173 research outputs found
Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV
We derive stellar masses, ages and star formation histories of massive
early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them
with those measured in a similarly mass-selected sample of field contemporaries
drawn from the GOODS South Field. Robust estimates of these parameters are
obtained by comparing a large grid of composite stellar population models with
8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the
entire relevant domain of emission of the different stellar populations.
Additionally, we present new, deep -band photometry of both fields, giving
access to the critical FUV rest-frame, in order to constrain empirically the
dependence on the environment of the most recent star formation processes. We
find that early-type galaxies, both in the cluster and in the field, show
analogous optical morphologies, follow comparable mass vs. size relation, have
congruent average surface stellar mass densities and lie on the same Kormendy
relation. We also that a fraction of early-type galaxies in the field employ
longer timescales, , to assemble their mass than their cluster
contemporaries. Hence we conclude that, while the formation epoch of early-type
only depends on their mass, the environment does regulate the timescales of
their star formation histories. Our deep -band imaging strongly supports
this conclusions. It shows that cluster galaxies are at least 0.5 mag fainter
than their field contemporaries of similar mass and optical-to-infrared colors,
implying that the last episode of star formation must have happened more
recently in the field than in the cluster.Comment: 20pages, 10 figures. to appear on Ap
The Red Sequence of High-Redshift Clusters: a Comparison with Cosmological Galaxy Formation Models
We compare the results from a semi-analytic model of galaxy formation with
spectro-photometric observations of distant galaxy clusters observed in the
range 0.8< z< 1.3. We investigate the properties of their red sequence (RS)
galaxies and compare them with those of the field at the same redshift. In our
model we find that i) a well-defined, narrow RS is obtained already by z= 1.2;
this is found to be more populated than the field RS, analogously to what
observed and predicted at z=0; ii) the predicted U-V rest-frame colors and
scatter of the cluster RS at z=1.2 have average values of 1 and 0.15
respectively, with a cluster-to-cluster variance of 0.2 and 0.06, respectively.
The scatter of the RS of cluster galaxies is around 5 times smaller than the
corresponding field value; iii) when the RS galaxies are considered, the mass
growth histories of field and cluster galaxies at z=1.2 are similar, with 90 %
of the stellar mass of RS galaxies at z=1.2 already formed at cosmic times
t=2.5 Gyr, and 50 % at t=1 Gyr; v) the predicted distribution of stellar ages
of RS galaxies at z=1.2 peaks at 3.7 Gyr for both cluster and field
populations; however, for the latter the distribution is significantly skewed
toward lower ages. When compared with observations, the above findings show an
overall consistency, although the average value 0.07 of the observed cluster RS
scatter (U-V colors) at z=1.2 is smaller than the corresponding model central
value. We discuss the physical origin and the significance of the above results
in the framework of cosmological galaxy formation.Comment: 14 pages, accepted for publication in ApJ. Updated one referenc
Deep Near-infrared Spectroscopy of Passively Evolving Galaxies at z âł 1.4
We present the results of new near-IR spectroscopic observations of passive galaxies at z âł 1.4 in a concentration of BzK-selected galaxies in the COSMOS field. The observations have been conducted with Subaru/MOIRCS, and have resulted in absorption lines and/or continuum detection for 18 out of 34 objects. This allows us to measure spectroscopic redshifts for a sample that is almost complete to K_AB = 21. COSMOS photometric redshifts are found in fair agreement overall with the spectroscopic redshifts, with a standard deviation of ~0.05; however, ~30% of objects have photometric redshifts systematically underestimated by up to ~25%. We show that these systematic offsets in photometric redshifts can be removed by using these objects as a training set. All galaxies fall in four distinct redshift spikes at z = 1.43, 1.53, 1.67, and 1.82, with this latter one including seven galaxies. SED fits to broadband fluxes indicate stellar masses in the range of ~4-40 Ă 10^10 M_â and that star formation was quenched ~1 Gyr before the cosmic epoch at which they are observed. The spectra of several individual galaxies have allowed us to measure their HÎŽ_F indices and the strengths of the 4000 Ă
break, which confirms their identification as passive galaxies, as does a composite spectrum resulting from the co-addition of 17 individual spectra. The effective radii of the galaxies have been measured on the COSMOS HST/ACS i_(F814W)-band image, confirming the coexistence at these redshifts of passive galaxies, which are substantially more compact than their local counterparts with others that follow the local effective radius-stellar mass relation. For the galaxy with the best signal-to-noise spectrum we were able to measure a velocity dispersion of 270 ± 105 km s^(â1) (error bar including systematic errors), indicating that this galaxy lies closely on the virial relation given its stellar mass and effective radius
A z~3 radio galaxy and its protocluster: evidence for a superstructure?
We present spectroscopic follow-up observations of Lyman Break Galaxies
(LBGs) selected in the field surrounding the radio galaxy MRC0316-257 at z~3.13
(0316). Robust spectroscopic redshifts are determined for 20 out of 24 objects.
Three of the spectroscopically confirmed galaxies have 3.12<z<3.13 indicating
that these objects reside in a protocluster structure previously found around
the radio galaxy. An additional 5 objects are found 1600 km/s blue-shifted with
respect to the main protocluster structure. This is in addition to three [OIII]
emitters found at this redshift in a previous study. This is further evidence
that a structure exists directly in front of the 0316 protocluster. We estimate
that the foreground structure is responsible for half of the surface
overdensity of LBGs found in the field as a whole. The foreground structure is
associated with a strong surface density peak 1.4 Mpc to the North-West of the
radio galaxy and a 2D Kolmogorov-Smirnov test indicates that the spatial
distributions of the 0316 and foreground galaxies differ at the 3 sigma level.
In addition, we compare the properties of protocluster, foreground structure
and field galaxies, but we find no significant differences. In terms of the
nature of the two structures, a merger scenario is a possible option. Simple
merger dynamics indicates that the observed relative velocity of 1600 km/s can
be reproduced if the two structures have masses of ~5x10^14 Msun and have
starting separations of around 2.5 to 3 Mpc. It is also possible that the
foreground structure is unrelated to the 0316 protocluster in which case the
two structures will not interact before z=0.Comment: 13 pages, 6 figures, accepted for publication in MNRA
Passive galaxies as tracers of cluster environments at z~2
Even 10 billion years ago, the cores of the first galaxy clusters are often
found to host a characteristic population of massive galaxies with already
suppressed star formation. Here we search for distant cluster candidates at z~2
using massive passive galaxies as tracers. With a sample of ~40
spectroscopically confirmed passive galaxies at 1.3<z<2.1, we tune photometric
redshifts of several thousands passive sources in the full 2 sq.deg. COSMOS
field. This allows us to map their density in redshift slices, probing the
large scale structure in the COSMOS field as traced by passive sources. We
report here on the three strongest passive galaxy overdensities that we
identify in the redshift range 1.5<z<2.5. While the actual nature of these
concentrations is still to be confirmed, we discuss their identification
procedure, and the arguments supporting them as candidate galaxy clusters
(likely mid-10^13 M_sun range). Although this search approach is likely biased
towards more evolved structures, it has the potential to select still rare,
cluster-like environments close to their epoch of first appearance, enabling
new investigations of the evolution of galaxies in the context of structure
growth.Comment: 5 pages, 5 figures; A&A Letters, in pres
A Chandra view of the z=1.62 galaxy cluster IRC-0218A
Context: Very few z > 1.5 clusters of galaxies are known. It is important to
study the properties of galaxies in these clusters and the ICM and, further, to
cross-check the reliability of the various mass estimates. This will help to
clarify the process of structure formation and how distant clusters may be used
to constrain cosmology. AIMS: We present a 84 ks Chandra observation of
IRC-0218A, a cluster of galaxies inferred by the presence of a galaxy
overdensity in the infrared at a redshift of 1.62 and associated with some XMM
emission Methods: Spatial analysis of the Chandra X-ray photon distribution.
Results: The Chandra observation of IRC-0218A appears to be entirely dominated
by a point-source located at the centroid of the MIR galaxy density. In
addition, we detect weak extended emission (2.3 sigma) out to a radius of 25"
with a flux of ~ 3 10E-15 erg/s/cm2 in the [0.3-2]keV band. Assuming that
clusters evolve similarly, we infer a virial mass of M200 =7.7+/-3.8 10E13Mo.
This is marginally compatible with our current estimate of the cluster
dynamical mass (based on 10 redshifts), although there is no evidence that the
galaxy peculiar velocities correspond to the motions of a virialized structure.
The stellar mass enclosed in the inferred X-ray virial radius is estimated to
1-2 10E12 Mo. We provide a detailed account of 28 X-ray point-sources detected
in the field.Comment: 6 pages, 8 figures, accepted for publication in A&A (minor changes
with respect to the submitted version
Galaxy Evolution in Overdense Environments at High Redshift: Passive Early-type Galaxies in a Cluster at z ~ 2
We present a study of galaxy populations in the central region of the IRAC-selected, X-ray-detected galaxy cluster Cl J1449+0856 at z = 2. Based on a sample of spectroscopic and photometric cluster members, we investigate stellar populations and the morphological structure of cluster galaxies over an area of ~0.7 Mpc^2 around the cluster core. The cluster stands out as a clear overdensity both in redshift space and in the spatial distribution of galaxies close to the center of the extended X-ray emission. The cluster core region (r < 200 kpc) shows a clearly enhanced passive fraction with respect to field levels. However, together with a population of massive, passive galaxies mostly with early-type morphologies, the cluster core also hosts massive, actively star-forming, often highly dust reddened sources. Close to the cluster center, a multi-component system of passive and star-forming galaxies could represent the future brightest cluster galaxy still forming. We observe a clear correlation between passive stellar populations and an early-type morphology, in agreement with field studies at similar redshift. Passive early-type galaxies in this cluster are typically a factor of 2-3 smaller than similarly massive early types at z ~ 0. On the other hand, these same objects are on average larger by a factor of ~2 than field early-types at similar redshift, lending support to recent claims of an accelerated structural evolution in high-redshift dense environments. These results point toward the early formation of a population of massive galaxies, already evolved both in their structure and stellar populations, coexisting with still actively forming massive galaxies in the central regions of young clusters 10 billion years ago
Satellite content and quenching of star formation in galaxy groups at z ~ 1.8
We study the properties of satellites in the environment of massive star-forming galaxies at z ~ 1.8 in the COSMOS field, using a sample of 215 galaxies on the main sequence of star formation with an average mass of ~1011Mâ. At z> 1.5, these galaxies typically trace halos of mass âł1013Mâ. We use optical-near-infrared photometry to estimate stellar masses and star formation rates (SFR) of centrals and satellites down to ~ 6 Ă 109Mâ. We stack data around 215 central galaxies to statistically detect their satellite halos, finding an average of ~3 galaxies in excess of the background density. We fit the radial profiles of satellites with simple ÎČ-models, and compare their integrated properties to model predictions. We find that the total stellar mass of satellites amounts to ~68% of the central galaxy, while spectral energy distribution modeling and far-infrared photometry consistently show their total SFR to be 25-35% of the central's rate. We also see significant variation in the specific SFR of satellites within the halo with, in particular, a sharp decrease at <100 kpc. After considering different potential explanations, we conclude that this is likely an environmental signature of the hot inner halo. This effect can be explained in the first order by a simple free-fall scenario, suggesting that these low-mass environments can shut down star formation in satellites on relatively short timescales of ~0.3 Gyr
THE RED SEQUENCE AT BIRTH IN THE GALAXY CLUSTER Cl J1449+0856 AT z=2
We use. Hubble Space Telescope/WFC3 imaging to study the red population in the IR-selected, X-ray detected, low-mass cluster Cl J1449+0856 at z = 2, one of the few bona fide established clusters discovered at this redshift, and likely a typical progenitor of an average massive cluster today. This study explores the presence and significance of an early red sequence in the core of this structure, investigating the nature of red-sequence galaxies, highlighting environmental effects on cluster galaxy populations at high redshift, and at the same time underlining similarities and differences with other distant dense environments. Our results suggest that the red population in the core of Cl J1449+0856 is made of a mixture of quiescent and dusty star-forming galaxies, with a seedling of the future red sequence already growing in the very central cluster region, and already characterizing the inner cluster core with respect to lower-density environments. On the other hand, the color-magnitude diagram of this cluster is definitely different from that of lower-redshift z less than or similar to 1 clusters, as well as of some rare particularly evolved massive clusters at similar redshift, and it is suggestive of a transition phase between active star formation and passive evolution occurring in the protocluster and established lower-redshift cluster regimes.Peer reviewe
VLT and ACS observations of RDCS J1252.9-2927: dynamical structure and galaxy populations in a massive cluster at z=1.237
We present results from an extensive spectroscopic survey, carried out with
VLT FORS, and from an extensive multiwavelength imaging data set from the HST
Advanced Camera for Surveys and ground based facilities, of the cluster of
galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster
members in the redshift range 1.22 < z < 1.25. A cluster median redshift of
z=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s are
obtained. Using the 38 confirmed redshifts, we were able to resolve, for the
first time at z > 1, kinematic structure. The velocity distribution, which is
not Gaussian at the 95% confidence level, is consistent with two groups that
are also responsible for the projected east-west elongation of the cluster. The
groups are composed of 26 and 12 galaxies with velocity dispersions of
486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation is
also seen in the intracluster gas and the dark matter distribution. This leads
us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state.
We extend the analysis of the color-magnitude diagram of spectroscopic members
to more than 1 Mpc from the cluster center. The scatter and slope of
non-[OII]-emitting cluster members in the near-IR red sequence is similar to
that seen in clusters at lower redshift. Furthermore, most of the galaxies with
luminosities greater than ~ K_s*+1.5 do not show any [OII], indicating that
these more luminous, redder galaxies have stopped forming stars earlier than
the fainter, bluer galaxies. Our observations provide detailed dynamical and
spectrophotometric information on galaxies in this exceptional high-redshift
cluster, delivering an in-depth view of structure formation at this epoch only
5 Gyr after the Big Bang.Comment: 29 pages. 16 figures. ApJ accepted. Tables 2,3 and 5, figure 1 and
the full figure 5 will be available in the paper and electronic editions from
ApJ. v2: minor corrections to the abstract and text to match the Journal's
versio
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