arXiv subjects

C. Porciani

Publications and source records attributed to C. Porciani.

46 records · Page 3Linked to original sources

arXiv2007

The angular correlations of galaxies in the COSMOS field

We present measurements of the two-point galaxy angular correlation function w(θ) in the COSMOS field. Independent determinations of w(θ) as a function of magnitude limit are presented for both the HST ACS catalog and also for the ground-based data from Subaru and the CFHT. Despite having significantly different masks, these three determinations agree well. At bright magnitudes (IAB<22), our data generally match very well with existing measurements and with mock catalogs based on semi-analytic galaxy formation calculations of Kitzbichler and White from the Millennium Simulation. The exception is that our result is at the upper end of the expected cosmic variance scatter for θ> 10 arcmin, which we attribute to a particularly rich structure known to exist at z~0.8. For fainter samples, however, the level of clustering is somewhat higher than reported by some previous studies: in all three catalogues we find w(θ=1')~0.014 at a median IAB magnitude of 24. At these very faintest magnitudes, our measurements agree well with the latest determinations from the Canada-France Legacy Survey. This level of clustering is approximately double what is predicted by the semi-analytic catalogs (at all angles). The semi-analytic results allow an estimate of cosmic variance, which is too small to account for the discrepancy. We therefore conclude that the mean amplitude of clustering at this level is higher than previously estimated.

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arXiv2007

The First Release COSMOS Optical and Near-IR Data and Catalog

We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3um and 2.4um. These include data taken on the Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.

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arXiv2006

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.

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arXiv2006

The Zurich Extragalactic Bayesian Redshift Analyzer (ZEBRA) and its first application: COSMOS

We present ZEBRA, the Zurich Extragalactic Bayesian Redshift Analyzer. The current version of ZEBRA combines and extends several of the classical approaches to produce accurate photometric redshifts down to faint magnitudes. In particular, ZEBRA uses the template-fitting approach to produce Maximum Likelihood and Bayesian redshift estimates based on: (1.) An automatic iterative technique to correct the original set of galaxy templates to best represent the SEDs of real galaxies at different redshifts; (2.) A training set of spectroscopic redshifts for a small fraction of the photometric sample; and (3.) An iterative technique for Bayesian redshift estimates, which extracts the full two-dimensional redshift and template probability function for each galaxy. We demonstrate the performance of ZEBRA by applying it to a sample of 866 I_AB <= 22.5 COSMOS galaxies with available u*, B, V, g', r', i', z' and K_s photometry and zCOSMOS spectroscopic redshifts in the range 0 < z < 1.3. Adopting a 5-sigma-clipping that excludes less than 10 galaxies, both the Maximum Likelihood and Bayesian estimates for this sample have an accuracy dz/1+z better than 0.03. Similar accuracies are recovered using mock galaxies.

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arXiv2006

The evolution of the number density of large disk galaxies in COSMOS

We study a sample of approximately 16,500 galaxies with I_AB <= 22.5 in the COSMOS field. Structural information on the galaxies is derived by fitting single Sersic models to their two-dimensional surface brightness distributions. We investigate the evolution of the number density of disk galaxies larger than 5 kpc between redshift z~1 and the present epoch. To this end, we use the measurements of the half-light radii to construct, as a function of redshift, the size function of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find that: (i) The number density of disk galaxies with intermediate sizes (r_{1/2}~5-7 kpc) remains nearly constant from z~1 to today. (ii) The number density of the largest disks (r_{1/2}>7 kpc) decreases by a factor of about two out to z~1. (iii) There is a constancy in 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 hypothesis that secular evolution processes produce - or at least add stellar mass to - the bulge components of disk galaxies.

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arXiv2006

The Canada-France Deep Fields III: Photometric Redshift Distribution to I(AB) ~ 24

We compute accurate redshift distributions to I(AB) = 24 and R(AB) = 24.5 using photometric redshifts estimated from six-band UBVRIZ photometry in the Canada-France Deep Fields-Photometric Redshift Survey (CFDF-PRS). Our photometric redshift algorithm is calibrated using hundreds of CFRS spectroscopic redshifts in the same fields. The dispersion in redshift is σ/(1+z) \la 0.04 to the CFRS depth of I(AB) = 22.5, rising to σ/(1+z) \la 0.06 at our nominal magnitude and redshift limits of I(AB) = 24 and z \le 1.3, respectively. We describe a new method to compute N(z) that incorporates the full redshift likelihood functions in a Bayesian iterative analysis and we demonstrate in extensive Monte Carlo simulations that it is superior to distributions calculated using simple maximum likelihood redshifts. The field-to-field differences in the redshift distributions, while not unexpected theoretically, are substantial even on 30' scales. We provide I(AB) and R(AB) redshift distributions, median redshifts, and parametrized fits of our results in various magnitude ranges, accounting for both random and systematic errors in the analysis.

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arXiv2003

The Halo Distribution of 2dF Galaxies

We use the clustering results obtained by Madgwick et al. (2003) for a sample of 96,791 2dF galaxies with redshift 0.01 \lt z \lt 0.15 to study the distribution of late-type and early-type galaxies within dark matter haloes of different mass. Within the framework of our models, galaxies of both classes are found to be as spatially concentrated as the dark matter within haloes even though, while the distribution of star-forming galaxies can also allow for some steeper profiles, this is drastically ruled out in the case of early-type galaxies. We also find evidence for morphological segregation, as late-type galaxies appear to be distributed within haloes of mass scales corresponding to groups and clusters up to about two virial radii, while passive objects show a preference to reside closer to the halo centre. If we assume a broken power-law of the form =(m/m_0)^{alpha_1} for m_{cut}\le m =(m/m_0)^{alpha_2} at higher masses to describe the dependence of the average number of galaxies within haloes on the halo mass, fits to the data show that star-forming galaxies start appearing in haloes of masses m_{cut}~10^{11}m_{sun}, much smaller than what is obtained for early-type galaxies (m_{cut}~10^{12.6}m_{sun}). In the high-mass regime m\ge m_0, $ increases with halo mass more slowly (alpha_2~0.7) in the case of late-type galaxies than for passive objects which present alpha_2~1.1. We stress that there is no degeneracy in the determination of the best functional forms for rho(r) and , as they affect the behaviour of the galaxy-galaxy correlation function on different scales.

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arXiv2000

Spin Profile of Galactic Halos and Disk Formation

We summarize recent developments in the study of the origin of halo spin profiles and preliminary implications on disk formation. The specific angular-momentum distributions within halos in N-body simulations match a universal shape, M(<j) \propto j/(j_0+j). It is characterized by a power law over most of the mass, and one shape parameter in addition to the spin parameter λ. The angular momentum tends to be aligned throughout the halo and of cylindrical symmetry. Even if angular momentum is conserved during baryonic infall, the resultant disk density profile is predicted to deviate from exponential, with a denser core and an extended tail. A slightly corrected version of the scaling relation due to linear tidal-torque theory is used to explain the origin of a typical power-law profile in shells, j(M) \propto M^s with s \gsim 1. While linear theory crudely predicts the amplitudes of halo spins, it is not a good predictor of their directions. Independently, mergers of halos are found to produce a similar profile due to j transfer from the orbit to the product halo via dynamical friction and tidal stripping. The halo spin is correlated with having a recent major merger, though this correlation is weakened by mass loss. These two effects are due to a correlation between the spins of neighboring halos and their orbit, leading to prograde mergers.

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arXiv1998

The bias field of dark matter haloes

This paper presents a stochastic approach to the clustering evolution of dark matter haloes in the Universe. Haloes, identified by a Press-Schechter-type algorithm in Lagrangian space, are described in terms of `counting fields', acting as non-linear operators on the underlying Gaussian density fluctuations. By ensemble averaging these counting fields, the standard Press-Schechter mass function as well as analytic expressions for the halo correlation function and corresponding bias factors of linear theory are obtained, thereby extending the recent results by Mo and White. The non-linear evolution of our halo population is then followed by solving the continuity equation, under the sole hypothesis that haloes move by the action of gravity. This leads to an exact and general formula for the bias field of dark matter haloes, defined as the local ratio between their number density contrast and the mass density fluctuation. Besides being a function of position and `observation' redshift, this random field depends upon the mass and formation epoch of the objects and is both non-linear and non-local. The latter features are expected to leave a detectable imprint on the spatial clustering of galaxies, as described, for instance, by statistics like bispectrum and skewness. Our algorithm may have several interesting applications, among which the possibility of generating mock halo catalogues from low-resolution N-body simulations.

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arXiv1996

Physical constraints on the halo mass function

We analyse the effect of two relevant physical constraints on the mass multiplicity function of dark matter halos in a Press--Schechter type algorithm. Considering the random--walk of linear Gaussian density fluctuations as a function of the smoothing scale, we simultaneously i) account for mass semi--positivity and ii) avoid the cloud--in--cloud problem. It is shown that the former constraint implies a severe cutoff of low--mass objects, balanced by an increase on larger mass scales. The analysis is performed both for scale--free power--spectra and for the standard cold dark matter model. Our approach shows that the well--known ``infrared" divergence of the standard Press--Schechter mass function is caused by unphysical, negative mass events which inevitably occur in a Gaussian distribution of density fluctuations.

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