Left: R1 vs stellar mass for AURIGA (orange), HESTIA (blue), NIHAO (green), FIRE-2 (pink), EAGLE (red), and Trujillo+20 (grey). Right: Stellar vs total mass (STMR) within R1 for AURIGA, HESTIA, NIHAO and FIRE-2. In grey, STMR within 3R1/2 for comparison.
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Authors
Elena
Arjona Gálvez
Salvador
Cardona Barrero
Robert J. J. Grand
Arianna
Di Cintio
Claudio
Dalla Vecchia
et al.
References
Measuring galaxy sizes is essential for understanding how they were formed and evolved across time. However, traditional methods based on light concentration or isophotal densities often lack a clear physical meaning. A recent study from Trujillo+20 explores a more physically motivated definition: the radius R1, where the stellar surface density falls to 1 solar masses per parsec square —roughly the threshold for gas to form stars in galaxies like the Milky Way. In this work, Arjona-Gálvez+25 uses over 1,000 galaxies from several state-of-the-art cosmological simulations (AURIGA, HESTIA, NIHAO and FIRE), showing that the R1–stellar mass relation is remarkably consistent across different galaxy types, evolutionary stages, and simulation models, reflecting a fundamental aspect of how galaxies grow. This relation displays significantly less scatter than traditional size indicators and remains stable across cosmic time.
The results also reveal a tight connection between the total mass enclosed within R1 and a galaxy’s stellar mass, suggesting that R1 could also act like a potential observational tracer of the galaxy’s dark matter halo properties. In other words, measuring a galaxy’s size in this way could also tell us about the total matter shaping it.
