skip to main content

Title: Deep forest: neural network reconstruction of intergalactic medium temperature

We explore the use of deep learning to infer the temperature of the intergalactic medium from the transmitted flux in the high-redshift Ly α forest. We train neural networks on sets of simulated spectra from redshift z = 2–3 outputs of cosmological hydrodynamic simulations, including high-temperature regions added in post-processing to approximate bubbles heated by He ii reionization. We evaluate how well the trained networks are able to reconstruct the temperature from the effect of Doppler broadening in the simulated input Ly α forest absorption spectra. We find that for spectra with high resolution (10 $\, {\rm km}\, {\rm s}^{-1}$ pixel) and moderate signal-to-noise ratio (20–50), the neural network is able to reconstruct the intergalactic medium temperature smoothed on scales of $\sim 6 \, h^{-1}\, {\rm Mpc}$ quite well. Concentrating on discontinuities, we find that high-temperature regions of width $25 \, h^{-1}\, {\rm Mpc}$ and temperature $20\, 000$ K can be fairly easily detected and characterized. We show an example where multiple sightlines are combined to yield tomographic images of hot bubbles. Deep learning techniques may be useful in this way to help us understand the complex temperature structure of the intergalactic medium around the time of helium reionization.

more » « less
Award ID(s):
Author(s) / Creator(s):
; ;
Publisher / Repository:
Oxford University Press
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range / eLocation ID:
p. 1568-1579
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract We present a new investigation of the intergalactic medium (IGM) near the end of reionization using “dark gaps” in the Ly α forest. Using spectra of 55 QSOs at z 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 Mpc bins falls below 5%. Nine ultralong ( L > 80 h −1 Mpc) dark gaps are identified at z < 6. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than 30 h −1 Mpc, F 30 , as a function of redshift. We measure F 30 ≃ 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 ≃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 ≲6 are also potentially consistent with the data. Overall, our results suggest that signatures 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 ≃ 5.3. 
    more » « less
  2. ABSTRACT We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model, the new model provides a better match to a large number of up-to-date empirical constraints, including: (1) new galaxy and AGN luminosity functions; (2) stellar spectra including binary stars; (3) obscured and unobscured AGN; (4) a measurement of the non-ionizing UV background; (5) measurements of the intergalactic H i and He ii photoionization rates at z ∼ 0−6; (6) the local X-ray background; and (7) improved measurements of the intergalactic opacity. In this model, AGN dominate the H i ionizing background at z ≲ 3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z ∼ 2, the slow evolution of the H i ionization rate inferred from the high-redshift H i Ly α forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of $\approx 1{{\ \rm per\ cent}}$ suffices to ionize the intergalactic medium at z = 3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the He ii Ly α forest in hydrodynamic simulations. 
    more » « less

    Our understanding of reionization has advanced considerably over the past decade, with several results now demonstrating that the intergalactic medium transitioned from substantially neutral at z = 7 to largely reionized at z = 6. However, little remains known about the sizes of ionized bubbles at z ≳ 7 as well as the galaxy overdensities which drive their growth. Fortunately, rest-ultraviolet (UV) spectroscopic observations offer a pathway towards characterizing these ionized bubbles thanks to the resonant nature of Lyman-alpha photons. In a previous work, we presented Ly α detections from three closely separated Lyman-break galaxies at z ≃ 6.8, suggesting the presence of a large (R > 1 physical Mpc) ionized bubble in the 1.5 deg2 COSMOS field. Here, we present new deep Ly α spectra of 10 UV-bright ($\mathrm{\mathit{ M}}_{\mathrm{UV}}^{} \le -20.4$) z ≃ 6.6–6.9 galaxies in the surrounding area, enabling us to better characterize this potential ionized bubble. We confidently detect (S/N > 7) Ly α emission at z = 6.701–6.882 in nine of ten observed galaxies, revealing that the large-scale volume spanned by these sources (characteristic radius R = 3.2 physical Mpc) traces a strong galaxy overdensity (N/〈N〉 ≳ 3). Our data additionally confirm that the Ly α emission of UV-bright galaxies in this volume is significantly enhanced, with 40 per cent (4/10) showing strong Ly α emission (equivalent width >25 Å) compared to the 8–9 per cent found on average at z ∼ 7. The median Ly α equivalent width of our observed galaxies is also ≈2 times that typical at z ∼ 7, consistent with expectations if a very large (R ∼ 3 physical Mpc) ionized bubble is allowing the Ly α photons to cosmologically redshift far into the damping wing before encountering H i.

    more » « less
  4. Abstract

    We present a new investigation of the intergalactic medium near reionization using dark gaps in the Lyβforest. With its lower optical depth, Lyβoffers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Lyαline. We identify dark gaps in the Lyβforest using spectra of 42 QSOs atzem> 5.5, including new data from the XQR-30 VLT Large Programme. Approximately 40% of these QSO spectra exhibit dark gaps longer than 10h−1Mpc atz≃ 5.8. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Lyαforest are caused solely by ionizing ultraviolet background fluctuations and with models that include large neutral hydrogen patches atz< 6 due to a late end to reionization. Of particular interest is a very long (L= 28h−1Mpc) and dark (τeff≳ 6) gap persisting down toz≃ 5.5 in the Lyβforest of thez= 5.85 QSO PSO J025−11. This gap may support late reionization models with a volume-weighted average neutral hydrogen fraction of 〈xH I〉 ≳ 5% byz= 5.6. Finally, we infer constraints on 〈xH I〉 over 5.5 ≲z≲ 6.0 based on the observed Lyβdark gap length distribution and a conservative relationship between gap length and neutral fraction derived from simulations. We find 〈xH I〉 ≤ 0.05, 0.17, and 0.29 atz≃ 5.55, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later thanz= 6.

    more » « less
  5. Abstract The absorption by neutral hydrogen in the intergalactic medium (IGM) produces the Ly α forest in the spectra of quasars. The Ly α forest absorbers have a broad distribution of neutral hydrogen column density N H I and Doppler b parameter. The narrowest Ly α absorption lines (of lowest b ) with neutral hydrogen column density above ∼10 13 cm −2 are dominated by thermal broadening, which can be used to constrain the thermal state of the IGM. Here we constrain the temperature-density relation T = T 0 ( ρ / ρ ¯ ) γ − 1 of the IGM at 1.6 < z < 3.6 by using N H I and b parameters measured from 24 high-resolution and high-signal-to-noise quasar spectra and by employing an analytic model to model the N H I -dependent low- b cutoff in the b distribution. In each N H I bin, the b cutoff is estimated using two methods, one non-parametric method from computing the cumulative b distribution and a parametric method from fitting the full b distribution. We find that the IGM temperature T 0 at the mean gas density ρ ¯ shows a peak of ∼1.5 × 10 4 K at z ∼ 2.7–2.9. At redshift higher than this, the index γ approximately remains constant, and it starts to increase toward lower redshifts. The evolution in both parameters is in good agreement with constraints from completely different approaches, which signals that He ii reionization completes around z ∼ 3. 
    more » « less