分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim5.5-9.5$ with $-17.05.5$ spectra. We find that the emission line ratios exhibited by these $z\sim5.5-9.5$ galaxies occupy clearly distinct regions of line-ratio space compared to typical z~0-3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [OIII]/[OII] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [OIII]/[OII]>10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [OII]/H$\beta$ and [OIII]/[OII], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [NII] in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log $U\sim-1.5$), and a range of metallicities spanning from $\sim0.1\times Z_\odot$ to higher than $\sim0.3\times Z_\odot$, suggesting we are probing low-metallicity systems undergoing periods of rapid star-formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
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
摘要: We present the radio properties of 66 spectroscopically-confirmed normal
star-forming galaxies (SFGs) at $4.4
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present JADES JWST/NIRSpec spectroscopy of GN-z11, the most luminous candidate $z>10$ Lyman break galaxy in the GOODS-North field with $M_{UV}=-21.5$. We derive a redshift of $z=10.603$ (lower than previous determinations) based on multiple emission lines in our low and medium resolution spectra over $0.8-5.3\,\mu$m. We significantly detect the continuum and measure a blue rest-UV spectral slope of $\beta=-2.4$. Remarkably, we see spatially-extended Lyman-$\alpha$ in emission (despite the highly-neutral IGM expected at this early epoch), offset 555 km/s redward of the systemic redshift. From our measurements of collisionally-excited lines of both low- and high-ionization (including [O II] $\lambda3727$, [Ne III] $\lambda 3869$ and C III] $\lambda1909$) we infer a high ionization parameter ($\log U\sim -2$). We detect the rarely-seen N IV] $\lambda1486$ and N III]$\lambda1748$ lines in both our low and medium resolution spectra, with other high ionization lines seen in low resolution spectrum such as He II (blended with O III]) and C IV (with a possible P-Cygni profile). Based on the observed rest-UV line ratios, we cannot conclusively rule out photoionization from AGN. The high C III]/He II ratios, however, suggest a likely star-formation explanation. If the observed emission lines are powered by star formation, then the strong N III] $\lambda1748$ observed may imply an unusually high $N/O$ abundance. Balmer emission lines (H$\gamma$, H$\delta$) are also detected, and if powered by star formation rather than an AGN we infer a star formation rate of $\sim 20-30 M_{\odot}\,\rm yr^{-1}$ (depending on the IMF) and low dust attenuation. Our NIRSpec spectroscopy confirms that GN-z11 is a remarkable galaxy with extreme properties seen 430 Myr after the Big Bang.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Finding and characterising the first galaxies that illuminated the early
Universe at cosmic dawn is pivotal to understand the physical conditions and
the processes that led to the formation of the first stars. In the first few
months of operations, imaging from the James Webb Space Telescope (JWST) have
been used to identify tens of candidates of galaxies at redshift (z) greater
than 10, less than 450 million years after the Big Bang. However, none of these
candidates has yet been confirmed spectroscopically, leaving open the
possibility that they are actually low-redshift interlopers. Here we present
spectroscopic confirmation and analysis of four galaxies unambiguously detected
at redshift 10.3