*Speaker:* Shmuel Bialy (Center For Astrophysics) *Date:* Thursday, February 21th *Time:* 12:00-1:00 pm Includes Pizza. *Title: *The Multiphase HI Gas - from Solar to Low Metallicities *Abstract: *The neutral atomic ISM is found to be multiphased, composed of the cold neutral medium (CNM, T ~100 K) and the warm neutral medium (WNM, T ~ 10^4 K), which coexist at approximately the same thermal pressure. Such a multiphase structure is predicted by models of heating-cooling thermal balance, where heating is dominated by photoelectric heating from dust-grains (and/or cosmic-ray heating), and cooling by Ly\alpha line emission (for the WNN, T ~10^4 K) and [C II] and [O I] line-emission (for the CNM, T ~100 K). Intermediate temperatures are thermally unstable. The multiphase phenomena and the thermal instability zone, may play a key role in regulating the star-formation properties in galaxies. I will present recent results (Bialy & Sternberg, in prep) discussing the thermal properties of HI gas, as functions of the UV intensity, the cosmic-ray ionization rate, and gas metallicity, from solar to primordial metallicities.I will discuss the importance of the residual H2 in controlling the thermal structure of the predominantly HI gas. Despite its low equilibrium abundance (H2/HI ~ 10^{-6}), we find that H2 cooling and heating mechanisms (dominated by H2 ro-vibrational cooling, UV pumping heating, and H2 formation heating) strongly affect (or totally dominate) the phase structure at low metallicities and/or when the UV intensity to cosmic-ray ionization rate ratio is low. *Location: *B-106 @ Center for Astrophysics (60 Garden Street <https://maps.google.com/?q=60+Garden+Street&entry=gmail&source=g>) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.