Galaxy formation and evolution

Abstract:

We develop diagnostics based on gas kinematics to identify the presence of a bar in an edge-on spiral galaxy and determine its orientation. We use position-velocity diagrams (PVDs) obtained by projecting edge-on two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy potential. We show that when a nuclear spiral is formed, the presence of a gap in the PVDs, between the signature of the nuclear spiral and that of the outer parts of the disk, reliably indicates the presence of a bar. This gap is due to the presence of shocks and inflows in the simulations, leading to a depletion of the gas in the outer bar region. If no nuclear spiral signature is present in a PVD, only indirect arguments can be used to argue for the presence of a bar. The shape of the signature of the nuclear spiral, and to a lesser extent that of the outer bar region, allows a determination of the orientation of the bar with respect to the line of sight. The presence of dust can also help to discriminate between viewing angles on either side of the bar. Simulations covering a large fraction of parameter space constrain the bar properties and mass distribution of observed galaxies. The strongest constraint comes from the presence or absence of the signature of a nuclear spiral in the PVD.

Abstract:

We develop diagnostics to detect the presence and orientation of a bar in an edge-on disk, using its kinematical signature in the position-velocity diagram (PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral galaxy mass model, we briefly review the orbital properties of two-dimensional nonaxisymmetric disks and identify the main families of periodic orbits. We use those families as building blocks to model real galaxies and calculate the PVDs obtained for various realistic combinations of periodic orbit families and for a number of viewing angles with respect to the bar. We show that the global structure of the PVD is a reliable bar diagnostic in edge-on disks. Specifically, the presence of a gap between the signatures of the families of periodic orbits in the PVD follows directly from the nonhomogeneous distribution of the orbits in a barred galaxy. Similarly, material in the two so-called forbidden quadrants of the PVD results from the elongated shape of the orbits. We show how the shape of the signatures of the dominant x1 and x2 families of periodic orbits in the PVD can be used efficiently to determine the viewing angle with respect to the bar, and to a lesser extent to constrain the mass distribution of an observed galaxy. We also address the limitations of the models when interpreting observational data.

Abstract:

The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near-UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ∼4.5 between 1991 and 1993 and has decreased its luminosity by a factor ∼2.0 between 1993 and 1996. FOS spectroscopy extending from the near-UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (∼3 x 105 L⊙), the spike is definitely placed among active galactic nuclei (AGNs) by current diagnostics based on the emission-line intensity ratios, being just on the borderline between Seyfert galaxies and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modeled with a combination of broad and narrow components, with FWHM of ∼3000 km s-1 and ∼700 km s-1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a star in a close flyby with the BH. The 1996 broad Hα luminosity of this mini-AGN is ∼5.6 x 1037 ergs s-1, about a factor of 2 less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN. Combining all observational constraints, we estimate the mass of the BH at the center of NGC 4552 to be in the range between 3 x 108 and 2 x 109 L⊙. The relevance for the demography of BHs in galaxies of the high (HST) resolution imaging and spectroscopy capable of revealing an extremely low level AGN activity in normal galaxies is briefly discussed.