Evolutionary paths among different red galaxy types at 0.3 < z < 1.5 and the late buildup of massive E-S0's through major mergers

TL;DR

This study provides observational evidence that major mergers significantly contributed to the late formation and evolution of massive E-S0 galaxies between redshifts 0.3 and 1.5, supporting hierarchical galaxy formation models.

Contribution

It identifies a dominant evolutionary pathway from irregular disks to spheroids among massive red galaxies, confirming major mergers' role in their buildup.

Findings

Massive red regular galaxies at low z originate from irregular progenitors at higher z.

Major mergers primarily drove the migration of blue disks to the Red Sequence between z~1.5 and z~0.6.

Formation of the most massive E-S0's via gas-rich mergers has ceased since z~0.6.

Abstract

Some recent observations seem to disagree with hierarchical theories of galaxy formation about the role played by major mergers in the late buildup of massive E-S0's. We re-address this question by analysing the morphology, structural distortion level, and star formation enhancement of a sample of massive galaxies (M_* > 5 * 10^10 Msun) lying on the Red Sequence and its surroundings at 0.3 < z < 1.5. We have used an initial sample of ~1800 sources with K_s < 20.5 mag over an area ~155 arcmin^2 on the Groth Strip, combining data from the Rainbow Extragalactic Database and the GOYA Survey. Red galaxy classes that can be directly associated to intermediate stages of major mergers and to their final products have been defined. We report observational evidence of the existence of a dominant evolutionary path among massive red galaxies at 0.6<z<1.5, consisting in the conversion of irregular disks into irregular spheroids, and of these ones into regular spheroids. This result implies that: 1) the massive red regular galaxies at low redshifts derive from the irregular ones populating the Red Sequence and its neighbourhood at earlier epochs up to z~1.5; 2) the progenitors of the bulk of present-day massive red regular galaxies have been blue disks that have migrated to the Red Sequence mostly through major mergers at 0.6 < z < 1.2 (these mergers thus starting at z~1.5); and 3) the formation of E-S0's that end up with M_*> 10^11 Msun at z=0 through gas-rich major mergers has frozen since z~0.6. All these facts support that major mergers have played the dominant role in the definitive buildup of present-day E-S0's with M_*> 10^11 Msun at 0.6<z<1.2, in good agreement with hierarchical scenarios of galaxy formation.