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The Warm-hot Gaseous Halo of the Milky Way
Smita MathurThe Ohio State University
WithAnjali Gupta, Yair Krongold,
Fabrizio Nicastro, M. Galeazzi
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In the low-redshift Universe, baryons are missing on all scales
Galaxies Clusters Universe
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Where are the Galactic missing baryons?
Local WHIM filamentscale length > 1 Mpc
Local Group Medium scale length of ~ 1 Mpc
Circum-Galactic Medium: extending up to the virial radius (~250kpc)
105 K < T < 107 K
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Simulations of the CGM
Stinson et al. 2011
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Diffuse Warm-hot CGM
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z = 0 z = 0 X-ray Absorption
In several high S/N Chandra In several high S/N Chandra and XMM spectraand XMM spectra
• OVII absorption (@21.60 Å)OVII absorption (@21.60 Å)
• Sometimes OVIII (@18.96 Å)Sometimes OVIII (@18.96 Å) and/or Ne IX (@13.44 Å)and/or Ne IX (@13.44 Å)
Mathur et al. 2003
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Z=0 X-ray absorption
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PKS 2155-304 PKS 2155-304 Nicastro et al. 2002Nicastro et al. 2002
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z = 0 X-ray Absorption• In several high S/N Chandra spectra:
– Mkn 421 (Williams et al. 2005) – Mkn 279 (Williams et al. 2006) – PKS 2155-304 (Nicastro et al. 2002)– 3C 273 (Fang et al. 2003)– Other sightlines with lower significance
(McKernan et al 2004)
• Always OVII, sometimes other species• Chandra-LETG resolution 700 km/s, so all
lines are unresolved
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Origin
• Hot Galactic Halo/Corona…– From galaxy formation / feedback processes– Some OVII seen within 50 kpc (Wang et al.
2005)
• … or Local Warm-Hot IGM?– Predicted by simulations (Kravtsov et al. 2002)– Upper limit on OVII emission implies very low
density (Rasmussen et al. 2003)
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Our Chandra Survey of Our Chandra Survey of OOVIIVII and O and OVIIIVIII
Chandra sight lines Previous OVII detection
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Our Chandra Survey of Our Chandra Survey of OVII and OVIIIOVII and OVIII
• 29 sight lines with good S/N near OVII z=0 29 sight lines with good S/N near OVII z=0 regionregion• OOVIIVII detection in 21 sight lines detection in 21 sight lines• OOVIIIVIII detection in 8 sight lines detection in 8 sight lines
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OVII EW DistributionOVII EW Distribution ___ OVII detection ----- OVII upper limit
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OVII OVIII
OVII and OVIII z=0 AbsorptionOVII and OVIII z=0 Absorption
• Log T = 6.1-6.4 KLog T = 6.1-6.4 K
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Column Density Measurement• Optically thin line
N(ion) = 1.3 × 1020 (EW/f λ2) cm-2
• For OVII EW(Kβ)/EW(Kα) = 0.156
Most of OVII KMost of OVII Kαα are saturated are saturated
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log(Nlog(NOVIIOVII) and b contour) and b contour
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• Log NLog NOVIIOVII = 16.19 ± 0.08 cm = 16.19 ± 0.08 cm-2-2
2-3 times higher than previous estimates2-3 times higher than previous estimates
• Column density NColumn density NHH==μμ n neeL L
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Galactic Halo Emission MeasureGalactic Halo Emission Measure
Henley et al. (2010) using XMM-Newton
Yoshino et al. (2009) using Suzaku• Galactic Halo temperature is fairly constant
T = (1.8 -2.4) × 106 K
• Halo emission measure varies by an order of magnitude
EM = (0.0005 – 0.005) cm-6 pc
EM = 0.003 (Z/ZΘ) cm-6 pc
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Combining Absorption and Emission Measurement Breaks the degenaracy
nnee = (2.0 ± 0.6 )× 10 = (2.0 ± 0.6 )× 10−4−4(0.5/f(0.5/fOVIIOVII))−1−1 cm cm−3−3
R = (71.8 ± 30.2) x
(8.51 × 104/(AO/AH))(0.5/f(0.5/fOVIIOVII))22(Z(ZΘΘ/Z) kpc/Z) kpc
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Mass Probed by OVII and OVIII X-ray Absorbing/Emitting Gas Phase
M M totaltotal > 1.7 × 10 > 1.7 × 1099 (fc/0.72) (8.51 × 10-4/(AO/AH))3(0.5/fOVII)5(ZΘ/Z)3MMΘΘ
For Z = 0.3ZFor Z = 0.3ZΘΘ
L > 138 kpcL > 138 kpc
MMtotaltotal > 6.1 × 10 > 6.1 × 101010 M MΘΘ
Gupta, Mathur + 2012, 2014
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Courtesy: Chandra presss office
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Fang, Bullock +2012
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This is a robust result!
• Is the z=0 absorption mostly from the Galactic disk? No.
• What about the uniform density profile? No problem: gives a lower limit on mass.
• Are the emission and absorption at different temperatures? No.
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….. no anticorrelation with Galactic latitude
In emission In absorption
Henley et al 2013 (XMM) Gupta et al. 2014
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Future directions• Probing the anisotropy: emission and
absorption along the same sightline.
-- New Suzaku observations
-- New XMM-Newton Observations.
• Different density and temperature profiles: e.g. Maller-Bullock profile in NFW halo.
• Probing the multi-phase medium: other ions dominant at different temperatures.
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ConclusionConclusion• X-rays provides evidence for hot (T>106 K) gas in and around the Milky Way.
• X-ray OVII and O VIII absorption lines at z=0 probe the hot gas extending over a large region around the Milky Way, with a radius of over 100 kpc.
•The mass content of this phase is over 1010 MΘ.
•A large fraction of Galactic missing baryons are in this hot phase.
•Appears to be a robust result supported by theoretical models.