Old age and super-solar metallicity in a massive z~1.4 early-type galaxy from VLT/X-Shooter spectroscopy

TL;DR

This study analyzes a high-redshift early-type galaxy using VLT/X-Shooter spectroscopy, revealing it is old, metal-rich, and formed rapidly at high redshift, supporting early galaxy formation models.

Contribution

First detailed spectroscopic analysis of a z~1.4 galaxy's age and metallicity, confirming rapid early formation of massive galaxies.

Findings

Galaxy is older than 3 Gyr

Metallicity exceeds solar ([Z/H] > 0.5)

Formed stars at z > 5 within 0.1 Gyr

Abstract

We present the first estimate of age, stellar metallicity and chemical abundance ratios, for an individual early-type galaxy at high-redshift (z = 1.426) in the COSMOS field. Our analysis is based on observations obtained with the X-Shooter instrument at the VLT, which cover the visual and near infrared spectrum at high (R >5000) spectral resolution. We measure the values of several spectral absorptions tracing chemical species, in particular Magnesium and Iron, besides determining the age-sensitive D4000 break. We compare the measured indices to stellar population models, finding good agreement. We find that our target is an old (t > 3 Gyr), high-metallicity ([Z/H] > 0.5) galaxy which formed its stars at z_{form} > 5 within a short time scale ~0.1 Gyr, as testified by the strong [\alpha/Fe] ratio ( > 0.4), and has passively evolved in the first > 3-4 Gyr of its life. We have verified that this result is robust against the choice and number of fitted spectral features, and stellar population model. The result of an old age and high-metallicity has important implications for galaxy formation and evolution confirming an early and rapid formation of the most massive galaxies in the Universe.