UV slope
Introduction
The ultraviolet (UV) continuum slope (\(\beta\), where \(f_{\lambda} \propto \lambda^{\beta}\)) serves as a key diagnostic for understanding the physical properties of high-redshift galaxies. An observable sensitive to dust attenuation, stellar populations, metallicity, and star formation history, \(\beta\) can provide valuable insight into galaxy evolution across cosmic time. The UV slope is also one of the few quantities readily observable in high-redshift Lyman break galaxies, since the rest-frame UV continuum is shifted into the optical/near-IR and the continuum slope can be measured via photometry alone. JWST has provided new insight into UV slopes at high redshifts (\(z > 7\)), probing deeper into the early universe than previously possible with Hubble Space Telescope (HST).
History
Pre-JWST history, e.g. Calzetti, Kinney, and Storchi-Bergmann (1994), Finkelstein et al. (2012), Dunlop et al. (2012), Wilkins et al. (2013)
Measurement
Broadly, the UV slope can be measured two ways:
- Direct power-law fits to the data (fitting \(f_\lambda \propto \lambda^\beta\))
- SED model fitting, in a full Bayesian context, where \(\beta\) is derived from the posterior SED
Each technique has their pros and cons, and can show systematic differences in \(\beta\) between the two.
It is possible to measure \(\beta\) with photometry alone, but this can create some issues. The choice of broadband filters can impact the derived \(\beta\). Contamination from emission lines can also impact broadband-derived slopes at specific redshifts.
Spectroscopy makes it easier, if SNR is sufficient. Calzetti windows to avoid emission line contribution.
Nebular continuum emission can significantly impact \(\beta\) measurements in very young galaxies. This is primarily due to free-bound emission, which is quite red at UV wavelengths. If the UV light is dominated by the nebular component, the slope can appear redder than the intrinsic stellar continuum slope.
Key JWST Observations
UV Slope Measurements at \(z > 10\)
JWST has enabled robust measurements of \(\beta\) in galaxies at \(z > 10\)
Evolution of \(\beta\) with Redshift
- Overall trend of bluer (more negative) \(\beta\) values with increasing redshift from \(z \sim 4\) to \(z \sim 12\)
Implications for Stellar Populations and Dust
- JWST observations indicate extremely low dust content in the earliest galaxies (\(z > 10\)), with \(A_V\) typically \(< 0.1\)
- First evidence for non-zero dust attenuation in some bright \(z > 10\) systems, challenging assumptions about dust formation timescales
- Extremely blue slopes (\(\beta < -2.7\)) in some high-\(z\) sources suggest very young stellar populations with ages \(< 10\) Myr
Connection to Ionizing Photon Escape
- Blue UV slopes support scenarios where low-mass galaxies dominate the ionizing photon budget during reionization
- Correlations between \(\beta\) and inferred Lyman continuum escape fraction (\(f_{\rm esc}\)) in some samples
- Constraints on the production rate of ionizing photons (\(\dot{n}_{\rm ion}\)) based on JWST-measured UV slopes and luminosity functions