L’émergence des galaxies passives massives à z~3

Cette thèse a pour objectif la compréhension de la nature des galaxies rouges lointaines dans l’univers jeune,dans le cadre de la recherche des galaxies massives passives ou "mortes". Dans l’univers local, les galaxiespassives sont parmi les plus massives, compte tenu de tous les types de galaxies. Elles se caractèrisent par despopulations stellaires extrêmement vieilles, distribuées selon une structure interne homologue de sphéroïde.L’étude détaillé de leurs couleurs et de leurs spectres a permis de dater la formation de ces galaxies et de laplacer à z~2, quand l’univers avait 25% de son âge actuel. Toutefois, des galaxies passives ont été découvertesdéjà massives et déjà "éteintes" à cette époque là. À z~2, ces galaxies sont composées d’étoiles de 1-2 milliardsd’années, enfermées dans des structures 10-100 fois plus denses, car leurs rayons de demi-lumière typiques sonten moyen 3-5 fois plus petits que dans l’univers local. L’un des défis les plus considérables dans l’astrophysiqueobservationnelle est de comprendre la nature des objets rouges lointains, car ils peuvent être galaxies à forteformation stellaire mais fortement obscurcies par la poussière, dont la distribution spectrale d’énergie (SED)peut ressembler à celle des galaxies passives. Cette thèse s’attaque au problème avec un échantillon de 10candidats galaxies passive dans l’univers lointain, sélectionnés dans le champ COSMOS entre 2.4 < z < 3.3...This thesis investigates the nature of distant red galaxies in the early universe in search for massive quenchedgalaxies (QGs). In the local universe, QGs are the most massive among all galaxy types, having extremely oldstellar populations, self-similar structures and extremely low star-formation rates (SFRs) at fixed stellar mass ifcompared to the typical relation describing the normal mode of star-formation in galaxies, the so-called MainSequence. Archaeological information from their spectra in the nearby universe placed their formation at z~2,however such quenched galaxies were later found to be already in place at such early epochs. They were foundwith suppressed SFRs and evolved stellar populations (1-2 Gyr) but with smaller sizes (by a factor of 3-5) andhigher stellar densities (by a factor of 10-100). Tracing the emergence of the massive quiescent galaxy populationhas since then been an increasingly hotter research topic, since it required understanding the interplay betweenmass assembly, star formation and quenching, interplay which is still highly debated today. One of the majorchallenges of observational astrophysics is disentangling the nature of faint distant red objects, as they can bepassively evolving systems lacking blue short-lived massive stars; or dusty star-forming galaxies whose high levelof dust reddening can often mimic the spectral energy distribution (SED) and colors of quiescent objects. Thisthesis addresses this issue by characterizing a sample of QG candidates in the distant universe (z~3). I reducedand analyzed the Hubble Space Telescope WFC3/G141 near-Infrared grism spectra of 10 QG candidates at 2.4 <z < 3.3 in the COSMOS field. Successful spectroscopic confirmation was accomplished for the entire sample. Theirrest-frame nearUV-optical spectra, combined with their broadband UV to NIR photometry, point towards theirquiescent nature, albeit the galaxies being consistent with having had their star-formation shut down veryrecently. Such sample constitutes one of the widest and most robust samples of quiescent galaxies available todate in the distant universe (D’Eugenio et al 2021, submitted to A&A). The strong Balmer absorption lines, whichflag the presence of young A-type stars, were better quantified through the creation of an high signal-to-noiseaverage spectrum, confirming a mean age of ~300 Myr and the presence of post-starburst features. Suchconfirmation allowed us to extend the poststarburst classification to the majority of classically selected quiescentgalaxies at z~3, marking their convergence to the quenching epoch. The presence of a weak [O II]l3727 emissionline suggests that the rapid age evolution with respect to intermediate redshift analogs is not followed by anincrease in specific star formation rate or, equivalently, by an increase in gas fraction (D’Eugenio et al. 2020, ApJLetters). Despite HST data do not provide spectral coverage for Hbeta, Halpha, [NII] and [OIII], median stackemission from 3GHz data helps excluding the presence of strong star-forming interlopers and might be indicativeof a substantial evolution in radio-mode AGN activity with respect to z~1.5. However, we caution that deep mm(ALMA/NOEMA) data are needed to individually assess the level of residual dust-obscured star formation giventhe shallow upper-limits from the FIR. Individual X-ray detections (30% of the sample) point towards an enhancedblack hole accretion rate in recently quenched objects with respect to either similarly massive star forminggalaxies at the same epoch or QGs at lower redshifts. The widespread compact, bulge-dominated morphologies,coupled with the young mass-weighted ages, suggest that the mechanisms responsible for structural andmorphological transformation might be preceding quiescence, similarly to what seen in compact star forminggalaxies at high-z