Measurements of the Thermal and Ionization State of the Intergalactic Medium during the Cosmic Afternoon
arxiv(2023)
摘要
We perform the first measurement of the thermal and ionization state of the
intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 \lya absorption lines
fitted from 12 HST STIS quasar spectra, with a total pathlength of \Delta
z=2.1. We employ the machine-learning-based inference method that uses joint
b-N distributions obtained from \lyaf decomposition. Our results show that the
HI photoionization rates, \Gamma, are in good agreement with the recent UV
background synthesis models, with \log (\Gamma/s^{-1})={-11.79}^{0.18}_{-0.15},
-11.98}^{0.09}_{-0.09}, and {-12.32}^{0.10}_{-0.12} at z=1.4, 1.2, and 1
respectively. We obtain the IGM temperature at the mean density, T_0, and the
adiabatic index, \gamma, as [\log (T_0/K), \gamma]= [{4.13}^{+0.12}_{-0.10},
{1.34}^{+0.10}_{-0.15}], [{3.79}^{+0.11}_{-0.11}, {1.70}^{+0.09}_{-0.09}] and
[{4.12}^{+0.15}_{-0.25}, {1.34}^{+0.21}_{-0.26}] at z=1.4, 1.2 and 1
respectively. Our measurements of T_0 at z=1.4 and 1.2 are consistent with the
expected trend from z<3 temperature measurements as well as theoretical
expectations that, in the absence of any non-standard heating, the IGM should
cool down after HeII reionization. Whereas, our T_0 measurements at z=1 show
unexpectedly high IGM temperature. However, because of the relatively large
uncertainty in these measurements of the order of \Delta T_0~5000 K, mostly
emanating from the limited redshift path length of available data in these
bins, we can not definitively conclude whether the IGM cools down at z<1.5.
Lastly, we generate a mock dataset to test the constraining power of future
measurement with larger datasets. The results demonstrate that, with redshift
pathlength \Delta z \sim 2 for each redshift bin, three times the current
dataset, we can constrain the T_0 of IGM within 1500K. Such precision would be
sufficient to conclusively constrain the history of IGM thermal evolution at z
< 1.5.
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