The Molecular Hydrogen Explorer (H2EX)

In answer to the ESA Cosmic Vision 2015-2025 Call, we have submitted (PI Francois Boulanger, IAS) a proposal for a medium class astronomy mission to survey for molecular hydrogen across the universe by direct, high resolution, spectroscopy of H2 utilising its lowest rotational transitions. H2EX will be the first mission dedicated to the observation of the most abundant, but elusive, molecule in space. Its payload will be the first wide field spectro-imager built for the observation of molecular gas. The Molecular Hydrogen Explorer, H2EX, is designed to detect H2 at high spectral resolution, in its pure rotational emission lines, over wide fields in our Galaxy and through cosmic time. H2EX will provide an inventory of molecular gas in a representative choice of star forming regions of the Milky Way, in a sample of galaxies of the local Universe, and further out to redshifts 2-3. With its 20'x20' field of view, H2EX will measure the mass, density and temperature of warm (T > 100K) molecular gas in a wide variety of objects, including nearby regions of star and planet formation, galactic molecular clouds, nearby galaxies, infrared luminous galaxies, and active galactic nuclei. The high spectral resolution of H2EX makes it uniquely suited to study the dynamics of H2 in the environment of newly born stars, in their planet forming disks, and on larger scales at the boundaries of galaxies and the intergalactic medium. Furthermore, H2EX is eminently suited to search for spectral signatures from organic molecules without permanent dipole moments that thereby lack microwave rotational emission. Through these observations H2EX can address key questions within two of the main Cosmic Vision themes: How is the luminous Universe taking shape? What is the H2 contribution to the missing baryons? What controls star formation efficiency? How do giant planets form? Where do complex organic molecules form? Pioneering space borne observations of H2 rotational lines were performed both with the Infrared Space Observatory (ISO) and substantially expanded with Spitzer; however, reduced imaging capability limited these missions to the study of individual objects and very little mapping. The H2EX imaging spectrometer is designed to observe H2 in the 0-0 S(0) to S(3) rotational lines, at 28.2, 17.0, 12.3 and 9.6 µm, over a 20'x20' field, at a spectral resolution of up to 30000, through narrow-band filters. In another mode, at medium and low resolution, the 8 to 25 µm range will be continuously accessible to enable observation of red shifted H2. In infrared luminous galaxies, the S(5) line at 6.9 µm is observed to be comparably bright to the S(1) and S(3) lines. H2EX can observe this line up to redshift 2.5. The rich array of molecular, atomic and ionic lines, as well as solid state features available in this spectral range brings additional science dimensions to H2EX. The spectral resolution can be tuned to each source type: circumstellar disks, galactic molecular clouds, local and distant galaxies, so as to resolve the line widths. H2EX will be placed in an L2 orbit to passively cool the telescope and spectrometer and thereby minimize the thermal background noise. Cryogenics are needed to keep the detectors and cold filters at 5 K. The specific thermal requirements and pointing accuracy are met with the space-qualified Planck cryogenic chain and the Herschel attitude orbit control system. Leading institutes from ESA member states, the United States and Canada are partners in this project. The mission plan is to perform legacy programs based on the full scientific potential of the H2EX spectro-imager to achieve the mission key science goals, within a nominal two years mission. The observations will produce data cubes that combine spatial and spectral information. The data will be archived and made available to the scientific community as soon as they have been scientifically validated, in order to maximize the scientific return. The archive will have great potential for serendipitous discoveries. It will have a lasting legacy value for future research on planet formation, interstellar matter physics and chemistry, star formation and galaxy evolution. It will provide new classes of targets selected on their spectral characteristics for follow-up observations with MIRI, ALMA and extremely large optical telescopes. The Italian participation is foreseen in the definition of scientific objectives (galaxy build-up), software development (post basic calibrated data) and hardware development (fore-optics segment and spectrometer calibration).  

People: A. Bressan (Italian PI), A. Baruffolo, C. Bonoli, F. Bortoletto, M. Clemens, G. De Zotti, G.L. Granato, O. Vega

Collaboration: L. Danese (SISSA), C. Pernechele (INAF OA Cagliari), L. Silva (INAF OA Trieste), H2EX Consortium

Publications: Boulanger et al. (2008),   ExA 23,277