STructuring by Inversion the Local InterStellar Medium – STILISM

Absorption measurements for stars distributed in distance and direction allow to infer the three-dimensional distribution of the interstellar absorbing matter. Such absorption datasets can be inverted by means of Bayesian methods, and allow to build maps whose spatial resolution depends on the target star distribution.

Newly calculated 3D maps are available with navigation tools on the project site. stilism.obspm.fr . The maps have been widely extended and are more detailed. Several articles describe maps, locating particular or known structures. The Local Galactic Arm or Orion Arm appears very different from existing patterns. Other articles describe new techniques for acquiring absorption and extinction measures. Finally, others describe parallel results: tomography of nearby opaque cloud using diffuse interstellar bands (DIBs) or, on the contrary, tomographie of external galaxies using the DIBs, heierarchical classification of DIBs according to the phases of the MIS and the radiation field, newly established relationships on multi-wavelength emissions of the various phases.
From the technical point of view, many tools have been developed to extract lines and absorption bands from all types of observations. The inversion codes have been adapted and offer new possibilities.
The synergy between stellar spectrophotometry-photometry and the study of the interstellar medium worked perfectly. The maps were used to select non reddened stars which then entered the Gaia G-band calibration. In return, the new calibration laws have made it possible to extract new distance-roughed measurements to feed more precise and extended maps. The process can be iterated.

A large part of STILISM's initial objectives, in particular the improvement of maps, underlying bases and inversion tools, have been achieved. The transition to 2D-3D assimilation has been delayed and will be achieved later using absorption data accumulated during the project, through the use of CO radio spectral cubes and dense phase absorbers, especially neutral potassium KI. On the other hand, the project has contributed to the development of new studies and results on the links between the observable quantities related to gas phases and the emission and absorption properties of grains. Independently of their utility for the characterization of the foreground at CMB, these laws could be exploited in the future for combined emission-absorption studies in 3D. In addition, unexpected developments have been made in the use of diffuse interstellar bands.
The project thus ends with new results and tools and in an increasingly animated context. Indeed, interest in 3D tomography has grown steadily in recent years, as we had anticipated from the beginning of the proposals (the project was accepted on the fourth attempt). Several teams work from different absorption sources and are waiting for the Gaia catalogues. Much progress is to be expected. For our part, we have an untapped kinematic database and spectroscopic and inversion tools that we will try to use as best we can to make an original contribution to the maps that will continue to improve.

30 articles in journals with referee were published (+1 in review) of which 22 mono and 9 multi-partners. Two international symposiums were organized on the subject (one as main organizer, the second as co-organizer). Several catalogues resulting from the analyses have been published.
Some of the main titles of the publications are:

#3D maps of interstellar dust in the Local Arm: using Gaia, 2MASS and APOGEE-DR14: Lallement R., L. Capitanio, L. Ruiz-Dern, C. Danielski, C. Babusiaux, J.L. Vergely, M. Elyajouri, F. Arenou, N. Leclerc, soumis à A&A (cartes 3D disponibles sur site stilism.obspm.fr)
#Cosmic-rays, gas, and dust in nearby anticentre clouds : III – Dust extinction, émission and grain properties: Remy Q., I.A. Grenier, D.J. Marshall, J.M. Casandjian, A, A&A, sous presse (2018)
#Empirical photometric calibration of the Gaia Red Clump: colours, effective temperature and absolute magnitude: Ruiz-Dern, L.; Babusiaux, C.; Arenou, F.; Turon, C.; Lallement, R.A&A, 609,IdA116, (2017)
#Three-dimensional mapping of the local interstellar medium with composite data: Capitanio L.; Lallement R.; Vergely J. L.; Elyajouri M.; Monreal-Ibero A., A&A, 606,65 (2017)
#The 15 273 Å diffuse interstellar band in the dark cloud Barnard 68: ElYajouri M., Cox, N.L.J., Lallement R., A&A, 605L, 10 (2017)
# On the distance to the North Polar Spur and the local CO-H2 factor: Lallement R., Snowden, S.; Kuntz, K. D.; Dame, T. M.; Koutroumpa, D.; Grenier, I.; Casandjian, J. M.: A&A, 595, 131 (2016)
# Near-infrared diffuse interstellar bands in APOGEE telluric standard star spectra . Weak bands and comparisons with optical counterparts: ElYajouri M., Monreal-Ibero A., Remy Q., Lallement R., , ApJS, 225, 19 (2016)
#The Gaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra. DIB-based interstellar medium line-of-sight structures at the kpc scale: Puspitarini L., Lallement R., Babusiaux, C.; +6, Monreal-Ibero, A.;+18, A&A, 573, 35, (2015)

Our objective is to provide a realistic and detailed three-dimensional (3D) mapping of the interstellar matter (ISM) distribution in the Sun vicinity, within a kiloparsec wide volume. This 3D distribution of gas and dust, based on the inversion of distance-limited data and their combination with integrated 2D maps would be a general product useful for various astronomical analyses and available for inclusion in larger scale Galactic models of gas, dust and radiation field that lack precise positioning of the ISM clouds in the Sun vicinity.
This 3D ISM distribution would represent a preparatory step toward a fuller 3D galactic map to be achieved thanks to the ESA cornerstone space mission Gaia and its related ground-based surveys. A detailed 3D map of the galactic ISM will strongly enhance the scientific return of Gaia by providing the combined stellar and interstellar history, and is otherwise mandatory to break the degeneracy between the star temperature and the reddening for the fainter, more distant objects. The work we propose would allow to develop and test new methods of spectral analysis, inversion techniques and combination of 3D and 2D data that would be directly applicable to future larger scale mappings. It would timely benefit from the new data of the Gaia-ESO Spectroscopic Survey and the ESO-AMBRE project, in complement to all other archival data. Radio spectral cubes with increased resolution have been made available, allowing more detailed comparisons between absorption and emission data. The proposed mapping, whose specificities are the use of individual sight lines and of the gas kinematics, would be an ideal complement to other extinction mapping techniques based on surveys and stellar population statistics.
Our proposed approach would for the first time combine several diagnostics: -i) gas absorption lines, -ii) diffuse interstellar absorption bands (DIBs) that correlate with the gas and/or the dust, –iii) colour excesses imprinted by interstellar (IS) matter in the stellar spectra and derived from spectro-photometry, -iv) “Doppler cubes” of radio line emission surveys at high spatial resolution and large sky coverage, -v) total gas column densities as traced in gamma rays for all gas phases including the “dark” atomic to molecular interface, -vi) total dust column densities as traced by sub-mm to infrared emission. The first three diagnostics have the unique advantage of providing information on the distance to the absorbing matter. These three datasets would be inverted separately and simultaneously to produce gas and dust distributions. The fourth diagnostic lacks distance information but provides a wealth of information on the angular distribution and kinematics. We intent to do major efforts to cross-identify velocity and angular structures in radio maps with absorption line velocities and inverted 3D clouds respectively. Finally, the resulting gas distributions would be calibrated, using the recent determinations of the gas phase composition maps based on Planck, Fermi, and IR surveys, including the "dark gas" phase that is not detected in radio.
Reaching self-consistency between the emission maps and the distribution of gas and dust is an entirely new goal, and to achieve it, the proposing team associates experts in interstellar emission data at all wavelengths, interstellar spectroscopy, stellar photometry, light extinction, and cross correlations between the tracers at all wavelengths. It includes experts with a strong experience in sophisticated inversion methods who have already initiated the development of 3D maps of the solar neighbourhood based on distance-limited data, and others who have worked on the kinematics of the local clouds. Finally, the team also includes members of the Gaia consortium and the Gaia-ESO survey whose experience in stellar spectroscopy and galactic stellar populations will be precious for the preparation of the Gaia ISM tools.