An international team led by researchers at Swiss Federal Institute of Technology in Zürich observed massive dead galaxies in the universe 4 billion years after the Big Bang with the Subaru Telescope's Multi-Object InfraRed Camera and Spectrograph (MOIRCS). They discovered that the stellar content of these galaxies is strikingly similar to that of massive elliptical galaxies seen locally. Furthermore, they identified progenitors of these dead galaxies when they were forming stars at an earlier cosmic epoch, unveiling the formation and evolution of massive galaxies across 11 billion years of cosmic time.
Analysis of the composite spectrum shows that the age of the galaxies is already 1 billion years old when observed 4 billion years after the Big Bang. They host 1.7 times more heavy elements relative to the amount of hydrogen and their α-elements are twice enhanced relative to iron than the solar values. It is the first time that the α-element abundance in stars is measured in such distant dead galaxies, and it tells us that the duration of star formation in these galaxies was shorter than 1 billion years. These results reveal that these massive dead galaxies have evolved to today without further star formation (Figure 2).
What do massive dead galaxies look like when they are forming stars? To answer this, the team investigated the progenitors of their sample based on their spectral analysis. The progenitors must be star-forming galaxies in the universe 1 billion years before the observed epoch for the dead galaxies. Indeed, they do find similarly massive star-forming galaxies at the right epoch and with the right star formation rate expected from the spectra. If these active galaxies continue to create stars at the same rate, they will immediately become more massive than seen in the present universe. Therefore, these galaxies will cease star formation soon and simply age.
Member of the research team (as of the publication of Onodera et al. 2015):
- Masato Onodera, C. Marcella Carollo, Sandro Tacchella (ETH Zürich, Swizerland)
- Alvio Renzini (INAF-Padova, Italy)
- Michele Cappellari (Oxford University, UK)
- Chiara Mancini (Padova University, Italy)
- Nobuo Arimoto, Yoshihiko Yamada (Subaru Telescope, Japan)
- Emanuele Daddi (CEA/Saclay, France)
- Raphaël Gobat (KIAS, South Korea)
- Veronica Strazzullo (Ludwig Maximilians University, Germany)
- α-elements are elements which have an atomic number that is a multiple of 4, i.e., of the helium nucleus. In this article, it refers to elements produced by Type II supernovae such as oxygen, neon, magnesium, silicon, sulfur, calcium, and titanium.