Galaxy clustering at z ∼ 2 and halo radii Roukema, B. F.; Valls-Gabaud, D.; Mobasher, B. ...
Monthly notices of the Royal Astronomical Society,
19/May , Volume:
305, Issue:
1
Journal Article
Peer reviewed
Open access
The amplitude of the angular two-point galaxy autocorrelation function w(θ) for galaxies at z ̃ 2 is estimated for galaxies in the Hubble Deep Field by using a U<27 complete subsample. The U-band ...selection ensures little contamination from z>2.5 galaxies, while photometric redshifts minimize the contribution from low redshift galaxies. It is confirmed that the amplitude of the correlation can be corrected for the integral constraint (lack of large-scale variance) without having to make assumptions about the shape of the correlation function and by avoiding the introduction of linear error terms. The estimate using this technique is w(θ≈5 arcsec)=0.10±0.09. Estimators that assume a power law of a given slope and include linear error terms would double this value. If the biases introduced in faint galaxy selection owing to obscuration by large objects are not corrected for by masking areas around them, then the estimate would be w(θ≈5 arcsec)=0.16±0.07. The effective (three-dimensional) galaxy pair separation at 5 arcsec and this redshift range is ≈25–250 h−1 kpc, so the correction to the spatial correlation function ξ(r) owing to exclusion of overlapping galaxy dark matter haloes should be considered. For stable clustering in proper units in an Ω=1,λ=0 universe, our w(5 arcsec) estimate (a) implies a present-day correlation length of r0̃2.6+1.1−1.7h−1 Mpc if halo overlapping is ignored, but (b) for a present-day correlation length of r0=5.5 h−1 Mpc implies that a typical halo exclusion radius is rhalo=70+420−30h−1 kpc. For Ω0=0.1,λ0=0.9, the corresponding values are (a) r0̃5.8+2.4−3.9h−1 Mpc and (b) rhalo<210 h−1 kpc (1σ upper limit). The decreasing correlation period (DCP) of a high initial bias in the spatial correlation function is not detected at this redshift. For an Ω=1,λ=0 universe and stable clustering in proper units, possible detections of the DCP in other work would imply that the values of ξ at redshifts greater than zt=1.7±0.9 would be (1+z)/(1+zt)2.1±3.6 times their values at zt, which is consistent with our lack of detection at z̃2.
During these past decades, our knowledge of evolutionary and structural properties of stars of different mass and chemical composition is significantly improved. This result has been achieved as a ...consequence of our improved capability in understanding and describing the physical behaviour of matter in the different thermal regimes characteristic of the various stellar mass ranges and evolutionary stages. This notwithstanding, the current generation of stellar models is still affected by significant and, usually, not negligible uncertainties. These uncertainties are related to our poor knowledge of some physical proceses occurring in the real stars such as, for instance, some thermodynamical processes, nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks of stellar models have to be properly taken into account when comparing theory with observations in order to derive relevant information about the properties of both resolved and unresolved stellar populations. On the other hand, observations of both field and cluster stars can provide fundamental benchmarks for constraining the reliability and accuracy of the theoretical framework. In the following we review some important evolutionary and structural properties of very-low and low-mass stars, as well as the most important uncertainties affecting the stellar models for such stars. We show what are the main sources of uncertainty along the main evolutionary stages, and discuss the present level of agreement between theory and observations.
Simple Linear Regression Grégoire, G.
EAS Publications Series,
01/2014, Volume:
66
Journal Article
This chapter deals with the very simple situation where the mean of a variable, the response variable, usually denoted Y, is linearly depending on another variable, the regressor, here denoted x1. ...The least squared method is used to get the parameter estimators and estimates of their precisions. This leads to design confidence and prediction intervals, significance tests, anova table. Residuals, diagnostics to identify influent observations and outliers are presented. Methods to detect departures from the model's assumptions and ways of dealing with these departures are addressed. Along the chapter a data set is used to illustrate the methods with the sofware R.
Present studies on the evolution of the Milky Way are driven and shaped by how we conceive its stellar populations, in an on going process started by W. Baade seventy years ago. Despite much progress ...and advances in our understanding of these populations, inspection of their main properties is however hardly indicative of the path the Milky Way has followed to build up its mass. This is not only a matter of (stellar) age measurement, but more so the consequence of how we interprete the structures that we see in our Galaxy, often through the filter of our definitions of stellar populations. The panorama presented in the following pages opens the possibility that the present “filter” is not fully adequate. I start these Lectures with a summary of the main properties of the disks, bulge, and halo, and then present some of the new directions in the interpretation of the structure and evolution of the disk(s), with emphasis on chemical evolution. I discuss recent results in our understanding of the bulge, its stellar components and chemical evolution. Finally, I present the ideas currently proposed to explain the formation of the Galactic stellar halo. I conclude by examining how deeply all these new results question our present definition of stellar populations.
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