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Register a new userGalaxy Zoo: The interplay of quenching mechanisms in the group environment
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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 $sim2.5~rm{Gyr}$ from star forming to complete quiescence, during an average infall time (from $sim 10R_{200}$ to $0.01R_{200}$) of $sim 2.6~rm{Gyr}$. 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.
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We have used Galaxy Zoo DECaLS (GZD) to study strong and weak bars in disk galaxies. Out of the 314,000 galaxies in GZD, we created a volume-limited sample (0.01 < z < 0.05, Mr < -18.96) which contains 1,867 galaxies with reliable volunteer bar classifications in the ALFALFA footprint. In keeping with previous Galaxy Zoo surveys (such as GZ2), the morphological classifications from GZD agree well with previous morphological surveys. GZD considers galaxies to either have a strong bar (15.5%), a weak bar (28.1%) or no bar (56.4%), based on volunteer classifications on images obtained from the DECaLS survey. This places GZD in a unique position to assess differences between strong and weak bars. We find that the strong bar fraction is typically higher in quiescent galaxies than in star forming galaxies, while the weak bar fraction is similar. Moreover, we have found that strong bars facilitate the quenching process in star forming galaxies, finding higher fibre SFRs, lower gas masses and shorter depletion timescales in these galaxies compared to unbarred galaxies. However, we also found that any differences between strong and weak bars disappear when controlling for bar length. Based on this, we conclude that weak and strong bars are not fundamentally different phenomena. Instead, we propose that there is a continuum of bar types, which varies from weakest to strongest.
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 the 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~0 major merger quenched galaxies are unlikely to constitute the majority of galaxies that transition the green valley. Furthermore, between z~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~0 nor within the last 5 Gyr. The majority of red galaxies must have been quenched through an alternative quenching mechanism which causes a slow blue to red evolution.
Using the Sloan Digital Sky Survey, we adopt the sSFR-$Sigma_{1kpc}$ diagram as a diagnostic tool to understand quenching in different environments. sSFR is the specific star formation rate, and $Sigma_{1kpc}$ is the stellar surface density in the inner kpc. Although both the host halo mass and group-centric distance affect the satellite population, we find that these can be characterised by a single number, the quenched fraction, such that key features of the sSFR-$Sigma_{1kpc}$ diagram vary smoothly with this proxy for the environment. Particularly, the sSFR of star-forming galaxies decreases smoothly with this quenched fraction, the sSFR of satellites being 0.1 dex lower than in the field. Furthermore, $Sigma_{1kpc}$ of the transition galaxies (i.e., the green valley or GV) decreases smoothly with the environment, by as much as 0.2 dex for $M_* = 10^{9.75-10} M_{odot}$ from the field, and decreasing for satellites in larger halos and at smaller radial distances within same-mass halos. We interpret this shift as indicating the relative importance of todays field quenching track vs. the cluster quenching track. These environmental effects in the sSFR-$Sigma_{1kpc}$ diagram are most significant in our lowest mass range ($9.75 < log M_{*}/M_{odot} < 10$). One feature that is shared between all environments is that at a given $M_{*}$ quenched galaxies have about 0.2-0.3 dex higher $Sigma_{1kpc}$ than the star-forming population. These results indicate that either $Sigma_{1kpc}$ increases (subsequent to satellite quenching), or $Sigma_{1kpc}$ for individual galaxies remains unchanged, but the original $M_*$ or the time of quenching is significantly different from those now in the GV.
We investigate the role of environment on radio galaxy properties by constructing a sample of large ($gtrsim100$~kpc), nearby ($z<0.3$) radio sources identified as part of the Radio Galaxy Zoo citizen science project. Our sample consists of 16 Fanaroff-Riley Type II (FR-II) sources, 6 FR-I sources, and one source with a hybrid morphology. FR-I sources appear to be hosted by more massive galaxies, consistent with previous studies. In the FR-II sample, we compare the degree of asymmetry in radio lobe properties to asymmetry in the radio source environment, quantified through optical galaxy clustering. We find that the length of radio lobes in FR-II sources is anti-correlated with both galaxy clustering and lobe luminosity. These results are in quantitative agreement with predictions from radio source dynamical models, and suggest that galaxy clustering provides a useful proxy for the ambient gas density distribution encountered by the radio lobes.
We study the properties of a sample of 3967 LINER galaxies selected from SDSS-DR7, respect to their proximity to galaxy groups. The host galaxies of LINER have been analysed and compared with a well defined control sample of 3841 non-LINER galaxies matched in redshift, luminosity, colour, morphology, age and stellar mass content. We find no difference between LINER and control galaxies in terms of colour and age of stellar population as function of the virial mass and distance to the geometric centre of the group. However, we find that LINER are more likely to populate low density environments in spite of their morphology, which is typical of high density regions such as rich galaxy clusters. For rich (poor) galaxy groups, the occurrence of LINER is $sim$2 times lower (higher) than the occurrence of matched, non-LINER galaxies. Moreover, LINER hosts do not seem to follow the expected morphology-density relation in groups of high virial mass. The high frequency of LINERS in low density regions could be due to the combination of a sufficiently ample gas reservoir to power the low ionization emission and/or enhanced galaxy interaction rates benefiting the gas flow toward their central regions.
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