The signature of dissipation in the mass-size relation: are bulges simply spheroids wrapped in a disc?

Abstract

The relation between the stellar mass and size of a galaxy's structural subcomponents, such as discs and spheroids, is a powerful way to understand the processes involved in their formation. Using very large catalogues of photometric bulge+disc structural decompositions and stellar masses from the Sloan Digital Sky Survey Data Release Seven, we carefully define two large subsamples of spheroids in a quantitative manner such that both samples share similar characteristics with one important exception: the 'bulges' are embedded in a disc and the 'pure spheroids' are galaxies with a single structural component. Our bulge and pure spheroid subsample sizes are 76,012 and 171,243 respectively. Above a stellar mass of ~$10^{10}$ M${}_{\odot }$, the mass-size relations of both subsamples are parallel to one another and are close to lines of constant surface mass density. However, the relations are offset by a factor of 1.4, which may be explained by the dominance of dissipation in their formation processes. Whereas the size-mass relation of bulges in discs is consistent with gas-rich mergers, pure spheroids appear to have been formed via a combination of 'dry' and 'wet' mergers.

Accepted for publication in MNRAS, 6 pages, 3 figures

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• Deep learning predictions of galaxy merger stage and the importance of observational realism
• Extending the evolution of the stellar mass-size relation at $z\le 2$ to low stellar mass galaxies from HFF and CANDELS
• Convolutional neural network identification of galaxy post-mergers in UNIONS using IllustrisTNG
• The fate of high-redshift massive compact galaxies
• Barred Galaxies in the IllustrisTNG Simulation
• Bulge plus disc and Sérsic decomposition catalogues for 16,908 galaxies in the SDSS Stripe 82 co-adds: A detailed study of the $ugriz$ structural measurements
• Star formation characteristics of CNN-identified post-mergers in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS)
• AGN in post-mergers from the Ultraviolet Near Infrared Optical Northern Survey
• Galaxy merger challenge: A comparison study between machine learning-based detection methods
• The life cycle of starbursting circumnuclear gas discs