The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals J. Tumlinson, et al. Science 334, 948 (2011); DOI: 10.1126/science.1209840 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 www.sciencemag.org on November 27, 2011 www.sciencemag.org (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: http://www.sciencemag.org/content/334/6058/948.full.html Supporting Online Material can be found at: http://www.sciencemag.org/content/suppl/2011/11/16/334.6058.948.DC1.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/334/6058/948.full.html#related This article cites 32 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/334/6058/948.full.html#ref-list-1 This article has been cited by 1 articles hosted by HighWire Press; see: http://www.sciencemag.org/content/334/6058/948.full.html#related-urls This article appears in the following subject collections: Astronomy http://www.sciencemag.org/cgi/collection/astronomyScience (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.
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Biol. 7, 1156 (2000). 190, 5862 (2008). 10.1126/science.1211204 REPORTS The Large, Oxygen-Rich Halos of medium (CGM)—loosely defined as gas surround- ing galaxies within their own halos of dark mat- ter (out to 100 to 300 kpc)—lies at the nexus of Star-Forming Galaxies Are a Major accretion and outflow, but the structure of the CGM and its relation to galaxy properties are Reservoir of Galactic Metals still uncertain. Galactic outflows are observed at both low (2–4) and high (5–7) redshift, but it J. Tumlinson,1* C. Thom,1 J. K. Werk,2 J. X. Prochaska,2 T. M. Tripp,3 D. H. Weinberg,4 is unclear how far they propagate, what level M. S. Peeples,5 J. M. O’Meara,6 B. D. Oppenheimer,7 J. D. Meiring,3 N. S. Katz,3 R. Davé,8 of heavy-element enrichment they possess, and A. B. Ford,8 K. R. Sembach1 whether the gas escapes the halo or eventually returns to fuel later star formation. Models of The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained 1 by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph Space Telescope Science Institute, Baltimore, MD 21218, USA. 2 University of California Observatories–Lick Observatory, Santa onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of Cruz, CA 95064, USA. 3Department of Astronomy, University of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around Massachusetts, Amherst, MA 01003, USA. 4Department of galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy Astronomy, Ohio State University, Columbus, OH 43210, USA. 5 elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data Department of Physics and Astronomy, University of Cali- fornia, Los Angeles, CA 90095, USA. 6Department of Chemistry indicate that it is a basic component of nearly all star-forming galaxies that is removed or and Physics, Saint Michael’s College, Colchester, VT 05439, transformed during the quenching of star formation and the transition to passive evolution. USA. 7Leiden Observatory, Leiden University, NL-2300 RA Leiden, Netherlands. 8Steward Observatory, University of alaxies grow by accreting gas from the sions release gas enriched with heavy elements G Arizona, Tucson, AZ 85721, USA. intergalactic medium (IGM) and convert- [or metals (1)], some of which is ejected in *To whom correspondence should be addressed. E-mail: ing it to stars. Stellar winds and explo- galactic-scale outflows (2). The circumgalactic firstname.lastname@example.org 18 NOVEMBER 2011 VOL 334 SCIENCE www.sciencemag.org
REPORTSgalaxy evolution require efficient outflows to the Hubble Space Telescope to directly map the Sun. The QSO sightlines probe projected radialexplain observed galaxy masses and chemical CGM by absorption-line spectroscopy, in which distances to the galaxies (i.e., impact parameters)abundances and to account for metals observed a diffuse gas is detected by its absorption of of R = 14 to 155 kpc. We used the COS data toin the more diffuse IGM (8, 9). The CGM may light from a background source. Our background measure the O VI column densities (NOVI in cm−2),also reflect the theoretically predicted transition sources are ultraviolet-bright quasi-stellar objects line profiles, and velocities with respect to thefrom filamentary streams of cold gas that feed (QSOs), which are the luminous active nuclei of target galaxies (Fig. 1) (21). We measured thelow-mass galaxies to hot, quasi-static envelopes galaxies lying far behind the galaxies of interest. precise redshift, star formation rate (SFR in M◉that surround high-mass galaxies (10, 11). Both We focus on the ultraviolet 1032, 1038 Å doublet of year−1), and metallicity for each of our sampleoutflow and accretion through the CGM may be O VI (O+5), the most accessible tracer of hot and/or galaxies by means of low-resolution spectrosco-intimately connected to the observed dichotomy highly ionized gas at redshift z < 0.5. O VI has py from the Keck Observatory Low-Resolutionbetween blue, star-forming, disk-dominated gal- been used to trace missing baryons in the IGM Imaging Spectrograph (LRIS) and the Las Campanasaxies and red, passively evolving, elliptical galaxies (13–16), the association of metals with galaxies Observatory Magellan Echellette (MagE) spec-with little or no star formation (12). However, the (17–19), and coronal gas in the Milky Way halo (20). trograph (21, 22).low density of the CGM makes it extremely dif- The high sensitivity of COS enables a QSO Our systematic sampling of galaxy propertiesficult to probe directly; thus, models of its structure absorption-line survey of halos around galaxies allows us to investigate the connection betweenand influences are typically constrained indirect- with a predetermined set of properties. We have galaxies themselves and the CGM. The O VI de-ly by its effects on the visible portions of galaxies, selected 42 sample galaxies (tables S1 and S2) tections extend to R = 150 kpc away from thenot usually by observations of the gas itself. that span redshifts zgal = 0.10 to 0.36 and stellar targeted galaxies, but the whole sample shows no Downloaded from www.sciencemag.org on November 27, 2011 We have undertaken a large program with the masses [log(M*/M◉)] = 9.5 to 11.5, where M* is obvious trend with radius R (Fig. 2). The strongnew Cosmic Origins Spectrograph (COS) aboard the galaxy stellar mass and M◉ is the mass of the clustering of detections within T200 km s−1 of the 3 Observed Wavelength (Å)Fig. 1. An illustration of our sampling technique and data. (A) An SDSS composite magnitude of 18.1. (C and D) The redshifted O VI 1032, 1038 Å doublet forimage of the field around the QSO J1016+4706 with two targeted galaxies, galaxies G1 (C) and G2 (D). (E and F) The full sample showing the locations oflabeled G1 and G2, which are both in the star-forming subsample. (B) The all sightlines in position angle and impact parameter R with respect to thecomplete COS count-rate spectrum (counts s−1) versus observed wavelength. targeted galaxies, for the star-forming (E) and passively evolving (F)This QSO lies at redshift zQSO = 0.822 and has an observed far-ultraviolet subsamples. The circles mark R = 50, 100, and 150 kpc. www.sciencemag.org SCIENCE VOL 334 18 NOVEMBER 2011 949
REPORTS A B Downloaded from www.sciencemag.org on November 27, 2011 Impact parameter [kpc] log (Mhalo /M ) Fig. 2. O VI association with galaxies. (A) O VI column density, NOVI, versus R respect to galaxy systemic redshift for O VI detections, versus inferred dark- for the star-forming (blue) and passive (red) subsamples. Solid and open matter halo mass. The range bars mark the full range of O VI absorption for symbols mark O VI detections and 3s upper limits, respectively. The each system. The inset shows a histogram of the component velocities. The detections in the star-forming galaxies maintain log NOVI ≈ 14.5 to R ≈ dashed lines mark the mass-dependent escape velocity at R = 50, 100, and 150 kpc, the outer limit of our survey. (B) Component centroid velocities with 150 kpc from outside to inside. Fig. 3. O VI correlation A B with galaxy properties. -9 (A) O VI column density versus sSFR (≡ M*/SFR). Star-forming galaxies log (sSFR [yr -1]) log (NOVI [cm-2]) -10 are divided from passive- ly evolving galaxies by sSFR ≈ 10−11 year−1; our -11 detection limit is sSFR ≈ 5 × 10−12 year−1. (B) The galaxy color-magnitude -12 diagram (sSFR versus M*) for SDSS+GALEX galaxies Star-forming galaxies from (23). Passive galaxies -13 9 10 11 sSFR [yr -1] log (M /M ) galaxy systemic velocities indicates a close phys- and passive subsamples overlap, rejects at >99% the hit rate correction fhit computed separately ical and/or gravitational association. confidence the null hypothesis that they draw in three 50-kpc annuli (Figs. 1 and 2). This mass CGM gas as traced by O VI reflects the un- from the same parent distribution of NOVI (fig. of oxygen is strictly a lower limit because we derlying bimodality of the general galaxy popu- S2). We therefore conclude that the basic dichot- have scaled to the maximum fOVI = 0.2 (Fig. 4). lation (12, 23). We found a correlation of NOVI omy between star-forming (“blue-cloud”) and The corresponding total mass of circumgalactic with specific star formation rate sSFR (≡ SFR/M*) passive (“red-sequence”) galaxies is strongly re- gas is (Fig. 3). For the 30 galaxies with sSFR ≥ 10−11 flected in their gaseous halos, and that the CGM Z⊙ year−1, there were 27 detections with a typical out to at least 150 kpc either directly influences or Mgas ¼ 177 MO Z column density log NOVI = 14.5 (24) and a is directly affected by star formation. Z⊙ 0:2 high covering fraction fhit ≈ 0.8 to 1 maintained O VI is a fragile ionization state that never ¼ 2 Â 109 M⊙ ð2Þ Z fOVI all the way out to R = 150 kpc (Fig. 2). For the exceeds a fraction fOVI = 0.2 of the total oxygen 12 galaxies in the passive subsample (sSFR ≤ for the physical conditions of halo gas and is where Z is the gas metallicity, and the solar 10−11 year−1), there were only four detections with frequently much less abundant (Fig. 4). Our ob- oxygen abundance is nO/nH = 5 × 10−4 (26). lower typical NOVI than the star-forming sub- servations imply a typical CGM oxygen mass Even for the most conservative ionization cor- sample (25). Accounting for the upper limits in MO, for star-forming galaxies, of rection ( fOVI = 0.2), the OVI-traced CGM con- NOVI and sSFR, we can reject the null hypothesis tains a mass of metals and gas that is substantial 0:2 that there is no correlation between NOVI and M O ¼ 5pR2 〈N OVI 〉mO fhit relative to other reservoirs of interstellar and cir- sSFR at 99.9% confidence for the whole sample fOVI cumgalactic gas. If our sample galaxies lie on the 7 0:2 and 98% for each of the 50-kpc annuli shown in ¼ 1:2 Â 10 M⊙ ð1Þ mean trend of gas fraction for low-z galaxies Fig. 1 (21). This effect remained even when we fOVI (27), they have interstellar medium (ISM) gas controlled for stellar mass: A Kolmogorov-Smirnov where we have taken a typical mean column masses of MISM = 5 × 109 to 10 × 109 M◉ and test over log M* 10.5, where the star-forming density 〈NOVI〉 = 1014.5 cm−2 and R = 150 kpc, and contain M O = 2 × 107 to 10 × 107 M◉ of ISM950 18 NOVEMBER 2011 VOL 334 SCIENCE www.sciencemag.org
REPORTSA B metals retained in the ISM. Thus, the detected oxygen could be the cumulative effect of steady enrichment over the preceding several billion years, the product of sporadic flows driven by rapid starbursts and an active nucleus (33), or the fossil remains of outflows from as early as z ≈ 1.5 to 3 (7, 31). Although the exact origin of the mass-metallicity relation of galaxies is not yet known, models that explain it in terms of preferential loss of metals imply that a substantial fraction of the metals produced by star formation must be ejected from the galaxy rather than retained in the ISM (28). The CGM detected here could be a major reservoir of this ejected ma- terial, with important consequences for models of galactic chemical evolution.Fig. 4. CGM oxygen masses compared to galactic reservoirs. (A) The curves and the axis labels at right The O VI we observe arises in bulk flows ofshow the fraction of gas-phase oxygen in the O VI ionization state fOVI as a function of temperature, for gas over 100 to 400 km s−1, but the relative Downloaded from www.sciencemag.org on November 27, 2011three overdensities relative to the cosmic mean, r/r. All values of r/ r ≥ 1000 track the black curve on velocities are usually below halo escape speedswhich collisional ionization dominates, whereas for lower values, photoionization by the extragalactic (Fig. 2), even when we take projection effectsbackground can increase fOVI at low T. For gas that traces dark matter, r/ r = 1000 is typical at R ≈ 100 into account (fig. S1). Thus, much of the mate-kpc; r/ r = 50 to 100 for the outskirts of the halo. The pale green band shows the expected oxygen mass of rial driven into the halo by star formation couldthe galaxies’ ISM if they lie on the standard relation between MISM and M* and follow the mass-metallicity eventually be reacquired by the galaxy in “re-relation (MZR). The green dashed line shows the oxygen mass produced by 3 × 109 M◉ of star formation.The yellow band shows the expected oxygen mass for the extreme assumption that the typical host dark- cycled winds,” which may be an important sourcematter halos (2 × 1011 to 1012 M◉) have the universal baryon fraction and solar metallicity. (B) The CGM of fuel for ongoing star formation (34). It is un-oxygen masses compared with the interstellar oxygen mass as a function of M*. Points with range bars likely that the detected gas is predominantly freshshow the CGM oxygen mass MO implied by Eq. 1 for fOVI = 0.2, calculated separately for star-forming material accreting from the IGM because models(blue) and passive (red) galaxies according to the hit rates in four bins of stellar mass. The purple curves of “cold mode” accretion predict very low me-show the calculated MISM for typical star-forming galaxies in the SDSS, accounting for the mean MZR in O tallicity and low covering fractions fhit ≈ 10 tothe central curve and its uncertainties in the shaded region. The data points increase their mass in inverse 20% (35, 36), and “hot mode” accretion typicallyproportion to fOVI. involves gas at temperatures T 106 K with undetectably low fOVI.oxygen, taking into account the observed corre- to produce a 1014.5 cm−2 column density within The passive galaxies in our sample oncelation between galaxy stellar mass and ISM the confines of a galactic halo, especially if the formed stars; thus, it follows that they wouldmetallicity (Fig. 4) (21). The minimum CGM metallicity is low (fig. S5). Thus, fOVI = 0.02 and once have possessed halos of ionized, metal-oxygen mass is thus 10 to 70% of the ISM ox- Z = 0.1Z◉ are plausible conditions for the O VI– enriched gas visible in O VI. The relative paucityygen (Fig. 4 and fig. S4). The covering fractions traced gas, but it is unlikely that both conditions of O VI around these galaxies implies that thisand column densities we find for star-forming hold simultaneously. However, if either condition material was transformed by processes that plau-galaxies are insensitive to M*, whereas the ISM holds, the CGM detected here could represent an sibly accompany the quenching of star formationmetal masses decline steeply with M* according important contribution to the cosmic budgets of (37), such as tidal stripping in group environ-to the mass-metallicity relation. Thus, the ratio metals and baryons. In either case, Mgas is com- ments, reaccretion onto the galaxy in ionizedof CGM metals to ISM metals appears to increase parable to the total ~3 × 1010 M◉ inside R = 300 kpc form, or heating or cooling to a temperature atfor lower-mass galaxies (assuming constant fOVI), inferred from H I measurements at low redshift which O VI is too rare to detect. Our findingsperhaps indicating that metals more easily escape (19) and to the ~4 × 1010 M◉ inferred for the present a quantitative challenge for theoreticalfrom their shallower gravitational potentials. The CGM surrounding rapidly star-forming galaxies models of galaxy growth and feedback, whichimplied total mass of circumgalactic gas Mgas is at z ≈ 2 to 3 (31). By generalizing our typical MO must explain both the ubiquitous presence of mas-more uncertain because it can strictly take on any to all star-forming galaxies with M* 109.5 M◉, sive, metal-enriched ionized halos around star-metallicity; for a fiducial solar metallicity, Eq. 2 we estimate that the halos of such galaxies con- forming galaxies and the fate of these metals afterimplies a total CGM mass comparable to MISM tain 15% × (0.02/fOVI) of the oxygen in the uni- star formation ends.and several times the total mass inferred for Milky verse and 2% × (0.02/fOVI) × (Z◉/Z) of theWay “high-velocity clouds” (28, 29) or for low- baryons in the universe. References and Notesionization (Mg II) gas surrounding low-redshift The metals detected out to R ≈ 150 kpc must 1. 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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 email@example.com and thus the flows could be much larger. Second, absorption components.952 18 NOVEMBER 2011 VOL 334 SCIENCE www.sciencemag.org