Skip to main content

Connecting the Extreme

Periodic Reporting for period 3 - ConTExt (Connecting the Extreme)

Reporting period: 2018-09-01 to 2020-02-29

This project investigates if there exist a proposed evolutionary sequence for the full cosmic evolution of massive galaxies.
The CONTEXT model (connecting the extremes) hypothesizes that some of the most extreme observed galactic phenomena are different evolutionary phases of the same massive galaxies undergoing cosmic evolution.
In the early Universe, intense nuclear starbursts are ignited as galaxies merge. In this phase the starbursts are detectable as sub-mm galaxies. Shortly thereafter the galaxies are quenched, either due to gas depletion or feedback from the starburst or a supermassive central black hole (in which case they may be detectable as quasars). Shortly thereafter the galaxies appears as compact quiescent galaxies, which evolve into local elliptical galaxies primarily through merging with minor companions.

The project performs detailed investigations of all of these extreme different galaxy types through out cosmic history, to test if this CONTEXT model is a good description of the evolution of massive galaxies.
1: Through stacking of far infrared and radio data of a large number of quiescent galaxy candidates as a function of both their mass and redshift, we have shown that genuinely quiescent galaxies exist all the way back to z=3, when the Universe was just 2 Gyr old. We also showed that quiescent galaxies have higher radio emission than expected from their low star formation rates, suggesting that they harbour radio loud active galactic nuclei that may be involved in quenching them.
This is an important result because it is inherently difficult to confirm the quiescent nature of galaxies from their UV-NIR emission alone, due to the dust/age degeneracy.
In a second study, published in Nature Astronomy, we show that the average dust mass and gas mass in high redshift quiescent galaxies is surprisingly high, given that they are not forming stars.

2: We have played in a key role in a large ALMA study of galaxies from z=0-3, probing their dust and gas. This study for the first time derived the accretion rates of gas onto galaxies from observations, and showed that these are very large. This further strengthens the mystery of early quenched galaxies. Not only is it strange that they finish their starformation so early in the history of the universe, but also that they remained quenched, despite the large amount of gas (fuel of star formation) that is pouring onto them from the cosmic filaments.

3. We have performed a high resolution study of the stellar populations and dusty starformation regions in a sample of the highest redshift starburst galaxies known. We find that the properties of these starburst galaxies are consistent with being progenitors of the highest redshift quiescent galaxies, once they have completed their nuclear starbursts and merged with minor companions.

4: A fourth major result is derived from a detailed study of a gravitationally lensed quiescent galaxy at z=2. The lensing makes it possible for the first time to resolve its inner structure and stellar populations, both photometrically and spectroscopically. The galaxy turns out to be a rotationally supported disk galaxy, rather than a the dispersion dominated proto-bulge that was expected from theory. This has major consequences for how it formed. A major merger would not lead to a compact disk. We were able to measure gradients in its stellar populations, showing for the first time that the galaxy quenched inside out over a timescale of 300 Myr. We also found evidence for outflows from a central AGN which could explain the quenching.
This study is published in Nature.
Our stacking of FIR data for quiescent galaxies is beyond the state of the art, as it clearly demonstrates that dead galaxies truly exists in the early universe.
While the FIR emission is weak, we do detect it. This has paved the way to get approved observations with e.g. ALMA to directly detect gas in early quenched galaxies, which will provide strong constraints for the first time on what quenched them.

We have pioneered observations of gravitationally lensed quiecent galaxies, and for the first time studied their inner structure, both spectroscopically and photometrically.
This has led to a highly surprising result, which is acknowledged by publication in Nature, one of the highest impact journals, and the massive interrest from peers at presentation of the result at conferences.