A Morphological Method to Determine Co-Rotation Radii in Spiral Galaxies

Shock induced star formation in a stellar density wave scenario produces an azimuthal gradient of ages across the spiral arms which has opposite signs on either side of the corotation resonance (CR). We present a method based on the Fourier analysis of azimuthal profiles, to locate the CR and determine the arm character (trailing or leading) in spiral galaxies. Basically, we compare the behavior of the phase angle of the two-armed spiral in blue and infrared colors which pick out respectively young and older disk stellar population. We illustrate the method using theoretical leading and trailing, spirals. We have also applied the method to the spiral galaxies NGC 7479, for which we confirm the reported leading arms, and NGC 1832. In these galaxies we find two and three CRs respectively.Comment: 9 pages, accepted for publication in ApJL, figures 4 and 6 avaliables at ftp://ftp.inaoep.mx/pub/salida/puerari, full paper also avaliable at http://www.inaoep.mx/~puerar

Lopsided spiral galaxies: evidence for gas accretion

We quantify the degree of lopsidedness for a sample of 149 galaxies observed in the near-infrared from the OSUBGS sample, and try to explain the physical origin for the observed disk lopsidedness. We confirm previous studies, but now for a larger sample, that a large fraction of galaxies show significant lopsidedness in their stellar disks, measured as the Fourier amplitude of the m=1 component, normalised to the average or m=0 component, in the surface density. Late-type galaxies are found to be more lopsided, while the presence of m=2 spiral arms and bars is correlated. The m=1 amplitude is found to be uncorrelated with the tidal forces acting on a galaxy via nearby companions. Numerical simulations are carried out to study the generation of m=1 via different processes: galaxy tidal encounters, galaxy mergers, and external gas accretion and subsequent star formation. The simulations show that galaxy interactions and mergers can trigger strong lopsidedness, but do not explain several independent statistical properties of observed galaxies. To explain all the observational results, it is required that a large fraction of lopsidedness results from cosmological accretion of gas on galactic disks, which can create strongly lopsided disks when this accretion is asymmetrical enough.Comment: accepted for publication in Astronomy and Astrophysics - Final version after language editio

Formation of rings in galactic discs by infalling small companions

We use N-body simulations to study the formation of rings in a disc galaxy by the impact of a small spherical companion. Both barred and nonbarred target discs are considered. We discuss the effect of the properties of the target disc (distribution of mass in the disc, velocity dispersion, etc.) as well as of the mass and orbit of the companion on the properties of the rings, such as their amplitude, width, shape, expansion velocity and lifetime. In particular the amplitude, width, lifetime and expansion velocity of the first ring increase considerably with companion mass, and so does the expansion velocity of the particles in it and the total extent of the disc after the interaction. We also discuss the formation and evolution of spokes and show that they can be caused by companions of relatively small mass. In our three examples of oblique impacts on barred target galaxies we note important transient displacements of the bar, as well as changes of its pattern speed and size. An asymmetric pseudoring is formed in each case, and during the first stages of its evolution the bar forms part of it.Comment: 21 pages Latex, 22 figures, of which 7 as separate gif-files. Requires mn.sty, accepted for publication in MNRAS. High resolution version at http://www-obs.cnrs-mrs.fr/dynamique/pap/ring.htm

Dust-penetrated morphology in the high-redshift universe: clues from NGC 922

Results from the Hubble Deep Field (HDF) North and South show a large percentage of high-redshift galaxies whose appearance falls outside traditional classification systems. The nature of these objects is poorly understood, but sub-mm observations indicate that at least some of these systems are heavily obscured (Sanders 2000). This raises the intriguing possibility that a physically meaningful classification system for high-redshift galaxies might be more easily devised at rest-frame infrared wavelengths, rather than in the optical regime. Practical realization of this idea will become possible with the advent of the Next Generation Space Telescope (NGST). In order to explore the capability of NGST for undertaking such science, we present NASA-IRTF and SCUBA observations of NGC 922, a chaotic system in our local Universe which bears a striking resemblance to objects such as HDF 2-86 (z=0.749) in the HDF North. If objects such as NGC 922 are common at high-redshifts, then this galaxy may serve as a local morphological `Rosetta stone' bridging low and high-redshift populations. In this paper we demonstrate that quantitative measures of galactic structure are recoverable in the rest-frame infrared for NGC 922 seen at high redshifts using NGST, by simulating the appearance of this galaxy at redshifts z=0.7 and z=1.2 in rest-frame K'. Our results suggest that the capability of efficiently exploring the rest-wavelength IR morphology of high-z galaxies should probably be a key factor in deciding the final choice of instruments for the NGST.Comment: 7 pages, 12 Figures. Accepted for publication in A&A. Better version of the figures can be found at http://www.inaoep.mx/~puerari/ngs