Dr Jacopo Chevallard

Abstract:

GHZ2 is among the highest-redshift galaxies discovered to date, exhibiting a spectrum rich with prominent emission lines in the rest-frame ultraviolet (UV) and optical. These features raise critical questions about the mechanism powering this nebular emission, in particular the extremely strong C iv λ1548+1551 emission (rest-frame equivalent width of 45 Å). Here we aim to quantify the active galactic nucleus (AGN) contribution within this system using the BEAGLE-AGN tool to simultaneously fit the spectrum and photometry of GHZ2. We consider a range of models with and without AGN components, allowing us to disentangle the stellar and AGN contribution of GHZ2 for the first time simultaneously using all observed emission lines. We conclude that a partial contribution by an AGN is significantly favored based on χ2 comparisons between models with and without an AGN component, measuring an AGN contribution of 97 −2+1 %, and 64 −12+23 % for the C IV λ1548+1551 and C III] λ1908 emission lines, respectively. We estimate the black hole mass using the accretion luminosity (Lacc) from the best-fit BEAGLE-AGN model, inferring a value of log10(MBH/M⊙) = 7.19 −0.03+0.03 , assuming an Eddington ratio of η = 0.5 (with a much larger systematic uncertainty of ∼1 dex). The inferred black hole mass to stellar mass ratio is 0.06 −0.02+0.02 (statistically), consistent with other high-redshift AGN systems. If the black hole interpretation is confirmed, GHZ2 would represent the most distant black hole identified to date, making it an ideal laboratory to study AGN growth and their role in shaping high-redshift galactic evolution.

Abstract:

Abstract JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, $Z < 1~{{\ \rm per\ cent}}~\mathrm{Z}_\odot$) at redshift z > 3, mostly through serendipitous NIRSpec discoveries and blind slitless spectroscopy. To accelerate our understanding of pristine star formation, we further develop a methodology to identify EMPG candidates from photometry, utilizing the extensive deep NIRCam medium-band imaging from JADES. Our EMPG candidates at 2.5 < z < 6.5 exhibit strong photometric boosts by Hα, yet correspondingly weak boosts by [O iii] + Hβ, likely indicating extremely low metallicity to explain their lack of [O iii] emission. We further demand our EMPG candidates to have strong Balmer jumps, as revealed by medium-band imaging, to ensure that they are young starbursts, as opposed to broad-line AGN/LRDs, though contamination by dusty/dense-gas starbursts and highly-obscured AGN remains a concern. SED-fitting with near-pristine models (~0.1–$1~{{\ \rm per\ cent}}~\mathrm{Z}_\odot$) indicates that our 14 EMPG candidates are low-mass (median M* ≈ 106.7 M⊙), faint dwarf galaxies (MUV ≈ −16.9), with high ionizing photon production efficiencies (log (ξion, obs/(Hz erg−1)) ≈ 26.0). Hence these are plausible sites of near-pristine star formation, constituting ~0.02–0.4% of 2.5 < z < 6.5 galaxies at −19 < MUV < −16. We discuss this extremely metal-poor extension to the mass–metallicity relation. We forecast that deep (~28 h) NIRCam slitless spectroscopy can identify bright EMPGs through strong Hβ but lack of [O iii] emission, or secure the redshifts of fainter systems through Hα detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to z = 10, without requiring deep NIRCam medium-band and/or MIRI imaging to identify secure candidates.

Abstract:

Our understanding of the physical properties of star-forming galaxies during the Epoch of Reionization (EoR; at z > 6) suffers from degeneracies among the apparent properties of the stars, nebular gas, and dust. These degeneracies are most prominent with photometry, which has insufficient (1) spectral resolution and (2) rest-frame spectral coverage. We explore ways to break these degeneracies with a sample of N = 22 high-redshift star-forming galaxies at 7 < zphot ≤ 9, using some of the deepest existing imaging from JWST/NIRCam and JWST/MIRI with JADES. Key to this study is the imaging from JWST/MIRI at 7.7 μm, which provides coverage of the rest-frame I band at the observed redshifts. We infer stellar population properties and rest-frame colors using a variety of filter sets and star formation history (SFH) assumptions to explore the impact of these choices. Evaluating these quantities both with and without the 7.7 μm data point shows that dense spectral coverage with JWST/NIRCam (eight or more filters, including at least one medium-band at λobs ≈ 4–5 μm) can compensate for lacking the rest-frame I-band coverage for the vast majority (≈80%) of our sample. Furthermore, these galaxy properties are most consistently determined by assuming the delayed-tau SFH, which provides the smallest offsets and scatters around these offsets when including JWST/MIRI. Within extragalactic surveys like JADES and CEERS, our findings suggest that robust characterization of the stellar population properties and rest-frame colors for typical high-redshift star-forming galaxies (i.e., galaxies with MUV ≈ −20 and βUV ≈ −2) is possible with JWST/NIRCam alone at z ≈ 8.

Abstract:

JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI (tobs ≈ 51 hr). This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at z = 14.18. Most notably, we detect [O III]λλ4959, 5007 at ≈14σ and Hα at ≈4σ with these ultradeep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star formation rate of SFR ≈ 8 ± 2M⊙ yr−1, star formation rate surface density of ΣSFR ≈ 20 ± 5M⊙ yr−1 kpc−2, and ionizing photon production efficiency of ξion ≈ 1025.3±0.1 Hz erg−1. Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of log10(O/H)+12≈7.5±0.2 (≈6%Z⊙), carbon-to-oxygen ratio of [C/O] ≈ −0.4 ± 0.2, ionization parameter of log10(U)≳−2.4 , and density of nH ≈ 690 ± 200 cm−3. Using detailed photoionization modeling, we instead derive log10(O/H)+12≈8.5−0.4+0.4 (≈60%Z⊙), log10(U)≈−1.4−0.4+0.3 , and nH≈540−320+520cm−3 . The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at z > 10 with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. These results suggest extreme ionization conditions and rapid metal enrichment less than 300 Myr after the Big Bang. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. Future work will focus on the detection of the rest-frame optical continuum and its interpretation for the stellar population properties of JADES-GS-z14-0.

Abstract:

We present a sample of 2081 sources selected at a photometric redshift zphot > 8 across the JADES DR5 data release in GOODS-S and GOODS-N over a total area of 469 arcmin2. These sources range from MUV = −22 to MUV = −16, with 19 objects at zphot > 14. We estimate the UV slopes for the full sample from fits to the photometry and find evidence for a steepening of the relationship between the UV continuum slope and MUV to higher redshifts, a result that differs from prior analyses of brighter samples in the literature. We provide evidence that over one-quarter of our sources have evidence for being morphologically extended, with many galaxies showing multiple bright knots or clumps even out to z ∼ 13–14, an indication of how galaxies at cosmic dawn are growing and evolving. We discuss JADES-GN+189.15982+62.28899, a GOODS-N F200W dropout galaxy at zphot ∼ 15–18, which has been observed spectroscopically with JWST/NIRSpec in prism mode, resulting in a very low signal-to-noise spectrum that is consistent with the photometry and rules out a number of low-redshift solutions for the source. Finally, we use a subsample of 123 objects in our sample with spectroscopic redshifts to explore the usage of alternate fitting templates and a prescription for Lyα damping wing absorption, finding that both produce significant improvements to the estimated photometric redshifts.