La guerre de Sécession

La guerre de Sécession (1861-1865) fut le conflit le plus sanglant du monde occidental entre les guerres napoléoniennes et la Première Guerre mondiale. Les hostilités furent déclenchées lorsque onze États du Sud entamèrent un processus de sécession des États-Unis pour former les États confédérés d’Amérique, présidés par Jefferson Davis. Le gouvernement fédéral, dirigé par le président Abraham Lincoln, déclara cette sécession illégale et engagea la guerre contre ces États. Après la perte de pr..

Nicholas GUYATT, Providence and the Invention of the United States, 1607-1876

En 1755, le Dictionnaire de langue anglaise de Samuel Johnson définissait avec concision la Providence comme « la responsabilité de Dieu à l’égard des êtres qu’il a créés ; le gouvernement divin ; la prudence, la frugalité et la prévoyance ». Le Dictionnaire américain de la langue anglaise de Noah Webster (1828) élargit non seulement le sens de la notion en offrant quatre définitions, mais il rétrograde également quelque peu Dieu en ne le citant qu’en troisième position dans sa classification..

Nicholas GUYATT, Providence and the Invention of the United States, 1607-1876

En 1755, le Dictionnaire de langue anglaise de Samuel Johnson définissait avec concision la Providence comme « la responsabilité de Dieu à l’égard des êtres qu’il a créés ; le gouvernement divin ; la prudence, la frugalité et la prévoyance ». Le Dictionnaire américain de la langue anglaise de Noah Webster (1828) élargit non seulement le sens de la notion en offrant quatre définitions, mais il rétrograde également quelque peu Dieu en ne le citant qu’en troisième position dans sa classification..

Brightest galaxies as halo centre tracers in SDSS DR7

Determining the positions of halo centres in large-scale structure surveys is crucial for many cosmological studies. A common assumption is that halo centres correspond to the location of their brightest member galaxies. In this paper, we study the dynamics of brightest galaxies with respect to other halo members in the Sloan Digital Sky Survey DR7. Specifically, we look at the line-of-sight velocity and spatial offsets between brightest galaxies and their neighbours. We compare those to detailed mock catalogues, constructed from high-resolution, dark-matter-only $N$-body simulations, in which it is assumed that satellite galaxies trace dark matter subhaloes. This allows us to place constraints on the fraction $f_{\rm BNC}$ of haloes in which the brightest galaxy is not the central. Compared to previous studies we explicitly take into account the unrelaxed state of the host haloes, velocity offsets of halo cores and correlations between $f_{\rm BNC}$ and the satellite occupation. We find that $f_{\rm BNC}$ strongly decreases with the luminosity of the brightest galaxy and increases with the mass of the host halo. Overall, in the halo mass range $10^{13} - 10^{14.5} h^{-1} M_\odot$ we find $f_{\rm BNC} \sim 30\%$, in good agreement with a previous study by Skibba et al. We discuss the implications of these findings for studies inferring the galaxy--halo connection from satellite kinematics, models of the conditional luminosity function and galaxy formation in general.Comment: 24 pages, 15 figures. Accepted for publication in MNRA

How to Optimally Constrain Galaxy Assembly Bias: Supplement Projected Correlation Functions with Count-in-cells Statistics

Most models for the connection between galaxies and their haloes ignore the possibility that galaxy properties may be correlated with halo properties other than mass, a phenomenon known as galaxy assembly bias. Yet, it is known that such correlations can lead to systematic errors in the interpretation of survey data. At present, the degree to which galaxy assembly bias may be present in the real Universe, and the best strategies for constraining it remain uncertain. We study the ability of several observables to constrain galaxy assembly bias from redshift survey data using the decorated halo occupation distribution (dHOD), an empirical model of the galaxy--halo connection that incorporates assembly bias. We cover an expansive set of observables, including the projected two-point correlation function $w_{\mathrm{p}}(r_{\mathrm{p}})$, the galaxy--galaxy lensing signal $\Delta \Sigma(r_{\mathrm{p}})$, the void probability function $\mathrm{VPF}(r)$, the distributions of counts-in-cylinders $P(N_{\mathrm{CIC}})$, and counts-in-annuli $P(N_{\mathrm{CIA}})$, and the distribution of the ratio of counts in cylinders of different sizes $P(N_2/N_5)$. We find that despite the frequent use of the combination $w_{\mathrm{p}}(r_{\mathrm{p}})+\Delta \Sigma(r_{\mathrm{p}})$ in interpreting galaxy data, the count statistics, $P(N_{\mathrm{CIC}})$ and $P(N_{\mathrm{CIA}})$, are generally more efficient in constraining galaxy assembly bias when combined with $w_{\mathrm{p}}(r_{\mathrm{p}})$. Constraints based upon $w_{\mathrm{p}}(r_{\mathrm{p}})$ and $\Delta \Sigma(r_{\mathrm{p}})$ share common degeneracy directions in the parameter space, while combinations of $w_{\mathrm{p}}(r_{\mathrm{p}})$ with the count statistics are more complementary. Therefore, we strongly suggest that count statistics should be used to complement the canonical observables in future studies of the galaxy--halo connection.Comment: Figures 3 and 4 show the main results. Published in Monthly Notices of the Royal Astronomical Societ