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Non-solar chemical composition in themultiphase hot halo of Milky Way
Sanskriti Das1 ([email protected]), Smita Mathur1,2
(1)Department of Astronomy, (2)Center for Cosmology & Astroparticle Physics, The Ohio State University
Introduction
The halo of Milky Way is known to be hot, extended,massive and anisotropic [1, 2, 3]. The halo is picturedas an isothermal sphere at T = 106.3 K, the virial tem-perature (fig. 1), and is assumed to have solar chem-ical composition. Shallow observations are unable toprobe any finer thermal and chemical information.
Figure 1:Hot gaseous halo of Milky Way [1] (https://www.nasa.gov/mission_pages/chandra/multimedia/hot_gas_halo.html)
Objective
Characterize the hot (> 106 K) halo of MilkyWay using Ms longimaging spectra in emission and grating spectra in absorption
Summary
•There are 3–4 discrete phases at 105.8K< T 6 107.4K coexisting in the halo ofMilky Way [5].
•The abundance ratios in the hot haloof Milky Way are non-solar. This givesimportant insights into the chemicalevolution of the Galaxy [4].
•The mass of the multiple temperaturecomponents and the non-solarchemical composition have greatimplications for missing baryons andmissing metals.
Future directions
Our Chandra archival proposal to extend this study alongmany sightlines has been accepted
AcknowledgementsWe thank our collaborators, Anjali Gupta, Yair Krongold, and Fabrizio Nicastro. The grants from NASAand the data from ESA are gratefully acknowledged.
References
[1] Gupta, A., Mathur, S. et al., 2012, APJL, 756, 8[2] Gupta, A., Mathur, S. et al., 2014, Ap&SS, 352, 775[3] Gupta, A., Mathur, S. & Krongold, Y., 2017, ApJ, 836, 243[4]Das, S., Mathur, S. et al. 2019, ApJL, 882, L23[5]Das, S., Mathur, S. et al. 2019, ApJ, in press
Result-I: The hot halo of Milky Way is NOT a single phase [5]
We analyze deep (texp = 1.85 Ms) XMM-Newton spectra toward and around the blazar 1ES 1553+113probing the hot Milky Way halo in absorption and emission with unprecedented sensitivity.
10 15 20 25 30
Wavelength [Å]
10-11
FE [
erg
s cm
−2 s−
1 k
eV−
1] NeX Lyα
NeIX HeαOVII Heβ
OVIII Lyα
OVII Heα
NVII Lyα
NVI HeβNVI Heα
RGS1 RGS2
Figure 2:The unfolded spectra of 1ES 1553+113 with the absorptionfeatures from the Milky Way halo, and the best-fit model with twohalo components: T=106.07−6.13K and T=106.96−7.15K.
0.4 0.5 0.6 0.7 0.8 1.0 1.2
Energy [keV]
10-14
10-13
10-12
10-11
10-10
FE [
erg
s cm
−2 s−
1ke
V−
1]
CV
I Lyβ
OV
II H
eα
OV
III Lyα
+O
VII H
eβ
OV
III Lyβ
OV
III Ly
(γ,δ,ε
)
NeIX
Heα
NeX
Lyα
NV
II L
yα
+N
VI H
eβ
Total
Foreground
Galactic halo 1
Galactic halo 2
Background
Instrumental
Figure 3:The emission spectra around the sightline toward1ES 1553+113, and the best-fit model with two halo compo-nents: T=106.25−6.42K and T=106.68−6.92K.
5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4log (Temperature) [K]
T1 T2 T3 T4
T′3
Absorption:3σ Absorption:1σ Emission
Figure 4:Multiple temperature components in the hot halo of Milky Way [5]
Result-II: The abundance ratios in the hot halo are NOT solar [4]
11.9 12.0 12.1 12.2 12.30.7
0.8
0.9
1.0
1.1
1.2
Norm
aliz
ed F
lux
NeX
13.3 13.4 13.5 13.6 13.70.7
0.8
0.9
1.0
1.1
1.2
NeIX
[Ne/O] ¯
[Ne/O] = ¯
24.5 24.6 24.7 24.8
Wavelength [Å]
0.7
0.8
0.9
1.0
1.1
1.2
Norm
aliz
ed F
lux
NVIIRGS1
RGS2
28.65 28.75 28.85 28.95 29.05
Wavelength [Å]
0.7
0.8
0.9
1.0
1.1
1.2
NVI
[N/O] ¯
[N/O] = ¯
Figure 5:The absorption lines of the detected neon and nitrogenlines for the best-fit non-solar (yellow lines) vs. solar (blue lines)abundance ratios relative to oxygen.
• [Ne/O] and [N/O] are super-solar, and[Ne/N] is solar: [Ne/O]=[N/O]=0.7+1.6
−0.2 (>99.73%confidence). This implies a) Type-II Supernovae en-richment, b) AGB stars enrichment, c) oxygen’s efficientcooling and depletion onto dust.
11.25 11.75 12.25 12.75
Wavelength [Å]
0.7
0.8
0.9
1.0
1.1
Norm
aliz
ed f
lux
NeFe ¯
NeFe = ¯
Figure 6:The absorption profile for the solar [Ne/Fe] (yellow lines)vs. the best-fit non-solar [Ne/Fe] (blue lines). The vertical blackline at 12.13 Å corresponds to Nex Ly-α line.
• [Ne/Fe] is super-solar: [Ne/Fe] = 1.6+1.2−0.4
(>99.73% confidence). [O/Fe] = 0.9+0.7−0.3 (>99.73%
confidence) is super-solar too. This indicates α-enhancement, and possible depletion of iron onto inter-stellar and/or circumgalactic dust.
