Undergraduate Honors Theses

Thesis Defended

Spring 2019

Document Type


Type of Thesis

Departmental Honors


Engineering Physics

First Advisor

Michael Shull

Second Advisor

Noah Finkelstein

Third Advisor

Paul Beale


The Cosmic Origins Spectrograph observations and The Hubble Space Telescope has detected OVI absorbers in the Circumgalactic Medium (CGM) of external galaxies, providiing evidence for large gas clouds outside galactic disks. These clouds may fall into the galactic disk, replenishing the gas that is consumed by forming stars. Large-scale galactic outflows are a signature of an active star-forming galaxy and are observed at all redshifts (Veilleux et al. 2005). Exploring a radially dependent pressure distribution of the confining pressure, Pcgm, suggests that outflows reach the outer halo or exit the galaxy if we assume an outflow velocity, v = 200 km s−1. This could be the source of over-densities in the CGM. Cooling times tcool are calculated for CGM number densities in the range 10−4 − 10−6 cm−3 and temperatures 104 to 106 K. A value tcool = 35 Myr for gas near the peak of the cooling curve suggests that slightly cooler, over-dense CGM gas will condense to form clouds of kpc-scale sizes. Analysis of the cooling time here provides a possible source of the observed cool CGM clouds gas. The maximum distance one kpc clouds can form in the CGM is calculated to be 150 kpc, using a beta model to describe the fall off of gas density with radius. Upon condensing, these clouds may lose pressure support and fall towards the galactic disk plane. Here, we find the minimum cloud hydrogen column density for precipitation, NH > 6 × 10^16 cm−2. This analysis is intended to provide a better understanding of the governing principals of cloud formation in the CGM and assess CGM clouds ability to transport gas into the galactic disk.