1. Gas and dust in the ISM towards the Center of the Galaxy through X-ray Spectroscopy
Ciro Pinto(1)
J. S. Kaastra(1,2), E. Costantini(1), F. Verbunt(1,2)
(1) SRON – Netherlands Institute for Space Research
(2) Astronomical Institute, Utrecht University

2. Interstellar medium (ISM) and Galactic life cycle
Crucial role in the evolution
of the entire Galaxy
Stellar evolution
Heavy elements
ISM
Metallicity gradient towards the G. center
Evolution of the Galaxy

3. Interstellar medium: 3-phase structure
Chemical composition close to Solar abundances

4. X-ray Spectroscopy: a powerful tool
Abundances + Constituents
Absorption edges and lines
Column densities
Recent results:
Ne/O ratio higher than Solar value
Metals depleted: ≥ 70% into dust
Oxygen depleted: 10-40% into dust
Dust grains mostly made of Silicates
Galactic Ionized Gas

5. Best Targets Best Instrument The Gratings
1) XRB GS , 4U ,
4U , GX 339-4,
2) Nebula: Crab
3) Nova: V2491 Cyg
Best Targets
Best Instrument
The Gratings
on board:
XMM-Newton +
Chandra
Ne, O, Mg & Fe edges

6. GS 1826-238: neutral gas Absorption: Cold neutral gas → Ne, O, Mg & Fe
Fitting package SPEX
(Kaastra et al. 1996)
Absorption:
Cold neutral gas
→ Ne, O, Mg & Fe
Continuum:
(Thompson 2008)
Residuals:
17.5 Å → dust
22.9 Å → dust
21.6 Å → O VII
23.3 Å → O II

7. GS 1826-238: ionized gas Model 2: 3-phase Gas 3 distinct phases
Cold, warm and hot phases
Abundances same for all phases
3 distinct phases
Pure-gas not fully satisfactory
Oxygen Edge
COLD GAS: v ~ 10 km/s
WARM GAS: v ~ 60 km/s
HOT GAS: v ~ 150 km/s
103
104
105
106
T (K)
NH (1021 atoms cm-2)

8. GS 1826-238: dust and molecules
Iron Edge
GS : dust and molecules
Pure-gas model doesn't work
→ need gas + dust !!!
Our current models:
- Shielding by dust grains
- Structure by molecules
Models in development !!!
Oxygen Edge

9. GS 1826-238: dust and molecules
Fe → shielding by grains
O → CO, H20 ice, silicates ..
Models in development !!!
> 90 % of iron in dust
10-20 % oxygen in dust

10. Contributions to interstellar oxygen

11. Analysis extended: several sight-lines
Crab nebula → Kaastra et al. (2009)
Cyg X → Yao et al. (2009)
4U → Costantini et al. (2010)
GS → Pinto et al. (2010) I
All sources → Pinto et al. (2010) II in prep.
Analysis extended: several sight-lines

12. How is oxygen distributed ?
Crab nebula → Kaastra et al. (2009)
Cyg X → Yao et al. (2009)
4U → Costantini et al. (2010)
GS → Pinto et al. (2010) I
All sources → Pinto et al. (2010) II in prep.
How is oxygen distributed ?
Ionized gas provides < 10 % of O-column
Dust silicates provide ~ 10 – 15 %

13. Abundances confirm the metallicity gradient
GS → 6-7 kpc vs. G. center
4U → 7-8 kpc vs. G. center
All elements are over-abundant
with respect to Solar composition
Over-abundances almost
agree Metallicity gradient

14. Abundances confirm the metallicity gradient
Crab nebula → Kaastra (2009)
Cyg X → Yao (2009)
4U → Costantini (2010)
GS → Pinto et al. (2010) I
All sources → Pinto et al. (2010) II
Average result:
Abundances increase
towards the center
of the Galaxy
But Ne doesn't show a clear change …

15. Conclusion and future Chemical analysis of ISM
Evolutionary effects, e.g. metallicity gradient
Study compounds like dust
Future: ASTRO-H, IXO, Lab. data
→ distinguish several compounds
→ mapping ISM
→ know better the evolution of our Galaxy