H-ATLAS/GAMA: Quantifying the Morphological Evolution of the Galaxy Population Using Cosmic Calorimetry

TL;DR

This study uses cosmic calorimetry and deep surveys to quantify galaxy morphological evolution, revealing that most stars formed in late-type galaxies, but many transitioned to early-type galaxies over cosmic time.

Contribution

It introduces a calorimetric method to measure the historical importance of galaxy morphological transformation using the extragalactic background radiation.

Findings

83% of stellar mass-density formed in late-type galaxies.

51% of local stellar mass-density is in early-type galaxies.

Major transformation of late-type into early-type galaxies occurred after most stars formed.

Abstract

Using results from the Herschel Astrophysical Terrahertz Large-Area Survey and the Galaxy and Mass Assembly project, we show that, for galaxy masses above approximately 1.0e8 solar masses, 51% of the stellar mass-density in the local Universe is in early-type galaxies (ETGs: Sersic n > 2.5) while 89% of the rate of production of stellar mass-density is occurring in late-type galaxies (LTGs: Sersic n < 2.5). From this zero-redshift benchmark, we have used a calorimetric technique to quantify the importance of the morphological transformation of galaxies over the history of the Universe. The extragalactic background radiation contains all the energy generated by nuclear fusion in stars since the Big Bang. By resolving this background radiation into individual galaxies using the deepest far-infrared survey with the Herschel Space Observatory and a deep near-infrared/optical survey with the Hubble Space Telescope (HST), and using measurements of the Sersic index of these galaxies derived from the HST images, we estimate that approximately 83% of the stellar mass-density formed over the history of the Universe occurred in LTGs. The difference between this and the fraction of the stellar mass-density that is in LTGs today implies there must have been a major transformation of LTGs into ETGs after the formation of most of the stars.