Galaxy quenching:
cessation of star formation in galaxies

Most of the massive galaxies do not form new stars at significant rates: Why do galaxies cease forming their stars, i.e. "quench" their star formation? This is one key questions galaxy evolution studies try to answer and is related to how galaxies regulate their star formation. Galaxy quenching is need for two reasons. First, quenching ensures that the most massive galaxies are the oldest ones: this is not expected in a bottom-up universe, in which more massive galaxies are expected to be younger, since they assemble at later times. Second, quenching can explain the difference between the observed galaxy mass function and the theoretical halo mass function at high masses. Although there is a strong motivation for quenching, we still lack a physical understanding of it. Physical processes responsible for quenching include galaxy-internal (such as stellar and black hole feedback) and galaxy-external (such as virial shock heating through the dark matter halo) processes. Observationally, it is difficult to make progress because we cannot observe individual galaxies evolving (the dynamical timescale of galaxies are 100s of Myr) and thereby quenching. We focus on (i) observing quenching in action in massive star-forming galaxies and (ii) studying the stellar populations of quiescent galaxies, which allows to re-construct their past evolutionary paths.

Galaxies quench inside-out 3 billion years after the Big Bang

New ground-based instrumentation with adaptive optics (AO) capabilities in conjuncture with HST has shifted the focus of studies of distant galaxies: from measuring integrated properties to detailed measurements on spatially resolved scales. We conducted several investigations based on the largest and deepest AO-assisted near-infrared integral field unit (IFU) spectroscopic program, which led to several breakthroughs that shaped our current understanding of galaxy evolution at cosmic noon (top figure). Specifically, my observational work – published in Science (Tacchella et al. 2015a) – led to the discovery that the most massive systems at cosmic noon (3 billion years after the Big Bang) sustain their high star-formation rates at large radii in rotating disk components, while hosting fully-grown and already quiescent bulges in their cores (bottom panel). Theoretically, this inside-out quenching phase can be reproduced with both model of quenching with and without feedback from a black hole (Tacchella et al. 2016b; Nelson, Tacchella et al. 2021). Observationally, we find signatures of strong nuclear outflows, indicating that a black hole is active in the core of these galaxies (Förster Schreiber et al. 2014; Genzel et al. 2014).

From these data, we gained additional insights into the following:

  • bright, star-forming disk components outshine dense, old bulges in massive star-forming galaxies at cosmic noon (Tacchella et al. 2015b);

  • the dust attenuation peaks in the central regions, though the amount of attenuation is not enough to explain the suppression of the Halpha emission and therefore the dip in the inferred star-formation rate (Tacchella et al. 2018b);

  • detailed kinematics analysis revealed that these star-forming galaxies are strongly baryon dominated (Genzel et al. 2017; Lang et al. 2017);

  • most of these galaxies are rotationally supported, consistent with the idea that these galaxies sustain their high star-formation rate through extended periods of continuous accretion (Förster Schreiber et al. 2018).

Fast, Slow, Early, Late: Diverse quenching paths of massive galaxies

In stead of observing galaxies in the quenching phase (see above), we can also re-construct the galaxies' past history by carefully measuring their stellar populations (ages and metallicities of the stars). We have done this kind of archeological analysis for z~1 galaxies (about 6 Billion years after the Big Bang) of the HALO7D survey. This survey is a collaboration of people studying stars in the Milky Way halo and us, who are interested in high-redshift galaxies. The HALO7D survey provides us with deep spectroscopy taken at the Keck Observatory in Hawaii, USA. Additionally, these galaxies have been observed with the Hubble Space Telescope, allowing us to perform a careful morphological analysis.

Using the fully Bayesian framework Prospector, in Tacchella et al. (2022) we perform a careful modeling of both the spectroscopic and photometric data with an advanced physical model (including non-parametric star-formation histories, emission lines, variable dust attenuation law, and dust and AGN emission) together with an uncertainty and outlier model. We show that both spectroscopy and photometry are needed to break the dust-age-metallicity degeneracy. We find a large diversity of star-formation histories: although the most massive galaxies formed the earliest, lower-mass galaxies have a wide range of formation redshifts. Star-forming galaxies evolve about the star-forming main sequence, crossing the ridgeline several times in their past. Quiescent galaxies show a wide range and continuous distribution of quenching timescales and of quenching epochs (figure on the left). This large diversity of quenching timescales and epochs points toward a combination of internal and external quenching mechanisms.