Damn You, Little h! (or, Real-World Applications Of The Hubble Constant Using Observed And Simulated Data)

The Hubble constant, H0, or its dimensionless equivalent, "little h", is a fundamental cosmological property that is now known to an accuracy better than a few percent. Despite its cosmological nature, little h commonly appears in the measured properties of individual galaxies. This can pose unique challenges for users of such data, particularly with survey data. In this paper we show how little h arises in the measurement of galaxies, how to compare like-properties from different datasets that have assumed different little h cosmologies, and how to fairly compare theoretical data with observed data, where little h can manifest in vastly different ways. This last point is particularly important when observations are used to calibrate galaxy formation models, as calibrating with the wrong (or no) little h can lead to disastrous results when the model is later converted to the correct h cosmology. We argue that in this modern age little h is an anachronism, being one of least uncertain parameters in astrophysics, and we propose that observers and theorists instead treat this uncertainty like any other. We conclude with a "cheat sheet" of nine points that should be followed when dealing with little h in data analysis.Comment: A guide to dealing with little h in data analysis (theory and observation), targeted at students, in the spirit of Hogg's "Distance Measures in Cosmology" (arXiv:astro-ph/9905116). 10 pages, 3 figures, 2 tables. Accepted for publication in PAS

Halo assembly bias and its effects on galaxy clustering

The clustering of dark halos depends not only on their mass but also on their assembly history, a dependence we term `assembly bias'. Using a galaxy formation model grafted onto the Millennium Simulation of the LCDM cosmogony, we study how assembly bias affects galaxy clustering. We compare the original simulation to `shuffled' versions where the galaxy populations are randomly swapped among halos of similar mass, thus isolating the effects of correlations between assembly history and environment at fixed mass. Such correlations are ignored in the halo occupation distribution models often used populate dark matter simulations with galaxies, but they are significant in our more realistic simulation. Assembly bias enhances 2-point correlations by 10% for galaxies with M_bJ-5logh brighter than -17, but suppresses them by a similar amount for galaxies brighter than -20. When such samples are split by colour, assembly bias is 5% stronger for red galaxies and 5% weaker for blue ones. Halo central galaxies are differently affected by assembly bias than are galaxies of all types. It almost doubles the correlation amplitude for faint red central galaxies. Shuffling galaxies among halos of fixed formation redshift or concentration in addition to fixed mass produces biases which are not much smaller than when mass alone is fixed. Assembly bias must reflect a correlation of environment with aspects of halo assembly which are not encoded in either of these parameters. It induces effects which could compromise precision measurements of cosmological parameters from large galaxy surveys.Comment: 8 pages, 4 figures, accepted for publication in MNRA

Galaxy Formation and Evolution

We take a multi-faceted approach to study galaxy populations in the local universe, using the completed Two Degree Field Galaxy Redshift Survey (2dFGRS), the ``Millennium Run'' LCDM N-body simulation, and a semi-analytic model of galaxy formation. Our investigation covers both small and large scale aspects of the galaxy distribution. This work can be broken into three sections, outlined below. Using the 2dFGRS we explore the higher-order clustering properties of local galaxies to quantify both (i) the linear and non-linear bias of the distribution relative to the underlying matter field, and (ii) the nature of hierarchical scaling in the clustering moments of the galaxy distribution. This last point is the expected signature of an initially Gaussian distribution of matter density fluctuations that evolved under the action of gravitational instability. We show in Chapters 2, 3, and 4 that the 2dFGRS higher-order clustering moments are indeed hierarchical, which we measure up to sixth order for galaxies brighter than M_bJ-5log10 h=-17 and which sample the survey volume out to z~0.3. The moments are found to be well described by the negative binomial probability distribution function, and we rule out, at high significance, other models of galaxy clustering, such as the lognormal distribution. This result holds in redshift space on all scales where we obtain a good statistical signal, typically 0.5< R (h^-1 Mpc) < 30 (i.e. from strongly non-linear to quasi-linear regimes). Interestingly, we find that the moments on larger scales can be significantly altered by two massive superclusters present in the 2dFGRS. The skewness of the galaxy distribution is found to have a weak dependence on galaxy luminosity. We show that a simple linear biasing model provides an inadequate description of the higher order results, suggesting that non-linear biasing is present in the clustering moments of the 2dFGRS. The large-scale distribution of structure within the 2dFGRS allows us to study the properties of the galaxy population as a function of local environment. In Chapter 5 we measure the luminosity function of early and late-types galaxies in survey regions ranging from sparse voids to dense clusters to reveal the dominant population in each. Fitting each luminosity function with a Schechter function allows us to quantify how the bright and faint populations transform with changing density contrast. We find that (i) the population in voids is dominated by late types, with a noticeable deficit of intermediate and bright galaxies relative to the mean, and (ii) cluster regions have an excess of very bright early-type galaxies relative to the mean. When directly comparing faint early and late type galaxies in void and cluster regions, the cluster population shows comparable abundances of both types, whereas in voids the late types dominate by almost an order of magnitude. Of interest to many galaxy formation models is our measurement that reveals that the faint-end slope of the overall luminosity function depends at most weakly on density environment. Finally, in Chapter 6, we develop a self-consistent model of galaxy formation and couple this to the Millennium Run LCDM N-body simulation. This simulation represents a significant step forward in both size and resolution, allowing us to follow the the complete evolutionary histories of approximately 20 million galaxies down to luminosities as faint as the Small Magellanic Cloud in a volume comparable to that sampled by the 2dFGRS. In our galaxy formation model we supplement previous treatments of the growth and activity of central black holes with a new model for `radio' feedback from those active galactic nuclei that lie at the centre of a quasistatic X-ray emitting atmosphere in a galaxy group or cluster. With this we can simultaneously explain (i) the low observed mass drop-out rate in cooling flows, (ii) the exponential cut-off at the bright end of the galaxy luminosity function, and (iii) the fact that the most massive galaxies tend to be bulge-dominated systems in clusters and contain systematically older stars than lower mass galaxies. This success occurs because static hot atmospheres form only in the most massive structures, and radio feedback (in contrast, for example, to supernova or starburst feedback) can suppress further cooling and thus star formation without itself requiring star formation. Matching galaxy formation models with such observations has previously proved quite challenging

A diversity of progenitors and histories for isolated spiral galaxies

We analyze a suite of 33 cosmological simulations of the evolution of Milky Way-mass galaxies in low-density environments. Our sample spans a broad range of Hubble types at z=0, from nearly bulgeless disks to bulge-dominated galaxies. Despite the fact that a large fraction of the bulge is typically in place by z=1, we find no significant correlation between the morphology at z=1 and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies, including smooth disks, unstable disks, interacting galaxies and bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place and the progenitor morphology is correlated with the final morphology. We next focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These show a correlation between B/T at z=0 and the mass ratio of the largest merger at z1. We find that the galaxies with the lowest B/T tend to have a quiet baryon input history, with no major mergers at z<2, and with a low and constant gas accretion rate that keeps a stable angular-momentum direction. More violent merger or gas accretion histories lead to galaxies with more prominent bulges. Most disk galaxies have a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend to have histories of intense gas accretion and disk instability rather than active mergers.Comment: Accepted for publication in ApJ. 29 pages, 32 figure