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O VI Absorbers at z =2-3 Photoionized by Quasars or Tracers of Hot Gas?

O VI Absorbers at z =2-3 Photoionized by Quasars or Tracers of Hot Gas?. Andrew Fox (ESO-Chile) Jacqueline Bergeron & Patrick Petitjean (IAP-Paris). Metal lines as tracers of ionization level. Energy.

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O VI Absorbers at z =2-3 Photoionized by Quasars or Tracers of Hot Gas?

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  1. O VI Absorbers at z=2-3Photoionized by Quasars or Tracers of Hot Gas? Andrew Fox (ESO-Chile) Jacqueline Bergeron & Patrick Petitjean (IAP-Paris)

  2. Metal lines as tracers of ionization level Energy • H I–H IISi III-Si IVC III-C IV He II-He IIIN IV-N V O V-O VI 13.6 eV 33.5 eV 47.9 eV 54 eV 77.9 eV 113.9 eV • O VI advantages: • O VI is most highly ionized line available in rest-frame UV • Oxygen is most abundant metal in Universe • O VI doublet at 1031, 1037 Å is intrinsically strong • O VI disadvantage: • O VI falls in Ly-a forest  blending/contamination. Only detectable at z2-3.

  3. O VI probes IGM ionization and enrichment • VLT/UVES, Keck/HIRES studies • Schaye et al. 2000 • Bergeron et al. 2002 • Carswell et al. 2002 • Simcoe et al. 2002,2004,2006 • Levshakov et al. 2003 • Reimers et al. 2001, 2006 • Bergeron & Herbert-Fort 2005 • Lopez et al. 2007 • Gonçalves et al. 2008 O VI absorbers have power-law column density distribution (Bergeron & Herbert-Fort 2005) “Associated” or “proximate” absorbers (at dv<5000 km s-1 from QSO) often removed from sample  affected by ionization conditions close to QSO. This talk: Examine this practice (Fox, Bergeron, & Petitjean 2008, MNRAS) Is there a proximity effect in O VI?

  4. UVES Spectra • VLT/UVES Large Program • 20 QSOs, high resolution (FWHM 6.6 km s-1) and high S/N (~40–60) • Searched for O VI absorbers within 8000 km s-1 of zQSO. • zQSO is determined from several QSO emission lines, allowing for systematic shifts (Tytler & Fan 1992) • 35 proximate O VI systems detected: • 26 weak systems • 9 strong systems -200 0 km/s 200 -200 0 km/s 200

  5. Two Populations of Proximate O VI • STRONG • log N(O VI) ≥ 15 • Strong N V and C IV • Multiple components • Velocities clustered around zQSO • Occasional evidence for partial coverage of continuum source. • Truly intrinsic: inflow/outflow near AGN central engine (several mini-BALs) • WEAK • log N(O VI)≤14.5 • Weak N V and C IV • 1 or 2 components • Velocities < zQSO • No evidence for partial coverage

  6. “Proximity Effect”: change in dN/dz at 2000 km s-1 Intervening systems (Bergeron & Herbert-Fort 2005) • Proximity zone extends over ~2000 km s-1, not 5000 km s-1.

  7. Weak O VI absorbers: trends with proximity At 2000 km s-1, see change in N(H I) and in N(C IV) but not in N(O VI)

  8. Furthermore, O VI/HI offsets are observed Significant velocity centroid offsets between O VI and H Iare seen in ~50% of the weak O VIabsorbers  two ions are not co-spatial. (similar fraction of low-z O VI absorbers show offsets; Tripp et al. 2008)

  9. O VI Component Line Width Distribution b=(2kT/m + b2non-thermal) • Median b-values • O VI <2000 km s-1 from QSO: b=12.3 km s-1 • Intervening O VI: b =12.7 km s-1 •  T <1.6x105 K • Intervening N V: b =6.0 km s-1 • (Fechner & Richter 2009) •  O VI and N V trace different regions

  10. O VI absorbers (even narrow) may not be photoionized; can be formed in non-equilibrium cooling gas Frozen-in ionization can lead to O VI being present in gas down to ~104 K if the metallicity is close to solar Results of Gnat & Sternberg (2007)

  11. Are there any physical reasons why such hot gas should exist at z=2-3? YES: Galactic Winds YES: Hot-mode accretion Simulations from Kawata & Rauch (2007) Simulations from Dekel & Birnboim (2007) See also Fangano, Ferrara, & Richter (2007)

  12. Comparison of high-ion ratios Observations vs theory (Gnat & Sternberg) Cooling gas models can explain data if elemental abundance ratios are non-solar: Need -1.8<[N/O]<0.4 -1.9 <[C/O]<0.6

  13. Implications for O VI absorbers in general • Single-phase photoionization models for IGM O VI absorbers are too simplistic, because • O VI-H I velocity offsets imply O5+ and H0 occupy different regions • O5+ may be collisionally- rather than photo-ionized • Don’t know EGB shape above 100 eV that well Use caution when combining O VI/H I ratio + CLOUDY  IGM metallicity What you see in O VI EGB O6+, O7+ T≥106 K O5+,T~105 K H0, O5+,T~104 K H0,T~104 K What you see in H I

  14. N(H I)~1015 N(O VI)~1013.5 N(H I)~1014 N(O VI)~1013.5 QSO 2000 km/s Proximity Warm plasma photoionized as you approach z(QSO), not hot plasma

  15. Almost 1 dex uncertainty inJnat 113.9 eV!!! Simcoe et al. (2004)

  16. Sawtooth modulation by He II Lyman series exacerbates the situation We don’t really know what’s happening out here! Madau & Haardt (2009)

  17. Survey for Proximate O VI: Summary • In 20 high-quality QSO spectra from UVES, we search for O VI within 8000 kms-1 of zQSO, finding • 9 strong absorbers (truly intrinsic, gas near AGN) • 26 weak absorbers Among weak O VI absorbers: • dN/dz increases by factor of 3 inside 2000 km s-1 • dN/dz in range 2000-8000 km s-1 matches intervening. • N(H I) and N(C IV) show a proximity effect (dependence on Dv), N(O VI) does not. • O VI-H Ivelocity centroid offsets imply at least half the absorbers are multiphase. • Cannot use O VI absorbers to probe high-energy tail of EGB: too many systematic uncertainties. Narrow O VI can form in radiatively-cooling hot gas, in interface regions that result from galactic winds/hot-mode accretion

  18. Partial Coverage of Continuum Source

  19. O VI absorber size is <200 kpc, based on lack of Hubble broadening. Simcoe et al. 2002

  20. Is there a line-of-sight proximity effect in O VI? • Strong O VI: • Yes, we see strong O VI clustered around zQSO • Weak O VI: • Yes, we see dN/dz increase by a factor of three within 2000 km s-1 (but galaxies are clustered near quasars). • No, the internal properties (b-values, log N) of the O VI absorbers do not depend on Dv, unlike H I and C IV Is there observational evidence (at z=2-3) for an extended (~10 Mpc) QSO proximity zone of E>100 eV photons? • No: properties of weak O VI do not require photons at E>100 eV (you can create the O VI with [cooled] hot gas)

  21. C IV ionization fraction in hot gas • Proximity zone extends over ~2000 km s-1, not 5000 km s-1. • convert QSO B-magnitude and zQSO to L912 (Rollinde et al. 2005) • Determine size of “Stromgren Sphere” where QSO radiation density exceeds estimated EGB radiation density at z=2.5 ( ~10 Mpc) • Convert size to velocity assuming Hubble Flow and H(z=2.5)=250 km s-1 Mpc-1 (1500-2500 km s-1)

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