The Recent Burstiness of Star Formation in Galaxies at z ∼ 4.5 from Hα Measurements

Abstract

The redshift range z = 4 − 6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 z ∼ 4.5 main-sequence galaxies at log(M/M ) > 9.7 by comparing their ultraviolet (UV) continuum, Hα luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6 µm]−[4.5 µm] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between Hα and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in Hα compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between Hα EW and stellar mass, ranging from 1000Å at log(M/M ) < 10 to below 100Å at log(M/M ) > 11. Consulting models suggests that most z ∼ 4.5 galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z = 4. We identify differential dust attenuation (f ) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing Hα EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z > 4 consistent with the extrapolation of measurements out to z = 2.

AstraeusIV : Quantifying the star formation histories of galaxies in the Epoch of Reionization

“…We note that this theoretical definition of stochastic star formation differs from estimates from observations (e.g. Faisst et al 2019). While we can follow the SFHs of galaxies at each epoch, observational estimates of SFHs are typically based on two data points (SFR in the last 10 and 100 Myrs through measuring the H𝛼 line and UV luminosity, respectively) and a chosen SFH shape.…”

“…The trend of stochastic star formation being prevalent in lower mass galaxies is in rough agreement with observational findings. Using a sample of observed galaxies with stellar masses of 10 8.5 M < M ★ < 10 11.5 M at 𝑧 ∼ 4.5, Faisst et al (2019) find that the excess of H𝛼 luminosity compared to UV luminosity decreases with increasing stellar mass, indicating a reduced burstiness of the SFH of massive galaxies. However, in their sample, high mass galaxies (𝑀 ★ > 10 10 M ) show signs of recent bursts of star formation, which are not present in our simulations.…”

“…This suggests that star formation in early galaxies increases with time during the EoR, transitioning from being stochastic to continuous (see e.g. Dayal et al 2013;Kimm & Cen 2014;Faisst et al 2019;Emami et al 2019) as their gravitational potentials become deep enough to withstand SN and radiative feedback. Indeed, hydrodynamical simulations assuming a homogeneous photoionization background at 𝑧 > 6 have found that the average SFH is smoothly rising with time and scales with stellar mass, and that the SFH shape is scale-invariant (Finlator et al 2011).…”

The ALPINE-ALMA [CII] Survey: Investigation of 10 Galaxies at $z\sim4.5$ with [OII] and [CII] Line Emission $-$ ISM Properties and [OII]$-$SFR Relation

“…The differential dust attenuation between nebular emission and stellar continuum is by far the largest contribution to the total uncertainty. There is observational evidence that the 𝑓 −factor of 𝑧 > 2 galaxies is closer to 0.7 than the locally measured value of 0.44 (Kashino et al 2017;Faisst et al 2019;Rodriguez-Munoz et al prep). In the case of 𝑓 = 0.7, the dust attenuation factors would decrease by a factor of 1.5 (0.18 dex change in luminosity) on average for our sample.…”

The Spitzer/IRAC Legacy over the GOODS Fields: Full-Depth 3.6, 4.5, 5.8 and 8.0um Mosaics and Photometry for > 9000 Galaxies at z~3.5-10 from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS)

“…A growing number of observations have identified a tight relation (σ 0.3 dex - (Daddi et al 2007;Whitaker et al 2012;Schreiber et al 2015;Kurczynski et al 2016) between the SFR and M (the so-called main sequence of star-forming galaxies - Noeske et al 2007) up to z ∼ 6−7 (e.g., Labbé et al 2010b;Bouwens et al 2012;Stark et al 2013;González et al 2014;Duncan et al 2014;Steinhardt et al 2014;Salmon et al 2015;Song et al 2016;Jiang et al 2016;Santini et al 2017;Iyer et al 2018;Pearson et al 2018;Khusanova et al 2020;Faisst et al 2019). Indeed, a correlation between the SFR and M is predicted by models (e.g., Finlator et al 2007Finlator et al , 2011Finlator et al , 2018Dekel et al 2009;Davé et al 2011;Lu et al 2014;Mancuso et al 2016;Wilkins et al 2017;Ma et al 2018;Ceverino et al 2018;Rosdahl et al 2018;Tacchella et al 2018).…”