Milky-Way-like stars in a galaxy core 8 billion years ago revealed by gravitational lensing

TL;DR

This study uses gravitational lensing to analyze a galaxy core 8 billion years ago, revealing a stellar initial mass function similar to the Milky Way's and challenging classical bulge formation theories.

Contribution

It presents the discovery of the smallest known quadruply lensed quasar at high redshift and constrains the galactic initial mass function in a new mass and redshift regime.

Findings

Galactic core's initial mass function matches the Milky Way's.

Lensing galaxy at redshift 1.055 has a mass of ~2x10^10 M_sun.

Core growth may have been slow or disrupted early.

Abstract

The assembly of stellar-dominated cores in elliptical galaxies is key to understanding how cosmic structures evolved. Gravitational lensing offers unique insights into the nature of their stars. We report the discovery of the smallest known quadruply lensed quasar (radius ~0.2"), whose lensing galaxy at redshift 1.055 (5.5 billion years after the Big Bang) features a lensing mass of only ~2x10^10 M_sun. A Bayesian analysis, based on the system's exceptional properties and standard scaling relations, allowed us to sample the central galactic initial mass function with unmatched accuracy and in a previously uncharted regime in terms of mass and redshift. We found it consistent with the Milky Way one, while excluding bottom-heavy functions. This suggests that the core either grew slowly or underwent early disruptive events altering its stellar build-up, in contrast with the classical view that bulges form rapidly and remain unchanged by later interactions.