Sébastien Comerón


Archæology of Thick discs (ArcThick)


Galaxy discs are made of two components: the thin and the thick disc. The thin disc, with a scaleheight of a few hundred parsecs, is where star formation takes place. The Sun sits in the thin disc of the Milky Way. The thick disc has a scaleheight of about a kiloparsec and is entirely made of old and relatively metal-poor stars.

Because they contain some of the oldest stars in the Universe, studying the thick discs opens a path to unveil the earliest stages of galaxy evolution. This information will constitute vital input for the next generation of cosmological models needed to understand the elusive dark matter and the ethereal dark energy.

The formation mechanisms of thick discs are disputed. Are they the remains of the early violent galaxy formation processes or the effect of a slow evolution over a cosmic time? Are their stars accreted in galaxy mergers or have they formed within their host galaxy?

Each of these possibilities has been modelled and found to leave distinct chemo-dynamical signatures in galaxies. Thus, by combining observations and theory it is possible to unveil the origin of thick discs.

However, to study thick discs is technically challenging because their very low surface brightness. This problem is exacerbated for thick disc spectroscopy, which can only be done with 8-meter class telescopes.

In ArcThick we aim to finally unveil the origin of thick discs by using a multi-technique approach combining Integral Field Unit (IFU) spectroscopy, deep imaging, and N-body modelling.

Our main successes are to prove that thick discs host a large fraction of the baryons in galaxies and to produce the first IFU studies of thick discs. To access our MUSE data-products please click here.

The team

Team leader: Sébastien Comerón, Universidad de La Laguna and Instituto de Astrofísica de Canarias (Spain)

Bruce G. Elemgreen, IBM (USA)

Johan H. Knapen, Instituto de Astrofísica de Canarias (Spain)

Eija Laurikainen, University of Oulu (Finland)

Reynier F. Peletier, University of Groningen (The Netherlands)

Heikki Salo, University of Oulu (Finland)

The articles

10 A prediction about the age of thick discs as a function of the stellar mass of the host galaxy , Comerón, S. 2021 A&A, 645, L13

Main results: Based on theoretical arguments, we made a prediction about the age of thick disc as a function of the stellar mass of the host galaxy if the thick discs formed in situ at high redshift. According to our prediction, the thick discs in massive galaxies such as the Milky Way should be about 10 Gyrs old, whereas those in low-mass galaxies could be as young as 5 Gyrs.

9 The kinematics of local thick discs do not support an accretion origin, Comerón, S., Salo, H., Knapen, J. H., Peletier, R. F. 2019 A&A, 623, A89

Main results: We study the kinematics of the thick discs of eight edge-on galaxies obtained from MUSE data. We use Jeans modelling combined with N-body models to determine their fraction of retrograde material. We find that the thick discs contain a negligible fraction of retrograde stars. This is suggestive of an internal origin for the stars in the thick discs.

8 The reports of thick discs' deaths are greatly exaggerated. Thick discs are NOT artefacts caused by diffuse scattered light, Comerón, S., Salo, H., Knapen, J. H. 2018 A&A, 610, A5

Main results: After claims in the literature that thick discs are actually thin disc scattered light, we revisit the decompositions of edge-on galaxies made for the papers 3, 4, and 5 of this list. We improve the decomposition algorithm and we use an improved Point Spread Function (PSF) modelling that accounts for extended PSF wings. We also more than double the sample of studied galaxies. We confirm and strengthen the conclusions of our previous papers and prove that thick discs are not an artefact caused by the extended wings of the PSF.

7 A monolithic collapse origin for the thin and thick disc structure of the S0 galaxy ESO 243-49, Comerón, S., Salo, H., Peletier, R. F., Mentz, J. 2016 A&A, 593, L6

Main results: We study an edge-on S0 galaxy with the MUSE IFU. We find that the thick disc contains little or no retrograde stars, which is suggestive of an internal origin. We also find that both the thin and the thick disc are very old. We propose that the thin/thick disc structure was already in place ∼10 Gyr ago when star formation stopped due to the ram stripping of the gas when the galaxy entered in the Abell 2877 cluster.

6 Galactic archaeology of a thick disc: Excavating ESO 533-4 with VIMOS, Comerón, S., Salo, H., Janz, J., Laurikainen, E., & Yoachim, P. 2015, A&A, 584, A34

Main results: We present the first ever IFU study of a thick disc. We found that the thick disc of ESO 533-4 has little or no retrograde material, which is suggestive that the stars in the thick disc have an internal origin (either the thick disc is born hot a high redshift or it is a consequence of a secularly dynamically heated pre-existing thin component).

5 Evidence for the concurrent growth of thick discs and central mass concentrations from S4G imaging, Comerón, S., Elmegreen, B. G., Salo, H., et al. 2014, A&A, 571, A58

Main results: We suggest that the relative masses of the different components of galaxies (thin discs, thick discs, gas discs, and central mass concentrations) are compatible with the fact that both the thick disc and the central mass concentration were born simultaneously in an early turbulent disc. The thin disc would have formed afterwards from material accreted through cold flows.

4 Breaks in Thin and Thick Disks of Edge-on Galaxies Imaged in the Spitzer Survey Stellar Structure in Galaxies (S4G), Comerón, S., Elmegreen, B. G., Salo, H., et al. 2012, ApJ, 759, 98

Main results: Some galaxies have antitruncated (up-bending) face-on radial luminosity profiles. We prove that the cause of the antitruncations for about 50% of those galaxies is the superposition of a thin and a thick disc with a different scale-length where the thick disc scale-length is the longest.

3 Thick Disks of Edge-on Galaxies Seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of Missing Baryons?, Comerón, S., Elmegreen, B. G., Knapen, J. H., et al. 2011, ApJ, 741, 28

Main results: We found that thick discs are much more massive than what was previously thought and that in some cases they can be as massive as thin discs. We suggest that those faint but massive thick discs might be the place where a fraction of the missing baryons are. We also confirm the previously known result that the thick/thin disc mass ratio is the largest for the least massive galaxies.

2 The Unusual Vertical Mass Distribution of NGC 4013 Seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G), Comerón, S., Elmegreen, B. G., Knapen, J. H., et al. 2011, ApJ, 738, L17

Main results: We studied the structure of a rare galaxy with two thick discs. We suggest that this peculiar structure is due to an early merger event which dynamically heated the original galaxy to form the most extended of the thick discs. A canonical thin+thick disc structure formed afterwards within the extended component.

1 The Thick Disk in the Galaxy NGC 4244 from S4G Imaging, Comerón, S., Knapen, J. H., Sheth, K., et al. 2011, ApJ, 729, 18

Main results: We used deep mid-infrared imaging to find evidence for a thick disc in one of the few edge-on disc galaxy which was claimed not to have one according to the literature.