Exploring early universe with neutral hydrogen atom

1. Exploring early universe with neutral hydrogen @上海天文台（2021/7/22） ©Aman Chokshi Hayato Shimabukuro(島袋隼士) （云南大学・SWIFAR） Aman Chokshi

2. •Born in Okinawa(冲绳) •Ph.D from Nagoya university(2016) •Postdoc at Paris observatory(2016-2018) About me •Postdoc at Tsinghua University(2018-)

3. •Born in Okinawa(冲绳) •Ph.D from Nagoya university(2016) •Postdoc at Paris observatory(2016-2018) About me •Postdoc at Tsinghua University(2018-) •云南⼤学（2019ー）

4. Outline • Introduction • Basics of 21cm line • Current 21cm cosmology status and future • My works • Challenge • Summary

5. Introduction

6. Current universe ©Hubble space telescope

7. Current universe Stars, galaxies, bright universe ©Hubble space telescope

8. Past universe

9. Past universe No stars, galaxies, dark universe

10. Past universe No stars, galaxies, dark universe How did the universe evolve from dark ages to present universe?

11. The history of the universe Present Past https://universe-review.ca/ Epoch of Reionization Dark ages Dark Ages・・・No luminous object exists. Epoch of Reionization(EoR)・・・UV photons by luminous objects ionize neutral hydrogen in the IGM (z~6-15). Cosmic Dawn・・・First stars and galaxies form (z~20-30).

12. The history of the universe Present Past https://universe-review.ca/ Epoch of Reionization Dark ages Dark Ages・・・No luminous object exists. Epoch of Reionization(EoR)・・・UV photons by luminous objects ionize neutral hydrogen in the IGM (z~6-15). Cosmic Dawn・・・First stars and galaxies form (z~20-30).

13. （C)Kenji Hasegawa（Nagoya University） Credit: M. Alvarez, R. Kaehler and T.Abel

14. （C)Kenji Hasegawa（Nagoya University） Credit: M. Alvarez, R. Kaehler and T.Abel

15. Current observations for EoR •Lyman alpha emitter galaxies（LAE） •Lyman alpha forest Konno et al (2014) (http://pages.astronomy.ua.edu/keel/agn/forest.html) QSO HI cloud >The number of ionizing photons Sensitive to neutral hydrogen fraction

18. Current observations tell us (Greig et al 2017) Constraints on average neutral(ionized) hydrogen fraction (Global history).

19. Current observations tell us (Greig et al 2017) Constraints on average neutral(ionized) hydrogen fraction (Global history). Current observations tell us only global history (at late stage of EoR).

20. We want to know EoR much more (ex) •EoR theory •Morphology and topology of ionized bubble •Ionizing sources •Relation to galaxy formation and evolution etc…

21. We want to know EoR much more We should observe IGM at the EoR directly ! (ex) •EoR theory •Morphology and topology of ionized bubble •Ionizing sources •Relation to galaxy formation and evolution etc…

22. We want to know EoR much more We should observe IGM at the EoR directly ! (ex) •EoR theory •Morphology and topology of ionized bubble •Ionizing sources •Relation to galaxy formation and evolution etc…

23. We want to know EoR much more We should observe IGM at the EoR directly ! (ex) •EoR theory •Morphology and topology of ionized bubble •Ionizing sources •Relation to galaxy formation and evolution etc… Synergy with galaxy observation by ALMA, JWST, Subaru

24. We want to know EoR much more We should observe IGM at the EoR directly ! (ex) •EoR theory •Morphology and topology of ionized bubble •Ionizing sources •Relation to galaxy formation and evolution etc… ✕ Synergy with galaxy observation by ALMA, JWST, Subaru

26. Basics of 21cm line

27. 21cm line •21cm line radiation : Neutral hydrogen atom in IGM emits the radiation due to the hyperfine structure. z=6 → 1.5m or 202 MHz z=20 → 4.4m or 68MHz Radio wavelength. We have yet to observe 21cm signal at EoR and cosmic dawn! We can map neutral hydrogen atom in the IGM. However…

28. 21cm line •21cm line radiation : Neutral hydrogen atom in IGM emits the radiation due to the hyperfine structure. z=6 → 1.5m or 202 MHz z=20 → 4.4m or 68MHz Radio wavelength. We have yet to observe 21cm signal at EoR and cosmic dawn! We can map neutral hydrogen atom in the IGM. However… We have not observed 21cm line at high redshift yet !

29. Spin temperature n"" n"# = 3 exp ✓ h⌫21cm kTS ◆ Key quantity in 21cm line physics T 1 S = T 1 CMB + xcT 1 K + x↵T 1 c 1 + xc + x↵ Spin temperature is determined by •interaction with CMB photons •collision with hydrogen atoms •interaction with Ly-alpha photons (TCMB) (TK, xc) (Tc ⇠ TK, x↵) Properties of X-ray sources (e.g. spectral energy distribution (SED)) Relevant astrophysics Properties of first stars (e.g. Initial mass function)

30. Mesinger et al 2010 heating WF eﬀect Wouthuysen-Field(WF) effect Spin temperature couples to IGM kinetic temperature via Ly-alpha photons from first stars. Thermal history X-ray heating X-ray photons drastically heat kinetic temperature of the IGM Spin temperature Kinetic temperature CMB temperature

31. Wouthuysen Field (WF)effect •The spin temperature couples Lyman-alpha color temperature (and also kinetic temperature) (Wouthuysen 1952,Field 1959). •The hyperfine state is changed through 2P state by Lyman-alpha photons (121.6nm) Solid lines : allowed path Dashed lines : not allowed path •Lyman alpha photon is emitted from first stars

32. 21cm line signal Red : cosmology Blue : astrophysics Global signal has characteristic peaks and troughs according to key epochs Tb = TS T 1 + z (1 exp(⌧⌫)) ⇠ 27xH(1 + m) ✓ H dvr/dr + H ◆ ✓ 1 T TS ◆ ✓ 1 + z 10 0.15 ⌦mh2 ◆1/2 ✓ ⌦bh2 0.023 ◆ [mK] Brightness temperature Global signal (sky averaged brightness temperature) *We actually observe brightness temperature

33. Images by 21cm line Mellema et al (2013) We can see how ionised regions are distributed by 21cm image. xi = 0.8 xi = 0.5 Ionised regions However, it is difficult to observe 21cm image by current observations due to specification…

34. 21cm power spectrum (PS) : Scale dependence Pober et al (2014) EoR X-ray heating WF effect z Redshift dependence 21cm power spectrum h Tb(k) Tb(k 0 )i = (2⇡)3 (k + k 0 )P21 We first try to detect the 21cm line signal statistically with ongoing telescopes.

35. Current 21cm cosmology status and future project

36. 1. Theoretical studies

37. 21cm statistical approaches Greig & Mesinger (2016), Park et al (2018) •EoR parameter estimation with Bayesian statistics •21cm signal analysis with machine learning Shimabukuro & Semelin (2017), Schmit et al (2018) •21cm higher order statistics (bispectrum, skewness) Shimabukuro et al (2015,2016,2017), Yoshiura et al (2015) Watkinson et al (2017), Majumadar et al (2018) Constraints on EoR models Park et al 2018 Shimabukuro & Semelin (2017)

38. Morphology of ionized bubble •Minkowski functionals •Betti number •Granulometry Gleser et al (2006),Lee et al (2008), Friedrich et al (2011),Hong et al (2014),Yoshiura et al (2017) Giri et al (2021), Kapahtia et al (2021) Kakiichi et al (2017) Gleser et al (2006) Giri et al (2021) Evaluate ionized bubble topologically and morphologically •Artificial neural network Shimabukuro & Semelin al (2020),Yoshiura et al (2021)

39. First luminous object & EoR •21cm line signal from first stars Yajima & Li (2013),Tanaka et al (2018) •Galaxy formation & EoR Yajima & Li (2013) Yajima & Li (2013) Hasegawa & Semelin (2013), Hutter et al (2020) How do first luminous objects affect EoR?

40. Synergy between 21cm & other lines Kubota et al (2018) •21cm line and Lyman-alpha galaxies Yoshiura et al (2018), Kubota et al (2018) Moriwaki et al (2019) Yoshiura et al (2019) Cross-correlation between 21cm line and other lines. Reducing systematic errors Ma et al (2018) Yoshiura et al (2019) •21cm line and[OIII] galaxies •21cm line and CMB •21cm line and X-ray background

41. 21cm absorption lines (21cm forest) •Dark matter, inflation and other cosmology Shimabukuro et al (2014,2019,2020a), Villanueva & Ichiki (2021), Kawasaki et al (2020) 21cm forest is powerful tool for EoR, cosmology and galaxy formation •Thermal state of the IGM Furlanetto & Loeb (2002), Ciardi et al (2015), Semelin (2015) •Galaxy formation Xu et al (2011), Aditya et al (2021)

42. 2. Observational status

43. Current 21cm experiments MWA LOFAR HERA GMRT Radio interferometer •Array of radio telescope antennas •Measure time delay between antennas •Work together as a single telescope

44. Current upper limits on 21cm PS HERA collaboration 2019 Current 21cm experiments put upper limit of the 21cm line power spectrum 2-3 order of magnitude higher than theoretical expectation. Challenges: ionosphere, RFI, foreground, etc

45. EDGES (Bouman et al 2018) Too deep trough Too flat We detected the 21cm line signal? Very strange result ! Need exotic physics? mis-calibration? unknown systematics?

46. Did we detect the 21cm global signal ? EDGES (Bouman et al 2018) Too deep trough Too flat We detected the 21cm line signal? Very strange result ! Need exotic physics? mis-calibration? unknown systematics?

47. 2021/6/29

48. 2021/6/29

49. 2021/6/29

50. SKA-Low •Frequency 50-350MHz(z=3~27) •Resolution : ~3.3-23 arcsec •FoV :~ tens -a few hundreds of square degree •Effective collecting area : ~300’000 m2 •China is a membership of SKA High resolution & High sensitivity Wide FoV &

51. Images by 21cm line Mellema et al (2013) •~ a few arc-minutes resolution •~ a few degree FoV (Minimum) required specification for imaging xi = 0.8 xi = 0.5 •Enough sensitivity

52. Images by 21cm line Mellema et al (2013) •~ a few arc-minutes resolution •~ a few degree FoV (Minimum) required specification for imaging xi = 0.8 xi = 0.5 SKA can do ! •Enough sensitivity

53. My works

54. What I have done so far •Cosmology at small scales with 21cm forest (Warm dark matter, axion dark matter and so on) [Shimabukuro et al.(2014), Shimabukuro, Ichiki & Kadota (2020a,2020c)] •21cm statistics (bispectrum, one point statistics) [Shimabukuro et al.(2015), (2016), (2017a)] •21cm signal analysis with artificial neural network (ANN) [Shimabukuro & Semelin (2017b), Shimabukuro, Mao & Tan (2020b)]

59. 21cm signal analysis with machine learning

60. 1.EoR parameter estimation with ANN

61. Statistical challenge in 21cm cosmology (Mesinger 2018) Cosmology CMB map (angular) power spectrum cosmological parameter 21cm 21cm 3D map 21cm power spectrum astrophysical parameter Based on Bayesian inference

62. Statistical challenge in 21cm cosmology (Mesinger 2018) Cosmology CMB map (angular) power spectrum cosmological parameter 21cm 21cm 3D map 21cm power spectrum astrophysical parameter Based on Bayesian inference We proposed alternative method.

63. Artificial Neural Network (ANN) An ANN is a mathematical model of human brain network. ex.) Rumelhart et. al (1986) LeCun et. al (1989) Recently, it has been applied to field of astronomy.

64. Artificial Neural Network (ANN) •Training network with training dataset, ANN can approximate any function which associates input and output values. y = f(x) • Applying trained network to unknown data for prediction. yANN = f(xtest) • ANN consists of input layer, hidden layer and output layer. Each layer has neurons. Regression Problem

65. Dataset ⇣ : the ionizing efficiency. : the minimum viral temperature of halos producing ionizing photons : the mean free path of ionizing photons through the IGM (Maximum HII bubble size) Tvir Rmfp ~ d = [P(k), ~ ✓] 21cm power spectrum (input) EoR parameter (output) EoR Parameter ✓EoR = f(P21)

66. z=11, PS without any noise Reconstructed by 21cmPS at z=11 10 20 30 40 50 60 10 20 30 40 50 60 R mfp,ANN [Mpc] Rmfp,true[Mpc] z=12 10 20 30 40 50 60 10 20 30 40 50 60 ANN true z=12 1 10 100 1 10 100 T vir,ANN [K/10 3 ] Tvir,true[K/103 ] z=12 14 neurons, 100’000 iterations • True value .vs. Reconstructed value •The scatter of is large. Rmfp ⇣ Tvir •Other reconstructed parameters match true one relatively well. Shimabukuro & Semelin (2017) Rmfp

67. z=11, PS without any noise Reconstructed by 21cmPS at z=11 10 20 30 40 50 60 10 20 30 40 50 60 R mfp,ANN [Mpc] Rmfp,true[Mpc] z=12 10 20 30 40 50 60 10 20 30 40 50 60 ANN true z=12 1 10 100 1 10 100 T vir,ANN [K/10 3 ] Tvir,true[K/103 ] z=12 14 neurons, 100’000 iterations • True value .vs. Reconstructed value •The scatter of is large. Rmfp ⇣ Tvir •Other reconstructed parameters match true one relatively well. Shimabukuro & Semelin (2017) Rmfp

68. z=9, 10, 11. PS with thermal noise and cosmic variance Reconstructed by 21cm PS at z=9,10,11 Rmfp ⇣ Tvir 10 20 30 40 50 60 10 20 30 40 50 60 R mfp,ANN [Mpc] Rmfp,true[Mpc] 10 20 30 40 50 60 10 20 30 40 50 60 ANN true 1 10 100 1 10 100 T vir,ANN [K/10 3 ] Tvir,true[K/10 3 ] Red : z=9,10,11 Blue : z=9 The parameters obtained by the ANN match true values. ANN work well !

69. 2.Recovering HII bubble size distribution with ANN

70. Bubble size distribution (BSD) ''How large bubbles are distributed ?’' Giri 2019 What can we learn from BSD? Giri et al 2017 •EoR source (galaxy or AGN?) •ionizing efficiency, recombination, radiative feedback. (ex.)

71. BSD from 21cm observation Kakiichi et al 2017 IFT 21cm Image BSD Incomplete IFT due to limited number of antenna in interferometer. visibility We do not observe 21cm image directly by radio interferometer! We first observe visibility and perform Inverse Fourier Transformation (IFT) to obtain 21cm image. Then, compute BSD.

72. BSD from 21cm PS Kakiichi et al 2017 21cm power spectrum BSD visibility We can directly compute 21cm power spectrum from visibility without Inverse Fourier Transformation. Avoid information loss by incomplete IFT.

73. BSD from 21cm PS Kakiichi et al 2017 21cm power spectrum BSD visibility We can directly compute 21cm power spectrum from visibility without Inverse Fourier Transformation. Can we recover BSD from 21cm PS ? Avoid information loss by incomplete IFT.

74. 21cm power spectrum Input Output ionised bubble size distribution Our datasets consist of 21cm power spectrum as input data and bubble size distribution as output data. Our strategy We try to recover ionised bubble size distribution from 21cm PS

75. Recovered BSD Black: Distribution obtained by 21cm 3D image directly. Red: Distribution obtained by ANN. xHI = 0.39

76. Different stage of reionization

77. Effect of thermal noise 21cm PS with thermal noises (SKA level) Errors are estimated by 10 realizations thermal noises xHI = 0.39

78. Challenging issue

79. Foreground problem Jelic et al 2008 The 21cm signal is buried under strong foreground ! Remove foreground ? or Avoid (strong)foreground? Santos 2005 ~8 order Dillon et al 2013

80. Take home messages of my talk are…

81. Take home messages of my talk are… •The epoch from the Dark Ages to cosmic reionization is the frontier in the history of the universe.

82. Take home messages of my talk are… •The epoch from the Dark Ages to cosmic reionization is the frontier in the history of the universe. •21cm signal is a promising tool to study this epoch.

83. Take home messages of my talk are… •The epoch from the Dark Ages to cosmic reionization is the frontier in the history of the universe. •21cm signal is a promising tool to study this epoch. •SKA will bring us fruitful information on the epoch through Dark Ages to EoR

84. Take home messages of my talk are… •The epoch from the Dark Ages to cosmic reionization is the frontier in the history of the universe. •21cm signal is a promising tool to study this epoch. •SKA will bring us fruitful information on the epoch through Dark Ages to EoR •We proposed a method based on machine learning to analyze the 21cm line signal.

85. •"21cm cosmology" (Prithcard & Loeb, astro-ph/1109.6012) Textbook Review paper •"Cosmology at low frequencies" (Furlanetto et al, astro-ph/0608032) •''In the beginning : the first sources of light and the deionization of the universe” R,Bakana & A,Loeb (astro-ph/0010468) References

86. bakcup

87. Accuracy for all test data Relative error between two size distributions at fixed bubble radius for all test data. Good recovery for all test data.

88. n"" n"# = 3 exp ✓ h⌫21cm kTS ◆ The spin temperature is determined by following equilibrium T 1 S = T 1 CMB + xcT 1 K + x↵T 1 c 1 + xc + x↵ de-excitation rate by collision de-excitation rate by UV photons excitation rate by UV photons excitation rate by collision Stimulated by CMB photons Spontaneous de- excitation with Einstein coefficient

89. Wouthuysen Field (WF)effect •The mechanism that couples the spin temperature of neutral hydrogen atom to Lyman-alpha photons(Wouthuysen 1952,Field 1959) •The hyperfine state is changed via 2P state Solid lines : allowed path Dashed lines : not allowed path

90. 21cm PS with SKA SKA covers wide epoch and range of the 21cm PS !! Redshift evolution Scale dependence z=8.95 z=15.98 Koopmans et al. (2014) Pritchard et al. (2014)

91. • 1000 EoR models • 48000 training datasets (20% of which is used for validation) • 2000 test datasets • 21cm PS is ranged from k=0.11/Mpc to 1.1/Mpc with 14 bins • 5 hidden layers • 212 neurons at each hidden layer • 2000 iterations Setup

92. Evaluate accuracy: noise We evaluate accuracy of obtained parameters by chi-square. Smaller chi-square means better accuracy. single z As expected, accuracy becomes worse if we add noise to 21cm power spectrum. without noise with noise

93. Evaluate accuracy: redshift We evaluate accuracy of obtained parameters by chi-square. Smaller chi-square means better accuracy. multiple z The accuracy of parameter estimation is improved when we consider redshift evolution of 21cm power spectrum. Single z Both include noise

94. (Ex.) Emulator EoR parameters 21cmPS ANN MCMC Before : 2.5days on 6 cores After: 4minutes speed up by 3 orders of magnitude (Schmit et al 2018) (input) (output)

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