UMi dSph underwent a complex formation process, contrary to the simple formation history of dwarf galaxies previously thought.

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arXiv2025

Constraints on asteroid-mass primordial black holes in dwarf galaxies using Hubble Space Telescope photometry

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and until now unconstrained dark matter (DM) candidate. If such PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion such as ultra-faint dwarf galaxies (UFDs). The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. As a result, the presence of PBHs in UFDs would alter their stellar mass functions. Using photometric observations of three ultra-faint dwarf galaxies from the Hubble Space Telescope, we show that it is unlikely that their mass functions have been significantly modified by PBHs, and we place constraints on the PBH abundance. In the ultra-faint dwarf galaxy Triangulum II, PBHs around $10^{19}$g are excluded at the $2σ$ ($3σ$) level from constituting more than $\sim55\%$ ($\sim78\%$) of the dark matter, while the possibility that PBHs represent the entirety of the DM is excluded at the $3.7σ$ level.

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arXiv2025

The structure of the stellar halo of the Andromeda galaxy explored with the NB515 for Subaru/HSC. I.: New Insights on the stellar halo up to 120 kpc

We analyse the M31 halo and its substructure within a projected radius of 120 kpc using a combination of Subaru/HSC $\textit{NB515}$ and CFHT/MegaCam $\textit{g}$- \& $\textit{i}$-bands. We succeed in separating M31's halo stars from foreground contamination with $\sim$ 90 \% accuracy by using the surface gravity sensitive $\textit{NB515}$ filter. Based on the selected M31 halo stars, we discover three new substructures, which associate with the Giant Southern Stream (GSS) based on their photometric metallicity estimates. We also produce the distance and photometric metallicity estimates for the known substructures. While these quantities for the GSS are reproduced in our study, we find that the North-Western stream shows a steeper distance gradient than found in an earlier study, suggesting that it is likely to have formed in an orbit closer to the Milky Way. For two streams in the eastern halo (Stream C and D), we identify distance gradients that had not been resolved. Finally, we investigate the global halo photometric metallicity distribution and surface brightness profile using the $\textit{NB515}$-selected halo stars. We find that the surface brightness of the metal-poor and metal-rich halo populations, and the all population can be fitted to a power-law profile with an index of $α=-1.65\pm0.02$, $-2.82\pm0.01$, and $-2.44\pm0.01$, respectively. In contrast to the relative smoothness of the halo profile, its photometric metallicity distribution appears to be spatially non-uniform with nonmonotonic trends with radius, suggesting that the halo population had insufficient time to dynamically homogenize the accreted populations.

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arXiv2024

What can we learn from the Nitrogen abundance of High-z galaxies?

We present measurements of the gas-phase Oxygen and Nitrogen abundances obtained by applying the direct method to JWST NIRspec $R\sim1000$ spectroscopy for 6 galaxies at redshift greater than 3. Our measurements are based on rest-frame optical Nitrogen [N II]$_{λ\lambda6548,6583}$ lines and are complemented by 6 additional objects from the literature at $3\leq z \leq 6$. We find that 9 out of 12 objects have values of log(N/O) that are compatible with those found for low-redshift, metal-poor, dwarf galaxies and for HII regions of more luminous local galaxies. However, 3 out of 12 objects have log(N/O) values that are overabundant compared to what is expected on the basis of their Oxygen abundance. We explore a few standard scenarios to explain the observations and conclude that, within the limited statistics available to us, none of them can be definitely excluded even though we prefer dilution by pristine gas infall in between star formation bursts as this is predicted by simulations to take place as a natural part of bursty star formation.

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arXiv2024

Uncertainty of line-of-sight velocity measurement of faint stars from low and medium resolution optical spectra

Massively multiplexed spectrographs will soon gather large statistical samples of stellar spectra. The accurate estimation of uncertainties on derived parameters, such as line-of-sight velocity $v_\mathrm{los}$, especially for spectra with low signal-to-noise ratios, is paramount. We generated an ensemble of simulated optical spectra of stars as if they were observed with low- and medium-resolution fiber-fed instruments on an 8-meter class telescope, similar to the Subaru Prime Focus Spectrograph, and determined $v_\mathrm{los}$ by fitting stellar templates to the simulations. We compared the empirical errors of the derived parameters -- calculated from an ensemble of simulations -- to the asymptotic error determined from the Fisher matrix, as well as from Monte Carlo sampling of the posterior probability. We confirm that the uncertainty of $v_\mathrm{los}$ scales with the inverse square root of $S/N$, but also show how this scaling breaks down at low $S/N$ and analyze the error and bias caused by template mismatch. We outline a computationally optimized algorithm to fit multi-exposure data and provide the mathematical model of stellar spectrum fitting that maximizes the so called significance, which allows for calculating the error from the Fisher matrix analytically. We also introduce the effective line count, and provide a scaling relation to estimate the error of $v_\mathrm{los}$ measurement based on the stellar type. Our analysis covers a range of stellar types with parameters that are typical of the Galactic outer disk and halo, together with analogs of stars in M31 and in satellite dwarf spheroidal galaxies around the Milky Way.

astro-ph.IM↗

arXiv2024

Triangulum IV: A Possible Ultra-Diffuse Satellite of M33

We report the detection of a dwarf satellite candidate (Triangulum IV: Tri IV) of the Triangulum galaxy (M33) using the deep imaging of Subaru/Hyper Suprime-Cam (HSC). From the apparent magnitude of the horizontal branch in Tri IV, the heliocentric distance of Tri IV is estimated to be $932^{+49}_{-43}$ kpc, indicating that Tri IV is located at the distance of $75^{+48}_{-40}$ kpc from the M33 center. This means that Tri IV is the probable satellite of M33, because its distance from M33 is within the virial radius of M33. We also estimate its surface brightness of $μ_{\it V} = 29.72^{+0.10}_{-0.10}$ mag arcsec$^{-2}$, and half-light radius of $r_h = 1749^{+523}_{-425}$ pc, suggesting that Tri IV is an ultra-diffuse galaxy or dynamically heated galaxy. The surface brightness of Tri IV is too low to be detected in the previous survey, so this detection suggests that much fainter satellites may be present in the outskirts of M33.

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arXiv2024

A Detailed Chemical Study of the Extreme Velocity Stars in the Galaxy

Two decades on, the study of hypervelocity stars is still in its infancy. These stars can provide novel constraints on the total mass of the Galaxy and its Dark Matter distribution. However how these stars are accelerated to such high velocities is unclear. Various proposed production mechanisms for these stars can be distinguished using chemo-dynamic tagging. The advent of Gaia and other large surveys have provided hundreds of candidate hyper velocity objects to target for ground based high resolution follow-up observations. We conduct high resolution spectroscopic follow-up observations of 16 candidate late-type hyper velocity stars using the Apache Point Observatory and the McDonald Observatory. We derive atmospheric parameters and chemical abundances for these stars. We measure up to 22 elements, including the following nucleosynthetic families: α (Mg, Si, Ca, Ti), light/Odd-Z (Na, Al, V, Cu, Sc), Fe-peak (Fe, Cr, Mn, Co, Ni, Zn), and Neutron Capture (Sr, Y, Zr, Ba, La, Nd, Eu). Our kinematic analysis shows one candidate is unbound, two are marginally bound, and the remainder are bound to the Galaxy. Finally, for the three unbound or marginally bound stars, we perform orbit integration to locate possible globular cluster or dwarf galaxy progenitors. We do not find any likely candidate systems for these stars and conclude that the unbound stars are likely from the the stellar halo, in agreement with the chemical results. The remaining bound stars are all chemically consistent with the stellar halo as well.

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arXiv2024

The Low-Mass Stellar Initial Mass Function in Nearby Ultrafaint Dwarf Galaxies

The stellar initial mass function (IMF) describes the distribution of stellar masses that form in a given star formation event. The long main-sequence lifetimes of low-mass stars mean that the IMF in this regime (below $\sim 1 \rm{M}_\odot$) can be investigated through star counts. Ultrafaint dwarf galaxies are low-luminosity systems with ancient, metal-poor stellar populations. We investigate the low-mass IMF in four such systems (Reticulum II, Ursa Major II, Triangulum II, and Segue 1), using Hubble Space Telescope imaging data that reaches to $\lesssim 0.2 \rm{M}_\odot$ in each galaxy. The analysis techniques that we adopt depend on the number of low-mass stars in each sample. We use Kolmogorov-Smirnov tests for all four galaxies to determine whether their observed apparent magnitude distributions can reject a given combination of IMF parameters and binary fraction for the underlying population. We forward model 1000 synthetic populations for each combination of parameters, and reject those parameters only if each of the 1000 realizations reject the null hypothesis. We find that all four galaxies reject a variety of IMFs, and the IMFs that they cannot reject include those that are identical, or similar, to that of the stellar populations of the Milky Way. We determine the best-fit parameter values for the IMF in Reticulum II and Ursa Major II and find that the IMF in Reticulum II is generally consistent with that of the Milky Way, while the IMF in Ursa Major II is more bottom heavy. The interpretation of the results for Ursa Major II is complicated by possible contamination from two known background galaxy clusters.

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arXiv2024

Detection of a Spatially Extended Stellar Population in M33: A Shallow Stellar Halo?

We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center using Subaru/HSC multi-color imaging. We identify Red Giant Branch (RGB) stars and Red Clump (RC) stars using the surface gravity sensitive $NB515$ filter for the RGB sample, and a multi-color selection for both samples. We construct the radial surface density profile of these RGB and RC stars, and find that M33 has an extended stellar population with a shallow power-law index of $α> -3$, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo which has been detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e. inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profile of RC/RGB stars, and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component is $μ_{\it V} = 35.72 \pm 0.08$ mag arcsec$^{-2}$. If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations.

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arXiv2023

Revealing mass distributions of dwarf spheroidal galaxies in the Subaru-PFS era

The Galactic dwarf spheroidal galaxies (dSphs) provide valuable insight into dark matter (DM) properties and its role in galaxy formation. Their close proximity enables the measurement of line-of-sight velocities for resolved stars, which allows us to study DM halo structure. However, uncertainties in DM mass profile determination persist due to the degeneracy between DM mass density and velocity dispersion tensor anisotropy. Overcoming this requires large kinematic samples and identification of foreground contamination. With 1.25 deg$^2$ and 2394 fibers, PFS plus pre-imaging with Hyper Suprime Cam will make significant progress in this undertaking.

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arXiv2022

Faint Stars in a Faint Galaxy: II. The Low Mass Stellar Initial Mass Function of the Boötes I Ultrafaint Dwarf Spheroidal Galaxy

This paper presents improved constraints on the low-mass stellar initial mass function (IMF) of the Boötes I (Boo~I) ultrafaint dwarf galaxy, based on our analysis of recent deep imaging from the Hubble Space Telescope. The identification of candidate stellar members of Boo~I in the photometric catalog produced from these data was achieved using a Bayesian approach, informed by complementary archival imaging data for the Hubble Ultra Deep Field. Additionally, the existence of earlier-epoch data for the fields in Boo~I allowed us to derive proper motions for a subset of the sources and thus identify and remove likely Milky Way stars. We were also able to determine the absolute proper motion of Boo~I, and our result is in agreement with, but completely independent of, the measurement(s) by \textit{Gaia}. The best-fitting parameter values of three different forms of the low-mass IMF were then obtained through forward modeling of the color-magnitude data for likely Boo~I member stars within an approximate Bayesian computation Markov chain Monte Carlo algorithm. The best-fitting single power-law IMF slope is $α= -1.95_{-0.28}^{+0.32}$, while the best-fitting broken power-law slopes are $α_1 = -1.67_{-0.57}^{+0.48}$ and $α_2 = -2.57_{-1.04}^{+0.93}$. The best-fitting lognormal characteristic mass and width parameters are $\rm{M}_{\rm{c}} = 0.17_{-0.11}^{+0.05} \cal M_\odot$ and $σ=0.49_{-0.20}^{+0.13}$. These broken power-law and lognormal IMF parameters for Boo~I are consistent with published results for the stars within the Milky Way and thus it is plausible that Bo{ö}tes I and the Milky Way are populated by the same stellar IMF.

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