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IXO and WHIM. Science of WHIM Status of WHIM search (X-ray and UV) Suzaku results on LSS and cluster outskirts Expectation with IXO XGS Summary. T. Ohashi (Tokyo Metropolitan University). Galaxies. Clusters. Reionization. 1st stars. Large –scale structure. Light elements. Big bang.

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ixo and whim


  • Science of WHIM
  • Status of WHIM search (X-ray and UV)
  • Suzaku results on LSS and cluster outskirts
  • Expectation with IXO XGS
  • Summary

T. Ohashi (Tokyo Metropolitan University)

thermal history of the universe




1st stars

Large –scale structure

Light elements

Big bang



IGM temperature

Reionization by UV and X

106 K

104 K

Shock heating

Cooling by expansion

Metal synthesis

102 K







Age of the Universe

Thermal history of the universe

WHIM (warm-hot intergalactic medium) will tell us the evolution of the hot-phase material in the universe

baryon phase
Baryon phase

Wide area in the baryon phase space is unexplored

Branchini et al. 09

Oxygen line probes the dark baryon efficiently


Bregman 07

cosmic structure

Dark matter

Galaxies (~104K)

Cosmic structure

WHIM (105-107 K) traces the cosmic large-scale structure = “Missing baryon”

Typical matter density: d (=n/nB) = 10 - 100

Yoshikawa et al. 2001, ApJ, 558, 520

size = 30 h-1 Mpc 5 deg at z=0.1

IGM (105-107K)

Cluster gas (107K)

sculptor wall by chandra and xmm
Sculptor Wall by Chandra and XMM

Fang et al. 2010, ApJ 714, 1715

  • Scupltor wall (z = 0.03  0.002) via absorption of blazar H2356-309 (z = 0.165)
  • XMM RGS (130 ks) + Chandra LETG (520 ks) combined
  • OVII (21.6 A) absorption line detected at z = 0.03 with 4.0s significance(EW = 26 mA)
  • If depth of L = 15 Mpc (filling the wall) and ZO = 0.1 solar assumed, overdensity becomes d 30
  • Consistent with WHIM filling the filament


fuse and xmm detection of whim
FUSE and XMM detection of WHIM

Nicastro et al. 2010, ApJ 715, 854


  • PKS0558-504 (z = 0.1372) shows OVIII (XMM) and HI Lyb (FUSE) absorption lines at z = 0.117
  • XMM RGS data with 900 ksec in total show 2.8s significance
  • FUSE data show Lyb (4s) but no OVI feature  T > 3.3  106 K
  • Metallicity 1% - 4% solar (absorption unlikely to be AGN outflow) and NH 1.5  1021 cm-2
  • Estimated T infers overdensity  300 and L 5 Mpc

OVIII (z = 0.116)


HI Lyb (z = 0.118)

cos detection of nearby whim
COS detection of nearby WHIM

Narayanan et al. 2010 ApJ, 721, 960

  • Broad Lya (COS) and OVI (FUSE) at z = 0.0103 against Mrk 290 (z = 0.0296)
  • Line width: T = 1.4  105 K
  • OVI is likely to be photoionized (combination of non-equilibrium coll. ion. and photoionization necessary)
  • Large-scale filament (20) exists
  • Possible connection with NGC5987, at about 400 kpc away from the absorption system
cos study of distant whim
COS study of distant WHIM

Narayanan et al. 2011 ApJ, 730, ---

  • NeVIII and OVI (both with COS) absorption at z = 0.495 against PKS 0405-123 (z = 0.57)
  • Line data: T 5  105 K, NH = 1019-20 cm-2, ZNe  0.25 solar
  • NeVIII probes collisionally ionized gas, while OVI is likely to be photoionized
  • Hot halo of a galaxy or WHIM
  • NeVIII useful in studying coll. ion. gas
suzaku search for whim
Suzaku search for WHIM

Suzaku is searching for WHIM in cluster outskirts and in superclusters, with no positive detection of redshifted O lines yet

Suzaku is giving fairly low upper limits (d < 300 assuming line-of-sight depth of 2 Mpc and ZO = 0.1 solar)

Suzaku will continue searching for WHIM in all possible locations

cluster outskirts with suzaku
Cluster outskirts with Suzaku

A2142 outskirt

A1413 outskirt

Rvir(2.7 Mpc)


Akamatsu et al. 2011 in prep

Hoshino et al. 2010 PASJ

  • Significant X-ray emission detected out to Rvir
  • Systematic errors due to fluctuation of CXB, Galactic background, non X-ray background, XIS filter contamination evaluated
  • Effect of stray light from bright center less than 7% of the observed flux
cluster outskirts with suzaku1
Cluster outskirts with Suzaku

Akamatsu et al. 2011 in prep

"Uniform" temperature profile

by Burns et al. 2010, ApJ 721, 1105

K = kT/n2/3



  • Entropy drop at > 0.5 rvir is seen in several clusters
  • Clumpiness explains only a few % drop
  • Non equilibrium (Ti > Te) more likely?

Nagai and Lau 2011 ApJL

filament junction by suzaku
Filament junction by Suzaku

Search for (and detection of ) X-ray emission from large-scale junction of filaments

Kawahara et al. 2011 ApJL 727, L38

DisPerSE : filament extractor

Sousbie 2010, Sousbie, Pichon, & Kawahara 2010

RASS map with SDSS galaxies


suzaku j1552 2739 a group of galaxies
Suzaku J1552+2739: a group of galaxies

X-ray intensity contour

Hardness ratio map

1.64 keV

"hot spot"3.91 keV

NXB subtracted & vignetting corrected image

LX = 2.4  1043 erg/s

  • Likely to be a group undergoing major merger
  • Filament junction could host many undetected X-ray groups
  • These should explain some of the missing baryons
ixo grating spectrometer
IXO: grating spectrometer
  • Big jump from Chandra and XMM-Newton grating spectrometers by factor of > 10 in effective area and 3 - 7 times higher resolution
  • Sensitive to Equivalent Width  2 meV
  • R = 3000 can resolve v = 100 km/s, such as structures of galactic winds


ixo absorption study
IXO: absorption study
  • OVII, OVIII absorptions: WHIM presence beyond any doubt
  • Predicted dN/dz of OVII, OVIII clouds (normalized with OVI results): several absorption systems per AGN will be detected
  • There are > 100 useful AGNs for absorption study (FX > 5  10-11 cgs), and observation of 30 AGNs will reveal 100 WHIM clouds

IXO: 600 ksec













EW (mÅ)

Bregman et al. IXO-WP

whim emission study
WHIM emission study




  • Grasp of XMS: equally large as those of dedicated missions
  • 5” angular resolution resolves 16 kpc at z = 0.2 (galactic outflow scale)


Simulated spectrum

whim search using resonance scatting
WHIM search using resonance scatting
  • Churazov et al. 2001, MN 323, 93Churazov, 2010, Space Sci. Rev. 157, 193
  • WHIM can be surrounding bright AGNs or groups
  • Part of continuum X-rays undergoes resonant scattering by WHIM gas
  • For d ~ 30, a few Mpc region around bright AGNs will shine in OVII and/or OVIII lines
  • Rough estimation shows that an AGN with LX = 1046 erg/s at z = 0.2 can produce about 100 OVII counts with IXO over ~ 5 arcmin region in 100 ksec observation
  • Weaker or comparable to the thermal line intensity, but this is an additional emission scaling with n
resonance scattering from whim
Resonance scattering from WHIM

Mpc scale halo

O line

  • AGN (or gal. group) X-rays will be resonantly scattered by OVII and OVIII ions in surrounding WHIM
  • A few Mpc scale extended halo of O lines is expected
  • Radial profile is much flatter than thermal emission from group gas



S. brightness

OVII from group



WHIM halo


  • WHIM will give us unique information about thermal and chemical evolution of the universe
  • Absorption and emission studies with IXO will reveal all phases of warm-hot baryons within z 0.5
  • Absorption study with IXO will probe low-density WHIM clouds (d < 100)
  • Emission measurement will also show us the spatial structure (filaments, outflows) of WHIM
  • Resonance scattering may show oxygen-line halos with ~ Mpc scale around bright objects