The galaxy starburst/main-sequence bimodality over five decades in stellar mass at z ~ 3-6.5

Abstract

We study the relation between stellar mass (M * ) and star formation rate (SFR) for star-forming galaxies over approximately five decades in stellar mass (5.5 log 10 (M * /M ) 10.5) at z ≈ 3 − 6.5. This unprecedented coverage has been possible thanks to the joint analysis of blank non-lensed fields (COSMOS/SMUVS) and cluster lensing fields (Hubble Frontier Fields) which allow us to reach very low stellar masses. Previous works have revealed the existence of a clear bimodality in the SFR − M * plane with a star-formation Main Sequence and a starburst cloud at z ≈ 4 − 5. Here we show that this bimodality extends to all star-forming galaxies and is valid in the whole redshift range z ≈ 3 − 6.5. We find that starbursts constitute at least ≈ 20% of all star-forming galaxies with M * 10 9 M at these redshifts and reach a peak of 40% at z = 4−5. More importantly, 60% to 90% of the total SFR budget at these redshifts is contained in starburst galaxies, indicating that the starburst mode of star-formation is dominant at high redshifts. Almost all the low stellar-mass starbursts with log 10 (M * /M ) 8.5 have ages comparable to the typical timescales of a starburst event, suggesting that these galaxies are being caught in the process of formation. Interestingly, galaxy formation models fail to predict the starburst/main-sequence bimodality and starbursts overall, suggesting that the starburst phenomenon may be driven by physical processes occurring at smaller scales than those probed by these models.

Still at odds with conventional galaxy evolution: the star formation history of ultradiffuse galaxy Dragonfly 44

“…As was shown in Fig. 7, the early SFR of DF44 is more typical of normal (MW-like) star forming galaxies at 𝑧 > 3 (Rinaldi et al 2021). The implication is that it is not the early, extreme SFH that makes DF44 unusual among 𝑧 = 0 galaxies, but rather its sudden quenching.…”

Chemical Evolution of the Universe and its Consequences for Gravitational‐Wave Astrophysics

“…• What is the contribution of starbursts (galaxies with abnormally high star formation) to the low metallicity star formation? Recent studies report a significantly higher fraction of starburst galaxies among the star forming population than previously found [162,163,164]. Furthermore, their prevalence seems to increase towards low galaxy masses and high redshifts.…”

“…However, given that the star formation rate and metallicity of star forming galaxies appears to be anti-correlated, one may expect that starbursts (significantly) contribute to star formation at relatively low metallicity (see [22] and references therein). Note that galaxies with such properties are lacking in the current cosmological simulations [165,166,164]. This should be kept in mind when their results are applied to model f SFR (Z,z) and used to characterize the populations of GW sources.…”

“…Recent studies report a significantly higher fraction of starburst galaxies among the star forming population than previously found. [162][163][164] Furthermore, their prevalence seems to increase toward low galaxy masses and high redshifts. There is limited direct information about the metallicity of those galaxies.…”

“…Note that galaxies with such properties are lacking in the current cosmological simulations. [164][165][166] This should be kept in mind when their results are applied to model f SFR (Z,z) and used to characterize the populations of GW sources.…”