REVIEW
The DEEP2 Galaxy Redshift Survey: The Relationship Between Galaxy Properties and Environment at z ~ 1
Astrophysics · Submitted 7 Mar 2006 · 92 references
Pith's verdict: A careful z~1 environmental study whose headline blue luminosity-environment slope is probably real but not yet separated from the color-magnitude relation, with a factor-of-20 inconsistency in the reported slope.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
At z ~ 1, brighter blue galaxies already live in denser regions, unlike today
The reading
What carries the argument
The central object is the projected 3rd-nearest-neighbor surface density, $\Sigma_3 = 3/(\pi D_{p,3}^2)$, where $D_{p,3}$ is the projected distance to the third-nearest galaxy within $\pm1000$ km/s in redshift. This estimator is corrected for spatially variable redshift completeness, for redshift-dependent sampling using the median density in redshift bins, and for survey edges, and is validated on mock catalogs. Mean relations are weighted by inverse selection volume $1/V_{\rm max}$ to reduce Malmquist bias. The analysis separates field, group, and cluster populations using a group finder to isolate where the environment trends originate.
Extended reading notes
Core claim
The paper's central claim is that at z ~ 1 the local galaxy overdensity, estimated by the projected third-nearest-neighbor surface density $\Sigma_3$, depends on rest-frame color the same way it does locally, but that the luminosity dependence is different for blue galaxies. Specifically, while local studies find the mean environment of blue galaxies to be independent of luminosity, DEEP2 measures a slope of about $-1.26$ in $\log(1+\delta_3)$ per B-band magnitude for blue galaxies at z~1, with brighter blue galaxies more overdense. Red galaxies show a local-like luminosity trend over the magnitude range probed. The paper argues that the bright, overdense blue galaxies are exactly the population that will be quenched onto the red sequence by z~0, so the disappearance of the blue slope locally is the signature of 'downsizing of quenching'.
Load-bearing premise
The load-bearing premise is that the projected third-nearest-neighbor density, after completeness and edge corrections, measures true local density without a luminosity-dependent bias; if the mock catalogs used to validate it miss real selection effects at the faint end, the blue-galaxy luminosity slope could be an artifact of target selection rather than an astrophysical signal.
Editorial extensions
If this is right
- If the blue luminosity-environment slope is real, the bright blue galaxies in overdense regions at z~1 are the progenitors of today's massive red-sequence galaxies, so most of the red sequence was built by quenching, not by merging of already-red galaxies.
- Quenching must operate efficiently in group-scale halos (masses around a few times $10^{11}\,M_\odot$), because cluster-specific processes such as ram-pressure stripping cannot explain the trends in a sample with only ~2% cluster members.
- The observed relations are well described by a separable model in which mean log-overdensity is a sum of a color term and a luminosity term, so color and luminosity contribute independently to environment at z~1.
- Rest-frame $(U-B)$ color is the single best predictor of environment among the properties tested, and [OII] equivalent width adds no independent information beyond color.
- The color-environment relation, once established, evolves mildly from z~1 to z~0, with the red-blue density contrast expected to grow as blue galaxies in dense regions are quenched.
Reading between the lines
- Beyond the paper, a direct testable extension: applying the same density estimator and analysis to fainter, higher-completeness samples at z~1 (for example with JWST spectroscopy) should reproduce the blue luminosity slope if it is astrophysical; its disappearance at the faint end would indicate a selection bias.
- Beyond the paper, the results imply that group-scale pre-processing was already effective at z~1, so galaxies falling into clusters today were likely quenched in groups first; this can be checked by measuring satellite quenched fractions as a function of host halo mass in simulations.
- Beyond the paper, if quenching tracks a critical halo mass near a few times $10^{11}\,M_\odot$, the blue luminosity-environment slope should steepen toward z~2, a prediction testable with upcoming wide spectroscopic surveys.
- Beyond the paper, the measured slope could partly reflect mass-dependent halo occupation (brighter galaxies live in more massive halos even when star-forming) rather than quenching alone; separating these requires stellar-mass-matched samples, which the paper flags for future work.
Original abstract
We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame color, luminosity, and [OII] equivalent width. The local galaxy overdensity for >14,000 galaxies at 0.75 < z < 1.35 is estimated using the projected 3rd-nearest-neighbor surface density. Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy color; all major features of the correlation between mean overdensity and rest-frame color observed in the local universe were already in place at z ~ 1. In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star-forming galaxies at z ~ 1, with brighter blue galaxies being found on average in regions of greater overdensity. We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a ``downsizing of quenching'' from z ~ 1 to z ~ 0. Our results add weight to existing evidence that the mechanism(s) that result in star-formation quenching are efficient in group environments as well as clusters. This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame color, luminosity, and [OII] equivalent width. The local galaxy overdensity for >14,000 galaxies at 0.75 < z < 1.35 is estimated using the projected 3rd-nearest-neighbor surface density. Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy color; all major features of the correlation between mean overdensity and rest-frame color observed in the local universe were already in place at z ~ 1. In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star-forming galaxies at z ~ 1, with brighter blue galaxies being found on average in regions of greater overdensity. We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a ``downsizing of quenching'' from z ~ 1 to z ~ 0. Our results add weight to existing evidence that the mechanism(s) that result in star-formation quenching are efficient in group environments as well as clusters. This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame color, luminosity, and [OII] equivalent width. | contribution 1.00 | 0.95 | 0.03 | asserted 0.73 |
| s1 | The local galaxy overdensity for >14,000 galaxies at 0.75 < z < 1.35 is estimated using the projected 3rd-nearest-neighbor surface density. | method 1.00 | 0.94 | 0.02 | asserted 1.00 |
| s2 | Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy color; all major features of the correlation between mean overdensity and rest-frame color observed in the local universe were already in place at z ~ 1. | result 1.00 | 0.96 | 0.70 | asserted 1.00 |
| s3 | In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star-forming galaxies at z ~ 1, with brighter blue galaxies being found on average in regions of greater overdensity. | result 1.00 | 0.97 | 0.12 | asserted 1.00 |
| s4 | We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a ``downsizing of quenching'' from z ~ 1 to z ~ 0. | interpretation 0.69 | 0.65 | 0.06 | unclear 0.58 |
| s5 | Our results add weight to existing evidence that the mechanism(s) that result in star-formation quenching are efficient in group environments as well as clusters. | interpretation 0.97 | 0.61 | 0.14 | hedged possibility 0.56 |
| s6 | This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1. | future work 0.49 | 0.44 | 0.03 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:astro-ph/0603177 by the authors.
References
How the literature uses it
1 arXiv paper cite this in 2 sentences
Discussion
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