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Journal club 1211.0758 "On the Hot Gas Content of the Milky Way Halo"
Transcript of Journal club 1211.0758 "On the Hot Gas Content of the Milky Way Halo"
Virial mass of Milky Way halo is (1-2)x10^12 Msun
within the virial radius 260-330 kpc.
So there should be (1.65-3.3)x10^11 Msun of baryons inside the MW, contrary to observed baryon mass ~0.65x10^11 Msun.
About 10^11 Msun of baryons is missing? Extensive studies of cold has have not found the missing baryons.
Plausible hypothesis - they are dispersed throughout the halo
In this case, the gas should be HOT (thermal velocities ~100 km/s = temperature ~50eV).
The average density within 300 kpc sphere
is very small, n ~ 4x10^-5 cm^-3 (R/300kpc)^-3.
Can we detect it? Missing baryons = X-ray hot gas? XMM and Suzaku measured thermal bremsstrahlung
from X-ray halo; they derived emission measure
EM = n_e^2 x R ~ (5-50)x10^-4 cm^-6 pc.
Our naive estimate gives
EM = 4.8x10^-4 cm^-6 pc (R/300kpc)^-5.
To derive n_e and R, one needs another independent
measurable: Stydies of X-ray halo Absorption lines from distant blazars (1205.5037): Recent X-ray observations of X-ray emission from
Galactic halo and absorption lines from distant blazars have revealed the presence of large quantities of X-ray gas, can explain all missing baryons (~10^11 Msun) in MW!
Further investigation is possible with wide-field
imaging spectrometer (e.g. based on microcalorimeter
technology, see e.g. http://adsabs.harvard.edu/abs/2012PDU.....1..136B) OVII (574eV) and OVIII (654eV) lines are detected in 21 of 29 high-quality observations by Chandra HETG/LETG.
This gives OVII column density N_e~ n_e x R - possible
to measure n_e and R SEPARATELY!
From logN(OVII) = 16.19+/-0.08 cm^-2,
f_OVII = 0.5 and A_0/A_H = 8.5x10^-4
one obtains N_e = 11 cm^-3 pc, consistent with our estimate N_e = 9.6 cm^-3 pc (R/300kpc)^-3!