Stellar haloes of disc galaxies at z ∼ 1

Taking advantage of the ultradeep near-infrared imaging obtained with the Hubble Space Telescope on the Hubble Ultra Deep Field, we detect and explore for the first time the properties of the stellar haloes of two Milky Way-like galaxies at z ∼ 1. We find that the structural properties of those haloes (size and shape) are similar to the ones found in the local Universe. However, these high-z stellar haloes are approximately three magnitudes brighter and exhibit bluer colours ((g − r) 0.3 mag) than their local counterparts. The stellar populations of z ∼ 1 stellar haloes are compatible with having ages 1 Gyr. This implies that the stars in those haloes were formed basically at 1 < z < 2. This result matches very well the theoretical predictions that locate most of the formation of the stellar haloes at those early epochs. A pure passive evolutionary scenario, where the stellar populations of our high-z haloes simply fade to match the stellar halo properties found in the local universe, is consistent with our data.