Martin Bureau

Abstract:

ABSTRACT We present a study of molecular structures (clumps and clouds) in the dwarf galaxy NGC 404 using high-resolution (≈0.86 × 0.51 pc2) Atacama Large Millimeter/sub-millimeter Array 12CO(2-1) observations. We find two distinct regions in NGC 404: a gravitationally stable central region (Toomre parameter Q = 3–30) and a gravitationally unstable molecular ring (Q ≲ 1). The molecular structures in the central region have a steeper size–linewidth relation and larger virial parameters than those in the molecular ring, suggesting gas is more turbulent in the former. In the molecular ring, clumps exhibit a shallower mass–size relation and larger virial parameters than clouds, implying density structures and dynamics are regulated by different physical mechanisms at different spatial scales. We construct an analytical model of clump–clump collisions to explain the results in the molecular ring. We propose that clump–clump collisions are driven by gravitational instabilities coupled with galactic shear, which lead to a population of clumps whose accumulation lengths (i.e. average separations) are approximately equal to their tidal radii. Our model-predicted clump masses and sizes (and mass–size relation) and turbulence energy injection rates (and size–linewidth relation) match the observations in the molecular ring very well, suggesting clump–clump collisions are the main mechanism regulating clump properties and gas turbulence in that region. As expected, our collision model does not apply to the central region, where turbulence is likely driven by clump migration.

Abstract:

<jats:title>ABSTRACT</jats:title> <jats:p>The mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) is probing supermassive black holes (SMBHs) in galaxies across the Hubble sequence via molecular gas dynamics. We present the first WISDOM study of a luminous infrared galaxy with an active galactic nuclei (AGNs): Fairall 49. We use new ALMA observations of the CO(2 − 1) line with a spatial resolution of ∼80 pc together with ancillary HST imaging. We reach the following results: (1) The CO kinematics are well described by a regularly rotating gas disc with a radial inflow motion, suggesting weak feedback on the cold gas from both AGN and starburst activity; (2) The dynamically inferred SMBH mass is 1.6 ± 0.4(rnd) ± 0.8(sys) × 108 M⊙ assuming that we have accurately subtracted the AGN and starburst light contributions, which have a luminosity of ∼109 L⊙; (3) The SMBH mass agrees with the SMBH−stellar mass relation but is ∼50 times higher than previous estimates from X-ray variability; (4) The dynamically inferred molecular gas mass is 30 times smaller than that inferred from adopting the Galactic CO-to-H2 conversion factor (XCO) for thermalized gas, suggesting low values of XCO; (5) the molecular gas inflow rate increases steadily with radius and may be as high as ∼5 M⊙ yr−1. This work highlights the potential of using high-resolution CO data to estimate, in addition to SMBH masses, the XCO factor, and gas inflow rates in nearby galaxies.</jats:p>