Lyman continuum leakage versus quenching with the James Webb Space Telescope: the spectral signatures of quenched star formation activity in reionization-epoch galaxies

Abstract

In this paper, we study the effects of a recent drop in star formation rate (SFR) on the spectra of epoch of reionization (EoR) galaxies, and the resulting degeneracy with the spectral features produced by extreme Lyman continuum leakage. In order to study these effects in the wavelength range relevant for the upcoming James Webb Space Telescope (JWST), we utilize synthetic spectra of simulated EoR galaxies from cosmological simulations together with synthetic spectra of partially quenched mock galaxies. We find that rapid declines in the SFR of EoR galaxies could seriously affect the applicability of methods that utilize the equivalent width of Balmer lines and the ultraviolet spectral slope to assess the escape fraction of EoR galaxies. In order to determine if the aforementioned degeneracy can be avoided by using the overall shape of the spectrum, we generate mock NIRCam observations and utilize a classification algorithm to identify galaxies that have undergone quenching. We find that while there are problematic cases, JWST/NIRCam or NIRSpec should be able to reliably identify galaxies with redshifts z ∼ 7 that have experienced a significant decrease in the SFR (by a factor 10-100) in the past 50-100 Myr with a success rate 85%. We also find that uncertainties in the dust-reddening effects on EoR galaxies significantly affect the performance of the results of the classification algorithm. We argue that studies that aim to characterize the dust extinction law most representative in the EoR would be extremely useful.

Intense C III] λλ1907,1909 emission from a strong Lyman continuum emitting galaxy

“…Clearly this method is complicated in practice, since it requires the determination of the intrinsic UV slope, which depends on the a priori unknown amount of UV attenuation and the attenuation law -especially at high redshift (see e.g. Capak et al 2015;Popping et al 2017;Narayanan et al 2018) -and is sensitive to several parameters including the metallicity, star-formation history, and age (Raiter et al 2010;Binggeli et al 2018). For J1154+2443 we find a UV slope β ∼ −1.9 ± 0.3 from the STIS spectrum, considerably redder than the range of UV slopes predicted by Zackrisson et al (2013), and which is clearly affected by dust attenuation, as shown by Izotov et al (2018a).…”

The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics

“…Moreover, many local LyC emitters (LCEs) exhibiting high Balmer emission line EWs have been found (Izotov et al 2016b(Izotov et al , 2018a(Izotov et al , 2018b. In any case, Balmer-line EWs are sensitive not only to nebula size but also to starburst age and star formation history (Zackrisson et al 2017;Binggeli et al 2018;Alavi et al 2020). Plagued by such degeneracies, the EW diagnostic may be best when paired with another indicator.…”

The Low-Redshift Lyman Continuum Survey II: New Insights into LyC Diagnostics

“…Moreover, many local LyC emitters (LCEs) exhibiting high Balmer emission line EWs have been found (Izotov et al 2016b(Izotov et al , 2018a. In any case, Balmer line EWs are sensitive not only to nebula size but also starburst age and star formation history (Zackrisson et al 2017;Binggeli et al 2018;Alavi et al 2020). Plagued by such degeneracies, the EW diagnostic may be best when paired with another indicator.…”