The formation pathways of compact elliptical galaxies

• Published in: Monthly Notices of the Royal Astronomical Society
• Main Authors: Deeley, Simon, Drinkwater, Michael J, Sweet, Sarah M, Bekki, Kenji, Couch, Warrick J, Forbes, Duncan A
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2023
• Subjects: Space and Planetary Science; Astronomy and Astrophysics; sami
• Online Access: http://dx.doi.org/10.1093/mnras/stad2313; https://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stad2313/51027614/stad2313.pdf

Abstract Compact elliptical (cE) galaxies remain an elusively difficult galaxy class to study. Recent observations have suggested that isolated and host-associated cEs have different formation pathways, while simulation studies have also shown different pathways can lead to a cE galaxy. However a solid link has not been established, and the relative contributions of each pathway in a cosmological context remains unknown. Here we combine a spatially-resolved observational sample of cEs taken from the SAMI galaxy survey with a matched sample of galaxies within the IllustrisTNG cosmological simulation to establish an overall picture of how these galaxies form. The observed cEs located near a host galaxy appear redder, smaller and older than isolated cEs, supporting previous evidence for multiple formation pathways. Tracing the simulated cEs back through time, we find two main formation pathways; 32 ± 5 percent formed via the stripping of a spiral galaxy by a larger host galaxy, while 68 ± 4 percent formed through a gradual build-up of stellar mass in isolated environments. We confirm that cEs in different environments do indeed form via different pathways, with all isolated cEs in our sample having formed via in-situ formation (i.e. none were ejected from a previous host), and 77 ± 6 percent of host-associated cEs having formed via tidal stripping. Separating them by their formation pathway, we are able to reproduce the observed differences between isolated and host-associated cEs, showing that these differences can be fully explained by the different formation pathways dominating in each environment.