Starbursts hiding in the main sequence: a pathway toward quenching?

Abstract

Star-forming galaxies spend most of their lifetimes on the star-forming main sequence, which establishes a tight empirical and statistical relation between stellar mass and star-formation rate. Occasional episodes of rapid star formation can push them temporarily above this sequence, turning them into starbursts. Yet some galaxies display starburst-like traits -- rapid, dense, and compact star formation -- while still remaining within the scatter of the main sequence. These "starbursts in the main sequence" (SBMSs) reveal the complexity and diversity of star formation modes, making them crucial for understanding how galaxies evolve and transition between different regimes. In this paper, we identify SBMSs in the cosmological simulation NewHorizon and follow their evolution across time to uncover their physical origins and the role of this special regime in shaping galaxy evolution. We explain the existence of SBMSs by a comparatively earlier assembly of their stellar mass, driven in particular by more frequent and repeated mergers as the other galaxies, as well as exceptionally productive starburst events triggered by these interactions. As a result, this regime appears preferentially -- though not exclusively -- in the most massive galaxies. The SBMS behavior is not continuous within individual galaxies but instead arises intermittently as a short-lived (~ 30 Myr) evolutionary mode. Nevertheless, such SBMS episodes exist throughout cosmic time across the galaxy population... [abridged]

Figures (16)

• Figure 1: Schematic classification of star-forming galaxies based on their timescale of star formation (traced by the depletion time $\tau_\mathrm{dep}$) and their offset from the main sequence ($\Delta\mathrm{SFR}_\mathrm{MS}$, i.e., the difference between their SFR and the SFR of the main sequence for their stellar mass). The vertical dotted line represents the median depletion time, and the horizontal one is $\Delta\mathrm{SFR}_\mathrm{MS}\xspace=0$. Shaded areas indicate $\pm 1$ times the standard deviation. We highlight four classes and propose an analogy with rivers: the width of the river is $\Delta\mathrm{SFR}_\mathrm{MS}$, and the speed of the runoff corresponds to the depletion time. A class-1 galaxy (hereafter a main sequence galaxy) has an average SFR for both its stellar and gas masses, analogous to an average river flow. A class-2 galaxy (hereafter a starburst) forms large amounts of stars at an accelerated pace, analogous to rapid and wide waterfalls. As visible in this diagram, the continuous evolution of $\Delta\mathrm{SFR}_\mathrm{MS}$ and $\tau_\mathrm{dep}$ imposes that a galaxy cannot experience a direct transition between classes 1 and 2: it must necessarily pass through classes 3 or 4. Class 3 includes galaxies with rapid star formation but producing average amounts of stars, similar to a tall but narrow waterfall. This work aims to understand the origins of these "starbursts in the main sequence" galaxies (SBMSs). Finally, class-4 galaxies produce large amounts of stars but at a normal rate, analogous to floods. We thus introduce the term "starflood" to name them.
• Figure 2: Distribution of galaxies in the stellar mass–SFR plane at $z=3$ (for illustration). The color indicates the standardized depletion time (i.e., the difference from the median value of the entire population, divided by the robust standard deviation; see Equation \ref{['eqn:std']}). The solid line represents the best least absolute deviation fit, and the shaded area indicates $\pm 1$ times the mean absolute deviation from this relation, defining the main sequence of the simulation at this redshift. For comparison, the dotted and dashed lines show the empirical main sequence relations from fits of observed galaxy populations in Schreiber2015 and Popesso2023, respectively (see Dubois2021 for a discussion on the main sequence in NewHorizon). The horizontal features at low SFR originate from the finite mass resolution of the simulation.
• Figure 3: Depletion times of the total gas of star-forming galaxies as a function of stellar mass (left) and sSFR (right), shown here at $z=3$. The horizontal line and shaded area show the median $\tau_\mathrm{dep}$ and its dispersion at this redshift. Not surprisingly, the SBMSs are found toward the lower end of the distribution of sSFR of galaxies with short depletion times. This is not a systematic feature since the main sequence is not defined using the sSFR but the best-fit relation in the $M_\star$–SFR plane.
• Figure 4: Number of star-forming galaxies considered in our sample as a function of redshift (grey line and its smoothed version in black, left axis), and fraction of these galaxies that have experienced at least one passage in the SBMS regime in their lifetime (red solid line, and linear fit in dashed red, right axis).
• Figure 5: Top three panels: evolution of the median of the logarithmic stellar mass, the half-mass radius of the star-forming gas, and the gas fraction for galaxies in the main sequence, in the starburst regime, in the SBMS regime and their analogues (i.e., the non-starburst main sequence galaxy with the most similar SFR and $M_\star$), and in the starflood regime (see Section \ref{['sec:semantics']} for details). Penultimate panel: evolution of the number fractions of each regime over the entire population of star-forming galaxies. Also shown is the fraction of SBMSs among galaxies with short depletion times (i.e., starbursts and SBMSs). Bottom panel: median depletion time of the SBMSs normalized by that of the starburst galaxies, and the same ratio between the analogues and the main sequence galaxies, and between the analogues and the SBMSs. The horizontal line indicates unity. All curves have been smoothed using a Savitzky–Golay algorithm to improve readability, with the original measurements shown by the semi-transparent lines.