Green Galaxies Research Articles

Signatures of Active Galactic Nucleus Feedback Modes: A Green Bean Galaxy with 150 kpc Jet-induced Radio Emission

Jetted active galactic nuclei (AGN) hosting extended photoionized nebulae provide us with a unique view of the timescales associated with AGN activity. Here, we present a new green bean galaxy at z = 0.304458 ± 0.000007 with large-scale jet-induced radio emission. The spectral energy distributions of the radio components show steep spectral indices (α = −0.85 to −0.92 for the extended regions, and α = −1.02 for the faint radio core), and spectral age modeling of the extended radio emission indicates that the lobes are >6 Myr old. It is unclear whether the jet is active, or is a remnant with an off-time of 2–3 Myr. Several detached clouds lie around the host galaxy up to 37.8 kpc away from the nucleus, and their ionization profile indicates a decline (∼2 dex) in the AGN ionizing photon production over the past ∼0.15 Myr. Furthermore, we measure a blueshift for one of the clouds that is spatially coincident with the path of the radio jet. The cloud is likely illuminated by the photoionizing AGN, and potentially underwent an interaction with the relativistic jet. Our multiwavelength analysis suggests that RGB1 was in a phase of jet production prior to the radiatively efficient accretion phase traced by the detached cloud emission. It is unclear whether RGB1 transitioned into a low-excitation radio galaxy or an inactive galaxy over the past ∼0.15 Myr, or whether the extended radio and optical emission trace distinct accretion phases that occurred in succession.

The Atacama Cosmology Telescope: Cosmology from Cross-correlations of unWISE Galaxies and ACT DR6 CMB Lensing

We present tomographic measurements of structure growth using cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck cosmic microwave background (CMB) lensing maps with the unWISE Blue and Green galaxy samples, which span the redshift ranges 0.2 ≲ z ≲ 1.1 and 0.3 ≲ z ≲ 1.8, respectively. We improve on prior unWISE cross-correlations not just by making use of the new, high-precision ACT DR6 lensing maps, but also by including additional spectroscopic data for redshift calibration and by analyzing our measurements with a more flexible theoretical model. We determine the amplitude of matter fluctuations at low redshifts (z ≃ 0.2–1.6), finding S8≡σ8(Ωm/0.3)0.5=0.813±0.021 using the ACT cross-correlation alone and S 8 = 0.810 ± 0.015 with a combination of Planck and ACT cross-correlations; these measurements are fully consistent with the predictions from primary CMB measurements assuming standard structure growth. The addition of baryon acoustic oscillation data breaks the degeneracy between σ 8 and Ω m , allowing us to measure σ 8 = 0.813 ± 0.020 from the cross-correlation of unWISE with ACT and σ 8 = 0.813 ± 0.015 from the combination of cross-correlations with ACT and Planck. These results also agree with the expectations from primary CMB extrapolations in ΛCDM cosmology; the consistency of σ 8 derived from our two redshift samples at z ∼ 0.6 and 1.1 provides a further check of our cosmological model. Our results suggest that structure formation on linear scales is well described by ΛCDM even down to low redshifts z ≲ 1.

Shaping physical properties of galaxy subtypes in the VIPERS survey: Environment matters

Aims. This study aims to explore the relation between the physical properties of different galaxy sub-classes, from red passive to blue star-forming, and their environment. Our work is based on the analysis of 31 631 galaxies from the VIMOS Public Extragalactic Redshift Survey (VIPERS), observed at 0.5 < z < 0.9. The unprecedented volume of VIPERS and the wealth of auxiliary derived data allow us to associate sub-classes of the main galaxy populations with their possibly different evolutionary paths. This is the first time such a study is performed with such statistical precision. Methods. We use the results of an unsupervised clustering algorithm to distinguish 11 subclasses of VIPERS galaxies, based on the multi-dimensional feature space, defined by rest-frame UV to NIR colours. We investigate the relationship between the properties of these sub-classes of galaxies and their local environment, defined as the galaxy density contrast, δ, derived from the fifth nearest neighbour technique. Results. We confirm that the galaxy population-density relation is already in place at z ∼ 0.9, with the blue galaxy fraction decreasing with density, compensated by an increase in the red fraction. We demonstrate how the properties of red, green, and blue galaxy subclasses are altered as they assemble into denser regions, and we attempt to interpret this in the context of their evolution. On average, red galaxies in the high-density environment are larger by 28% than those in low-density environments. In particular, we find one group of galaxies, sub-class C3, whose increase of size with time can be explained mainly as the result of mergers; for other red subclasses, mergers would not seem to play the major role (subclass C2) or would play a negligible role (sub-class C1). The properties of the green galaxies (sub-classes C4–6) depend on whether their stellar mass is above or below a transition mass, log(Mstar/M⊙)=10.6. Low-mass green (9.5 ≲ log(Mstar/M⊙)≲10.6) galaxies appear to have grown through secular processes, while in high-mass (10.6 ≲ log(Mstar/M⊙)≲11.5) green galaxies, mass assembly appears to be dominated by mergers. When it comes to blue galaxies, the trend of decreasing fraction with denser environments seen for the group as a whole (sub-classes C7–11) is found to be driven mostly by one (the most numerous) group of galaxies; sub-class C10. These are compact low-mass galaxies with high, specific star formation rates, which are preferentially found in low-density environments. However, the remaining blue galaxies (sub-classes C7–9) are larger and appear in denser environments than galaxies within C10.

Probing Galaxy Evolution in Massive Clusters Using ACT and DES: Splashback as a Cosmic Clock

Abstract We measure the projected number density profiles of galaxies and the splashback feature in clusters selected by the Sunyaev–Zel’dovich effect from the Advanced Atacama Cosmology Telescope (AdvACT) survey using galaxies observed by the Dark Energy Survey (DES). The splashback radius is consistent with CDM-only simulations and is located at 2.4 − 0.4 + 0.3 Mpc h − 1 . We split the galaxies on color and find significant differences in their profile shapes. Red and green-valley galaxies show a splashback-like minimum in their slope profile consistent with theory, while the bluest galaxies show a weak feature at a smaller radius. We develop a mapping of galaxies to subhalos in simulations and assign colors based on infall time onto their hosts. We find that the shift in location of the steepest slope and different profile shapes can be mapped to the average time of infall of galaxies of different colors. The steepest slope traces a discontinuity in the phase space of dark matter halos. By relating spatial profiles to infall time, we can use splashback as a clock to understand galaxy quenching. We find that red galaxies have on average been in clusters over 3.2 Gyr, green galaxies about 2.2 Gyr, while blue galaxies have been accreted most recently and have not reached apocenter. Using the full radial profiles, we fit a simple quenching model and find that the onset of galaxy quenching occurs after a delay of about a gigayear and that galaxies quench rapidly thereafter with an exponential timescale of 0.6 Gyr.

Green valley galaxies in the cosmic web: internal versus environmental quenching

We analyze the SDSS data to classify the galaxies based on their colour using a fuzzy set-theoretic method and quantify their environments using the local dimension. We find that the fraction of the green galaxies does not depend on the environment and 10%–20% of the galaxies at each environment are in the green valley depending on the stellar mass range chosen. Approximately 10% of the green galaxies at each environment host an AGN. Combining data from the Galaxy Zoo, we find that ∼ 95% of the green galaxies are spirals and ∼ 5% are ellipticals at each environment. Only ∼ 8% of green galaxies exhibit signs of interactions and mergers, ∼ 1% have dominant bulge, and ∼ 6% host a bar. We show that the stellar mass distributions for the red and green galaxies are quite similar at each environment. Our analysis suggests that the majority of the green galaxies must curtail their star formation using physical mechanism(s) other than interactions, mergers, and those driven by bulge, bar and AGN activity. We speculate that these are the massive galaxies that have grown only via smooth accretion and suppressed the star formation primarily through mass driven quenching. Using a Kolmogorov-Smirnov test, we do not find any statistically significant difference between the properties of green galaxies in different environments. We conclude that the environmental factors play a minor role and the internal processes play the dominant role in quenching star formation in the green valley galaxies.

Conditions for Superwind Classes of Super Star Clusters

Abstract Mechanical feedback from young massive stars in super star clusters contributes to the formation of superwinds and superbubbles in star-forming regions. We conduct hydrodynamic simulations using the non-equilibrium ionization package maihem to explore how outflow velocity, mass-loading, metallicity, and ambient density can affect the occurrence of catastrophic cooling. To predict optical and UV emission lines, we apply the photoionization code cloudy to the physical conditions predicted by our hydrodynamic simulations. Our results could be useful for characterizing catastrophic cooling in starburst regions like the nearby Green Peas and distant star-forming galaxies.

The Hα Dots Survey. II. A Second List of Faint Emission-line Objects

Abstract We present the second catalog of serendipitously discovered compact extragalactic emission-line sources—Hα Dots. These objects have been discovered in searches of moderately deep narrow-band images acquired for the ALFALFA Hα project. In addition to cataloging 119 new Hα Dots, we also present follow-up spectral data for the full sample. These spectra allow us to confirm the nature of these objects as true extragalactic emission-line objects, to classify them in terms of activity type (star-forming or AGN), and to identify the emission line via which they were discovered. We tabulate photometric and spectroscopic data for the all objects, and we present an overview of the properties of the full Hα Dot sample. The Hα Dots represent a broad range of star-forming and active galaxies detected via several different emission lines over a wide range of redshifts. The sample includes Hα-detected blue compact dwarf galaxies at low redshift, [O iii]-detected Seyfert 2 and Green Pea-like galaxies at intermediate redshifts, and QSOs detected via one of several UV emission lines, including Lyα. Despite the heterogeneous appearance of the resulting catalog of objects, we show that our selection method leads to well-defined samples of specific classes of emission-line objects with properties that allow for statistical studies of each class.

The Effect of Environment on AGN Activity: The Properties of Radio and Optical AGN in Void, Isolated, and Group Galaxies

Abstract The evolution of galaxies depends on their environment. In this work, active galactic nucleus (AGN) activity in different environments has been studied. The fractions of radio and optical AGN in four different environments have been compared using samples of void, isolated, group member, and the brightest group galaxies. Galaxies in voids show significantly lower stellar ages, concentrations, colors, and surface mass densities, and they experience more one-on-one interactions compared to the isolated galaxies and galaxies in groups. To study pure environmental effects, the biases caused by the stellar mass and galaxy type quantified by 4000 Å break strength have been removed. While the results confirm no dependence of the optical AGN activity on environment in blue galaxies and with lower significance in green galaxies, a higher fraction of optical AGN has been observed for the massive red galaxies in voids compared to the galaxies in dense environments. This may be related to the higher amount of one-on-one interaction observed in the void galaxies, or it may reflect more fundamental differences in the host galaxies or environments of the voids. The radio-mode AGN activity increases in the dense environment for red galaxies. No changes in the radio-loud AGN fraction have been observed for the blue and green galaxies. This shows that the effect of environment on AGN activity is not significant in the presence of cold gas in galaxies. We also discuss whether the efficiency of gas accretion depends on the properties of the host galaxy.

A method for classification of red, blue, and green galaxies using fuzzy set theory

ABSTRACT Red and blue galaxies are traditionally classified using some specific cuts in colour or other galaxy properties, which are supported by empirical arguments. The vagueness associated with such cuts are likely to introduce a significant contamination in these samples. Fuzzy sets are vague boundary sets that can efficiently capture the classification uncertainty in the absence of any precise boundary. We propose a method for classification of galaxies according to their colours using fuzzy set theory. We use data from the Sloan Digital Sky Survey (SDSS) to construct a fuzzy set for red galaxies with its members having different degrees of ‘redness’. We show that the fuzzy sets for the blue and green galaxies can be obtained from it using different fuzzy operations. We also explore the possibility of using fuzzy relation to study the relationship between different galaxy properties and discuss its strengths and limitations.

A mathematical approach for creative graphics design

Mathematics is most beautiful thing in our daily life but when it comes to equations and formulae people seems to be unpleasant. This equations and formulae exhibits interactive properties and relation in our daily life. It is sure that the almost all beautiful thing in our nature even Universe unveils mathematical properties. These properties produce many patterns and shape and is directly related to geometric algebra as we can see in nature for example, fluorescent green beetle and colourful galaxies. These gorgeous properties of nature can be transformed into equations and logics. Therefore, the main aim of this paper is to generate creative graphics using mathematical equations such as number theory, position vectors and trigonometry functions. The sectional organization of this paper is as follows: section 1 gives the introduction of interactive graphic design and related work. Mathematical approach for graphic design has been briefed in section 2. In section 2 adopted algorithms and Graphical visualization of design is given. Graphic designs are applied to Indian ethnic wear and T-shirts. Last Section gives the conclusions and further scope of this work.

Chemodynamics of green pea galaxies – I. Outflows and turbulence driving the escape of ionizing photons and chemical enrichment

ABSTRACT We investigate the ionized gas kinematics, physical properties, and chemical abundances of Sloan Digital Sky Survey J142947, a Green Pea galaxy at redshift z∼ 0.17 with strong, double-peak Ly α emission and indirect evidence of Lyman continuum (LyC) leakage. Using high-dispersion spectroscopy, we perform a multicomponent analysis of emission-line profiles. Our model consistently fits all lines as a narrow component with intrinsic velocity dispersion σ ∼ 40 km s−1, and two broader blue-shifted components with σ ∼ 90 and ∼ 250 km s−1. We find electron densities and temperatures, ionization conditions, and direct O/H and N/O abundances for each component. A highly ionized, metal-poor, young and compact starburst dominates narrow emission, showing evidence of hard radiation fields and elevated N/O. The blue-shifted broader components are consistent with highly turbulent, possibly clumpy ionized gas at the base of a strong photoionized outflow, which accounts for ≳50 per cent of the integrated emission-line fluxes. The outflow is dense and metal-enriched compared to the H ii regions, with expansion velocities larger than those obtained from UV interstellar absorption lines under standard assumptions. Some of these metals may be able to escape, with outflows loading factors comparable to those found in high-z galaxies of similar SFR/Area. Our findings depict a two-stage starburst picture; hard radiation fields from young star clusters illuminate a turbulent and clumpy ISM that has been eroded by SNe feedback. Whilst UV data suggest an extended Ly α halo with high average H i column density, LyC photons could only escape from SDSS J142947 through low H i density channels or filaments in the ISM approaching density-bounded conditions, traced by outflowing gas.

The Mass Dependence of Structure, Star Formation Rate, and Mass Assembly Mode at 0.5 < z < 2.5

Abstract To investigate the mass dependence of structural transformation and star formation quenching, we construct three galaxy samples using massive (M * > 1010 M ⊙) red, green, and blue galaxy populations at 0.5 < z < 2.5 in five 3D–HST/CANDELS fields. The structural parameters, including effective radius (r e), galaxy compactness (Σ1.5), and second-order moment of 20% brightest pixels (M 20), are found to be correlated with stellar mass. Sérsic index (n), concentration (C), and Gini coefficient (G) seem to be insensitive to stellar mass. The morphological distinction between blue and red galaxies is found at a fixed mass bin, suggesting that quenching processes should be accompanied with transformations of galaxy structure and morphology. Except for r e and Σ1.5 at the high-mass end, structural parameters of green galaxies are intermediate between red and blue galaxies in each stellar mass bin at z < 2, indicating that green galaxies are at a transitional phase when blue galaxies are being quenched into quiescent statuses. The similar sizes and compactness for the blue and green galaxies at the high-mass end imply that some of these galaxies will not appear to be significantly shrunk until they are completely quenched into red quiescent galaxies. For the green galaxies at 0.5 < z < 1.5, a morphological transformation sequence of bulge buildup can be seen as their star formation activities are gradually shut down, while a faster morphological transformation is verified for the green galaxies at 1.5 < z < 2.5.

J-PLUS: Impact of bars on quenching timescales in nearby green valley disc galaxies

Context. Between the blue cloud and the red sequence peaks on the galaxy colour–magnitude diagram there is a region sparsely populated by galaxies called the green valley. In a framework where galaxies mostly migrate on the colour–magnitude diagram from star forming to quiescent, the green valley is considered a transitional galaxy stage. The details of the processes that drive galaxies from star-forming to passive systems still remain unknown. Aims. We aim to measure the transitional timescales of nearby galaxies across the green valley, through the analysis of Galaxy Evolution Explorer and Javalambre Photometric of Local Universe Survey photometric data. Specifically, we seek to study the impact of bars on the quenching timescales. Methods. We developed a method that estimates empirically the star formation quenching timescales of green valley galaxies, assuming an exponential decay model of the star formation histories and through a combination of narrow and broad bands from the Javalambre Photometric of Local Universe Survey and Galaxy Evolution Explorer. We correlated these quenching timescales with the presence of bars. Results. We find that the Javalambre Photometric of Local Universe Survey colours F0395 −g and F0410 −g are sensitive to different star formation histories, showing, consequently, a clear correlation with the Dn(4000) and Hδ, A spectral indices. We measured quenching timescales based on these colours and we find that quenching timescales obtained with our new approach are in agreement with those determined using spectral indices. We also compared the quenching timescales of green valley disc galaxies as a function of the probability of hosting a bar. We find that galaxies with high bar probability tend to quench their star formation slowly. Conclusions. We conclude that: (1) Javalambre Photometric of Local Universe Survey filters can be used to measure quenching timescales in nearby green valley galaxies; and (2) the resulting star formation quenching timescales are longer for barred green valley galaxies. Considering that the presence of a bar indicates that more violent processes (e.g. major mergers) are absent in host galaxies, we conclude that the presence of a bar can be used as a morphological signature for slow star formation quenching.

Galaxy And Mass Assembly (GAMA): Timescales for galaxies crossing the green valley

We explore the constraints that can be placed on the evolutionary timescales for typical low redshift galaxies evolving from the blue cloud through the green valley and onto the red sequence. We utilise galaxies from the GAMA survey with 0.1 < z < 0.2 and classify them according to the intrinsic (u-r?) colours of their stellar populations, as determined by fits to their multi-wavelength spectral energy distributions. Using these fits to also determine stellar population ages and star formation timescales, we argue that our results are consistent with a green valley population dominated by galaxies that are simply decreasing their star formation (running out of gas) over a timescale of 2-4 Gyr which are seen at a specific epoch in their evolution (approximately 1.6 e-folding times after their peak in star formation). If their fitted star formation histories are extrapolated forwards, the green galaxies will further redden over time, until they attain the colours of a passive population. In this picture, no specific quenching event which cuts-off their star formation is required, though it remains possible that the decline in star formation in green galaxies may be expedited by internal or external forces. However, there is no evidence that green galaxies have recently changed their star formation timescales relative to their previous longer term star formation histories.

Compact Galaxies at intermediate redshifts quench faster than normal-sized Galaxies

Massive quiescent compact galaxies have been discovered at high redshifts, associated with rapid compaction and cessation of star formation (SF). In this work we set out to quantify the time-scales in which SF is quenched in compact galaxies at intermediate redshifts. For this, we select a sample of green valley galaxies within the COSMOS field in the midst of quenching their SF at $0.5<z<1.0$ that exhibit varying degrees of compactness. Based on the H$\delta$ absorption line and the 4000 \AA \ break of coadded zCOSMOS spectra for sub-samples of normal-sized and compact galaxies we determine quenching time-scales as a function of compactness. We find that the SF quenching time-scales in green valley compact galaxies are much shorter than in normal-sized ones. In an effort to understand this trend, we use the Illustris simulation to trace the evolution of the SF history, the growth rate of the central super massive black hole (SMBH) {\bf and the AGN-feedback in compact and normal-sized galaxies. We find that the key difference between their SF quenching time-scales is linked to the mode of the AGN-feedback. In the compact galaxies predominates the kinematic-mode, which is highly efficient at quenching the SF by depleting the internal gas. On the normal-sized galaxies, the prevailing thermal-mode injects energy in the circumgalactic gas, impeding the cold gas supply and quenching the SF via the slower strangulation mechanism.} These results are consistent with the violent disk instability and gas-rich mergers scenarios, followed by strong AGN and stellar feedback. Although this kind of event is most expected to occur at $z=2-3$, we find evidences that the formation of compact quiescent galaxies can occur at $z<1$.

Investigating the Lyman photon escape in local starburst galaxies with the Cosmic Origins Spectrograph★

We present a study of 7 star-forming galaxies from the Cosmic Evolution Survey (COSMOS) observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). The galaxies are located at relatively low redshifts, $z\sim$0.3, with morphologies ranging from extended and disturbed to compact and smooth. To complement the HST observations we also analyze observations taken with the VIMOS spectrograph on the Very Large Telescope (VLT). In our galaxy sample we identify three objects with double peak Lyman-$\alpha$ profiles similar to those seen in Green Pea compact galaxies and measure peak separations of 655, 374, and 275 km s$^{-1}$. We measure Lyman-$\alpha$ escape fractions with values ranging between 5-13\%. Given the low flux levels in the individual COS exposures we apply a weighted stacking approach to obtain a single spectrum. From this COS combined spectrum we infer upper limits for the absolute and relative Lyman continuum escape fractions of $f_{\rm abs}(\rm LyC)$ = 0.4$^{+10.1}_{-0.4}$\% and $f_{\rm res}(\rm LyC)$ = 1.7$^{+15.2}_{-1.7}$\%, respectively. Finally, we find that most of these galaxies have moderate UV and optical SFRs (SFRs $\lesssim$ 10 M$_{\odot}$ yr$^{-1}$).

Galaxy and Mass Assembly (GAMA): variation in galaxy structure across the green valley

Using a sample of 472 local Universe (z<0.06) galaxies in the stellar mass range 10.25 < log M*/M_sun < 10.75, we explore the variation in galaxy structure as a function of morphology and galaxy colour. Our sample of galaxies is sub-divided into red, green and blue colour groups and into elliptical and non-elliptical (disk-type) morphologies. Using KiDS and VIKING derived postage stamp images, a group of eight volunteers visually classified bars, rings, morphological lenses, tidal streams, shells and signs of merger activity for all systems. We find a significant surplus of rings ($2.3\sigma$) and lenses ($2.9\sigma$) in disk-type galaxies as they transition across the green valley. Combined, this implies a joint ring/lens green valley surplus significance of $3.3\sigma$ relative to equivalent disk-types within either the blue cloud or the red sequence. We recover a bar fraction of ~44% which remains flat with colour, however, we find that the presence of a bar acts to modulate the incidence of rings and (to a lesser extent) lenses, with rings in barred disk-type galaxies more common by ~20-30 percentage points relative to their unbarred counterparts, regardless of colour. Additionally, green valley disk-type galaxies with a bar exhibit a significant $3.0\sigma$ surplus of lenses relative to their blue/red analogues. The existence of such structures rules out violent transformative events as the primary end-of-life evolutionary mechanism, with a more passive scenario the favoured candidate for the majority of galaxies rapidly transitioning across the green valley.

The Morphological Evolution, AGN Fractions, Dust Content, Environments, and Downsizing of Massive Green Valley Galaxies at 0.5 &lt; z &lt; 2.5 in 3D-HST/CANDELS

Abstract To explore the evolutionary connection among red, green, and blue galaxy populations, based on a sample of massive ( ) galaxies at 0.5 &lt; z &lt; 2.5 in five 3D-HST/CANDELS fields, we investigate the dust content, morphologies, structures, active galactic nucleus (AGN) fractions, and environments of these three populations. Green valley galaxies are found to have intermediate dust attenuation and reside in the middle of the regions occupied by quiescent and star-forming galaxies in the UVJ diagram. Compared with blue and red galaxy populations at z &lt; 2, green galaxies have intermediate compactness and morphological parameters. The above findings seem to favor the scenario that green galaxies are at a transitional phase when star-forming galaxies are being quenched into quiescent status. The green galaxies at z &lt; 2 show the highest AGN fraction, suggesting that AGN feedback may have played an important role in star formation quenching. For the massive galaxies at 2 &lt; z &lt; 2.5, both red and green galaxies are found to have a similarly higher AGN fraction than the blue ones, which implies that AGN feedback may help to keep quiescence of red galaxies at z &gt; 2. A significant environmental difference is found between green and red galaxies at z &lt; 1.5. Green and blue galaxies at z &gt; 0.5 seem to have similar local density distributions, suggesting that environment quenching is not the major mechanism to cease star formation at z &gt; 0.5. The fractions of three populations as functions of mass support a “downsizing” quenching picture that the bulk of star formation in more massive galaxies is completed earlier than that of lower-mass galaxies.

Galaxy and Mass Assembly (GAMA): Morphological transformation of galaxies across the green valley

We explore constraints on the joint photometric and morphological evolution of typical low redshift galaxies as they move from the blue cloud through the green valley and onto the red sequence. We select GAMA survey galaxies with $10.25<{\rm log}(M_*/M_\odot)<10.75$ and $z<0.2$ classified according to their intrinsic $u^*-r^*$ colour. From single component S\'ersic fits, we find that the stellar mass-sensitive $K-$band profiles of red and green galaxy populations are very similar, while $g-$band profiles indicate more disk-like morphologies for the green galaxies: apparent (optical) morphological differences arise primarily from radial mass-to-light ratio variations. Two-component fits show that most green galaxies have significant bulge and disk components and that the blue to red evolution is driven by colour change in the disk. Together, these strongly suggest that galaxies evolve from blue to red through secular disk fading and that a strong bulge is present prior to any decline in star formation. The relative abundance of the green population implies a typical timescale for traversing the green valley $\sim 1-2$~Gyr and is independent of environment, unlike that of the red and blue populations. While environment likely plays a r\^ole in triggering the passage across the green valley, it appears to have little effect on time taken. These results are consistent with a green valley population dominated by (early type) disk galaxies that are insufficiently supplied with gas to maintain previous levels of disk star formation, eventually attaining passive colours. No single event is needed quench their star formation.

Radio Galaxy Zoo: A Search for Hybrid Morphology Radio Galaxies

Abstract Hybrid morphology radio sources (HyMoRS) are a rare type of radio galaxy that display different Fanaroff–Riley classes on opposite sides of their nuclei. To enhance the statistical analysis of HyMoRS, we embarked on a large-scale search of these sources within the international citizen science project, Radio Galaxy Zoo (RGZ). Here, we present 25 new candidate hybrid morphology radio galaxies. Our selected candidates are moderate power radio galaxies ( W Hz−1 sr−1) at redshifts . Hosts of nine candidates have spectroscopic observations, of which six are classified as quasars, one as high- and two as low-excitation galaxies. Two candidate HyMoRS are giant ( Mpc) radio galaxies, one resides at the center of a galaxy cluster, and one is hosted by a rare green bean galaxy. Although the origin of the hybrid morphology radio galaxies is still unclear, this type of radio source starts depicting itself as a rather diverse class. We discuss hybrid radio morphology formation in terms of the radio source environment (nurture) and intrinsically occurring phenomena (nature; activity cessation and amplification), showing that these peculiar radio galaxies can be formed by both mechanisms. While high angular resolution follow-up observations are still necessary to confirm our candidates, we demonstrate the efficacy of the RGZ in the pre-selection of these sources from all-sky radio surveys, and report the reliability of citizen scientists in identifying and classifying complex radio sources.