Resonance Rings in Early SB Galaxies: Estimating Bar Strength, Disk Star Formation, Mass Density and M/L
Gene BYRD
University of Alabama
Résumé :
We use our HST observations of the inner (r) and outer (R1 ) rings of the galaxy
NGC3081 (Buta, Byrd, & Freeman 2003) with n-body simulations to confirm a new
analytical ring formulation (Byrd, Freeman & Buta 2006). The formulation takes
into account the strong response for these rings which are just inside and outside
co-rotation of the bar and orbital motion. We show how r and R1 sizes and shapes
(plus the rotation curve) give the strength and pattern speed of the bar potential
perturbation. The NGC3081 HST observations show individual clusters in the inner
ring, some of which appear to be as old as 400 Myr. We see evidence of aging in the
cluster luminosity function and colors in orbital angle from the inner ring tips.
Disk surface mass density (µ) and mass-to-light ratio (M/L) is known with real
certainty only at the solar orbital radius for the Milky Way Galaxy (MW) (Flynn et
al 2006). For other disk galaxies, there are mutually contradictory needs for
accurate
velocity dispersion and thickness measurements normal to the disk. We derive how
to use r and outer R1 rings to get disk µ\x{2019}s. Using Fourier surface brightness
components, we obtained disk µ\x{2019}s from the r through R1 ring radii for NGC3081 and
new results for NGC6782. As with the MW, we also find the disk dynamics of both
galaxies to be halo-dominated. Disk M/L's are also calculated. Contrary to
common assumptions, we find that the disk M/L is not constant for either NGC 3081
or 6782 but increases with radius. Having a variety of M/L\x{2019}s at different radii
shows promise for studying secondary M/L indicators and disk star formation
history.
