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Before the last years of the past Millennium there was no clear evidence of a possible connection between the formation of galaxies and quasars (objects whose luminosity is powered by gravitational accretion onto a super-massive black hole). The paper by Silk and Rees in 1998 put forward for the first time the idea that the evolutionary tracks of these two types of cosmic objects were not independent and that actually quasars and their host galaxies mutually influence each other through a series of so-called feedback (or self-regulation) processes. This proposal was certainly stimulated by puzzling observations, described below, that were hard to understand without the premises introduced by the paper. The study is a textbook example of how the complexity of a natural phenomenon recorded in experimental data can be interpreted and clarified by the astonishingly simple and yet profound beauty of a one-line formula, i.e. Eq. (1) of the paper. This formula essentially expresses the relationship between the mass of the super-massive black hole and that of the galactic spheroid hosting it. This is one of the most important formulas in modern cosmology, and the physical idea behind it will be discussed later on. The work was first presented at the 13th IAP Astrophysics Colloquium in Paris in July 1997. While listening to Prof. Silk describe the paper’s results, I got the clear perception that this apparently simple idea would change the field forever, and indeed it did so. Sparse evidence of a possible connection between quasars and galaxies was already present before 1998. For example, it was already clear that, rather than being permanent but rare phenomena, quasars are short-lived phenomena occurring in a large fraction of galaxies (if not all of them). If so, the most promising sites to search for quasar remnants are the high-density central regions of giant galaxies where the high concentrations of matter would make formation of super-massive black holes more probable. Hints in that direction came from the work of Faber’s group in October 1996 (Faber et al. 1997). They showed that luminous, “hot” (a proxy for spiral bulges) galaxies have a core whose origin was most likely explained by the action of a central black hole erasing the central density cusp. It was also tentatively argued that the BH mass per spheroid was a fixed fraction of the stellar mass of the core, thus providing the first link between the galaxy and the central black-hole mass. The year 1997, blessed by landmark IAU Symposia in Kyoto where the results were first presented, witnessed the final burgeoning of experimental data, with two (and perhaps more) independent groups coming to similar, albeit different, conclusions (Magorrian et al. 1998; Ford et al. 1998). In short, the main idea behind these experiments, made possible by the aberrationcorrected Hubble Space Telescope, was to measure the motions of stars and gas with great precision within the influence region (50–100 pc) of the central black hole, thereby determining the rise of the gravitational potential associated with the compact object. The sub-arcsecond resolution required by the experiment was previously not achievable from the ground and had to wait for space telescopes like HST. Although these observations made clear that most galaxies indeed host a (dormant) black hole in their centers, a vigorous debate was concentrated on the quantity to which the black-hole mass, M•, was correlated. While one group found no clear evidence of a correlation with the spheroid luminosity, the other one showed that indeed M• was correlating with the spheroid mass according to the linear scaling M• = 0.002 Msph. However, few ideas were available to explain this evidence prior to the paper by Silk and Rees. In addition, an associated problem needed to be addressed in the same context. During those years quasars powered by supermassive (M• > 108 M ) black holes beyond redshift z = 4 were already known (the record-holding quasar today having z = 6.4, Fan et al. 2003). This evidence posed the problem of the rapid build-up of these objects from available seeds. Such black holes could have formed from the merging of smaller black holes left over by the death of massive Pop III stars (Madau & Rees 2001); alternatively, their seed could have formed via the homologous collapse of giant, pristine gas clouds (Loeb & Rasio 1994). Both mechanisms have their own weaknesses. The first must postulate the presence of massive stars and might require too long a dynamical timescale to coagulate small black holes into a super-massive one in galactic mergings. The latter requires a UV background-induced suppression of molecular hydrogen that could lead to a fragmentation of the cloud before it reaches substantial densities leading to runaway collapse into a compact object. Like today, this question has not yet received a final answer, although a large set of scientific papers has been produced on the subject, most likely stimulated by the importance of the question that is central to the Silk and Rees scenario. The authors favored this second channel for black-hole formation in the center of galaxies and started from there to explore the growth of the black hole seed fed by material brought in by galaxy mergings and regulated by a wind driven by quasar luminosity. Once the above hypothesis is made, the physical understanding of the observed relation between black-hole and spheroid mass in galaxies becomes crystal clear. The basic physics of the idea is the following. Suppose that a black hole is present

How quasars and galaxies get to know each other