Beyond the Merger-Quasar-Quench Paradigm I: Mergers are neither necessary nor sufficient to quench central galaxies in IllustrisTNG

Abstract

The cessation of star formation in galaxies, known as 'quenching', is a complex, multi-scale process which has been theorized to be linked to galaxy mergers. In this paper, we investigate the potential role of mergers in quenching galaxies in the IllustrisTNG cosmological hydrodynamical simulation. We track the evolution of over 11,000 central galaxies in the simulation with stellar mass $M_\star \ge 10^9 M_\odot$ at $z = 0$ throughout the entirety of cosmic history. We compare their star formation and merger histories to test whether mergers are necessary or sufficient for inducing quenching in the simulation. Only a very small fraction of mergers (about 3 per cent of major mergers and about 12 per cent of all mergers) lead to quenching within 1 Gyr, indicating that mergers are not sufficient by themselves to cause quenching. Furthermore, the vast majority of quenching events are not preceded by a merger within 1 Gyr. Once random coincidences are accounted for and a stellar mass-matched control sample is applied, no merger excess is observed. Hence, mergers are clearly not necessary for quenching to occur in the simulation. Finally, we perform a series of random forest classification and regression analyses to assess the integrated role of mergers in galaxy quenching and supermassive black hole growth in IllustrisTNG. We determine that secular processes dominate the growth of supermassive black holes and the quenching of central galaxies in this simulation, in stark contrast to prior theoretical expectations from idealized hydrodynamical simulations.

Figures (25)

• Figure 1: Effective merger-ratio completeness implied by our resolved-merger criterion. Requiring $M_{\star,\mathrm{sec}}\ge 10^7\,M_\odot$ sets a host-mass-dependent minimum ratio $r_{\rm min}(M_{\star,\mathrm{main}})=10^7/M_{\star,\mathrm{main}}$ (solid black line). Mergers above this limit are included (green shaded region), while those below are excluded (purple shaded region). The vertical dashed line marks $\log_{10}(M_{\star,\mathrm{main}}/M_\odot)=10.5$, near the stellar-mass scale where central galaxies typically begin quenching, illustrating completeness in merger ratio down to $\sim$1:3000 at that mass.
• Figure 2: Examples of the KDE-based procedure used to fit the main sequence (MS) and estimate its scatter at three snapshots (top to bottom: $z=0,\,0.5,\,1$, as labeled). Right: KDE contours in the $\log_{10}(\mathrm{sSFR}/\mathrm{yr}) - \log_{10}(M_\bigstar/M_\odot)$ plane with the adopted HDR boundary (dark blue), the linear MS fit (dashed blue; the reported Slope+1 corresponds to $a+1$ for a fit of the form $y = a\,x + b$), and the population boundaries defined relative to the MS as $y - y_{\rm MS} = -2\sigma$ and $y - y_{\rm MS} = -5\sigma$ (dashed green), with shaded star-forming (blue), green-valley (green), and quenched (pink) regions. Vertical dashed black lines mark the stellar-mass percentiles $p_{10}$, $p_{50}$, and $p_{90}$ of $x=\log_{10}(M_\bigstar/M_\odot)$ for the sample in that snapshot. Left: the collapsed distribution of $y\equiv\log_{10}(\mathrm{sSFR}/\mathrm{yr})$ for galaxies inside the HDR boundary (relative frequency) and the Gaussian fit used to define $\mu$ and $\sigma$; the inset also reports $p_{50}$ evaluated on the MS (i.e., $y_{\rm MS}$ at the median stellar mass) and the offset $|\mu - p_{50}|$.
• Figure 3: Star formation histories for three example galaxy types: a star forming galaxy (top panel), a fast-quenched galaxy (center panel), and a slow-quenched galaxy (bottom panel). On each panel, the $Y$-axis shows the evolution of ${\mathrm{r(sSFR)}}$ as a function of the age of the Universe (bottom $X$-axis) and redshift (top $X$-axis). The ${\mathrm{r(sSFR)}}$ values are measured as offsets from the main sequence at each epoch, and are normalized by the standard deviation ($\sigma$) obtained from a Gaussian fit to the sSFR distributions (see eq. 3). The shaded regions represent different states of star formation (as defined in Figure \ref{['sSFR_Peaks']}). The blue region corresponds to star forming systems, the green region to quenching systems, and the red region to fully quenched systems. The vertical dashed and solid lines indicate times when minor and major mergers occur, respectively. The blue points represent the individual ${\mathrm{r(sSFR)}}$ values at each epoch, while the smoothed solid curve (displayed as a solid blue line on each panel) is obtained using a Savitzky-Golay filter to highlight the overall trend, averaging out stochasticity due to burstiness in star formation. The two solid red vertical lines indicate the times when each galaxy begins and completes quenching, defined by entering and departing the green valley region.
• Figure 4: Distributions of quenching starting times and durations in IllustrisTNG. Top-left-hand panel: Distribution of galaxy quenching starting times as a function of the age of the Universe (bottom axis) and corresponding redshift (top axis). The histogram is overlaid with a 5th-order polynomial fit (purple curve), highlighting a broad peak around $\sim10$ Gyr ($z \sim 0.33$), indicating the typical time at which galaxies quench. Bottom-left-hand panel: Stellar mass-dependent distributions of quenching initiation times (i.e., quenching time distributions computed within stellar mass bins). Each curve is a 5th-order polynomial fit to the distribution of quenching start times within individual stellar mass bins. The systematic leftward shift of the distribution peaks with increasing stellar mass illustrates that more massive galaxies typically initiate quenching at earlier cosmic epochs. Top-right-hand panel: Distribution of quenching durations, exhibiting a clear bimodal structure. Two Gaussian fits identify peaks centered at $\mu = 8.29$ and $\mu = 9.11$ in $\log_{10}(\Delta\tau_Q/\text{yrs})$, with corresponding standard deviations of $\sigma = 0.28$ and $\sigma = 0.30$, respectively. An intermediate threshold at $\log_{10}(\Delta\tau_Q/\text{yrs}) = 8.75$ (equivalent to 0.56 Gyr) is used to distinguish between fast- and slow-quenching populations. Bottom-right-hand panel: Stellar mass–dependent quenching duration distributions, each fitted with a 5th-order polynomial. The parameter $f_{\text{slow}}$ denotes the fraction of slow-quenching galaxies.
• Figure 5: Fractional composition of our central galaxy sample (left-hand panel) and the breakdown of quenching timescales for quenched galaxies (right-hand panel). The left-hand panel shows that $\sim76\%$ remain star-forming, $\sim11\%$ are fully quenched, $\sim11\%$ lie in the green valley, and $\sim3\%$ experience rejuvenation (by $z = 0$). The right-hand panel indicates that $\sim62\%$ of quenched galaxies follow a fast-quenching pathway, while $\sim38\%$ quench more slowly.