In order to understand the formation and subsequent evolution of galaxies one must first distinguish between the two main morphological classes of massive systems: spirals and early-type systems. This paper introduces a project, Galaxy Zoo, which provides visual morphological classifications for nearly one million galaxies, extracted from the Sloan Digital Sky Survey (SDSS). This achievement was made possible by inviting the general public to visually inspect and classify these galaxies via the internet. The project has obtained more than 4 × 10 7 individual classifications made by ∼10 5 participants. We discuss the motivation and strategy for this project, and detail how the classifications were performed and processed. We find that Galaxy Zoo results are consistent with those for subsets of SDSS galaxies classified by professional astronomers, thus demonstrating that our data provide a robust morphological catalogue. Obtaining morphologies by direct visual inspection avoids introducing biases associated with proxies for morphology such as colour, concentration or structural parameters. In addition, this catalogue can be used to directly compare SDSS morphologies with older data sets. The colour-magnitude diagrams for each morphological class are shown, and we illustrate how these distributions differ from those inferred using colour alone as a proxy for morphology.

The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell PublishingA major amendment in recent models of hierarchical galaxy formation is the inclusion of so-called active galactic nucleus (AGN) feedback. The energy input from an active central massive black hole is invoked to suppress star formation in early-type galaxies at later epochs. A major problem is that this process is poorly understood, and compelling observational evidence for its mere existence is still missing. In search for signatures of AGN feedback, we have compiled a sample of 16 000 early-type galaxies in the redshift range 0.05 200 km s ) and roughly evenly distributed between star formation and AGN at intermediate and low (?? <100 km s ) masses. The objects with emission (???20 per cent) are offset from the red sequence and form a well-defined pattern in the colour-mass diagram. Star-forming early-types inhabit the blue cloud, while early-types with AGN are located considerably closer to and almost on the red sequence. Star formation-AGN composites are found right between these two extremes. We further derive galaxy star formation histories using a novel method that combines multiwavelength photometry from near-ultraviolet (UV) to near-infrared (IR) and stellar absorption indices. We find that in those objects deviating from the red sequence star formation occurred several 100 Myr in the past involving 1-10 per cent of the total stellar mass. We identify an evolutionary sequence from star formation via nuclear activity to quiescence. This transition process lasts about 1 Gyr, and the peak AGN phase occurs roughly half a Gyr after the starburst. The most likely interpretation is that star formation is suppressed by nuclear activity in these objects before they settle on the red sequence. This is empirical evidence for the occurrence of AGN feedback in early-type galaxies at recent epochs. ?? 2007 The Authors. Journal compilation ?? 2007 RAS

We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in lowredshift 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 earlytype galaxies move rapidly across the green valley after the morphologies are transformed from disk to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant disks, 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 disk to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disk to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly AGN feedback.

The definitive article can be found at: http://www3.interscience.wiley.com/ Copyright Royal Astronomical SocietyThe environment is known to affect the formation and evolution of galaxies considerably best visible through the well-known morphology???density relationship. It is less clear, though, whether the environment is equally important at a given galaxy morphology. In this paper, we study the effect of environment on the evolution of early-type galaxies as imprinted in the fossil record by analysing the stellar population properties of 3360 galaxies morphologically selected by visual inspection from the Sloan Digital Sky Survey in a narrow redshift range ( 0.05 ???z??? 0.06 ). The morphological selection algorithm is critical, as it does not bias against recent star formation. We find that the distribution of ages is bimodal with a strong peak at old ages and a secondary peak at young ages around ???2.5 Gyr containing about 10 per cent of the objects. This is an analogue to 'red sequence' and 'blue cloud' identified in galaxy populations usually containing both early- and late-type galaxies. The fraction of the young, rejuvenated galaxies increases with both decreasing galaxy mass and decreasing environmental density up to about 45 per cent, which implies that the impact of environment increases with decreasing galaxy mass. The rejuvenated galaxies have lower ??/Fe ratios than the average and most of them show signs of ongoing star formation through their emission line spectra. All objects that host active galactic nuclei in their centres without star formation are part of the red sequence population. We confirm and statistically strengthen earlier results that luminosity weighted ages, metallicities and ??/Fe element ratios of the red sequence population correlate well with velocity dispersion and galaxy mass. Most interestingly, however, these scaling relations are not sensitive to environmental densities and are only driven by galaxy mass. We infer that early-type galaxy formation has undergone a phase transition a few billion years ago around z??? 0.2 . A self-regulated formation phase without environmental dependence has recently been superseded by a rejuvenation phase, in which the environment plays a decisive role possibly through galaxy mergers and interactions

???The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.15383.xWe 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]??5007 ??, with an unusually large equivalent width of up to ???1000 ??. 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??? 108.5???1010 M???) with high star formation rates (???10 M??? yr???1) , low metallicities (log[O/H]+ 12 ??? 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 ???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?? 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

Hard X-ray (10 keV) observations of active galactic nuclei (AGNs) can shed light on some of the most obscured episodes of accretion onto supermassive black holes. The 70-month Swift/BAT all-sky survey, which probes the 14-195 keV energy range, has currently detected 838 AGNs. We report here on the broadband X-ray (0.3-150 keV) characteristics of these AGNs, obtained by combining XMM-Newton, Swift/XRT, ASCA, Chandra, and Suzaku observations in the soft X-ray band ( 10  keV) with 70-month averaged Swift/BAT data. The nonblazar AGNs of our sample are almost equally divided into unobscured (N 10 cm H 22 2 < -) and obscured (N 10 cm H 22 2  -) AGNs, and their Swift/BAT continuum is systematically steeper than the 0.3-10 keV emission, which suggests that the presence of a high-energy cutoff is almost ubiquitous. We discuss the main X-ray spectral parameters obtained, such as the photon index, the reflection parameter, the energy of the cutoff, neutral and ionized absorbers, and the soft excess for both obscured and unobscured AGNs.

Does the environment of a galaxy directly influence the quenching history of a galaxy? Here we investigate the detailed morphological structures and star formation histories of a sample of SDSS group galaxies with both classifications from Galaxy Zoo 2 and NUV detections in GALEX. We use the optical and NUV colours to infer the quenching time and rate describing a simple exponentially declining SFH for each galaxy, along with a control sample of field galaxies. We find that the time since quenching and the rate of quenching do not correlate with the relative velocity of a satellite but are correlated with the group potential. This quenching occurs within an average quenching timescale of ∼ 2.5 Gyr from star forming to complete quiescence, during an average infall time (from ∼ 10R 200 to 0.01R 200) of ∼ 2.6 Gyr. Our results suggest that the environment does play a direct role in galaxy quenching through quenching mechanisms which are correlated with the group potential, such as harassment, interactions or starvation. Environmental quenching mechanisms which are correlated with satellite velocity, such as ram pressure stripping, are not the main cause of quenching in the group environment. We find that no single mechanism dominates over another, except in the most extreme environments or masses. Instead an interplay of mergers , mass & morphological quenching and environment driven quenching mechanisms dependent on the group potential drive galaxy evolution in groups.

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. 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 Galaxy Zoo 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 ( 90%) 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 http://data.galaxyzoo.org.