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Banerji M.,University College London | Banerji M.,University of Cambridge | Lahav O.,University College London | Lintott C.J.,Denys Wilkinson Building | And 10 more authors.
Monthly Notices of the Royal Astronomical Society

We present morphological classifications obtained using machine learning for objects in the Sloan Digital Sky Survey DR6 that have been classified by Galaxy Zoo into three classes, namely early types, spirals and point sources/artefacts. An artificial neural network is trained on a subset of objects classified by the human eye, and we test whether the machine-learning algorithm can reproduce the human classifications for the rest of the sample. We find that the success of the neural network in matching the human classifications depends crucially on the set of input parameters chosen for the machine-learning algorithm. The colours and parameters associated with profile fitting are reasonable in separating the objects into three classes. However, these results are considerably improved when adding adaptive shape parameters as well as concentration and texture. The adaptive moments, concentration and texture parameters alone cannot distinguish between early type galaxies and the point sources/artefacts. Using a set of 12 parameters, the neural network is able to reproduce the human classifications to better than 90 per cent for all three morphological classes. We find that using a training set that is incomplete in magnitude does not degrade our results given our particular choice of the input parameters to the network. We conclude that it is promising to use machine-learning algorithms to perform morphological classification for the next generation of wide-field imaging surveys and that the Galaxy Zoo catalogue provides an invaluable training set for such purposes. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Masters K.L.,University of Portsmouth | Mosleh M.,University of Sussex | Mosleh M.,Leiden University | Romer A.K.,University of Sussex | And 15 more authors.
Monthly Notices of the Royal Astronomical Society

We study the spectroscopic properties and environments of red (or passive) spiral galaxies found by the Galaxy Zoo project. By carefully selecting face-on disc-dominated spirals, we construct a sample of truly passive discs (i.e. they are not dust reddened spirals, nor are they dominated by old stellar populations in a bulge). As such, our red spirals represent an interesting set of possible transition objects between normal blue spiral galaxies and red early types, making up ∼6 per cent of late-type spirals. We use optical images and spectra from Sloan Digital Sky Survey to investigate the physical processes which could have turned these objects red without disturbing their morphology. We find red spirals preferentially in intermediate density regimes. However, there are no obvious correlations between red spiral properties and environment suggesting that environment alone is not sufficient to determine whether a galaxy will become a red spiral. Red spirals are a very small fraction of all spirals at low masses (M{black star} < 1010 M⊙), but are a significant fraction of the spiral population at large stellar masses showing that massive galaxies are red independent of morphology. We confirm that as expected, red spirals have older stellar populations and less recent star formation than the main spiral population. While the presence of spiral arms suggests that a major star formation could not have ceased a long ago (not more than a few Gyr), we show that these are also not recent post-starburst objects (having had no significant star formation in the last Gyr), so star formation must have ceased gradually. Intriguingly, red spirals are roughly four times as likely than the normal spiral population to host optically identified Seyfert/low-ionization nuclear emission region (LINER; at a given stellar mass and even accounting for low-luminosity lines hidden by star formation), with most of the difference coming from the objects with LINER-like emission. We also find a curiously large optical bar fraction in the red spirals (70 ± 5 verses 27 ± 5 per cent in blue spirals) suggesting that the cessation of star formation and bar instabilities in spirals are strongly correlated. We conclude by discussing the possible origins of these red spirals. We suggest that they may represent the very oldest spiral galaxies which have already used up their reserves of gas - probably aided by strangulation or starvation, and perhaps also by the effect of bar instabilities moving material around in the disc. We provide an online table listing our full sample of red spirals along with the normal/blue spirals used for comparison. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Lintott C.,Oxford Astrophysics | Schawinski K.,Yale University | Bamford S.,University of Nottingham | Slosar A.,Brookhaven National Laboratory | And 10 more authors.
Monthly Notices of the Royal Astronomical Society

Morphology is a powerful indicator of a galaxy's dynamical and merger history. It is strongly correlated with many physical parameters, including mass, star formation history and the distribution of mass. The Galaxy Zoo project collected simple morphological classifications of nearly 900 000 galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of thousands of volunteers. This large number of classifications allows us to exclude classifier error, and measure the influence of subtle biases inherent in morphological classification. This paper presents the data collected by the project, alongside measures of classification accuracy and bias. The data are now publicly available and full catalogues can be downloaded in electronic format from © 2010 The Authors. Monthly Notices of the Royal Astronomical Society © 2010 RAS. Source

Jimenez R.,ICREA and Institute of Space science CSIC IEEC | Jimenez R.,Lawrence Berkeley National Laboratory | Slosar A.,Lawrence Berkeley National Laboratory | Slosar A.,University of Oxford | And 13 more authors.
Monthly Notices of the Royal Astronomical Society

We report on the finding of a correlation between the past star formation activity of galaxies and the degree to which the rotation axes of neighbouring galaxies are aligned. This is obtained by cross-correlating star formation histories, derived using the multiple optimized parameter estimation and data compression (MOPED) algorithm, and the spatial coherence of spin direction (chirality), as determined by the Galaxy Zoo project, for a sample of Sloan Digital Sky Survey (SDSS) galaxies. Our findings suggest that spiral galaxies, which formed the majority of their stars early (z > 2), tend to display coherent rotation over scales of ∼10 Mpc h-1. The correlation is weaker for galaxies with significant recent star formation. We find evidence for this alignment at more than the 5σ level, but no correlation with other galaxy stellar properties. This finding can be explained within the context of hierarchical tidal-torque theory if the SDSS galaxies harbouring the majority of the old stellar population were formed in the past, in the same filament and at about the same time. Galaxies with significant recent star formation instead are in the field, thus influenced by the general tidal field that will align them in random directions, or have had a recent merger that would promote star formation but change the spin direction. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Wong O.I.,CSIRO | Wong O.I.,Yale University | Schawinski K.,Yale University | Kaviraj S.,Imperial College London | And 14 more authors.
Monthly Notices of the Royal Astronomical Society

We present a study of local post-starburst galaxies (PSGs) using the photometric and spectroscopic observations from the Sloan Digital Sky Survey and the results from the Galaxy Zoo project. We find that the majority of our local PSG population have neither early- nor late-type morphologies but occupy a well-defined space within the colour-stellar mass diagram, most notably, the low-mass end of the 'green valley' below the transition mass thought to be the mass division between low-mass star-forming galaxies and high-mass passively evolving bulge-dominated galaxies. Our analysis suggests that it is likely that local PSGs will quickly transform into 'red', low-mass early-type galaxies as the stellar morphologies of the 'green' PSGs largely resemble that of the early-type galaxies within the same mass range. We propose that the current population of PSGs represents a population of galaxies which is rapidly transitioning between the star-forming and the passively evolving phases. Subsequently, these PSGs will contribute towards the build-up of the low-mass end of the 'red sequence' once the current population of young stars fade and stars are no longer being formed. These results are consistent with the idea of 'downsizing' where the build-up of smaller galaxies occurs at later epochs. © 2012 CSIRO Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

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