Stellar haloes of disc galaxies at z~1

TL;DR

This study uses ultra-deep Hubble imaging to analyze stellar haloes of Milky Way-like galaxies at z~1, revealing they are brighter, bluer, and younger than local counterparts, consistent with early formation models.

Contribution

First observational detection of high-redshift stellar haloes with detailed properties, confirming early formation and passive evolution scenarios.

Findings

High-z stellar haloes are brighter and bluer than local ones.

Stellar populations in z~1 haloes are younger than 1 Gyr.

Structural properties of haloes are similar across cosmic time.

Abstract

Taking advantage of the ultra-deep 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.