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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• 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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• 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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• 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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• 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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• 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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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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• 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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• 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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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 ..

1 probing the high redshift (2-3) igm through ovi absorption sowgat muzahid (iucaa, india)...

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