[Paper Review] NIRSpec View of the Appearance and Evolution of Balmer Breaks and the Transition from Bursty to Smooth Star Formation Histories from Deep Within the Epoch of Reionization to Cosmic Noon

Danial Langeroodi, J. Hjorth|arXiv (Cornell University)|Apr 19, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study uses NIRSpec prism spectroscopy of 631 high-redshift galaxies (3 < z < 14) to trace the evolution of Balmer breaks and star formation burstiness from the Epoch of Reionization to cosmic noon. It finds that bursty star formation peaks at z > 6 and β_UV < -2.25, with sSFR > 30 Gyr⁻¹, while a transition to smooth star formation occurs by z < 4, marked by redder UV colors, stronger Balmer breaks, and lower sSFRs.

ABSTRACT

Theoretical models and observational evidence suggest that high-redshift galaxies grow under the bursty mode of star formation, with large temporal star formation rate (SFR) fluctuations around some mean value. From an observational perspective, it has not been clear at which redshift and stellar population characteristics the transition from bursty to smooth star formation occurs. Here, we investigate these using a uniformly reduced sample of NIRSpec prism spectra of 631 galaxies at $3 &lt; z_{ m spec} &lt; 14$, stacked in 8 redshift and 8 UV slope bins. We evaluate the burstiness of star formation histories using the Balmer break strengths as well as the ratios of SFRs as measured from the emission lines to those measured from the UV continua. The break strength increases monotonically from $z = 10$ to $z = 3$, and from $β_{ m UV} = -3.0$ to $β_{ m UV} = 0.0$. The break strength is tightly anti-correlated with specific SFR (sSFR), and in dusty galaxies, strongly correlated with dust attenuation. Based on the SFR ratios, we find that bursty star formation thrives in the highest redshift, bluest, and lowest stellar mass galaxies, which exhibit the highest sSFRs. The burstiness appears to plateau at $z &gt; 6$, suggesting that we might be observing the peak of star formation burstiness at these redshifts. The $z &lt; 4$ galaxies do not appear particularly bursty, suggesting that the smooth mode of star formation starts taking over right before cosmic noon. As galaxies mature and develop redder UV colors and more pronounced Balmer breaks, their ability to sustain star formation over longer timescales increases, signalling their transition from bursty to smooth star formation.

Motivation & Objective

• To determine the redshift and stellar population conditions at which high-redshift galaxies transition from bursty to smooth star formation histories.
• To quantify the burstiness of star formation using Balmer break strength and SFR ratios from UV continuum and emission lines.
• To assess the role of dust attenuation and UV slope in shaping observed spectral features and star formation indicators.
• To identify the redshift and galaxy properties where the transition from bursty to smooth star formation becomes dominant.
• To calibrate the use of UV slope as a proxy for Balmer break strength to break degeneracies in photometric redshift estimation.

Proposed method

• Stacked NIRSpec prism spectra of 631 galaxies in 8 redshift and 8 UV slope bins to enhance signal-to-noise for spectral feature analysis.
• Measured Balmer break strength as f_ν(4225Å)/f_ν(3565Å) to quantify the relative contribution of young and old stellar populations.
• Calculated SFR ratios by comparing emission-line-based SFR (from Hα, Hβ, etc.) to UV-continuum-based SFR to assess burstiness.
• Used dust attenuation (A_V) and specific SFR (sSFR) as diagnostics to correlate with Balmer break strength and star formation history.
• Applied redshift-dependent spectral modeling and photometric redshift calibration to correct for observational biases in high-z stacks.
• Employed UV slope (β_UV) as a proxy to predict Balmer break strength and mitigate degeneracies between emission lines and breaks in photometry.

Experimental results

Research questions

• RQ1 At what redshift and galaxy properties does the transition from bursty to smooth star formation occur in high-redshift galaxies?
• RQ2 How does Balmer break strength evolve from z ≈ 10 to z ≈ 3, and what does it reveal about stellar population age and SFR history?
• RQ3 To what extent is burstiness in star formation correlated with sSFR, dust attenuation, and UV slope in high-redshift galaxies?
• RQ4 Why do z < 4 galaxies not exhibit strong burstiness despite high SFRs, and what does this imply about the onset of smooth star formation?
• RQ5 Can UV slope be reliably used as a proxy for Balmer break strength to improve photometric redshift accuracy in high-redshift surveys?

Key findings

• Balmer break strength increases monotonically from z = 10 to z = 3, with the strongest breaks observed in dusty, reddest galaxies (β_UV > -1.25) and lowest sSFRs (< 10 Gyr⁻¹).
• The most bursty galaxies are found at z > 6, β_UV < -2.25, and sSFR > 30 Gyr⁻¹, with negative Balmer breaks (f_ν(4225Å)/f_ν(3565Å) < 1) and negligible dust attenuation.
• Burstiness plateaus at z > 6, indicating that the peak of star formation burstiness is observed in the earliest cosmic epochs.
• Galaxies with z < 4 show no significant burstiness, suggesting that the smooth mode of star formation dominates by cosmic noon.
• As galaxies mature, they develop redder UV colors, stronger Balmer breaks, higher dust attenuation (A_V > 0.2 mag), and lower sSFRs, indicating a transition to sustained, long-timescale star formation.
• The strongest Balmer breaks, highest dust attenuation, reddest UV colors, and lowest sSFRs are found in the least bursty, most mature stacks, confirming the transition from bursty to smooth star formation.