11 research outputs found
The degeneracy between star-formation parameters in dwarf galaxy simulations and the Mstar-Mhalo relation
We present results based on a set of N-Body/SPH simulations of isolated dwarf
galaxies. The simulations take into account star formation, stellar feedback,
radiative cooling and metal enrichment. The dark matter halo initially has a
cusped profile, but, at least in these simulations, starting from idealised,
spherically symmetric initial conditions, a natural conversion to a core is
observed due to gas dynamics and stellar feedback.
A degeneracy between the efficiency with which the interstellar medium
absorbs energy feedback from supernovae and stellar winds on the one hand, and
the density threshold for star formation on the other, is found. We performed a
parameter survey to determine, with the aid of the observed kinematic and
photometric scaling relations, which combinations of these two parameters
produce simulated galaxies that are in agreement with the observations.
With the implemented physics we are unable to reproduce the relation between
the stellar mass and the halo mass as determined by Guo et al. (2010), however
we do reproduce the slope of this relation.Comment: Accepted for publication in MNRAS | 12 pages, 8 figure
Simulations of the formation and evolution of isolated dwarf galaxies - II. Angular momentum as a second parameter
We show results based on a large suite of N-Body/SPH simulations of isolated,
flat dwarf galaxies, both rotating and non-rotating. The main goal is to
investigate possible mechanisms to explain the observed dichotomy in radial
stellar metallicity profiles of dwarf galaxies: dwarf irregulars (dIrr) and
flat, rotating dwarf ellipticals (dE) generally possess flat metallicity
profiles, while rounder and non-rotating dEs show strong negative metallicity
gradients. These simulations show that flattening by rotation is key to
reproducing the observed characteristics of flat dwarf galaxies, proving
particularly efficient in erasing metallicity gradients. We propose a
"centrifugal barrier mechanism" as an alternative to the previously suggested
"fountain mechanism" for explaining the flat metallicity profiles of dIrrs and
flat, rotating dEs. While only flattening the dark-matter halo has little
influence, the addition of angular momentum slows down the infall of gas, so
that star formation (SF) and the ensuing feedback are less centrally
concentrated, occurring galaxy-wide. Additionally, this leads to more
continuous SFHs by preventing large-scale oscillations in the SFR
("breathing"), and creates low density holes in the ISM, in agreement with
observations of dIrrs. Our general conclusion is that rotation has a
significant influence on the evolution and appearance of dwarf galaxies, and we
suggest angular momentum as a second parameter (after galaxy mass as the
dominant parameter) in dwarf galaxy evolution. Angular momentum differentiates
between SF modes, making our fast rotating models qualitatively resemble dIrrs,
which does not seem possible without rotation.Comment: Accepted for publication in MNRAS | 19 pages, 20 figures | extra
online content available (animations) : on the publisher's website / on the
YouTube channel for the astronomy department of the University of Ghent :
http://www.youtube.com/user/AstroUGent / YouTube playlist specifically for
this article :
http://www.youtube.com/user/AstroUGent#grid/user/EFAA5AAE5C5E474
Stellar orbits and the survival of metallicity gradients in simulated dwarf galaxies
We present a detailed analysis of the formation, evolution, and possible
longevity of metallicity gradients in simulated dwarf galaxies. Specifically,
we investigate the role of potentially orbit-changing processes such as radial
stellar migration and dynamical heating in shaping or destroying these
gradients. We also consider the influence of the star formation density
threshold, investigating both a low and high value (0.1 amu/ - 100
amu/).
The Nbody-SPH models that we use to self-consistently form and evolve dwarf
galaxies in isolation show that, in the absence of significant angular
momentum, metallicity gradients are gradually built up during the evolution of
the dwarf galaxy, by ever more centrally concentrated star formation adding to
the overall gradient. Once formed, they are robust, survive easily in the
absence of external disturbances and hardly decline over several Gyr, and they
agree well with observed metallicity gradients of dwarf galaxies in the Local
Group. The underlying orbital displacement of stars is quite limited in our
models, being of the order of only fractions of the half light radius over
time-spans of 5 to 10 Gyr in all star formation schemes. This is contrary to
the strong radial migration found in massive disc galaxies, caused by
scattering of stars off the corotation resonance of spiral structures. In the
dwarf regime the stellar body only seems to undergo mild dynamical heating, due
to the lack of long-lived spiral structures and/or discs.
The density threshold has profound influences on the star formation mode of
the models, but only a minor influence on the evolution of metallicity
gradients. Increasing the threshold 1000-fold causes comparatively stronger
dynamical heating of the stellar body due to the increased turbulence and the
scattering of stars off dense gas clouds, but the effect remains very limited
in absolute terms.Comment: Accepted for publication in MNRAS | 20 pages, 19 figures | animations
available on the YouTube playlist for this article :
http://www.youtube.com/playlist?list=PL-DZsb1G8F_lDNn3G-9ACGgrinen8aSQs
(YouTube channel of the department of astronomy at Ghent University:
http://www.youtube.com/user/AstroUGent