Disk-halo interaction: the molecular clouds in the galactic center region

Resumen

The formation and evolution of galaxies is one of the hot topics in Astrophysics. In particular, processes like gas accretion through disk-halo interaction and bar potential, together with galactic winds and superwinds of massive star formation, are thought to play a key role in the evolution of galaxies. The Milky Way, and in particular the Galactic center (GC), offers an excellent opportunity to explore in detail the chemical and physical processes associated to the accretion and ejection phenomena present in galactic nuclei. In this thesis, we study two kinds of phenomena which are presumably occurring in the GC region, and are believed to be partly responsible for the gas accretion toward the nuclear region of the Galaxy: the barred potential, and the Giant Molecular Loops (GMLs). Binney et al. (1991) have shown that the large-scale gas kinematics in the central regions of the Galaxy can be accounted for by a barred galactic potential, in which there are two major families of orbits inside the bar: the X1 orbits parallel to the bar, and the inner X2 orbits, orthogonal to it. The gas falls from X1 to X2 orbits when it self-intersects in the X1-X2 orbits transition. The other phenomenon are the huge loop structures (GMLs) found by Fukui et al. (2006) and thought to be formed by a Parker instability. The gas of the loops is thought to flow down along the magnetic field lines, and join the Galactic plane, generating shock fronts at the ¿foot points¿ of the loops. However, so far there is no clear observational evidence of gas accretion from the X1 orbits and/or from high latitudes in the GMLs. In this thesis the high altitude clouds found in the GML will be referred as "halo" to differentiate from the molecular clouds in the Galactic plane. In this thesis, we first performed a large-scale study of the GC region in SiO(2-1), HCO+(1-0), and H13CO+(1-0) (Riquelme et al 2010b ,A&A, 523, A45) using the 4 m-NANTEN telescope. We found an increased emission in SiO as compared to the HCO+ line intensity at the foot points of the GMLs and toward the 1.3 complex, which indicates the presence of shocks (Martín-Pintado et al., 1992, 1997). From this study we selected 7 positions as places where the SiO emission shows an enhancement with respect to the HCO+ emission. The positions are considered in this thesis as the "interaction regions", because they mark the places where gas in the GC could be interacting with gas coming from higher latitude (disk-halo interaction) or from larger galactocentric radius, according to the GMLs scenario and the bar potential model, respectively. They were selected including five positions in the GMLs and two positions in the X1-X2 orbits interaction places. The positions placed in the GMLs were called halo as previously mentioned. Among them, 3 positions are placed where the gas in the GMLs joins the gas in the disk (foot points of the loops), and 2 positions are placed at the top of the loops. Each of the positions located at the X1-X2 orbits interaction places has two main kinematical components, one associated to the X1 orbits and the other to the X2 orbits. They were called as disk X1, and disk X2. Finally, we selected 2 additional positions close to b = 0 that we called disk which are not associated to neither the GMLs nor the locations of the orbits interaction, and therefore they should trace the typical GC clouds. To test if the GMLs scenario is correct, or if the gas is being ejected from the disk in the GC, we studied the 12C/13C isotopic ratio to trace gas accretion/ejection. Using the IRAM 30m-telescope, we observed the J = 1-0 rotational transition of HCO+, HCN, HNC, and their 13C isotopic substitutions toward the 9 selected positions. While 12C is predicted to be formed in first-generation, massive stars on rapid timescales, 13C is produced primarily via CNO processing of 12C seeds from earlier stellar generations, on a longer timescale. The 12C/13C isotopic ratio shows, therefore, the relative degree of primary to secondary processing in stars. We found a systematically higher 12C/13C isotopic ratio (> 40) toward the halo and the X1 orbits than for the GC standard molecular clouds (20-25). The high isotopic ratios are consistent with the accretion of the gas from the halo and from the outskirts of the Galactic disk (Riquelme et al 2010a, A&A, 523, A51). The next step was to study the physical conditions of the molecular gas in the interaction regions of the GC. Using the Effelsberg 100m-telescope, we measured the metastable inversion transitions of NH3 from (1,1) to (6,6) in 6 positions that can be observed from this site. Using rotational diagrams and large velocity gradient calculations, we estimated for the first time, the kinetic temperatures of the molecular clouds in these regions. We derive two kinetic temperature regimes (one warm at ~200 K and one cold at ~40 K) for all the positions, except for the halo where only the warm component is present. Using the 30m-telescope, we also observed molecular tracers of different physical processes like SiO, HNCO, CS, C18O, and 13CO (shocks, photodissociation, dense gas). The fractional abundances derived from the different molecules support the shock origin of the heating mechanism in the GC (Riquelme et al 2012a, A&A, submitted). Finally, we used the Mopra telescope to map one molecular cloud (M-3.8 + 0.9) at the foot points of the GMLs in 3mm lines. The maps reveal structures at small scales in the SiO emission, and large differences between the spatial distribution of the SiO and the HCO+. The SiO emission in the M-3.8 + 0.9 cloud presents narrow profiles (20 km/s) in comparison with the HCO+ profiles (50 km/s), thus, shocked gas is dynamically more confined than the HCO+ emission (Riquelme et al 2012b, in prep.). Overall, the results presented and discussed in this thesis give important clues to the understanding of the nature of the molecular clouds in the Galactic center region. They evidence the presence of shocks and are consistent with the shock origin of the heating mechanisms. Furthermore, this work strongly supports the accretion of the gas towards the GC (through bar potential and GMLs) as the cause of such shocks.