12 research outputs found
The length of stellar bars in SB galaxies and N-body simulations
We have investigated the accuracy and reliability of six methods used to
determine the length of stellar bars in galaxies or N-body simulations. All
these methods use ellipse fitting and Fourier decomposition of the surface
brightness. We have applied them to N-body simulations that include stars, gas,
star formation, and feedback. Stellar particles were photometrically calibrated
to make B and K-band mock images. Dust absorption is also included. We discuss
the advantages and drawbacks of each method, the effects of projection and
resolution, as well as the uncertainties introduced by the presence of dust.
The use of N-body simulations allows us to compare the location of Ultra
Harmonic Resonance (UHR or 4/1) and corotation (CR) with measured bar lengths.
We show that the minimum of ellipticity located just outside the bulk of the
bar is correlated with the corotation, whereas the location of the UHR can be
approximated using the phase of the fitted ellipses or the phase of the m=2
Fourier development of the surface brightness. We give evidence that the
classification of slow/fast bars, based on the ratio R = Rcr/Rbar could
increase from 1 (fast bar) to 1.4 (slow bar) just by a change of method. We
thus conclude that one has to select the right bar-length estimator depending
on the application, since these various estimators do not define the same
physical area.Comment: Major revision, A&A in pres
Formation of young boxy/peanut bulges in ringed barred galaxies
We investigate whether the formation mechanism of boxy and peanut-shaped
(B/PS) bulges could depend on the gas content of the galaxy. We have performed
N-body simulations with and without a gaseous component. In the second case
star formation/feedback recipes have also been implemented to create new
stellar populations. As in many previous studies, in our N-body collisionless
simulation, the B/PS is due to the classical break in the z mirror symmetry
lasting roughly 200 Myr. When a gaseous component and star formation recipes
are added to the simulation, the bulge-growing mechanism is quite different.
The young stellar population that is born in the thin gaseous disc rapidly
populates vertical resonant orbits triggered by the combined effects of the
linear horizontal and vertical ILRs. This leads to a B/PS bulge mainly made of
stellar material younger than the surrounding population. The non-linear
analysis of the orbital structure shows that the main orbit family responsible
for the B/PS is not the same in the two cases. The 2:2:1 orbits prevail in the
collisionless simulation whereas additional asymmetrical families contribute to
the B/PS if a dissipative component is present and can form new stars. We found
that 2:3:1 and 2:5:1 orbits trap a significant fraction of the mass. A flat
ringed discy stellar component also appears simultaneously with the thickening
of the young population. It is due to the star formation in a nuclear gaseous
disc located in the central kpc, inside the ILR, and accumulated there by the
torques exerted by the large-scale bar. Remarkably, it remains flat throughout
the simulation although it develops a nuclear bar, leading to a double-barred
galaxy. We predict that two populations of B/PS bulges could exist and even
coexist in the same galaxy.Comment: 10 pages, 11 Postscript figures, published in A&
Photometric and dynamic evolution of an isolated disc galaxy simulation
We present a detailed analysis of the evolution of a simulated isolated disc
galaxy. The simulation includes stars, gas, star formation and simple chemical
yields. Stellar particles are split in two populations: the old one is present
at the beginning of the simulation and is calibrated according to various ages
and metallicities; the new population borns in the course of the simulation and
inherits the metallicity of the gas particles. The results have been calibrated
in four wavebands with the spectro-photometric evolutionary model GISSEL2000
(Bruzual & Charlot 1993). Dust extinction has also been taken into account. A
rest-frame morphological and bidimensional photometric analysis has been
performed on simulated images, with the same tools as for observations. The
effects of the stellar bar formation and the linked star formation episode on
the global properties of the galaxy (mass and luminosity distribution, colours,
isophotal radii) have been analysed. In particular, we have disentangled the
effects of stellar evolution from dynamic evolution to explain the cause of the
isophotal radii variations. We show that the dynamic properties (e.g. mass) of
the area enclosed by any isophotal radius depends on the waveband and on the
level of star formation activity. It is also shown that the bar isophotes
remain thinner than mass isodensities a long time (> 0.7 Gyr) after the maximum
of star formation rate. We show that bar ellipticity is very wavelength
dependent as suggested by real observations. Effects of dust extinction on
photometric and morphological measurements are systematically quantified.Comment: 14 pages, 16 figures (13 in eps, 3 in jpg format). Accepted for
publication in A&
The population of barred galaxies in the local universe I. Detection and characterisation of bars
(Abridge) Bars are very common in the centre of the disc galaxies, and they
drive the evolution of their structure. A volume-limited sample of 2106 disc
galaxies extracted from the Sloan Digital Sky Survey Data Release 5 was studied
to derive the bar fraction, length, and strength as a function of the
morphology, size, local galaxy density, light concentration, and colour of the
host galaxy. The bars were detected using the ellipse fitting method and
Fourier analysis method. The ellipse fitting method was found to be more
efficient in detecting bars in spiral galaxies. The fraction of barred galaxies
turned out to be 45%. A bar was found in 29% of the lenticular galaxies, in 55%
and 54% of the early- and late-type spirals, respectively. The bar length
(normalised by the galaxy size) of late-type spirals is shorter than in
early-type or lenticular ones. A correlation between the bar length and galaxy
size was found with longer bars hosted by larger galaxies. The bars of the
lenticular galaxies are weaker than those in spirals. Moreover, the unimodal
distribution of the bar strength found for all the galaxy types argues against
a quick transition between the barred and unbarred statues. There is no
difference between the local galaxy density of barred and unbarred galaxies.
Besides, neither the length nor strength of the bars are correlated with the
local density of the galaxy neighbourhoods. In contrast, a statistical
significant difference between the central light concentration and colour of
barred and unbarred galaxies was found. Bars are mostly located in less
concentrated and bluer galaxies. These results indicate that the properties of
bars are strongly related to those of their host galaxies, but do not depend on
the local environment.Comment: 15 pages, 13 figures. Accepted for publication in A&
The ATLAS project - XII : Recovery of the mass-to-light ratio of simulated early-type barred galaxies with axisymmetric dynamical models
We investigate the accuracy in the recovery of the stellar dynamics of barred galaxies when using axisymmetric dynamical models. We do this by trying to recover the mass-to-light ratio (M/L) and the anisotropy of realistic galaxy simulations using the Jeans Anisotropic Multi-Gaussian Expansion (JAM) modelling method. However, given that the biases we find are mostly due to an application of an axisymmetric modelling algorithm to a non-axisymmetric system and in particular to inaccuracies in the deprojected mass model, our results are relevant for general axisymmetric modelling methods. We run N-body collisionless simulations to build a library with various luminosity distribution, constructed to mimic real individual galaxies, with realistic anisotropy. The final result of our evolved library of simulations contains both barred and unbarred galaxies. The JAM method assumes an axisymmetric mass distribution, and we adopt a spatially constant M/L and anisotropy distributions. The models are fitted to two-dimensional maps of the second velocity moments of the simulations for various viewing angles [position angle (PA) of the bar and inclination of the galaxy]. We find that the inclination is generally well recovered by the JAM models, for both barred and unbarred simulations. For unbarred simulations the M/L is also accurately recovered, with negligible median bias and with a maximum one of just ?(M/L) < 1.5 per cent when the galaxy is not too close to face on. At very low inclinations () the M/L can be significantly overestimated (9 per cent in our tests, but errors can be larger for very face-on views). This is in agreement with previous studies. For barred simulations the M/L is on average (when PA = 45 degrees) essentially unbiased, but we measure an over/underestimation of up to ?(M/L) = 15 per cent in our tests. The sign of the M/L bias depends on the PA of the bar as expected: overestimation occurs when the bar is closer to end-on, due to the increased stellar motion along the line-of-sight, and underestimation otherwise. For unbarred simulations, the JAM models are able to recover the mean value of the anisotropy with bias , within the region constrained by the kinematics. However when a bar is present, or for nearly face-on models, the recovered anisotropy varies wildly, with biases up to ?beta z similar to 0.3.Peer reviewe