Taking a Break at Cosmic Noon: Continuum-selected Low-mass Galaxies Require Long Burst Cycles

Abstract

While bursty star formation in low-mass galaxies has been observed in local populations and reproduced in simulations, the dormant phase of the burst cycle has not been well studied beyond the local Universe due to observational limitations. We present a unique sample of 43 JWST PRISM spectra of low-mass galaxies ($M_\star < 10^{9.5}\,M_\odot$) at cosmic noon ($1<z<3$), uniformly selected on F200W magnitude and precise photometric redshifts enabled by 20-band JWST photometry from the UNCOVER and MegaScience surveys. The spectra reveal numerous strong Balmer breaks, which are negatively correlated with the galaxies' H$α$ equivalent width. By comparing these observations to synthetic samples of spectra generated using a simple parametrization of bursty star formation histories, we show that star formation in low-mass galaxies at cosmic noon is likely dominated by burst cycles with long timescales ($\gtrsim 100$ Myr) and large deviations below the star-forming main sequence ($\gtrsim 0.8$ dex). Our results suggest that galaxies in this population--at least those within our detection limits--should not be classified solely by their current star formation rates, but instead viewed as a unified population undergoing dynamic movement above and below the star-forming main sequence. The derived constraints demonstrate that long-timescale fluctuations are important for this class of galaxies, indicating that galaxy-scale gas cycles--rather than molecular-cloud-scale stochasticity--are the primary regulators of star formation variability in low-mass galaxies at cosmic noon.

Figures (12)

• Figure 1: Left: Stellar mass versus redshift for the sources in the PRISM sample and parent sample, colored by their F200W magnitude. The PRISM sources are plotted at the position of their spectroscopic redshift while the parent sample uses photometric redshifts. The PRISM sample was selected on F200W magnitude (F200W $<$ 27) and photometric redshift ($1<z_\text{phot} <3$). Right: Log specific star formation rate versus log stellar mass for the PRISM sample and parent sample, colored by redshift. The solid lines and shaded region show the median sSFR as a function of mass for the parent sample in two redshift bins: $z\in[1,2]$ in blue and $z\in[2,3]$ in red. The PRISM sample spans the sSFR range, with a significant number of sources above and below the inner quartile range.
• Figure 2: Top: PRISM spectra and MegaScience JWST photometry for four galaxies in our low-mass cosmic noon sample. The galaxies are ordered by decreasing Balmer break strength and increasing H$\alpha$ EW. The 20 bands of broadband and medium-band photometry are plotted over the PRISM spectra, with broadbands shown in black and medium bands shown with colors corresponding to the those of the medium-band filter transmission curves at the top of the figure. The SED-fitted stellar mass, spectroscopic redshift, H$\alpha$ EW, and Balmer break strength are listed for each galaxy at the left of each panel. The Balmer break and some notable emission lines are indicated. Bottom: Stacked Balmer breaks and H$\alpha$ emission for all 43 of the PRISM spectra in the sample. The spectra were locally normalized, interpolated to a common wavelength grid, and then combined. The spectra exhibit notable Balmer breaks; the median Balmer break strength of the sample is $\sim1.8$.
• Figure 3: An example of the method we employ to measure Balmer break strength and H$\alpha$ EW from the PRISM spectra. Left: The blue and red regions indicate the wavelength range over which the continua are fit on either side of the break. The vertical dashed lines show the wavelengths at which we evaluate the break strength. Right: The blue and red points indicate the wavelength range used to fit the continuum under H$\alpha$. The black points show the integration range we use to calculate the EW.
• Figure 4: Bursty SFHs for a galaxy with $M_\star=10^9\,M_\odot$ at $z=2$ with fixed amplitude ($A_\textrm{SFMS}$$=1$ dex) and varying timescale ($\delta_\textrm{burst}$). Top: The SFR over the most recent 1 Gyr for bursty SFHs with $\delta_\textrm{burst}$$=0.05, 0.1, 0.5,$ and 1 Gyr plotted in different colors. The dashed line shows the evolution of a burst-free galaxy on the Popesso2023 star-forming main sequence and the shaded region indicates the area 1 dex above and below the main sequence. Middle: The log H$\alpha$ EW as a function of time for the various timescales with marginal distributions shown at the right. Bottom: The same as the middle panel, but for the Balmer break strength. Top right: The evolution of a galaxy following the various bursty SFHs in the Balmer break vs. log H$\alpha$ EW space. The points shown are evenly time spaced and sampled over a full period in phase, with $t_0$ running from 0 to $\delta_\textrm{burst}$ as described in Section \ref{['subsec:sfhs']}. Strong breaks are only achieved in SFHs with long-timescale bursts; SFHs with shorter bursts always sustain a population of young, massive stars and so never develop strong Balmer breaks, which are characteristic of a stellar population dominated by A stars.
• Figure 5: The same as \ref{['fig:sfhs_db']}, but varying $A_\textrm{SFMS}$ with $\delta_\textrm{burst}$ held constant at 200 Myr. The shaded regions show the symmetric logarithmic deviations of $A_\textrm{SFMS}$ from the main sequence, which are not matched to the range of the final SFHs due to the mass normalization. Strong breaks are only achieved by SFHs with significant deviation from the SFMS. Low-amplitude bursty SFHs never reach sufficiently low SFRs, even in their dormant periods.