16680

Ultra Diffuse Galaxies (UDGs) are galaxies with large effective radii but very low surface brightness. They were recently discovered in unprecedented numbers in the Coma Cluster comma and are now being found in galaxy groups comma and in isolation in the field. Their origin remains far from clear. In this proposal comma we request time to observe a QSO that lies behind a gas rich UDG comma using the COS G130M grating comma in order to search for absorption lines from the interstellar and circumgalactic medium of the galaxy. The sightline to the QSO LBQS 1431+0142 passes within 11 kpc of the UDG ACG 242019 comma a galaxy with an HI mass of 6x10elev8 solar masses comma recovered recently in the ALFALFA survey from its 21 cm emission. This HI mass favors the detection of HI with a high column density in absorption towards the background QSO comma which comma in combination with metal lines comma can be used to measure the metallicity of the CGM gas in the UDG. Such data can help constrain the origin of the galaxy. Comparison of our results with those from the population of Damped Lyman-alpha systems at higher redshift will begin to examine whether UDGs might contribute significantly to the population of QSO Absorption Line Systems in the universe.