Modal approach to the morphology of spiral galaxies. I - Basic structure and astrophysical viability.

view Abstract Citations (238) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Modal Approach to the Morphology of Spiral Galaxies. I. Basic Structure and Astrophysical Viability Bertin, G. ; Lin, C. C. ; Lowe, S. A. ; Thurstans, R. P. Abstract The principal objective of the investigations described in the present paper is the demonstration of the viability of the modal approach to global spiral structures in galaxies of all Hubble morphological types. This is done through (1) the identification of the appropriate basic states of galaxy models from the dynamical point of view, and (2) the demonstration of their compatibility with observations from the physical point of view. The modal approach is preferred to a direct evolutionary approach because it is believed that the observed spiral structure in the majority of galaxies is associated with the late states of an evolutionary process and that the pitch angle of the spiral arms of a galaxy can be used as a criterion for determining its Hubble type. From a dynamical point of view, it is shown that barred spiral modes are likely to occur for relatively large disk masses. Specifically, the ratio R_A_ for active disk/total mass (see Section III) within four exponential scale lengths for such spiral galaxies should be on the order of 30% or larger. There is in general only a single important unstable mode; thus, a regular quasi-stationary barlike structure may be expected. Normal spirals occur for lower active disk masses, especially those with lower pitch angles (Sa and Sb galaxies). From the physical point of view, this lower active disk mass is associated with the three-dimensional distribution of the stellar component, including the relatively large nuclear bulge. A long-standing challenge in the case of normal spirals of small pitch angle is resolved. A new perception of the basic structure of these galaxies is proposed. For such normal spirals, the gaseous component plays an essential role, both in the excitation of the unstable spiral modes and in the stable maintenance of the final spiral structure. Indeed, dynamical studies indicate that a condition of marginal instability must be realized at large galactocentric distances and that a large part of the active galactic disk, with the possible exception of the central part, must be relatively "cool." Physically, this is realistic because of the three-dimensional distribution of stellar mass, especially in the central regions. The length scale of the active mass distribution is often found to be about twice as large as the exponential length scale of the optical disk. For the type of basic states considered, we expect in many cases two-armed spiral structures extending over a few exponential length scales, often with multiple winding. In particular, when the number of important spiral modes is small, the spiral structure is expected to have a highly regular grand design and to evolve in time in a quasistationary manner. Publication: The Astrophysical Journal Pub Date: March 1989 DOI: 10.1086/167182 Bibcode: 1989ApJ...338...78B Keywords: Astronomical Models; Cosmology; Galactic Structure; Spiral Galaxies; Astronomical Spectroscopy; Dark Matter; Density Distribution; Galactic Bulge; Galactic Evolution; Galactic Mass; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE; STARS: STELLAR DYNAMICS full text sources ADS | data products SIMBAD (13) NED (13) Related Materials (2) Part 2: 1989ApJ...338..104B Part 3: 1994ApJ...427..184L