627 research outputs found
Revisiting the Scale Length-mu0 Plane and the Freeman Law in the Local Universe
We have used Virtual Observatory technology to analyse the disk scale length
and central surface brightness for a sample of 29955 bright disk galaxies from
the Sloan Digital Sky Survey. We use the results in the r-band and revisit the
relation between these parameters and the galaxy morphology, and find the
average disk surface brightness of 20.2(0.7) mag/arcsec^2. We confirm that late
type spirals populate the lower left corner of the scale length-mu0 plane and
that the early and intermediate spirals are mixed in this diagram, with disky
ellipticals at the top left corner. We further investigate the Freeman Law and
affirm that it indeed defines an upper limit for the disk central surface
brightness in bright galaxies, and that disks in late type spirals have fainter
central surface brightness. Our results are based on a volume corrected sample
of galaxies in the local universe (z < 0.3) that is two orders of magnitudes
larger than any sample previously studied, and deliver statistically
significant implications that provide a comprehensive test bed for future
theoretical studies and numerical simulations of galaxy formation and
evolution.Comment: ApJL, in pres
An N-body/SPH Study of Isolated Galaxy Mass Density Profiles
We investigate the evolution of mass density profiles in secular disk galaxy
models, paying special attention to the development of a two-component profile
from a single initial exponential disk free of cosmological evolution (i.e., no
accretion or interactions). As the source of density profile variations, we
examine the parameter space of the spin parameter, halo concentration, virial
mass, disk mass and bulge mass, for a total of 162 simulations in the context
of a plausible model of star formation and feedback (GADGET-2). The evolution
of the galaxy mass density profile, including the development of a
two-component profile with an inner and outer segment, is controlled by the
ratio of the disk mass fraction, , to the halo spin parameter,
. The location of the break between the two components and speed at
which it develops is directly proportional to ; the amplitude of
the transition between the inner and outer regions is however controlled by the
ratio of halo concentration to virial velocity. The location of the divide
between the inner and outer profile does not change with time. (Abridged)Comment: 27 pages, 31 figures. Accepted for publication at MNRAS. A
high-resolution version of the paper with figures can be found here
http://www.mpia-hd.mpg.de/~foyle/papers/MN-07-1491-MJ.R1.pd
Star formation thresholds and galaxy edges: why and where
We study global star formation thresholds in the outer parts of galaxies by
investigating the stability of disk galaxies embedded in dark halos. The disks
are self-gravitating, contain metals and dust, and are exposed to UV radiation.
We find that the critical surface density for the existence of a cold
interstellar phase depends only weakly on the parameters of the model and
coincides with the empirically derived surface density threshold for star
formation. Furthermore, it is shown that the drop in the thermal velocity
dispersion associated with the transition from the warm to the cold gas phase
triggers gravitational instability on a wide range of scales. The presence of
strong turbulence does not undermine this conclusion if the disk is
self-gravitating. Models based on the hypothesis that the onset of thermal
instability determines the star formation threshold in the outer parts of
galaxies can reproduce many observations, including the threshold radii, column
densities, and the sizes of stellar disks as a function of disk scale length
and mass. Finally, prescriptions are given for implementing star formation
thresholds in (semi-)analytic models and three-dimensional hydrodynamical
simulations of galaxy formation.Comment: 16 pages, 6 figures, accepted for publication in the Astrophysical
Journal. Version 2: text significantly revised (major improvements), physics
unchanged. Version 3: minor correction
The dark matter halo shape of edge-on disk galaxies - II. Modelling the HI observations: methods
This is the second paper of a series in which we attempt to put constraints
on the flattening of dark halos in disk galaxies. For this purpose, we observe
the HI in edge-on galaxies, where it is in principle possible to measure the
force field in the halo vertically and radially from gas layer flaring and
rotation curve decomposition respectively. To calculate the force fields, we
need to analyse the observed XV diagrams to accurately measure all three
functions that describe the planar kinematics and distribution of a galaxy: the
radial HI surface density, the rotation curve and the HI velocity dispersion.
In this paper, we discuss the improvements and limitations of the methods
previously used to measure these HI properties. We extend the constant velocity
dispersion method to include determination of the HI velocity dispersion as a
function of galactocentric radius and perform extensive tests on the quality of
the fits. We will apply this 'radial decomposition XV modelling method' to our
HI observations of 8 HI-rich, late-type, edge-on galaxies in the third paper of
this series.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12566.pd
The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087
We have analysed the globular cluster (GC) system of the nucleated dwarf
elliptical galaxy VCC 1087 in the Virgo cluster, based on Keck/LRIS
spectroscopy and archival HST/ACS imaging. We estimate VCC 1087 hosts a total
population of 77+/-19 GCs, which corresponds to a relatively high V-band
specific frequency of 5.8+/-1.4. The g-z color distribution of the GCs shows a
blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in
color to those seen in luminous ellipticals. Spectroscopy of a subsample of 12
GCs suggests that the GC system is old and coeval (~10 Gyr), with a fairly
broad metallicity distribution (-1.8<[m/H]<-0.8). In contrast, an integrated
spectrum of the underlying galaxy starlight reveals that its optical luminosity
is dominated by metal-rich, intermediate-aged stars. Radial velocities of the
GCs suggest rotation close to the major axis of the galaxy, and this rotation
is dynamically significant with (v/sigma)^* >1. A compilation of the kinematics
of the GC systems of 9 early-type galaxies shows surprising diversity in the
v/sigma parameter for GC systems. In this context, the GC system of VCC 1087
exhibits the most significant rotation to velocity dispersion signature.
Modeling the velocity dispersion profile of the GCs and galaxy stars suggest
fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present
observations can entertain both baryonic and non-baryonic solutions, and GC
velocities at larger radii would be most valuable with regard to this issue. We
discuss the evolution of VCC 1087 in terms of the galaxy ``harassment''
scenario, and conclude that this galaxy may well be the remains of a faded,
tidally perturbed Sc spiral [abridged].Comment: 17 pages, 13 figures, to appear in the A
Truncations of stellar disk and warps of HI-layers in edge-on spiral galaxies
Edge-on spiral galaxies often have stellar disks with relatively sharp
truncations and warped HI-layers in the outer parts. Warps appear to start
preferentially near the optical boundaries of the disks. Here we make a
comparative study of warps and truncations in edge-on galaxies. The
Garc\'{\i}a-Ruiz et al. (2002) sample with extensive HI-mapping is complemented
with luminosity distributions from the Sloan Digital Sky Survey. The method to
identify truncations has been tested using the sample of edge-on galaxies of
van der Kruit & Searle. Results are: (i.) The majority (17 out of 23) of the
galaxies show evidence for truncations. (ii.) When an HI-warp is present it
starts at 1.1 truncation radii. (iii.) This supplements the rules for warps
formulated by Briggs (1990), if the Holmberg radius is replaced for edge-on
systems with the truncation radius. (iv.) The truncation radius and the onset
of the warps coincide radially with features in the rotation curve and the HI
surface density. The latter is also true for less inclined systems. (v.) Inner
disks are very flat and the onset of the warp just beyond the truncation radius
is abrupt and discontinuous. These findings suggest that the inner flat disk
and the outer warped disk are distinct components with quite different
formation histories, probably involving quite different epochs. The inner disk
forms initially and the warped outer disk forms as a result of much later
infall of gas with a higher angular momentum in a different orientation. In an
appendix the Holmberg radius is discussed. Contrary to what is often assumed
Holmberg radii are not corrected for inclination.Comment: Accepted for pubication by Astronomy & Astrophysic
Forecasting chronic mastitis using automatic milking system sensor data and gradient-boosting classifiers
Although most of the losses due to mastitis per case in dairy production are estimated to be caused by clinical cases, subclinical cases, especially chronic, can also be problematic due to milk production losses and the risk of transmission of pathogens. Knowing which subclinical mastitis cases will become chronic at an early stage would be helpful in intervening in these cases. Automatic milking systems (AMS) can collect data on mastitis indicators such as conductivity, Somatic cell count (SCC), and blood in the milk for each milking. The aim of this study was to develop a sensor-based prediction model using SCC, conductivity, blood in the milk, parity, milk diversion, time interval between milkings, milk yield and DIM that forecasts the chronicity in subclinical mastitis cases after an initial increase in SCC. We used sensor data from 14 European and North American dairy farms (with herd sizes of lactating cows ranging from 55 to 638 cows and herd mean parities between 2.00 and 3.19) with an AMS and an online cell counter, measuring SCC. Typically, a threshold of 200,000 SCC/ml has been used to distin- guish cows with subclinical mastitis from healthy cows. We used gradient-boosting trees and sensor data to forecast whether the SCC would decrease structurally below 200,000 SCC/ml in 50 days after the day at which the prediction was performed. Data from 30 and 15 days prior to the day where the forecast was made, was used. The model was trained on data from seven randomly selected dairy farms from the dataset and the data of the remaining seven dairy farms were used to estimate the predictive performance. These results were compared with two approaches that simulate how farmers would diagnose chronic mastitis with a simple prediction rule based on close-to-daily SCC (frequent sampling approach), and on less frequent monthly SCC (monthly sampling approach). We used accuracy, Matthew’s correlation coefficient (MCC), and Area under the Curve (AUC) as metrics to assess the forecasting performance of the chronic mastitis prediction model. On average, the forecast model, using 30 days of sensor data prior to the day of prediction, outperformed the approaches according to the accuracy (chronic mastitis prediction model: 0.888, frequent sampling approach: 0.848, and monthly sampling approach: 0.865), MCC (chronic mastitis prediction model: 0.712, frequent sampling approach: 0.630, and monthly sampling approach: 0.552), and AUC metrics (chronic mastitis prediction model: 0.964 and frequent sampling approach: 0.941) metrics. The results also indicate that shortening the input requirement from 30 days of prior sensor data to 15 days has a limited effect on the performance of the model. Overall, this study shows that it is possible with a high accuracy to predict the future chronic mastitis status using past sensor data and machine learning models
Diagnostic properties of milk diversion and farmer-reported mastitis to indicate clinical mastitis status in dairy cows using Bayesian latent class analysis
The development of digital farming gives bovine mastitis research and management tools access to large datasets. However, the quality of registered data on clinical mastitis cases or treatments may be inadequate (e.g. due to missing records). In automatic milking systems, the decision to divert milk from the bulk milk tank during milking is registered (i.e. milk diversion indicator) for every milking and could potentially indicate a clinical mastitis case. This study accordingly estimated the diagnostic performance of a milk diversion indicator in relation to farmer-recorded clinical mastitis cases in the absence of a “gold standard”. Data on milk diversion and farmer-reported clinical mastitis from 3,443 lactations in 13 herds were analyzed. Each cow lactation was split into 30-DIM periods in which it was registered whether milk was diverted and whether clinical mastitis was reported. One 30-DIM period was randomly sampled for each lactation and this was the unit of analysis, this procedure was repeated 300 times, resulting in 300 datasets to create autocorrelation-robust results during analysis. We used Bayesian latent class analysis to assess the diagnostic properties of milk diversion and farmer-reported clinical status. We analyzed different episode lengths of milk diversion of 1 or more milk diversion days until 10 or more milk diversion days for two scenarios: farmers with poor-quality (51% sensitivity, 99% specificity) and high-quality (90% sensitivity, 99% specificity) mastitis registrations. The analysis was done for all 300 datasets. The results showed that for the scenario where the quality of clinical mastitis reporting was high, the sensitivity was similar for milk-diversion threshold durations of 1–4 days (0.843 to 0.793 versus 0.893). Specificity increased when the number of days of milk diversion increased and was ≥98% at a milk-diversion threshold durations of 8 or more consecutive milk diversion days. In the scenario where the quality of clinical mastitis reporting was low, the sensitivity of milk diversion and reported clinical mastitis cases was similar at milk-diversion threshold durations of 1–7 days (0.687 to 0.448 versus 0.503 to 0.504) while specificity exceeded the 98% at milk-diversion threshold durations of 7 or more consecutive milk diversion days. In both scenarios, a milk diversion threshold duration of 4–7 days achieved the most desirable combined sensitivity and specificity. This study concluded that milk diversion can be a valid alternative to farmer-reported clinical mastitis as it performs similarly in indicating actual clinical mastitis
Radial Profiles of Star Formation in the Far Outer Regions of Galaxy Disks
Star formation in galaxies is triggered by a combination of processes,
including gravitational instabilities, spiral wave shocks, stellar compression,
and turbulence compression. Some of these persist in the far outer regions
where the column density is far below the threshold for instabilities, making
the outer disk cutoff somewhat gradual. We show that in a galaxy with a single
exponential gas profile the star formation rate can have a double exponential
with a shallow one in the inner part and a steep one in the outer part. Such
double exponentials have been observed recently in the broad-band intensity
profiles of spiral and dwarf Irregular galaxies. The break radius in our model
occurs slightly outside the threshold for instabilities provided the Mach
number for compressive motions remains of order unity to large radii. The ratio
of the break radius to the inner exponential scale length increases for higher
surface brightness disks because the unstable part extends further out. This is
also in agreement with observations. Galaxies with extended outer gas disks
that fall more slowly than a single exponential, such as 1/R, can have their
star formation rate scale approximately as a single exponential with radius,
even out to 10 disk scale lengths. Halpha profiles should drop much faster than
the star formation rate as a result of the rapidly decreasing ambient density.Comment: To appear in ApJ. Available from
ftp.lowell.edu/pub/dah/papers/sfouterdisks
The Extended Main-Sequence Turn-off Clusters of the Large Magellanic Cloud - Missing links in Globular Cluster Evolution
Recent observations of intermediate age (1 - 3 Gyr) massive star clusters in
the Large Magellanic Cloud (LMC) have revealed that the majority possess
bifurcated or extended main-sequence turn-off (EMSTO) morphologies. This effect
can be understood to arise from subsequent star formation amongst the stellar
population with age differences between constituent stars amounting to 50 - 300
Myr. Age spreads of this order are similarly invoked to explain the light
element abundance variations witnessed in ancient globular clusters. In this
paper we explore the proposition that the clusters exhibiting the EMSTO
phenomenon are a general phase in the evolution of massive clusters, one that
naturally leads to the particular chemical properties of the ancient globular
cluster population.
We show that the isolation of EMSTO clusters to intermediate ages is the
consequence of observational selection effects. In our proposed scenario, the
EMSTO phenomenon is identical to that which establishes the light element
abundance variations that are ubiquitous in the ancient globular cluster
population. Our scenario makes a strong prediction: EMSTO clusters will exhibit
abundance variations in the light elements characteristic of the ancient GC
population.Comment: ApJ accepted. 33 pages, 5 figure
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