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Science 2011-tripp-952-5

  1. 1. The Hidden Mass and Large Spatial Extent of a Post-Starburst Galaxy Outflow Todd M. Tripp, et al. Science 334, 952 (2011); DOI: 10.1126/science.1209850 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at Downloaded from on November 27, 2011 (this infomation is current as of November 27, 2011 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: Supporting Online Material can be found at: A list of selected additional articles on the Science Web sites related to this article can be found at: This article cites 46 articles, 1 of which can be accessed free: This article has been cited by 1 articles hosted by HighWire Press; see: This article appears in the following subject collections: Astronomy (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright2011 by the American Association for the Advancement of Science; all rights reserved. The title Science is aregistered trademark of AAAS.
  2. 2. REPORTS 10. D. Keres, N. Katz, D. H. Weinberg, R. Davé, Mon. Not. R. (14, 15, 38) and is higher than the mean value (14.0) 39. J. N. Bregman, E. D. Miller, A. E. Athey, J. A. Irwin, Astron. Soc. 363, 2 (2005). measured by the Far Ultraviolet Spectroscopic Explorer Astrophys. J. 635, 1031 (2005). 11. A. Dekel, Y. Birnboim, Mon. Not. R. Astron. Soc. 368, through the halo of the Milky Way (20), which would Acknowledgments: We thank the anonymous reviewers 2 (2006). belong in our star-forming sample. for constructive comments. This work is based on 12. G. Kauffmann et al., Mon. Not. R. Astron. Soc. 341, 25. O VI emission is seen in elliptical galaxies (39), but observations made for program GO11598 with the 33 (2003). this gas is most likely associated with the ISM and not NASA/ESA Hubble Space Telescope, obtained at the 13. T. M. Tripp, B. D. Savage, E. B. Jenkins, Astrophys. J. the CGM. Space Telescope Science Institute, operated by AURA 534, L1 (2000). 26. M. Asplund, N. Grevesse, A. J. Sauval, P. Scott, Annu. Rev. under NASA contract NAS 5-26555, and at the 14. C. W. Danforth, J. M. Shull, Astrophys. J. 679, 194 Astron. Astrophys. 47, 481 (2009). W. M. Keck Observatory, operated as a scientific (2008). 27. M. S. Peeples, F. Shankar, Mon. Not. R. Astron. Soc. 417, partnership of the California Institute of Technology, the 15. C. Thom, H.-W. Chen, Astrophys. J. 683, 22 (2008). 2962 (2011). University of California, and NASA. The Observatory was 16. J. N. Bregman, Annu. Rev. Astron. Astrophys. 45, 28. M. E. Putman, Astrophys. J. 645, 1164 (2006). made possible by the generous financial support of the 221 (2007). 29. N. Lehner, J. C. Howk, Science 334, 955 (2011); W. M. Keck Foundation. The Hubble data are available 17. J. T. Stocke et al., Astrophys. J. 641, 217 (2006). 10.1126/science.1209069. from the MAST archive at 18. H.-W. Chen, J. S. Mulchaey, Astrophys. J. 701, 1219 30. H.-W. Chen et al., Astrophys. J. 714, 1521 (2010). M.S.P. was supported by the Southern California Center (2009). 31. C. C. Steidel et al., Astrophys. J. 717, 289 (2010). for Galaxy Evolution, a multicampus research program 19. J. X. Prochaska, B. Weiner, H.-W. Chen, J. S. Mulchaey, 32. D. Thomas, L. Greggio, R. Bender, Mon. Not. R. funded by the UC Office of Research. K. L. Cooksey, (2011). Astron. Soc. 296, 119 (1998). 20. K. R. Sembach et al., Astrophys. J. Suppl. Ser. 146, 33. T. M. Tripp et al., Science 334, 952 (2011). 165 (2003). 34. B. D. Oppenheimer et al., Mon. Not. R. Astron. Soc. 406, Supporting Online Material 21. See supporting material on Science Online. 2325 (2010). 22. J. K. Werk et al., 35. K. R. Stewart et al., Astrophys. J. 735, L1 (2011). Downloaded from on November 27, 2011 SOM Text (2011). 36. M. Fumagalli et al., Figs. S1 to S5 23. D. Schiminovich et al., Astrophys. J. Suppl. Ser. 173, (2011). Tables S1 and S2 315 (2007). 37. J. M. Gabor, R. Davé, K. Finlator, B. D. Oppenheimer, References (40–62) 24. The typical log NOVI = 14.5 to 15.0 for star-forming Mon. Not. R. Astron. Soc. 407, 749 (2010). galaxies resembles the high end of the column-density 38. T. M. Tripp et al., Astrophys. J. Suppl. Ser. 177, 15 June 2011; accepted 27 September 2011 distribution seen in blind surveys of intergalactic clouds 39 (2008). 10.1126/science.1209840 the total column density and mass of the outflows The Hidden Mass and Large Spatial are poorly constrained. Previous outflow obser- vations were often limited to low-resolution spec- Extent of a Post-Starburst Galaxy Outflow tra of only one or two ions (e.g., Na I or Mg II) or relied on composite spectra that cannot yield precise Todd M. Tripp,1* Joseph D. Meiring,1 J. Xavier Prochaska,2 Christopher N. A. Willmer,3 column densities. Without any constraints on hydro- J. Christopher Howk,4 Jessica K. Werk,2 Edward B. Jenkins,5 David V. Bowen,5 Nicolas Lehner,4 gen (the vast bulk of the mass) or other elements Kenneth R. Sembach,6 Christopher Thom,6 Jason Tumlinson6 and ions, these studies were forced to make highly uncertain assumptions to correct for ionization, Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, elemental abundances, and depletion of species but key aspects of these outflows have not been probed with observations. By using ultraviolet by dust. Lastly, galactic winds contain multiple absorption spectroscopy, we show that “warm-hot” plasma at 105.5 kelvin contains 10 to 150 times phases with a broad range of physical conditions more mass than the cold gas in a post-starburst galaxy wind. This wind extends to distances > 68 (6), and wind gas in the key temperature range kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of between 105 to 106 K (where radiative cooling is the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of maximized) is too cool to be observed in x-rays; the cold matter with a hotter (unseen) phase at >>106 kelvin. Such multiphase winds can detection of this so-called “warm-hot” phase remove substantial masses and alter the evolution of post-starburst galaxies. requires observations in the ultraviolet (UV). To study the more extended gas around gal- alaxies do not evolve in isolation. They in- galaxies (2) and eventually into elliptical-type axies, including regions affected by outflows, we G teract with other galaxies and, more subtly, with the gas in their immediate environ- ments. Mergers of comparable-mass, gas-rich galaxies with little or no star formation (3). Mergers are not required to propel galaxy evo- lution, however. Even relatively secluded galaxies used the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) to obtain high-resolution spectra of the quasi-stellar object galaxies trigger star-formation bursts by driving accrete matter from the intergalactic medium (QSO) PG1206+459 (at redshift zQSO = 1.1625). matter into galaxy centers, but theory predicts that (IGM), form stars, and drive matter outflows into By exploiting absorption lines imprinted on the such starbursts are short-lived: The central gas is their halos or out of the galaxies entirely (4, 5). QSO spectrum by foreground gaseous material, rapidly driven away by escaping galactic winds In either case, the competing processes of gas we can detect the low-density outer gaseous en- powered by massive stars and supernova explo- inflows and outflows are expected to regulate velopes of galaxies, regions inaccessible to other sions or by a central supermassive black hole galaxy evolution. techniques. We focus on far-ultraviolet (FUV) ab- (1). Such feedback mechanisms could trans- Outflows are evident in some nearby objects sorption lines at rest wavelengths lrest < 912 Å. form gas-rich spiral galaxies into post-starburst (6–9) and are ubiquitous in some types of gal- This FUV wavelength range is rich in diagnostic axies (10–15); their speeds can exceed the escape transitions (23), including the Ne VIII 770.409, 1 velocity. Nevertheless, their broader impact on 780.324 Å doublet, a robust probe of warm-hot Department of Astronomy, University of Massachusetts, Am- herst, MA 01003, USA. 2University of California Observatories/ galaxy evolution is poorly understood. First, their gas, as well as banks of adjacent ionization stages. Lick Observatory, University of California, Santa Cruz, CA 95064, full spatial extent is unknown. Previous studies The sight line to PG1206+459 pierces an absorp- USA. 3Steward Observatory, University of Arizona, Tucson, AZ (6, 9, 16–22) have revealed flows with spatial tion system, at redshift zabs = 0.927, that provides 85721, USA. 4Department of Physics, University of Notre Dame, extents ranging from a few parsecs up to ~20 kilo- insights about galactic outflows. This absorber Notre Dame, IN 46556, USA. 5Princeton University Obser- vatory, Princeton, NJ 08544, USA. 6Space Telescope Science parsecs (kpc). However, because of their low has been studied before (24), but previous obser- Institute, Baltimore, MD 21218, USA. densities, outer regions of outflows may not have vations did not cover Ne VIII and could not pro- *To whom correspondence should be addressed. E-mail: been detected with previously used techniques, vide accurate constraints on H I in the individual and thus the flows could be much larger. Second, absorption components.952 18 NOVEMBER 2011 VOL 334 SCIENCE
  3. 3. REPORTS This absorber is illustrated in Figs. 1 to 3, in- er Lyman series lines are not saturated), which H I lines were detected in at least nine compo-cluding the COS data (25). The absorber is a enables accurate H I column density [N(H I)] mea- nents (25) spanning a large velocity range from“partial” Lyman limit (LL) system (i.e., the high- surement (Fig. 1). A wide variety of metals and −317 to +1131 km s−1 (Figs. 1 and 2). The Ne VIII doublet was unambiguously detected (Fig. 2) with a total N(Ne VIII) = 1014.9 cm−2 (25), which is ~10 times higher than any previous N(Ne VIII) measurements in intervening absorbers (26, 27). The component at +1131 km s−1 exceeds vescape of any individual galaxy, and the other compo- nents have very similar properties to the +1131 km s−1 component (25), suggesting a common origin. Whether the other components have v > vescape depends on the (unknown) potential well, but allowing for projection effects and noting that the gas is already far from the affiliated gal- axy (see below), several of the other components could also be escaping. Combined with detec- tion of Ne VIII, the detections of banks of adjacent Downloaded from on November 27, 2011 ions (N II, N III, N IV, N V; O III, O IV; S III, S IV, S V) place tight constraints on physical condi- tions of the gas. Notably, the velocity centroids and profile shapes of lower and higher ioniza- tion stages are quite similar (Fig. 3). This strong Ne VIII/LL absorber is affiliated with a galaxy near the QSO sight line (24, 25).Fig. 1. (Top) Small portion of the Keck HIRES spectrum of PG1206+459 (24). Tick marks at top indicate This galaxy, which we refer to as 177_9, is thecomponents detected at various velocities in the Mg II 2803.53 Å transition. A velocity scale in the rest type of galaxy expected to drive a galactic su-frame of the affiliated galaxy 177_9 is inset at bottom. Gray indicates a feature not due to Mg II 2803.53 Å. perwind (Fig. 4). Like post-starburst (11) and(Bottom) Small portion of the ultraviolet spectrum of PG1206+459 recorded with the COS on HST that ultraluminous infrared galaxies (28), galaxy 177_9shows H I Lyman series absorption lines (marked with ticks and labels) at the redshift of the Mg II complex in is very luminous and blue (29); based on thethe top graph, including H I Lyz through Lys (highest lines are marked but not labeled). characteristic magnitude (M*) of the z ~ 1 lumi- nosity function from the Deep Evolutionary Exploratory Probe 2 (DEEP2) (30), the galaxy luminosity L = 1.8 L*. The Multiple Mirror Tel- escope (MMT) spectrum in Fig. 4 is also similar to those of the post-starburst galaxies in (11), with higher Balmer series absorption lines, [O II] emission and [Ne V] emission indicative of an active galactic nucleus (AGN) (25). Most impor- tantly, the galaxy has a large impact parameter from the QSO sight line, r = 68 kpc (31), which implies that the gaseous envelope of 177_9 has a large spatial extent. The component-to-component similarity of the absorption lines (Fig. 3) suggests a related origin. To further investigate the nature of this absorber, we used photoionization models (32) to derive ionization corrections and elemental abun- dances (25). These models indicate that the indi- vidual components have high abundances ranging from ~0.5 to 3 times those in the Sun (table S2). Such high abundances (or metallicities) favor an origin in outflowing ejecta enriched by nucleo- synthesis products from stars; at the large impact parameter of 177_9, corotating outer-disk or halo gas or tidal debris from a low-mass satellite gal- axy would be expected to have much lower me- tallicity. Tidal debris from a massive galaxy couldFig. 2. Continuum-normalized absorption profiles (black lines) of various species detected in the LL /Mg II have high metallicity, but we are currently awareabsorber shown in Fig. 1, plotted in velocity with respect to the galaxy 177_9 redshift (i.e., v = 0 km s−1 at of only one luminous galaxy near the sight line atz = 0.927). Labels below each absorption profile indicate the species and rest wavelength. We fitted nine the absorber redshift (33); another luminous gal-components to the COS and Space Telescope Imaging Spectrograph data (24). Component centroids are axy interacting with 177_9 is not evident. Theindicated by gray lines, and the Voigt-profile fits are overplotted with red lines (25). Yellow lines indicate absorber could also be intragroup gas, but some-contaminating features from other redshifts or transitions. The two graphs at lower left compare apparent how it must have been metal-enriched, so somecolumn density profiles (39) of the N V and Ne VIII doublets. type of galactic outflow is implicated in any case. SCIENCE VOL 334 18 NOVEMBER 2011 953
  4. 4. REPORTS The photoionization models also constrain the total hydrogen column (i.e., H I and H II), and, combined with spatial extent ≥ 68 kpc, this allows mass estimates. By using fiducial thin- shell models (25), we find that the mass of cool, photoionized gas in individual components ranges from 0.6 × 108 to 14 × 108 solar masses (M◉). However, photoionization fails (sometimes by orders of magnitude) to produce enough S V, N V, and Ne VIII; these species must arise in hot gas at temperature T > 105 K. By using equi- librium and nonequilibrium collision ionization models (25), we find that the warm-hot gas con- tains much more mass than the cold gas, with individual components harboring 10 × 108 to 400 × 108 M◉ in hot material. These are rough estimates with many uncertainties. For example, if the absorption arises in thin filaments analo- Downloaded from on November 27, 2011 gous to those seen in starburst galaxies (6) or AGN bubbles (34), the cold-gas mass could reduce to ~106 M◉ per component. However, as in the thin-shell models, the warm-hot gas could harbor 10 to 150 times more mass in such filaments (25). In either case (shells or filaments), given the similarity of the cold and warm-hot absorption lines (Fig. 3), the Ne VIII–bearing plasma must be a transitional phase that links the colder and hotter material and thus provides insights on the outflow physics. The Ne VIII/N V phase is not photoionized, so it must be generated through interaction of the cold gas with a hotter ambient medium analogous to x-ray–emitting regions seen in nearby galaxies. How this occurs is an open question; the absorbers could be Fig. 3. Comparison of apparent column density profiles (39) of the LL absorber affiliated with material cooling from the hot phase down to the galaxy 177_9. In each graph, the C II 687.05 Å profile (black histogram) is compared to another cool gas, or the cool clouds could have a hotter species (colored circles) as labeled at upper left; the comparison species profile is also scaled by the and more-ionized surface that is evaporating. factor in parentheses after the species label. Gray lines indicate regions contaminated by unrelated Low-density plasma in the T = 105 to 106 K absorption. As in Fig. 2, v = 0 km s−1 at z = 0.927. range has been effectively hidden from most Fig. 4. Montage of observations of the galaxy at zgal = 0.927 that drives a large-scale outflow of metal-enriched plasma. (Top left) The galaxy, and the background QSO that reveals the outflow via absorption spectroscopy, is shown in a multicolor image obtained with the Large Binocular Telescope. This galaxy, which we refer to as 177_9, is the red object 8.63 arc sec south of the bright QSO PG1206 +459 (zQSO = 1.1625) at a position angle of 177° (N through E) from the QSO. At the galaxy redshift, the angular separation from the QSO sight line corre- sponds to an impact parameter of 68 kpc. (Top right) The large red circle indicates the rest-frame U-B color and absolute B magnitude of 177_9 compared to all galaxies from the DEEP2 survey (gray scale) (30) and DEEP2 galaxies within T0.05 of z(177_9) (cyan points). The small purple circles show post- starburst galaxies from (11). (Bottom) An MMT optical spectrum of 177_9 (upper trace) with its 1s uncertainty (lower trace). The strong feature at ≈ 7600 Å is partially due to telluric absorption.954 18 NOVEMBER 2011 VOL 334 SCIENCE
  5. 5. REPORTSoutflow studies. In principle, the O VI 1032,1038 Å 8. D. S. Rupke, S. Veilleux, D. B. Sanders, Astrophys. J. 36. C. Tremonti, A. M. Diamond-Stanic, J. Moustakas, indoublet can reveal such gas, but it is unclear Suppl. Ser. 160, 87 (2005). Galaxy Evolution: Emerging Insights and Future 9. C. L. Martin, Astrophys. J. 647, 222 (2006). Challenges, S. Jogee, I. Marinova, L. Hao, G. Blanc, Eds,whether the O VI arises in photoionized 104 K 10. M. Pettini et al., Astrophys. J. 554, 981 (2001). (Astronomical Society of the Pacific Conference Series,gas, hotter material at ~105.5 K, or both (35). The 11. C. A. Tremonti, J. Moustakas, A. M. Diamond-Stanic, San Francisco, 2009), vol. 419, pp. 369–376.Ne VIII doublet avoids this ambiguity, and we Astrophys. J. 663, L77 (2007). 37. A. L. Coil et al., found that this warm-hot matter is a sub- 12. C. C. Steidel et al., Astrophys. J. 717, 289 (2010). (2011). 13. K. H. R. Rubin et al., Astrophys. J. 719, 1503 (2010). 38. J. Tumlinson et al., Science 334, 948 (2011).stantial component in the mass inventory of a 14. F. Hamann, G. Ferland, Annu. Rev. Astron. Astrophys. 37, 39. B. D. Savage, K. R. Sembach, Astrophys. J. 379, 245galactic wind. Moreover, this wind has a large 487 (1999). (1991).spatial extent, and the mass carried away by the 15. J. P. Grimes et al., Astrophys. J. Suppl. Ser. 181, 272 (2009). Acknowledgments: This study has its basis in observationsoutflow will affect the evolution of the galaxy. 16. K. H. R. Rubin, J. X. Prochaska, D. C. Koo, A. C. Phillips, made with the NASA/European Space Agency Hubble B. J. Weiner, Astrophys. J. 712, 574 (2010). Space Telescope (HST); the MMT, a joint facilityWhereas earlier studies of poststarburst outflows 17. M. Moe, N. Arav, M. A. Bautista, K. T. Korista, operated by the Smithsonian Astrophysical Observatoryfocused on Mg II and could not precisely con- Astrophys. J. 706, 525 (2009). and the University of Arizona; and the Large Binocularstrain the metallicity, hydrogen column, and 18. J. P. Dunn et al., Astrophys. J. 709, 611 (2010). Telescope, an international collaboration amongmass, these studies do indicate that post-starburst 19. D. Edmonds et al., Astrophys. J. 739, 7 (2011). institutions in the United States, Italy, and Germany. 20. F. Hamann et al., Mon. Not. R. Astron. Soc. 410, 1957 Support for HST program number 11741 was providedoutflows are common: 22/35 of the post-starbursts (2011). by NASA through a grant from the Space Telescopein (36) showed outflowing Mg II absorption with 21. G. A. Kriss et al., Astron. Astrophys. 534, 41 (2011). Science Institute, which is operated by the Associationmaximum (radial) velocities of 500 to 2400 km s−1, 22. D. M. Capellupo, F. Hamann, J. C. Shields, P. Rodríguez of Universities for Research in Astronomy, Incorporated,similar to the absorption near 177_9 (Fig. 1), and Hidalgo, T. Barlow, Mon. Not. R. Astron. Soc. 413, 908 under NASA contract NAS5-26555. Additional support Downloaded from on November 27, 201177 and 100% of the post-starburst and AGN (2011). was provided by NASA grant NNX08AJ44G. The DEEP2 23. D. A. Verner, D. Tytler, P. D. Barthel, Astrophys. J. 430, survey was supported by NSF grants AST 95-29098,galaxies, respectively, in (37) drive outflows but 186 (1994). 00-711098, 05-07483, 08-08133, 00-71048,with lower maximum velocities, which may be 24. J. Ding, J. C. Charlton, C. W. Churchill, C. Palma, 05-07428, and 08-07630. Funding for the Sloan Digitaldue to selection of wind-driving galaxies in a Astrophys. J. 590, 746 (2003). Sky Survey has been provided by the Alfred P. Sloanlater evolutionary stage. With existing COS data, 25. See further information in supporting material on Science Foundation, the Participating Institutions, NASA, NSF, the Online. U.S. Department of Energy Office of Science, thethe effects of large-scale outflows on galaxy evo- 26. B. D. Savage, N. Lehner, B. P. Wakker, K. R. Sembach, Japanese Monbukagakusho, and the Max Planck Society.lution can be studied with the techniques pre- T. M. Tripp, Astrophys. J. 626, 776 (2005). We thank C. Churchill for providing the archival Kecksented here but with larger samples (38), with 27. A. Narayanan et al., Astrophys. J. 730, 15 (2011). data and the referees for review comments thatwhich it will be possible to statistically track how 28. Y. Chen, J. D. Lowenthal, M. S. Yun, Astrophys. J. 712, significantly improved this paper. We are also grateful to 1385 (2010). the Hawaiian people for graciously allowing us to conductoutflows affect galaxies. 29. The galaxy appears to be red in Fig. 4 because of its observations from Mauna Kea, a revered place in redshift; in the rest frame of the galaxy, it has a very the culture of Hawaii. The HST data in this paper References and Notes ultraviolet-blue (U-B) color. are available from the Multimission Archive at the 1. P. F. Hopkins et al., Astrophys. J. Suppl. Ser. 163, 30. C. N. A. Willmer et al., Astrophys. J. 647, 853 (2006). Space Telescope Science Institute (MAST) at 1 (2006). 31. For distance calculations, we assume a cold dark matter 2. A. I. Zabludoff et al., Astrophys. J. 466, 104 (1996). cosmology with Hubble constant H0 = 70 km s−1 Mpc−1 3. G. F. Snyder, T. J. Cox, C. C. Hayward, L. Hernquist, and dimensionless density parameters Ωm = 0.30, ΩL = P. Jonsson, Astrophys. J. 741, 77 (2011). 0.70. Supporting Online Material 4. D. Kereš, N. Katz, R. Davé, M. Fardal, D. H. Weinberg, 32. G. J. Ferland et al., Publ. Astron. Soc. Pac. 110, 761 (1998). Materials and Methods Mon. Not. R. Astron. Soc. 396, 2332 (2009). 5. B. D. Oppenheimer et al., Mon. Not. R. Astron. Soc. 406, 33. As discussed in the supporting online material, the yellow SOM Text galaxy northwest of the QSO (Fig. 4) does not have a Figs. S1 to S5 2325 (2010). Tables S1 and S2 6. S. Veilleux, G. Cecil, J. Bland-Hawthorn, Annu. Rev. spectroscopic redshift but is likely to have z << 0.927. Astron. Astrophys. 43, 769 (2005). 34. A. C. Fabian et al., Nature 454, 968 (2008). References (40–54) 7. T. M. Heckman, M. D. Lehnert, D. K. Strickland, L. Armus, 35. T. M. Tripp et al., Astrophys. J. Suppl. Ser. 177, 39 15 June 2011; accepted 26 October 2011 Astrophys. J. Suppl. Ser. 129, 493 (2000). (2008). 10.1126/science.1209850 metal-poor gas (metallicity that is less than 10%A Reservoir of Ionized Gas in the of that of the Sun, or Z ≲ 0.1 Z◉) to flow onto galaxies along dense intergalactic filaments (1).Galactic Halo to Sustain Star However, galaxies may also exchange mass with the local intergalactic medium (IGM) through outflows driven by galactic “feedback,” galacticFormation in the Milky Way winds powered by massive stars and their death and from massive black holes. Some of this ma-Nicolas Lehner* and J. Christopher Howk terial may return to the central galaxy as recycled infalling matter—the galactic fountain mechanismWithout a source of new gas, our Galaxy would exhaust its supply of gas through the formation (2, 3). The circumgalactic medium about a gal-of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their axy is thus a complicated blend of outflowingdistances are key for placing them in the galactic halo and unraveling their role. We have used metal-rich and infalling metal-poor gas. The rela-the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the tive importance of these processes is poorly con-foreground of galactic stars. We show that iHVCs with 90 ≤ |vLSR| ≲ 170 kilometers per second strained observationally. Here, we demonstrate(where vLSR is the velocity in the local standard of rest frame) are within one galactic radius of the that ionized gas in the local galactic halo providesSun and have enough mass to maintain star formation, whereas iHVCs with |vLSR| ≳ 170 kilometers a major supply of matter for fueling ongoing starper second are at larger distances. These may be the next wave of infalling material. formation. he time scale for gas consumption via star eous fuel in the disks of galaxies for continuedT formation in spiral galaxies is far shorter than a Hubble time (13.8 billion years),requiring an ongoing replenishment of the gas- star formation. Analytical models and hydrody- namical simulations have emphasized the impor- tance of cold-stream accretion as a means for Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA. *To whom correspondence should be addressed. E-mail: SCIENCE VOL 334 18 NOVEMBER 2011 955