82 research outputs found
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
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
Red giant masses and ages derived from carbon and nitrogen abundances
We show that the masses of red giant stars can be well predicted from their photospheric carbon and nitrogen abundances, in conjunction with their spectroscopic stellar labels log g, Teff, and [Fe/H]. This is qualitatively expected from mass-dependent post-main-sequence evolution. We here establish an empirical relation between these quantities by drawing on 1475 red giants with asteroseismic mass estimates from Kepler that also have spectroscopic labels from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR12. We assess the accuracy of our model, and find that it predicts stellar masses with fractional rms errors of about 14 per cent (typically 0.2 M⊙). From these masses, we derive ages with rms errors of 40 per cent. This empirical model allows us for the first time to make age determinations (in the range 1-13 Gyr) for vast numbers of giant stars across the Galaxy. We apply our model to ˜52 000 stars in APOGEE DR12, for which no direct mass and age information was previously available. We find that these estimates highlight the vertical age structure of the Milky Way disc, and that the relation of age with [α/M] and metallicity is broadly consistent with established expectations based on detailed studies of the solar neighbourhood
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