840 research outputs found
Comparison of bar strengths in optical and near-infrared for the OSUBSGS sample
We use a gravitational bar torque method to compare bar strengths (the
maximum tangential force normalized by radial force) in B and H-band images of
152 galaxies from the Ohio State University Bright Spiral Galaxy Survey. Our
main motivation is to check how much the difference in the rest-frame
wavelength could affect comparisons of bar strengths in low and high redshift
observations. Between these two bands we find an average bar strength ratio
Q_B/H= 1.25 which factor is nearly independent of the morphological type. We
show that Q_B/H > 1 is mostly due to reduced bulge dilution of radial forces in
the B-band. The bar torque method needs an estimate for the vertical scale
height of the galaxy, based on the radial scale length of the disk and the
galaxy's morphological type. Since these two might not always be possible to
determine at high redshifts in a reliable manner, we also checked that similar
results are obtained with vertical scale heights estimated from the radii
corresponding to the K-band surface brightness of 20 mag/arcsec^2. Also, we
made a simple test of the usability of the bar torque method at high redshifts
by checking the effects of image degradation (nearest neighbour sampling
without any adjustment of noise levels): we found that the estimated bar
strengths varied by +/- 10% at most as long as the total extent of the bar was
at least 10 pixels. Overall, we show that the gravitational bar torque method
should provide a proficient tool for quantifying bar strengths also at high
redshifts.Comment: 15 pages, 9 figures, 1 table, accepted to MNRA
On the nature of the barlens component in barred galaxies: what do boxy/peanut bulges look like when viewed face-on?
Barred galaxies have interesting morphological features whose presence and
properties set constraints on galactic evolution. Here we examine barlenses,
i.e. lens-like components whose extent along the bar major axis is shorter than
that of the bar and whose outline is oval or circular. We identify and analyse
barlenses in -body plus SPH simulations, compare them extensively with those
from the NIRS0S (Near-IR S0 galaxy survey) and the SG samples (Spitzer
Survey of Stellar Structure in Galaxies) and find very good agreement. We
observe barlenses in our simulations from different viewing angles. This
reveals that barlenses are the vertically thick part of the bar seen face-on,
i.e. a barlens seen edge-on is a boxy/peanut/X bulge. In morphological studies,
and in the absence of kinematics or photometry, a barlens, or part of it, may
be mistaken for a classical bulge. Thus the true importance of classical
bulges, both in numbers and mass, is smaller than currently assumed, which has
implications for galaxy formation studies. Finally, using the shape of the
isodensity curves, we propose a rule of thumb for measuring the barlens extent
along the bar major axis of moderately inclined galaxies, thus providing an
estimate of which part of the bar is thicker.Comment: 21 pages, 11 figures, revised version as published in MNRA
Decreased Frequency of Strong Bars in S0 Galaxies: Evidence for Secular Evolution?
Using data from the Near-Infrared S0 Survey (NIRS0S) of nearby, early-type
galaxies, we examine the distribution of bar strengths in S0 galaxies as
compared to S0/a and Sa galaxies, and as compared to previously published bar
strength data for Ohio State University Bright Spiral Galaxy Survey (OSUBSGS)
spiral galaxies. Bar strengths based on the gravitational torque method are
derived from 2.2 micron Ks-band images for a statistical sample of 138 (98 S0,
40 S0/a,Sa) galaxies having a mean total blue magnitude <= 12.5 and
generally inclined less than 65 degrees. We find that S0 galaxies have weaker
bars on average than spiral galaxies in general, even compared to their closest
spiral counterparts, S0/a and Sa galaxies. The differences are significant and
cannot be due entirely to uncertainties in the assumed vertical scale-heights
or in the assumption of constant mass-to-light ratios. Part of the difference
is likely due simply to the dilution of the bar torques by the higher mass
bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by
Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of
gas accretion according to this theory. If S0s are stripped former spirals, or
else are evolved from former spirals due to internal secular dynamical
processes which deplete the gas as well as grow the bulges, then the weaker
bars and the prevalence of lenses in S0 galaxies could further indicate that
bar evolution continues to proceed during and even after gas depletionComment: Accepted for publication in the Astrophysical Journal, September 2010
issue (LaTex, 29 pages + 3 figures, uses aastex.cls
Catalogue of the morphological features in the Spitzer Survey of Stellar Structure in Galaxies (SG)
A catalogue of the morphological features for the complete Spitzer Survey of
Stellar Structure in Galaxies (SG), including 2352 nearby galaxies, is
presented. The measurements are made using 3.6 m images, largely tracing
the old stellar population; at this wavelength the effects of dust are also
minimal. The measured features are the sizes, ellipticities, and orientations
of bars, rings, ringlenses, and lenses. Measured in a similar manner are also
barlenses (lens-like structures embedded in the bars), which are not lenses in
the usual sense, being rather the more face-on counterparts of the boxy/peanut
structures in the edge-on view. In addition, pitch angles of spiral arm
segments are measured for those galaxies where they can be reliably traced.
More than one pitch angle may appear for a single galaxy. All measurements are
made in a human-supervised manner so that attention is paid to each galaxy. We
used isophotal analysis, unsharp masking, and fitting ellipses to measured
structures. We find that the sizes of the inner rings and lenses normalized to
barlength correlate with the galaxy mass: the normalized sizes increase toward
the less massive galaxies; it has been suggested that this is related to the
larger dark matter content in the bar region in these systems. Bars in the low
mass galaxies are also less concentrated, likely to be connected to the mass
cut-off in the appearance of the nuclear rings and lenses. We also show
observational evidence that barlenses indeed form part of the bar, and that a
large fraction of the inner lenses in the non-barred galaxies could be former
barlenses in which the thin outer bar component has dissolved.Comment: 17 pages, 12 figures, accepted for publication in A&
Statistics of the structure components in S0s: implications for bar induced secular evolution
The fractions and dimension of bars, rings and lenses are studied in the
Near-IR S0 galaxy Survey (NIRS0S). We find evidence that multiple lenses in
some barred S0s are related to bar resonances in a similar manner as the inner
and outer rings, for which the outer/inner length ratio 2. Inner lenses in the
non-barred galaxies normalized to galaxy diameter are clearly smaller than
those in the barred systems. Interestingly, these small lenses in the
non-barred galaxies have similar sizes as barlenses (lens-like structures
embedded in a bar), and therefore might actually be barlenses in former barred
galaxies, in which the outer, more elongated bar component, has been destroyed.
We also find that fully developed inner lenses are on average a factor 1.3
larger than bars, whereas inner rings have similar sizes as bars. The fraction
of inner lenses is found to be constant in all family classes (A, AB, B).
Nuclear bars appear most frequently among the weakly barred (AB) galaxies,
which is consistent with the theoretical models by Maciejewski & Athanassoula
(2008). Similar sized bars as the nuclear bars were detected in seven
'non-barred' S0s. Galaxy luminosity does not uniquely define the sizes of bars
or bar-related structures, neither is there any upper limit in galaxy
luminosity for bar formation. Although all the family classes cover the same
range of galaxy luminosity, the non-barred (A) galaxies are on average 0.6 mag
brighter than the strongly barred (B) systems. Overall, our results are
consistent with the idea that bars play an important role in the formation of
the structure components of galaxies. The fact that multiple lenses are common
in S0s, and that at least the inner lenses can have very old stellar
populations, implies that the last destructive merger, or major gas accretion
event, must have taken place at a fairly high redshift.Comment: 36 pages (include 13 figures, 11 tables). Accepted to MNRAS 2013 Jan
2
Evidence for the concurrent growth of thick discs and central mass concentrations from SG imaging
We have produced vertically integrated radial
luminosity profiles of 69 edge-on galaxies from the Spitzer Survey of Stellar
Structure in Galaxies (SG). We decomposed the luminosity profiles into a
disc and a central mass concentration (CMC). These fits, combined with
thin/thick disc decompositions from our previous studies, allow us to estimate
the masses of the CMCs, the thick discs, and the thin discs (, , and ). We obtained atomic
disc masses () from the literature. We then consider the
CMC and the thick disc to be dynamically hot components and the thin disc and
the gas disc to be dynamically cold components. We find that the ratio between
the mass of the hot components and that of the cold components,
, does not depend on the total galaxy mass as described
by circular velocities (). We also find that the higher the , the more concentrated the hot component of a galaxy. We suggest that our
results are compatible with having CMCs and thick discs built in a short and
early high star forming intensity phase. These components were born thick
because of the large scale height of the turbulent gas disc in which they
originated. Our results indicate that the ratio between the star forming rate
in the former phase and that of the formation of the thin disc is of the order
of 10, but the value depends on the duration of the high star forming intensity
phase.Comment: Accepted for publication in A&
Near-IR Atlas of S0-Sa galaxies (NIRS0S)
An atlas of Ks-band images of 206 early-type galaxies is presented, including
160 S0-S0/a galaxies, 12 ellipticals, and 33 Sa galaxies. A majority of the
Atlas galaxies belong to a magnitude-limited (mB<12.5 mag) sample of 185 NIRS0S
(Near-IR S0 galaxy Survey) galaxies. To assure that mis-classified S0s are not
omitted, 25 ellipticals from RC3 classified as S0s in the Carnegie Atlas were
included in the sample. The images are 2-3 mag deeper than 2MASS images. Both
visual and photometric classifications are made. Special attention is paid to
the classification of lenses, coded in a systematic manner. A new lens-type,
called a 'barlens', is introduced. Also, boxy/peanut/x-shaped structures are
identified in many barred galaxies, even-though the galaxies are not seen in
edge-on view, indicating that vertical thickening is not enough to explain
them. Multiple lenses appear in 25% of the Atlas galaxies, which is a challenge
to the hierarchical evolutionary picture of galaxies. Such models need to
explain how the lenses were formed and survived in multiple merger events that
galaxies may have suffered during their lifetimes. Following the early
suggestion by van den Bergh, candidates of S0c galaxies are shown, which
galaxies are expected to be former Sc-type spirals stripped out of gas.Comment: 67 pages (include 16 figures and 6 tables). Accepted to MNRAS 2011
June 1
Variation of Bar Strength with Central Velocity Dispersion in Spiral Galaxies
We investigate the variation of bar strength with central velocity dispersion
in a sample of barred spiral galaxies. The bar strength is characterized by
, the maximal tangential perturbation associated with the bar, normalized
by the mean axisymmetric force. It is derived from the galaxy potentials which
are obtained using near-infrared images of the galaxies. However, is
sensitive to bulge mass. Hence we also estimated bar strengths from the
relative Fourier intensity amplitude () of bars in near-infrared images.
The central velocity dispersions were obtained from integral field spectroscopy
observations of the velocity fields in the centers of these galaxies; it was
normalized by the rotation curve amplitude obtained from HI line width for each
galaxy. We found a correlation between bar strengths (both and )
and the normalized central velocity dispersions in our sample. This suggests
that bars weaken as their central components become kinematically hotter. This
may have important implications for the secular evolution of barred galaxies.Comment: To appear in Ap&S
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