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Register a new userGreen valley galaxies in the cosmic web: internal versus environmental quenching
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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 $sim 95%$ of the green galaxies are spirals and $sim 5%$ are ellipticals at each environment. Only $sim 8%$ of green galaxies exhibit signs of interactions and mergers, $sim 1%$ have dominant bulge, and $sim 6%$ 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.
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A clear transition feature of galaxy quenching is identified in the multi-parameter space of stellar mass ($M_*$), bulge to total mass ratio ($B/T_{rm m}$), halo mass ($M_{rm h}$) and halo-centric distance ($r/r_{180}$). For given halo mass, the characteristic stellar mass ($M_{*, rm ch}$) for the transition is about one-fifth of that of the corresponding central galaxy, and almost independent of $B/T_{rm m}$. Once $B/T_{rm m}$ is fixed, the quenched fraction of galaxies with $M_{*} < M_{*, rm ch}$ increases with $M_{rm h}$, but decreases with $M_*$ in the inner part of halos ($r/r_{180} < 0.5$). In the outer part ($r/r_{180} > 0.5$), the trend with $M_{rm h}$ remains but the correlation with $M_*$ is absent or becomes positive. For galaxies above $M_{rm *, ch}$ and with $B/T_{rm m}$ fixed, the quenched fraction increases with $M_{rm *}$, but depends only weakly on $M_{rm h}$ in both the inner and outer regions. At fixed $B/T_{rm m}$ and $M_*$, the quenched fraction increases with decreasing $r/r_{180}$ for galaxies with $M_{*} < M_{*, rm ch}$, and depends only weakly on $r/r_{180}$ for galaxies with $M_{*} > M_{*, rm ch}$. Our finding provides a physically-motivated way to classify galaxies in halos into two classes based on their quenching properties: an `upper class with $M_{*} > M_{rm *,ch}$ and a `lower class with $M_{*} < M_{rm *,ch}$. Environmental quenching is important for `lower class galaxies, while internal quenching plays the dominating role for the `upper class.
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. We developed a method that estimates empirically the star formation quenching times-scales of green valley galaxies, assuming an exponential decay model of the SFH and through a combination of narrow and broad bands from J-PLUS and GALEX. We correlate these quenching time-scales with the presence of bars. We find that the J-PLUS colours F0395-g and F0415-g are sensitive to different SFH, showing, a clear correlation with the Dn(4000) and H-delta,A spectral indices. We find that quenching time-scales obtained with our new approach are in agreement with those determined using spectral indices. We also find that galaxies with high bar probability tend to quench their star formation slowly. We conclude that: 1) J-PLUS filters can be used to measure quenching timescales in nearby green valley galaxies; and 2) the resulting star formation quenching time-scales 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.
We study radial profiles in H$alpha$ equivalent width and specific star formation rate (sSFR) derived from spatially-resolved SDSS-IV MaNGA spectroscopy to gain insight on the physical mechanisms that suppress star formation and determine a galaxys location in the SFR-$rm M_star$ diagram. Even within the star-forming `main sequence, the measured sSFR decreases with stellar mass, both in an integrated and spatially-resolved sense. Flat sSFR radial profiles are observed for $rm log(M_star/ M_odot) < 10.5$, while star-forming galaxies of higher mass show a significant decrease in sSFR in the central regions, a likely consequence of both larger bulges and an inside-out growth history. Our primary focus is the green valley, constituted by galaxies lying below the star formation main sequence, but not fully passive. In the green valley we find sSFR profiles that are suppressed with respect to star-forming galaxies of the same mass at all galactocentric distances out to 2 effective radii. The responsible quenching mechanism therefore appears to affect the entire galaxy, not simply an expanding central region. The majority of green valley galaxies of $rm log(M_star/ M_odot) > 10.0$ are classified spectroscopically as central low-ionisation emission-line regions (cLIERs). Despite displaying a higher central stellar mass concentration, the sSFR suppression observed in cLIER galaxies is not simply due to the larger mass of the bulge. Drawing a comparison sample of star forming galaxies with the same $rm M_star$ and $rm Sigma_{1~kpc}$ (the mass surface density within 1 kpc), we show that a high $rm Sigma_{1~kpc}$ is not a sufficient condition for determining central quiescence.
We use SDSS+textit{GALEX}+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from 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 strikingly anisotropic large-scale distribution of matter made of an extended network of voids delimited by sheets, themselves segmented by filaments, within which matter flows towards compact nodes where they intersect, imprints its geometry on the dynamics of cosmic flows, ultimately shaping the distribution of galaxies and the redshift evolution of their properties. The (filament-type) saddle points of this cosmic web provide a local frame in which to quantify the induced physical and morphological evolution of galaxies on large scales. The properties of virtual galaxies within the Horizon-AGN simulation are stacked in such a frame. The iso-contours of the galactic number density, mass, specific star formation rate (sSFR), kinematics and age are clearly aligned with the filament axis with steep gradients perpendicular to the filaments. A comparison to a simulation without feedback from active galactic nuclei (AGN) illustrates its impact on quenching star formation of centrals away from the saddles. The redshift evolution of the properties of galaxies and their age distribution are consistent with the geometry of the bulk flow within that frame. They compare well with expectations from constrained Gaussian random fields and the scaling with the mass of non-linearity, modulo the redshift dependent impact of feedback processes. Physical properties such as sSFR and kinematics seem not to depend only on mean halo mass and density: the residuals trace the geometry of the saddle, which could point to other environment-sensitive physical processes, such as spin advection, and AGN feedback at high mass.
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