Integrated Light Spectra Research Articles

Measuring the Chemodynamics and Ages of the M32 and M110 Dwarf Galaxies with APOGEE

We present a full-spectrum-fitting analysis of the central kinematics and chemistry of the Andromeda dwarf satellite galaxies M32 and M110. We use an Markov Chain Monte Carlo routine to fit high-resolution, near-infrared, integrated-light spectra from APOGEE with empirical simple stellar population templates constructed from individual APOGEE stellar spectra. This yields the best-fitting mean radial velocity, velocity dispersion, metallicity, α abundance, and age for each spectrum. In general, our results are consistent with literature values where available, and we explore possible reasons where offsets are measured. This study was presented in a poster at the 243rd meeting of the American Astronomical Society in 2024 January.

Detailed chemical composition of the globular cluster Sextans A GC-1 on the outskirts of the Local Group

The chemical composition of globular clusters (GCs) across the Local Group provides information on chemical abundance trends. Studying GCs in isolated systems in particular provides us with important initial conditions plausibly unperturbed by mergers and tidal forces from the large Local Group spirals. We present a detailed chemical abundance analysis of Sextans A GC-1. The host galaxy, Sextans A, is a low-surface-brightness dwarf irregular galaxy located on the edge of the Local Group. We derive the dynamical mass of the GC together with the mass-to-light ratio and the abundances of the alpha , Fe-peak, and heavy elements. Abundance ratios were determined from the analysis of an optical integrated-light spectrum of Sextans A GC-1, obtained with UVES on the VLT. We apply non-local thermodynamic equilibrium (NLTE) corrections to Mg, Ca, Ti, Fe, and Ni. The GC appears to be younger and more metal-poor than the majority of the GCs of the Milky Way, with an age of 8.6pm 2.7 Gyr and $ Fe/H dex. The calculated dynamical mass is dyn odot $, which results in an atypically high value of the mass-to-light ratio, 4.35pm 1.40 M$_ odot $/L$_ V odot $. Sextans A GC-1 has varying alpha elements -- the Mg abundance is extremely low, Ca and Ti are solar-scaled or mildly enhanced, and Si is enhanced. The measured values are $ Mg/Fe Ca/Fe Ti/Fe and $ Si/Fe which makes the mean alpha abundance (excluding Mg) to be enhanced $ <Si,Ca,Ti>/Fe NLTE The Fe-peak elements are consistent with scaled-solar or slightly enhanced abundances: $ Cr/Fe Mn/Fe Sc/Fe and $ Ni/Fe The heavy elements measured are Ba, Cu, Zn, and Eu. Ba and Cu have sub-solar abundance ratios ($ Ba/Fe and $ Cu/Fe <-0.343$), while Zn and Eu are consistent with their upper limits being solar-scaled and enhanced, $ Zn/Fe <+0.171$ and $ Eu/Fe The composition of Sextans A GC-1 resembles the overall pattern and behaviour of GCs in the Local Group. The anomalous values are the mass-to-light ratio and the depleted abundance of Mg. There is no definite explanation for such an extreme abundance value. Variations in the initial mass function or the presence of an intermediate-mass black hole might explain the high mass-to-light ratio value.

Horizontal branch structure, age, and chemical composition for very metal-poor extragalactic globular clusters

ABSTRACT This paper presents the results of analysing the integrated light (IL) low-resolution spectra of globular clusters (GCs) in the M31 and Centaurus A groups of galaxies. The sample consists of eight very metal-poor GCs ($\rm [Fe/H]\le -2$ dex) with high signal-to-noise ratio spectra acquired with the telescopes: the 6-m SAO RAS (BTA), the Southern African Large (SALT), and the 6.5-m Magellan (MMT). We study the influence of contribution of the horizontal branch stars on the hydrogen Balmer line profiles in the IL spectra. By modelling the Balmer lines, as well as the metal lines in the observed spectra, we determine the optimum parameters of stellar evolution isochrones and, consequently, the parameters of the atmospheres of the cluster stars. For all the studied GCs, the parameters of horizontal branch stars set by the selected isochrones, the corresponding ages, and carbon abundances are presented for the first time. The abundances of several other elements (Mg, Ca, Ti, Cr, and Mn) were determined for five GCs for the first time. All the studied GCs have blue horizontal branches and are older than 10 Gyr. Their chemical abundances, with the exception of Mg and Mn, are in good agreement with the abundances of stars in the Galactic field. The reasons of low [Mg/Fe] and of high [Mn/Fe] are discussed. Study of the fundamental properties of stellar populations in old GCs facilitates a better understanding of the formation processes of their parent galaxies and nucleosynthesis in the early Universe.

The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated-light Spectroscopy

We present an analysis of nearly 1000 near-infrared, integrated-light spectra from APOGEE in the inner ∼7 kpc of M31. We utilize full-spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [α/M]. With this, we determine the mean kinematics, metallicities, α abundances, and ages of the stellar populations of M31's bar, bulge, and inner disk (∼4–7 kpc). We find a nonaxisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is less metal-rich (mean [M/H] = dex) than the disk, features minima in metallicity on either side of the bar ([M/H] ∼ −0.2), and is enhanced in α abundance (mean [α/M] = ). The disk of M31 within ∼7 kpc is enhanced in both metallicity ([M/H] = ) and α abundance ([α/M] = ). Both of these structural components are uniformly old at ≃12 Gyr. We find the mean metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis (0.043 ± 0.021 dex kpc−1). This gradient is the result of changing light contributions from the bulge and disk. The chemodynamics of stellar populations encodes information about a galaxy’s chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies.

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 developed a 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 introduced 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 applied the methods to data of the star cluster M 54. We show that the SVD-MCMC inferences match those from standard MCMC, but they are a factor 5–10 faster to compute. We apply ULoG to the inferred M 54 age/metallicity distributions, identifying between two or three significant, distinct populations amongst its stars.

KDG 64: a large dwarf spheroidal or a small ultradiffuse satellite of Messier 81

ABSTRACTLow-mass early-type galaxies, including dwarf spheroidals (dSph) and brighter dwarf ellipticals (dE), dominate the galaxy population in groups and clusters. Recently, an additional early-type population of more extended ultradiffuse galaxies (UDGs) has been identified, sparking a discussion on the potential morphological and evolutionary connections between the three classifications. Here, we present the first measurements of spatially resolved stellar kinematics from deep integrated-light spectra of KDG 64 (UGC 5442), a large dSph galaxy in the M 81 group. From these data, we infer stellar population properties and dark matter halo parameters using Jeans dynamical modelling. We find an old, metal-poor stellar population with no young stars and a dark matter mass fraction of ∼90 per cent within the half-light radius. These properties and the position of KDG 64 on the Fundamental Plane indicate that it is a local analogue of smaller UDGs in the Coma and Virgo clusters and is probably a transitional dSph-UDG object. Its evolutionary path cannot be uniquely established from the existing data, but we argue that supernovae feedback and tidal heating played key roles in shaping KDG 64.

Chemical composition of the young massive cluster NGC 1569-B

Context.The chemical composition of young massive clusters (YMCs) provides stellar population information on their host galaxy. As potential precursors of globular clusters (GCs), their properties can help us understand the origins of GCs and their evolution.Aims.We present a detailed chemical abundance analysis of the YMC NGC 1569-B. The host galaxy, NGC 1569, is a dwarf irregular starburst galaxy at a distance of 3.36 ± 0.20 Mpc. We derive the abundances of theα, Fe-peak, and heavy elements.Methods.We determined the abundance ratios from the analysis of an optical integrated-light (IL) spectrum of NGC 1569-B, obtained with the HIRES echelle spectrograph on the Keck I telescope. We considered different red-to-blue supergiant ratios (NRSG/NBSG), namely: the ratio obtained from a theoretical isochrone (NRSG/NBSG = 1.24), the ratio obtained from a resolved colour–magnitude diagram of the YMC (NRSG/NBSG = 1.53), and the ratio that minimises theχ2when comparing our model spectra with the observations (NRSG/NBSG = 1.90). We adopted the latter ratio for our resulting chemical abundances.Results.The derived iron abundance is sub-solar with [Fe/H] = −0.74 ± 0.05. In relation to the scaled solar composition, we find enhancedα-element abundances, [⟨Mg, Si, Ca, Ti⟩/Fe]= + 0.25 ± 0.11, with a particularly high Ti abundance of +0.49 ± 0.05. Other super-solar elements include [Cr/Fe] = +0.50 ± 0.11, [Sc/Fe] = +0.78 ± 0.20, and [Ba/Fe] = +1.28 ± 0.14, while other Fe-peak elements are close to scaled solar abundances: ([Mn/Fe] = −0.22 ± 0.12 and [Ni/Fe] = +0.13 ± 0.11).Conclusions.The composition of NGC 1569-B resembles the stellar populations of the YMC NGC 1705-1, located in a blue compact dwarf galaxy. The two YMCs agree with regard toα-elements and the majority of the Fe-peak elements, except for Sc and Ba, which are extremely super-solar in NGC 1569-B – and higher than in any YMC studied so far. The blue part of the optical spectrum of a young population is still a very challenging wavelength region to analyse using IL spectroscopic studies. This is due to the uncertain contribution to the light from blue supergiant stars, which can be difficult to disentangle from turn-off stars, even when resolved photometry is available. We suggest that the comparison of model fits at different wavelengths offers a route to determining the red-to-blue supergiant ratio from IL spectroscopy.

Chemical Abundances of Young Massive Clusters in NGC 1313∗ ∗ Based on observations made with ESO telescopes at the La Silla Paranal Observatory under program ID 084.B-0468(A).

We analyze spectroscopic observations of five young massive clusters (YMCs) in the barred spiral galaxy NGC 1313 to obtain detailed abundances from their integrated light. Our sample of YMCs was observed with the X-shooter spectrograph on the Very Large Telescope. We make use of theoretical isochrones to generate synthetic integrated-light spectra, iterating on the individual elemental abundances until converging on the best fit to the observations. We measure abundance ratios for [Ca/Fe], [Ti/Fe], [Mg/Fe], [Cr/Fe], and [Ni/Fe]. We estimate an Fe abundance gradient of −0.124 ± 0.034 dex kpc−1, and a slightly shallower α gradient of −0.093 ± 0.009 dex kpc−1. This is in contrast to previous metallicity studies that focused on the gas-phase abundances, which have found NGC 1313 to be the highest-mass barred galaxy known not to have a radial abundance gradient. We propose that the gradient discrepancy between the different studies originates from the metallicity calibrations used to study the gas-phase abundances. We also observe an age–metallicity trend that supports a scenario of constant star formation throughout the galaxy, with a possible burst in star formation in the southwest region where YMC NGC 1313–379 is located.

Disentangling the Signatures of Blended-light Atmospheres in L/T Transition Brown Dwarfs

We present a technique to identify spectrophotometrically variable L7−T3 brown dwarfs with single-epoch, low-resolution, near-infrared SpeX spectra. We calculated spectral indices on known variable brown dwarfs and used them to select 11 index–index parameter spaces where known variables can be distinguished from the rest of the general population of brown dwarfs. We find 62 candidate variables, 12 of which show significant variability amplitude in independent photometric monitoring surveys. This technique constitutes the first formal method to identify a time-dependent effect such as variability from peculiarities in their integrated light spectra. This technique will be a useful tool to prioritize targets for future photometric and spectroscopic monitoring in the era of the James Webb Space Telescope and 30 m-class telescopes.

The chemical composition of globular clusters in the Local Group

We present detailed chemical abundance measurements for 45 globular clusters (GCs) associated with galaxies in (and, in one case, beyond) the Local Group. The measurements are based on new high-resolution integrated-light spectra of GCs in the galaxies NGC 185, NGC 205, M 31, M 33, and NGC 2403, combined with reanalysis of previously published observations of GCs in the Fornax dSph, WLM, NGC 147, NGC 6822, and the Milky Way. The GCs cover the range −2.8 < [Fe/H] < −0.1 and we determined abundances for Fe, Na, Mg, Si, Ca, Sc, Ti, Cr, Mn, Ni, Cu, Zn, Zr, Ba, and Eu. Corrections for non local thermodynamic equilibrium effects are included for Na, Mg, Ca, Ti, Mn, Fe, Ni, and Ba, building on a recently developed procedure. For several of the galaxies, our measurements provide the first quantitative constraints on the detailed composition of their metal-poor stellar populations. Overall, the GCs in different galaxies exhibit remarkably uniform abundance patterns of the α, iron-peak, and neutron-capture elements, with a dispersion of less than 0.1 dex in [α/Fe] for the full sample. There is a hint that GCs in dwarf galaxies are slightly less α-enhanced (by ∼0.04 dex on average) than those in larger galaxies. One GC in M 33 (HM33-B) resembles the most metal-rich GCs in the Fornax dSph (Fornax 4) and NGC 6822 (SC7) by having α-element abundances closer to scaled-solar values, possibly hinting at an accretion origin. A principal components analysis shows that the α-element abundances strongly correlate with those of Na, Sc, Ni, and Zn. Several GCs with [Fe/H] < −1.5 are deficient in Mg compared to other α-elements. We find no GCs with strongly enhanced r-process abundances as reported for metal-poor stars in some ultra-faint dwarfs and the Magellanic Clouds. The similarity of the abundance patterns for metal-poor GCs in different environments points to similar early enrichment histories and only allow for minor variations in the initial mass function.

Chemical composition and ages of four globular clusters in M31 from the analysis of their integrated-light spectra

Abstract We compare the results on the chemical composition of four globular clusters (GCs) in M31 (Bol 6, Bol 20, Bol 45, and Bol 50) (Maricheva M. 2021. Study of integrated spectra of four globular clusters in M 31. Astrophys. Bull. 76:389–404. doi: https://doi.org/10.1134/S199034132104009X) to the available literature data on integrated-light spectroscopy of GCs with similar ages and chemical abundances in our Galaxy and M31 and on the chemical abundances of stars in two galaxies. The clusters and their literature analogues are of moderate metallicity − 1.1 < [ Fe/H ] < − 0.75 -1.1\lt \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}]\lt -0.75 dex and old (10 Gyr). Mg, Ca, and C abundances of four GCs are higher than literature estimates for the GCs in M31 with [ Fe/H ] ∼ − 1 \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}] \sim -1 dex obtained using high-resolution integrated-light spectroscopy methods. Using literature data, we did not find complete analogues for the studied clusters in our Galaxy and M31 in terms of age, helium mass fraction ( Y Y ), and chemical composition. The alpha-element abundances in four clusters are about 0.2 dex higher than the average for stars in the Galactic field at [ Fe/H ] ∼ − 1 \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}] \sim -1 dex. We suggest that these and M. Maricheva’s (Maricheva M. 2021) findings about lower metallicities of the studied objects than the average metallicity of red giants in the M31 halo and about the abundances of alpha-process elements in them corresponding to the average value for stars in the M31 inner halo likely indicate that the star formation process in the vicinity of M31 at the time of our sample cluster formation was complex with the inflow of fresh gas from the intergalactic medium and violent star forming events associated with SNe II bursts.

How well can we determine ages and chemical abundances from spectral fitting of integrated light spectra?

ABSTRACT The pixel-to-pixel spectral fitting technique is often used in studies of stellar populations. It enables the user to infer several parameters from integrated light spectra such as ages and chemical abundances. In this paper, we examine the question of how the inferred parameters change with the choice of wavelength range used. We have employed two different libraries of integrated light spectra of globular clusters (GCs) from the literature and fitted them to stellar population models using the code Starlight. We performed tests using different regions of the spectra to infer reddening, ages, [Fe/H], and [α/Fe]. Comparing our results to age values obtained from isochrone fitting and chemical abundances from high-resolution spectroscopy, we find that: (1) the inferred parameters change with the wavelength range used; (2) the method in general retrieves good reddening estimates, specially when a wider wavelength range is fitted; (3) the ideal spectral regions for determination of age, [Fe/H], and [α/Fe] are 4170–5540, 5280–7020, and 4828–5364 Å, respectively; (4) the retrieved age values for old metal-poor objects can be several Gyr younger than those resulting from isochrone fitting. We conclude that, depending on the parameter of interest and the accuracy requirements, fitting the largest possible wavelength range may not necessarily be the best strategy.

Age and chemical composition of the globular cluster NGC 6652

We present the results of determination of the age, helium mass fraction (Y), metallicity ([Fe/H]), and abundances of the elements C, N, O, Na, Mg, Ca, Ti, C and Mn for the Galactic globular cluster NGC 6652. We use its medium-resolution integrated-light spectrum from the library of Schiavon and our population synthesis method to fulfill this task. We select the evolutionary isochrone and stellar mass function for our analysis, which provide the best approximation to the shapes and intensities of the observed Balmer line profiles. The determined elemental abundances, age and metallicity are characteristic of stellar populations in the Galactic Bulge.

Analysis of Integrated-Light Spectra of Galactic Globular Clusters

We present the results of determination of the age, helium mass fraction (Y) in terms of the used stellar evolutionary models, metallicity ([Fe/H]), and abundances of the elements C, O, Na, Mg, Ca, Ti, Cr and Mn for 26 globular clusters of the Galaxy. In this work, we apply a method developed by us that employs medium-resolution integrated-light spectra of globular clusters and models of stellar atmospheres and it is supplemented in this paper by the automatic calculation of microturbulence velocities of stars in the studied objects. Based on the data obtained for 26 objects, as well as on the results of our previous studies, it is shown that the abundances of chemical elements, that we measured, with the exception of carbon, are consistent with the literature estimates from the analysis of integrated-light spectra of clusters and from high-resolution spectroscopic observations of their brightest stars. Our estimates of [C/Fe] are consistent with the literature values obtained from the integrated-light spectra of clusters. We interpret the systematic difference between the derived [C/Fe] for globular clusters and the literature [C/Fe] values for the brightest stars of the clusters as a change of the chemical composition in the atmospheres of stars during their evolution. The estimated absolute ages and average Y for the clusters are in a reasonable agreement with the literature data from the analysis of color magnitude diagrams of the objects.

Using the Age and Metallicity of 8 Star Clusters to confirm the Chemical Enrichment History of the LMC

In this paper, we find the ages and metallicities of 8 Large Magellanic Cloud (LMC) star clusters obtained from integrated light spectra and use them to obtain the Chemical Enrichment History of the host galaxy. Our results are in good agreement with those obtained from resolved studies. This confirms that this method can be used for far away galaxies for which their star clusters are not resolved.

NLTE modelling of integrated light spectra

Aims.We study the effects of non-local thermodynamic equilibrium (NLTE) on the abundance analysis of barium, magnesium, and manganese from integrated light spectroscopy, as typically applied to the analysis of extra-galactic star clusters and galaxies. In this paper, our reference object is a synthetic simple stellar population (SSP) representing a mono-metallicα-enhanced globular cluster with the metallicity [Fe/H] = −2.0 and the age of 11 Gyr.Methods.We used the MULTI2.3 program to compute LTE and NLTE equivalent widths of spectral lines of Mg I, Mn I, and Ba II ions, which are commonly used in abundance analyses of extra-galactic stellar populations. We used ATLAS12 model atmospheres for stellar parameters sampled from a model isochrone to represent individual stars in the model SSP. The NLTE and LTE equivalent widths calculated for the individual stars were combined to calculate the SSP NLTE corrections.Results.We find that the NLTE abundance corrections for the integrated light spectra of the metal-poor globular cluster are significant in many cases, and often exceed 0.1 dex. In particular, LTE abundances of Mn are consistently under-estimated by 0.3 dex for all optical lines of Mn I studied in this work. On the other hand, Ba II, and Mg I lines show a strong differential effect: the NLTE abundance corrections for the individual stars and integrated light spectra are close to zero for the low-excitation lines, but they amount to − 0.15 dex for the strong high-excitation lines. Our results emphasise the need to take NLTE effects into account in the analysis of spectra of individual stars and integrated light spectra of stellar populations.

Modeling and analysis of medium-resolution integrated-light spectra of globular clusters in dwarf galaxies

AbstractThe study of ages, helium mass fraction (Y) and chemical composition of globular clusters in dwarf galaxies is important for understanding the physical conditions at the main evolutionary stages of the host galaxies and for constraining the build-up histories of large galaxies. We present the analysis of integrated-light spectra of 8 extragalactic and 20 Galactic globular clusters (GCs) using our population synthesis method. We calculate synthetic spectra of GCs according to the defined stellar mass functions using model atmospheres and stellar parameters ([Fe/H], Teff, and logg) set by theoretical isochrones. The main advantage of our method is the ability to determine not only chemical composition but also the age and mean Y in a cluster by modelling and analysis of Balmer absorption lines. The knowledge of Y and anomalies of light elements in star clusters is one of the key points for understanding the phenomenon of multiple stellar populations.

On collision course: The nature of the binary star cluster NGC2006/SL 538

Context. The Large Magellanic Cloud (LMC) is known to be the host of a rich variety of star clusters of all ages. A large number of them is seen in close projected proximity. Ages have been derived for few of them showing differences up to few million years, hinting at being binary star clusters. However, final confirmation through spectroscopy measurements and dynamical analysis is needed. Aims. In the present work we focus on one of these LMC cluster pairs (NGC 2006–SL 538) and aim to determine whether the star cluster pair is a bound entity and, therefore, a binary star cluster or a chance alignment. Methods. We used the Magellan Inamori Kyocera Echelle (MIKE) high-resolution spectrograph on the 6.5 m Magellan-II Clay telescope at Las Campanas Observatory to acquire integrated-light spectra of the two clusters, measuring their radial velocities with individual absorption features and cross-correlation of each spectrum with a stellar spectral library. Results. We measured radial velocities by two methods: first by direct line-profile measurement yields νr = 300.3 ± 5 ± 6 km s−1 for NGC 2006 and νr = 310.2 ± 4 ± 6 km s−1 for SL 538. The second one is derived by comparing observed spectra with synthetic bootstrapped spectra yielding νr = 311.0 ± 0.6 km s−1 for NGC 2006 and νr = 309.4 ± 0.5 km s−1 for SL 538. Finally when spectra are directly compared, we find a Δν = 1.08 ± 0.47 km s−1. Full-spectrum spectral energy distribution fits reveal that the stellar population ages of both clusters lie in the range 13–21 Myr with a metallicity of Z = 0.008. We find indications for differences in the chemical abundance patterns as revealed by the helium absorption lines between the two clusters. The dynamical analysis of the system shows that the two clusters are likely to merge within the next ∼150 Myr to form a star cluster with a stellar mass of ∼104 M⊙. Conclusions. The NGC 2006–SL 538 cluster pair shows radial velocities, stellar population and dynamical parameters consistent with a gravitational bound entity and, considering that the velocity dispersion of the stars in LMC is ≲20 km s−1, we reject them as a chance alignment. We conclude that this is a genuine binary cluster pair, and we propose that their differences in ages and stellar population chemistry is most likely due to variances in their chemical enrichment history within their environment. We suggest that they may have formed in a loosely bound star-formation complex which saw initial fragmentation but then had its clusters become a gravitationally bound pair by tidal capture.

NLTE Stellar Population Synthesis of Globular Clusters Using Synthetic Integrated Light Spectra. II. Expanded Photometry and Sensitivity of Near-IR Spectral Features to Cluster Age and Metallicity

Abstract We present an expanded investigation of the library of globular cluster (GC) synthetic integrated light (IL) spectra of Young & Short, focusing on the impact of non-local thermodynamic equilibrium (NLTE) modeling effects on cluster parameters derived from photometric colors and sensitivity of near-IR spectral features to cluster age and metallicity. Johnson–Cousins–Bessel UBVIJK photometric colors are produced for 910 synthetic IL spectra with two degrees of α enhancement, in both NLTE and local thermodynamic equilibrium (LTE). These color values are used to investigate the GC age–metallicity degeneracy and compare NLTE and LTE derived [M/H] values for NGC 104, NGC 5139, and NGC 6205. For a given age, derived [M/H] values are shown to increase by up to 0.05 dex when modeled in NLTE. A total of 86 spectral lines in the range λ = 12000–22000 Å, representing 14 different atomic species, were identified as sensitive to either cluster age or metallicity, 12 of which were identified as sensitive to both. Equivalent widths of the lines are measured in NLTE and LTE spectra, with NLTE effects changing the widths by up to Å depending on the atomic species.

Modelling of integrated-light spectra from the optical to the near-infrared: the globular cluster G280 in M31

Context. In previous papers, we introduced our method for measuring chemical abundances from integrated-light spectra of globular clusters and applied it to a variety of extragalactic star clusters. Our work so far, however, has concentrated primarily on the optical range 4200 Å –6200 Å. Aims. Here we extend our analysis technique to the infrared and test it on an H-band spectrum of the massive globular cluster G280 in M 31. Methods. We simultaneously analyse an optical spectrum of G280, obtained with the HIRES spectrograph on the Keck I telescope, and an H-band spectrum obtained with NIRSPEC on Keck II. We discuss the sensitivity of our results to various modifications of the input assumptions, such as different line lists and isochrones and the possible presence of a metallicity spread in G280. Results. When using the most recent version of the Kurucz line list, we measure iron abundances of [Fe/H] = −0.68±0.02 from the optical spectrum and [Fe/H] = −0.60±0.07 from the infrared spectrum. These values agree well with previous spectroscopic determinations of the metallicity of G280. While the small difference between the optical and infrared measurements is insignificant given the uncertainties, it is also consistent with a metallicity spread similar to that observed in massive GCs such as ω Cen and G1, and also hinted at by the colour-magnitude diagram of G280. The optical and infrared spectra both indicate an α-enhancement of about 0.3–0.4 dex relative to solar-scaled abundances, as typically also observed in Milky Way GCs. Conclusions. From this analysis, it appears that our integrated-light analysis technique also performs well in the H-band. However, complications due to the presence of molecular bands and telluric contamination are more severe in the infrared, and accurate modelling of the coolest giants is more critical.