Molecular Astronomy

Galaxies possess large reservoirs of molecular gas where the formation of new stars is continuously powered. The gas content of a galaxy is composed from simply atoms to complex molecules, which form and survive in the cold medium exposed to destructive ultraviolet radiation. In that sense molecules are a diagnostic probe of the excitation, kinematics, temperature, density and chemistry of the interstellar medium (ISM) of a galaxy. All these physical and chemical characteristics of a galaxy are crucial to derminate its morphology, formation and evolution.

Thanks to the periodical improvement of the existent facilities, during the last decades a growing number of molecules have been identified in the interstellar medium (ISM). The CO emission lines have been used  as the common cold gas tracer, basically because CO the most abundant molecule after H₂ molecule and it has a small dipole moment, so the CO transition lines are easily excited at low densities and low temperatures. However, other complex molecules such as HCN, NHC, HCO⁺, CN, CS, OH and so on, are used trace the highest-density gas regions in the ISM. Our group have carried out extensive observational campaigns in order to study the cold molecular in galaxies. For these observations we have used some of the current facilities such as the 30m IRAM single dish, PdBI, ALMA, APEX and JCMT.

Our studies includes the systematic CO (1-0) characterization of the largest sample of (U)LIRGs so far, as well as a CO (1-0) survey towards a sample of type 2 quasars at intermediate redshift carried out using the 30m IRAM telescope. The molecular gas content of these objects was quantified based in their CO luminosity. Such results were also helpful to inquiry a possible connection between the QSO2, QSO1 and ULIRG-quasar evolutionary scenarios (see Villar-Martín et al., 2013, and Rodríguez et al., 2104).

In a more detailed way, a H₂CO survey was carried out using both IRAM facilities the 30m single dish and the Plateau de Bure interferometer (PdBI) toward a two starburst galaxies (SB).  We used three emission lines (at millimeter wavelengths) of the H₂CO molecule. Our main results lead to the conclusion that H₂CO is tracing the warm  and high-density molecular gas component in those galaxies. These results, confirm that the selected formaldehyde emission lines can be used as a very powerful tool to determine the physical conditions (i.e. temperature and density) in SB galaxies. We are currently woking in the publication of these results.

The new generations of telescopes in the radio regime (ALMA, NOEMA) will allows us to expand our knowledge about the molecular content of galaxies. At the unprecedented high resolution and sensitivity offered by these instruments we will be able to derive the physical conditions of the ISM, the efficiency star formation, the gas kinematics, and the dynamical masses of the galaxies in function of the redshifts. Such results will provide us the clues to inquirer the various processes of galaxy formation and their evolution.