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Galactic Rings Revisited. I. CVRHS Classifications of 3962 A MNRAS 000, 1–?? (2016) Preprint 21 July 2017 Compiled using MNRAS L TEX style file v3.0 Galactic Rings Revisited. I. CVRHS Classifications of 3962 Ringed Galaxies from the Galaxy Zoo 2 Database Ronald J. Buta⋆ Department of Physics & Astronomy,University of Alabama, Box 870324, Tuscaloosa, AL 35487 Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Rings are important and characteristic features of disc-shaped galaxies. This paper is the first in a series which re-visits galactic rings with the goals of further understand- ing the nature of the features and for examining their role in the secular evolution of galaxy structure. The series begins with a new sample of 3962 galaxies drawn from the Galaxy Zoo 2 citizen science database, selected because zoo volunteers recognized a ring-shaped pattern in the morphology as seen in Sloan Digital Sky Survey colour images. The galaxies are classified within the framework of the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. It is found that zoo volun- teers cued on the same kinds of ring-like features that were recognized in the 1995 Catalogue of Southern Ringed Galaxies (CSRG). This paper presents the full cata- logue of morphological classifications, comparisons with other sources of classifications, and some histograms designed mainly to highlight the content of the catalogue. The advantages of the sample are its large size and the generally good quality of the images; the main disadvantage is the low physical resolution which limits the detectability of linearly small rings such as nuclear rings. The catalogue includes mainly inner and outer disc rings and lenses. Cataclysmic (“encounter-driven”) rings (such as ring and polar ring galaxies) are recognized in less than 1% of the sample. Key words: galaxies: general – galaxies: structure – galaxies: spiral 1 INTRODUCTION mation”) κ = m(Ω−Ωp), m being an integer. A galaxy whose shape is dominated by resonances can look very distinctive One approach to understanding galaxy evolution is system- compared to ones which are not (e. g., NGC 3081, Buta et atic examination of the fine details of galaxy morphology. al. 2004). Resonances can lead to slow changes in the struc- Hidden in these details are not only clues to the basic fac- ture of a galaxy, making them an important factor in the tors that shape galaxies, but also how different galaxy types secular evolutionary process (Kormendy 2012). might be related in an evolutionary sense. This is the domain of the emerging paradigm of galaxy secular evolution, which Rings have attracted attention because they are fairly is believed to be the predominant mechanism of change in common features of disc galaxies, and because the morpho- logical and star formation characteristics of distinct types of arXiv:1707.06589v1 [astro-ph.GA] 20 Jul 2017 the present epoch (Kormendy & Kennicutt 2004). Rings are features that have the potential to tell us a rings seem to link them to specific resonances, mainly the great deal about galaxy evolution and dynamics (Kormendy inner and outer Lindblad resonances (ILR and OLR, respec- m ± 1979, 2012; Buta & Combes 1996). Rings are mainly a phe- tively, for which = 2) and the inner and outer 4:1 reso- m ± nomenon of disc galaxies, which by virtue of their high an- nances (I4R and O4R, respectively, for which = 4). Res- gular momentum content are prone to the development of onances are not the only way of explaining the properties well-defined internal perturbations such as bars, ovals, and of galactic rings. The recently proposed “manifold theory” global spirals. These patterns are thought to rotate at a (Athanassoula et al. 2009a,b; 2010) has also had consider- able success in explaining ring morphologies. Deciding which uniform rate, the pattern speed Ωp, while the stars and gas clouds in a galaxy rotate differentially with an angular ve- of these views is most favored will depend on observations locity Ω(r) and a radial oscillation frequency κ(r), where r is beyond the scope of this paper. The emphasis here will be the galactocentric radius. This will set up orbital resonances on the resonance interpretations of the rings. at positions where (in the first order or “epicyclic approxi- Normal galactic rings occur in at least three basic types: nuclear, inner, and outer rings. Nuclear rings are the small rings often found at the centers of early-to-intermediate type ⋆ E-mail: [email protected] barred galaxies. In the most extensive study to date, the At- c 2016 The Authors 2 Ronald J. Buta las of Nuclear Rings (AINUR), Comer´on et al. (2010) found such as inner, outer, and nuclear lenses, nuclear rings and that nuclear rings occur in 20%±2% of galaxies in the type bars, bar ansae, barlenses, X patterns, thick discs, extrapla- range S0− to Sd, have typical sizes ranging from 0.2-2kpc, nar discs, and other features. The system is described in the and may be physically smaller in galaxies with longer bars. de Vaucouleurs Atlas of Galaxies (Buta et al. 2007=deVA) Nuclear rings can also be the sites of intense starbursts (e. and is extended further by Buta et al. (2015). The main goal g., Benedict et al. 2002). Inner rings are intermediate-sized of this classification is to provide a northern hemisphere sam- (major axis diameter ≈2-12kpc) features which envelop a ple of ringed galaxies comparable in scope and size to the bar if present, while outer rings are large, diffuse structures CSRG. This catalogue will be called the “GZ2 Catalogue of about twice the size of a bar or inner ring. In many cases Northern Ringed Galaxies,” or GZ2-CNRG. we can see that a ring is actually made of tightly-wrapped Section 2 describes the Galaxy Zoo 2 project and how spiral arms, leading to the concept of “pseudorings.” it was used to compile the GZ2-CNRG. Section 3 discusses Considerable information on the intrinsic shapes, rel- the sample selection and classification procedure. An analy- ative bar-ring orientations, and relative sizes of inner and sis of the morphological information is presented in section outer rings, pseudorings, and lenses is provided by the Cata- 4, while section 5 presents small tables and illustrations of logue of Southern Ringed galaxies (CSRG; Buta 1995=B95), examples of different morphologies from the catalog. A sum- the Atlas of Resonance Rings as known in the S4G (AR- mary is given in section 6. RAKIS; Comer´on et al. 2014), and the Near-Infrared S0 Sur- vey (NIRS0S; Laurikainen et al. 2011). The CSRG is based on a visual search for ringed galaxies using film copies of 2 GALAXY ZOO 2 the photographic ESO/SRC IIIa-J (≈B-band) Southern Sky Survey, while the ARRAKIS is based on digital images taken The Galaxy Zoo Project (Lintott et al. 2008) has been and at the mid-infrared wavelength of 3.6µm for the Spitzer Sur- continues to be a remarkably effective amateur-professional 4 vey of Stellar Structure in Galaxies (S G, Sheth et al. 2010). collaboration designed to provide basic classifications for the The NIRS0S is a compilation of 2.2µm groundbased digital hundreds of thousands of relatively well-resolved galaxies images of 206 nearby early-type galaxies, of which 193 are imaged in the SDSS (DR 7). The colour images examined − in the type range S0 to Sa. by zoo volunteers are constructed from separate images ob- Both the ARRAKIS and the NIRS0S catalogues are tained in green (g-band, 477 nm), red (r-band, 623 nm), statistically well-defined samples. In the ARRAKIS study, and near-infrared (i-band, 762.5 nm) filters. In these images, Comer´on et al. (2014) found that inner rings and pseudor- isophotes of only a few percent of the night sky brightness ings occur in 35% ± 1% of the sample galaxies, with a high- (corresponding to surface brightnesses of ≈26.0, 25.5, and est abundance of 60% for stages S0+ to Sb, and that outer 25.0 mag arcsec−2 in g, r, and i, respectively) are readily de- rings occur in 16% ± 1% of the galaxies, rising to 40% when tectable in an image display. The typical full width at half restricted to stages S0+ to Sab. For stages later than Sb, maximum of the point spread function is 3.1 ± 0.5 pix, 3.0 ± 4 outer rings are much rarer than inner rings in the S G sam- 0.4 pix, and 2.9 ± 0.4 pix for g, r, and i, respectively, where ple, dropping below the 10% level. In the NIRS0S study, 1 pixel = 0.′′396. Laurikainen et al. (2011) found that 61% of the barred and The philosophy of Galaxy Zoo is that human visual clas- 38% of the nonbarred galaxies in the sample have lenses, and sification is superior to computer classification with respect that multiple lenses are common among S0 to S0/a galaxies. to reliable identification of major and minor classes, and has 4 Although the CSRG, S G, and NIRS0S provide a great the added advantage that it can allow recognition of spe- deal of information on galactic rings and related features, cial cases of interest (e.g., Hanny’s Voorwerp, Lintott et al. all have limitations. The CSRG only includes galaxies south 2009; Schawinski et al. 2010) that a computer program likely of declination −20o, and as a result very few galaxies in would overlook. Computer classification also often ignores the catalogue have publicly available digital imaging. Also, the finer details of galaxy morphology because these are dif- 4 the S G and the NIRS0S have sharp limits to inclusion, the ficult to characterize compared to the more basic galactic former emphasizing extreme late-type, very nearby galaxies components.
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