Browsing by Author "Kaviraj, Sugata"
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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 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: CANDELS barred discs and bar fractions(Oxford University Press, 2014) Simmons, B. D.; Melvin, Thomas; Lintott, Chris; Masters, Karen L.; Willett, Kyle W.; Keel, William C.; Smethurst, R. J.; Cheung, Edmond; Nichol, Robert C.; Schawinski, Kevin; Rutkowski, Michael; Kartaltepe, Jeyhan S.; Bell, Eric F.; Casteels, Kevin R. V.; Conselice, Christopher J.; Almaini, Omar; Ferguson, Henry C.; Fortson, Lucy; Hartley, William; Kocevski, Dale; Koekemoer, Anton M.; McIntosh, Daniel H.; Mortlock, Alice; Newman, Jeffrey A.; Ownsworth, Jamie; Bamford, Steven; Dahlen, Tomas; Faber, Sandra M.; Finkelstein, Steven L.; Fontana, Adriano; Galametz, Audrey; Grogin, N. A.; Gruetzbauch, Ruth; Guo, Yicheng; Haeussler, Boris; Jek, Kian J.; Kaviraj, Sugata; Lucas, Ray A.; Peth, Michael; Salvato, Mara; Wiklind, Tommy; Wuyts, Stijn; University of Oxford; University of Portsmouth; University of Southampton; University of Minnesota System; University of Minnesota Twin Cities; University of Alabama Tuscaloosa; University of California System; University of California Santa Cruz; Swiss Federal Institutes of Technology Domain; ETH Zurich; National Optical Astronomy Observatory; University of Michigan System; University of Michigan; Institut d'Estudis Espacials de Catalunya (IEEC); University of Barcelona; University of Nottingham; Space Telescope Science Institute; University of Kentucky; University of Missouri System; University of Missouri Kansas City; Pennsylvania Commonwealth System of Higher Education (PCSHE); University of Pittsburgh; University of Texas System; University of Texas Austin; Istituto Nazionale Astrofisica (INAF); Universidade de Lisboa; University of Hertfordshire; Johns Hopkins University; Max Planck Society; European Southern ObservatoryThe formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z similar to 1, and by z > 1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z similar to 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 <= z <= 2 (f(bar) = 10.7(-3.5)(+6.3) per cent after correcting for incompleteness) does not significantly evolve. We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion years.Item Galaxy Zoo: dust and molecular gas in early-type galaxies with prominent dust lanes(Oxford University Press, 2012) Kaviraj, Sugata; Ting, Yuan-Sen; Bureau, Martin; Shabala, Stanislav S.; Crockett, R. Mark; Silk, Joseph; Lintott, Chris; Smith, Arfon; Keel, William C.; Masters, Karen L.; Schawinski, Kevin; Bamford, Steven P.; Imperial College London; University of Oxford; Institut Polytechnique de Paris; University of Tasmania; University of Alabama Tuscaloosa; University of Portsmouth; Yale University; University of NottinghamWe explore the properties of dust and associated molecular gas in 352 nearby (0.01 < z < 0.07) early-type galaxies (ETGs) with prominent dust lanes, drawn from the Sloan Digital Sky Survey (SDSS). Two-thirds of these dusty ETGs (D-ETGs) are morphologically disturbed, which suggests a merger origin, making these galaxies ideal test beds for studying the merger process at low redshift. The D-ETGs preferentially reside in lower density environments, compared to a control sample drawn from the general ETG population. Around 80 per cent of D-ETGs inhabit the field (compared to 60 per cent of the control ETGs) and less than 2 per cent inhabit clusters (compared to 10 per cent of the control ETGs). Compared to their control-sample counterparts, D-ETGs exhibit bluer ultravioletoptical colours (indicating enhanced levels of star formation) and an active galactic nucleus fraction that is more than an order of magnitude greater (indicating a strikingly higher incidence of nuclear activity). The mass of clumpy dust residing in large-scale dust features is estimated, using the SDSS r-band images, to be in the range 104.5106.5 M?. A comparison to the total (clumpy + diffuse) dust masses calculated using the far-infrared fluxes of 15 per cent of the D-ETGs that are detected by the Infrared Astronomical Satellite (IRAS) indicates that only 20 per cent of the dust is typically contained in these large-scale dust features. The dust masses are several times larger than the maximum value expected from stellar mass loss, ruling out an internal origin. The dust content shows no correlation with the blue luminosity, indicating that it is not related to a galactic scale cooling flow. Furthermore, no correlation is found with the age of the recent starburst, suggesting that the dust is accreted directly in the merger rather than being produced in situ by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in the literature, we estimate that the median current molecular gas fraction in the IRAS-detected ETGs is similar to 1.3 per cent. Adopting reasonable values for gas depletion time-scales and starburst ages, the median initial gas fraction in these D-ETGs is similar to 4 per cent. Recent work has suggested that the merger activity in nearby ETGs largely involves minor mergers (dry ETG + gas-rich dwarf), with mass ratios between 1:10 and 1:4. If the IRAS-detected D-ETGs have formed via this channel, then the original gas fractions of the accreted satellites are between 20 and 44 per cent.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: 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 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.