Browsing by Author "Buta, R. J."

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    AINUR: Atlas of Images of NUclear Rings
    (Oxford University Press, 2010-03-11) Comeron, S.; Knapen, J. H.; Beckman, J. E.; Laurikainen, E.; Salo, H.; Martinez-Valpuesta, I.; Buta, R. J.; Instituto de Astrofisica de Canarias; Universidad de la Laguna; University of Oulu; University of Alabama Tuscaloosa
    We present the most complete atlas of nuclear rings to date. We include 113 rings found in 107 galaxies, six of which are elliptical galaxies, five are highly inclined disc galaxies, 18 are unbarred disc galaxies and 78 are barred disc galaxies. Star-forming nuclear rings occur in 20 +/- 2 per cent of disc galaxies with types between T = -3 and T = 7. We aim to explore possible relationships between the size and morphology of the rings and various galactic parameters. We also aim to establish whether ultra-compact nuclear rings are a distinct population of nuclear rings or if they are merely the low-end tail of the nuclear ring size distribution. We produce colour index and structure maps, as well as H alpha and Pa alpha continuum-subtracted images from Hubble Space Telescope archival data. We derive ellipticity profiles from H-band Two-Micron All-Sky Survey images in order to detect bars and find their metric parameters. We measure the non-axisymmetric torque parameter, Q(g), and search for correlations between bar and ring metric parameters, and Q(g). Our atlas of nuclear rings includes star-forming and dust rings. Nuclear rings span a range from a few tens of parsecs to a few kiloparsecs in radius. Star-forming nuclear rings can be found in a wide range of morphological types, from S0 to Sd, with a peak in the distribution between Sab and Sb and without strong preference for barred galaxies. The ellipticities of rings found in disc galaxies range from c(r) = 0 to c(r) = 0.4, assuming that nuclear rings lie in the galactic plane. Dust nuclear rings are found in elliptical and S0 galaxies. For barred galaxies, the maximum radius that a nuclear ring can reach is a quarter of the bar radius. We found a nearly random distribution of position angle offsets between nuclear rings and bars. There is some evidence that nuclear ring ellipticity is limited by bar ellipticity. We confirm that the maximum relative size of a star-forming nuclear ring is inversely proportional to the non-axisymmetric torque parameter, Q(g) ('stronger bars host smaller rings') and that the origin of nuclear rings, even the ones in non-barred hosts, is closely linked to the existence of dynamical resonances. Ultra-compact nuclear rings constitute the low-radius portion of the nuclear ring size distribution. We discuss implications for the lifetimes of nuclear rings and for their origin and evolution.
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    RING STAR FORMATION RATES IN BARRED AND NONBARRED GALAXIES
    (IOP Publishing, 2010-05-10) Grouchy, R. D.; Buta, R. J.; Salo, H.; Laurikainen, E.; National Optical Astronomy Observatory; University of Alabama Tuscaloosa; University of Oulu; University of Turku
    Nonbarred ringed galaxies are relatively normal galaxies showing bright rings of star formation in spite of lacking a strong bar. This morphology is interesting because it is generally accepted that a typical galactic disk ring forms when material collects near a resonance, set up by the pattern speed of a bar or bar-like perturbation. Our goal in this paper is to examine whether the star formation properties of rings are related to the strength of a bar or, in the absence of a bar, to the non-axisymmetric gravity potential in general. For this purpose, we obtained Ha emission line images and calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA galaxies and four weakly barred SAB galaxies with rings. For comparison, we combine our new observations with a re-analysis of previously published data on five SA, seven SAB, and 15 SB galaxies with rings, three of which are duplicates from our sample. With these data, we examine what role a bar may play in the star formation process in rings. Compared to barred ringed galaxies, we find that the inner ring SFRs and H alpha+[N II] equivalent widths in nonbarred ringed galaxies show a similar range and trend with absolute blue magnitude, revised Hubble type, and other parameters. On the whole, the star formation properties of inner rings, excluding the distribution of H II regions, are independent of the ring shapes and the bar strength in our small samples. We confirm that the deprojected axis ratios of inner rings correlate with maximum relative gravitational force Q(g); however, if we consider all rings, a better correlation is found when a local bar forcing at the radius of the ring, Q(r), is used. Individual cases are described and other correlations are discussed. By studying the physical properties of these galaxies, we hope to gain a better understanding of their placement in the scheme of the Hubble sequence and how they formed rings without the driving force of a bar.