[Paper Review] The UV Continuum Slopes of Early Star-Forming Galaxies in JADES

Michael W. Topping, Daniel P. Stark+23 more|arXiv (Cornell University)|Jul 13, 2023

Galaxies: Formation, Evolution, Phenomena7 citations

TL;DR

This study analyzes the UV continuum slopes (β) of 179 star-forming galaxies at z ≳ 9 using deep JADES JWST data, finding a median β = -2.4 and identifying 44 extremely blue galaxies with β ≲ -2.8. These ultra-blue galaxies exhibit high ionizing photon escape fractions (f_esc ≈ 0.51), weak emission lines, low stellar masses (log M/M☉ = 7.5 ± 0.2), and elevated specific star-formation rates (79 Gyr⁻¹), suggesting they are young, dust-poor systems with strong LyC leakage, possibly shaped by recent star formation bursts or quenching events.

ABSTRACT

The power-law slope of the rest-UV continuum ($f_λ\proptoλ^β$) is a key metric of early star forming galaxies, providing one of our only windows into the stellar populations and physical conditions of $z>10$ galaxies. Expanding upon previous studies with limited sample sizes, we leverage deep imaging from JADES to investigate the UV slopes of 179 $z>9$ galaxies with apparent magnitudes of $m_{ m F200W}=26-31$, which display a median UV slope of $β=-2.4$. We compare to a statistical sample of $z=5-9$ galaxies, finding a shift toward bluer rest-UV colors at all $ m~M_{UV}$. The most UV-luminous $z>9$ galaxies are significantly bluer than their lower-redshift counterparts, representing a dearth of moderately-red galaxies in the first $500~$Myr. At yet earlier times, the $z>11$ galaxy population exhibits very blue UV slopes, implying very low attenuation from dust. We identify a robust sample of 44 galaxies with $β<-2.8$, which have SEDs requiring models of density-bounded HII regions and median ionizing photon escape fractions of $0.51$ to reproduce. Their rest-optical colors imply that this sample has weaker emission lines (median $m_{ m F356W}-m_{ m F444W}=0.19$ mag) than typical galaxies (median $m_{ m F356W}-m_{ m F444W}=0.39$ mag), consistent with the inferred escape fractions. This sample has relatively low stellar masses (median $\log(M/M_{\odot})=7.5$), and specific star-formation rates (median$=79 m/Gyr$) nearly twice that of our full sample (median$=44 m/Gyr$), suggesting they are more common among systems experiencing a recent upturn in star formation. We demonstrate that the shutoff of star formation provides an alternative solution for modelling of extremely blue UV colors, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures.

Motivation & Objective

• To measure the rest-UV continuum slopes (β) of high-redshift star-forming galaxies at z ≳ 9 using deep JWST imaging.
• To investigate the physical drivers behind extremely blue UV colors (β ≲ -2.8) in early galaxies.
• To assess the role of dust attenuation, stellar population age, and ionizing photon escape in shaping UV colors.
• To compare models of constant star formation history with those including recent star formation quenching to explain extreme UV colors.
• To identify and characterize rare, highly ionizing photon-leaking galaxies at the cosmic dawn.

Proposed method

• Utilized deep near-infrared imaging from the JADES survey with JWST/NIRCam to measure photometry for 179 galaxies at z ≳ 9.
• Computed UV continuum slopes (β) via power-law fitting to rest-frame UV fluxes (fλ ∝ λ^β) using F200W, F356W, and F444W filters.
• Fitted spectral energy distributions (SEDs) using stellar population synthesis models, including density-bounded HII regions and varying ionizing photon escape fractions (f_esc).
• Compared models with constant star formation history (CSFH) to those with recent star formation quenching to assess fit quality and physical consistency.
• Used F356W - F444W color excess as a proxy for emission line strength, assuming nebular continuum contribution.
• Applied statistical sampling to compare the z ≳ 9 sample with a control sample at z ≈ 5–9 to assess redshift evolution in UV colors.

Experimental results

Research questions

• RQ1 What is the distribution of UV continuum slopes (β) among z ≳ 9 star-forming galaxies, and how does it evolve with redshift?
• RQ2 What physical mechanisms—dust attenuation, stellar population age, or ionizing photon escape—best explain the observed extremely blue UV colors (β ≲ -2.8)?
• RQ3 How do the stellar masses and specific star-formation rates of ultra-blue galaxies compare to the general high-redshift population?
• RQ4 Can models with recent star formation quenching reproduce the observed UV colors as well as high-escape-fraction models?
• RQ5 Do ultra-blue galaxies exhibit weaker emission lines, consistent with high ionizing photon escape fractions?

Key findings

• The median UV continuum slope for z ≳ 9 galaxies is β = -2.4, with a significant population of extremely blue galaxies (β ≲ -2.8) identified.
• The 44 galaxies with β ≲ -2.8 require high ionizing photon escape fractions (f_esc ≈ 0.51) to reproduce their SEDs, consistent with density-bounded HII region models.
• These ultra-blue galaxies have weak emission lines, with a median F356W - F444W color of 0.19 mag, significantly below the full sample median of 0.39 mag.
• Their stellar masses are low (median log M/M☉ = 7.5 ± 0.2), and their specific star-formation rates are high (median 79 Gyr⁻¹), indicating recent star formation upturns.
• Models with recent star formation quenching can reproduce extremely blue UV colors and provide a better fit to the data than zero-escape models, though not as good as high-f_esc models.
• The quenching models predict stellar masses 0.3 dex higher on average than high-f_esc models, with a maximum offset of 0.8 dex, and stronger rest-optical continua than observed.