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Angus Mok, Michael L. Balogh,
Sean L. McGee,
David J. Wilman,
Alexis Finoguenov,
Masayuki Tanaka,
Stefania Giodini,
Richard G. Bower,
Jennifer L. Connelly,
Annie Hou,
John S. Mulchaey,
Laura C. Parker
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ABSTRACT: We present deep GMOS-S spectroscopy for 11 galaxy groups at 0.8<z<1.0, for
galaxies with r_{AB}<24.75. Our sample is highly complete (>66%) for eight of
the eleven groups. Using an optical-NIR colour-colour diagram, the galaxies in
the sample were separated with a dust insensitive method into three categories:
passive (red), star-forming (blue), and intermediate (green). The strongest
environmental dependence is observed in the fraction of passive galaxies, which
make up only ~20 per cent of the field in the mass range
10^{10.3}<M_{star}/M_\odot<10^{11.0} but are the dominant component of groups.
If we assume that the properties of the field are similar to those of the
`pre-accreted' population, the environment quenching efficiency (\epsilon_\rho)
is defined as the fraction of field galaxies required to be quenched in order
to match the observed red fraction inside groups. The efficiency obtained is
~0.4, similar to its value in intermediate-density environments locally. While
green (intermediate) galaxies represent ~20 per cent of the star-forming
population in both the group and field, at all stellar masses, the average sSFR
of the group population is lower by a factor of ~3. The green population does
not show strong H-delta absorption that is characteristic of starburst
galaxies. Finally, the high fraction of passive galaxies in groups, when
combined with satellite accretion models, require that most accreted galaxies
have been affected by their environment. Thus, any delay between accretion and
the onset of truncation of star formation (\tau) must be <2 Gyr, shorter than
the 3-7 Gyr required to fit data at z=0. The relatively small fraction of
intermediate galaxies requires that the actual quenching process occurs
quickly, with an exponential decay timescale of \tau_q<1 Gyr.
02/2013;
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Jennifer L. Connelly,
David J. Wilman,
Alexis Finoguenov,
Annie Hou,
John S. Mulchaey,
Sean L. McGee, Michael L. Balogh,
Laura C. Parker,
Roberto Saglia,
Robert D. E. Henderson,
Richard G. Bower
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ABSTRACT: We present the global group properties of two samples of galaxy groups
containing 39 high quality X-ray selected systems and 38 optically
(spectroscopically) selected systems in coincident spatial regions at
0.12<z<0.79. Only nine optical systems are associable with X-ray systems. We
discuss the confusion inherent in the matching of both galaxies to extended
X-ray emission and of X-ray emission to already identified optical systems.
Extensive spectroscopy has been obtained and the resultant redshift catalog and
group membership are provided here. X-ray, dynamical, and total stellar masses
of the groups are also derived and presented. We explore the effects of
applying three different kinds of radial cut to our systems: a constant cut of
1 Mpc and two r200 cuts, one based on the velocity dispersion of the system and
the other on the X-ray emission. We find that an X-ray based r200 results in
less scatter in scaling relations and less dynamical complexity as evidenced by
results of the Anderson-Darling and Dressler-Schectman tests, indicating that
this radius tends to isolate the virialized part of the system. The constant
and velocity dispersion based cuts can overestimate membership and can work to
inflate velocity dispersion and dynamical and stellar mass. We find Lx-sigma
and Mstellar-Lx scaling relations for X-ray and optically selected systems are
not dissimilar. The mean fraction of mass found in stars for our systems is
approximately 0.014 with a logarithmic standard deviation of 0.398 dex. We also
define and investigate a sample of groups which are X-ray underluminous given
the total group stellar mass. For these systems the fraction of stellar mass
contributed by the most massive galaxy is typically lower than that found for
the total population of groups implying that there may be less IGM contributed
from the most massive member in these systems. (Abridged)
08/2012;
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ABSTRACT: We present a deep [OII] emission line survey of faint galaxies (22.5<KAB<24)
in the Chandra Deep Field South and the FIRES field. With these data we measure
the star formation rate (SFR) in galaxies in the stellar mass range 8.85 <
log(M*/Msun) < 9.5 at 0.62<z<0.885, to a limit of SFR = 0.1Msun/yr. The
presence of a massive cluster (MS1054-03) in the FIRES field, and of
significant large scale structure in the CDFS field, allows us to study the
environmental dependence of SFRs amongst this population of low-mass galaxies.
Comparing our results with more massive galaxies at this epoch, with our
previous survey (ROLES) at the higher redshift z=1, and with SDSS Stripe 82
data, we find no significant evolution of the stellar mass function of
star-forming galaxies between z=0 and z=1, and no evidence that its shape
depends on environment. The correlation between specific star formation rate
(sSFR) and stellar mass at z=0.75 has a power-law slope of beta=-0.2, with
evidence for a steeper relation at the lowest masses. The normalization of this
correlation lies as expected between that corresponding to z=1 and the present
day. The global SFR density is consistent with an evolution of the form (1+z)^2
over 0<z<1, with no evidence for a dependence on stellar mass. The sSFR of
these star-forming galaxies at z=0.75 does not depend upon the density of their
local environment. Considering just high-density environments, the low-mass end
of the sSFR-M* relation in our data is steeper than that in Stripe 82 at z=0,
and shallower than that measured by ROLES at z=1. Evolution of low-mass
galaxies in dense environments appears to be more rapid than in the general
field.
07/2012;
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ABSTRACT: The presence of substructure in galaxy groups and clusters is believed to be
a sign of recent galaxy accretion and can be used not only to probe the
assembly history of these structures, but also the evolution of their member
galaxies. Using the Dressler-Shectman (DS) Test, we study substructure in a
sample of intermediate redshift (z ~ 0.4) galaxy groups from the Group
Environment and Evolution Collaboration (GEEC) group catalog. We find that 4 of
the 15 rich GEEC groups, with an average velocity dispersion of ~525 km s-1,
are identified as having significant substructure. The identified regions of
localized substructure lie on the group outskirts and in some cases appear to
be infalling. In a comparison of galaxy properties for the members of groups
with and without substructure, we find that the groups with substructure have a
significantly higher fraction of blue and star-forming galaxies and a parent
colour distribution that resembles that of the field population rather than the
overall group population. In addition, we observe correlations between the
detection of substructure and other dynamical measures, such as velocity
distributions and velocity dispersion profiles. Based on this analysis, we
conclude that some galaxy groups contain significant substructure and that
these groups have properties and galaxy populations that differ from groups
with no detected substructure. These results indicate that the substructure
galaxies, which lie preferentially on the group outskirts and could be
infalling, do not exhibit signs of environmental effects, since little or no
star-formation quenching is observed in these systems.
01/2012;
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ABSTRACT: We present measurements of the specific star formation rate (SSFR)–stellar mass relation for star-forming galaxies. Our deep spectroscopic samples are based on the Redshift One LDSS3 Emission line Survey (ROLES) and European Southern Observatory (ESO) public spectroscopy at z= 1, and on the Sloan Digital Sky Survey (SDSS) at z= 0.1. These data sets cover an equally deep mass range of 8.5 ≲ log(M*/M⊙) ≲ 11 at both epochs. We find that the SSFR–mass relation evolves in a way which is remarkably independent of stellar mass, as we previously found for the SFR density (SFRD)–mass relation. However, we see a subtle upturn in SSFR–mass for the lowest mass galaxies (which may at least partly be driven by mass-incompleteness in the K-selected sample). This upturn is suggestive of greater evolution for lower mass galaxies, which may be explained by less massive galaxies forming their stars later and on longer time-scales than higher mass galaxies, as implied by the ‘cosmic downsizing’ scenario. Parametrizing the e-folding time-scale and formation redshift as simple functions of baryonic mass gives best-fitting parametrizations of τ(Mb) ∝M−1.01b and 1 +zf(Mb) ∝M0.30b. This subtle upturn is also seen in the SFRD as a function of stellar mass. At higher masses, such as those probed by previous surveys, the evolution in SSFR–mass is almost independent of stellar mass. At higher masses [log(M*/M⊙) > 10] the shapes of the cumulative cosmic SFRDs are very similar at both z= 0.1 and 1.0, both showing 70 per cent of the total SFRD above a mass of log(M*/M⊙) > 10. Mass functions are constructed for star-forming galaxies and found to evolve by only <35 per cent between z= 1 and 0.1 over the whole mass range. The evolution is such that the mass function decreases with increasing cosmic time, confirming that galaxies are leaving the star-forming sequence/blue cloud. The observational results are extended to z∼ 2 by adding two recent Lyman break galaxy samples, and data at these three epochs (z= 0.1, 1, 2) are compared with the GALFORM semi-analytic model of galaxy formation. GALFORM predicts an overall SFRD as a function of stellar mass in reasonable agreement with the observations. The star formation time-scales inferred from 1/SSFR also give reasonable overall agreement, with the agreement becoming worse at the lowest and highest masses. The models do not reproduce the SSFR upturn seen in our data at low masses, where the effects of extinction and active galactic nuclei feedback should be minimal and the comparison should be most robust.
Monthly Notices of the Royal Astronomical Society 03/2011; 414(1):304 - 320. · 4.90 Impact Factor
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ABSTRACT: We present measurements of the specific star-formation rate (SSFR)-stellar
mass relation for star-forming galaxies. Our deep spectroscopic samples are
based on the Redshift One LDSS3 Emission line Survey, ROLES, and European
Southern Observatory, ESO, public spectroscopy at z=1, and on the Sloan Digital
Sky Survey (SDSS) at z=0.1. These datasets cover an equally deep mass range of
8.5<~log(M*/Msun)<~11 at both epochs. We find that the SSFR--mass relation
evolves in a way which is remarkably independent of stellar mass, as we
previously found for the star-formation rate density (SFRD)--mass relation. At
higher masses, such as those probed by previous surveys, the evolution in
SSFR--mass is almost independent of stellar mass. At higher masses
(log(M*/Msun)>10) the shapes of the cumulative cosmic SFRDs are very similar at
both z=0.1 and z=1.0, both showing 70% of the total SFRD above a mass of
log(M*/Msun)>10. Mass functions are constructed for star-forming galaxies and
found to evolve by only <35% between z=1 and z=0.1 over the whole mass range.
The evolution is such that the mass function decreases with increasing cosmic
time, confirming that galaxies are leaving the star-forming sequence/blue
cloud. The observational results are extended to z~2 by adding two recent Lyman
break galaxy samples, and data at these three epochs (z=0.1, 1, 2) are compared
with the GALFORM semi-analytic model of galaxy formation. GALFORM predicts an
overall SFR density (SFRD) as a function of stellar mass in reasonable
agreement with the observations. The star formation timescales inferred from
1/SSFR also give reasonable overall agreement, with the agreement becoming
worse at the lowest and highest masses. [abridged]
01/2011;
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ABSTRACT: We examine the star formation properties of group and field galaxies in two surveys, the Sloan Digital Sky Survey (SDSS; at z ~ 0.08) and the Group Environment and Evolution Collaboration (GEEC; at z ~ 0.4). Using UV imaging from the GALEX space telescope, along with optical and, for GEEC, near infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star forming galaxies of all environments undergo a systematic lowering of their star formation rate between z=0.4 and z=0.08 regardless of mass. Nonetheless, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass-dependent, in the sense the the difference is larger at low masses. However, the star formation properties of star forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z=0.4 and z=0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyrs after falling into a ~ 10E13 Msun group. This long timescale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star forming galaxies appear similar in both the group and field, as observed. Comment: 19 pages, MNRAS, in press
12/2010;
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ABSTRACT: (abridged) We introduce our survey of galaxy groups at 0.85<z<1, as an
extension of the Group Environment and Evolution Collaboration (GEEC). Here we
present the first results, based on Gemini GMOS-S nod-and-shuffle spectroscopy
of seven galaxy groups selected from spectroscopically confirmed, extended XMM
detections in COSMOS. In total we have over 100 confirmed group members, and
four of the groups have >15 members. The dynamical mass estimates are in good
agreement with the masses estimated from the X-ray luminosity, with most of the
groups having 13<log(Mdyn/Msun)<14. Our spectroscopic sample is statistically
complete for all galaxies with Mstar>1E10.1 Msun, and for blue galaxies we
sample masses as low as Mstar=1E8.8 Msun. Like lower-redshift groups, these
systems are dominated by red galaxies, at all stellar masses Mstar>1E10.1 Msun.
Few group galaxies inhabit the "blue cloud" that dominates the surrounding
field; instead, we find a large and possibly distinct population of galaxies
with intermediate colours. The "green valley" that exists at low redshift is
instead well-populated in these groups, containing ~30 per cent of galaxies.
These do not appear to be exceptionally dusty galaxies, and about half show
prominent Balmer-absorption lines. Furthermore, their HST morphologies appear
to be intermediate between those of red-sequence and blue-cloud galaxies of the
same stellar mass. We postulate that these are a transient population,
migrating from the blue cloud to the red sequence, with a star formation rate
that declines with an exponential timescale 0.6 Gyr< tau < 2 Gyr. Their
prominence among the group galaxy population, and the marked lack of blue,
star-forming galaxies, provides evidence that the group environment either
directly reduces star formation in member galaxies, or at least prevents its
rejuvenation during the normal cycle of galaxy evolution.
11/2010;
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ABSTRACT: We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3E14<M/Msun<6E14, as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, AAT and CFHT to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction. Comment: MNRAS, in press. Replacement to correct error caught in proofs. Two of the undetected clusters do not have Chandra or XMM data and the limits are based on ROSAT data
11/2010;
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ABSTRACT: We probe the diffuse stellar mass in a sample of 1401 low-redshift galaxy groups (1013–1014 h−1 M⊙) by examining the rate of hostless Type Ia supernovae (SNe Ia) within the groups. We correlate the sample of confirmed SNe Ia from the Sloan Digital Sky Survey SN survey with the positions of our galaxy groups, as well as with the resolved galaxies within them. We find that 19 of the 59 SNe Ia within the group sample have no detectable host galaxy, with another three ambiguous instances. This gives a robust upper limit that a maximum of 2.69+1.58−1.34 per cent of the group's total mass arises from diffuse stars in the intragroup medium. After correcting for a contribution from ‘prompt’ SNe occurring within galaxies, and including a contribution from those which arise in dwarf galaxies below our photometric limit, we find that only 1.32+0.78−0.70 per cent of the group's total mass is likely in the form of diffuse stellar mass. Combining this result with the galaxy stellar mass functions of Yang, Mo and van den Bosch, we find that 47+16−15 per cent of the stellar mass in our groups is in the form of diffuse light, so that stars make up a fraction 0.028+0.011−0.010 of the total group mass. Galaxy groups appear to be very efficient in disrupting stellar mass into a diffuse component; however, stars still make up a small fraction of the group mass, comparable to that seen in rich clusters. This remains a challenge to galaxy formation models.
Monthly Notices of the Royal Astronomical Society Letters 03/2010; 403(1):L79 - L83.
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ABSTRACT: We examine the dust distribution around a sample of 70,000 low redshift galaxy groups and clusters derived from the Sloan Digital Sky Survey. By correlating spectroscopically identified background quasars with the galaxy groups we obtain the relative colour excess due to dust reddening. We present a significant detection of dust out to a clustercentric distance of 30 Mpc/h in all four independent SDSS colours, consistent with the expectations of weak lensing masses of similar mass halos and excess galaxy counts. The wavelength dependence of this colour excess is consistent with the expectations of a Milky Way dust law with R_V=3.1. Further, we find that the halo mass dependence of the dust content is much smaller than would be expected by a simple scaling, implying that the dust-to-gas ratio of the most massive clusters (~10E14 Msun/h) is ~3% of the local ISM value, while in small groups (~10E12.7 Msun/h) it is ~55% of the local ISM value. We also find that the dust must have a covering fraction on the order of 10% to explain the observed color differences, which means the dust is not just confined to the most massive galaxies. Comparing the dust profile with the excess galaxy profile, we find that the implied dust-to-galaxy ratio falls significantly towards the group or cluster center. This has a significant halo mass dependence, such that the more massive groups and clusters show a stronger reduction. This suggests that either dust is destroyed by thermal sputtering of the dust grains by the hot, dense gas or the intrinsic dust production is reduced in these galaxies. Comment: 10 pages, MNRAS, in press
03/2010;
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ABSTRACT: We explore the use of simple star-formation rate (SFR) indicators (such as
may be used in high-redshift galaxy surveys) in the local Universe using [OII],
Ha, and u-band luminosities from the deeper 275 deg^2 Stripe 82 subsample of
the Sloan Digital Sky Survey (SDSS) coupled with UV data from the Galaxy
Evolution EXplorer satellite (GALEX). We examine the consistency of such
methods using the star-formation rate density (SFRD) as a function of stellar
mass in this local volume, and quantify the accuracy of corrections for dust
and metallicity on the various indicators. Rest-frame u-band promises to be a
particularly good SFR estimator for high redshift studies since it does not
require a particularly large or sensitive extinction correction, yet yields
results broadly consistent with more observationally expensive methods. We
suggest that the [OII]-derived SFR, commonly used at higher redshifts (z~1),
can be used to reliably estimate SFRs for ensembles of galaxies, but for high
mass galaxies (log(M*/Msun)>10), a larger correction than is typically used is
required to compensate for the effects of metallicity dependence and dust
extinction. We provide a new empirical mass-dependent correction for the
[OII]-SFR.
02/2010;
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ABSTRACT: Motivated by suggestions of 'cosmic downsizing', in which the dominant contribution to the cosmic star formation rate density (SFRD) proceeds from higher to lower mass galaxies with increasing cosmic time, we describe the design and implementation of the Redshift One LDSS3 Emission line Survey (ROLES). ROLES is a K-selected (22.5 < K_AB < 24.0) survey for dwarf galaxies [8.5<log(M*/Msun)< 9.5] at 0.89 < z < 1.15 drawn from two extremely deep fields (GOODS-S and MS1054-FIRES). Using the [OII]3727 emission line, we obtain redshifts and star-formation rates (SFRs) for star-forming galaxies down to a limit of ~0.3 Msun/yr. We present the [OII] luminosity function measured in ROLES and find a faint end slope of alpha_faint ~ -1.5, similar to that measured at z~0.1 in the SDSS. By combining ROLES with higher mass surveys, we measure the SFRD as a function of stellar mass using [OII] (with and without various empirical corrections), and using SED-fitting to obtain the SFR from the rest-frame UV luminosity for galaxies with spectroscopic redshifts. Our best estimate of the corrected [OII]-SFRD and UV SFRD both independently show that the SFRD evolves equally for galaxies of all masses between z~1 and z~0.1. The exact evolution in normalisation depends on the indicator used, with the [OII]-based estimate showing a change of a factor of ~2.6 and the UV-based a factor of ~6. We discuss possible reasons for the discrepancy in normalisation between the indicators, but note that the magnitude of this uncertainty is comparable to the discrepancy between indicators seen in other z~1 works. Our result that the shape of the SFRD as a function of stellar mass (and hence the mass range of galaxies dominating the SFRD) does not evolve between z~1 and z~0.1 is robust to the choice of indicator. [abridged] Comment: Resubmitted to MNRAS following first referee report. 20 pages, 16 figures. High resolution version available at http://astro.uwaterloo.ca/~dgilbank/papers/roles2.pdf
02/2010;
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ABSTRACT: We present photometric, spectroscopic and weak lensing analysis of the large-scale structure and dynamics of the most X-ray luminous galaxy cluster known, RX J1347-1145, at z=0.451. We spectroscopically confirmed 47 new members with LDSS3 on the Magellan telescope. Together with previously known members, we measure a new velocity dispersion of 1163(+/-97) km/s. The mass inferred from our velocity dispersion is M200 = 1.16^{+0.32}_{-0.27}x10^{15} solar mass, with r200=1.85Mpc, under the assumption of a singular isothermal sphere. We also present a weak lensing analysis using deep CFHT data on this cluster, and find a deprojected mass of 1.47^{+0.46}_{-0.43}x10^{15} solar mass within r200, in excellent agreement with our dynamical estimate. Thus, our new dynamical mass estimate is consistent with that from weak lensing and X-ray studies in the literature, resolving a previously claimed discrepancy. We photometrically detect and spectroscopically confirm another massive cluster with sigma=780(+/-100) km/s and M200=3.4^{+1.4}_{-1.1}x10^{14} solar mass ~7Mpc south-west of RX J1347-1145, which we refer to as RXJ1347-SW. Our spectroscopic survey reveals a possible excess of galaxies in velocity space in the region between RX J1347-1145 and RXJ1347-SW; comparing with simulations, this excess appears consistent with that expected from a large filamentary structure traced by galaxies connecting these two clusters. Comment: version submitted to MNRAS, after incorporating referee comments
12/2009;
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ABSTRACT: We measure the diversity of galaxy groups and clusters with mass M>1E13/h Msun, in terms of the star formation history of their galaxy populations, for the purpose of constraining the mass scale at which environmentally-important processes play a role in galaxy evolution. We consider three different group catalogues, selected in different ways, with photometry and spectroscopy from the Sloan Digital Sky Survey. For each system we measure the fraction of passively-evolving galaxies within R200 and brighter than either Mr=-18 (and with z<0.05) or Mr=-20 (and z<0.1). We use the (u-g) and (r-i) galaxy colours to distinguish between star-forming and passively-evolving galaxies. By considering the binomial distribution expected from the observed number of members in each cluster, we are able to either recover the intrinsic scatter in this fraction, or put robust 95% confidence upper-limits on its value. The intrinsic standard deviation in the fraction of passive galaxies is consistent with a small value of <0.1 in most mass bins for all three samples. There is no strong trend with mass; even groups with M=1E13/h Msun are consistent with such a small, intrinsic distribution. We compare these results with theoretical models of the accretion history to show that, if environment plays a role in transforming galaxies, such effects must occur first at mass scales far below that of rich clusters, at most M=1E13 Msun. Comment: 5 pages, MNRAS Letters, in press
12/2009;
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ABSTRACT: We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. (2009) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (10E12.9 to 10E15.3 Msun/h), and selected from each of four redshift epochs (z=0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 10E14.5 Msun/h mass cluster at z=0 has, on average, accreted ~ 40% of its galaxies (Mstellar > 10E9 Msun/h) from halos with masses greater than 10E13 Msun/h. Further, the galaxies which are accreted through groups are more massive, on average, than galaxies accreted through smaller halos or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies, from 5-6 % per Gyr at z=0 to 15% per Gyr at z=1.5. We find that combining the existence of a Butcher Oemler effect at z=0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions. Namely, that the dominant environmental process must begin to occur in halos of 10E12 -- 10E13 Msun/h, and act over timescales of > 2 Gyrs. This argues in favor of a mechanism like "strangulation", in which the hot halo of a galaxy is stripped upon infalling into a more massive halo . This simple model predicts that by z=1.5 galaxy groups and clusters will display little to no environmental effects. Comment: Accepted for publication in MNRAS. 15 pages, 13 figures
08/2009;
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ABSTRACT: The global star formation rate has decreased significantly since z ~ 1, for reasons that are not well understood. Red-sequence galaxies, dominating in galaxy clusters, represent the population that have had their star formation shut off, and may therefore be the key to this problem. In this work, we select 127 rich galaxy clusters at 0.17<z<0.36, from 119 square degrees of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) optical imaging data, and construct the r'-band red-sequence luminosity functions (LFs). We show that the faint end of the LF is very sensitive to how red-sequence galaxies are selected, and an optimal way to minimise the contamination from the blue cloud is to mirror galaxies on the redder side of the colour-magnitude relation (CMR). The LFs of our sample have a significant inflexion centred at Mr' ~- 18.5, suggesting a mixture of two populations. Combining our survey with low redshift samples constructed from the Sloan Digital Sky Survey, we show that there is no strong evolution of the faint end of the LF (or the red-sequence dwarf-to-giant ratio) over the redshift range 0.2 < z < 0.4, but from z ~ 0.2 to z ~ 0 the relative number of red-sequence dwarf galaxies has increased by a factor of ~3, implying a significant build-up of the faint end of the cluster red-sequence over the last 2.5 Gyr. Comment: MNRAS submitted, revised after referee's comments, 23 pages, 19 figures
05/2009;
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ABSTRACT: (Abridged) We present new optical and near-infrared imaging for a sample of 98 spectroscopically-selected galaxy groups at 0.25<z<0.55. We measure accurate colours for group members and the surrounding field population, statistically complete above a stellar mass limit of M=1E10 Msun. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by 20+/-7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in HST imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. We show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group sample, and shows little dependence on near infrared luminosity. Thus, the fraction of bright group members with no sign of star formation or AGN activity is 54+/-6 per cent. Our field sample, which includes galaxies in all environments, contains 35+/-3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense environments at z<0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations. Comment: Resubmitted to MNRAS following referee report
05/2009;
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ABSTRACT: We present the results of a wide-field survey for Hα-emitting galaxies in the cluster AC 114 at z = 0.32. Spectra centered on Hα at the cluster redshift have been obtained for 586 galaxies to Itot ~ 22 out to a radius of ~2 h Mpc. At most, only ~10% of these were found to be Hα-emitting cluster members. These objects are predominantly blue and of late-type spiral morphology, consistent with them hosting star formation. However, ~65% of the cluster members classified morphologically as spirals (with HST) have no detectable Hα emission; star formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the Hα detection rate and the strength of Hα emission with environment (as traced by local galaxy density) are found to be weak within the region studied. Star formation within the cluster members is also found to be strongly and uniformly suppressed with the rates inferred from the Hα emission not exceeding 4 M☉ yr-1, and AC 114's Hα luminosity function being an order of magnitude below that observed for field galaxies at the same redshift. None of the galaxies detected have the high star formation rates associated with "starburst" galaxies; however, this may still be reconcilable with the known (8% ± 3%) fraction of "post-starburst" galaxies within AC 114, given the poorly determined but short lifetimes of starbursts and the possibility that much of the associated star formation is obscured by dust.
The Astrophysical Journal 12/2008; 549(2):820. · 6.02 Impact Factor
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ABSTRACT: We analyze the u - r color distribution of 24,346 galaxies with Mr ≤ -18 and z < 0.08, drawn from the Sloan Digital Sky Survey first data release, as a function of luminosity and environment. The color distribution is well fitted with two Gaussian distributions, which we use to divide the sample into a blue and red population. At fixed luminosity, the mean color of the blue (red) distribution is nearly independent of environment, with a weakly significant (~3 σ) detection of a trend for colors to become redder by 0.1-0.14 (0.03-0.06) mag with a factor of ~100 increase in local density, as characterized by the surface density of galaxies within a ±1000 km s-1 redshift slice. In contrast, at fixed luminosity the fraction of galaxies in the red distribution is a strong function of local density, increasing from ~10%-30% of the population in the lowest density environments to ~70% at the highest densities. The strength of this trend is similar for both the brightest (-23 < Mr < -22) and faintest (-19 < Mr < -18) galaxies in our sample. The fraction of red galaxies within the virialized regions of clusters shows no significant dependence on velocity dispersion. Even at the lowest densities explored, a substantial population of red galaxies exists, which might be fossil groups. We propose that most star-forming galaxies today evolve at a rate that is determined primarily by their intrinsic properties and independent of their environment. Any environmentally triggered transformations from blue to red colors must occur either on a short timescale or preferentially at high redshift to preserve the simple Gaussian nature of the color distribution. The mechanism must be effective for both bright and faint galaxies.
The Astrophysical Journal 12/2008; 615(2):L101. · 6.02 Impact Factor
