We present an algorithm using principal component analysis (PCA) to subtract galaxies from imaging data and also two algorithms to find strong, galaxy-scale gravitational lenses in the resulting ...residual image. The combined method is optimised to find full or partial Einstein rings. Starting from a pre-selection of potential massive galaxies, we first perform a PCA to build a set of basis vectors. The galaxy images are reconstructed using the PCA basis and subtracted from the data. We then filter the residual image with two different methods. The first uses a curvelet (curved wavelets) filter of the residual images to enhance any curved/ring feature. The resulting image is transformed in polar coordinates, centred on the lens galaxy. In these coordinates, a ring is turned into a line, allowing us to detect very faint rings by taking advantage of the integrated signal-to-noise in the ring (a line in polar coordinates). The second way of analysing the PCA-subtracted images identifies structures in the residual images and assesses whether they are lensed images according to their orientation, multiplicity, and elongation. We applied the two methods to a sample of simulated Einstein rings as they would be observed with the ESA Euclid satellite in the VIS band. The polar coordinate transform allowed us to reach a completeness of 90% for a purity of 86%, as soon as the signal-to-noise integrated in the ring was higher than 30 and almost independent of the size of the Einstein ring. Finally, we show with real data that our PCA-based galaxy subtraction scheme performs better than traditional subtraction based on model fitting to the data. Our algorithm can be developed and improved further using machine learning and dictionary learning methods, which would extend the capabilities of the method to more complex and diverse galaxy shapes.
We present a novel, virtual X-ray observatory designed to obtain synthetic observations from hydro-numerical simulations, named phox. In particular, we provide a description of the code constituting ...the photon simulator and of the new approach implemented. We apply phox to simulated galaxy clusters in order to demonstrate its capabilities. In fact, X-ray observations of clusters of galaxies continue to provide us with an increasingly detailed picture of their structure and of the underlying physical phenomena governing the gaseous component, which dominates their baryonic content. Therefore, it is fundamental to find the most direct and faithful way to compare such observational data with hydrodynamical simulations of cluster-like objects, which can currently include various complex physical processes. Here, we present and analyse synthetic Suzaku observations of two cluster-size haloes obtained by processing with phox the hydrodynamical simulation of the large-scale, filament-like region in which they reside. Taking advantage of the simulated data, we test the results inferred from the X-ray analysis of the mock observations against the underlying, known solution. Remarkably, we are able to recover the theoretical temperature distribution of the two haloes by means of the multi-temperature fitting of the synthetic spectra. Moreover, the shapes of the reconstructed distributions allow us to trace the different thermal structure that distinguishes the dynamical state of the two haloes.
We investigate trends between the recent star formation history and black hole growth in galaxy bulges in the Sloan Digital Sky Survey. The galaxies lie at 0.01 < z < 0.07 where the fibre aperture ...covers only the central 0.6–4.0 kpc diameter of the galaxy. We find strong trends between black hole growth, as measured by dust-attenuation-corrected O iii luminosity, and the recent star formation history of the bulges. 56 per cent of the bulges are quiescent with no signs of recent or ongoing star formation and, while almost half of all active galactic nuclei (AGN) lie within these bulges, they contribute only ∼10 per cent to the total black hole growth in the local Universe. At the other extreme, the AGN contained within the ∼4 per cent of galaxy bulges that are undergoing or have recently undergone the strongest starbursts, contribute at least 10–20 per cent of the total black hole growth. Much of this growth occurs in AGN with high amounts of dust extinction and thus the precise numbers remain uncertain. The remainder of the black hole growth (>60 per cent) is contributed by bulges with more moderate recent or ongoing star formation. The strongest accreting black holes reside in bulges with a wide range in recent star formation history. We conclude that our results support the popular hypothesis for black hole growth occurring through gas inflow into the central regions of galaxies, followed by a starburst and triggering of the AGN. However, while this is a significant pathway for the growth of black holes, it is not the dominant one in the present-day Universe. More unspectacular processes are apparently responsible for the majority of this growth. In order to arrive at these conclusions we have developed a set of new high signal-to-noise ratio (S/N) optical spectral indicators, designed to allow a detailed study of stellar populations which have undergone recent enhanced star formation. Working in the rest-frame wavelength range 3750–4150 Å, ideally suited to many recent and ongoing spectroscopic surveys at low and high redshift, the first two indices are equivalent to the previously well-studied 4000-Å break strength and Hδ equivalent width. The primary advantage of this new method is a greatly improved S/N for the latter index, allowing the present study to use spectra with S/N per pixel as low as 8. The third index measures the excess strength of Ca ii (H&K), which is particularly sensitive to the transition of a post-starburst spectrum from A to F stars, and allows the degeneracy between time of burst and strength of burst to be broken.
The Millennium Run Observatory: first light Overzier, R; Lemson, G; Angulo, R. E ...
Monthly notices of the Royal Astronomical Society,
2013, Letnik:
428, Številka:
1
Journal Article
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Simulations of galaxy evolution aim to capture our current understanding as well as to make predictions for testing by future experiments. Simulations and observations are often compared in an ...indirect fashion: physical quantities are estimated from the observational data and compared to models. However, many applications can benefit from a more direct approach, where the observing process is also simulated, so that the models are seen fully from the observer's perspective. To facilitate this, we have developed the Millennium Run Observatory (MRObs), a theoretical virtual observatory which uses virtual telescopes to 'observe' semi-analytic galaxy formation simulations based on the suite of Millennium Run (MR) dark matter simulations. The MRObs produces data that can be processed and analysed using the standard observational software packages developed for real observations. At present, we produce images in 40 filters covering the rest-frame ultraviolet to infrared for two stellar population synthesis models, for three different models of absorption by the intergalactic medium, and in two cosmologies (Wilkinson Microwave Anisotropy Probe year 1 and 7). Galaxy distributions for a large number of mock light cones can be 'observed' using models of major ground- and space-based telescopes. The data include light cone catalogues linked to structural properties of galaxies, pre-observation model images, mock telescope images and Source Extractor products that can all be traced back to the higher level dark matter, semi-analytic galaxy and light cone catalogues available in the MR data base. Here, we describe our methods and announce a first public release of simulated observations that emulate a large number of extragalactic surveys e.g. Sloan Digital Sky Survey, Canada-France-Hawaii Telescope Legacy Survey (CFHT-LS), Great Observatories Origins Deep Survey (GOODS), GOODS/Early Release Science (ERS), Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) and Hubble Ultra Deep Field (HUDF). The MRObs browser, an online tool, further facilitates exploration of the simulated data. We demonstrate the benefits of a direct approach through a number of example applications: (1) deep galaxy number counts in the CANDELS survey; (2) observed properties of galaxy clusters; (3) structural parameters of galaxies; and (4) identification of dropout galaxies. The MRObs enhances the range of questions that can be asked of semi-analytic models, allowing observers and theorists to work towards each other with virtually complete freedom of where to meet.
We identify 42 "candidate groups" lying between 1.8 < z < 3.0 from a sample of 3502 galaxies with spectroscopic redshifts in the zCOSMOS-deep redshift survey within this same redshift interval. Based ...on extensive analysis of mock catalogs that have been generated from the Millennium simulation, we examine the likely nature of these systems at the time of observation, and what they will evolve into down to the present epoch. We also examine present-day halos and ask whether their progenitors would have been seen among our candidate groups. There is a clear excess of massive galaxies above 10sup 10 M around the locations of the candidate groups in a large independent COSMOS photo-z sample, but we see no evidence in this latter data for any color differentiation with respect to the field. This is, however, consistent with the idea that such differentiation arises in satellite galaxies, as indicated at z < 1, if the candidate groups are indeed only starting to be assembled.
We use large N-body simulations of dissipationless gravitational clustering in cold dark matter (CDM) cosmologies to study whether the properties of dark matter haloes are affected by their ...environment. We look for correlations between the masses, formation redshifts, concentrations, shapes and spins of haloes and the overdensity of their local environment. We also look for correlations of these quantities with the local tidal field. Our conclusion is extremely simple. Only the mass distribution varies as a function of environment. This variation is well described by a simple analytic formula based on the conditional Press—Schechter theory. We find no significant dependence of any other halo property on environment. Our results do not depend on our choice of cosmology. According to current hierarchical models, the structure and evolutionary history of a galaxy are fully determined by the structure and evolutionary history of the dark halo in which it is embedded. If these models are correct, clustering variations between galaxies of differing morphological types, luminosities, colours and surface brightnesses must all arise because the halo mass function is skewed towards high-mass objects in overdense regions of the Universe and towards low-mass objects in underdense regions.
We study the biasing relation between dark matter haloes or galaxies and the underlying mass distribution, using cosmological N-body simulations in which galaxies are modelled via semi-analytic ...recipes. The non-linear, stochastic biasing is quantified in terms of the mean biasing function and the scatter about it as a function of time, scale and object properties. The biasing of galaxies and haloes shows a general similarity and a characteristic shape, with no galaxies in deep voids and a steep slope in moderately underdense regions. At a comoving scale of ∼8h−1Mpc, the non-linearity in the biasing relation is typically ≲10per cent and the stochasticity is a few tens of per cent, corresponding to ∼30per cent variations in the cosmological parameter β=Ω0.6/b Biasing depends weakly on halo mass, galaxy luminosity, and scale. The observed trend with luminosity is reproduced when dust extinction is included. The time evolution is rapid, with the mean biasing larger by a factor of a few at z∼3 compared with z= 0, and with a minimum for the non-linearity and stochasticity at an intermediate redshift. Biasing today is a weak function of the cosmological model, reflecting the weak dependence on the power-spectrum shape, but the time evolution is more cosmology-dependent, reflecting the effect of the growth rate. We provide predictions for the relative biasing of galaxies of different type and colour, to be compared with upcoming large redshift surveys. Analytic models in which the number of objects is conserved underestimate the evolution of biasing, while models that explicitly account for merging provide a good description of the biasing of haloes and its evolution, suggesting that merging is a crucial element in the evolution of biasing.
In hierarchical models of gravitational clustering, virialized haloes are biased tracers of the matter distribution. As discussed by Mo & White, this bias is non-linear and stochastic. They developed ...a model that allows one to write down analytic expressions for the mean of the bias relation, in the initial Lagrangian, and the evolved, Eulerian, spaces. We provide analytic expressions for the higher order moments as well. In the initial Lagrangian space, each halo occupies a volume that is proportional to its mass. Haloes cannot overlap initially, so this gives rise to volume exclusion effects which can have important consequences for the halo distribution, particularly on scales smaller than that of a typical halo. Our model allows one to include these volume exclusion effects explicitly when computing the mean and higher order statistics of the Lagrangian space halo distribution. As a result of dynamical evolution, the spatial distribution of haloes in the evolved Eulerian space is likely to be different from that in the initial Lagrangian space. When combined with the Mo & White spherical collapse model, the model developed here allows one to quantify the evolution of the mean and scatter of the bias relation. We also show how their approach can be extended to compute the evolution, not just of the haloes, but of the dark matter distribution itself. Biasing and its evolution depend on the initial power spectrum. Clustering from Poisson and white-noise Gaussian initial conditions is treated in detail, since, in these cases, exact analytical results are available. We conjecture that these results can be easily extended to provide an approximate but accurate model for the biasing associated with clustering from more general Gaussian initial conditions. For all initial power spectra studied here, the model predictions for the Eulerian bias relation are in reasonable agreement with numerical simulations of hierarchical gravitational clustering for haloes of a wide range of masses, whereas the predictions for the corresponding Lagrangian space quantities are accurate only for massive haloes.
We simulate the formation and evolution of the local galaxy population, starting from initial conditions with a smoothed linear density field which matches that derived from the IRAS 1.2‐Jy galaxy ...survey. Our simulations track the formation and evolution of all dark matter haloes more massive than 1011 M⊙ out to a distance of 8000 km s−1 from the Milky Way. We implement prescriptions similar to those of Kauffmann et al. to follow the assembly and evolution of the galaxies within these haloes. We focus on two variants of the CDM cosmology: a ΛCDM model and a τCDM model. Galaxy formation in each is adjusted to reproduce the I‐band Tully–Fisher relation of Giovanelli et al. We compare the present‐day luminosity functions, colours, morphology and spatial distribution of our simulated galaxies with those of the real local population, in particular with the Updated Zwicky Catalog, with the IRAS PSCz redshift survey, and with individual local clusters such as Coma, Virgo and Perseus. We also use the simulations to study the clustering bias between the dark matter and galaxies of differing type. Although some significant discrepancies remain, our simulations recover the observed intrinsic properties and the observed spatial distribution of local galaxies reasonably well. They can thus be used to calibrate methods which use the observed local galaxy population to estimate the cosmic density parameter or to draw conclusions about the mechanisms of galaxy formation. To facilitate such work, we publicly release our z=0 galaxy catalogues, together with the underlying mass distribution.
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