分类： 天文学 >> 天文学 提交时间： 2023-02-19
Aayush Saxena
Brant E. Robertson
Andrew J. Bunker
Ryan Endsley
Alex J. Cameron
Stephane Charlot
Charlotte Simmonds
Sandro Tacchella
Joris Witstok
Chris Willott
Stefano Carniani
Emma Curtis-Lake
Pierre Ferruit
Peter Jakobsen
Santiago Arribas
Jacopo Chevallard
Mirko Curti
Francesco D'Eugenio
Anna De Graaff
Gareth C. Jones
摘要： We report the discovery of a remarkable Ly$\alpha$ emitting galaxy at $z=7.278$, JADES-GS+53.16746-27.7720 (shortened to JADES-GS-z7-LA), with EW$_0$(Ly$\alpha$) $\approx400 \pm 90$A and UV magnitude $-16.7$. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$\beta$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$\alpha$ line is detected in both lower resolution ($R\sim100$) PRISM as well as medium resolution ($R\sim1000$) G140M grating spectra. The Ly$\alpha$ FWHM in the grating is $\approx360$ km s$^{-1}$ and the line peaks within $120$ km s$^{-1}$ of the systemic redshift, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$\alpha$ escape fraction to be $\sim100\%$. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of $8.8 \pm 1.1$ and ([O III]+[O II])/H$\beta$ ratio (R23) of $9.6\pm2.2$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by $0.23''$), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$\beta$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a `bursty' star-formation history, with a low stellar mass of $10^{7.15}$ M$_\odot$. The only explanation of the high EW Ly$\alpha$ emission seen in JADES-GS-z7-LA is if it resides in an ionized bubble with radius $\gtrsim 3$ pMpc. Owing to the faint nature of JADES-GS-z7-LA, we show that it is incapable of single-handedly ionizing a bubble large enough. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$, allowing us to study the process of reionization across both small and large scales.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Christina C. Williams
Sandro Tacchella
Michael V. Maseda
Brant E. Robertson
Benjamin D. Johnson
Chris J. Willott
Daniel J. Eisenstein
Christopher N. A. Willmer
Zhiyuan Ji
Kevin N. Hainline
Jakob M. Helton
Stacey Alberts
Stefi Baum
Rachana Bhatawdekar
Kristan Boyett
Andrew J. Bunker
Stefano Carniani
Stephane Charlot
Jacopo Chevallard
Emma Curtis-Lake
摘要： We present JEMS (JWST Extragalactic Medium-band Survey), the first public
medium-band imaging survey carried out using JWST/NIRCam and NIRISS. These
observations use $\sim2\mu$m and $\sim4\mu$m medium-band filters (NIRCam F182M,
F210M, F430M, F460M, F480M; and NIRISS F430M & F480M in parallel) over 15.6
square arcminutes in the Hubble Ultra Deep Field (UDF), thereby building on the
deepest multi-wavelength public datasets available anywhere on the sky. We
describe our science goals, survey design, NIRCam and NIRISS image reduction
methods, and describe our first data release of the science-ready mosaics. Our
chosen filters create a JWST imaging survey in the UDF that enables novel
analysis of a range of spectral features potentially across the redshift range
of $0.31$ magnitude)
across redshifts $1.5
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Context: Population III (Pop III) stars may be fast rotating. An expected consequence of fast rotation is strong internal mixing that deeply affects their evolutionary tracks in the Hertzsprung-Russell diagram and hence their ionising power. aims: We investigate the impact on the ionising power of Pop III stars in an extreme case of internal mixing, the one leading to chemically homogeneous evolution (CHE). In that situation, during the main sequence phase, the star keeps the same chemical composition from its center to its surface. Homogeneous stars have larger effective temperatures and luminosities than stars evolving non-homogeneously and thus are stronger ionising sources. Methods: The stellar evolution models are based on $n=3$ polytropes with a time varying hydrogen mass fraction. The ionisation model employs the self-similar champagne flow solution from Shu et al. (2002), as well as numerical simulations for the stochastic treatment of star clusters over a grid of redshifts and halo masses. Results: We find that haloes containing chemically homogeneous stars have an escape fraction of ionising photons about twice that of haloes containing classical Pop III stars. By extrapolating the high-$z$ ionisation history powered by Pop III stars (at $z\gtrsim 15$) to the post-reionisation epoch, we derive the Thomson scattering optical depth $\tau$, which is compared with the value measured by $\textit{Planck}$. We find that $\tau$ is overproduced by $\sim1.5- 5\sigma$, when all Pop III stars evolve homogeneously. This indicates that CHE is unlikely to be realised in the majority of Pop III stars, although the present study cannot exclude that a fraction of them undergoes CHE. Conclusions: Fast rotation might have a significant impact on the ionising budget of Pop III stars, and thus on early cosmic reionisation.
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