409 research outputs found
A close look at secular evolution: Boxy/peanut bulges reduce gas inflow to the central kiloparsec
In this letter we investigate the effect of boxy/peanut (b/p) bulges on
bar-induced gas inflow to the central kiloparsec, which plays a crucial role on
the evolution of disc galaxies. We carry out hydrodynamic gas response
simulations in realistic barred galaxy potentials, including or not the
geometry of a b/p bulge, to investigate the amount of gas inflow induced in the
different models. We find that b/p bulges can reduce the gas inflow rate to the
central kiloparsec by more than an order of magnitude, which leads to a
reduction in the amount of gas available in the central regions. We also
investigate the effect of the dark matter halo concentration on these results,
and find that for maximal discs, the effect of b/p bulges on gas inflow remains
significant. The reduced amount of gas reaching the central regions due to the
presence of b/p bulges could have significant repercussions on the formation of
discy- (pseudo-) bulges, on the amount of nuclear star formation and feedback,
on the fuel reservoir for AGN activity, and on the overall secular evolution of
galaxies.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Letters. 5 pages, 6 figure
The Effects of Boxy/Peanut Bulges on Galaxy Models
We examine the effects that the modelling of a Boxy/Peanut (B/P) bulge will
have on the estimates of the stellar gravitational potential, forces, orbital
structure and bar strength of barred galaxies. We present a method for
obtaining the potential of disc galaxies from surface density images, assuming
a vertical density distribution (height function), which is let to vary with
position, thus enabling it to represent the geometry of a B/P. We construct a
B/P height function after the results from a high-resolution, N-body+SPH
simulation of an isolated galaxy and compare the resulting dynamical model to
those obtained with the commonly used, position-independent "flat" height
functions. We show that methods that do not allow for a B/P can induce errors
in the forces in the bar region of up to 40% and demonstrate that this has a
significant impact on the orbital structure of the model, which in turn
determines its kinematics and morphology. Furthermore, we show that the bar
strength is reduced in the presence of a B/P. We conclude that neglecting the
vertical extent of a B/P can introduce considerable errors in the dynamical
modelling. We also examine the errors introduced in the model due to
uncertainties in the parameters of the B/P and show that even for generous but
realistic values of the uncertainties, the error will be noticeably less than
that of not modelling a B/P bulge at all.Comment: Accepted for publication in MNRA
Characterizing Bars at z~0 in the optical and NIR: Implications for the Evolution of Barred Disks with Redshift
Critical insights on galaxy evolution stem from the study of bars. With the
advent of HST surveys that trace bars in the rest-frame optical out to z~1, it
is critical to provide a reference baseline for bars at z~0 in the optical
band. We present results on bars at z~0 in the optical and NIR bands based on
180 spirals from OSUBSGS. (1) The deprojected bar fraction at z~0 is ~60% +/-6%
in the NIR H-band and ~44% +/-6% in the optical B-band. (2) The results before
and after deprojection are similar, which is encouraging for high-redshift
studies that forego deprojection. (3) Studies of bars at z~0.2-1.0 (lookback
time of 3-8 Gyr) have reported an optical bar fraction of ~30% +/-6%, after
applying cutoffs in absolute magnitude (M_V = 1.5
kpc), and bar ellipticity (e_bar >= 0.4). Applying these exact cutoffs to the
OSUBSGS data yields a comparable optical B-band bar fraction at z~0 of ~
34%+/-6%. This rules out scenarios where the optical bar fraction in bright
disks declines strongly with redshift. (4) Most (~70%) bars have moderate to
high strentgh or ellipticity (0.50 <= e_bar <= 0.75). There is no bimodality in
the distribution of e_bar. The H-band bar fraction and e_bar show no
substantial variation across RC3 Hubble types Sa to Scd. (5) RC3 bar types
should be used with caution. Many galaxies with RC3 types "AB" turn out to be
unbarred and RC3 bar classes "B" and "AB" have a significant overlap in e_bar.
(6) Most bars have sizes below 5 kpc. Bar and disk sizes correlate, and most
bars have a_bar/R_25~0.1-0.5. This suggests that the growths of bars and disks
are intimately tied.Comment: 11 pages, 17 figures, 3 tables, ApJ accepted, abridged abstract
below. Minor changes and shortened paper for ApJ limits. For high resolution
figures see http://www.as.utexas.edu/~marinova/paper1-highres.pd
Boxy/peanut/X bulges, barlenses and the thick part of galactic bars: What are they and how did they form?
Bars have a complex three-dimensional shape. In particular their inner part
is vertically much thicker than the parts further out. Viewed edge-on, the
thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge
and viewed face-on it is referred to as a barlens. These components are due to
disc and bar instabilities and are composed of disc material. I review here
their formation, evolution and dynamics, using simulations, orbital structure
theory and comparisons to observations.Comment: 21 pages, 7 figures, invited review to appear in "Galactic Bulges",
E. Laurikainen, R. Peletier, D. Gadotti, (eds.), Springe
Halo properties and secular evolution in barred galaxies
The halo plays a crucial role in the evolution of barred galaxies. Its
near-resonant material absorbs angular momentum emitted from some of the disc
particles and helps the bar become stronger. As a result, a bar (oval) forms in
the inner parts of the halo of strongly barred disc galaxies. It is thinner in
the inner parts (but still considerably fatter than the disc bar) and tends to
spherical at larger radii. Its length increases with time, while always staying
shorter than the disc bar. It is roughly aligned with the disc bar, which it
trails only slightly, and it turns with roughly the same pattern speed. The
bi-symmetric component of the halo density continues well outside the halo bar,
where it clearly trails behind the disc bar. The length and strength of the
disc and halo bars correlate; the former being always much stronger than the
latter. If the halo is composed of weakly interacting massive particles, then
the formation of the halo bar, by redistributing the matter in the halo and
changing its shape, could influence the expected annihilation signal. This is
indeed found to be the case if the halo has a core, but not if it has a steep
cusp. The formation and evolution of the bar strongly affect the halo orbits. A
fraction of them becomes near-resonant, similar to the disc near-resonant
orbits at the same resonance, while another fraction becomes chaotic. Finally,
a massive and responsive halo makes it harder for a central mass concentration
to destroy the disc bar.Comment: 6 pages, 3 figures, to appear in "Island Universes - Structure and
Evolution of Disk Galaxies" ed. R. S. de Jon
Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach
We develop diagnostics to detect the presence and orientation of a bar in an
edge-on disk, using its kinematical signature in the position-velocity diagram
(PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral
galaxy mass model, we briefly review the orbital properties of two-dimensional
non-axisymmetric disks and identify the main families of periodic orbits. We
use those families as building blocks to model real galaxies and calculate the
PVDs obtained for various realistic combinations of periodic orbit families and
for a number of viewing angles with respect to the bar. We show that the global
structure of the PVD is a reliable bar diagnostic in edge-on disks.
Specifically, the presence of a gap between the signatures of the families of
periodic orbits in the PVD follows directly from the non-homogeneous
distribution of the orbits in a barred galaxy. Similarly, material in the two
so-called forbidden quadrants of the PVD results from the elongated shape of
the orbits. We show how the shape of the signatures of the dominant x1 and x2
families of periodic orbits in the PVD can be used efficiently to determine the
viewing angle with respect to the bar and, to a lesser extent, to constrain the
mass distribution of an observed galaxy. We also address the limitations of the
models when interpreting observational data.Comment: 22 pages, 9 figures (AASTeX, aaspp4.sty). Accepted for publication in
The Astrophysical Journa
Bar Diagnostics in Edge-On Spiral Galaxies. II. Hydrodynamical Simulations
We develop diagnostics based on gas kinematics to identify the presence of a
bar in an edge-on spiral galaxy and determine its orientation. We use
position-velocity diagrams (PVDs) obtained by projecting edge-on
two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy
potential. We show that when a nuclear spiral is formed, the presence of a gap
in the PVDs, between the signature of the nuclear spiral and that of the outer
parts of the disk, reliably indicates the presence of a bar. This gap is due to
the presence of shocks and inflows in the simulations, leading to a depletion
of the gas in the outer bar region. If no nuclear spiral signature is present
in a PVD, only indirect arguments can be used to argue for the presence of a
bar. The shape of the signature of the nuclear spiral, and to a lesser extent
that of the outer bar region, allows to determine the orientation of the bar
with respect to the line-of-sight. The presence of dust can also help to
discriminate between viewing angles on either side of the bar. Simulations
covering a large fraction of parameter space constrain the bar properties and
mass distribution of observed galaxies. The strongest constraint comes from the
presence or absence of the signature of a nuclear spiral in the PVD.Comment: 25 pages (AASTeX, aaspp4.sty), 11 jpg figures. Accepted for
publication in The Astrophysical Journal. Online manuscript with PostScript
figures available at: http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Bar Diagnostics in Edge-On Spiral Galaxies. III. N-Body Simulations of Disks
Present in over 45% of local spirals, boxy and peanut-shaped bulges are
generally interpreted as edge-on bars and may represent a key phase in the
evolution of bulges. Aiming to test such claims, the kinematic properties of
self-consistent 3D N-body simulations of bar-unstable disks are studied. Using
Gauss-Hermite polynomials to describe the stellar kinematics, a number of
characteristic bar signatures are identified in edge-on disks: 1) a major-axis
light profile with a quasi-exponential central peak and a plateau at moderate
radii (Freeman Type II profile); 2) a ``double-hump'' rotation curve; 3) a
sometime flat central velocity dispersion peak with a plateau at moderate radii
and occasional local central minimum and secondary peak; 4) an h3-V correlation
over the projected bar length. All those kinematic features are spatially
correlated and can easily be understood from the orbital structure of barred
disks. They thus provide a reliable and easy-to-use tool to identify edge-on
bars. Interestingly, they are all produced without dissipation and are
increasingly realized to be common in spirals, lending support to bar-driven
evolution scenarios for bulge formation. So called ``figure-of-eight''
position-velocity diagrams are never observed, as expected for realistic
orbital configurations. Although not uniquely related to triaxiality,
line-of-sight velocity distributions with a high velocity tail (i.e. an h3-V
correlation) appear as particularly promising tracers of bars. The stellar
kinematic features identified grow in strength as the bar evolves and vary
little for small inclination variations. Many can be used to trace the bar
length. Comparisons with observations are encouraging and support the view that
boxy and peanut-shaped bulges are simply thick bars viewed edge-on.Comment: 32 pages, 4 figures, AASTeX preprint. Revised following referees'
comments. Now accepted for publication in The Astrophysical Journal. We
strongly suggest you download the version with full resolution figures at
http://www.astro.columbia.edu/~bureau/Publications/Nbody_ApJ04.ps.g
Environmental regulation of cloud and star formation in galactic bars
The strong time-dependence of the dynamics of galactic bars yields a complex
and rapidly evolving distribution of dense gas and star forming regions.
Although bars mainly host regions void of any star formation activity, their
extremities can gather the physical conditions for the formation of molecular
complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical
simulation of a Milky Way-like galaxy, we probe these conditions to explore how
and where bar (hydro-)dynamics favours the formation or destruction of
molecular clouds and stars. The interplay between the kpc-scale dynamics (gas
flows, shear) and the parsec-scale (turbulence) is key to this problem. We find
a strong dichotomy between the leading and trailing sides of the bar, in term
of cloud fragmentation and in the age distribution of the young stars. After
orbiting along the bar edge, these young structures slow down at the
extremities of the bar, where orbital crowding increases the probability of
cloud-cloud collision. We find that such events increase the Mach number of the
cloud, leading to an enhanced star formation efficiency and finally the
formation of massive stellar associations, in a fashion similar to
galaxy-galaxy interactions. We highlight the role of bar dynamics in decoupling
young stars from the clouds in which they form, and discuss the implications on
the injection of feedback into the interstellar medium, in particular in the
context of galaxy formation.Comment: MNRAS accepte
- …