85 research outputs found
A large-scale super-structure at z=0.65 in the UKIDSS Ultra-Deep Survey field
In hierarchical structure formation scenarios, galaxies accrete along high density filaments. Superclusters represent the largest density enhancements in the cosmic web with scales of 100 to 200 Mpc. As they represent the largest components of LSS, they are very powerful tools to constrain cosmological models. Since they also offer a wide range of density, from infalling group to high density cluster core, they are also the perfect laboratory to study the influence of environment on galaxy evolution. I will present a newly discovered large scale structure at z=0.65 in the UKIDSS UDS field. Although statistically predicted, the presence of such structure in UKIDSS, one of the most extensively covered and studied extragalactic field, remains a serendipity. Our follow-up confirmed more than 15 group members including at least three galaxy clusters with M200~10^14Msol . Deep spectroscopy of the quiescent core galaxies reveals that the most massive structure knots are at very different formation stage with a range of red sequence properties. Statistics allow us to map formation age across the structure denser knots and identify where quenching is most probably occurring across the LSS. Spectral diagnostics analysis also reveals an interesting population of transition galaxies we suspect are transforming from star-forming to quiescent galaxies
Star Formation and AGN Activity in Galaxy Clusters from : a Multi-wavelength Analysis Featuring /PACS
We present a detailed, multi-wavelength study of star formation (SF) and AGN
activity in 11 near-infrared (IR) selected, spectroscopically confirmed,
massive () galaxy clusters at . Using
new, deep /PACS imaging, we characterize the optical to far-IR
spectral energy distributions (SEDs) for IR-luminous cluster galaxies, finding
that they can, on average, be well described by field galaxy templates.
Identification and decomposition of AGN through SED fittings allows us to
include the contribution to cluster SF from AGN host galaxies. We quantify the
star-forming fraction, dust-obscured SF rates (SFRs), and specific-SFRs for
cluster galaxies as a function of cluster-centric radius and redshift. In good
agreement with previous studies, we find that SF in cluster galaxies at
is largely consistent with field galaxies at similar epochs,
indicating an era before significant quenching in the cluster cores
(Mpc). This is followed by a transition to lower SF activity as
environmental quenching dominates by . Enhanced SFRs are found in lower
mass () cluster galaxies. We
find significant variation in SF from cluster-to-cluster within our uniformly
selected sample, indicating that caution should be taken when evaluating
individual clusters. We examine AGN in clusters from , finding an
excess AGN fraction at , suggesting environmental triggering of AGN
during this epoch. We argue that our results a transition from field-like
to quenched SF, enhanced SF in lower mass galaxies in the cluster cores, and
excess AGN are consistent with a co-evolution between SF and AGN in
clusters and an increased merger rate in massive haloes at high redshift.Comment: 26 pages, 14 figures, 6 tables with appendix, accepted for
publication in the Astrophysical Journa
LUCIFER@LBT view of star-forming galaxies in the cluster 7C 1756+6520 at z~1.4
Galaxy clusters are key places to study the contribution of {\it nature}
(i.e. mass, morphology) and {\it nurture} (i.e.environment) in the formation
and evolution of galaxies. Recently, a number of clusters at z1, i.e.
corresponding to the first epochs of the cluster formation, has been discovered
and confirmed spectroscopically. We present new observations obtained with the
{\sc LUCIFER} spectrograph at Large Binocular Telescope (LBT) of a sample of
star-forming galaxies associated with a large scale structure around the radio
galaxy 7C1756+6520 at z=1.42. Combining our spectroscopic data and the
literature photometric data, we derived some of the properties of these
galaxies: star formation rate, metallicity and stellar mass. With the aim of
analyzing the effect of the cluster environment on galaxy evolution, we have
located the galaxies in the plane of the so-called Fundamental Metallically
Relation (FMR), which is known not to evolve with redshift up to z for
field galaxies, but it is still unexplored in rich environments at low and high
redshift. We found that the properties of the galaxies in the cluster 7C
1756+6520 are compatible with the FMR which suggests that the effect of the
environment on galaxy metallicity at this early epoch of cluster formation is
marginal. As a side study, we also report the spectroscopic analysis of a
bright AGN, belonging to the cluster, which shows a significant outflow of gas.Comment: Accepted for publication by MNRAS, 10 pages, 6 figures, 3 table
A large-scale galaxy structure at z = 2.02 associated with the radio galaxy MRC 0156-252
We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC 0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target ~80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (<6″) also show AGN signatures. Ten sources, including the radio galaxy, lie within | z − 2.020 | <0.015 (i.e., velocity offsets <1500 km s^-1) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC 0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger-scale structure
First results from the VIRIAL survey: the stellar content of -selected quiescent galaxies at from KMOS
We investigate the stellar populations of 25 massive, galaxies
() at using data obtained with
the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were
selected to be quiescent based on their broadband colors and redshifts using
data from the 3D-HST grism survey. The mean redshift of our sample is , where KMOS YJ-band data probe age- and metallicity-sensitive
absorption features in the rest-frame optical, including the band, Fe I,
and high-order Balmer lines. Fitting simple stellar population models to a
stack of our KMOS spectra, we derive a mean age of Gyr.
We confirm previous results suggesting a correlation between color and age for
quiescent galaxies, finding mean ages of Gyr and
Gyr for the reddest and bluest galaxies in our sample.
Combining our KMOS measurements with those obtained from previous studies at
we find evidence for a Gyr spread in the formation epoch of
massive galaxies. At the measured stellar ages are consistent with
passive evolution, while at they appear to saturate at
1 Gyr, which likely reflects changing demographics of the (mean)
progenitor population. By comparing to star-formation histories inferred for
"normal" star-forming galaxies, we show that the timescales required to form
massive galaxies at are consistent with the enhanced
-element abundances found in massive local early-type galaxies.Comment: 6 pages, 5 figures, accepted for publication in ApJ
Growing up in a megalopolis : environmental effects on galaxy evolution in a supercluster at z similar to 0.65 in UKIDSS UDS
We present a large-scale galaxy structure C1 J021734-0513 at z similar to 0.65 discovered in the UKIDSS UDS field, made of similar to 20 galaxy groups and clusters, spreading over 10 Mpc. We report on a VLT/VIMOS spectroscopic follow-up program that, combined with past spectroscopy, allowed us to confirm four galaxy clusters (M-200 similar to 10(14) M-circle dot) and a dozen associated groups and star-forming galaxy overdensities. Two additional filamentary structures at z similar to 0.62 and 0.69 and foreground and background clusters at 0.6 <z <0.7 were also confirmed along the line of sight. The structure subcomponents are at different formation stages. The clusters have a core dominated by passive galaxies and an established red sequence. The remaining structures are a mix of star-forming galaxy overdensities and forming groups. The presence of quiescent galaxies in the core of the latter shows that 'pre-processing' has already happened before the groups fall into their more massive neighbours. Our spectroscopy allows us to derive spectral index measurements e.g. emission/absorption line equivalent widths, strength of the 4000 angstrom break, valuable to investigate the star formation history of structure members. Based on these line measurements, we select a population of 'post-starburst' galaxies. These galaxies are preferentially found within the virial radius of clusters, supporting a scenario in which their recent quenching could be prompted by gas stripping by the dense intracluster medium. We derive stellar age estimates using Markov Chain Monte Carlo-based spectral fitting for quiescent galaxies and find a correlation between ages and colours/stellar masses which favours a top-down formation scenario of the red sequence. A catalogue of similar to 650 redshifts in UDS is released alongside the paper (via MNRAS online data).Peer reviewe
Structural Evolution of Early-type Galaxies to z=2.5 in CANDELS
Projected axis ratio measurements of 880 early-type galaxies at redshifts
1<z<2.5 selected from CANDELS are used to reconstruct and model their intrinsic
shapes. The sample is selected on the basis of multiple rest-frame colors to
reflect low star-formation activity. We demonstrate that these galaxies as an
ensemble are dust-poor and transparent and therefore likely have smooth light
profiles, similar to visually classified early-type galaxies. Similar to their
present-day counterparts, the z>1 early-type galaxies show a variety of
intrinsic shapes; even at a fixed mass, the projected axis ratio distributions
cannot be explained by the random projection of a set of galaxies with very
similar intrinsic shapes. However, a two-population model for the intrinsic
shapes, consisting of a triaxial, fairly round population, combined with a flat
(c/a~0.3) oblate population, adequately describes the projected axis ratio
distributions of both present-day and z>1 early-type galaxies. We find that the
proportion of oblate versus triaxial galaxies depends both on the galaxies'
stellar mass, and - at a given mass - on redshift. For present-day and z<1
early-type galaxies the oblate fraction strongly depends on galaxy mass. At z>1
this trend is much weaker over the mass range explored here
(10^10<M*/M_sun<10^11), because the oblate fraction among massive (M*~10^11
M_sun) was much higher in the past: 0.59+-0.10 at z>1, compared to 0.20+-0.02
at z~0.1. In contrast, the oblate fraction among low-mass early-type galaxies
(log(M*/M_sun)1 to
0.72+-0.06 at z=0. [Abridged]Comment: accepted for publication in ApJ; 14 pages; 10 figures; 4 table
The Mid-Infrared Environments of High-Redshift Radio Galaxies
Taking advantage of the impressive sensitivity of Spitzer to detect massive
galaxies at high redshift, we study the mid-infrared environments of powerful,
high-redshift radio galaxies at 1.2<z<3. Galaxy cluster member candidates were
isolated using a single Spitzer/IRAC mid-infrared color criterion,
[3.6]-[4.5]>-0.1 (AB), in the fields of 48 radio galaxies at 1.2<z<3. This
simple IRAC color selection is effective at identifying galaxies at z>1.2.
Using a counts-in-cell analysis, we identify a field as overdense when 15 or
more red IRAC sources are found within 1arcmin (i.e.,~0.5Mpc at 1.2<z<3) of the
radio galaxy to the 5sigma flux density limits of our IRAC data (f3.6=11.0uJy,
f4.5=13.4uJy). We find that radio galaxies lie preferentially in medium to
dense regions, with 73% of the targeted fields denser than average. Our
(shallow) 120s data permit the rediscovery of previously known clusters and
protoclusters associated with radio galaxies as well as the discovery of new
promising galaxy cluster candidates at z>1.2.Comment: 14 pages, 7 figures, 3 tables, accepted for publication in Ap
The Kinematics of Massive Quiescent Galaxies at 1.4 < z <2.1: Dark Matter Fractions, IMF Variation, and the Relation to Local Early-type Galaxies
We study the dynamical properties of massive quiescent galaxies at 1.4 DM[e]. Comparing our high-redshift sample to their likely descendants at low redshift, we find that f DM[e] has increased by a factor of more than 4 since z ≈ 1.8, from f DM[e] = 6.6% ± 1.0% to ~24%. The observed increase appears robust to changes in the methods used to estimate dynamical masses or match progenitors and descendants. We quantify possible variation of the stellar IMF through the offset parameter α, defined as the ratio of dynamical mass in stars to the stellar mass estimated using a Chabrier IMF. We demonstrate that the correlation between stellar velocity dispersion and α reported among quiescent galaxies at low redshift is already in place at z = 2, and we argue that subsequent evolution through (mostly minor) merging should act to preserve this relation while contributing significantly to galaxies' overall growth in size and stellar mass
- …