Properties of RR Lyrae stars in the inner regions of the Large Magellanic Cloud. II.The extended sample.

Context. All galaxies that have been adequately examined so far have shown an extended stellar halo. Aims. To search for such a halo in the LMC we have obtained low-resolution spectra for 100 LMC RR Lyrae stars, of which 87 are in the field and 13 in the clusters NGC 1835 and NGC 2019. Methods. We measured radial velocities for 87 LMC RR Lyrae stars, and metallicities for 78 RR Lyrae stars, nearly tripling the previous sample. These targets are located in 10 fields covering a wide range of distances, out to 2.5 degrees from the center of the LMC. Results. Our main result is that the mean velocity dispersion for the LMC RR Lyrae stars is $\sigma_{RV} = 50 \pm 2$ km s -1 . This quantity does not appear to vary with distance from the LMC center. The metallicity shows a Gaussian distribution, with mean [Fe/H] = –$1.53 \pm 0.02$ dex, and dispersion $\sigma_{\rm [Fe/H]} = 0.20\pm 0.02$ dex in the Harris metallicity scale, confirming that they represent a very homogeneous metal-poor population. There is no dependence between the kinematics and metallicity of the field RR Lyrae star population. Conclusions. Using good quality low-resolution spectra from FORS1, FORS2 and GEMINI-GMOS we have found that field RR Lyrae stars in the LMC show a large velocity dispersion and that this indicate the presence of old and metal-poor stellar halo. All the evidence so far for the halo, however, is from the spectroscopy of the inner LMC regions, similar to the inner flattened halo in our Galaxy. Further study is necessary to confirm this important result.