The chemodynamical evolution of the Milky Way disc -- A new modeling approach

Despite the recent advancements in the field of galaxy formation and evolution, fully self-consistent simulations are still unable to make the detailed predictions necessary for the planned and ongoing large spectroscopic and photometry surveys of the Milky Way disc. These difficulties arise from the very uncertain nature of sub-grid physical energy feedback within models, affecting both star formation rates and chemical enrichment. To avoid these problems, we have introduced a new approach which consists of fusing disc chemical evolution models with compatible numerical simulations. We demonstrate the power of this method by showing that a range of observational results can be explained by our new model. We show that due to radial migration from mergers at high redshift and the central bar at later times, a sizable fraction of old metal-poor, high-[alpha/Fe] stars can reach the solar vicinity. This naturally accounts for a number of recent observations related to both the thin and thick discs, despite the fact that we use thin-disc chemistry only. Within the framework of our model, the MW thick disc has emerged naturally from (i) stars born with high velocity dispersions at high redshift, (ii) stars migrating from the inner disc very early on due to strong merger activity, and (iii) further radial migration driven by the bar and spirals at later times. A significant fraction of old stars with thick-disc characteristics could have been born near the solar radius.Comment: Invited review at IAUS 298, Setting the scene for Gaia and LAMOST - the current and next generations of surveys and models, held in Lijiang, China, May 17-21, 2013. Will appear in IAU Symposium, vol 298, S. Feltzing, G. Zhao, N. Walton and P. Whitelock, ed

Numerical simulations of galaxy evolution in cosmological context

Large volume cosmological simulations succeed in reproducing the large-scale structure of the Universe. However, they lack resolution and may not take into account all relevant physical processes to test if the detail properties of galaxies can be explained by the CDM paradigm. On the other hand, galaxy-scale simulations could resolve this in a robust way but do not usually include a realistic cosmological context. To study galaxy evolution in cosmological context, we use a new method that consists in coupling cosmological simulations and galactic scale simulations. For this, we record merger and gas accretion histories from cosmological simulations and re-simulate at very high resolution the evolution of baryons and dark matter within the virial radius of a target galaxy. This allows us for example to better take into account gas evolution and associated star formation, to finely study the internal evolution of galaxies and their disks in a realistic cosmological context. We aim at obtaining a statistical view on galaxy evolution from z = 2 to 0, and we present here the first results of the study: we mainly stress the importance of taking into account gas accretion along filaments to understand galaxy evolution.Comment: 6 pages - Proceedings of IAU Symposium 254 "The Galaxy disk in cosmological context", Copenhagen, June 2008 - Movies available at http://aramis.obspm.fr/~bournaud/stargas35small.avi and http://aramis.obspm.fr/~bournaud/stargasZ35_small.av

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

CONGRESS AND THE APPELLATE JURISDICTION OF THE SUPREME COURT

A democratic government such as ours, based upon the theory of popular sovereignty, presents many curious political phenomena. For example: in order to insure a proper balance of the powers, it has been necessary for the Supreme Court to assume the onerous task of passing upon the constitutionality of congressional legislation. It is unfortunate, but necessary, that the Court be obliged to exercise this power of judicial review at a time when the entire country is suffering from the effects of a severe and sustained economic depression. It is unfortunate, too, that the legislation under judicial examination should involve questions of great economic, social, and political consequence. How much simpler it would be for the personnel of the Court if they could shirk this duty, and by their evasion avoid the censure and criticism which seems of necessity to follow any decision involving momentous issues. That the Court has had the courage to see its duty and to discharge it, is a factor which has contributed in no small measure to the success of our experiment in government by the people and it was an experiment at the time our Federal Constitution was drafted

The two-phase formation history of spiral galaxies traced by the cosmic evolution of the bar fraction

We study the evolution of galactic bars and the link with disk and spheroid formation in a sample of zoom-in cosmological simulations. Our simulation sample focuses on galaxies with present-day stellar masses in the 10^10-10^11 Msun range, in field and loose group environments, with a broad variety of mass growth histories. In our models, bars are almost absent from the progenitors of present-day spirals at z>1.5, and they remain rare and generally too weak to be observable down to z~1. After this characteristic epoch, the fractions of observable and strong bars raise rapidly, bars being present in 80% of spiral galaxies and easily observable in two thirds of these at z<0.5. This is quantitatively consistent with the redshift evolution of the observed bar fraction. Our models predict that the decrease in the bar fraction with increasing redshift should continue with a fraction of observable bars <10-15% in disk galaxies at z>1. Our models also predict later bar formation in lower-mass galaxies, in agreement with existing data. We find that the characteristic epoch of bar formation, namely redshift z~0.8-1, corresponds to the epoch at which today's spirals acquire their disk-dominated morphology. At higher redshift, disks tend to be rapidly destroyed by mergers and gravitational instabilities and rarely develop significant bars. The bar formation epoch corresponds to the transition between an early "violent" phase of spiral galaxy formation at z>1 and a late "secular" phase at z<0.8. In the secular phase, the presence of bars substantially contributes to the growth of the bulge, but the bulge mass budget remains statistically dominated by the contribution of mergers, interactions and disk instabilities at high redshift. Early bars at z>1 are often short-lived, while most of the bars formed at z<1 persist down to z=0, late cosmological gas infall being necessary to maintain some of them.Comment: 15 pages, 15 figures, ApJ accepte

BAYES-LOSVD: a bayesian framework for non-parametric extraction of the line-of-sight velocity distribution of galaxies

We introduce BAYES-LOSVD, a novel implementation of the non-parametric extraction of line-of-sight velocity distributions (LOSVDs) in galaxies. We employ bayesian inference to obtain robust LOSVDs and associated uncertainties. Our method relies on principal component analysis to reduce the dimensionality of the base of templates required for the extraction and thus increase the performance of the code. In addition, we implement several options to regularise the output solutions. Our tests, conducted on mock spectra, confirm the ability of our approach to model a wide range of LOSVD shapes, overcoming limitations of the most widely used parametric methods (e.g. Gauss-Hermite expansion). We present examples of LOSVD extractions for real galaxies with known peculiar LOSVD shapes, i.e. NGC4371, IC0719 and NGC4550, using MUSE and SAURON integral-field unit (IFU) data. Our implementation can also handle data from other popular IFU surveys (e.g. ATLAS3D, CALIFA, MaNGA, SAMI). Details of the code and relevant documentation are freely available to the community in the dedicated repositories.Comment: 13 pages, 7 figures. Accepted for publication in Astronomy & Astrophysics. Public repository with the code can be found at: https://github.com/jfalconbarroso/BAYES-LOSV