5,419 research outputs found
The large-scale Structure and dynamics of the local universe
This thesis investigates the dynamics of the local Universe with particular reference to discovering the source of the Local Group (LG) motion. A redshift survey of the Great Attractor (GA) region, thought responsible for a significant fraction of the LG motion, is presented. Over 3053 galaxies, located in both clusters and filaments, were targeted using the 2dF on the AAT. Velocity distributions and mass estimates for nine clusters are reported. Together with redshifts from the literature, this survey reveals the dominant feature in the core of the GA to be a large filament extending from Abell S0639 (l= 281º, b=+11º) towards a point at I ~ 5º b ~ -50º , encompassing the Cen-Crux, CIZA J1324.7-5736, Norma and Pavo II clusters. A new model of the local velocity field out to ՅՕՕh(^-1) Мрc is derived from the combined REFLEX, BCS and CIZA surveys: the RBC catalogue. This is the first all-sky, X-ray selected galaxy cluster sample. The reconstruction includes an intrinsic correction for the bias of clusters in tracing the total density field. The velocity fields from both this reconstruction and that of the PSCz survey are compared to the observed peculiar velocities of 98 local type la supernovae (SNIa). The best fits are respectively found for values of βRBC(= Ω(^0.6)m /(^b)RBC) = 0.39±0.20 and βι = 0.55±0.06. These results are found to be robust to culls of the SNIa sample by distance, host-galaxy extinction and the reference frame in which the comparison is carried out. As the PSCz preferentially samples late-type galaxies, the derived density field under- Samples the contributions from regions of greatest over density, precisely the regions traced by the RBC survey. When combined in the ratio 78% PSCz, 22% RBC these two complimentary reconstructions are a better fit to the ресuliar velocities of the same SNIa sample than either one alone. Compared to galaxy surveys, which only see contributions to the LG motion from structures within -60h(^-1) Мрc, previous cluster surveys have argued that sources at much greater distances (~150h(^-1) Mpc) influence local dynamics. However, the RBC reconstruction presented here shows similar contributions from the same depths as the PSCz, which is partly attributed to the intrinsic bias correction and inclusion of the Virgo cluster in the RBC. The extended GA region, defined as the volume enclosed by 250 < / <350º, 一45 < ь < 45º and 2000 <cz< 6000 km s (^-1) is found to be responsible for 65% of the LG motion, whilst the more distant (~ 145h(^-1) Mpc) SSC only accounts for 12%
The GHOSTS survey. II. The diversity of Halo Color and Metallicity Profiles of Massive Disk Galaxies
We study the stellar halo color properties of six nearby massive highly
inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3 observations in both F606W and F814W filters from the
GHOSTS survey. The observed fields, placed both along the minor and major axis
of each galaxy, probe the stellar outskirts out to projected distances of ~
50-70 kpc from their galactic centre along the minor axis. The 50% completeness
levels of the color magnitude diagrams are typically at two mag below the tip
of the red giant branch. We find that all galaxies have extended stellar halos
out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color
distribution and color profile for each galaxy using the median colors of stars
in the RGB. Within each galaxy we find variations in the median colors as a
function of radius which likely indicates population variations, reflecting
that their outskirts were built from several small accreted objects. We find
that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear
negative color gradient, reflecting a declining metallicity in their halos; the
other have no significant color or population gradient. In addition,
notwithstanding the modest sample size of galaxies, there is no strong
correlation between their halo color/metallicity or gradient with galaxy's
properties such as rotational velocity or stellar mass. The diversity in halo
color profiles observed in the GHOSTS galaxies qualitatively supports the
predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of
the stochasticity inherent in the assembling history of galaxies.Comment: Accepted by MNRAS, 31 pages, 18 figures. Appendix added and some
editions to match accepted version. Conclusions unchange
Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
We have examined the resolved stellar populations at large galactocentric
distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with
limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc
galaxies using HST data from the GHOSTS survey. We select red giant branch
stars to derive stellar halo density profiles. The projected minor axis density
profiles can be approximated by power laws with projected slopes of between
and and a diversity of stellar halo masses of , or of the total galaxy stellar masses. The typical
intrinsic scatter around a smooth power law fit is dex owing to
substructure. By comparing the minor and major axis profiles, we infer
projected axis ratios at kpc between . The GHOSTS
stellar haloes are diverse, lying between the extremes charted out by the
(rather atypical) haloes of the Milky Way and M31. We find a strong correlation
between the stellar halo metallicities and the stellar halo masses. We compare
our results with cosmological models, finding good agreement between our
observations and accretion-only models where the stellar haloes are formed by
the disruption of dwarf satellites. In particular, the strong observed
correlation between stellar halo metallicity and mass is naturally reproduced.
Low-resolution hydrodynamical models have unrealistically high stellar halo
masses. Current high-resolution hydrodynamical models appear to predict stellar
halo masses somewhat higher than observed but with reasonable metallicities,
metallicity gradients and density profiles.Comment: 26 pages, 17 figures. Accepted for publication in MNRA
Evolution in the Dust Lane Fraction of Edge-on L* Spiral Galaxies since z=0.8
The presence of a well-defined and narrow dust lane in an edge-on spiral
galaxy is the observational signature of a thin and dense molecular disk, in
which gravitational collapse has overcome turbulence. Using a sample of
galaxies out to z~1 extracted from the COSMOS survey, we identify the fraction
of massive disks that display a dust lane. Our goal is to explore the evolution
in the stability of the molecular ISM disks in spiral galaxies over a cosmic
timescale. We check the reliability of our morphological classifications
against changes in restframe wavelength, resolution, and cosmic dimming with
(artificially redshifted) images of local galaxies from SDSS. We find that the
fraction of L* disks with dust lanes in COSMOS is consistent with the local
fraction (~80%) out to z~0.7. At z=0.8, the dust lane fraction is only slightly
lower. A somewhat lower dust lane fraction in starbursting galaxies tentatively
supports the notion that a high specific star formation rate can efficiently
destroy or inhibit a dense molecular disk. A small subsample of higher redshift
COSMOS galaxies display low internal reddening (E[B-V]), as well as a low
incidence of dust lanes. These may be disks in which the growth of the dusty
ISM disk lags behind that of the stellar disk. We note that at z=0.8, the most
massive galaxies display a lower dust lane fraction than lower mass galaxies. A
small contribution of recent mergers or starbursts to this most massive
population may be responsible. The fact that the fraction of galaxies with dust
lanes in COSMOS is consistent with little or no evolution implies that models
to explain the Spectral Energy Distribution or the host galaxy dust extinction
of supernovae based on local galaxies are still applicable to higher redshift
spirals. It also suggests that dust lanes are long lived phenomena or can be
reformed over very short time-scales.Comment: 14 pages, 9 figures, 2 tables, accepted for publication by Ap
Extragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies
We study the individual evolution histories of three nearby low-mass edge-on
galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations,
we constructed star count density maps for populations of different ages and
analyzed the change of structural parameters with stellar age within each
galaxy.
We do not detect a separate thick disk in any of the three galaxies, even
though our observations cover a wider range in equivalent surface brightness
than any integrated light study. While scale heights increase with age, each
population can be well described by a single disk. Two of the galaxies contain
a very weak additional component, which we identify as the faint halo. The mass
of these faint halos is lower than 1% of the mass of the disk. The three
galaxies show low vertical heating rates, which are much lower than the heating
rate of the Milky Way. This indicates that heating agents, such as giant
molecular clouds and spiral structure, are weak in low-mass galaxies. All
populations in the three galaxies exhibit no or only little flaring. While this
finding is consistent with previous integrated light studies, it poses strong
constraints on galaxy simulations, where strong flaring is often found as a
result of interactions or radial migration.Comment: Accepted for publication in A&
Herschel/SPIRE observations of the dusty disk of NGC 4244
We present Herschel/SPIRE images at 250, 350, and 500 mu m of NGC 4244, a typical low-mass, disk-only and edge-on spiral galaxy. The dust disk is clumpy and shows signs of truncation at the break radius of the stellar disk. This disk coincides with the densest part of the Hi disk. We compare the spectral energy distribution (SED), including the new SPIRE fluxes, to 3D radiative transfer models; a smooth model disk and a clumpy model with embedded heating. Each model requires a very high value for the dust scale-length (h(d) = 2-5 h(*)), higher dust masses than previous models of NGC 4244 (M-d = 0.47-1.39 x 10(7) M-circle dot) and a face-on optical depth of tau(f.o.)(V) = 0.4-1.12, in agreement with previous disk opacity studies. The vertical scales of stars and dust are similar. The clumpy model much better mimics the general morphology in the sub-mm images and the general SED. The inferred gas-to-dust mass ratio is compatible with those of similar low-mass disks. The relatively large radial scale-length of the dust disk points to radial mixing of the dusty ISM within the stellar disk. The large vertical dust scale and the clumpy dust distribution of our SED model are both consistent with a scenario in which the vertical structure of the ISM is dictated by the balance of turbulence and self-gravity
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