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

TL;DR

The paper analyzes integrated-light spectra of eight very metal-poor extragalactic globular clusters to infer HB morphology, ages, and carbon abundances, underscoring how HB properties shape Balmer-line profiles in IL spectra.An automatic isochrone-selection framework combined with synthetic IL-spectrum modelling (CLUSTER with ATLAS9 atmospheres and BaSTI/B08 isochrones) yields ages, helium content, metallicities, and detailed chemical abundances for Fe, C, Mg, Ca, Ti, Cr, and Mn.All clusters are found to be older than 10 Gyr with blue HB morphologies, with HB Teff reaching up to about 12600 K in the most extreme cases, and with Mg generally depleted and Mn enhanced relative to Galactic field stars; C is enhanced due to evolutionary changes on the RGB.Comparisons with CMD-based Hess diagrams for HIII, B317, and EXT8 validate the isochrone selection and the IL-spectral approach, though some metallicity-scale differences and uncertainties in RGB mass loss and field-star contamination are acknowledged.Overall, the results illuminate early-universe nucleosynthesis and galaxy formation histories, while highlighting limitations intrinsic to IL-spectral analyses of extragalactic GCs.

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 globular clusters facilitates a better understanding of the formation processes of their parent galaxies and nucleosynthesis in the early Universe.

Figures (16)

• Figure 1: Variation of the measured shifts and broadening of the spectral lines in the twilight spectrum as a function of wavelength in observations with the SCORPIO-I and Hectospec spectrographs.
• Figure 2: Comparison of the spectra of the studied clusters with the model ones calculated with the methods described in Section \ref{['sec:3']}, with the isochrone parameters given in Table \ref{['tab:2']}, and the element abundances presented in Table \ref{['tab:3']}.
• Figure 3: Effective temperatures and luminosities of the MSTO stars and the hottest HB stars depending on age. The B08 isochrones are used for Z=0.0001. The data for different isochrones are highlighted in colour as explained in the legend.
• Figure 4: Variation of I$_\text{core}$ and FWHM with age for three Balmer hydrogen lines in the synthetic IL spectra of GCs with the metallicity $\text{Z} = 0.0004$. The spectra were obtained using the B08 isochrones (the solid, dashed, and dash-dotted lines for $\text{Y} = 0.23$, 0.26, and 0.30, respectively) ignoring the HB stars.
• Figure 5: Same as in the previous figure but with the HB stars included.