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Browsing by Author "Bennert, Nicola"

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    Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies
    (Wiley-Blackwell, 2009) Cardamone, Carolin; Schawinski, Kevin; Sarzi, Marc; Bamford, Steven P.; Bennert, Nicola; Urry, C. M.; Lintott, Chris; Keel, William C.; Parejko, John; Nichol, Robert C.; Thomas, Daniel; Andreescu, Dan; Murray, Phil; Raddick, M. Jordan; Slosar, Anze; Szalay, Alex; VandenBerg, Jan; Yale University; University of Hertfordshire; University of Nottingham; University of California System; University of California Santa Barbara; University of Oxford; University of Alabama Tuscaloosa; Drexel University; University of Portsmouth; Johns Hopkins University; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California Berkeley
    We investigate a class of rapidly growing emission line galaxies, known as 'Green Peas', first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in Sloan Digital Sky Survey imaging. Their appearance is due to very strong optical emission lines, namely [O iii] lambda 5007 A, with an unusually large equivalent width of up to similar to 1000 A. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some active galactic nuclei interlopers including eight newly discovered narrow-line Seyfert 1 galaxies. The star-forming Peas are low-mass galaxies (M similar to 108.5-1010 M(circle dot)) with high star formation rates (similar to 10 M(circle dot) yr-1), low metallicities (log[O/H] + 12 similar to 8.7) and low reddening [E(B - V) < 0.25] and they reside in low-density environments. They have some of the highest specific star formation rates (up to similar to 10-8 yr-1) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myr. The few star-forming Peas with Hubble Space Telescope imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to luminous blue compact galaxies. They are also similar to high-redshift ultraviolet-luminous galaxies, e.g. Lyman-break galaxies and Ly alpha emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe.
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    Galaxy Zoo: 'Hanny's Voorwerp', a quasar light echo?
    (Oxford University Press, 2009) Lintott, Chris J.; Schawinski, Kevin; Keel, William; van Arkel, Hanny; Bennert, Nicola; Edmondson, Edward; Thomas, Daniel; Smith, Daniel J. B.; Herbert, Peter D.; Jarvis, Matt J.; Virani, Shanil; Andreescu, Dan; Bamford, Steven P.; Land, Kate; Murray, Phil; Nichol, Robert C.; Raddick, M. Jordan; Slosar, Anze; Szalay, Alex; Vandenberg, Jan; University of Oxford; Yale University; University of Alabama Tuscaloosa; University of California System; University of California Riverside; University of California Santa Barbara; University of Portsmouth; Liverpool John Moores University; University of Hertfordshire; Johns Hopkins University; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California Berkeley
    We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in the SDSS g band due to unusually strong [O III]4959, 5007 emission lines. We present the results of the first targeted observations of the object in the optical, ultraviolet and X-ray, which show that the object contains highly ionized Gas. Although the line ratios are similar to extended emission-line regions near luminous active galactic nucleus (AGN), the source of this ionization is not apparent. The emission-fine properties, and lack of X-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy's own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10(5) yr. This object may thus represent the first direct probe of quasar history on these time-scales.