University of Louisville ThinkIR: The University of Louisville's Institutional Repository Faculty Scholarship 1-2013 Galaxy zoo : a catalog of overlapping galaxy pairs for dust studies. William C. Keel University of Alabama - Tuscaloosa Anna M. Manning University of Alabama - Tuscaloosa Benne W. Holwerda University of Louisville Massimo Mezzoprete Chris J. Lintott Oxford University See next page for additional authors Follow this and additional works at: https://ir.library.louisville.edu/faculty Part of the Astrophysics and Astronomy Commons Original Publication Information Keel, William C., et al. "Galaxy Zoo: A Catalog of Overlapping Galaxy Pairs for Dust Studies." 2013. Publications of the Astronomical Society of the Pacific 125(923): 2-16. This Article is brought to you for free and open access by ThinkIR: The University of Louisville's Institutional Repository. It has been accepted for inclusion in Faculty Scholarship by an authorized administrator of ThinkIR: The University of Louisville's Institutional Repository. For more information, please contact [email protected]. Authors William C. Keel, Anna M. Manning, Benne W. Holwerda, Massimo Mezzoprete, Chris J. Lintott, Kevin Schawinski, Pamela Gay, and Karen L. Masters This article is available at ThinkIR: The University of Louisville's Institutional Repository: https://ir.library.louisville.edu/ faculty/222 Galaxy Zoo: A Catalog of Overlapping Galaxy Pairs for Dust Studies William C. Keel1;2 and Anna M. Manning1;3 Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487 [email protected] Benne W. Holwerda ESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands Massimo Mezzoprete Rome, Italy Chris J. Lintott Astrophysics, Oxford University; and Adler Planetarium, 1300 S. Lakeshore Drive, Chicago, IL 60605 Kevin Schawinski4 Department of Physics, Yale University, New Haven, CT 06511 USA; and Yale Center for Astronomy and Astrophysics, Yale University, P.O.Box 208121, New Haven, CT 06520 and Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 16, CH-8093 Zurich, Switzerland Pamela Gay Center for STEM REO, Campus Box 2224, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026 and Karen L. Masters Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth, PO1 3FX, UK arXiv:1211.6723v1 [astro-ph.CO] 28 Nov 2012 ABSTRACT Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light. We present a catalog of 1990 such galaxy pairs selected from the Sloan Digital Sky Survey (SDSS) by volunteers of the Galaxy Zoo project. We highlight subsamples which are particularly useful for retrieving such properties of the dust distribution as UV extinction, the extent perpendicular to the disk plane, and extinction in the inner parts of disks. The sample spans wide ranges of morphology and surface brightness, opening up the possibility of using this technique to address systematic changes in dust extinction or distribution with galaxy type. This sample will form the basis for forthcoming work on the ranges of dust distributions in local disk galaxies, both for their astrophysical implications and as the low-redshift part of a study of the evolution of dust properties. Separate lists and figures show deep overlaps, where the inner regions of the foreground1 galaxy are backlit, and the relatively small number of previously-known overlapping pairs outside the SDSS DR7 sky coverage. Subject headings: galaxies: spiral | dust | galaxies: ISM 1. Introduction scale height of the dust is much less than that of the stars. Around the same time, Valentijn Dust composes only a small fraction of the (1990) claimed that extensive analysis of surface mass of the interstellar medium in galaxies, but photometry of galaxies from the ESO-LV sample it plays key roles in both their development and suggested nearly constant surface brightness with observed properties. Radiative cooling by dust disk inclination, and much higher internal opacity facilitates the collapse of massive clouds during than previously assumed. These results spurred a episodes of star formation, and grains remain as rebirth in work on this question, leading to a wide a key constituent of protoplanetary disks around range of studies converging on a general picture the nascent stars. At the other end of the stars of high opacity in spiral arms, resonance rings, lives, grains condense in the cooling and enriched and the centers of disks, with diffuse dust fading gaseous envelopes of supergiants, planetary nebu- to very small extinction at the edges of the opti- lae, and the ejecta of novae and supernovae. On cal disks (Thronson 1995). Still, it became clear grander scales, dust influences the evolution of that substantial variations exist among and within galaxies even as its absorption complicates our galaxies. Use of the Galaxy Zoo morphological ability to interpret the observed properties of classifications with color data suggested further galaxies. Correction for dust extinction within complications, such as very luminous disks being galaxies is important for the use of SN Ia as cos- comparatively dust-deficient (Masters et al. 2010). mological probes, especially if the extinction curve All these factors can be exacerbated for galax- varies among galaxies. It also affects distance ies at high redshift. Many have high rates of star measures via the Tully-Fisher relation, through formation, suggesting a rich and massive interstel- the required corrections for internal extinction as lar medium, and are observed most often in the a function of inclination to our line of sight. If emitted ultraviolet, where dust effects are more the extinction is high enough toward the cores of important, compared to the emitted optical range luminous spirals, many straightforward estimates where our basis for comparison is more extensive. of stellar masses and distributions come into ques- Different ways of correcting for internal extinction tion (Driver et al. 2008). We present an extensive give widely varying results, contributing to the dis- catalog of superimposed galaxy pairs, which are parate conclusions obtained by several groups on useful for measuring dust extinction and allow a the history of cosmic star formation, even when much wider study than previously available. starting from the same Hubble data in the deep For decades, the standard view on dust effects fields (e.g., Madau et al. 1996, Thompson, Wey- in galaxies derived from variants of the Holmberg mann, & Storrie-Lombardi 2001). (1958) approach, based on surface brightnesses of The dust content and distribution in galaxies disks viewed at different inclinations to the line of may be approached in several ways. Together with sight. Optically thin disks will increase in surface models of radiative transfer and constraints from brightness when viewed closer to edge-on, while the global energy budget, fits to the spectral en- in the optically thick limit, the surface brightness ergy distributions (SEDs) of galaxies have been will remain constant. Holmberg's data indicated a used to estimate the typical extinction at various mild inclination dependence and correspondingly wavelengths and total dust masses. Models must modest internal extinction. This comforting con- assume the relative distributions of stars and dust, clusion was eventually challenged by Disney et al. as set out by, for example, Witt, Thronson, & (1989) on theoretical grounds; models with very Capuano (1992); dust mixed with the stars gives dusty disks could reproduce the data if the disk much less overall extinction than a simple fore- ground screen. This must be taken into account 1Visiting Astronomer, WIYN Observatory, Kitt Peak National Observatory. KPNO is operated by AURA, Inc. in deriving extinction laws from comparison of ob- under contract to the National Science Foundation jects' spectra, as in the effective extinction law 2SARA Observatory derived by Calzetti, Kinney, & Storchi-Bergmann 3Now at Naval Oceanographic Office, Stennis Space (1999). SED fitting of edge-on spirals consistently Center, MS underestimates the face-on extinction and amount 4Einstein Fellow of UV radiation which is reprocessed (Baes et al. 2 2003, Baes et al. 2010, Kuchinski et al. 1998), so across whole galaxies (Alton et al. 2000, James improved measurements of these quantities are im- et al. 2002). portant in refining models of this kind. Absorption studies have the high angular res- Far-infrared emission provides very direct map- olution of optical or ultraviolet imaging, but are ping of the dust content of galaxies, modulated by limited to regions with background sources that the typical grain temperature. Very cold grains are adequate in solid-angle coverage and our un- remain difficult to detect, and large fractions of derstanding of their properties. When the back- the total dust mass can in principle be found only ground light arises within the galaxy under study, by careful spectral decomposition of the submil- other issues arise in our knowledge of the rela- limetre emission, in view of the dominant contri- tive distribution of stars and dust, and the role of bution of the warmest grains. Far-IR data have scattering; these are approached in such ways as hinted that some spirals have very extensive dust modeling the z-distribution of disk stars of various distributions, at least through a larger disk scale kinds to interpret