Galaxies are not only composed of stars. Between the stars (and also between galaxies) there is some matter, the interstellar medium consisting of gas and dust. The interstellar medium, like fog on Earth, absorbs the light from the stars : the stars will appear weaker or more distant than they are in reality : it is the interstellar extinction. This effect is highly chromatic : the blue light is more absorbed than the red light, and this is called interstellar reddening : the stars appear redder than they are in reality and we must correct this effect to determine the characteristics of their atmospheres. Finally, these gas, atoms or molecules, or dust produce lines in absorption or emission in the stellar spectra. Figure 1 shows examples of interstellar gas absorbing the light from stars located further away (the cone at the bottom of the figure) and material diffusing the light of bright blue stars (Photo ESO).
Figure 2 (© ESA/Gaia/DPAC/CU6/Observatoire de Paris-Meudon/Olivier Marchal, Carine Babusiaux & David Katz) shows the spectra obtained by the spectrograph of Gaia, the Radial Velocity Spectrometer (RVS) in a single observation (there will be about 40 such observations per star during the mission) and on one of the three bands of CCDs for three hot stars (temperatures between about 9000 and 20000 K). Many hydrogen lines (H), calcium (Ca), helium (He) or nitrogen (N) are clearly visible. These lines are produced in the atmosphere of stars and reflect the presence and abundance of these elements in the atmospheres of these supergiants of types B1,5, A2 and B2. Other lines, indicated by red arrows, are due to the presence of interstellar matter between the observed stars and Gaia. These lines are from the family of diffuse interstellar bands (DIBs), and it is presently not clear by which molecules they are created. The force (depth and width) of these lines is correlated with the amount of interstellar material present between the observed stars and Gaia. Spectra are sorted by order of increasing extinction downward. Extinction is characterized by the redenning of the starlight by the interstellar medium : the color excess E (B-V), increases from top to bottom, as well as the strength of DIBs. By comparison Figure 3 shows spectra obtained from the ground for more and more distant stars : the strength of DIBs increases steadily with the distance of the stellar groups considered, less than 1 kpc at the top, to 4-6 kpc at the bottom (Puspitarini, Lallement, Babusiaux et al 2015).
The combination of very precise distances expected from Gaia and the measured intensity of the DIBs will allow to build a 3D map of the interstellar medium of unprecedented accuracy. Indeed, the strength of the DIB depends on the amount of interstellar matter between the star and Gaia. The distance from the star gives an upper bound on the distance of the material that contributes to the DIB absorption line. By combining many stars distributed at different distances, one can be obtain by inversion, the distribution of the material along the line of sight (a method equivalent to tomography).
The comparison with estimates of extinction obtained from observations of the Gaia spectrophotometer will not only produce an unprecedented 3D mapping of the galactic interstellar medium, but also provide new constraints on the potential carriers of DIBs.
Reference
- Puspitarini, L., Lallement, R., Babusiaux, C., Chen, H.-C., Bonifacio, P., Sbordone, L., Caffau, E., Duffau, S., Hill, V., Monreal-Ibero, A., Royer, F., Arenou, F., Peralta, R., Drew, J. E., Bonito, R., Lopez-Santiago, J., Alfaro, E. J., Bensby, T., Bragaglia, A., Flaccomio, E., Lanzafame, A. C., Pancino, E., Recio-Blanco, A., Smiljanic, R., Costado, M. T., Lardo, C., de Laverny, P., Zwitter, T. : 2015, The Gaia-ESO Survey : Extracting diffuse interstellar bands from cool star spectra. DIB-based interstellar medium line-of-sight structures at the kpc scale Astronomy & Astrophysics, 573, A35