32 research outputs found
Diverse Structural Evolution at z > 1 in Cosmologically Simulated Galaxies
From mock Hubble Space Telescope images, we quantify non-parametric
statistics of galaxy morphology, thereby predicting the emergence of
relationships among stellar mass, star formation, and observed rest-frame
optical structure at 1 < z < 3. We measure automated diagnostics of galaxy
morphology in cosmological simulations of the formation of 22 central galaxies
with 9.3 < log10 M_*/M_sun < 10.7. These high-spatial-resolution zoom-in
calculations enable accurate modeling of the rest-frame UV and optical
morphology. Even with small numbers of galaxies, we find that structural
evolution is neither universal nor monotonic: galaxy interactions can trigger
either bulge or disc formation, and optically bulge-dominated galaxies at this
mass may not remain so forever. Simulated galaxies with M_* > 10^10 M_sun
contain relatively more disc-dominated light profiles than those with lower
mass, reflecting significant disc brightening in some haloes at 1 < z < 2. By
this epoch, simulated galaxies with specific star formation rates below 10^-9.7
yr^-1 are more likely than normal star-formers to have a broader mix of
structural types, especially at M_* > 10^10 M_sun. We analyze a cosmological
major merger at z ~ 1.5 and find that the newly proposed MID morphology
diagnostics trace later merger stages while G-M20 trace earlier ones. MID is
sensitive also to clumpy star-forming discs. The observability time of typical
MID-enhanced events in our simulation sample is less than 100 Myr. A larger
sample of cosmological assembly histories may be required to calibrate such
diagnostics in the face of their sensitivity to viewing angle, segmentation
algorithm, and various phenomena such as clumpy star formation and minor
mergers.Comment: 23 pages, 16 figures, MNRAS accepted versio
COSMOS morphological classification with ZEST (the Zurich Estimator of Structural Types) and the evolution since z=1 of the Luminosity Function of early-, disk-, and irregular galaxies
(ABRIDGED) Motivated by the desire to reliably and automatically classify
structure of thousands of COSMOS galaxies, we present ZEST, the Zurich
Estimator of Structural Types. To classify galaxy structure, ZEST uses: (i)
Five non-parametric diagnostics: asymmetry, concentration, Gini coefficient,
2nd-order moment of the brightest 20% of galaxy pixels, and ellipticity; and
(ii) The exponent n of single--Sersic fits to the 2D surface brightness
distributions. To fully exploit the wealth of information while reducing the
redundancy present in these diagnostics, ZEST performs a principal component
(PC) Analysis. We use a sample of ~56,000 I<24 COSMOS galaxies to show that the
first three PCs fully describe the key aspects of the galaxy structure, i.e.,
to calibrate a three-dimensional classification grid of axis PC_1, PC_2, and
PC_3. We demonstrate the robustness of the ZEST grid on the z=0 sample of Frei
et al. (1996). The ZEST classification breaks most of the degeneracy between
different galaxy populations that affects morphological classifications based
on only some of the diagnostics included in ZEST. As a first application, we
present the evolution since z~1 of the Luminosity Functions of COSMOS galaxies
of early, disk and irregular galaxies and, for disk galaxies, of different
bulge-to-disk ratios. Overall, we find that the LF up to a redshift z=1 is
consistent with a pure-luminosity evolution (of about 0.95 magnitudes at z
\~0.7). We highlight however two trends, that are in general agreement with a
down-sizing scenario for galaxy formation: (1.) A deficit of a factor of about
two at z~0.7 of MB>-20.5 structurally--classified early--type galaxies; and
(2.) An excess of a factor of about three, at a similar redshift, of irregular
galaxies.Comment: Accepted for publication in the ApJ COSMOS special issue. A version
with high resolution figures is available at
http://www.exp-astro.phys.ethz.ch/scarlata/papers/ApJS_ZEST.pd
Physical and morphological properties of z~3 LBGs: dependence on Lyalpha line emission
We investigate the physical and morphological properties of LBGs at z ~2.5 to
~3.5, to determine if and how they depend on the nature and strength of the
Lyalpha emission. We selected U-dropout galaxies from the z-detected GOODS
MUSIC catalog, by adapting the classical Lyman Break criteria on the GOODS
filter set. We kept only those galaxies with spectroscopic confirmation, mainly
from VIMOS and FORS public observations. Using the full multi-wavelength
14-bands photometry, we determined the physical properties of the galaxies,
through a standard spectral energy distribution fitting with the updated
Charlot & Bruzual (2009) templates. We also added other relevant observations,
i.e. the 24mu m observations from Spitzer/MIPS and the 2 MSec Chandra X-ray
observations. Finally, using non parametric diagnostics (Gini, Concentration,
Asymmetry, M_20 and ellipticity), we characterized the rest-frame UV morphology
of the galaxies. We then analyzed how these physical and morphological
properties correlate with the presence of the Lyalpha line in the optical
spectra. We find that, unlike at higher redshift, the dependence of physical
properties on the Lyalpha line is milder: galaxies without Lyalpha in emission
tend to be more massive and dustier than the rest of the sample, but all other
parameters, ages, SFRs, X-ray emission as well as UV morphology do not depend
strongly on the presence of the line emission. A simple scenario where all LBGs
have intrinsically high Lyalpha emission, but where dust and neutral hydrogen
content (which shape the final appearance of the Lyalpha) depend on the mass of
the galaxies, is able to reproduce the majority of the observed properties at
z~3. Some modification might be needed to account for the observed evolution of
these properties with cosmic epoch, which is also discussed.Comment: Accepted for publication on A&
Environmental effects in the interaction and merging of galaxies in zCOSMOS
We analyze the environments and galactic properties (morphologies and star-formation histories) of a sample of 153 close kinematic pairs in the redshift range 0.2 < z < 1 identified in the zCOSMOS-bright 10k spectroscopic sample of galaxies. Correcting for projection effects, the fraction of close kinematic pairs is three times higher in the top density quartile than in the lowest one. This translates to a three times higher merger rate because the merger timescales are shown, from mock catalogues based on the Millennium simulation, to be largely independent of environment once the same corrections for projection is applied. We then examine the morphologies and stellar populations of galaxies in the pairs, comparing them to control samples that are carefully matched in environment so as to remove as much as possible the well-known effects of environment on the properties of the parent population of galaxies. Once the environment is properly taken into account in this way, we find that the early-late morphology mix is the same as for the parent population, but that the fraction of irregular galaxies is boosted by 50-75%, with a disproportionate increase in the number of irregular-irregular pairs (factor of 4-8 times), due to the disturbance of disk galaxies. Future dry-mergers, involving elliptical galaxies comprise less than 5% of all close kinematic pairs. In the closest pairs, there is a boost in the specific star-formation rates of star-forming galaxies of a factor of 2-4, and there is also evidence for an increased incidence of post star-burst galaxies. Although significant for the galaxies involved, the "excess" star-formation associated with pairs represents only about 5% of the integrated star-formation activity in the parent sample. Although most pair galaxies are in dense environments, the effects of interaction appear to be largest in the lower density environments
CANDELS: The correlation between galaxy morphology and star formation activity at z~2
We discuss the state of the assembly of the Hubble Sequence in the mix of
bright galaxies at redshift 1.4< z \le 2.5 with a large sample of 1,671
galaxies down to H_{AB}~26, selected from the HST/ACS and WFC3 images of the
GOODS--South field obtained as part of the GOODS and CANDELS observations. We
investigate the relationship between the star formation properties and
morphology using various parametric diagnostics, such as the Sersic light
profile, Gini (G), M_{20}, Concentration (C), Asymmetry (A) and multiplicity
parameters. Our sample clearly separates into massive, red and passive galaxies
versus less massive, blue and star forming ones, and this dichotomy correlates
very well with the galaxies' morphological properties. Star--forming galaxies
show a broad variety of morphological features, including clumpy structures and
bulges mixed with faint low surface brightness features, generally
characterized by disky-type light profiles. Passively evolving galaxies, on the
other hand, very often have compact light distribution and morphology typical
of today's spheroidal systems. We also find that artificially redshifted local
galaxies have a similar distribution with z~2galaxies in a G-M_{20} plane.
Visual inspection between the rest-frame optical and UV images show that there
is a generally weak morphological k-correction for galaxies at z~2, but the
comparison with non-parametric measures show that galaxies in the rest-frame UV
are somewhat clumpier than rest-frame optical. Similar general trends are
observed in the local universe among massive galaxies, suggesting that the
backbone of the Hubble sequence was already in place at z~2.Comment: 22 pages, 19 figures, ApJ accepted (added 3 references
The Formation and Evolution of Virgo Cluster Galaxies - I. Broadband Optical & Infrared Colours
We use a combination of deep optical (gri) and near-infrared (H) photometry
to study the radially-resolved colours of a broad sample of 300 Virgo cluster
galaxies. For most galaxy types, we find that the median g-H colour gradient is
either flat (gas-poor giants and gas-rich dwarfs) or negative (i.e., colours
become bluer with increasing radius; gas-poor dwarfs, spirals, and gas-poor
peculiars). Later-type galaxies typically exhibit more negative gradients than
early-types. Given the lack of a correlation between the central colours and
axis ratios of Virgo spiral galaxies, we argue that dust likely plays a small
role, if at all, in setting those colour gradients. We search for possible
correlations between galaxy colour and photometric structure or environment and
find that the Virgo galaxy colours become redder with increasing concentration,
luminosity and surface brightness, while no dependence with cluster-centric
radius or local galaxy density is detected (over a range of ~2 Mpc and ~3-16
Mpc^-2, respectively). However, the colours of gas-rich Virgo galaxies do
correlate with neutral gas deficiency, such that these galaxies become redder
with higher deficiencies. Comparisons with stellar population models suggest
that these colour gradients arise principally from variations in stellar
metallicity within these galaxies, while age variations only make a significant
contribution to the colour gradients of Virgo irregulars. A detailed stellar
population analysis based on this material is presented in Roediger et al
(2011b; arXiv:1011.3511).Comment: 34 pages, 12 figures, 1 table, submitted to MNRAS; Paper II
(arXiv:1011.3511) has also been update
The Evolution of the Number Density of Large Disk Galaxies in COSMOS
We study a sample of approximately 16,500 galaxies with I_(ACS,AB) †22.5 in the central 38% of the COSMOS field, which are extracted from a catalog constructed from the Cycle 12 ACS F814W COSMOS data set. Structural information on the galaxies is derived by fitting single SĂ©rsic models to their two-dimensional surface brightness distributions. In this paper we focus on the disk galaxy population (as classified by the Zurich Estimator of Structural Types), and investigate the evolution of the number density of disk galaxies larger than approximately 5 kpc between redshift z ~ 1 and the present epoch. Specifically, we use the measurements of the half-light radii derived from the SĂ©rsic fits to construct, as a function of redshift, the size function Ί(r_(1/2), z) of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. In each redshift bin, the size function specifies the number of galaxies per unit comoving volume and per unit half-light radius r_(1/2). Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find the following: (1) The number density of disk galaxies with intermediate sizes (r_(1/2) ~ 5-7 kpc) remains nearly constant from z ~ 1 to today. Unless the growth and destruction of such systems exactly balanced in the last eight billion years, they must have neither grown nor been destroyed over this period. (2) The number density of the largest disks (r_(1/2) > 7 kpc) decreases by a factor of about 2 out to z ~ 1. (3) There is a constancyâor even slight increaseâin the number density of large bulgeless disks out to z ~ 1; the deficit of large disks at early epochs seems to arise from a smaller number of bulged disks. Our results indicate that the bulk of the large disk galaxy population has completed its growth by z ~ 1 and support the theory that secular evolution processes produceâor at least add stellar mass toâthe bulge components of disk galaxies
The zCOSMOS survey: the role of the environment in the evolution of the luminosity function of different galaxy types
Aims. An unbiased and detailed characterization of the galaxy luminosity function (LF) is a basic requirement in many astrophysical issues: it is of particular interest in assessing the role of the environment in the evolution of the LF of different galaxy types.
Methods. We studied the evolution in the B band LF to redshift z ~ 1 in the zCOSMOS 10k sample, for which both accurate galaxy classifications (spectrophotometric and morphological) and a detailed description of the local density field are available.
Results. The global B band LF exhibits a brightening of ~0.7 mag in M^* from z ~ 0.2 to z ~ 0.9. At low redshifts (z -20), while the bright end is populated mainly by spectrophotometric early types. At higher redshift, spectrophotometric late-type galaxies evolve significantly and, at redshift z ~ 1, the contribution from the various types to the bright end of the LF is comparable. The evolution for spectrophotometric early-type galaxies is in both luminosity and normalization: M* brightens by ~0.6 mag but Ï^â decreases by a factor ~1.7 between the first and the last redshift bin. A similar behaviour is exhibited by spectrophotometric late-type galaxies, but with an opposite trend for the normalization: a brightening of ~0.5 mag is present in M^*, while Ï^â increases by a factor ~1.8.â©Studying the role of the environment, we find that the global LF of galaxies in overdense regions has always a brighter M^* and a flatter slope. In low density environments, the main contribution to the LF is from blue galaxies, while for high density environments there is an important contribution from red galaxies to the bright end.â©The differences between the global LF in the two environments are not due to only a difference in the relative numbers of red and blue galaxies, but also to their relative luminosity distributions: the value of M^* for both types in underdense regions is always fainter than in overdense environments. These results indicate that galaxies of the same type in different environments have different properties.â©We also detect a differential evolution in blue galaxies in different environments: the evolution in their LF is similar in underdense and overdense regions between z ~ 0.25 and z ~ 0.55, and is mainly in luminosity. In contrast, between z ~ 0.55 and z ~ 0.85 there is little luminosity evolution but there is significant evolution in Ï^â, that is, however, different between the two environments: in overdense regions Ï^â increases by a factor ~1.6, while in underdense regions this increase reaches a factor ~2.8. Analyzing the blue galaxy population in more detail, we find that this evolution is driven mainly by the bluest types.
Conclusions. The âspecularâ evolution of late- and early-type galaxies is consistent with a scenario where a part of blue galaxies is transformed in red galaxies with increasing cosmic time, without significant changes in the fraction of intermediate-type galaxies. The bulk of this tranformation in overdense regions probably happened before z ~ 1, while it is still ongoing at lower redshifts in underdense environments
The bulk of the black hole growth since z ~ 1 occurs in a secular universe: no major merger-AGN connection
What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs)
activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and
a matched control sample of 1264 inactive galaxies over z ~ 0.3â1.0 and M_â < 10^(11.7) M_â with high-resolution
Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of
their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies
in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be
responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies
do not show strong distortions and (2) there is no significant difference in the distortion fractions between active
and inactive galaxies. Our findings provide the best direct evidence that, since z ~ 1, the bulk of black hole (BH)
accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,
internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth.We
also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability
of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement
of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major
mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of
a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low
levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions
The glue of the economic system: the effect of relational goods on trust and trustworthiness.
The role of ârelational goodsâ is almost unexplored in the literature, yet our experimental results document that, even in their weakest form (opportunity of meeting an unknown player at the end of an experimental game), they significantly affect important âlubricantsâ of economic activity such as trust and trustworthiness and generate significant departures from the standard Nash equilibrium outcome in trust (investment) games. Our findings suggest that relational goods are an important âsource of energyâ in economic interactions and that the study of this âneglected particleâ of socioeconomic life may produce significant advancements on both positive and normative economics.relational goods, trust, experimental games