ABSTRACT
We present the results of a model-independent investigation of the rest-frame UV spectra from a comprehensive sample of 394 quasars in the redshift range 1.5 ≤ z ≤ 7.5. We fit the main broad ...emission lines (BELs) in the rest-frame range 1280 Å ≤ λ ≤ 3000 Å (O i, C ii, Si iv, C iii, C iv, and Mg ii) with a lightly supervised spline fitting technique. Redshifts are derived from the peaks of each fitted BEL and used to compute relative velocity shifts. We show that our method gives unbiased velocity shifts and is insensitive to spectral resolution and instrumental parameters. It is found that the average blueshift of the C iv line with respect to several low-ionization lines in luminosity-matched samples does not significantly evolve over 1.5 ≤ z ≤ 6. However, the average blueshift increases significantly by a factor of ∼2.5 at z ≳ 6. We propose that this redshift evolution can be explained by C iv winds launched perpendicularly to an accretion disc with increased torus opacity at high redshift, coupled with a potential orientation-driven selection bias. Our results open new exciting avenues of investigation into young quasars in the reionization epoch.
ABSTRACT
The presence of excess scatter in the Ly-α forest at z ∼ 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 reionization. However, low data quality and systematic uncertainties complicate the use of Ly-α transmission as a precision probe of reionization’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 ≤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 < z < 6.1. Using all known systematics in a forward modelling analysis, we find excellent agreement between the observed Ly-α transmission distributions and the homogeneous-UVB simulations Sherwood and Nyx up to z ≤ 5.2 (<1σ), and mild tension (∼2.5σ) at z = 5.3. Homogeneous UVB models are ruled out by excess Ly-α transmission scatter at z ≥ 5.4 with high confidence (>3.5σ). Our results indicate that reionization-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 reionization.
ABSTRACT
The decline in abundance of Lyman-α (Lyα) emitting galaxies at z ≳ 6 is a powerful and commonly used probe to constrain the progress of cosmic reionization. We use the CoDaII simulation, ...which is a radiation hydrodynamic simulation featuring a box of ∼94 comoving Mpc side length, to compute the Lyα transmission properties of the intergalactic medium (IGM) at z ∼ 5.8 to 7. Our results mainly confirm previous studies, i.e. we find a declining Lyα transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Lyα peaks at high redshift, we also analyse the IGM transmission on the blue side, which shows a rapid decline at z ≳ 6 of the blue transmission. This low transmission can be attributed not only to the presence of neutral regions but also to the residual neutral hydrogen within ionized regions, for which a density even as low as $n_{\rm HI}\sim 10^{-9}\, \mathrm{cm}^{-3}$ (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that $\sim 1{{\ \rm per\ cent}}$ of sightlines towards M1600AB ∼ −21 galaxies at z ∼ 7 are transparent enough to allow a transmission of a blue Lyα peak. We discuss our results in the context of the interpretation of observations.
Signs of damping-wing absorption attenuating the Lyman α emission line of the first known z ∼ 7 quasar, ULAS J1120+0641, recently provided exciting evidence of a significantly neutral intergalactic ...medium (IGM). This long-awaited signature of reionization was inferred, in part, from a deficit of flux in the quasar's Lyman α emission line based on predictions from a composite of lower redshift quasars. The composite sample was chosen based on its C iv emission line properties; however, as the original study by Mortlock et al. noted, the composite contained a slight velocity offset in C iv compared to ULAS J1120+0641. Here we test whether this offset may be related to the predicted strength of the Lyman α emission line. We confirm the significant (∼10 per cent at rms) scatter in Lyman α flux for quasars of a given C iv velocity and equivalent width found by Mortlock et al. We further find that among lower redshift objects chosen to more closely match the C iv properties of ULAS J1120+0641, its Lyman α emission falls within the observed distribution of fluxes. Among lower redshift quasars chosen to more closely match in C iv velocity and equivalent width, we find that ULAS J1120+0641 falls within the observed distribution of Lyman α emission line strengths. This suggests that damping-wing absorption may not be present, potentially weakening the case for neutral gas around this object. Larger samples of z > 7 quasars may therefore be needed to establish a clearer picture of the IGM neutral fraction at these redshifts.
ABSTRACT
Observations of the early Universe suggest that reionization was complete by z ∼ 6, however, the exact history of this process is still unknown. One method for measuring the evolution of the ...neutral fraction throughout this epoch is via observing the Lyα damping wings of high-redshift quasars. In order to constrain the neutral fraction from quasar observations, one needs an accurate model of the quasar spectrum around Lyα, after the spectrum has been processed by its host galaxy but before it is altered by absorption and damping in the intervening intergalactic medium (IGM). In this paper, we present a novel machine learning approach, using artificial neural networks, to reconstruct quasar continua around Lyα. Our Quasar Spectra from Artificial Neural Network based predictive Regression Algorithm(QSANNdRA) improves the error in this reconstruction compared to the state-of-the-art principal component analysis (PCA) based model in the literature by 14.2 per cent on average, and provides an improvement of 6.1 per cent on average when compared to an extension thereof. In comparison with the extended PCA model, QSANNdRA further achieves an improvement of 22.1 per cent and 16.8 per cent when evaluated on low-redshift quasars most similar to the two high-redshift quasars under consideration, ULAS J1120+0641 at z = 7.0851 and ULAS J1342+0928 at z = 7.5413, respectively. Using our more accurate reconstructions of these two z > 7 quasars, we estimate the neutral fraction of the IGM using a homogeneous reionization model and find $\bar{x}_\mathrm{H\, \small{I}} = 0.25^{+0.05}_{-0.05}$ at z = 7.0851 and $\bar{x}_\mathrm{H\, \small{I}} = 0.60^{+0.11}_{-0.11}$ at z = 7.5413. Our results are consistent with the literature and favour a rapid end to reionization.
Abstract
The reionization epoch concludes when ionizing photons reach every corner of the universe. Reionization has generally been assumed to be limited primarily by the rate at which galaxies ...produce ionizing photons, but the recent measurement of a surprisingly short ionizing photon mean free path of
0.75
−
0.45
+
0.65
proper Mpc at
z
= 6 by Becker et al. suggests that absorption by residual neutral hydrogen in the otherwise ionized intergalactic medium may play a much larger role than previously expected. Here we show that consistency between this short mean free path and the coeval dark pixel fraction in the Ly
α
forest requires a cumulative output of
6.1
−
2.4
+
11
ionizing photons per baryon by reionization’s end, well above the typically required ∼1–3. This represents a dramatic increase in the ionizing photon budget over previous estimates, greatly exacerbating the tension with measurements of the ionizing output from galaxies at later times. Translating this constraint into the instantaneous ionizing production from galaxies in our model, we find
log
10
f
esc
ξ
ion
/
(
erg
/
Hz
)
−
1
=
25.02
−
0.21
+
0.45
at
z
∼ 6. Even with optimistic assumptions about the ionizing production efficiency of early stellar populations, and assuming the galaxy luminosity function extends to extremely faint sources (
M
UV
≤ − 11), complete reionization requires the escape fraction of ionizing photons to exceed 20% across the galaxy population. This is far larger than observed in any galaxy population at lower redshifts, requiring rapid evolution in galaxy properties after the first billion years of cosmic time. This tension cannot be completely relieved within existing observational constraints on the hydrogen neutral fraction and mean free path.
ABSTRACT
The distribution and abundance of metals in the diffuse intergalactic medium (IGM) have implications for galaxy formation and evolution models, and has been argued to be sensitive to the ...Universe’s reionization history. However, reduced sensitivity in the near-IR implies that probing IGM metals at z > 4 is currently out of reach with the traditional method of detecting individual absorbers. We present a new technique based on clustering analysis that enables the detection of these weak IGM absorbers. We investigate the two-point correlation function (2PCF) of the ${\rm C\, {\small IV}}$ forest as a probe of IGM metallicity and enrichment topology by simulating the z = 4.5 IGM with models of inhomogeneous metal distributions. The 2PCF of the ${\rm C\, {\small IV}}$ forest demonstrates a clear peak at a characteristic separation corresponding to the doublet separation of the ${\rm C\, {\small IV}}$ line.The peak amplitude scales quadratically with metallicity, while enrichment topology affects both the shape and amplitude of the 2PCF. For models consistent with the distribution of metals at z ∼ 3, we find that we can constrain C/H to within 0.2 dex, log$\, M_{\rm {min}}$ to within 0.4 dex, and R to within 15 per cent. We show that CGM absorbers can be reliably identified and masked, thus recovering the underlying IGM signal. The auto-correlation of the metal-line forest presents a compelling avenue to constrain the IGM metallicity and enrichment topology with high precision at z > 4, thereby pushing such measurements into the Epoch of Reionization.
Abstract
Quasar proximity zones at
correspond to overdense and overionized environments. Galaxies found inside proximity zones can therefore display features that would otherwise be masked by ...absorption in the intergalactic medium. We demonstrate the utility of this quasar-galaxy synergy by reporting the discovery of the first three “proximate Ly
α
emitters” (LAEs) within the proximity zone of quasar J0836+0054 at
z
= 5.795 (Aerith A, B, and C). Aerith A, located behind the quasar with an impact parameter
pkpc, provides the first detection of an Ly
α
transverse proximity effect. We model the transmission and show that it constrains the onset of J0836's quasar phase to
in the past. The second object, Aerith B at a distance
pkpc from the quasar, displays a bright and broad double-peaked Ly
α
emission line. The peak separation implies a low ionizing
. We fit the Ly
α
line with an outflowing shell model, finding a typical central density
, outflow velocity
km s
−1
, and gas temperature
compared to
analog LAEs. We detect object Aerith C via an Ly
α
emission line at
z
= 5.726. This corresponds with the edge of the quasar’s proximity zone (
), suggesting that the proximity zone is truncated by a density fluctuation. Via the analyses conducted here, we illustrate how proximate LAEs offer unique insight into the ionizing properties of both quasars and galaxies during hydrogen reionization.
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