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Register a new userThe GOGREEN survey: Transition Galaxies and The Evolution of Environmental Quenching
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We measure the rate of environmentally-driven star formation quenching in galaxies at $zsim 1$, using eleven massive ($Mapprox 2times10^{14},mathrm{M}_odot$) galaxy clusters spanning a redshift range $1.0<z<1.4$ from the GOGREEN sample. We identify three different types of transition galaxies: green valley (GV) galaxies identified from their rest-frame $(NUV-V)$ and $(V-J)$ colours; blue quiescent (BQ) galaxies, found at the blue end of the quiescent sequence in $(U-V)$ and $(V-J)$ colour; and spectroscopic post-starburst (PSB) galaxies. We measure the abundance of these galaxies as a function of stellar mass and environment. For high stellar mass galaxies ($log{M/mathrm{M}_odot}>10.5$) we do not find any significant excess of transition galaxies in clusters, relative to a comparison field sample at the same redshift. It is likely that such galaxies were quenched prior to their accretion in the cluster, in group, filament or protocluster environments. For lower stellar mass galaxies ($9.5<log{M/mathrm{M}_odot}<10.5$) there is a small but significant excess of transition galaxies in clusters, accounting for an additional $sim 5-10$ per cent of the population compared with the field. We show that our data are consistent with a scenario in which 20--30 per cent of low-mass, star-forming galaxies in clusters are environmentally quenched every Gyr, and that this rate slowly declines from $z=1$ to $z=0$. While environmental quenching of these galaxies may include a long delay time during which star formation declines slowly, in most cases this must end with a rapid ($tau<1$ Gyr) decline in star formation rate.
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We use photometric redshifts and statistical background subtraction to measure stellar mass functions in galaxy group-mass ($4.5-8times10^{13}~mathrm{M}_odot$) haloes at $1<z<1.5$. Groups are selected from COSMOS and SXDF, based on X-ray imaging and sparse spectroscopy. Stellar mass ($M_{mathrm{stellar}}$) functions are computed for quiescent and star-forming galaxies separately, based on their rest-frame $UVJ$ colours. From these we compute the quiescent fraction and quiescent fraction excess (QFE) relative to the field as a function of $M_{mathrm{stellar}}$. QFE increases with $M_{mathrm{stellar}}$, similar to more massive clusters at $1<z<1.5$. This contrasts with the apparent separability of $M_{mathrm{stellar}}$ and environmental factors on galaxy quiescent fractions at $zsim 0$. We then compare our results with higher mass clusters at $1<z<1.5$ and lower redshifts. We find a strong QFE dependence on halo mass at fixed $M_{mathrm{stellar}}$; well fit by a logarithmic slope of $mathrm{d}(mathrm{QFE})/mathrm{d}log (M_{mathrm{halo}}) sim 0.24 pm 0.04$ for all $M_{mathrm{stellar}}$ and redshift bins. This dependence is in remarkably good qualitative agreement with the hydrodynamic simulation BAHAMAS, but contradicts the observed dependence of QFE on $M_{mathrm{stellar}}$. We interpret the results using two toy models: one where a time delay until rapid (instantaneous) quenching begins upon accretion to the main progenitor (no pre-processing) and one where it starts upon first becoming a satellite (pre-processing). Delay times appear to be halo mass dependent, with a significantly stronger dependence required without pre-processing. We conclude that our results support models in which environmental quenching begins in low-mass ($<10^{14}M_odot$) haloes at $z>1$.
We study the star formation histories (SFHs) and mass-weighted ages of 331 UVJ-selected quiescent galaxies in 11 galaxy clusters and in the field at 1<z<1.5 from the Gemini Observations of Galaxies in Rich Early ENvironments (GOGREEN) survey. We determine the SFHs of individual galaxies by simultaneously fitting rest-frame optical spectroscopy and broadband photometry to stellar population models. We confirm that the SFHs are consistent with more massive galaxies having on average earlier formation times. Comparing galaxies found in massive clusters with those in the field, we find galaxies with $M_ast<10^{11.3}$ M$_{odot}$ in the field have more extended SFHs. From the SFHs we calculate the mass-weighted ages, and compare age distributions of galaxies between the two environments, at fixed mass. We constrain the difference in mass-weighted ages between field and cluster galaxies to $0.31_{^{-0.33}}^{_{+0.51}}$ Gyr, in the sense that cluster galaxies are older. We place this result in the context of two simple quenching models and show that neither environmental quenching based on time since infall (without pre-processing) nor a difference in formation times alone can reproduce both the average age difference and relative quenched fractions. This is distinctly different from local clusters, for which the majority of the quenched population is consistent with having been environmentally quenched upon infall. Our results suggest that quenched population in galaxy clusters at z>1 has been driven by different physical processes than those at play at z=0.
Using a sample of 4 galaxy clusters at $1.35 < z < 1.65$ and 10 galaxy clusters at $0.85 < z < 1.35$, we measure the environmental quenching timescale, $t_Q$, corresponding to the time required after a galaxy is accreted by a cluster for it to fully cease star formation. Cluster members are selected by a photometric-redshift criterion, and categorized as star-forming, quiescent, or intermediate according to their dust-corrected rest-frame colors and magnitudes. We employ a delayed-then-rapid quenching model that relates a simulated cluster mass accretion rate to the observed numbers of each type of galaxy in the cluster to constrain $t_Q$. For galaxies of mass $M_* gtrsim 10^{10.5}~ mathrm{M}_odot$, we find a quenching timescale of $t_Q=$ 1.24 Gyr in the $zsim1.5$ cluster sample, and $t_Q=$ 1.50 Gyr at $zsim1$. Using values drawn from the literature, we compare the redshift evolution of $t_Q$ to timescales predicted for different physical quenching mechanisms. We find $t_Q$ to depend on host halo mass such that quenching occurs over faster timescales in clusters relative to groups, suggesting that properties of the host halo are responsible for quenching high-mass galaxies. Between $z=0$ and $z=1.5$, we find that $t_Q$ evolves faster than the molecular gas depletion timescale and slower than an SFR-outflow timescale, but is consistent with the evolution of the dynamical time. This suggests that environmental quenching in these galaxies is driven by the motion of satellites relative to the cluster environment, although due to uncertainties in the atomic gas budget at high redshift, we cannot rule out quenching due to simple gas depletion.
We study the stellar mass functions (SMFs) of star-forming and quiescent galaxies in 11 galaxy clusters at 1.0<z<1.4, drawn from the Gemini Observations of Galaxies in Rich Early Environments (GOGREEN) survey. Based on more than 500 hours of Gemini/GMOS spectroscopy, and deep multi-band photometry taken with a range of observatories, we probe the SMFs down to a stellar mass limit of 10^9.7 Msun (10^9.5 Msun for star-forming galaxies). At this early epoch, the fraction of quiescent galaxies is already highly elevated in the clusters compared to the field at the same redshift. The quenched fraction excess (QFE) represents the fraction of galaxies that would be star-forming in the field, but are quenched due to their environment. The QFE is strongly mass dependent, and increases from ~30% at Mstar=10^9.7 Msun, to ~80% at Mstar=10^11.0 Msun. Nonetheless, the shapes of the SMFs of the two individual galaxy types, star-forming and quiescent galaxies, are identical between the clusters and the field - to high statistical precision. Yet, along with the different quiescent fractions is the total galaxy SMF environmentally dependent, with a relative deficit of low-mass galaxies in the clusters. These results are in stark contrast with findings in the local Universe, and thus require a substantially different quenching mode to operate at early times. We discuss these results in the light of several popular quenching models.
In the local Universe, there is a strong division in the star-forming properties of low-mass galaxies, with star formation largely ubiquitous amongst the field population while satellite systems are predominantly quenched. This dichotomy implies that environmental processes play the dominant role in suppressing star formation within this low-mass regime (${M}_{star} sim 10^{5.5-8}~{rm M}_{odot}$). As shown by observations of the Local Volume, however, there is a non-negligible population of passive systems in the field, which challenges our understanding of quenching at low masses. By applying the satellite quenching models of Fillingham et al. (2015) to subhalo populations in the Exploring the Local Volume In Simulations (ELVIS) suite, we investigate the role of environmental processes in quenching star formation within the nearby field. Using model parameters that reproduce the satellite quenched fraction in the Local Group, we predict a quenched fraction -- due solely to environmental effects -- of $sim 0.52 pm 0.26$ within $1< R/R_{rm vir} < 2$ of the Milky Way and M31. This is in good agreement with current observations of the Local Volume and suggests that the majority of the passive field systems observed at these distances are quenched via environmental mechanisms. Beyond $2~R_{rm vir}$, however, dwarf galaxy quenching becomes difficult to explain through an interaction with either the Milky Way or M31, such that more isolated, field dwarfs may be self-quenched as a result of star-formation feedback.
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