Undergraduate Honors Theses

Thesis Defended

Spring 2017

Document Type


Type of Thesis

Departmental Honors



First Advisor

Dr. Charles Danforth

Second Advisor

Dr. Erica Ellingson

Third Advisor

Dr. Paul Beale


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 bNT = 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 logTHI = 4.80 (with a ±1σ range of 4.44 – 5.11) and logTOVI = 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.