The Small Scale Physical Evolution of Molecular Gas in Nearby Galaxies

With the term of Interstellar Medium (or ISM) we refer to the gas and the dust that pervade the interstellar space. The ISM consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, dust grains, cosmic rays, and (galactic) magnetic fields. The interstellar medium plays a crucial role in astrophysics because it intermediates between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps to determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation. The distribution and mass of the different components of the ISM (the atomic gas, the cold dense molecular gas, the hot ionized gas, ...) change for different galaxies, according to their Hubble morphological types, peculia-rities, or properties of the environment. Since differences in stellar contents and light distributions of early- and late-type galaxies can be studied as differences in their ISM, the study of the interstellar medium is fundamental for understanding the nature of the galaxies. Observations of different energy ranges give us information on different gas components of the ISM: the optical lines trace ionized regions or HII regions, the infrared range gives information on the radiation from interstellar dust grains, the 2.6 mm emission line of the carbon monoxide molecule is a good tracer of the molecular gas, the 21 cm line traces the atomic gas, and in the meter and decimeter range we can detect strong sources of continuous synchrotron radiation. The study of the interstellar medium in galaxies is currently in a phase of strong technical progress, especially concerning the molecular gas component of the ISM. Both millimeter-wave single-dish telescopes (FCRAO 14m, IRAM 30m, NRO 45m) and interferometers (IRAM, CARMA, NRO, SMA) already perform high resolution observations (<10") resolving (sub)struc-tures of the molecular clouds present in the galaxies. The immediate future with the construction of new instruments, the Atacama Millimeter Array (ALMA) in particular, will bring a great revolution in millimeter astronomy. ALMA, operating at wavelengths between 0.3 to 9.6 millimeters and with resolutions as fine as 0.005", will play a role similar to HST in the optical domain by opening a completely new range of observational possibilities. In this PhD Thesis we focus on a important topic of the ALMA science, the molecular gas in external galaxies in order to prepare ourselves to this revolutionary instrument. We investigate the small scale excitation, fragmentation properties, distribution and kinematics of the molecular gas in two nearby galaxies (Messier 81 and NGC~3147) observed both with single-dish and interferometer instruments. We discuss the problem of the H2-CO conversion factor variation in external galaxies. The value of this factor, defined as X=N(H2)/ICO, is matter of discussion because it appears to depend on the particular physical and chemical conditions of the molecular gas (e.g. temperature, density, and metallicity). The line ratio is also used to derive the physical conditions of the gas, such as excitation temperature and optical depth. In combination and complementary to these two detailed studies on the molecular gas component in specific galaxies, in this Thesis we also present a statistical and interpretative work on the relations existing between different components of the ISM obtained considering a sample of galaxies of all the morphologies, nuclear activities, and type of interaction. Some relations, such as that known between CO and the 100 ?m fluxes for early-type galaxies, are confirmed and extended considering a large and heterogeneous sample of galaxies, other relationships lacking for particular typologies of galaxies are interpreted with the help of chemo-dynamical models. This double approach to the study of the interstellar medium - a detailed analysis of the molecular component of single galaxies and a statistical study of the relations between different components of the ISM for a large sample of galaxies - helped us to understand better the complex scenario of the ISM. Major efforts have been dedicated to the molecular gas component in the galaxies. The Thesis is organized as follows: in the first chapter we describe the principal characteristics of the different phases of the interstellar medium. The second chapter is completely dedicated to the molecular component of the ISM, describing the properties of the principal molecular gas structures (molecular clouds, molecular clumps, ...) and their link with the star formation process. We also introduce the problem of the H2-CO conversion factor variation, parameter always used in the studies of the molecular gas, and describe the molecular gas distribution in galaxies as a function of the morphological type, environment, redshift, and activity. In the third chapter we present a analysis of the relations existing between CO(1-0) line, 100 micron, B, and X-ray fluxes for a sample of 3000 galaxies. The fourth chapter is devoted to a detailed study of the molecular gas component of the M 81 galaxy center. In the fifth chapter we analyze the molecular gas of the Seyfert 2 NGC 3147 galaxy to study the mechanisms for gas fueling of its Active Galactic Nucleus (AGN). In the sixth chapter we summarize our conclusions and describe the role - at the present day - of a Thesis on the molecular gas.