University of Alabama Repository :: Browsing by Author "Keel, William C."

Browsing by Author "Keel, William C."

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About AGN ionization echoes, thermal echoes and ionization deficits in low-redshift Ly alpha blobs
(Oxford University Press, 2016-07-28) Schirmer, Mischa; Malhotra, Sangeeta; Levenson, Nancy A.; Fu, Hai; Davies, Rebecca L.; Keel, William C.; Torrey, Paul; Bennert, Vardha N.; Pancoast, Anna; Turner, James E. H.; Arizona State University; Arizona State University-Tempe; University of Iowa; Australian National University; University of Alabama Tuscaloosa; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; Massachusetts Institute of Technology (MIT); California State University System; California Polytechnic State University San Luis Obispo
We report the discovery of 14 Ly alpha blobs (LABs) at z similar to 0.3, existing at least 4-7 billion years later in the Universe than all other LABs known. Their optical diameters are 20-70 kpc, and GALEX data imply Ly alpha luminosities of (0.4-6.3) x 10(43) erg s(-1). Contrary to high-z LABs, they live in low-density areas. They are ionized by AGN, suggesting that cold accretion streams as a power source must deplete between z = 2 and 0.3. We also show that transient AGN naturally explain the ionization deficits observed in many LABs. Their Ly alpha and X-ray fluxes decorrelate below less than or similar to 10(6) years because of the delayed escape of resonantly scattering Ly alpha photons. High Ly alpha luminosities do not require currently powerful AGN, independent of obscuration. Chandra X-ray data reveal intrinsically weak AGN, confirming the luminous optical nebulae as impressive ionization echoes. For the first time, we also report mid-infrared thermal echoes from the dusty tori. We conclude that the AGN have faded by three to four orders of magnitude within the last 10(4-5) years, leaving fossil UV, optical and thermal radiation behind. The host galaxies belong to the group of previously discovered Green Bean galaxies (GBs). Gemini optical imaging reveals smooth spheres, mergers, spectacular outflows and ionization cones. Because of their proximity and high flux densities, GBs are perfect targets to study AGN feedback, mode switching and the Ly alpha escape. The fully calibrated, co-added optical FITS images are publicly available.
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 Tuscaloosa
We 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.
BROAD-LINE REVERBERATION IN THE KEPLER-FIELD SEYFERT GALAXY Zw 229-015
(IOP Publishing, 2011-05-10) Barth, Aaron J.; Nguyen, My L.; Malkan, Matthew A.; Filippenko, Alexei V.; Li, Weidong; Gorjian, Varoujan; Joner, Michael D.; Bennert, Vardha Nicola; Botyanszki, Janos; Cenko, S. Bradley; Childress, Michael; Choi, Jieun; Comerford, Julia M.; Cucciara, Antonino; da Silva, Robert; Duchene, Gaspard; Fumagalli, Michele; Ganeshalingam, Mohan; Gates, Elinor L.; Gerke, Brian F.; Griffith, Christopher V.; Harris, Chelsea; Hintz, Eric G.; Hsiao, Eric; Kandrashoff, Michael T.; Keel, William C.; Kirkman, David; Kleiser, Io K. W.; Laney, C. David; Lee, Jeffrey; Lopez, Liliana; Lowe, Thomas B.; Moody, J. Ward; Morton, Alekzandir; Nierenberg, A. M.; Nugent, Peter; Pancoast, Anna; Rex, Jacob; Rich, R. Michael; Silverman, Jeffrey M.; Smith, Graeme H.; Sonnenfeld, Alessandro; Suzuki, Nao; Tytler, David; Walsh, Jonelle L.; Woo, Jong-Hak; Yang, Yizhe; Zeisse, Carl; University of California System; University of California Irvine; University of California Los Angeles; University of California Berkeley; California Institute of Technology; National Aeronautics & Space Administration (NASA); NASA Jet Propulsion Laboratory (JPL); Brigham Young University; University of California Santa Barbara; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Texas System; University of Texas Austin; University of California Santa Cruz; Centre National de la Recherche Scientifique (CNRS); CNRS - National Institute for Earth Sciences & Astronomy (INSU); Institut de Planetologie et d'Astrophysique de Grenoble (IPAG); UDICE-French Research Universities; Communaute Universite Grenoble Alpes; Universite Grenoble Alpes (UGA); Stanford University; SLAC National Accelerator Laboratory; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; University of Alabama Tuscaloosa; University of California San Diego; Seoul National University (SNU)
The Seyfert 1 galaxy Zw 229-015 is among the brightest active galaxies being monitored by the Kepler mission. In order to determine the black hole mass in Zw 229-015 from H beta reverberation mapping, we have carried out nightly observations with the Kast Spectrograph at the Lick 3 m telescope during the dark runs from 2010 June through December, obtaining 54 spectroscopic observations in total. We have also obtained nightly V-band imaging with the Katzman Automatic Imaging Telescope at Lick Observatory and with the 0.9 m telescope at the Brigham Young University West Mountain Observatory over the same period. We detect strong variability in the source, which exhibited more than a factor of two change in broad H beta flux. From cross-correlation measurements, we find that the H beta light curve has a rest-frame lag of 3.86(-0.90)(+0.69) days with respect to the V-band continuum variations. We also measure reverberation lags for H alpha and H gamma and find an upper limit to the H delta lag. Combining the H beta lag measurement with a broad H beta width of sigma(line) = 1590 +/- 47 km s(-1) measured from the rms variability spectrum, we obtain a virial estimate of M-BH = 1.00(-0.24)(+0.19) x 10(7) M-circle dot for the black hole in Zw 229-015. As a Kepler target, Zw 229-015 will eventually have one of the highest-quality optical light curves ever measured for any active galaxy, and the black hole mass determined from reverberation mapping will serve as a benchmark for testing relationships between black hole mass and continuum variability characteristics in active galactic nuclei.
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 Concepcion
We 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.
THE DISAPPEARANCE OF Ly alpha BLOBS: A GALEX SEARCH AT z=0.8
(IOP Publishing, 2009-09) Keel, William C.; White, Raymond E., III; Chapman, Scott; Windhorst, Rogier A.; University of Alabama Tuscaloosa; University of Cambridge; Arizona State University; Arizona State University-Tempe
Ly alpha blobs-luminous, spatially extended emission-line nebulae, often lacking bright continuum counterparts-are common in dense environments at high redshift. Until recently, atmospheric absorption and filter technology have limited our knowledge of any similar objects at z <= 2. We use Galaxy Evolution Explorer slitless spectroscopy to search for similar objects in the rich environments of two known cluster and supercluster fields at z = 0.8, where the instrumental sensitivity peaks. The regions around Cl 1054-0321 and Cl 0023+0423 were each observed in slitless-spectrum mode for 10-19 ks, with accompanying direct images of 3-6 ks to assist in recognizing continuum sources. Using several detection techniques, we find no resolved Ly alpha emitters to a flux limit of (1.5-9) x 10(-15) erg cm(-2) s(-1), on size scales of 5-30 arcsec. This corresponds to line luminosities of (0.5-3) x 10(43) erg s(-1) for linear scales 35-200 kpc. Comparison with both blind and targeted surveys at higher redshifts indicates that the population must have evolved in comoving density at least as strongly as (1 + z)(3). These results suggest that the population of Ly alpha blobs is specific to the high-redshift universe.
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 Australia
We 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.
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 University
We 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.
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 Diego
We 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.
Galaxy Zoo and ALFALFA: atomic gas and the regulation of star formation in barred disc galaxies
(Oxford University Press, 2012) Masters, Karen L.; Nichol, Robert C.; Haynes, Martha P.; Keel, William C.; Lintott, Chris; Simmons, Brooke; Skibba, Ramin; Bamford, Steven; Giovanelli, Riccardo; Schawinski, Kevin; University of Portsmouth; Cornell University; University of Alabama Tuscaloosa; University of Oxford; Yale University; University of Arizona; University of Nottingham
We study the observed correlation between atomic gas content and the likelihood of hosting a large-scale bar in a sample of 2090 disc galaxies. Such a test has never been done before on this scale. We use data on morphologies from the Galaxy Zoo project and information on the galaxies H?I content from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) blind H?I survey. Our main result is that the bar fraction is significantly lower among gas-rich disc galaxies than gas-poor ones. This is not explained by known trends for more massive (stellar) and redder disc galaxies to host more bars and have lower gas fractions: we still see at fixed stellar mass a residual correlation between gas content and bar fraction. We discuss three possible causal explanations: (1) bars in disc galaxies cause atomic gas to be used up more quickly, (2) increasing the atomic gas content in a disc galaxy inhibits bar formation and (3) bar fraction and gas content are both driven by correlation with environmental effects (e.g. tidal triggering of bars, combined with strangulation removing gas). All three explanations are consistent with the observed correlations. In addition our observations suggest bars may reduce or halt star formation in the outer parts of discs by holding back the infall of external gas beyond bar co-rotation, reddening the global colours of barred disc galaxies. This suggests that secular evolution driven by the exchange of angular momentum between stars in the bar, and gas in the disc, acts as a feedback mechanism to regulate star formation in intermediate-mass disc galaxies.
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 Diego
In 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.
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.
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 Oxford
Some 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.
Galaxy Zoo: bars in disc galaxies
(Wiley-Blackwell, 2011) Masters, Karen L.; Nichol, Robert C.; Hoyle, Ben; Lintott, Chris; Bamford, Steven P.; Edmondson, Edward M.; Fortson, Lucy; Keel, William C.; Schawinski, Kevin; Smith, Arfon M.; Thomas, Daniel; University of Portsmouth; University of Barcelona; University of Oxford; University of Nottingham; University of Minnesota System; University of Minnesota Twin Cities; University of Alabama Tuscaloosa; Yale University
We present first results from Galaxy Zoo 2, the second phase of the highly successful Galaxy Zoo project (www.galaxyzoo.org). Using a volume-limited sample of 13 665 disc galaxies (0.01 < z < 0.06 and M-r < -19.38), we study the fraction of galaxies with bars as a function of global galaxy properties like colour, luminosity and bulge prominence. Overall, 29.4 +/- 0.5 per cent of galaxies in our sample have a bar, in excellent agreement with previous visually classified samples of galaxies (although this overall fraction is lower than that measured by automated bar-finding methods). We see a clear increase in the bar fraction with redder (g - r) colours, decreased luminosity and in galaxies with more prominent bulges, to the extent that over half of the red, bulge-dominated disc galaxies in our sample possess a bar. We see evidence for a colour bimodality for our sample of disc galaxies, with a 'red sequence' that is both bulge and bar dominated, and a 'blue cloud' which has little, or no, evidence for a (classical) bulge or bar. These results are consistent with similar trends for barred galaxies seen recently both locally and at higher redshift, and with early studies using the RC3. We discuss these results in the context of internal (secular) galaxy evolution scenarios and the possible links to the formation of bars and bulges in disc galaxies.
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 Observatory
The 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.
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 Nottingham
We 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.
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 Berkeley
We 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.
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 Diego
We 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.
Galaxy Zoo: Mergers - Dynamical models of interacting galaxies
(Oxford University Press, 2016-03-17) Holincheck, Anthony J.; Wallin, John F.; Borne, Kirk; Fortson, Lucy; Lintott, Chris; Smith, Arfon M.; Bamford, Steven; Keel, William C.; Parrish, Michael; George Mason University; Middle Tennessee State University; University of Minnesota System; University of Minnesota Twin Cities; University of Oxford; University of Nottingham; University of Alabama Tuscaloosa
The dynamical history of most merging galaxies is not well understood. Correlations between galaxy interaction and star formation have been found in previous studies, but require the context of the physical history of merging systems for full insight into the processes that lead to enhanced star formation. We present the results of simulations that reconstruct the orbit trajectories and disturbed morphologies of pairs of interacting galaxies. With the use of a restricted three-body simulation code and the help of citizen scientists, we sample 105 points in parameter space for each system. We demonstrate a successful recreation of the morphologies of 62 pairs of interacting galaxies through the review of more than 3 million simulations. We examine the level of convergence and uniqueness of the dynamical properties of each system. These simulations represent the largest collection of models of interacting galaxies to date, providing a valuable resource for the investigation of mergers. This paper presents the simulation parameters generated by the project. They are now publicly available in electronic format at http://data.galaxyzoo.org/mergers.html. Though our best-fitting model parameters are not an exact match to previously published models, our method for determining uncertainty measurements will aid future comparisons between models. The dynamical clocks from our models agree with previous results of the time since the onset of star formation from starburst models in interacting systems and suggest that tidally induced star formation is triggered very soon after closest approach.
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 University
We 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.
Galaxy Zoo: the environmental dependence of bars and bulges in disc galaxies
(Wiley-Blackwell, 2012) Skibba, Ramin A.; Masters, Karen L.; Nichol, Robert C.; Zehavi, Idit; Hoyle, Ben; Edmondson, Edward M.; Bamford, Steven P.; Cardamone, Carolin N.; Keel, William C.; Lintott, Chris; Schawinski, Kevin; University of Arizona; University of Portsmouth; Case Western Reserve University; Institut d'Estudis Espacials de Catalunya (IEEC); University of Barcelona; Helsinki Institute of Physics; University of Helsinki; University of Nottingham; Massachusetts Institute of Technology (MIT); Brown University; University of Alabama Tuscaloosa; University of Oxford; Yale University
We present an analysis of the environmental dependence of bars and bulges in disc galaxies, using a volume-limited catalogue of 15 810 galaxies at z < 0.06 from the Sloan Digital Sky Survey with visual morphologies from the Galaxy Zoo 2 project. We find that the likelihood of having a bar, or bulge, in disc galaxies increases when the galaxies have redder (optical) colours and larger stellar masses, and observe a transition in the bar and bulge likelihoods at M*= 2 x 10(10) M?, such that massive disc galaxies are more likely to host bars and bulges. In addition, while some barred and most bulge-dominated galaxies are on the red sequence of the colourmagnitude diagram, we see a wider variety of colours for galaxies that host bars. We use galaxy clustering methods to demonstrate statistically significant environmental correlations of barred, and bulge-dominated, galaxies, from projected separations of 150 kpc h-1 to 3 Mpc h-1. These environmental correlations appear to be independent of each other: i.e. bulge-dominated disc galaxies exhibit a significant barenvironment correlation, and barred disc galaxies show a bulgeenvironment correlation. As a result of sparse sampling tests our sample is nearly 20 times larger than those used previously we argue that previous studies that did not detect a barenvironment correlation were likely inhibited by small number statistics. We demonstrate that approximately half of the barenvironment correlation can be explained by the fact that more massive dark matter haloes host redder disc galaxies, which are then more likely to have bars; this fraction is estimated to be 50 +/- 10 per cent from a mock catalogue analysis and 60 +/- 5 per cent from the data. Likewise, we show that the environmental dependence of stellar mass can only explain a smaller fraction (25 +/- 10 per cent) of the barenvironment correlation. Therefore, a significant fraction of our observed environmental dependence of barred galaxies is not due to colour or stellar mass dependences, and hence must be due to another galaxy property, such as gas content, or to environmental influences. Finally, by analysing the projected clustering of barred and unbarred disc galaxies with halo occupation models, we argue that barred galaxies are in slightly higher mass haloes than unbarred ones, and some of them (approximately 25 per cent) are satellite galaxies in groups. We discuss the implications of our results on the effects of minor mergers and interactions on bar formation in disc galaxies.