On the spectral resolution of the MILES stellar library

Context. Empirical stellar libraries are extensively used to extract stellar kinematics in galaxies and to build stellar population models. An accurate knowledge of the spectral resolution of these libraries is critical for avoiding propagation errors and uncertain estimates of the intrinsic stellar velocity dispersion of stellar systems, such as galaxies and star clusters. Aims. In this research note we re-assess the spectral resolution of the MILES stellar library and of the stellar population models based on it. This exercise was performed, because of a recent controversy over the exact MILES resolution. Methods. We performed our test by comparing MILES stellar spectra with three different sets of higher resolution templates, one fully theoretical - the MARCS library - and two empirical ones, namely the Indo-US and ELODIE v3.1 libraries. The theoretical template has a well-defined, very high (R = 20 000) resolution, so errors in this theoretical value do not affect our conclusions. Our approach based on the MARCS library was also crucial for constraining the resolution of the two empirical templates. Results. We find that the MILES resolution is coarser than previously stated. We derive a new spectral resolution of 2.54 angstrom FWHM, instead of the nominal 2.3 angstrom. The reason for this difference is the previous overestimation of the resolution for the Indo-US library, which was used to estimate the MILES one. For the Indo-US library we obtain a new value of 1.35 angstrom FWHM. Most important, the results derived from the MARCS and ELODIE libraries are in very good agreement. Conclusions. These results are important for users of the MILES spectra library and for further development of stellar population models aimed at obtaining accurate stellar kinematics and spectroscopic properties of galaxies.