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tokens: less concentrated (i.e. low n ) again. Note that error bars correspond to 1σ confidence intervals of the posterior distributions. 5 DISCUSSION 5.1 Morphological trends: two modes of star formation In nearby naked red nuggets stellar populations with different ages show different radial distributions (Fig. 4 ). In particular, we have shown how the effective radius remains almost constant up until ∼8 Gyr ago, but then it increases for younger ages. Note that individual points in Fig. 4 are not integrated values; i.e. they do not show how the galaxy would look at that time, but how the stars born at that time are distributed. The observed trend in R e , increasing with decreasing age of the stars, is effectively equivalent to the larger sizes of galaxies observed through bluer photometric bands (e.g. Evans 1994 ; Cunow 2001 ; Möllenhoff, Popescu Tuffs 2006 ; Kelvin et al. 2012 ) as younger stars will dominate the flux at shorter wavelengths. The peak in the Sérsic index evolution reveals a more complex scenario. The bottom panel in Fig. 4 suggests that a compaction phase takes place during the early live of red nuggets. This initial compaction is a well-known theoretical expectation, where gas is efficiently channelled towards the centre of dark matter haloes, leading to massive, gas-rich, compact, and star-forming discs (e.g. Gammie 2001 ; Dekel, Sari Ceverino 2009 ; Krumholz Burkert 2010 ; Cacciato, Dekel Genel 2012 ; Dekel et al. 2013 ; Dekel Burkert 2014 ; Zolotov et al. 2015 ; Dekel, Lapiner Dubois 2019 ). Observations of massive, compact and star-forming galaxies at high- z support this scenario (e.g. Barro et al. 2013 , 2014 , 2017 ; Huertas-Company et al. 2015 ; van Dokkum et al. 2015 ), but linking this population of galaxies to their more extended progenitors is not trivial. Our approach, tracing back in time the structural evolution of individual objects, shows that for our sample of galaxies an initial compaction took place, reaching a maximum ∼8 Gyr ago ( z ∼ 1). In order to explore the connection between SFR and compaction, we show in Fig. 5 the average evolution of sSFR–log Σ 1 in our sample, colour coded by the Sérsic index at that time (from Fig. 4 ). A clear connection can be seen between star formation activity and galaxy structure. The initial compaction takes place during the star-forming phase, as expected in a gas rich environment. Later, the actual quenching takes place when the compaction is maximal. This could be naturally explained if the gas brought towards the centre or the galaxy efficiently feeds the central supermassive black hole. The black hole would become therefore energetic enough to shut down star formation, likely leading to an inner bulge-like component (e.g. Silk Rees 1998 ; King 2003 ; Di Matteo et al. 2005 ; Sijacki et al. 2007 ; Silk 2013 ). Note also that the buildup of the central component is likely associated with a strong stellar feedback even, which would also favour the quenching process. The