1 Probing the high redshift (2-3) IGM through OVI absorption Sowgat Muzahid (IUCAA, INDIA)...
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Transcript of 1 Probing the high redshift (2-3) IGM through OVI absorption Sowgat Muzahid (IUCAA, INDIA)...
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Probing the high redshift (2-3) IGM through OVI absorption
Sowgat Muzahid (IUCAA, INDIA)
Supervisor : R. Srianand (IUCAA, INDIA)
Collaborator : P. Petitjean (IAP, FRANCE)
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Plan of the talk :
• Introduction
• Issues we want to address
• Data Sample and Search procedure
• Statistical properties of OVI systems
• Conclusions
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Introduction
• OVI : fifth ionization state of Oxygen, I. P ~ 113.9 eV
• Strongest transitions OVI λλ 1032,1037 Å falls in the UV regime
• Collisional ionization fraction of OVI peaks at T ~ 3 × 105 K
OVI is the best species to probe :
1. Photo-ionized gas subject to hard ionizing photon .
2. Gas with fairly high temperature where collisional
ionization is important .
Gnat & Sternberg 2007
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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 K and δ ~ 5 – 100 .
(Fang & Bryan – 2001)
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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 ?
( Any Evolution ? ).
• 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 !!
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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 .
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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 .
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Data Sample
• Example of a type I (left) and a type II (right) system .
• We use our own Voigt profile fitting code .
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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
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Statistical Properties of OVI absorbers
• No redshift evolution of N(OVI) for 1.9 ≤ z ≤ 2.9 .
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Statistical Properties of OVI absorbers
• With the same spirit of OVI system classification we divide total 188 OVI components into two main categories ..
1. OVI with CIV : 87(188)
2. 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
(a)b-para = 14.4 km/s ( b ≥ 14.4 km/s is consistent with CIE)
(b) NOVI = 13.5 cm-2 ( which is the crossover column density according to the simulation of the low redshift OVI systems.)
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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) > 13.5
cm-2 is seen in a two sided KS test.
(only ~ 77% significance level)
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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.
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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.
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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 ~ NHI 1.20± 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
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Statistical Properties of OVI absorbers
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.
OVI systems from wide varieties of
astrophysical regions (LMC, SMC, HVCs,
Halo, Disk, Starburst, IGM) in low redshift
show b – N correlation .
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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 .
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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
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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
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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.
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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
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Thank You ..