Chemical abundances of two extragalactic young massive clusters^⋆

¹ Department of Astrophysics/IMAPPRadboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: shernandez@astro.ru.nl
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
³ Astronomical Institute Anton Pannekoek, Universiteit van Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands

Received: 2 February 2017
Accepted: 24 April 2017

Abstract

Aims. We use integrated-light spectroscopic observations to measure metallicities and chemical abundances for two extragalactic young massive star clusters (NGC 1313-379 and NGC 1705-1). The spectra were obtained with the X-shooter spectrograph on the ESO Very Large Telescope.

Methods. We compute synthetic integrated-light spectra, based on colour-magnitude diagrams (CMDs) for the brightest stars in the clusters from Hubble Space Telescope photometry and theoretical isochrones. Furthermore, we test the uncertainties arising from the use of CMD+Isochrone method compared to an Isochrone-Only method. The abundances of the model spectra are iteratively adjusted until the best fit to the observations is obtained. In this work we mainly focus on the optical part of the spectra.

Results. We find metallicities of [Fe/H] = −0.84 ± 0.07 and [Fe/H] = −0.78 ± 0.10 for NGC 1313-379 and NGC 1705-1, respectively. We measure [α/Fe] = +0.06 ± 0.11 for NGC 1313-379 and a super-solar [α/Fe] = +0.32 ± 0.12 for NGC 1705-1. The roughly solar [α/Fe] ratio in NGC 1313-379 resembles those for young stellar populations in the Milky Way (MW) and the Magellanic Clouds, whereas the enhanced [α/Fe] ratio in NGC 1705-1 is similar to that found for the cluster NGC 1569-B by previous studies. Such super-solar [α/Fe] ratios are also predicted by chemical evolution models that incorporate the bursty star formation histories of these dwarf galaxies. Furthermore, our α-element abundances agree with abundance measurements from H II regions in both galaxies. In general we derive Fe-peak abundances similar to those observed in the MW and Large Magellanic Cloud (LMC) for both young massive clusters. For these elements, however, we recommend higher-resolution observations to improve the Fe-peak abundance measurements.