Title：Deciphering the Thermal and Ionization State of the Intergalactic Medium over the Past 10 Billion Years  

Speaker：Teng Hu ( Laboratoire d'Astrophysique de Marseille)

Time: Aug. 30th, Friday, 3:00 pm

Tencent Meeting：712-184-221 password: 6360

Location: Lecture Hall, 3rd floor

Abstract：

One of the great successes of modern cosmology is the percent-level concordance between theory andobservations of the intergalactic medium (IGM) at z > 1.7. Yet, the Lyα forest at z < 1.7, which can only bestudied via HST UV spectra, has highlighted a puzzling discrepancy: the Doppler b-parameters of theseabsorption lines are, on average, about 10 km/s wider than those in any existing hydrodynamic simulation. Thisdiscrepancy suggests that the low-z IGM might be substantially hotter than expected, contradicting one of thefundamental predictions in current cosmology—that the IGM should cool down due to the Hubble expansionafter He II reionization (z < 2.5). Moreover, the thermal state of the IGM is interdependent with its ionizationstate, characterized by the ultraviolet (UV) background photoionization rate, Γ_HI, which influences theabundance of the Lyα absorbers. Such a dependency necessitates that any reliable measurement adopts ameticulous statistical inference procedure. To address these challenges, we employ a novel machine-learning-based inference framework to jointly measure the thermal and ionization state of the IGM, using the 2Ddistribution of b-parameter and H I column density. This method effectively resolves the degeneracies betweenparameters of the thermal and ionization state of the IGM and achieves high precision, even with limited-sizeddata. We apply this method to 94 archival HST COS and STIS quasar spectra distributed across seven redshiftbins, providing a comprehensive evolutionary history of the IGM thermal and ionization states at z < 1.5. Theresults suggest that the IGM may be significantly hotter than previously expected at low-z and is potentiallyisothermal, with an IGM temperature at the mean density, T_0 = 28,000K and the power-law index of thetemperature-density relation γ=1.06 at z=0.1. The inferred thermal history suggests that this unexpected IGMtemperature possibly emerges around z~1. Additionally, while the Γ_HI measurements align with the theoreticalmodel at z~1, the values measured at z < 0.5 are substantially lower than predicted, posing challenges to low-zUV background synthesis models.

CV：Teng Hu‘s Bio: Postdoc at Laboratoire d'Astrophysique de Marseille, France (2024-), PhD. in Physics, University of California, Santa Barbara (2024).