arXiv subjects

Yongda Zhu

Publications and source records attributed to Yongda Zhu.

113 records · Page 7Linked to original sources

arXiv2022

Hydrogen reionisation ends by $z=5.3$: Lyman-$α$ optical depth measured by the XQR-30 sample

The presence of excess scatter in the Ly-$α$ forest at $z\sim 5.5$, together with the existence of sporadic extended opaque Gunn-Peterson troughs, has started to provide robust evidence for a late end of hydrogen reionisation. However, low data quality and systematic uncertainties complicate the use of Ly-$α$ transmission as a precision probe of reionisation's end stages. In this paper, we assemble a sample of 67 quasar sightlines at $z>5.5$ with high signal-to-noise ratios of $>10$ per $\leq 15$ km s$^{-1}$ spectral pixel, relying largely on the new XQR-30 quasar sample. XQR-30 is a large program on VLT/X-Shooter which obtained deep (SNR $>20$ per pixel) spectra of 30 quasars at $z>5.7$. We carefully account for systematics in continuum reconstruction, instrumentation, and contamination by damped Ly-$α$ systems. We present improved measurements of the mean Ly-$α$ transmission over $4.9 3.5 σ$). Our results indicate that reionisation-related fluctuations, whether in the UVB, residual neutral hydrogen fraction, and/or IGM temperature, persist in the intergalactic medium until at least $z=5.3$ ($t=1.1$ Gyr after the Big Bang). This is further evidence for a late end to reionisation.

astro-ph.CO↗

arXiv2022

Chemical Abundance of z~6 Quasar Broad-Line Regions in the XQR-30 Sample

The elemental abundances in the broad-line regions of high-redshift quasars trace the chemical evolution in the nuclear regions of massive galaxies in the early universe. In this work, we study metallicity-sensitive broad emission-line flux ratios in rest-frame UV spectra of 25 high-redshift (5.8 < z < 7.5) quasars observed with the VLT/X-shooter and Gemini/GNIRS instruments, ranging over $\log(M_{\rm{BH}}/M_{\odot})= 8.4-9.8$ in black hole mass and $\log(L_{\rm{bol}}/\rm{erg\, s}^{-1})= 46.7-47.7$ in bolometric luminosity. We fit individual spectra and composites generated by binning across quasar properties: bolometric luminosity, black hole mass, and blueshift of the \civ\, line, finding no redshift evolution in the emission-line ratios by comparing our high-redshift quasars to lower-redshift (2.0 < z < 5.0) results presented in the literature. Using Cloudy-based locally optimally-emitting cloud photoionisation model relations between metallicity and emission-line flux ratios, we find the observable properties of the broad emission lines to be consistent with emission from gas clouds with metallicity that are at least 2-4 times solar. Our high-redshift measurements also confirm that the blueshift of the CIV emission line is correlated with its equivalent width, which influences line ratios normalised against CIV. When accounting for the CIV blueshift, we find that the rest-frame UV emission-line flux ratios do not correlate appreciably with the black hole mass or bolometric luminosity.

astro-ph.GA↗

arXiv2021

Constraints on the End of Reionization from the Density Fields Surrounding Two Highly Opaque Quasar Sightlines

The observed large-scale scatter in Lyman $α$ opacity of the intergalactic medium at $z<6$ implies large fluctuations in the neutral hydrogen fraction that are unexpected long after reionization has ended. A number of models have emerged to explain these fluctuations that make testable predictions for the relationship between Ly$α$ opacity and density. We present selections of $z=5.7$ Ly$α$-emitting galaxies (LAEs) in the fields surrounding two highly opaque quasar sightlines with long Ly$α$ troughs. The fields lie towards the $z=6.0$ quasar ULAS J0148+0600, for which we re-analyze previously published results using improved photometric selection, and towards the $z=6.15$ quasar SDSS J1250+3130, for which results are presented here for the first time. In both fields, we report a deficit of LAEs within 20 $h^{-1}$ Mpc of the quasar. The association of highly opaque sightlines with galaxy underdensities in these two fields is consistent with models in which the scatter in Ly$α$ opacity is driven by large-scale fluctuations in the ionizing UV background, or by an ultra-late reionization that has not yet concluded at $z=5.7$.

astro-ph.CO↗

arXiv2021

The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

The mean free path of ionizing photons, $λ_{\rm mfp}$, is a key factor in the photoionization of the intergalactic medium (IGM). At $z \gtrsim 5$, however, $λ_{\rm mfp}$ may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of $λ_{\rm mfp}$ that address this bias and extend up to $z \sim 6$ for the first time. Our measurements at $z \sim 5$ are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At $z \sim 6$ we use QSO spectra from Keck ESI and VLT X-Shooter. We measure $λ_{\rm mfp} = 9.09^{+1.62}_{-1.28}$ proper Mpc and $0.75^{+0.65}_{-0.45}$ proper Mpc (68% confidence) at $z = 5.1$ and 6.0, respectively. The results at $z = 5.1$ are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At $z = 6.0$, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at $z = 6.0$ falls well below extrapolations from lower redshifts, indicating rapid evolution in $λ_{\rm mfp}$ over $5 < z < 6$. This evolution disfavors models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by $z = 6$, but is qualitatively consistent with models wherein reionization completed at $z = 6$ or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20% neutral at $z=6$, although our measurement at $z = 6.0$ is even lower than these models prefer.

astro-ph.CO↗

arXiv2021

Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Ly$α$ Forest over $5 < z < 6$

We present a new investigation of the intergalactic medium (IGM) near the end of reionization using "dark gaps" in the Lyman-alpha (Ly$α$) forest. Using spectra of 55 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Ly$α$ forest where the transmission averaged over 1 comoving $h^{-1}\,{\rm Mpc}$ bins falls below 5%. Nine ultra-long ($L > 80~h^{-1}\,{\rm Mpc}$) dark gaps are identified at $z<6$. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than $30~h^{-1}\,{\rm Mpc}$, $F_{30}$, as a function of redshift. We measure $F_{30} \simeq 0.9$, 0.6, and 0.15 at $z = 6.0$, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to $z \simeq 5.3$. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at $z \lesssim 6$ are also potentially consistent with the data. Overall, our results suggest that signature of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least $z \simeq 5.3$.

astro-ph.CO↗