Tumlinson, JasonThom, ChristopherDave, RomeelWerk, Jessica K.Prochaska, J. XavierTripp, Todd M.Katz, NealOppenheimer, Benjamin D.Meiring, Joseph D.Ford, Amanda BradyO'Meara, John M.Peeples, Molly S.Sembach, Kenneth R.Weinberg, David H.2017-07-242017-07-242013Tumlinson, J. et al. (2013). The COS-Halos survey: Rationale, design and a census of circumgalactic neutral hydrogen. The Astrophysical Journal , 777 (59): 1-330004-637xhttp://hdl.handle.net/10566/3098We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15–0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of zem 0.5 QSOs with S/N ∼10–15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass logM∗/M = 9.5–11.5. We find that the circumgalactic medium exhibits strong Hi, averaging 1Å in Lyα equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of Hi. Kinematics indicate that the detected material is bound to the host galaxy, such that 90% of the detected column density is confined within ±200 km s−1 of the galaxies. This material generally exists well below the halo virial temperatures at T 105 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected Hi arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L ∗ galaxies and may harbor a total gaseous mass comparable to galactic stellar masses.enPublisher retains copyright. Authors may archive the published version in their institutional repository.GalaxiesFormation – galaxiesHalos – intergalactic medium – quasarsAbsorption linesArticle