Physical properties of giant molecular clouds in nearby starburst galaxies

In this dissertation, I have explored the properties of molecular gas in the nuclei of four nearby, gas-rich, actively star forming galaxies. Three barred galaxies (IC 342, Maffei 2, and NGC 6946) and one possible merger remnant (NGC 4826) have been mapped at high spatial resolution (∼3 , or 30–100 pc) in CO, CO, CO, 3 mm, and 1 mm continuum using the Owens Valley Millimeter Array. The two closest and brightest galaxies of the sample, IC 342 and Maffei 2, have been mapped in the and transitions of each isotopomer. ComJ p 2–1 J p 1–0 parisons between the and transitions provide J p 2–1 J p 1–0 the first determination of the physical conditions of individual giant molecular clouds (GMCs) in the centers of large spiral galaxies other than our own. Variations in physical properties of the clouds are related to their spatial locations within these galactic centers and to the star formation. The line ratios in IC 342 paradoxically imply optically DJ thin CO and optically thick CO. The assuredly optically thin CO gives yet another picture. A model is developed in which strong external radiation fields create photodissociation regions (PDRs), with high kinetic temperatures in the outer, UVilluminated cloud surfaces and progressively cooler gas toward the centers of the clouds. Because CO and CO have lower abundances and optical depths, they sample most of the molecular gas in the GMCs and hence are less influenced by the highly emissive outer edges of the molecular clouds compared to CO. These isotopomers yield a more robust measure of the true temperatures and densities of the molecular gas in these starburst regions than does the overluminous CO (D. S. Meier, J. L. Turner, & R. L. Hurt 2000, ApJ, 531, 200). The isotopic line ratios, such as CO(1–0)/CO(1–0), vary by as much as a factor of 4 over ∼100 pc scales in these galaxies. The isotopic ratio minima (CO/CO ∼ 3–4) tend to trace the locations of dense GMCs seen in HCN(1–0). In the three barred galaxies, there are large off-arm regions having very high isotopic line ratios (CO/CO 1 20), best explained by the presence of an extended low-density molecular gas component. For the conditions prevalent in these galactic centers ( –40 K and cm ), CO is warm and 2.5 4.5 3 T ∼ 10 n ∼ 10 –10 kin H2 thermalized while the rarer isotopomers are subthermally excited. Measurements of molecular cloud masses are derived from four independent methods—optically thin CO isotopomers, 1.3 mm dust continuum emission, near-IR dust extinction, and virial sizes and line widths—have been made in an effort to constrain the true gas column density in each galaxy. Masses obtained by three of the four methods generally agree. The exception is the estimated virial masses. The virial masses are always the largest; this may indicate that the line widths are dominated by systematic orbital motions and that the GMCs are in fact not virialized. At no location in any of the four galaxies is the Galactic conversion factor obtained. In IC 342 and Maffei 2, XCO appears to be lower than the Galactic value by factors of 3–4 toward the dense gas peaks and higher elsewhere. Similar values are derived for NGC 6946 and NGC 4826. Evidently, molecular masses in the centers of galaxies have been significantly overestimated by the use of the Galactic XCO (D. S. Meier & J. L. Turner 2001, ApJ, 551, 687). Active star formation occurs at the sites of the densest gas. However, a significant amount of dense gas is currently sterile. The main governor of physical conditions, particularly density, appears to be dynamical in nature. In all three barred galaxies, the molecular gas closely follows the predicted orbits found in barred potentials. The observed barlike features are 0.3–1.0 kpc in scale, smaller than any large-scale (galaxy class defining) bar, and are typically confined to the large-scale inner Lindblad resonance of each galaxy. CO and CO tend to be brighter relative to CO on the leading sides of the arms traced out by the orbitals. Toward the center of the three barred galaxies, x1 the gas appears to pile up into a ring, presumably associated with the perpendicular bar orbits. Comparisons with HCN x2 show that the densest gas (and star formation) picks out the intersections of the two orbital families. Modeled gas inflow rates of each nucleus match the observed star formation rates. Moreover, the theoretically predicted “spray regions” in barred potentials naturally explain the presence and locations of the extended low-density gas component inferred from the regions of high CO(1–0)/CO(1–0). These results suggest that these small nuclear bars are driving inflows of gas toward the centers of the galaxies, triggering the starburst. By contrast, in NGC 4826, the lone unbarred galaxy in the sample, there is no current starburst, and no noncircular motions or rapidly varying physical conditions are seen. The physical properties of both