arXiv subjects

Guy Worthey

Publications and source records attributed to Guy Worthey.

At least 19 recordsLinked to original sources

arXiv2025

Stellar Populations with MaNGA: Iron Kink and Nitrogen Fuzz

Recent analysis of 2968 MaNGA early type galaxies has yielded two notable trends with velocity dispersion ($\sigma$) not previously discussed in the literature. First, Fe abundance rises with $\sigma$, but only until $\sigma\approx100$ km s$^{-1}$, after which it falls. This kink is reproduced by TNG100 simulations, implying that hierarchical merger processes might explain it. Second, astrophysical scatter in N is high for galaxies with $\sigma < 100$ km s$^{-1}$. Due to the restricted list of nucleosynthetic sources for N, it is likely that asymptotic giant branch stars provide most of this N. A varied star formation history (compared to that of massive galaxies) along with variable retention and recycling of N-enriched gas might explain the fuzz of N abundance in low-$\sigma$ galaxies. Because a timescale argument seems necessary to explain the nitrogen fuzz, and an initial mass function argument is ruled out, similar timescale arguments for the [Mg/Fe] trend as a function of velocity dispersion are supported.

astro-ph.GA↗

arXiv2025

Revealing the Origins of Galactic Globular Clusters via Their Mg-Al Abundances

Many Galactic globular clusters (GCs) originated in diverse host galaxies before being subsequently incorporated into the Milky Way through hierarchical galaxy assembly. Identifying their origins is crucial for revealing galaxy properties at early times. Traditional classification methods relying on dynamical properties face inherent uncertainties stemming from the evolving Galactic potential and complex merger histories. Chemically driven classification confronts a distinct obstacle: multiple populations - abundance variations in light elements of GC members. In this Letter, we identify primordial populations exhibiting lower [Al/Fe] as reliable tracers of their birth environments' chemical evolution. A clear chemical dichotomy emerges between in-situ and accreted GC populations at [Fe/H] > -1.5, particularly in the [Mg/Fe]-[Al/Fe] plane, indicating that their progenitor galaxies have experienced fundamentally different enrichment histories. While our chemically driven classification demonstrates general consistency with dynamically driven classifications, notable discrepancies emerge: NGC 288 and M4 are reclassified as in-situ, and Terzan 9 as accreted. This chemically driven GC classification provides promising application for Galactic archaeology.

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arXiv2025

The Morphology of Dwarf Galaxies Hosting Variable Active Galactic Nuclei

We analyze Hubble Space Telescope (HST) optical imaging of eight low-mass galaxies hosting active galactic nuclei (AGN) identified via their photometric variability in \cite{baldassare_search_2020}. We use GALFIT to model the 2D galaxy light profiles, and find a diversity of morphologies. The galaxies with regular morphologies are best fit with pseudo-bulges and disks, rather than classical bulges. We estimate black hole masses using scaling relations and find black hole masses of 10$^{3.7-6.6}$ M$_\odot$. We compare this sample to dwarf galaxies with AGN selected via optical spectroscopy. On average, the variable host galaxies have lower mass black holes. We analyze the brightest point source in each galaxy and find their properties are not entirely consistent with star clusters, indicating that they are likely AGN. These point sources are found to have lower luminosities than spectroscopically selected dwarf AGN, but brighter than the point sources in dwarf galaxies not identified as AGN. Our detailed imaging analysis shows that variability selection has the potential to find lower mass black holes and lower luminosity AGN than optical spectroscopy. These active dwarfs may have been missed by spectroscopic searches due to star formation dilution or low gas content.

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arXiv2025

Radial Gradients and Intrinsic Scatter in MaNGA Galaxies

We derive stellar population parameters and radial gradients within 0.65 $R_e$ for spatially resolved spectra of 2968 early-type galaxies from MaNGA, spanning stellar velocity dispersions ($\sigma$) of 50--340 km s$^{-1}$. Light-weighted mean age and C, N, Na, Mg, and Fe abundances are obtained by inverting metallicity-composite stellar population models with isochrones that respond in $T_{\mathrm{eff}}$ to individual element abundance changes. Globally, $\log$(age) increases ($\sim$0.53 dex per decade), [Fe/H] declines slightly ($\sim$-0.06 dex per decade), and [X/Fe] for light elements rises (0.19--0.37 dex per decade for $\log\sigma < 2.0$ but steepens to nearly double slope for $\log\sigma > 2.0$) with $\log\sigma$. [Fe/H] peaks at $\log\sigma \sim 2.0$ and falls on either side. Light-element [X/Fe] anticorrelates with [Fe/H] ($\sim$-0.1 dex per decade). Astrophysical scatter is largest in low-$\sigma$ galaxies, especially for Fe and N. Internally, age gradients are nearly flat in low-$\sigma$ galaxies and slightly negative in high-$\sigma$ systems ($\sim$-0.04 dex per decade). [Fe/H] radial gradients steepen from -0.06 dex per decade to -0.15 dex per decade across $\sigma$, while light elements (except Na) show $\sim$-0.03 dex per decade gradients. Scatter in gradients peaks in high-$\sigma$ galaxies, most strongly for Fe ($\sim$0.23 dex), suggesting comparable numbers of inside-out and outside-in formation. A near-zero ($\sim$-0.03) age and light-element gradient plus mild [Fe/H] gradients supports hierarchical merging for ETG evolution. Simulations match the observed age structure, slope change at $\log\sigma \sim 2.0$, and flat gradients, though they overpredict absolute abundances.

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arXiv2024

On the Age Calibration of Open Clusters using Red Clump Stars

In this study, we extend the dust-independent Hatzidimitriou (1991) relation between cluster age and $d_{B-R}$ color difference between the red giant branch (RGB) and red clump to younger cluster ages. We perform membership analysis on fourteen galactic open clusters using Gaia DR3 astrometry, then compute the difference in color of the RGB and red clump $d_{B-R}$ using Gaia photometry. We also compute $d_{B-R}$ for five fields surrounding Small Magellanic Cloud (SMC) clusters. We find that the trend derived from older clusters does not extrapolate to younger ages and becomes double-valued. We confirm that $d_{B-R}$ is independent of metallicity. Current stellar evolutionary isochrones do not quantitatively reproduce the trend and furthermore predict an increased color gap with a decrease in metallicity that is not echoed in the data. Integrated light models based on current isochrones exaggerate the color change over the $-0.5 <$ [Fe/H] $< 0$ interval at the few-percent level.

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arXiv2023

(P)reheating Effects of the Kähler Moduli Inflation I Model

We investigate reheating in the string-theory-motivated Kähler Moduli Inflation I (KMII) potential, coupled to a light scalar field $χ$ and produce constraints and forecasts based on Cosmic Microwave Background (CMB) and gravitational wave observables. We implement a Markov Chain Monte Carlo (MCMC) sampling method to compute the adopted model's parameter ranges allowed by the current CMB observations. Floquet analysis and numerical lattice simulations are performed to analyze the nonlinear effects of the model's (p)reheating phase. We derive bounds on the $Λ$CDM parameters $A_s$, $n_s$, $n_{\mathrm{run}}$, and $r$ based on \textit{Planck} results, finding that correlations between model parameters severely constrain the range of these parameters allowed within this model. While the KMII potential's non-vanishing minimum may provide a possible source for the observed dark energy density $ρ_{\mathrm{DE}}$ this cannot be tested with current observations. We estimate the $95\%$ CI bounds on the inflaton mass $m_ϕ$ and reheating temperature $T_{\mathrm{reh}}$ to be $2.1 \times 10^{13} \, \mathrm{GeV} \lesssim m_ϕ \lesssim 3.2 \times 10^{13} \, \mathrm{GeV}$ and $T_{\mathrm{reh}} \gtrsim 1.8 \times 10^{3} \, \mathrm{GeV}$, respectively. We observe {both} self-resonance and parametric resonance instability band structures in our Floquet analysis results. Finally, we do not observe any formation of oscillon configurations in our lattice simulations; however, our results predict a stochastic gravitational wave background generated during preheating that would be observable today in the $10^{9}$ - $10^{11} \, \mathrm{Hz}$ frequency range.

astro-ph.CO↗

arXiv2023

HST Low Resolution Stellar Library

Hubble Space Telescope's (HST) Space Telescope Imaging Spectrograph (STIS) targeted 556 stars in a long-running program called Next Generation Spectral Library (NGSL) via proposals GO9088, GO9786, GO10222, and GO13776. Exposures through three low resolution gratings provide wavelength coverage from 0.2 $< λ<$ 1 $μ$m at $λ/Δλ\sim$ 1000, providing unique coverage in the ultraviolet (UV). The UV grating (G230LB) scatters red light and this results in unwanted flux that becomes especially troubling for cool stars. We applied scattered light corrections based on \cite{2022stis.rept....5W} and flux corrections arising from pointing errors relative to the center of the 0\farcs2 slit. We present 514 fully reduced spectra, fluxed, dereddened, and cross-correlated to zero velocity. Because of the broad spectral range, we can simultaneously study H$α$ and Mg II $λ$2800, indicators of chromospheric activity. Their behaviours are decoupled. Besides three cool dwarfs and one giant with mild flares in H$α$, only Be stars show strong H$α$ emission. Mg2800 emission, however, strongly anti-correlates with temperature such that warm stars show absorption and stars cooler than $5000 \: \! \rm{K}$ universally show chromospheric emission regardless of dwarf/giant status or metallicity. Transformed to Mg2800 flux emerging from the stellar surface, we find a correlation with temperature with approximately symmetric astrophysical scatter, in contrast to other workers who find a basal level with asymmetric scatter to strong values. Unsurprisingly, we confirm that Mg2800 activity is variable.

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arXiv2022

Scattered Light in STIS Grating G230LB

The G230LB grating used with STIS's CCD detector scatters red light. In red objects, the scattered light mingles with the ultraviolet signal, causing spurious short-wavelength flux and weakening absorption features. Recent calibration observations characterize the scattered light using duplicate observations with the MAMA detector and similar grating G230L. The full two-dimensional spectrum contains little helpful information to mitigate the scattered light problem. For one-dimensional, extracted spectra, the scattered light can be approximately modeled as a ramped pedestal whose amplitude is proportional to the object's V-band flux. We present formulae for scattered light corrections. For stars warmer than G0 spectral type, correction is superfluous. Off-slit-center positioning appears not to affect the properties of the scattered light. Therefore, we are able to extrapolate correction formulae for extended objects from the point source formulae. Polynomials for flux corrections due to off-center slit positioning in the 0.2 arcsec slit are also tabulated.

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arXiv2022

Chemically Peculiar Stars in Integrated Light Stellar Population Models and Local Group Galaxies

Integrated-light models that incorporate common types of chemically peculiar (CP) stars are assembled using synthetic spectra. Selected spectral features encode significant age information for populations with ages $\sim$50 Myr $<$ age $< \sim$2 Gyr. Due to the alleviation of template mismatch, the inclusion of CP star features in model spectra improves the accuracy of recovered stellar population parameters, but we are not able to show that new or unique age information can be extracted from the weak CP features compared to continuum fitting and strong-feature strengths, at least at the present state of the art. An age-extraction routine that recovers 2- and 3-burst age structures is employed to analyze the spectra of local group galaxies. NGC 224 (M31) has a stellar population too old for the types of CP stars we examine. NGC 221 (M32) also shows no CP spectral features. It appears to contain a component at age $\sim$1 Gyr at 1% by mass in addition to its dominant 4.7 Gyr population. Unlike SDSS galaxy spectrum averages, NGC 205 (M110) contains no features due to HgMn stars. This excludes the age range associated with HgMn production, and its near-nuclear spectrum is best fit by a 68$\pm$2 Myr population superimposed on an older population with a 1.85$\pm$0.1 Gyr component. Both NGC 205 and NGC 221 have an ancient component whose mass is not easy to constrain given the overwhelming light-dominance of the younger populations.

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arXiv2022

Individual element sensitivity for stellar evolutionary isochrones

Stellar evolution calculations with variable abundance ratios were used to gauge the effects on temperatures, luminosities, and lifetimes in various phases. Individual elements C, N, O, Mg, Si, and Fe were included. Most of the effect relevant to integrated light models is contained in the temperature variable, as opposed to the timescale or luminosity. We derive a recipe for including abundance-sensitive temperature effects that is applicable to existing isochrone grids. The resultant enhanced isochrones are incorporated into composite stellar population models and compared with galaxy data from the Sloan Digital Sky Survey. A severe oxygen-age degeneracy is apparent, 2 - 3 Gyr per 0.1 dex in [O/R], where R represents a heavy element such as Fe. Over the range of early-type galaxy velocity dispersion, the spans of all abundance ratios are reduced but the age range increases, systematically older. Allowing Fe-peak elements the freedom to vary accentuates this increase of age span. Overall, these results sharpen the age-mass correlation known as downsizing but decrease the steepness of abundance ratio gradients. Both of these observations, in turn, imply a more robust contribution from gas free mergers in the histories of typical elliptical galaxies.

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arXiv2021

(P)reheating Effects of a Constrained Kähler Moduli Inflation Model

In this talk, I discuss the effects, viability, and predictions of the string-theory-motivated Kähler Moduli Inflation I (KMII) potential, coupled to a light scalar field $χ$, which can provide a possible source for today's dark energy density due to the potential's non-vanishing minimum. Although the model is consistent with the current measured Cosmic Microwave Background (CMB) data, tighter constraints from future observations are required to test the viability of the KMII potential with its minimum equivalent to the observed cosmological constant's energy density $ρ_{Λ_{\mathrm{obs}}}$. We implement a Markov Chain Monte Carlo (MCMC) sampling method to compute the allowed model parameter ranges and bounds on the inflaton's mass $m_ϕ$ and reheating temperature $T_{\mathrm{reh}}$. Additionally, our lattice simulations predict stochastic gravitational-wave backgrounds generated during the inflaton oscillations that would be observable today in the $10^{9}$-$10^{11} \, \mathrm{Hz}$ frequency range. All the results and details will be included in our upcoming paper with the same title.

astro-ph.CO↗

arXiv2021

The frequency by mass of Galactic carbon stars inferred from Gaia measurements of star cluster members

We investigate the frequency of occurrence of Galactic carbon stars as a function of progenitor mass using Gaia data. Small number statistics limit fidelity, but C star frequency agrees with that observed in the Magellanic Clouds (MCs) down to $m \approx1.67$ M$_\odot$. At $m \approx 1.38$ M$_\odot$, the frequency rises by a factor of three even though the frequency appears to drop to zero for the MCs. In fact this is due to a lack of clusters at the key age range in the MCs. At $m \approx 1.24$ M$_\odot$ and below, no C stars are observed, corresponding to ages older than 4 Gyr. Within uncertainties, C~star frequency in M 31 is consistent with that of the Galaxy and the MCs. We find an ambiguous C-star candidate at $\sim$7 M$_\odot$.

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arXiv2018

Chemical and kinematic analysis of CN-strong Metal-poor Field Stars in LAMOST DR3

The large amount of chemical and kinematic information available in large spectroscopic surveys have inspired the search for chemically peculiar stars in the field. Though these metal-poor field stars ([Fe/H$]<-1$) are commonly enriched in nitrogen, their detailed spatial, kinematic, and chemical distributions suggest that various groups may exist, and thus their origin is still a mystery. To study these stars statistically, we increase the sample size by identifying new CN-strong stars with LAMOST DR3 for the first time. We use CN-CH bands around 4000 Å~to find CN-strong stars, and further separate them into CH-normal stars (44) and CH-strong (or CH) stars (35). The chemical abundances from our data-driven software and APOGEE DR 14 suggest that most CH-normal stars are N-rich, and it cannot be explained by only internal mixing process. The kinematics of our CH-normal stars indicate a substantial fraction of these stars are retrograding, pointing to an extragalactic origin. The chemistry and kinematics of CH-normal stars imply that they may be GC-dissolved stars, or accreted halo stars, or both.

astro-ph.GA↗

arXiv2018

Hot Star Extension to the Hubble Space Telescope Stellar Spectral Library

Libraries of stellar spectra find many uses in astrophysics, from photometric calibration to stellar population synthesis. We present low resolution spectra of 40 stars from 0.2 micrometers (ultraviolet) to 1.0 micrometers (near infrared) with excellent fluxing. The stars include normal O-type stars, helium-burning stars, and post-asymptotic giant branch (PAGB) stars. Spectra were obtained with the Space Telescope Imaging Spectrograph (STIS) installed in the Hubble Space Telescope (HST) using three low resolution gratings, G230LB, G430L, and G750L. Cosmic ray hits and fringing in the red were corrected. A correction for scattered light was applied, significant only for our coolest stars. Cross-correlation was used to bring the spectra to a common, final, zero velocity wavelength scale. Finally, synthetic stellar spectra were used to estimate line of sight dust extinction to each star, and a five-parameter dust extinction model was fit, or a one-parameter fit in the case of low extinction. These spectra dovetail with the similar Next Generation Stellar Library (NGSL) spectra, extending the NGSL's coverage of stellar parameters, and extending to helium burning stars and stars that do not fuse. The fitted dust extinction model showed considerable variation from star to star, indicating variations in dust properties for different lines of sight. Interstellar absorption lines are present in most stars, notably MgII.

astro-ph.SR↗

arXiv2017

Optical spectroscopy and initial mass function of $z=0.4$ red galaxies

Spectral absorption features can be used to constrain the stellar initial mass function (IMF) in the integrated light of galaxies. Spectral indices used at low redshift are in the far red, and therefore increasingly hard to detect at higher and higher redshifts as they pass out of atmospheric transmission and CCD detector wavelength windows. We employ IMF-sensitive indices at bluer wavelengths. We stack spectra of red, quiescent galaxies around $z=0.4$, from the DEEP2 Galaxy Redshift Survey. The $z=0.4$ red galaxies have 2 Gyr average ages so that they cannot be passively evolving precursors of nearby galaxies. They are slightly enhanced in C and Na, and slightly depressed in Ti. Split by luminosity, the fainter half appears to be older, a result that should be checked with larger samples in the future. We uncover no evidence for IMF evolution between $z=0.4$ and now, but we highlight the importance of sample selection, finding that an SDSS sample culled to select archetypal elliptical galaxies at z$\sim$0 is offset toward a more bottom heavy IMF. Other samples, including our DEEP2 sample, show an offset toward a more spiral galaxy-like IMF. All samples confirm that the reddest galaxies look bottom heavy compared with bluer ones. Sample selection also influences age-color trends: red, luminous galaxies always look old and metal-rich, but the bluer ones can be more metal-poor, the same abundance, or more metal-rich, depending on how they are selected.

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arXiv2015

On Disentangling IMF Degeneracies in Integrated Light

The study of extragalactic integrated light can yield partial information on stellar population ages, abundances, and the initial mass function (IMF). The power-law slope of the IMF has been studied in recent investigations with gravity-sensitive spectral indicators that hopefully measure the ratio between KM dwarfs and giants. We explore two additional effects that might mimic the effects of the IMF slope in integrated light, the low mass cutoff (LMCO) and a variable contribution of light from the asymptotic giant branch (AGB). We show that the spectral effects of these three (IMF slope, LMCO, AGB strength) are subtle compared to age-abundance effects. We illustrate parameter degeneracies and covariances and conclude that the three effects can be disentangled, but only in the regime of very accurate observations, with enhanced effectiveness if high-precision photometry is combined with spectroscopy.

astro-ph.GA↗

arXiv2015

Precision age indicators that exploit chemically peculiar stars

We would like to find a way to improve the determination of galaxy star formation history from integrated light spectroscopy. To this end, several classes of chemically peculiar (CP) stars arise during the course of normal evolution in single stars and noninteracting binary stars. An aging stellar population has periods of time in which CP stars contribute to the integrated light, and others in which the contributions fade. The HgMn stars, for example, occupy a narrow temperature range of 10500 to 16000 K, which maps to a narrow range of ages. Wolf-Rayet stars, He-poor stars, Bp-Ap stars, Am-Fm stars, and C stars all become very common in a normal stellar population at various ages between zero and several Gyr, fading in and out in a way that is analogous to features used in stellar spectral classification. We examine population fractions and light fractions in order to assess the feasibility of using CP stars as age tracers. We find that, even though CP stars do not usually dominate in number, there are enough of them so that the CP spectral features are detectable in high-quality integrated spectra of young and intermediate age stellar populations. The new technique should be calibratable and useful. Furthermore, using CP signatures as age dating tools sidesteps reliance on photometry that is susceptible to dust and Balmer features that are susceptible to nebular fill-in.

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arXiv2014

Composite Stellar Populations and Element by Element Abundances in the Milky Way Bulge and Elliptical Galaxies

This paper explores the integrated-light characteristics of the Milky Way (MW) bulge and to what extent they match those of elliptical galaxies in the local universe. We model composite stellar populations with realistic abundance distribution functions (ADFs), tracking the trends of individual elements as a function of overall heavy element abundance as actually observed in MW bulge stars. The resultant predictions for absorption feature strengths from the MW bulge mimic elliptical galaxies better than solar neighborhood stars do, but the MW bulge does not match elliptical galaxies, either. Comparing bulge versus elliptical galaxies, Fe, Ti, and Mg trend about the same for both but C, Na, and Ca seem irreconcilably different. Exploring the behavior of abundance compositeness leads to the concepts of "red lean" where a narrower ADF appears more metal rich than a wide one, and "red spread" where the spectral difference between wide and narrow ADFs increases as the ADF peak is moved to more metal-rich values. Tests on the systematics of recovering abundance, abundance pattern, and age from composite stellar populations using single stellar population models were performed. The chemical abundance pattern was recovered adequately, though a few minor systematic effects were uncovered. The prospects of measuring the width of the ADF of an old stellar population were investigated and seem bright using UV to IR photometry.

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