Spitzer Space Telescope - General Observer Proposal #40535


Glitters of warm H2 in cold diffuse molecular gas

Principal Investigator: Edith Falgarone
           Institution: Ecole Normale Superieure

     Technical Contact: Edith Falgarone, Ecole Normale Superieure

Co-Investigators: 
Francois Boulanger, IAS Orsay                                         
Francoise Combes, LERMA Paris                                         
Guillaume Pineau des Forets, IAS Orsay                                
Laurent Verstraete, IAS Orsay                                         

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IrsMap 
  Hours Approved: 7.1 

Abstract:
Cold molecular hydrogen, a dominant gas fraction in galaxies, does not radiate
due to the symmetry of the molecule. The only tracers of cold H2, the rotational
lines of CO and dust thermal emission operate only in metal-rich environments.
By detecting the lowest rotational lines of H2 at unexpected levels in the cold
diffuse medium of the Galaxy, ISO-SWS has challenged the traditional view of the
interstellar medium (ISM) by possibly revealing the existence of tiny gas
fractions within the cold ISM, warm enough to excite H2 lines. The heating
source of H2 there is the intermittent dissipation of supersonic turbulence,
pervading the entire ISM. These glitters of H2 line emission may become the
unique tracers of cold H2 in low metallicity environments. Given the fundamental
importance of probing large hidden masses of gas in galaxies, for their
implication on galaxy dynamics, star formation thresholds in metal-poor
environments, and the hypothesis of H2 as baryonic dark matter in galaxies, the
present SST/IRS proposal is dedicated to further search of this still elusive
emission. The proposed observations consist in several IRS LL pointings along
the major axis of two external galaxies with massive HI disks extending far
beyond their optical radius, and for which the HI rotation curve cannot be
accounted for by the stellar and visible gas components. These spectra also
observed in the direction of the galaxy nuclei, are meant to allow the
separation between the H2 emission with stellar-type excitation from that
originating in gas heated by turbulence dissipation. The goal of the proposal is
to strenghten the existence of pockets of warm H2 disseminated in the cold
diffuse medium of galaxies. These glitters of warm H2 would be a new tracer of
hitherto undetected amounts of cold H2 in low metallicity environments, and, as
a more exploratory facet, might probe the presence of large amounts of baryonic
dark matter in galaxies in the form of cold molecular hydrogen.