Professor Andrew Bunker

Abstract:

Ultracool T (Teff ≈ 500–1200 K) and Y dwarfs (Teff ⪅ 500 K) have historically been found only a few hundred parsecs from the Sun. The sensitivity and wavelength coverage of the NIRCam instrument on board the James Webb Space Telescope offer a unique method for finding low-temperature brown dwarfs in deep extragalactic datasets out to multiple kiloparsecs. Here we report on the selection of a sample of 41 brown dwarfs and brown dwarf candidates across the JWST Advanced Deep Extragalactic Survey in the GOODS-S and GOODS-N regions. We introduce a new open-source Bayesian tool, Near-Infrared Fitting for T and Y dwarfs (NIFTY), to derive effective temperatures, metallicities, and distances from JWST photometry. We find that 31 candidates have fits consistent with T dwarf temperatures out to 5–6 kpc, and 10 candidates have fits consistent with Y dwarf temperatures out to 1–2 kpc. The majority of the sources are best fit with subsolar metallicity models. We report proper motions for 10 brown dwarf candidates (three are newly presented), and calculate the number density of T and Y dwarfs as a function of temperature and distance above the Milky Way midplane. We further discuss how Y dwarfs can serve as contaminants in the search for ultra-high-redshift galaxies. Together, these results demonstrate the power of deep JWST extragalactic imaging to probe the coldest substellar populations far beyond the solar neighborhood, providing new constraints on the Milky Way’s structure and brown dwarf demographics.

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.