The Size and Shape of Voids in Three-Dimensional Galaxy Surveys

The sizes and shapes of voids in a galaxy survey depend not only on the physics of structure formation, but also on the sampling density of the survey and on the algorithm used to define voids. Using an N-body simulation with a CDM power spectrum, we study the properties of voids in samples with different number densities of galaxies, both in redshift space and in real space. When voids are defined as regions totally empty of galaxies, their characteristic volume is strongly dependent on sampling density; when they are defined as regions whose density is 0.2 times the mean galaxy density, the dependence is less strong. We compare two void-finding algorithms, one in which voids are nonoverlapping spheres, and one, based on the algorithm of Aikio and Mahonen, which does not predefine the shape of a void. Regardless of the algorithm chosen, the characteristic void size is larger in redshift space than in real space, and is larger for low sampling densities than for high sampling densities. We define an elongation statistic Q which measures the tendency of voids to be stretched or squashed along the line of sight. Using this statistic, we find that at sufficiently high sampling densities (comparable to the number densities of galaxies brighter than L_*), large voids tend to be slightly elongated along the line of sight in redshift space.Comment: LaTex, 21 pages (including 7 figures), ApJ, submitte

Stellar and Gas properties of High HI Mass-to-Light Ratio Galaxies in the Local Universe

We present a multi-wavelength study (BVRI band photometry and HI line interferometry) of nine late-type galaxies selected from the HIPASS Bright Galaxy Catalog on the basis of apparently high HI mass-to-light ratios (3 M_sun/L_sun < M_HI/L_B < 27 M_sun/L_sun). We found that most of the original estimates for M_HI/L_B based on available photographic magnitudes in the literature were too high, and conclude that genuine high HI mass-to-light ratio (>5 M_sun/L_sun) galaxies are rare in the Local Universe. Extreme high M_HI/L_B galaxies like ESO215-G?009 appear to have formed only the minimum number of stars necessary to maintain the stability of their HI disks, and could possibly be used to constrain galaxy formation models. They may to have been forming stars at a low, constant rate over their lifetimes. The best examples all have highly extended HI disks, are spatially isolated, and have normal baryonic content for their total masses but are deficent in stars. This suggests that high M_HI/L_B galaxies are not lacking the baryons to create stars, but are underluminous as they lack either the internal or external stimulation for more extensive star formation.Comment: 29 Pages, 59 Figures. Accepted for publication in AJ (to be published ~April 2006

Simulating JWST deep extragalactic imaging surveys and physical parameter recovery

International audienceWe present a new prospective analysis of deep multi-band imaging with the James Webb Space Telescope (JWST). In this work, we investigate the recovery of high-redshift 5  6 and redshifts of 0   5 galaxy samples can be reduced to < 0.01 arcmin−2 with a limited impact on galaxy completeness. We investigate multiple high-redshift galaxy selection techniques and find that the best compromise between completeness and purity at 5 <  z <  10 using the full redshift posterior probability distributions. In the EGS field, the galaxy completeness remains higher than 50% at magnitudes mUV <  27.5 and at all redshifts, and the purity is maintained above 80 and 60% at z ≤ 7 and 10, respectively. The faint-end slope of the galaxy UV luminosity function is recovered with a precision of 0.1–0.25, and the cosmic star formation rate density within 0.1 dex. We argue in favor of additional observing programs covering larger areas to better constrain the bright end

Predicting spectral features in galaxy spectra from broad-band photometry

We explore the prospects of predicting emission line features present in galaxy spectra given broad-band photometry alone. There is a general consent that colours, and spectral features, most notably the 4000 A break, can predict many properties of galaxies, including star formation rates and hence they could infer some of the line properties. We argue that these techniques have great prospects in helping us understand line emission in extragalactic objects and might speed up future galaxy redshift surveys if they are to target emission line objects only. We use two independent methods, Artifical Neural Neworks (based on the ANNz code) and Locally Weighted Regression (LWR), to retrieve correlations present in the colour N-dimensional space and to predict the equivalent widths present in the corresponding spectra. We also investigate how well it is possible to separate galaxies with and without lines from broad band photometry only. We find, unsurprisingly, that recombination lines can be well predicted by galaxy colours. However, among collisional lines some can and some cannot be predicted well from galaxy colours alone, without any further redshift information. We also use our techniques to estimate how much information contained in spectral diagnostic diagrams can be recovered from broad-band photometry alone. We find that it is possible to classify AGN and star formation objects relatively well using colours only. We suggest that this technique could be used to considerably improve redshift surveys such as the upcoming FMOS survey and the planned WFMOS survey.Comment: 10 pages 7 figures summitted to MNRA

Origin and evolution of halo bias in linear and non-linear regimes

We present results from a study of bias and its evolution for galaxy-size halos in a large, high-resolution simulation of a LCDM model. We consider the evolution of bias estimated using two-point correlation function (b_xi), power spectrum (b_P), and a direct correlation of smoothed halo and matter overdensity fields (b_d). We present accurate estimates of the evolution of the matter power spectrum probed deep into the stable clustering regime (k~[0.1-200]h/Mpc at z=0). The halo power spectrum evolves much slower than the power spectrum of matter and has a different shape which indicates that the bias is time- and scale-dependent. At z=0, the halo power spectrum is anti-biased with respect to the matter power spectrum at wavenumbers k~[0.15-30]h/Mpc, and provides an excellent match to the power spectrum of the APM galaxies at all probed k. In particular, it nicely matches the inflection observed in the APM power spectrum at k~0.15h/Mpc. We complement the power spectrum analysis with a direct estimate of bias using smoothed halo and matter overdensity fields and show that the evolution observed in the simulation in linear and mildly non-linear regimes can be well described by the analytical model of Mo & White (1996), if the distinction between formation redshift of halos and observation epoch is introduced into the model. We present arguments and evidence that at higher overdensities, the evolution of bias is significantly affected by dynamical friction and tidal stripping operating on the satellite halos in high-density regions of clusters and groups; we attribute the strong anti-bias observed in the halo correlation function and power spectrum to these effects. (Abridged)Comment: submitted to the Astrophys.Journal; 19 pages, 9 figures LaTeX (uses emulateapj.sty

Polar Ring Galaxies and the Tully Fisher relation: implications for the dark halo shape

We have investigated the Tully-Fisher relation for Polar Ring Galaxies (PRGs), based on near infrared, optical and HI data available for a sample of these peculiar objects. The total K-band luminosity, which mainly comes from the central host galaxy, and the measured HI linewidth at 20% of the peak line flux density, which traces the potential in the polar plane, place most polar rings of the sample far from the Tully-Fisher relation defined for spiral galaxies, with many PRGs showing larger HI linewidths than expected for the observed K band luminosity. This result is confirmed by a larger sample of objects, based on B-band data. This observational evidence may be related to the dark halo shape and orientation in these systems, which we study by numerical modeling of PRG formation and dynamics: the larger rotation velocities observed in PRGs can be explained by a flattened polar halo, aligned with the polar ring.Comment: 22 pages, 8 postscript figures, accepted for publication in Astrophysical Journa

Evidence for Evolving Spheroidals in the Hubble Deep Fields North and South

We investigate the dispersion in the internal colours of faint spheroidals in the HDFs North and South. We find that a remarkably large fraction ~30% of the morphologically classified spheroidals with I<24 mag show strong variations in internal colour, which we take as evidence for recent episodes of star-formation. In most cases these colour variations manifest themselves via the presence of blue cores, an effect of opposite sign to that expected from metallicity gradients. Examining similarly-selected ellipticals in five rich clusters with 0.37<z<0.83 we find a significant lower dispersion in their internal colours. This suggests that the colour inhomogeneities have a strong environmental dependence being weakest in dense environments where spheroidal formation was presumably accelerated at early times. We use the trends defined by the cluster sample to define an empirical model based on a high-redshift of formation and estimate that at z~1 about half the field spheroidals must be undergoing recent episodes of star-formation. Using spectral synthesis models, we construct the time dependence of the density of star-formation. Although the samples are currently small, we find evidence for an increase in $\rho_{SFR}$ between z=0 to z=1. We discuss the implications of this rise in the context of that observed in the similar rise in the abundance of galaxies with irregular morphology. Regardless of whether there is a connection our results provide strong evidence for the continued formation of field spheroidals over 0<z<1.Comment: 13 pages, 11 figures. To appear in MNRAS in response to referee's Report. Figures and paper also available at http://www.ast.cam.ac.uk/~fmenante/HDFs

The Velocity Function of Galaxies

We present a galaxy circular velocity function, Psi(log v), derived from existing luminosity functions and luminosity-velocity relations. Such a velocity function is desirable for several reasons. First, it enables an objective comparison of luminosity functions obtained in different bands and for different galaxy morphologies, with a statistical correction for dust extinction. In addition, the velocity function simplifies comparison of observations with predictions from high-resolution cosmological N-body simulations. We derive velocity functions from five different data sets and find rough agreement among them, but about a factor of 2 variation in amplitude. These velocity functions are then compared with N-body simulations of a LCDM model (corrected for baryonic infall) in order to demonstrate both the utility and current limitations of this approach. The number density of dark matter halos and the slope of the velocity function near v_*, the circular velocity corresponding to an ~L_* spiral galaxy, are found to be comparable to that of observed galaxies. The primary sources of uncertainty in construction of Psi(log v) from observations and N-body simulations are discussed and explanations are suggected to account for these discrepancies.Comment: Latex. 28 pages, 4 figures. Accepted by Ap

Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films

Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2\Delta \simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.Comment: 6 two-column pages, 6 figs., 26 Refs., to be published in Superconductor Science and Technolog

Evidence for the Hierarchical Formation of the Galactic Spheroid

The possibility that the Galactic spheroid was assembled from numerous chemically-distinct, proto-Galactic fragments is investigated using a Monte-Carlo technique designed to simulate the chemical evolution of the Galaxy in hierarchical formation scenarios which involve no gas dissipation. By comparing the observed and simulated metallicity distributions of Galactic globular clusters and halo field stars, we estimate the level of fragmentation in the collapsing proto-Galaxy. Although the formation process is highly stochastic, the simulations often show good agreement with the observed metallicity distributions, provided the luminosity function of proto-Galactic fragments had a power-law form with exponent ~ -2. While this steep slope is strongly at odds with the presently observed luminosity function of the Local Group, it is in close agreement with the predictions of semi-analytic and numerical models of hierarchical galaxy formation. We discuss a number of possible explanations for this discrepancy. These simulations suggest that the Galactic halo and its globular cluster system were assembled via the accretion and disruption of approximately 1000 metal-poor, proto-Galactic fragments by the dominant Galactic building block: a proto-bulge whose own metal-rich globular clusters system has been preferentially eroded by dynamical processes. We argue that the same process (ie, hierarchical growth involving little gas dissipation) is responsible for the formation of both giant elliptical galaxies and the bulge-halo components of spiral galaxies. (ABRIDGED).Comment: 20 pages, 9 postscript figures. Accepted for publication in the ApJ, April 10 2000 issu