ABSTRACT We study the effects of the local environment and stellar mass on galaxy properties using a mass complete sample of quiescent and star-forming systems in the COSMOS field at . We show that ...at the median star formation rate (SFR) and specific SFR (sSFR) of all galaxies depend on the environment, but they become independent of the environment at z 1. However, we find that only for star-forming galaxies, the median SFR and sSFR are similar in different environments regardless of redshift and stellar mass. We find that the quiescent fraction depends on the environment at z 1 and on stellar mass out to z ∼ 3. We show that at z 1 galaxies become quiescent faster in denser environments and that the overall environmental quenching efficiency increases with cosmic time. Environmental and mass quenching processes depend on each other. At z 1 denser environments more efficiently quench galaxies with higher masses (log( ) 10.7), possibly due to a higher merger rate of massive galaxies in denser environments. We also show that mass quenching is more efficient in denser regions. We show that the overall mass quenching efficiency ( ) for more massive galaxies (log( ) 10.2) rises with cosmic time until z ∼ 1 and then flattens out. However, for less massive galaxies, the rise in continues to the present time. Our results suggest that environmental quenching is only relevant at z 1 and is likely a fast process, whereas mass quenching is the dominant mechanism at z 1 with a possible stellar feedback physics.
We use a mass complete (log( ) ) sample of galaxies with accurate photometric redshifts in the COSMOS field to construct the density field and the cosmic web to z = 1.2. The comic web extraction ...relies on the density field Hessian matrix and breaks the density field into clusters, filaments, and the field. We provide the density field and cosmic web measures to the community. We show that at z 0.8, the median star formation rate (SFR) in the cosmic web gradually declines from the field to clusters and this decline is especially sharp for satellites (∼1 dex versus ∼0.5 dex for centrals). However, at z 0.8, the trend flattens out for the overall galaxy population and satellites. For star-forming (SF) galaxies only, the median SFR is constant at z 0.5 but declines by ∼0.3-0.4 dex from the field to clusters for satellites and centrals at z 0.5. We argue that for satellites, the main role of the cosmic web environment is to control their SF fraction, whereas for centrals, it is mainly to control their overall SFR at z 0.5 and to set their fraction at z 0.5. We suggest that most satellites experience a rapid quenching mechanism as they fall from the field into clusters through filaments, whereas centrals mostly undergo a slow environmental quenching at z 0.5 and a fast mechanism at higher redshifts. Our preliminary results highlight the importance of the large-scale cosmic web on galaxy evolution.
We report on the design and performance of the Keck Cosmic Web Imager (KCWI), a general purpose optical integral field spectrograph that has been installed at the Nasmyth port of the 10 m Keck II ...telescope on Maunakea, Hawaii. The novel design provides blue-optimized seeing-limited imaging from 350-560 nm with configurable spectral resolution from 1000-20,000 in a field of view up to 20″ × 33″. Selectable volume phase holographic (VPH) gratings and high-performance dielectric, multilayer silver, and enhanced-aluminum coatings provide end-to-end peak efficiency in excess of 45% while accommodating the future addition of a red channel that will extend wavelength coverage to 1 micron. KCWI takes full advantage of the excellent seeing and dark sky above Maunakea with an available nod-and-shuffle observing mode. The instrument is optimized for observations of faint, diffuse objects such as the intergalactic medium or cosmic web. In this paper, a detailed description of the instrument design is provided with measured performance results from the laboratory test program and 10 nights of on-sky commissioning during the spring of 2017. The KCWI team is lead by Caltech and JPL (project management, design, and implementation) in partnership with the University of California at Santa Cruz (camera optical and mechanical design) and the W. M. Keck Observatory (observatory interfaces).
Abstract
We present a sample of ∼1000 emission-line galaxies at z = 0.4–4.7 from the ∼0.7deg2 High-z Emission-Line Survey in the Boötes field identified with a suite of six narrow-band filters at ...≈0.4–2.1 μm. These galaxies have been selected on their Ly α (73), O ii (285), H β/O iii (387) or H α (362) emission line, and have been classified with optical to near-infrared colours. A subsample of 98 sources have reliable redshifts from multiple narrow-band (e.g. O ii–H α) detections and/or spectroscopy. In this survey paper, we present the observations, selection and catalogues of emitters. We measure number densities of Ly α, O ii, H β/O iii and H α and confirm strong luminosity evolution in star-forming galaxies from z ∼ 0.4 to ∼5, in agreement with previous results. To demonstrate the usefulness of dual-line emitters, we use the sample of dual O ii–H α emitters to measure the observed O ii/H α ratio at z = 1.47. The observed O ii/H α ratio increases significantly from 0.40 ± 0.01 at z = 0.1 to 0.52 ± 0.05 at z = 1.47, which we attribute to either decreasing dust attenuation with redshift, or due to a bias in the (typically) fibre measurements in the local Universe that only measure the central kpc regions. At the bright end, we find that both the H α and Ly α number densities at z ≈ 2.2 deviate significantly from a Schechter form, following a power law. We show that this is driven entirely by an increasing X-ray/active galactic nucleus fraction with line luminosity, which reaches ≈100 per cent at line luminosities L ≳ 3 × 1044 erg s−1.
We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to ...galaxies brighter than Hubble Space Telescope AB mag in the redshift range 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M 1011 M ) out to z ∼ 3.5 such that an overdense environment hosts 20% more massive quiescent galaxies than an underdense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient than stellar mass quenching for low-mass galaxies (M 1010 M ) at low and intermediate redshifts (z 1.2). Our findings concur thoroughly with the "overconsumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an overdense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with stellar mass.
We study the effects of the local environment on the molecular gas content of a large sample of log(M*/M ) 10 star-forming and starburst galaxies with specific star formation rates (sSFRs) on and ...above the main sequence (MS) to z ∼ 3.5. ALMA observations of the dust continuum in the COSMOS field are used to estimate molecular gas masses at z 0.5-3.5. We also use a local universe sample from the ALFALFA H i survey after converting it into molecular masses. The molecular mass (MISM) scaling relation shows a dependence on z, M*, and sSFR relative to the MS, but no dependence on environmental overdensity Δ(MISM ∝ Δ0.03). Similarly, gas mass fraction (fgas) and depletion timescale (τ) show no environmental dependence to z ∼ 3.5. At ∼ 1.8, the average , , and in densest regions is (1.6 0.2) × 1011 M , 55 2%, and 0.8 0.1 Gyr, respectively, similar to those in the lowest density bin. Independent of the environment, fgas decreases and τ increases with increasing cosmic time. Cosmic molecular mass density ( ) in the lowest density bins peaks at z ∼ 1-2, and this peak happens at z < 1 in densest bins. This differential evolution of across environments is likely due to the growth of the large-scale structure with cosmic time. Our results suggest that the molecular gas content and the subsequent star formation activity of log(M*/M ) 10 star-forming and starburst galaxies is primarily driven by internal processes, and not by their local environment since z ∼ 3.5.
Abstract
We study the mass–metallicity relation for 19 members of a spectroscopically confirmed protocluster in the COSMOS field at
z
= 2.2 (CC2.2), and compare it with that of 24 similarly selected ...field galaxies at the same redshift. Both samples are H
α
emitting sources, chosen from the HiZELS narrowband survey, with metallicities derived from the
N
2
NII
λ
6584
H
α
line ratio. For the mass-matched samples of protocluster and field galaxies, we find that protocluster galaxies with 10
9.9
M
⊙
≤
M
*
≤ 10
10.9
M
⊙
are metal deficient by 0.10 ± 0.04 dex (2.5
σ
significance) compared to their coeval field galaxies. This metal deficiency is absent for low-mass galaxies,
M
*
< 10
9.9
M
⊙
. Moreover, relying on both spectral energy distribution derived and H
α
(corrected for dust extinction based on M
*
) star formation rates (SFRs), we find no strong environmental dependence of the SFR–
M
*
relation; however, we are not able to rule out the existence of small dependence due to inherent uncertainties in both SFR estimators. The existence of 2.5
σ
significant metal deficiency for massive protocluster galaxies favors a model in which funneling of the primordial cold gas through filaments dilutes the metal content of protoclusters at high redshifts (
z
≳ 2). At these redshifts, gas reservoirs in filaments are dense enough to cool down rapidly and fall into the potential well of the protocluster to lower the gas-phase metallicity of galaxies. Moreover, part of this metal deficiency could be originated from galaxy interactions that are more prevalent in dense environments.
We examine the role of environment on the in situ star formation (SF) hosted by the progenitors of the most massive galaxies in the present-day universe, the brightest cluster galaxies (BCGs), from z ...∼ 3 to present in the COSMOS field. Progenitors are selected from the COSMOS field using a stellar mass cut motivated by the evolving cumulative comoving number density of progenitors within the Illustris simulation, as well as the Millennium-II simulation and a constant comoving number density method for comparison. We characterize each progenitor using far-ultraviolet-far-infrared observations taken from the COSMOS field and fitting stellar, dust, and active galactic nucleus components to their spectral energy distributions. Additionally, we compare the SF rates of our progenitor sample to the local density maps of the COSMOS field to identify the effects of environment. We find that BCG progenitors evolve in three stages, starting with an in situ SF-dominated phase (z > 2.25). This is followed by a phase until z ∼ 1.25 where mass growth is driven by in situ SF and stellar mass deposited by mergers (both gas rich and poor) on the same order of magnitude independent of local environment. Finally, at low redshift dry mergers are the dominant stellar mass generation process. We also identify this final transition period as the time when progenitors quench, exhibiting quiescent NUVrJ colors.
We report the spectroscopic confirmation of a new protocluster in the COSMOS field at z ∼ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as an overdensity of narrowband selected H emitting ...candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both H (∼1.47-1.81 m) and K (∼1.92-2.40 m) bands (∼1.5 hr each), we confirm 35 unique protocluster members with at least two emission lines detected with S/N > 3. Combined with 12 extra members from the zCOSMOS-deep spectroscopic survey (47 in total), we estimate a mean redshift and a line-of-sight velocity dispersion of zmean = 2.23224 0.00101 and los = 645 69 km s−1 for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of Mvir ∼ (1-2) ×1014M for the structure. We evaluate a number density enhancement of δg ∼ 7 for this system and we argue that the structure is likely not fully virialized at z ∼ 2.2. However, in a spherical collapse model, δg is expected to grow to a linear matter enhancement of ∼1.9 by z = 0, exceeding the collapse threshold of 1.69, and leading to a fully collapsed and virialized Coma-type structure with a total mass of Mdyn(z = 0) ∼ 9.2 × 1014M by now. This observationally efficient confirmation suggests that large narrowband emission-line galaxy surveys, when combined with ancillary photometric data, can be used to effectively trace the large-scale structure and protoclusters at a time when they are mostly dominated by star-forming galaxies.
We show that unsupervised machine learning techniques are a valuable tool for both visualizing and computationally accelerating the estimation of galaxy physical properties from photometric data. As ...a proof of concept, we use self-organizing maps (SOMs) to visualize a spectral energy distribution (SED) model library in the observed photometry space. The resulting visual maps allow for a better understanding of how the observed data maps to physical properties and allows for better optimization of the model libraries for a given set of observational data. Next, the SOMs are used to estimate the physical parameters of 14,000 z ∼ 1 galaxies in the COSMOS field and are found to be in agreement with those measured with SED fitting. However, the SOM method is able to estimate the full probability distribution functions for each galaxy up to ∼106 times faster than direct model fitting. We conclude by discussing how this acceleration, as well as learning how the galaxy data manifold maps to physical parameter space and visualizing this mapping in lower dimensions, helps overcome other challenges in galaxy formation and evolution.
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