"Fossils" of galaxies reveal the formation and evolution of massive galaxies
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.
In the local universe, massive galaxies hosting more than about 100
billion stars are predominantly dead elliptical galaxies, that is,
without any signs of star-formation activity. Many questions remain on
when, how and for how long star formation occurred in such galaxies
before the cessation of star formation, as well as what happened since
to form the dead elliptical galaxies seen today.
In order to address these issues, the research team made use of
fossil records imprinted by stars in the spectra of distant dead
galaxies which give important clues to their age, metal content, and
element abundances. Local massive dead galaxies are about 10 billion
years old and rich in heavy elements. Also, α-elements (Note 1),
which measure the duration of star formation, are more abundant than
iron, indicating that these galaxies formed a large amount of stars in a
very short period. The team investigated the stellar content of
galaxies in the distant universe 4 billion years after the Big Bang, in
order to study galaxy evolution much closer to their formation epoch.
The team took the advantage of the MOIRCS's capability to observe
multiple objects simultaneously, efficiently observing a sample of 24
faint galaxies. They created a composite spectrum that would have taken
200 hours of Subaru Telescope's time for a single spectrum of comparable
quality (Figure 1).
Figure 1: Composite spectrum of 24 massive dead
galaxies in the universe 4 billion years after the Big Bang. The spectra
is equivalent to 200 hours of Subaru Telescope's observing time.
Rectangles on the spectrum indicate spectral features, which are used to
calculate the ages, the amount of heavy elements and the α-element
abundance in the stellar populations of these galaxies. (Credit: ETH
Zürich/NAOJ)
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).
Figure 2: Cosmic evolution of the age (Left), the
abundance of heavy elements (Middle) and the abundance of α-elements
relative to iron (Right) of massive dead elliptical galaxies. Gray data
points show the results from previous works by other studies. The
colored strip in the left panel is a prediction of their evolution if
such massive elliptical galaxies formed 10 to 11 billion years ago
(redshift of about 2.3) and evolved without forming new stars to the
present universe (redshift of zero). The prediction agrees with the
observed trend very well. The middle and left panels clearly show that
chemical composition of massive elliptical galaxies does not evolve over
cosmic time. (Credit: ETH Zürich/NAOJ)
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.
This study establishes a consistent picture of the history of massive
galaxies over 11 billion years of cosmic time. Dr. Masato Onodera who
leads the team says, "We would like to explore galaxy evolution in more
detail by carrying out an object-by-object study and by extending the
method to an even earlier epoch."
This research was published on 1st August 2015 in The Astrophysical
Journal (Onodera et al. 2015 "The Ages, Metallicities, and Element
Abundance Ratios of Massive Quenched Galaxies at z~1.6"). This work was
supported by the Japan Society for the Promotion of Science (Grant ID:
23224005) and the Program for Leading Graduate Schools. The preprint of
the paper is available at this link.
Member of the research team (as of the publication of Onodera et al. 2015):
Notes:
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)
Notes:
- 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.
Source: Subaru Telescope