Nicolas Lehner

Research Associate Professor

B.Sc., Universite Louis Pasteur, France, 1994
M.Sc., ibid., 1995
Ph.D., The Queen’s University of Belfast, N. Ireland, 2000

E-mail: nlehner@nd.edu
Lehner’s Personal Web Page 
Address: NSH 338
Phone: (574) 631-5755
Fax: (574) 631-5952

Research Interests

My research focuses on understanding the physical processes that drive and regulate the growth of galaxies. Modern cosmological simulations cannot explain galaxies' properties without invoking large-scale inflows and outflows. Stars cannot continue to form in galaxies over billions of years without a replenishment of gas in galaxies from the intergalactic medium (IGM), while feedback from star formation and AGN activity can fuel massive outflows from a gas-rich galaxy that may choke off star formation. The competition between these regulating processes is played out in the circumgalactic medium (CGM), the interface between galaxies and the IGM, where outflows may cool and stall, where infalling gas may encounter winds or be shock-heated and be halted.

With the Hubble Space Telescope (HST),  Large Binocular Telescope, and other facilities, I study the infall and winds directly in the extended gaseous halos of galaxies using absorption lines of hydrogen and "metals" observed in the spectra of distant light (AGN) sources. I estimate the density, temperature, metallicity, kinematics, and mass of the inflowing and outflowing matter in galaxy halos, and correlate the gas-flow properties with the galaxy kinematics, morphologies, environments, metallicities, and star formations. 

One of our latest results demonstrates that the population of fast moving ionized gas clouds covering much of the Galactic sky resides in the Galactic halo within about a few tens of light-years. These clouds are close enough and their mass is large enough to support the star formation in the disk, essentially solving the missing fuel problem for continuing star formation in the Milky Way. We have also recently provided the first observational evidence for cold mode accretion onto galaxies as seen in cosmological hydrodynamical simulations where galaxies get their gas from infall of material along intergalactic filaments. 

Selected Publications

1. “A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way,” Lehner, N., and Howk, J.C., Science, 334, 955-958 (2011)

2. “The Hidden Mass and Large Spatial Extent of a Post-Starburst Galaxy Outflow,” Tripp, T.M., Meiring, J.D., Prochaska, J.X., Willmer, C.N.A., Howk, J.C., Werk, J.K., Jenkins, E.B., Bowen, D.V., Lehner, N., Sembach, K.R., Thom, C., and Tumlinson, J. Science, 334, 952-955 (2011)

3. “Evidence for Cold Accretion: Primitive Gas Flowing onto a Galaxy at z~0.274,” Ribaudo, J., Lehner, N., Howk, J.C., Werk, J.K., Tripp, T.M., Prochaska, J.X., Meiring, J.D., and Tumlinson, J., Astrophysical Journal, 743, 207 [7pp] (2011)

4. “A Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution,” Ribaudo, J., Lehner, N., Howk, J.C., Astrophysical Journal, 736, 42 [23pp] (2011)

5. “Observation of Interstellar Lithium in Low Metallicity Small Magellanic Cloud,” Howk, J.C., Lehner, N., Fields, B.C., Mathews, G., Nature, 489, 121 (2012)

Lehner full CV PDF file