Stellar shells as a test of cosmological models

Ivana Ebrova 1 , Michal  Bilek 2 , Pierre-Alain Duc 3

  • 1 Fzu – Institute Of Physics Of The Czech Academy Of Sciences, Prague
  • 2 Paris Observatory (LERMA), Paris
  • 3 Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg (ObAS), Strasbourg

Abstract

Shell galaxies constitute 10-20% of early-type galaxies and a few percent of spirals. The stellar shells are a special kind of tidal feature in the form of open, concentric arcs which do not cross each other. They arise predominantly from nearly radial minor or intermediate mergers of galaxies. Their unique kinematics bring many opportunities for their utilization, such as measurements of the gravitational potential of the host galaxy or estimating the time of the merger undergone by the host galaxy.
The spatial distribution of the shells in a galaxy can be translated to estimates of the merger time. The upcoming Large Survey of Space and Time (LSST) at the Vera C. Rubin Observatory will open the possibility to apply this procedure to thousands of shell galaxies. This way, we will obtain significant constraints on the recent merger history of our universe that can be compared with the predictions of cosmological models.
The outermost observed shells are of a special use. The size of the biggest shell in a galaxy is determined by the mass of the host galaxy, the pericentric velocity of the merger, and the internal stellar velocities of the accreted galaxy. We examined the largest shells around galaxies in the large-scale cosmological hydrodynamical simulation IllustrisTNG. At the surface brightness limit of 28 mag/arcsec² in g-band, the simulation predicts that 10-20% of massive galaxies should contain a shell larger than 100 kpc. This is a surprisingly high rate compared to the only known observed shell that exceeds this limit. Finding the missing big shells will be an important test of the current cosmological model.

Shell galaxies

Video: Gadget-2 (Springel 2005) simulation of 3.56 Gyr evolution of a shell-creating merger with 1:10 stellar-mass ratio, elliptical primary galaxy and secondary with inclined disk and 6 kpc impact parameter. Box size 140 kpc, one second of the video corresponds to 60 Myr. For more details, see Ebrová et al. (2020).

About 10-20% of all early-type galaxies (ETGs) possess shells (e.g., Malin & Carter 1983, Bílek et al. 2020). Shells are made of stars from a secondary galaxy accreted on a close-to-radial trajectory (Quinn 1984). 

The unique shell kinematics enables us to estimate the time since the merger such as in Bílek et al. (2014) for the richest shell system NGC 3923, in Ebrová et al. (2020) for NGC 4993 ⁠— the host of the famous gravitational-wave event GW170817, and in Bílek et al. (2022) for the spectacular shell galaxy NGC 474. 

The outermost observed shells are especially important as they put the most precise constraints on the time and orbital energy of the merger. The size of the biggest shell in a galaxy is determined by the mass of the host galaxy, the pericentric velocity of the merger, and the internal stellar velocities of the accreted galaxy.

Large shells in IllustrisTNG

We examined the largest shells around galaxies in TNG100-1 run of IllustrisTNG project (Nelson et al. 2019) ⁠— a set of large-scale cosmological simulations that follow the co-evolutions of dark and baryonic matter.

We examined g-band images with the surface brightness limit of 28 mag/arcsec2. Among 300 most massive galaxies in TNG100 we found that 10-20% of massive galaxies should contain a shell larger than 100 kpc. Six such examples: 

There is the only known observed shelllarger than 100 kpc (in NGC 3923; Prieur 1988, Bílek et al. 2016). Bílek et al. (2020) and Solo et al. (2022) inspected images of 179 ETGs in MATLAS (Duc et al. 2015) ⁠— an ultra deep imaging, complete magnitude- and volume-limited survey conducted at the 3.6 m CanadaFrance-Hawaii Telescope (CFHT) using the MegaCam imager; see also an affiliated poster. None of the shells found there exceeds 90 kpc, with the majority being smaller than 40 kpc. Finding the missing big shells will be an important test of the current cosmological models.

References & Acknowledgements

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Acknowledgements

We acknowledge the support from the Polish National Science Centre under the grat 2017/26/D/TS9/00449( IE and MB).This project has received funding from the European Union's Horizon Europe Research and Innovation programme under the Marie Skłodowska-Curie grant agreement No. 101067618-GalaxyMergers.

Disclaimer: Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor REA can be held responsible for them.