Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5<z<8.5$ JADES galaxies

Abstract

The discovery of high-redshift galaxies exhibiting a steep spectral UV downturn potentially indicative of two-photon continuum emission marks a turning point in our search for signatures of top-heavy star formation in the early Universe. We develop a photometric search method for identifying further nebular-dominated galaxy candidates, whose nebular continuum dominates over the starlight, due to the high ionising photon production efficiencies ${\xi }_{\mathrm{ion}}$ associated with massive star formation. We utilise the extensive medium-band imaging from JADES, which enables the identification of Balmer jumps across a wide range of redshifts ($1.5<z<8.5$), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts ($\lesssim 3$~Myr), we further demand a high observed $\log ({\xi }_{\mathrm{ion},\mathrm{obs}}/(\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1}))>25.60$ to power the strong nebular continuum, together with a relatively non-blue UV slope indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting $\sim$10% of galaxies at $z\sim 6$ (decreasing to $\sim$3% at $z\sim 2$, not completeness-corrected) are faint in the rest-frame optical (median $M_{\mathrm{opt}}=-17.95$) with extreme line emission (median ${\mathrm{EW}}_{\mathrm{H}\alpha ,\mathrm{rest}}=1567$ Å, ${\mathrm{EW}}_{[\mathrm{O}\mathrm{III}]+\mathrm{H}\beta ,\mathrm{rest}}=2244$ Å). However, hot H II region temperatures, collisionally-enhanced two-photon continuum emission, and strong UV lines are expected to accompany top-heavy star formation. Thus nebular-dominated galaxies do not necessarily exhibit the biggest Balmer jumps, nor the largest ${\xi }_{\mathrm{ion},\mathrm{obs}}$ or reddest UV slopes. Hence continuum spectroscopy is ultimately required to establish the presence of a two-photon downturn in our candidates, thus advancing our understanding of primordial star formation and AGN.

26 pages, 17 figures, 1 table

References in corpus (64)

• The James Webb Space Telescope
• EAZY: A Fast, Public Photometric Redshift Code
• The Hubble Ultra Deep Field
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities
• PyNeb: a new tool for analyzing emission lines. I. Code description and validation of results
• The James Webb Space Telescope Mission
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)
• Spectroscopic Confirmation of CEERS NIRCam-selected Galaxies at $\mathbf{z}\simeq 8-10$
• JADES NIRSpec Initial Data Release for the Hubble Ultra Deep Field: Redshifts and Line Fluxes of Distant Galaxies from the Deepest JWST Cycle 1 NIRSpec Multi-Object Spectroscopy
• CEERS Spectroscopic Confirmation of NIRCam-Selected z > 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of their Properties
• EIGER II. first spectroscopic characterisation of the young stars and ionised gas associated with strong H$\beta$ and [OIII] line-emission in galaxies at z=5-7 with JWST
• The Hubble Legacy Field GOODS-S Photometric Catalog
• JWST NIRCam+NIRSpec: Interstellar medium and stellar populations of young galaxies with rising star formation and evolving gas reservoirs
• Diagnostics for PopIII galaxies and Direct Collapse Black Holes in the early universe
• Searching for Extremely Blue UV Continuum Slopes at $z=7-11$ in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies
• RUBIES: a complete census of the bright and red distant Universe with JWST/NIRSpec
• The JWST-PRIMAL Legacy Survey. A JWST/NIRSpec reference sample for the physical properties and Lyman-$\alpha$ absorption and emission of $\sim 500$ galaxies at $z=5.5-13.4$
• First Sample of H$\alpha$+[O III] $\lambda$5007 Line Emitters at $z>6$ Through JWST/NIRCam Slitless Spectroscopy: Physical Properties and Line Luminosity Functions
• JWST-JADES. Possible Population III signatures at z=10.6 in the halo of GN-z11
• H-alpha emission in local galaxies: star formation, time variability and the diffuse ionized gas
• An extremely metal poor star complex in the reionization era: Approaching Population III stars with JWST
• The ionising photon production efficiency at z~6 for Lyman-alpha emitters using JEMS and MUSE
• Low-redshift compact star-forming galaxies as analogues of high-redshift star-forming galaxies
• Early results from GLASS-JWST. VII: evidence for lensed, gravitationally bound proto-globular clusters at z=4 in the Hubble Frontier Field A2744
• Witnessing the onset of reionisation via Lyman-$\alpha$ emission at redshift 13
• Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra
• The JWST Advanced Deep Extragalactic Survey: Discovery of an Extreme Galaxy Overdensity at $z=5.4$ with JWST/NIRCam in GOODS-S
• JWST FRESCO: a comprehensive census of H$\beta$+[OIII] emitters at 6.8<z<9.0 in the GOODS fields
• Identification of High-Redshift Galaxy Overdensities in GOODS-N and GOODS-S
• JADES + JEMS: A Detailed Look at the Buildup of Central Stellar Cores and Suppression of Star Formation in Galaxies at Redshifts 3 < z < 4.5
• JADES: Primaeval Lyman-$\alpha$ emitting galaxies reveal early sites of reionisation out to redshift $z\sim 9$
• First Peek with JWST/NIRCam Wide-Field Slitless Spectroscopy: Serendipitous Discovery of a Strong [O III]/H$\alpha$ Emitter at $z=6.11$
• The Challenges of Identifying Population III Stars in the Early Universe
• A strong He II $\lambda$1640 emitter with extremely blue UV spectral slope at $z=8.16$: presence of Pop III stars?
• Observable and ionizing properties of star-forming galaxies with very massive stars and different IMFs
• Spectroscopy with the JWST Advanced Deep Extragalactic Survey (JADES) -- the NIRSpec/NIRCam GTO galaxy evolution project
• Zapped then Napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic ${\beta }_{UV}$ slope at z=8.5
• Evidence for Very Massive Stars in extremely UV-bright star-forming galaxies at $z\sim 2.2-3.6$
• A black hole in a near-pristine galaxy 700 million years after the Big Bang
• Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies: Stochastic IMF Sampling and Departures from Case-B
• Physical properties of extreme emission-line galaxies at $z\sim 4-9$ from the JWST CEERS survey
• The density-bounded twilight of starbursts in the early Universe
• 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations
• Metal-Enriched Neutral Gas Reservoir around a Strongly-lensed, Low-mass Galaxy at $z=4$ Identified by JWST/NIRISS and VLT/MUSE
• A Metal-Free Galaxy at $z=3.19$? Evidence of Late Population III Star Formation at Cosmic Noon
• NEBULAR: A simple synthesis code for the hydrogen and helium nebular spectrum
• EMPRESS. XI. SDSS and JWST Search for Local and z~4-5 Extremely Metal-Poor Galaxies (EMPGs): Clustering and Chemical Properties of Local EMPGs
• Uncovering the physical origin of the prominent Lyman-$\alpha$ emission and absorption in GS9422 at $z=5.943$
• A Pristine Star-Forming Complex at z=4.19
• Strong Balmer break objects at z ~ 7-10 uncovered with JWST
• Before its time: a remarkably evolved protocluster core at z=7.88
• A systematic search for dormant galaxies at z~5-7 from the JWST NIRSpec archive
• 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
• JADES Data Release 4 -- Paper II: Data reduction, analysis and emission-line fluxes of the complete spectroscopic sample
• JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: Photometric Catalog
• On the origins of oxygen: ALMA and JWST characterise the multi-phase, metal-enriched, star-bursting medium within a 'normal' $z>11$ galaxy
• Pristine Massive Star Formation Caught at the Break of Cosmic Dawn
• JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: NIRCam Imaging in GOODS-S and GOODS-N
• First Light and Reionization Epoch Simulations (FLARES) -- XVIII: the ionising emissivities and hydrogen recombination line properties of early AGN
• SAPPHIRES: A Galaxy Over-Density in the Heart of Cosmic Reionization at $z=8.47$
• SAPPHIRES: Extremely Metal-Poor Galaxy Candidates with $12+\log (O/H)<7.0$ at $z\sim 5-7$ from Deep JWST/NIRCam Grism Observations
• MINERVA: A NIRCam Medium Band and MIRI Imaging Survey to Unlock the Hidden Gems of the Distant Universe
• JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: Wisp Subtraction with the Non-negative Matrix Factorization Algorithm
• JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: Photometrically Selected Galaxy Candidates at z > 8

Cited by in corpus (1)

• Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5<z<6.5$ with deep JADES medium-band imaging