Intergalactic HI and OVI:  A Survey of 259 low-z COS Absorbers with Reference to Ionization Mechanics

Osborne, Shannon

Undergraduate Honors Thesis

Intergalactic HI and OVI: A Survey of 259 low-z COS Absorbers with Reference to Ionization Mechanics Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/w0892b478

Abstract

Modern astrophysics still lacks a strong understanding of the Intergalactic Medium (IGM), particularly the Warm-Hot Intergalactic Medium (WHIM; T = 10⁵ – 10⁷ K). To better understand the distribution and temperature of this gas, absorption-line observations along AGN sight lines from the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (COS/HST) were analyzed (see Danforth et al. 2016). Here we present the largest analysis to date of IGM systems detected in both HI and OVI: 259 absorbers along 52 sight lines. We separate these systems into aligned and non-aligned categories based on the velocity separation of the HI and OVI absorption features. For the aligned HI and OVI absorption, we derive a median temperature of logT = 4.65 (with a ±1σ range of 4.22 – 5.03 in Kelvin) and a median value for the non-thermal motions in the clouds of b_NT = 22 km/s (with a ±1σ range of 17 – 34 km/s). This temperature is lower than expected for collisional ionization equilibrium and suggests that most aligned IGM OVI absorbers are either photoionized or originally shock-heated but then cooled (non-equilibrium ionization). We then statistically derive temperatures for the HI and OVI absorbers in the non-aligned cases which results in logT_HI = 4.80 (with a ±1σ range of 4.44 – 5.11) and logT_OVI = 5.99 (with a ±1σ range of 5.50 – 6.31). The higher OVI temperature distribution found here points to these non-aligned clouds being more likely collisionally ionized.

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