323 research outputs found
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
Stellar Bar Evolution in Cuspy and Flat-Cored Triaxial CDM Halos
We analyze the evolution of stellar bars in galactic disks in mildly triaxial
flat-core and cuspy CDM halos. We use tailored simulations of rigid and live
halos which include the feedback from disk/bar onto the halo in order to test
the work by El-Zant & Shlosman (2002). The latter used the Liapunov exponents
to analyze the fate of bars in analytical asymmetric halos. We find: (1) The
bar growth is similar in all rigid axisymmetric and triaxial halos. (2) Bars in
live models vertically buckle and form a pseudobulge with a boxy/peanut shape.
(3) In live axisymmetric halos, the bar strength varies little during the
secular evolution. The bar pattern speed anticorrelates with the halo core
size. The bar strength is larger for smaller disk-to-halo mass ratios within
disk radii, the bar size correlates with the halo core sizes, and the bar
pattern speeds -- with the halo central mass concentration. Bars embedded in
live triaxial halos have a starkly different fate: they dissolve on ~1.5-5 Gyr
due to the onset of chaos over continuous zones, leaving behind a weak oval
distortion. The onset of chaos is related to the halo triaxiality, the fast
rotating bar and the halo cuspiness. Before the bar dissolves, the region
outside it develops strong spiral structures, especially in the live triaxial
halos. (4) More angular momentum is absorbed by the triaxial halos as compared
to the axisymmetric models and its exchange is mediated by resonances. (5)
Cuspy halos are more susceptible than flat-core halos to having their
prolateness washed out by the bar. We analyze these results in terms of the
stability of trajectories and development of chaos. We set constraints on the
triaxiality of DM halos by comparing our predictions to recent observations of
bars out to z~1.Comment: 17 pages, 14 figures, Astrophysical Journal, in press, Vol. 637.
Updated version (text, references
Frequency and properties of bars in cluster and field galaxies at intermediate redshifts
We present a study of large-scale bars in field and cluster environments out
to redshifts of ~0.8 using a final sample of 945 moderately inclined disk
galaxies drawn from the EDisCS project. We characterize bars and their host
galaxies and look for relations between the presence of a bar and the
properties of the underlying disk. We investigate whether the fraction and
properties of bars in clusters are different from their counterparts in the
field. The total optical bar fraction in the redshift range z=0.4-0.8 (median
z=0.60), averaged over the entire sample, is 25% (20% for strong bars). For the
cluster and field subsamples, we measure bar fractions of 24% and 29%,
respectively. We find that bars in clusters are on average longer than in the
field and preferentially found close to the cluster center, where the bar
fraction is somewhat higher (~31%) than at larger distances (~18%). These
findings however rely on a relatively small subsample and might be affected by
small number statistics. In agreement with local studies, we find that
disk-dominated galaxies have a higher optical bar fraction (~45%) than
bulge-dominated galaxies (~15%). This result is based on Hubble types and
effective radii and does not change with redshift. The latter finding implies
that bar formation or dissolution is strongly connected to the emergence of the
morphological structure of a disk and is typically accompanied by a transition
in the Hubble type. (abridged)Comment: 17 pages, accepted for publication in A&
Evolution of optically faint AGN from COMBO-17 and GEMS
We have mapped the AGN luminosity function and its evolution between z=1 and
z=5 down to apparent magnitudes of . Within the GEMS project we have
analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South,
enabling us to assess the evolution of AGN host galaxy properties with cosmic
time.Comment: to appear in proceedings 'Multiwavelength AGN Surveys', Cozumel 200
Interacting Galaxies in the A901/902 Supercluster with STAGES
We present a study of galaxy mergers and the influence of environment in the
Abell 901/902 supercluster at z~0.165. We use HST ACS F606W data from the
STAGES survey, COMBO-17, Spitzer 24um, and XMM-Newton X-ray data. Our analysis
utilizes both a visual classification system, and quantitative CAS parameters
to identify systems which show evidence of a recent or ongoing merger of mass
ratio >1/10. Our results are: (1) After visual classification and minimizing
the contamination from false projection pairs, we find that the merger fraction
f_merge is 0.023+/-0.007. The estimated fractions of likely major mergers,
likely minor mergers, and ambiguous cases are 0.01+/-0.004, 0.006+/-0.003, and
0.007+/-0.003, respectively. (2) The mergers lie outside the cluster core of
radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large
galaxy velocity dispersion in the core. Mergers populate the region (0.25 Mpc <
R <= 2 Mpc) between the core and outskirt. In this region, the estimated
frequency of mergers is similar to those seen at typical group overdensities.
This suggests ongoing growth of the clusters via accretion of group and field
galaxies. (3) We compare our observed merger fraction with those reported in
other clusters and groups out to z~0.4. Existing data points on the merger
fraction for L<= L* galaxies in clusters allow for a range of evolutionary
scenarios. (4) The fraction of mergers, which lie on the blue cloud is
80%+/-18% versus 34%+/-7% for non-interacting galaxies, implying that
interacting galaxies are preferentially blue. (5) The average SFR, based on UV
or UV+IR data, is enhanced by a factor of ~1.5 to 2 in mergers compared to
non-interacting galaxies. However, mergers in the clusters contribute only a
small fraction (between 10% and 15%) of the total SFR density.(Abridged)Comment: Accepted for publication in ApJ. 34 pages, 16 figures. Version with
full resolution figures available at: http://www.as.utexas.edu/~alh/apj/int/
; updated abridged abstrac
History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS
We perform a comprehensive estimate of the frequency of galaxy mergers and
their impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr)
using 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data.
Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the
visually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%.
Lower limits on the major and minor merger fractions over this interval range
from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first
approximate empirical estimate of the frequency of minor mergers at z<1. For a
visibility timescale of ~0.5 Gyr, it follows that over T_b~3--7 Gyr, ~68% of
high mass systems have undergone a merger of mass ratio >1/10, with ~16%, 45%,
and 7% of these corresponding respectively to major, minor, and ambiguous
`major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2)
We compare the empirical merger fraction and rate for high mass galaxies to a
suite of Lambda CDM-based models: halo occupation distribution models,
semi-analytic models, and hydrodynamic SPH simulations. We find qualitative
agreement between observations and models such that the (major+minor) merger
fraction or rate from different models bracket the observations, and show a
factor of five dispersion. Near-future improvements can now start to rule out
certain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that
the mean SFR of visibly merging systems is only modestly enhanced compared to
non-interacting galaxies over z~0.24--0.80. Visibly merging systems only
account for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This
suggests that the behavior of the cosmic SFR density over the last 7 Gyr is
predominantly shaped by non-interacting galaxies.Comment: Accepted for Publication in the Astrophysical Journal. 17 pages of
text, 21 figures, 3 tables. Uses emulateapj5.st
Nuclear starburst-driven evolution of the central region in NGC 6764
We study the CO and the radiocontinuum emission in an active galaxy to
analyze the interplay between the central activity and the molecular gas. We
present new high-resolution observations of the CO(1-0) and CO(2-1) emission
lines, and 3.5 cm and 20 cm radio continuum emission in the central region of
the LINER/starburst galaxy NGC 6764. The galaxy has an outflow morphology in
radio continuum, spatially coincident with the CO and H emission, and
centered slightly off the radio continuum peak at the LINER nucleus. The total
molecular gas mass in the center is about 7x10^8 \msun, using a CO luminosity
to total molecular gas conversion factor that is three times lower than the
standard one. CO(1-0) emission is found near the boundaries of the radio
continuum emission cone. The outflow has a projected expansion velocity of 25
km/s relative to the systemic velocity of NGC6764. About 4x 10^6 \msun of
molecular gas is detected in the outflow. The approximate location (~1 kpc) of
the dynamical inner Lindblad resonance has been derived from the rotation
curve. The peak of the CO emission is slightly (< 200 pc) offset from the peak
of the radio continuum.
The molecular gas has most likely been ejected by the stellar winds from the
recent starburst, but the CO line ratios show indication of an interaction with
the AGN. The energy released by the nuclear starburst is sufficient to explain
the observed outflow, even if the data cannot exclude the AGN from being the
major energy source. Comparison of the outflow with hydrodynamical simulations
suggests that the nuclear starburst is 3--7 Myr old and the bubble-like outflow
is still confined and not freely expanding.Comment: Accepted for publication in A&
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