Browsing by Author "Lintott, Chris J."
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Item Black Hole Growth and Host Galaxy Morphology(2010) Schawinski, Kevin; Urry, C. Megan; Virani, Shanil; Coppi, Paolo; Bamford, Steven P.; Treister, Ezequiel; Lintott, Chris J.; Sarzi, Marc; Keel, William C.; Kaviraj, Sugata; Cardamone, Carolin N.; Masters, Karen L.; Ross, Nicholas P.; Galaxy Zoo Team; University of Alabama TuscaloosaWe use data from large surveys of the local universe (SDSS+Galaxy Zoo) to show that the galaxy–black hole connection is linked to host morphology at a fundamental level. The fraction of early-type galaxies with actively growing black holes, and therefore the AGN duty cycle, declines significantly with increasing black hole mass. Late-type galaxies exhibit the opposite trend: the fraction of actively growing black holes increases with black hole mass.Item CHANDRA OBSERVATIONS OF GALAXY ZOO MERGERS: FREQUENCY OF BINARY ACTIVE NUCLEI IN MASSIVE MERGERS(IOP Publishing, 2012-07-10) Teng, Stacy H.; Schawinski, Kevin; Urry, C. Megan; Darg, Dan W.; Kaviraj, Sugata; Oh, Kyuseok; Bonning, Erin W.; Cardamone, Carolin N.; Keel, William C.; Lintott, Chris J.; Simmons, Brooke D.; Treister, Ezequiel; National Aeronautics & Space Administration (NASA); NASA Goddard Space Flight Center; University System of Maryland; University of Maryland College Park; Yale University; University of Oxford; Yonsei University; Brown University; University of Alabama Tuscaloosa; Universidad de ConcepcionWe present the results from a Chandra pilot study of 12 massive galaxy mergers selected from Galaxy Zoo. The sample includes major mergers down to a host galaxy mass of 10(11) M-circle dot that already have optical active galactic nucleus (AGN) signatures in at least one of the progenitors. We find that the coincidences of optically selected active nuclei with mildly obscured (N-H less than or similar to 1.1 x 10(22) cm(-2)) X-ray nuclei are relatively common (8/12), but the detections are too faint (<40 counts per nucleus; f(2-10 keV) less than or similar to 1.2 x 10(-13) erg s(-1) cm(-2)) to reliably separate starburst and nuclear activity as the origin of the X-ray emission. Only one merger is found to have confirmed binary X-ray nuclei, though the X-ray emission from its southern nucleus could be due solely to star formation. Thus, the occurrences of binary AGNs in these mergers are rare (0%-8%), unless most merger-induced active nuclei are very heavily obscured or Compton thick.Item Extended X-ray emission in the IC 2497-Hanny's Voorwerp system: energy injection in the gas around a fading AGN(Oxford University Press, 2016-01-29) Sartori, Lia F.; Schawinski, Kevin; Koss, Michael; Treister, Ezequiel; Maksym, W. Peter; Keel, William C.; Urry, C. Megan; Lintott, Chris J.; Wong, O. Ivy; Swiss Federal Institutes of Technology Domain; ETH Zurich; Universidad de Concepcion; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of Alabama Tuscaloosa; Yale University; University of Oxford; University of Western AustraliaWe present deep Chandra X-ray observations of the core of IC 2497, the galaxy associated with Hanny's Voorwerp and hosting a fading AGN. We find extended soft X-ray emission from hot gas around the low intrinsic luminosity (unobscured) AGN (L-bol similar to 10(42)-10(44) erg s(-1)). The temperature structure in the hot gas suggests the presence of a bubble or cavity around the fading AGN (E-bub similar to 10(54)-10(55) erg). A possible scenario is that this bubble is inflated by the fading AGN, which after changing accretion state is now in a kinetic mode. Other possibilities are that the bubble has been inflated by the past luminous quasar (L-bol similar to 10(46) erg s(-1)), or that the temperature gradient is an indication of a shock front from a superwind driven by the AGN. We discuss the possible scenarios and the implications for the AGN-host galaxy interaction, as well as an analogy between AGN and X-ray binaries lifecycles. We conclude that the AGN could inject mechanical energy into the host galaxy at the end of its lifecycle, and thus provide a source for mechanical feedback, in a similar way as observed for X-ray binaries.Item Fading AGN Candidates: AGN Histories and Outflow Signatures(IOP Publishing, 2017-02-01) Keel, William C.; Lintott, Chris J.; Maksym, W. Peter; Bennert, Vardha N.; Chojnowski, S. Drew; Moiseev, Alexei; Smirnova, Aleksandrina; Schawinski, Kevin; Sartori, Lia F.; Urry, C. Megan; Pancoast, Anna; Schirmer, Mischa; Scott, Bryan; Showley, Charles; Flatland, Kelsi; University of Alabama Tuscaloosa; University of Oxford; California State University System; California Polytechnic State University San Luis Obispo; New Mexico State University; Russian Academy of Sciences; Special Astrophysics Observatory of the Russian Academy of Sciences; Swiss Federal Institutes of Technology Domain; ETH Zurich; Yale UniversityWe consider the energy budgets and radiative history of eight fading active galactic nuclei (AGNs), identified from an energy shortfall between the requirements to ionize very extended (radius > 10 kpc) ionized clouds and the luminosity of the nucleus as we view it directly. All show evidence of significant fading on timescales of approximate to 50,000 yr. We explore the use of minimum ionizing luminosity Q(ion) derived from photoionization balance in the brightest pixels in H alpha at each projected radius. Tests using presumably constant Palomar-Green QSOs, and one of our targets with detailed photoionization modeling, suggest that we can derive useful histories of individual AGNs, with the caveat that the minimum ionizing luminosity is always an underestimate and subject to uncertainties about fine structure in the ionized material. These consistency tests suggest that the degree of underestimation from the upper envelope of reconstructed Q(ion) values is roughly constant for a given object and therefore does not prevent such derivation. The AGNs in our sample show a range of behaviors, with rapid drops and standstills; the common feature is a rapid drop in the last approximate to 2 x 10(4) yr before the direct view of the nucleus. The e-folding timescales for ionizing luminosity are mostly in the thousands of years, with a few episodes as short as 400 yr. In the limit of largely obscured AGNs, we find additional evidence for fading from the shortfall between even the lower limits from recombination balance and the maximum luminosities derived from far-infrared fluxes. We compare these long-term light curves, and the occurrence of these fading objects among all optically identified AGNs, to simulations of AGN accretion; the strongest variations over these timespans are seen in models with strong and local (parsec-scale) feedback. We present Gemini integral-field optical spectroscopy, which shows a very limited role for outflows in these ionized structures. While rings and loops of emission, morphologically suggestive of outflow, are common, their kinematic structure shows some to be in regular rotation. UGC 7342 exhibits local signatures of outflows <300 km s(-1), largely associated with very diffuse emission, and possibly entraining gas in one of the clouds seen in Hubble Space Telescope images. Only in the Teacup AGN do we see outflow signatures of the order of 1000 km s(-1). In contrast to the extended emission regions around many radio-loud AGNs, the clouds around these fading AGNs consist largely of tidal debris being externally illuminated but not displaced by AGN outflows.Item Galaxy Zoo 2: detailed morphological classifications for 304 122 galaxies from the Sloan Digital Sky Survey(Oxford University Press, 2013-09-22) Willett, Kyle W.; Lintott, Chris J.; Bamford, Steven P.; Masters, Karen L.; Simmons, Brooke D.; Casteels, Kevin R. V.; Edmondson, Edward M.; Fortson, Lucy F.; Kaviraj, Sugata; Keel, William C.; Melvin, Thomas; Nichol, Robert C.; Raddick, M. Jordan; Schawinski, Kevin; Simpson, Robert J.; Skibba, Ramin A.; Smith, Arfon M.; Thomas, Daniel; University of Minnesota System; University of Minnesota Twin Cities; University of Oxford; University of Nottingham; University of Portsmouth; Institut d'Estudis Espacials de Catalunya (IEEC); University of Barcelona; University of Hertfordshire; University of Alabama Tuscaloosa; Johns Hopkins University; Swiss Federal Institutes of Technology Domain; ETH Zurich; University of California System; University of California San DiegoWe present the data release for Galaxy Zoo 2 (GZ2), a citizen science project with more than 16 million morphological classifications of 304 122 galaxies drawn from the Sloan Digital Sky Survey (SDSS). Morphology is a powerful probe for quantifying a galaxy's dynamical history; however, automatic classifications of morphology (either by computer analysis of images or by using other physical parameters as proxies) still have drawbacks when compared to visual inspection. The large number of images available in current surveys makes visual inspection of each galaxy impractical for individual astronomers. GZ2 uses classifications from volunteer citizen scientists to measure morphologies for all galaxies in the DR7 Legacy survey with m(r) > 17, in addition to deeper images from SDSS Stripe 82. While the original GZ2 project identified galaxies as early-types, late-types or mergers, GZ2 measures finer morphological features. These include bars, bulges and the shapes of edge-on disks, as well as quantifying the relative strengths of galactic bulges and spiral arms. This paper presents the full public data release for the project, including measures of accuracy and bias. The majority (greater than or similar to 90 per cent) of GZ2 classifications agree with those made by professional astronomers, especially for morphological T-types, strong bars and arm curvature. Both the raw and reduced data products can be obtained in electronic format at ext-link-type="uri" xlink:href="http://data.galaxyzoo.org" xmlns:xlink="http://www.w3.org/1999/xlink">http://data.galaxyzoo.org.Item Galaxy Zoo and sparcfire: constraints on spiral arm formation mechanisms from spiral arm number and pitch angles(Oxford University Press, 2017-08-22) Hart, Ross E.; Bamford, Steven P.; Hayes, Wayne B.; Cardamone, Carolin N.; Keel, William C.; Kruk, Sandor J.; Lintott, Chris J.; Masters, Karen L.; Simmons, Brooke D.; Smethurst, Rebecca J.; University of Nottingham; University of California System; University of California Irvine; University of Alabama Tuscaloosa; University of Oxford; University of Portsmouth; University of California San DiegoIn this paper, we study the morphological properties of spiral galaxies, including measurements of spiral arm number and pitch angle. Using Galaxy Zoo 2, a stellar mass-complete sample of 6222 SDSS spiral galaxies is selected. We use the machine vision algorithm SPARCFIRE to identify spiral arm features and measure their associated geometries. A support vector machine classifier is employed to identify reliable spiral features, with which we are able to estimate pitch angles for half of our sample. We use these machine measurements to calibrate visual estimates of arm tightness, and hence estimate pitch angles for our entire sample. The properties of spiral arms are compared with respect to various galaxy properties. The star formation properties of galaxies vary significantly with arm number, but not pitch angle. We find that galaxies hosting strong bars have spiral arms substantially (4 degrees-6 degrees) looser than unbarred galaxies. Accounting for this, spiral arms associated with many-armed structures are looser (by 2 degrees) than those in two-armed galaxies. In contrast to this average trend, galaxies with greater bulge-to-total stellar mass ratios display both fewer and looser spiral arms. This effect is primarily driven by the galaxy disc, such that galaxies with more massive discs contain more spiral arms with tighter pitch angles. This implies that galaxy central mass concentration is not the dominant cause of pitch angle and arm number variations between galaxies, which in turn suggests that not all spiral arms are governed by classical density waves or modal theories.Item The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black hole accretion events(Oxford University Press, 2012) Keel, William C.; Chojnowski, S. Drew; Bennert, Vardha N.; Schawinski, Kevin; Lintott, Chris J.; Lynn, Stuart; Pancoast, Anna; Harris, Chelsea; Nierenberg, A. M.; Sonnenfeld, Alessandro; Proctor, Richard; University of Alabama Tuscaloosa; Florida Institute of Technology; Texas Christian University; University of California System; University of California Santa Barbara; California State University System; California Polytechnic State University San Luis Obispo; Yale University; University of OxfordSome active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 0.2-2 x 10(5) years in duration. Motivated by the discovery of the spectacular nebula known as Hanny's Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within approximate to 10(5) years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies. We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii r(proj) > 10 kpc. This search recovered known EELRs (such as Mrk 78, Mrk 266 and NGC 5252) and identified additional previously unknown cases, one with detected emission to r = 37 kpc. One new Sy 2 was identified. At least 14/19 are in interacting or merging systems, suggesting that tidal tails are a prime source of distant gas out of the galaxy plane to be ionized by an AGN. We see a mix of one- and two-sided structures, with observed cone angles from 23 degrees to 112 degrees. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, on the assumption of a non-variable nuclear source, ranges from 0.02 to > 12; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which only a small fraction of the ionizing output escapes to be traced by surrounding gas. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation has occurred; this is the only current technique for addressing these time-scales in AGN history. The relative numbers of faded and non-faded objects we infer, and the projected extents of the ionized regions, give our estimate (0.2-2 x 10(5) years) for the length of individual bright phases.Item 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 BerkeleyWe 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.Item Galaxy Zoo: dust in spiral galaxies star(Oxford University Press, 2010) Masters, Karen L.; Nichol, Robert; Bamford, Steven; Mosleh, Moein; Lintott, Chris J.; Andreescu, Dan; Edmondson, Edward M.; Keel, William C.; Murray, Phil; Raddick, M. Jordan; Schawinski, Kevin; Slosar, Anze; Szalay, Alexander S.; Thomas, Daniel; Vandenberg, Jan; University of Portsmouth; University of Nottingham; University of Sussex; Leiden University; Leiden University - Excl LUMC; University of Oxford; University of Alabama Tuscaloosa; Johns Hopkins University; Yale University; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California System; University of California BerkeleyWe investigate the effect of dust on spiral galaxies by measuring the inclination dependence of optical colours for 24 276 well-resolved Sloan Digital Sky Survey (SDSS) galaxies visually classified via the Galaxy Zoo project. We find clear trends of reddening with inclination which imply a total extinction from face-on to edge-on of 0.7, 0.6, 0.5 and 0.4 mag for the ugri passbands (estimating 0.3 mag of extinction in z band). We split the sample into 'bulgy' (early-type) and 'discy' (late-type) spirals using the SDSS fracdeV (or f(DeV)) parameter and show that the average face-on colour of 'bulgy' spirals is redder than the average edge-on colour of 'discy' spirals. This shows that the observed optical colour of a spiral galaxy is determined almost equally by the spiral type (via the bulge-disc ratio and stellar populations), and reddening due to dust. We find that both luminosity and spiral type affect the total amount of extinction, with discy spirals at M-r similar to -21.5 mag having the most reddening - more than twice as much as both the lowest luminosity and most massive, bulge-dominated spirals. An increase in dust content is well known for more luminous galaxies, but the decrease of the trend for the most luminous has not been observed before and may be related to their lower levels of recent star formation. We compare our results with the latest dust attenuation models of Tuffs et al. We find that the model reproduces the observed trends reasonably well but overpredicts the amount of u-band attenuation in edge-on galaxies. This could be an inadequacy in the Milky Way extinction law (when applied to external galaxies), but more likely indicates the need for a wider range of dust-star geometries. We end by discussing the effects of dust on large galaxy surveys and emphasize that these effects will become important as we push to higher precision measurements of galaxy properties and their clustering.Item Galaxy Zoo: Major Galaxy Mergers Are Not a Significant Quenching Pathway(IOP Publishing, 2017) Weigel, Anna K.; Schawinski, Kevin; Caplar, Neven; Carpineti, Alfredo; Hart, Ross E.; Kaviraj, Sugata; Keel, William C.; Kruk, Sandor J.; Lintott, Chris J.; Nichol, Robert C.; Simmons, Brooke D.; Smethurst, Rebecca J.; Swiss Federal Institutes of Technology Domain; ETH Zurich; Imperial College London; University of Nottingham; University of Hertfordshire; University of Oxford; University of Alabama Tuscaloosa; University of Portsmouth; University of California System; University of California San DiegoWe use stellar mass functions to study the properties and the significance of quenching through major galaxy mergers. In addition to SDSS DR7 and Galaxy Zoo 1 data, we use samples of visually selected major galaxy mergers and post-merger galaxies. We determine the stellar mass functions of the stages that we would expect major-merger-quenched galaxies to pass through on their way from the blue cloud to the red sequence: (1) major merger, (2) post-merger, (3) blue early type, (4) green early type, and (5) red early type. Based on their similar mass function shapes, we conclude that major mergers are likely to form an evolutionary sequence from star formation to quiescence via quenching. Relative to all blue galaxies, the major-merger fraction increases as a function of stellar mass. Major-merger quenching is inconsistent with the mass and environment quenching model. At z similar to 0, major-merger-quenched galaxies are unlikely to constitute the majority of galaxies that transition through the green valley. Furthermore, between z similar to 0 - 0.5, major-merger-quenched galaxies account for 1%-5% of all quenched galaxies at a given stellar mass. Major galaxy mergers are therefore not a significant quenching pathway, neither at z similar to 0 nor within the last 5 Gyr. The majority of red galaxies must have been quenched through an alternative quenching mechanism that causes a slow blue to red evolution.Item Galaxy Zoo: quantifying morphological indicators of galaxy interaction(Oxford University Press, 2013) Casteels, Kevin. R. V.; Bamford, Steven P.; Skibba, Ramin A.; Masters, Karen L.; Lintott, Chris J.; Keel, William C.; Schawinski, Kevin; Nichol, Robert C.; Smith, Arfon M.; University of Barcelona; University of Nottingham; University of Arizona; University of Portsmouth; University of Oxford; University of Alabama Tuscaloosa; Yale UniversityWe use Galaxy Zoo 2 visual classifications to study the morphological signatures of interaction between similar-mass galaxy pairs in the Sloan Digital Sky Survey. We find that many observable features correlate with projected pair separation - not only obvious indicators of merging, disturbance and tidal tails, but also more regular features, such as spiral arms and bars. These trends are robustly quantified, using a control sample to account for observational biases, producing measurements of the strength and separation scale of various morphological responses to pair interaction. For example, we find that the presence of spiral features is enhanced at scales less than or similar to 70 h(70)(-1) kpc, probably due to both increased star formation and the formation of tidal tails. On the other hand, the likelihood of identifying a bar decreases significantly in pairs with separations less than or similar to 30 h(70)(-1) kpc, suggesting that bars are suppressed by close interactions between galaxies of similar mass. We go on to show how morphological indicators of physical interactions provide a way of significantly refining standard estimates for the frequency of close pair interactions, based on velocity offset and projected separation. The presence of loosely wound spiral arms is found to be a particularly reliable signal of an interaction, for projected pair separations up to similar to 100 h(70)(-1) kpc. We use this indicator to demonstrate our method, constraining the fraction of low-redshift galaxies in truly interacting pairs, with M-* > 10(9.5) M-circle dot and mass ratio <4, to be between 0.4 and 2.7 per cent.Item GALAXY ZOO: THE FUNDAMENTALLY DIFFERENT CO-EVOLUTION OF SUPERMASSIVE BLACK HOLES AND THEIR EARLY-AND LATE-TYPE HOST GALAXIES(IOP Publishing, 2010-03-01) Schawinski, Kevin; Urry, Megan; Virani, Shanil; Coppi, Paolo; Bamford, Steven P.; Treister, Ezequiel; Lintott, Chris J.; Sarzi, Marc; Keel, William C.; Kaviraj, Sugata; Cardamone, Carolin N.; Masters, Karen L.; Ross, Nicholas P.; Andreescu, Dan; Murray, Phil; Nichol, Robert C.; Raddick, M. Jordan; Slosar, Anze; Szalay, Alex S.; Thomas, Daniel; Vandenberg, Jan; Yale University; University of Nottingham; University of Hawaii System; University of Oxford; University of Hertfordshire; University of Alabama Tuscaloosa; Imperial College London; University of Portsmouth; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; Johns Hopkins University; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California System; University of California Berkeley; University of LjubljanaWe use data from the Sloan Digital Sky Survey and visual classifications of morphology from the Galaxy Zoo project to study black hole growth in the nearby universe (z 0.05) and to break down the active galactic nucleus (AGN) host galaxy population by color, stellar mass, and morphology. We find that the black hole growth at luminosities L[O III] > 10(40) erg s(-1) in early- and late-type galaxies is fundamentally different. AGN host galaxies as a population have a broad range of stellar masses (10(10)-10(11) M-circle dot), reside in the green valley of the color-mass diagram and their central black holes have median masses around 10(6.5) M-circle dot. However, by comparing early- and late-type AGN host galaxies to their non-active counterparts, we find several key differences: in early-type galaxies, it is preferentially the galaxies with the least massive black holes that are growing, while in late-type galaxies, it is preferentially the most massive black holes that are growing. The duty cycle of AGNs in early-type galaxies is strongly peaked in the green valley below the low-mass end (10(10) M-circle dot) of the red sequence at stellar masses where there is a steady supply of blue cloud progenitors. The duty cycle of AGNs in late-type galaxies on the other hand peaks in massive (10(11) M-circle dot) green and red late-types which generally do not have a corresponding blue cloud population of similar mass. At high-Eddington ratios (L/L-Edd > 0.1), the only population with a substantial fraction of AGNs are the low-mass green valley early-type galaxies. Finally, the Milky Way likely resides in the "sweet spot" on the color-mass diagram where the AGN duty cycle of late-type galaxies is highest. We discuss the implications of these results for our understanding of the role of AGNs in the evolution of galaxies.Item The green valley is a red herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies star(Oxford University Press, 2014-03-15) Schawinski, Kevin; Urry, C. Megan; Simmons, Brooke D.; Fortson, Lucy; Kaviraj, Sugata; Keel, William C.; Lintott, Chris J.; Masters, Karen L.; Nichol, Robert C.; Sarzi, Marc; Skibba, Ramin; Treister, Ezequiel; Willett, Kyle W.; Wong, O. Ivy; Yi, Sukyoung K.; Swiss Federal Institutes of Technology Domain; ETH Zurich; Yale University; University of Oxford; University of Minnesota System; University of Minnesota Twin Cities; University of Hertfordshire; University of Alabama Tuscaloosa; University of Portsmouth; University of California System; University of California San Diego; Universidad de Concepcion; Commonwealth Scientific & Industrial Research Organisation (CSIRO); Yonsei UniversityWe use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from disc to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant discs, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV-optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly active galactic nuclei feedback.Item THE HISTORY AND ENVIRONMENT OF A FADED QUASAR: HUBBLE SPACE TELESCOPE OBSERVATIONS OF HANNY'S VOORWERP AND IC 2497(IOP Publishing, 2012-08) Keel, William C.; Lintott, Chris J.; Schawinski, Kevin; Bennert, Vardha N.; Thomas, Daniel; Manning, Anna; Chojnowski, S. Drew; van Arkel, Hanny; Lynn, Stuart; University of Alabama Tuscaloosa; University of Oxford; Yale University; University of California System; University of California Santa Barbara; University of Portsmouth; University of VirginiaWe present Hubble Space Telescope imaging and spectroscopy, along with supporting Galaxy Evolution Explorer and ground-based data, for the extended high-ionization cloud known as Hanny's Voorwerp, near the spiral galaxy IC 2497. Wide Field Camera 3 images show complex dust absorption near the nucleus of IC 2497. The galaxy core in these data is, within the errors, coincident with the very long baseline interferometry core component marking the active nucleus. Space Telescope Imaging Spectrograph (STIS) optical spectra show the active galactic nucleus (AGN) to be a type 2 Seyfert galaxy of rather low luminosity. The derived ionization parameter log U =- 3.5 is in accordance with the weak X-ray emission from the AGN. We find no high- ionization gas near the nucleus, adding to the evidence that the AGN is currently at a low radiative output (perhaps with the central black hole having switched to a mode dominated by kinetic energy). The nucleus is accompanied by an expanding ring of ionized gas approximate to 500 pc in projected diameter on the side opposite Hanny's Voorwerp. Where sampled by the STIS slit, this ring has Doppler offset approximate to 300 km s(-1) from the nucleus, implying a kinematic age < 7 x 10(5) years. Narrowband [O III] and H alpha+[N II] Advanced Camera for Surveys images show fine structure in Hanny's Voorwerp, including limb-brightened sections suggesting modest interaction with a galactic outflow and small areas where Ha is strong. We identify these latter regions as regions ionized by recent star formation, in contrast to the AGN ionization of the entire cloud. These candidate "normal" H II regions contain blue continuum objects, whose colors are consistent with young stellar populations; they appear only in a 2 kpc region toward IC 2497 in projection, perhaps meaning that the star formation was triggered by compression from a narrow outflow. The ionization-sensitive ratio [O III]/H alpha shows broad bands across the object at a skew angle to the galaxy nucleus, and no discernible pattern near the prominent "hole" in the ionized gas. The independence of ionization and surface brightness (SB) suggests that there is substantial spatial structure which remains unresolved, to such an extent that the SB samples the number of denser filaments rather than the characteristic density in emission regions; this might be a typical feature of gas in tidal tails, currently measurable only when such gas is highly ionized. These results fit with our picture of an ionization echo from an AGN whose ionizing luminosity has dropped by a factor > 100 (and possibly much more) within the last (1-2) x 10(5) years; we suggest a tentative sequence of events in IC 2497 and discuss implications of such rapid fluctuations in luminosity for our understanding of AGN demographics.Item HST IMAGING OF FADING AGN CANDIDATES. I. HOST-GALAXY PROPERTIES AND ORIGIN OF THE EXTENDED GAS(IOP Publishing, 2015-05) Keel, William C.; Maksym, W. Peter; Bennert, Vardha N.; Lintott, Chris J.; Chojnowski, S. Drew; Moiseev, Alexei; Smirnova, Aleksandrina; Schawinski, Kevin; Urry, C. Megan; Evans, Daniel A.; Pancoast, Anna; Scott, Bryan; Showley, Charles; Flatland, Kelsi; University of Alabama Tuscaloosa; California State University System; California Polytechnic State University San Luis Obispo; New Mexico State University; Russian Academy of Sciences; Special Astrophysics Observatory of the Russian Academy of Sciences; Swiss Federal Institutes of Technology Domain; ETH Zurich; Yale University; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of California System; University of California Santa Barbara; California Institute of Technology; National Aeronautics & Space Administration (NASA); NASA Jet Propulsion Laboratory (JPL); San Francisco State UniversityWe present narrow- and medium-band Hubble Space Telescope imaging, with additional supporting ground-based imaging, spectrophotometry, and Fabry-Perot interferometric data, for eight galaxies identified as hosting a fading active galactic nucleus (AGN). These are selected to have AGN-ionized gas projected > 10 kpc from the nucleus and energy budgets with a significant shortfall of ionizing radiation between the requirement to ionize the distant gas and the AGN as observed directly, indicating fading of the AGN on approximate to 50,000 yr timescales. This paper focuses on the host-galaxy properties and origin of the gas. In every galaxy, we identify evidence of ongoing or past interactions, including tidal tails, shells, and warped or chaotic dust structures; a similarly selected sample of obscured AGNs with extended ionized clouds shares this high incidence of disturbed morphologies. Several systems show multiple dust lanes in different orientations, broadly fit by differentially precessing disks of accreted material viewed similar to 1.5 Gyr after its initial arrival. The host systems are of early Hubble type; most show nearly pure de Vaucouleurs surface brightness profiles and Sersic indices appropriate for classical bulges, with one S0 and one SB0 galaxy. The gas has a systematically lower metallicity than the nuclei; three systems have abundances uniformly well below solar, consistent with an origin in tidally disrupted low-luminosity galaxies, while some systems have more nearly solar abundances (accompanied by such signatures as multiple Doppler components), which may suggest redistribution of gas by outflows within the host galaxies themselves. These aspects are consistent with a tidal origin for the extended gas in most systems, although the ionized gas and stellar tidal features do not always match closely. Unlike extended emission regions around many radio-loud AGNs, these clouds are kinematically dominated by rotation, in some cases in warped disks. Outflows can play important kinematic roles only in localized regions near some of the AGNs. We find only a few sets of young star clusters potentially triggered by AGN outflows. In UGC 7342 and UGC 11185, multiple luminous star clusters are seen just within the projected ionization cones, potentially marking star formation triggered by outflows. As in the discovery example, Hanny's Voorwerp/IC 2497, there are regions in these clouds where the lack of a strong correlation between Ha surface brightness and ionization parameter indicates that there is unresolved fine structure in the clouds. Together with thin coherent filaments spanning several kpc, persistence of these structures over their orbital lifetimes may require a role for magnetic confinement. Overall, we find that the sample of fading AGNs occur in interacting and merging systems, that the very extended ionized gas is composed of tidal debris rather than galactic winds, and that these host systems are bulge-dominated and show no strong evidence of triggered star formation in luminous clusters.Item PLANET HUNTERS: A TRANSITING CIRCUMBINARY PLANET IN A QUADRUPLE STAR SYSTEM(IOP Publishing, 2013-05-10) Schwamb, Megan E.; Orosz, Jerome A.; Carter, Joshua A.; Welsh, William F.; Fischer, Debra A.; Torres, Guillermo; Howard, Andrew W.; Crepp, Justin R.; Keel, William C.; Lintott, Chris J.; Kaib, Nathan A.; Terrell, Dirk; Gagliano, Robert; Jek, Kian J.; Parrish, Michael; Smith, Arfon M.; Lynn, Stuart; Simpson, Robert J.; Giguere, Matthew J.; Schawinski, Kevin; Yale University; California State University System; San Diego State University; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of Hawaii System; California Institute of Technology; University of Notre Dame; University of Alabama Tuscaloosa; Florida Institute of Technology; University of Oxford; Northwestern University; Swiss Federal Institutes of Technology Domain; ETH ZurichWe report the discovery and confirmation of a transiting circumbinary planet (PH1b) around KIC 4862625, an eclipsing binary in the Kepler field. The planet was discovered by volunteers searching the first six Quarters of publicly available Kepler data as part of the Planet Hunters citizen science project. Transits of the planet across the larger and brighter of the eclipsing stars are detectable by visual inspection every similar to 137 days, with seven transits identified in Quarters 1-11. The physical and orbital parameters of both the host stars and planet were obtained via a photometric-dynamical model, simultaneously fitting both the measured radial velocities and the Kepler light curve of KIC 4862625. The 6.18 +/- 0.17 R-circle plus planet orbits outside the 20 day orbit of an eclipsing binary consisting of an F dwarf (1.734 +/- 0.044 R-circle dot, 1.528 +/- 0.087 M-circle dot) and M dwarf (0.378 +/- 0.023 R-circle dot, 0.408 +/- 0.024 M-circle dot). For the planet, we find an upper mass limit of 169 M-circle plus (0.531 Jupiter masses) at the 99.7% confidence level. With a radius and mass less than that of Jupiter, PH1b is well within the planetary regime. Outside the planet's orbit, at similar to 1000 AU, a previously unknown visual binary has been identified that is likely bound to the planetary system, making this the first known case of a quadruple star system with a transiting planet.Item Probing Quasar Shutdown Timescales with Hanny's Voorwerp(2012) Evans, Daniel A.; Schawinski, Kevin; Virani, Shanil; Urry, C. Megan; Keel, William C.; Natarajan, Priyamvada; Lintott, Chris J.; Manning, Anna; Coppi, Paolo; Kaviraj, Sugata; Bamford, Steven P.; Jozsa, Gyula I. G.; van Arkel, Hanny; Gay, Pamela; Fortson, Lucy; Garrett, Michael; University of Alabama TuscaloosaGalaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, however, unknown. We present the first measurement of the shutdown timescale for an individual quasar using Suzaku and XMM-Newton X-ray observations of the nearby galaxy IC 2497, which hosted a luminous quasar no more than ∼230,000 years ago that is still seen as a light echo in 'Hanny's Voorwerp', but whose presentday radiative output is lower by at least 2 and more likely by over 4 orders of magnitude. This extremely rapid shutdown provides new insights into the physics of accretion in supermassive black holes, and may signal a transition of the accretion disk to a radiatively inefficient state. These results were first presented by [1]. © 2012 American Institute of Physics.Item THE SUDDEN DEATH OF THE NEAREST QUASAR(IOP Publishing, 2010-11-20) Schawinski, Kevin; Evans, Daniel A.; Virani, Shanil; Urry, C. Megan; Keel, William C.; Natarajan, Priyamvada; Lintott, Chris J.; Manning, Anna; Coppi, Paolo; Kaviraj, Sugata; Bamford, Steven P.; Jozsa, Gyula I. G.; Garrett, Michael; van Arkel, Hanny; Gay, Pamela; Fortson, Lucy; Yale University; Massachusetts Institute of Technology (MIT); Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; Elon University; University of Alabama Tuscaloosa; University of Oxford; Imperial College London; University of Nottingham; University of Bonn; Leiden University; Leiden University - Excl LUMC; Swinburne University of Technology; Southern Illinois University System; Southern Illinois University Edwardsville; University of Minnesota System; University of Minnesota Twin CitiesGalaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, however, unknown. We present the first measurement of the shutdown timescale for an individual quasar using X-ray observations of the nearby galaxy IC 2497, which hosted a luminous quasar no more than 70,000 years ago that is still seen as a light echo in "Hanny's Voorwerp," but whose present-day radiative output is lower by at least two, and more likely by over four, orders of magnitude. This extremely rapid shutdown provides new insight into the physics of accretion in supermassive black holes and may signal a transition of the accretion disk to a radiatively inefficient state.Item THE ULTRAVIOLET ATTENUATION LAW IN BACKLIT SPIRAL GALAXIES(IOP Publishing, 2014-02) Keel, William C.; Manning, Anna M.; Holwerda, Benne W.; Lintott, Chris J.; Schawinski, Kevin; University of Alabama Tuscaloosa; European Space Agency; Leiden University; Leiden University - Excl LUMC; University of Oxford; Swiss Federal Institutes of Technology Domain; ETH ZurichThe effective extinction law (attenuation behavior) in galaxies in the emitted ultraviolet (UV) regime is well known only for actively star-forming objects and combines effects of the grain properties, fine structure in the dust distribution, and relative distributions of stars and dust. We use Galaxy Evolution Explorer, XMM Optical Monitor, and Hubble Space Telescope (HST) data to explore the UV attenuation in the outer parts of spiral disks which are backlit by other UV-bright galaxies, starting with the candidate list of pairs provided by Galaxy Zoo participants. New optical images help to constrain the geometry and structure of the target galaxies. Our analysis incorporates galaxy symmetry, using non-overlapping regions of each galaxy to derive error estimates on the attenuation measurements. The entire sample has an attenuation law across the optical and UV that is close to the Calzetti et al. form; the UV slope for the overall sample is substantially shallower than found by Wild et al., which is a reasonable match to the more distant galaxies in our sample but not to the weighted combination including NGC 2207. The nearby, bright spiral NGC 2207 alone gives an accuracy almost equal to the rest of our sample, and its outer arms have a very low level of foreground starlight. Thus, this widespread, fairly "gray" law can be produced from the distribution of dust alone, without a necessary contribution from differential escape of stars from dense clouds. Our results indicate that the extrapolation needed to compare attenuation between backlit galaxies at moderate redshifts from HST data, and local systems from Sloan Digital Sky Survey and similar data, is mild enough to allow the use of galaxy overlaps to trace the cosmic history of dust in galaxies. For NGC 2207, HST data in the near-UV F336W band show that the covering factor of clouds with small optical attenuation becomes a dominant factor farther into the UV, which opens the possibility that widespread diffuse dust dominates over dust in star-forming regions deep into the UV. Comparison with published radiative-transfer models indicates that the role of dust clumping dominates over differences in grain populations at this coarse spatial resolution.