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Probing high redshift IGM through OVI absorption

This talk provides an introduction to OVI absorption, discusses the issues and data sample, analyzes statistical properties of OVI systems, and draws conclusions about the high redshift intergalactic medium.

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Probing high redshift IGM through OVI absorption

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  1. Probing the high redshift (2-3) IGM through OVI absorption Sowgat Muzahid (IUCAA, INDIA) Supervisor : R. Srianand (IUCAA, INDIA) Collaborator : P. Petitjean (IAP, FRANCE)

  2. Plan of the talk : • Introduction • Issues we want to address • Data Sample and Search procedure • Statistical properties of OVI systems • Conclusions

  3. Introduction • OVI : fifth ionization state of Oxygen, I. P ~ 113.9 eV • Strongest transitions OVI λλ1032,1037 Å falls in the UV regime • Collisional ionization fraction of OVI peaks at T ~ 3 × 105 K • OVI is the best species to probe : • Photo-ionized gas subject to hard ionizing photon . • Gas with fairly high temperature where collisional ionization is important . Gnat & Sternberg 2007

  4. Introduction • Census of baryons at low redshift (z< 0.5) implies that ~ 50% of the baryonic mass (as predicted by BBN) is yet to be detected . (Fukugita et al. 1998) • Recent numerical simulations predict that a substantial fraction of this “missing baryons” could reside in a warm – hot phase of the IGM . • ( [WHIM ] , T ~ 105 – 107 K) (Cen & Ostriker 1999 ; Dave’ et al. 2001) • Relatively cooler phase of the WHIM can be probed by OVI absorption . • OVI lines with rest frame EW > 40 mÅ are primarily produced by collisionally ionized gas at : • T ~ few 105 Kand δ ~ 5 – 100 . (Fang & Bryan – 2001)

  5. Issues we are interested in .. • Spatial distribution of OVI absorbers hence the high temperature regions and/or regions affected by hard ionizing photons . • Physical properties of OVI absorbers at high redshift. Is there any fundamental difference in the properties of what is seen in the local universe ? • ( AnyEvolution ? ). • Estimating the contribution of OVI absorbers to the baryon inventory around redshift 2 - 3 . • Absorption study is indirect in nature . Big challenge is to relate the LOS properties to the global picture of the absorber. • Large homogeneous sample is needed !!

  6. Data Sample • We have ~ 100 high resolution QSO absorption spectra from VLT/UVES . • 18 best quality spectra have been picked up to analyze . • These data were obtained in the course of the large programme “The Cosmic Evolution of the IGM” . Typical resolution ~ 45,000 (6.6 km/s) and S/R ~ 70 /pixel, wave length coverage 3200 Å to 10,000 Å . • This provide a homogeneous sample of QSO sight lines in the redshift range 2.1 - 3.3 . • These sight lines allow us to study OVI systems for redshift ~ 1.9 - 3.0 where the Ly-alpha forest is not too severe .

  7. Data Sample • We search OVI systems mainly in two ways .. • Guided (by other metal lines) search : • Blind search : • We classify OVI systems mainly into three categories .. • Type I : OVI lines are accompanied by other metal lines . • Type II : OVI with only Lyman series lines . • Type III : OVI with consistent profiles without metal lines • and Lyman series lines . • This classification is motivated by the facts that .. • Type I >> representative of photoionized gas . • Type II >> representative of high temp. gas . • Type III >> representative of highly ionized and high temp. gas .

  8. Data Sample • Example of a type I (left) and a type II (right) system . • We use our own Voigt profile fitting code .

  9. Data Sample • We have identified more than 70 OVI systems ( Biggest OVI sample ever reported ! ). • We fit 51 OVI systems comprised of 188 components from 14 LOS. • Type I : 45 Type II : 06 Type III : 00 • Type II & III systems are always affected by possible Ly-series contaminations which leads to false detections !! • Highest redshift : 2.9075 • Lowest redshift : 1.9643 • Median redshift : 2.32 • Median N(HI) : 14.19 cm-2

  10. Statistical Properties of OVI absorbers • No redshift evolution of N(OVI) for 1.9 ≤ z ≤ 2.9 .

  11. Statistical Properties of OVI absorbers • With the same spirit of OVI system classification we divide total 188 OVI components into two main categories .. • OVI with CIV : 87(188) • OVI without CIV : 101(188) • This is just to see if there is any difference in properties in this two sub samples which are supposed to trace photoionized and collisionally ionized gas respectively . • We will use two indicators for further analysis • b-para = 14.4 km/s ( b ≥ 14.4 km/s is consistent with CIE) • NOVI =13.5 cm-2 ( which is the crossover column density according to the simulation of the low redshift OVI systems.)

  12. Statistical Properties of OVI absorbers • 107(188) i.e ~ 57% of total OVI • 53(87) i.e ~ 61% of OVI with CIV • 54(101) i.e ~ 53% of OVI without CIV • components show N(OVI) > 13.5 cm-2 • No significant difference between OVI components with and without CIV for N(OVI) > 13.5cm-2 is seen in a two sided KS test. • (only ~ 77% significance level)

  13. Statistical Properties of OVI absorbers • 88(188) i.e ~ 47 % of total OVI • 38(87) i.e ~ 44 % OVI with CIV • 50(101) i.e ~ 50 % OVI without CIV • components show b-parameter consistent with CIE i.e b > 14.4 km/s ( T > 2×105 K) • A two sided KS test does not show any significant difference between components with and without CIV for b > 14.4 km/s.

  14. Statistical Properties of OVI absorbers • 64(87) ~ 74% components show bOVI > bCIV • 22(93) ~ 24% components show bOVI > bHI • CIV and OVI are appear to be associated kinematically but originally trace different phases of the (multiphase!) IGM.

  15. Statistical Properties of OVI absorbers • NOVI almost constant for 7 decades variation in NHI . • If the HI and OVI phases were well mixed, we would expect multiphase ratio (NHI/NOVI) to be constant with NHI . • Green points are taken from Fox et al. 2007. They have studied hot halos in high redshift protogalaxies . • Its intriguing that nowhere (from low density Ly-alpha forests to high density DLAs) OVI is varying that much. • NHI /NOVI ~ NHI1.20± 0.01 • Danforth & Shull shown that such correlation exists at low redshift z < 0.15 . They found : • NHI /NOVI ~ NHI 0.9±0.1 Danforth & Shull-05

  16. Statistical Properties of OVI absorbers b – N correlation ?? OVI systems from wide varieties of astrophysical regions (LMC, SMC, HVCs, Halo, Disk, Starburst, IGM) in low redshift show b – N correlation . Heckman et al. -02 • Radiatively cooling hot gas passing through coronal regime gives rise to such correlation. • For log (b) > 1.6 , NOVI increases linearly with temp.

  17. Statistical Properties of OVI absorbers b – N correlation ?? • b – N correlation is well known in case of HI (eqn. of state) • Here we find mild b-N correlation. • rs = 0.5 is good enough to rule out the null hypothesis . • Bias ??? • Low column with large ‘b’ will be affected by S/N .

  18. Statistical Properties of OVI absorbers b – N correlation ?? • Spearman Rank coefficient: 0.500 • Slope = 2.00 ± 0.24 • Intercept = 11.20 ± 0.27 • Spearman Rank coefficient: 0.537 • Slope = 2.02 ± 0.20 • Intercept = 11.29 ± 0.23

  19. Statistical Properties of OVI absorbers A simple model • We run CLOUDY v07.02 to model 51 OVI systems . • Assumption : a) cloud is optically thin • b) cloud is in single phase ! • CLOUDY parameters : Stop column density : N(HI) = 15.0 cm-2 • HM-05 EGB at redshift 2.32 • log Z ~ -3.0 to -1.0 ; log nH~ -5.0 to -3.5 assuming photoionization !! QSO + GAL QSO

  20. Conclusions • There is no redshift evolution of NOVI between 1.9 < z < 2.9 . • There is no significant difference in column density distributions between OVI with and without CIV for NOVI > 13.5 cm-2 . • There is no significant difference in b-parameter distributions between OVI with and without CIV for b > 14.4 km/s . • Almost 75% cases we find bOVI > bCIV which indeed imply CIV and OVI probe different phases of the IGM . • Increase of multiphase ratio NHI /NOVI with NHI suggests that IGM has at least two phases (WHIM & WNM) and they are not well mixed . • Mild log b – log NOVI correlation is there with slope ~ 2.0 which is not due to any bias !! • b – NOVI correlation is possibly due to local physics of heating and cooling . • A simple model of the OVI systems gives metallicity ~ -3.0 to -1.0 in log and δ ~ 15 – 60 assuming photoionization by Haardt-Madau EGB.

  21. References • Fukugita, M ., Hogan, C. J., Peebles, P. J. E ., 1998, ApJ, 503, 518 • Cen, R., Ostriker, J. P., 1998, ApJ, 514, 1 • Dave´, R., et al., 2001, ApJ, 552, 473 • Fang, T. & Bryan, G. L., 2001, ApJ, 561, L31 • Danforth, C.W. & Shull, M.J., ApJ, 624:560, 2005 • Heckman., et al., ApJ, 577:691-700, 2002 • Bergeron, J., Aracil, B., Petitjean, P., Pichon, C., A&A 396,L11-15,02 • Bergeron, J. & Herbert-Fort., Proceeding IAU Colloquium No 199,2005 • Gnat, O. & Strenberg, A., ApJ, 168:213 – 230, 2007 • Fox, A. J., et al. A&A 465, 171-184(2007) • Haardt, F., & Madau, P. 1996, ApJ, 461, 20 • Ferland, G. J., et al., 1998, PASP, 110, 761

  22. Thank You ..

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