DOLORES gravitational lensing data reveal Milky Way-like stars in a distant galaxy core 8 billion years ago

Spectroscopic observations obtained with the spectrograph DOLORES installed at the Telescopio Nazionale Galileo enabled astronomers to determine the redshift of one of the most distant lensing galaxies known to date.

The system, J1453+0529, consists of a quasar at redshift 2.82 whose light is gravitationally lensed by a foreground elliptical galaxy at redshift 1.055. The lens produces a rare quadruple configuration with an extremely small separation between the images. High-resolution imaging clearly shows the four quasar images arranged around the central lensing galaxy (see Figure 1). This configuration allows astronomers to measure the Einstein radius with great precision and determine the total mass enclosed within it, providing strong constraints on the stellar initial mass function in the galaxy’s central region.

A key step in the analysis was determining the redshift of the lensing galaxy. This measurement relies crucially on spectroscopy carried out at the Telescopio Nazionale Galileo. The integrated spectrum (Figure 2) revealed strong narrow absorption lines imprinted on the quasar continuum by interstellar material in the foreground galaxy. These features allowed the team to measure the lens redshift, and, in turn, precisely measure the total mass budget within the Einstein radius.

The recent discovery of the most compact quadruply lensed quasar known so far has opened a unique opportunity to investigate the stellar population in the core of a distant elliptical galaxy. The initial mass function of the bulge of this galaxy resembles that of the Milky Way, suggesting that it either grew slowly or underwent early disruptive events altering its stellar build-up, challenging classical models of galaxy formation that predict that bulges form rapidly and remain unchanged by later interactions.

This result is particularly notable because classical “two-phase” models predict that the cores of elliptical galaxies should form rapidly at early times and contain a large fraction of low-mass stars. Instead, the observations suggest that this galaxy’s central stellar population was already relatively bottom-light when the Universe was only about 5.5 billion years old.

“This suggests that the assembly history of the galaxy may have been more complex than previously thought,” says Quirino D’Amato (INAF - Osservatorio Astrofisico di Arcetri) principal author of the study.

The significance of this result is further enhanced by the context in which the observations were obtained. The data were collected during an observing stage at the Telescopio Nazionale Galileo carried out by high school students winners of the I edition of the Young Astronomers e Telescopio Nazionale Galileo competition, having earned this opportunity through an observing proposal focused on these gravitationally lensed systems.

“TNG dedicates a significant portion of its time to educational activities, with the main goal of bringing young people closer to astronomy,” says Gloria Andreuzzi (INAF), coordinator of education activities at TNG. “While educational observations are not usually expected to lead to results like this, this project clearly shows how education can turn into real scientific research,” she concludes.

Link to the paper.