633 research outputs found
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
Ring Star Formation Rates in Barred and Nonbarred Galaxies
Nonbarred ringed galaxies are relatively normal galaxies showing bright rings
of star formation in spite of lacking a strong bar. This morphology is
interesting because it is generally accepted that a typical ring forms when
material collects near a resonance, set up by the pattern speed of a bar or
bar-like perturbation. Our goal in this paper is to examine whether the ring
star formation properties are related to the non-axisymmetric gravity potential
in general. For this purpose, we obtained H{\alpha} emission line images and
calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA
galaxies and four weakly barred SAB galaxies with rings. For comparison, we
combine our observations with a re-analysis of previously published data on
five SA, seven SAB, and 15 SB galaxies with rings, three of which are
duplicates from our sample. With these data, we examine what role a bar may
play in the star formation process in rings. Compared to barred ringed
galaxies, we find that the inner ring SFRs and H{\alpha}+[N ii] equivalent
widths in nonbarred ringed galaxies show a similar range and trend with
absolute blue magnitude, revised Hubble type, and other parameters. On the
whole, the star formation properties of inner rings, excluding the distribution
of H ii regions, are independent of the ring shapes and the bar strength in our
small samples. We confirm that the deprojected axis ratios of inner rings
correlate with maximum relative gravitational force Q_g; however, if we
consider all rings, a better correlation is found when local bar forcing at the
radius of the ring, Q_r, is used. Individual cases are described and other
correlations are discussed. By studying the physical properties of these
galaxies, we hope to gain a better understanding of their placement in the
scheme of the Hubble sequence and how they formed rings without the driving
force of a bar.Comment: 55 pages; 21 figures and 9 tables. Article has been accepted for
publication in the Astronomical Journa
Bars from the Inside Out: An HST Study of their Dusty Circumnuclear Regions
The results of bar-driven mass inflow are directly observable in
high-resolution HST observations of their circumnuclear regions. These
observations reveal a wealth of structures dominated by dust lanes, often with
a spiral-like morphology, and recent star formation. Recent work has shown that
some of these structures are correlated with the presence or absence of a bar.
I extend this work with an investigation of circumnuclear morphology as a
function of bar strength for a sample of 48 galaxies with both measured bar
strengths and ``structure maps'' computed from HST images. The structure maps
for these galaxies, which have projected spatial resolutions of 2 - 15 pc, show
that the fraction of galaxies with grand-design (GD) circumnuclear dust spirals
increases significantly with bar strength, while tightly wound dust spirals are
only present in the most axisymmetric galaxies. GD structure is only found at
the centers of galaxies classified as SB(s) or SB(rs) and not SB(r). SB(s)
galaxies on average have stronger bars than SB(r) galaxies. There is also a
modest increase in the fraction of loosely wound dust spirals at later
morphological types, which may reflect an increase in the fraction of galaxies
with circumnuclear, gaseous disks. (abridged)Comment: 10 pages, 6 figures. To appear in "Penetrating Bars through Masks of
Cosmic Dust: The Hubble Tuning Fork strikes a New Note" held June 7-12th,
2004 in Pilanesberg National Park, South Africa. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/safrica04.pd
On the 3D dynamics and morphology of inner rings
We argue that inner rings in barred spiral galaxies are associated with
specific 2D and 3D families of periodic orbits located just beyond the end of
the bar. These are families located between the inner radial ultraharmonic 4:1
resonance and corotation. They are found in the upper part of a type-2 gap of
the x1 characteristic, and can account for the observed ring morphologies
without any help from families of the x1-tree. Due to the evolution of the
stability of all these families, the ring shapes that are favored are mainly
ovals, as well as polygons with `corners' on the minor axis, on the sides of
the bar. On the other hand pentagonal rings, or rings of the NGC 7020 type
hexagon, should be less probable. The orbits that make the rings belong in
their vast majority to 3D families of periodic orbits and orbits trapped around
them.Comment: 11 pages, 12 figures, to appear in MNRA
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
Reconstructing the Stellar Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 μm Images. I. Correcting for Contamination by Polycyclic Aromatic Hydrocarbons, Hot Dust, and Intermediate-age Stars
With the aim of constructing accurate two-dimensional maps of the stellar mass distribution in nearby galaxies from Spitzer Survey of Stellar Structure in Galaxies 3.6 and 4.5 μm images, we report on the separation of the light from old stars from the emission contributed by contaminants. Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust content, and star formation histories are presented to demonstrate our approach. To isolate the old stellar light from contaminant emission (e.g., hot dust and the 3.3 μm polycyclic aromatic hydrocarbon (PAH) feature) in the IRAC 3.6 and 4.5 μm bands we use an independent component analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. The contaminants are studied by comparison with the non-stellar emission imaged at 8 μm, which is dominated by the broad PAH feature. Using the measured 3.6 μm/8 μm ratio to select individual contaminants, we find that hot dust and PAHs together contribute between ~5% and 15% to the integrated light at 3.6 μm, while light from regions dominated by intermediate-age (AGB and RSG) stars accounts for only 1%-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate-age stars contribute upward of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing stellar mass, as will be the focus in a companion paper
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
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
Grand Design and Flocculent Spirals in the Spitzer Survey of Stellar Structure in Galaxies (S4G)
Spiral arm properties of 46 galaxies in the Spitzer Survey of Stellar
Structure in Galaxies (S4G) were measured at 3.6mu, where extinction is small
and the old stars dominate. The sample includes flocculent, multiple arm, and
grand design types with a wide range of Hubble and bar types. We find that most
optically flocculent galaxies are also flocculent in the mid-IR because of star
formation uncorrelated with stellar density waves, whereas multiple arm and
grand design galaxies have underlying stellar waves. Arm-interarm contrasts
increase from flocculent to multiple arm to grand design galaxies and with
later Hubble types. Structure can be traced further out in the disk than in
previous surveys. Some spirals peak at mid-radius while others continuously
rise or fall, depending on Hubble and bar type. We find evidence for regular
and symmetric modulations of the arm strength in NGC 4321. Bars tend to be
long, high amplitude, and flat-profiled in early type spirals, with arm
contrasts that decrease with radius beyond the end of the bar, and they tend to
be short, low amplitude, and exponential-profiled in late Hubble types, with
arm contrasts that are constant or increase with radius. Longer bars tend to
have larger amplitudes and stronger arms.Comment: 31 pages, 14 figures, ApJ in pres
Comparison of bar strengths in active and non-active galaxies
Bar strengths are compared between active and non-active galaxies for a
sample of 43 barred galaxies. The relative bar torques are determined using a
new technique (Buta and Block 2001), where maximum tangential forces are
calculated in the bar region, normalized to the axisymmetric radial force
field. We use JHK images of the 2 Micron All Sky Survey. We show a first clear
empirical indication that the ellipticies of bars are correlated with the
non-axisymmetric forces in the bar regions. We found that nuclear activity
appears preferentially in those early type galaxies in which the maximum bar
torques are weak and appear at quite large distances from the galactic center.
Most suprisingly the galaxies with the strongest bars are non-active. Our
results imply that the bulges may be important for the onset of nuclear
activity, but that the correlation between the nuclear activity and the early
type galaxies is not straightforward.Comment: MNRAS macro in tex format, 9 pages, 10 figure
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