Browsing by Author "Freeman, T"

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    A Hubble Space Telescope study of star formation in the inner resonance ring of NGC 3081
    (IOP Publishing, 2004) Buta, RJ; Byrd, GG; Freeman, T; University of Alabama Tuscaloosa
    We present Hubble Space Telescope Wide Field Planetary Camera 2 images of the inner regions of NGC 3081, an absolute magnitude M-B = - 20.0 early-type barred spiral having four well-defined resonance rings: a nuclear ring, an inner ring, an outer R-1 ring, and an outer R-2' pseudoring. Here we focus on a photometric study of the inner ring, a feature likely associated with an inner 4: 1 resonance near the ends of the bar. The ring is notable for its high contrast and sharp definition, which is due to a significant degree of active star formation. The ring is also notable for its significant intrinsic elongation and parallel alignment with the bar. These characteristics influence the way star-forming sites are distributed around the ring. The ring is lined by numerous blue sources, many of which appear to be slightly diffuse compared with the stellar point-spread function. These blue sources are strongly concentrated within +/-60degrees of the bar axis and follow the Halpha distribution well. The blue sources are much larger than typical Galactic open or globular clusters and may represent young massive clusters like the "populous clusters'' of the LMC and objects seen previously mainly in intermediate- to late-type spiral galaxies. We also present an analysis of the integrated light of the inner ring, to deduce information on its star formation history. A profile analysis is used to separate the ring from the background old disk starlight. High-resolution Fourier analysis is used to search for wavelength-dependent phase shifts along the ring to determine if star-forming sites stay in the ring as they age. The results give an intriguing picture of a galaxy in an advanced evolutionary state where periodic orbits are clearly manifested in the morphology.
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    The inner resonance ring of NGC 3081. II. Star formation, bar strength, disk surface mass density, and mass-to-light ratio
    (University of Chicago Press, 2006-03) Byrd, GG; Freeman, T; Buta, RJ; University of Alabama Tuscaloosa
    We complement our Hubble Space Telescope (HST) observations of the inner ring of the galaxy NGC 3081 using an analytical approach and n-body simulations. We find that a gas cloud inner (r) ring forms under a rotating bar perturbation with very strong azimuthal cloud crowding where the ring crosses the bar major axis. Thus, star formation results near to and "downstream'' of the major axis. From the dust distribution and radial velocities, the disk rotates counterclockwise (CCW) on the sky like the bar pattern speed. We explain the observed CCW color asymmetry crossing the major axis as due to the increasing age of stellar associations inside the r ring major axis. These move faster than the pattern speed. The exterior point of the r ring at the bar major axis has an orbital rate equal to the pattern speed. We show how the perturbation strength can be estimated from the ring shapes and relative spacing over the inner through outer ring regions. The bar strength ( maximum tangential/radial force) appears to be constant from 6 to 15 kpc. We derive how the perturbation, the fractional long wavelength m 2 intensity, and the rotation curve can be used to calculate the disk surface mass density versus radius. The disk surface density at 7 kpc is 13 M-circle dot pc(-2) rising to 19 at 13 kpc. The latter is insufficient by a factor of seven to generate the observed rotation curve, implying halo domination. The surface density may have been reduced at 7 kpc due to inner ring gas cloud scattering. The surface density plus the observed surface brightness gives a disk mass-to-light (M/L) ratio which increases from 7 kpc through 13 kpc, contradicting the usual assumption in bar strength calculations. The simulation ring lifetime of several billion years is consistent with our similar to 400 Myr HST estimates. With a sufficiently high gas cloud surface mass density, our simulations form gas cloud "associations'' near the ends of the bar as observed. Too high a density destroys the ring.
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    The ringed spiral galaxy NGC 4622. I. Photometry, kinematics, and the case for two strong leading outer spiral arms
    (IOP Publishing, 2003-02) Buta, RJ; Byrd, GG; Freeman, T; University of Alabama Tuscaloosa
    The intriguing nearly face-on southern ringed spiral galaxy NGC 4622, the first galaxy definitively shown to have leading spiral structure, is revisited in this paper with new images from the Hubble Space Telescopes (HST) WFPC2, together with ground-based optical and near-IR imaging, and a Fabry-Perot Halpha velocity field. The data provide new information on the disk/bulge/halo mix, rotation curve, star formation in the galaxy, and the sense of winding of its prominent spiral arms. Previously, we suggested that the weaker, inner single arm most likely has the leading sense, based on a numerical simulation. Now, taking advantage of HST resolution and using de Vaucouleurs standard extinction and reddening technique to determine the near side of the galaxy's slightly tilted disk, we come to the more surprising conclusion that the two strong outer arms have the leading sense. We suggest that this highly unusual configuration may be the result of a past minor merger or mild tidal encounter. Possible evidence for a minor merger is found in a short, central dust lane, although this is purely circumstantial and an unrelated interaction with a different companion could also be relevant. The leading arms may be allowed to persist because NGC 4622 is dark halo dominated (i.e., not maximum disk in the inner regions) and displays a significantly rising rotation curve. The new HST observations also reveal a rich globular cluster system in the galaxy. The mean color of these clusters is (V-I)(0) = 1.04, and the specific frequency is 3.4 +/- 0.6. The luminosity function of these clusters confirms the membership of NGC 4622 in the Centaurus Cluster.