1,663 research outputs found
Galaxy threshing and the origin of intracluster stellar objects
We numerically investigate dynamical evolution of non-nucleated dwarf
elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of
galaxies in order to understand the origin of intracluster stellar objects,
such as intracluster stars (ICSs), GCs (ICGCs), and ``ultra-compact dwarf''
(UCDs) recently discovered by all-object spectroscopic survey centred on the
Fornax cluster of galaxies. We find that the outer stellar components of a
nucleated dwarf are removed by the strong tidal field of the cluster, whereas
the nucleus manages to survive as a result of its initially compact nature. The
developed naked nucleus is found to have physical properties (e.g., size and
mass) similar to those observed for UCDs. We also find that the UCD formation
processes does depend on the radial density profile of the dark halo in the
sense that UCDs are less likely to be formed from dwarfs embedded in dark
matter halos with central `cuspy' density profiles. Our simulations also
suggest that very massive and compact stellar systems can be rapidly and
efficiently formed in the central regions of dwarfs through the merging of
smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be
stripped to form ICGCs.Comment: 6 pages and 3 figures (JPG file for Fig. 1), in the proceedings of
IAU 217 ``Recycling intergalactic and interstellar matter'
Hepatic acute phase response protects the brain from focal inflammation during postnatal window of susceptibility
Perinatal inflammation is known to contribute to neurodevelopmental diseases. Animal models of perinatal inflammation have revealed that the inflammatory response within the brain is age dependent, but the regulators of this variation remain unclear. In the adult, the peripheral acute phase response (APR) is known to be pivotal in the downstream recruitment of leukocytes to the injured brain. The relationship between perinatal brain injury and the APR has not been established. Here, we generated focal inflammation in the brain using interleukin (IL)-1ÎČ at postnatal day (P)7, P14, P21 and P56 and studied both the central nervous system (CNS) and hepatic inflammatory responses at 4âŻh. We found that there is a significant window of susceptibility in mice at P14, when compared to mice at P7, P21 and P56. This was reflected in increased neutrophil recruitment to the CNS, as well as an increase in bloodâbrain barrier permeability. To investigate phenomena underlying this window of susceptibility, we performed a dose response of IL-1ÎČ. Whilst induction of endogenous IL-1ÎČ or intercellular adhesion molecule (ICAM)-1 in the brain and induction of a hepatic APR were dose dependent, the recruitment of neutrophils and associated bloodâbrain barrier breakdown was inversely proportional. Furthermore, in contrast to adult animals, an additional peripheral challenge (intravenous IL-1ÎČ) reduced the degree of CNS inflammation, rather than exacerbating it. Together these results suggest a unique window of susceptibility to CNS injury, meaning that suppressing systemic inflammation after brain injury may exacerbate the damage caused, in an age-dependent manner
The Forestecology R Package for Fitting and Assessing Neighborhood Models of the Effect of Interspecific Competition on the Growth of Trees
Neighborhood competition models are powerful tools to measure the effect of interspecific competition. Statistical methods to ease the application of these models are currently lacking. We present the forestecology package providing methods to (a) specify neighborhood competition models, (b) evaluate the effect of competitor species identity using permutation tests, and (cs) measure model performance using spatial cross-validation. Following Allen and Kim (PLoS One, 15, 2020, e0229930), we implement a Bayesian linear regression neighborhood competition model. We demonstrate the package\u27s functionality using data from the Smithsonian Conservation Biology Institute\u27s large forest dynamics plot, part of the ForestGEO global network of research sites. Given ForestGEOâs data collection protocols and data formatting standards, the package was designed with cross-site compatibility in mind. We highlight the importance of spatial cross-validation when interpreting model results. The package features (a) tidyverse-like structure whereby verb-named functions can be modularly âpipedâ in sequence, (b) functions with standardized inputs/outputs of simple features sf package class, and (c) an S3 object-oriented implementation of the Bayesian linear regression model. These three facts allow for clear articulation of all the steps in the sequence of analysis and easy wrangling and visualization of the geospatial data. Furthermore, while the package only has Bayesian linear regression implemented, the package was designed with extensibility to other methods in mind
Observations of Stripped Edge-on Virgo Cluster Galaxies
We present observations of highly inclined, HI deficient, Virgo cluster
spiral galaxies. Our high-resolution VLA HI observations of edge-on galaxies
allow us to distinguish extraplanar gas from disk gas. All of our galaxies have
truncated H-alpha disks, with little or no disk gas beyond a truncation radius.
While all the gas disks are truncated, the observations show evidence for a
continuum of stripping states: symmetric, undisturbed truncated gas disks
indicate galaxies that were stripped long ago, while more asymmetric disks
suggest ongoing or more recent stripping. We compare these timescale estimates
with results obtained from two-dimensional stellar spectroscopy of the outer
disks of galaxies in our sample. One of the galaxies in our sample, NGC 4522 is
a clear example of active ram-pressure stripping, with 40% of its detected HI
being extraplanar. As expected, the outer disk stellar populations of this
galaxy show clear signs of recent (and, in fact, ongoing) stripping. Somewhat
less expected, however, is the fact that the spectrum of the outer disk of this
galaxy, with very strong Balmer absorption and no observable emission, would be
classified as ``k+a'' if observed at higher redshift. Our observations of NGC
4522 and other galaxies at a range of cluster radii allow us to better
understand the role that clusters play in the structure and evolution of disk
galaxies.Comment: 4 pages, 2 figures, to appear in the proceedings of the Island
Universes conference held in Terschelling, Netherlands, July 2005, ed. R. de
Jong, version with high resolution figures can be downloaded from
ftp://ftp.astro.yale.edu/pub/hugh/papers/iu_crowl_h.ps.g
Detection of weak lensing by a cluster of galaxies at z=0.83
We report the detection of weak gravitational lensing of faint, distant
background galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03
at z=0.83. This is the first measurement of weak lensing by a bona fide cluster
at such a high redshift. We detect tangential shear at the 5% - 10% level over
a range of radii 50'' < r < 250'' centered on the optical position of the
cluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I <
25.5 shows a strong peak which coincides with the peak of the smoothed cluster
light distribution. Splitting this sample by magnitude (at I = 23.5) and color
(at R-I = 0.7), we find that the brighter and redder subsamples are only very
weakly distorted, indicating that the faint blue galaxies (FBG's), which
dominate the shear signal, are relatively more distant. The derived cluster
mass is quite sensitive to the N(z) for the FBG's. At one extreme, if all the
FBG's are at z_s = 3, then the mass within a Mpc aperture is \h1 , and the mass-to-light ratio is in solar units. For the derived mass is 70\%
higher and . If follows the no evolution model (in
shape) then , and if all the FBG's lie at z_s\la 1 the
required exceeds . These data provide clear evidence that large,
dense mass concentrations existed at early epochs; that they can be weighed
efficiently by weak lensing observations; and that most of the FBG's are at
high redshift.Comment: Submitted to ApJ, 15 pages (incl 8 figs, 3 of which are plates).
Plate images not included, but are available from
ftp://hubble.ifa.hawaii.edu/pub/ger/ms1054/ms1054_fig[1,3,5].ps.
The spatial distribution of galaxies of different spectral types in the massive intermediate-redshift cluster MACSJ0717.5+3745
We present the results of a wide-field spectroscopic analysis of the galaxy
population of the massive cluster MACSJ0717.5+3745 and the surrounding
filamentary structure (z=0.55), as part of our systematic study of the 12 most
distant clusters in the MACS sample. Of 1368 galaxies spectroscopically
observed in this field, 563 are identified as cluster members; of those, 203
are classified as emission-line galaxies, 260 as absorption-line galaxies, and
17 as E+A galaxies (defined by \AA and no
detection of [OII] and in emission). The variation of the fraction
of emission- and absorption-line galaxies as a function of local projected
galaxy density confirms the well-known morphology-density relation, and becomes
flat at projected galaxy densities less than $\sim 20Mpc^{-2}. Interestingly,
16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping
radius around the cluster core, which we take to be direct evidence of
ram-pressure stripping being the primary mechanism that terminates
star-formation in the E+A population of galaxy clusters. This conclusion is
supported by the rarity of E+A galaxies in the filament which rules out galaxy
mergers as the dominant driver of evolution for E+A galaxies in clusters. In
addition, we find the 42 e(a) and 27 e(b) member galaxies, i.e., the
dusty-starburst and starburst galaxies respectively, to be spread out across
almost the entire study area. Their spatial distribution, which shows a strong
preference for the filament region, suggests that starbursts are triggered in
relatively low-density environments as galaxies are accreted from the field
population.Comment: 16 pages, 15 figures, accepted by Ap
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
A comparison of the galaxy populations in the Coma and distant clusters: the evolution of k+a galaxies and the role of the intracluster medium
The spectroscopic properties of galaxies in the Coma cluster are compared
with those of galaxies in rich clusters at , to investigate the
evolution of the star formation history in clusters. Luminous galaxies with
and post-starburst/post-starforming (k+a) spectra which
constitute a significant fraction of galaxies in distant cluster samples are
absent in Coma, where spectacular cases of k+a spectra are found instead at
and represent a significant proportion of the cluster dwarf galaxy
population. A simple inspection of their positions on the sky indicates that
this type of galaxy does not show a preferential location within the cluster,
but the bluest and strongest-lined group of k+a's lies in projection towards
the central 1.4 Mpc of Coma and have radial velocities significantly higher
than the cluster mean. We find a striking correlation between the positions of
these young and strong post-starburst galaxies and substructure in the hot
intracluster medium (ICM) identified from {\it XMM-Newton} data, with these
galaxies lying close to the edges of two infalling substructures. This result
strongly suggests that the interaction with the dense ICM could be responsible
for the quenching of the star formation (thus creating the k+a spectrum), and
possibly, for any previous starburst. The evolution with redshift of the
luminosity distribution of k+a galaxies can be explained by a ``downsizing
effect'', with the maximum luminosity/mass of actively star-forming galaxies
infalling onto clusters decreasing at lower redshift. We discuss the possible
physical origin of this downsizing effect and the implications of our results
for current scenarios of environmental effects on the star formation in
galaxies.Comment: 21 pages, 7 figures, to appear in ApJ, version after referee's
change
Disentangling the Dynamical Mechanisms for Cluster Galaxy Evolution
The determination of the dynamical causes of the morphological Butcher-Oemler
(BO) effect, or the rapid transformation of a large population of late-type
galaxies to earlier Hubble types in the rich cluster environment between
intermediate redshifts and the local universe, has been an important unsolved
problem which is central to our understanding of the general problems of galaxy
formation and evolution. In this article, we survey the existing proposed
mechanisms for cluster galaxy transformation, and discuss their relevance and
limitations to the explanation of the morphological BO effect. A new infrared
diagnostic approach is devised to disentangle the relative importance of
several major physical mechanisms to account for the BO effect, and an example
of the first application of this procedure to a single rich, intermediate
redshift galaxy cluster is given to demonstrate the viability of this approach.
The preliminary result of this analysis favors the interaction-enhanced secular
evolution process as the major cause of the cluster-galaxy morphological
transformation. This conclusion is also supported by a wide range of other
published results which are assembled here to highlight their implications on a
coherent physical origin for the morphological BO effect.Comment: Accepted for publication in the PAS
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