"Fossils" of galaxies reveal the formation and evolution of massive galaxies

"Fossils" of galaxies reveal the formation and evolution of massive galaxies
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

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 hosting more than about 100 billion stars are predominantly dead , 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, 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).

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).

"Fossils" of galaxies reveal the formation and evolution of massive galaxies
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 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 soon and simply age.

This study establishes a consistent picture of the history of 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."


Explore further

Image: Hubble observes galaxies' evolution in slow motion

More information: "The Ages, Metallicities, and Element Abundance Ratios of Massive Quenched Galaxies at z ~ 1.6," M. Onodera et al., 2015, Astrophysical Journal, DOI: 10.1088/0004-637X/808/2/161
Journal information: Astrophysical Journal

Provided by Subaru Telescope
Citation: "Fossils" of galaxies reveal the formation and evolution of massive galaxies (2015, September 25) retrieved 25 May 2019 from https://phys.org/news/2015-09-fossils-galaxies-reveal-formation-evolution.html
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Sep 25, 2015
Merger mania extrapolated into the ludicrous! Grasping at very limited data. Embarrassing.

Anyway, if these actual non-dead ellipticals continue instead to grow from within from their cores over time, eventually evolving into giant sphericals, then examining galaxies at the same growth stage at any epoch will yield a similar metal content, naturally. Starting with the wrong model as established fact leads to ludicrous conclusions.

Sep 26, 2015
@Tuxford:

It is your trolling which is embarrassing.

- What would be the extrapolation in the purely new observations?
- How do dead galaxies 'grow' without having mergers or new gas gravitationally assembling, and how wouldn't that supply or encourage new star formation and change the metal content (which is not observed)?
- And why do you think your arm waving without doing any of the quantifiable work the experts do would even raise to any level of concern in them or other science observers?

Pathetic trolls are pathetic. (If Tuxford isn't one, he may provide his calculations supporting his scientese, and then to go on to publish the refutation of his claimed 'wrong model'.)


Sep 26, 2015
@Tuxford

It is your trolling which is embarrassing.

- And why do you think your arm waving without doing any of the quantifiable work the experts do would even raise to any level of concern in them or other science observers?

Pathetic trolls are pathetic. (If Tuxford isn't one, he may provide his calculations supporting his scientese, and then to go on to publish the refutation of his claimed 'wrong model'.


TL, cease it with the name calling & stick to the science as set forth by the topic under discussion. It's just so obvious you want to be popular with a certain cadre to get 5 Star votes, so you do what they do, name calling & profanity laced postings as you twaddle around in their inane world of pure psycho-babble.

Where are your published peer reviewed papers whereby you claim fame to yourself as some kind of scientist? Yet you criticize others as if you're the expert. I have 6 years of Engineering School education but don't claim to be an expert, you do.

Sep 26, 2015
What galaxies formations and evolution when big bang philosophers claim that the fictional expansion of the universe dominate the gravity?

Sep 26, 2015
There are no "fossils" of galaxies and metal content will be more or less the same! Galaxies are like living animals whose cells are continuously destroyed and new ones form. "It is entirely possible to speculate that some of the ellipticals transformed into spirals by forming proto-arms through ejection from the core, because unlike the case with the spirals, ejection along any direction will be slowed down by the existing bulk of matter. The spirals then slowly convert back to the SO or E type galaxies through gravitational relaxation. The different types of galaxies are dynamic structures in the various stages of formation/ dissolution,succession/inter-conversion to each other ...The morphology of the galaxies is therefore, mediated by the dialectical process of dispersion/ejection/deformation initiated by the catastrophic events of matter/antimatter annihilation and the regularizing effects of gravitational attraction." http://redshift.v...2MAL.pdf

Sep 27, 2015
From where comes the (new) Gas? Can the, at all, galaxies grow in the universe without creating new matter? How can a dwarf galaxy become a big (no connection), from where so much gas?
The universe is observed as a whole, not partially or through individual events.

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