3,184 research outputs found
The Globular Cluster Luminosity Function and Specific Frequency in Dwarf Elliptical Galaxies
The globular cluster luminosity function, specific globular cluster
frequency, S_N, specific globular cluster mass, T_MP, and globular cluster mass
fraction in dwarf elliptical galaxies are explored using the full 69 galaxy
sample of the HST WFPC2 Dwarf Elliptical Galaxy Snapshot Survey. The GCLFs of
the dEs are well-represented with a t_5 function with a peak at
M_{V,Z}^0(dE,HST) = -7.3 +/- 0.1. This is ~0.3 magnitudes fainter than the GCLF
peaks in giant spiral and elliptical galaxies, but the results are consistent
within the uncertainties. The bright-end slope of the luminosity distribution
has a power-law form with slope alpha = -1.9 +/- 0.1. The trend of increasing
S_N or T_MP with decreasing host galaxy luminosity is confirmed. The mean value
for T_MP in dE,N galaxies is about a factor of two higher than the mean value
for non-nucleated galaxies and the distributions of T_MP in dE,N and dE,noN
galaxies are statistically different. These data are combined with results from
the literature for a wide range of galaxy types and environments. At low host
galaxy masses the distribution of T_MP for dE,noN and dI galaxies are similar.
This supports the idea that one pathway for forming dE,noN galaxies is by the
stripping of dIs. The formation of nuclei and the larger values of T_MP in dE,N
galaxies may be due to higher star formation rates and star cluster formation
efficiencies due to interactions in galaxy cluster environments.Comment: 53 pages, 13 figures, 12 tables, accepted by the Astrophysical
Journa
Empirically modelled Pc3 activity based on solar wind parameters
It is known that under certain solar wind (SW)/interplanetary magnetic
field (IMF) conditions (e.g. high SW speed, low cone angle) the occurrence of
ground-level Pc3–4 pulsations is more likely. In this paper we demonstrate
that in the event of anomalously low SW particle density, Pc3 activity is
extremely low regardless of otherwise favourable SW speed and cone angle. We
re-investigate the SW control of Pc3 pulsation activity through a statistical
analysis and two empirical models with emphasis on the influence of SW
density on Pc3 activity. We utilise SW and IMF measurements from the OMNI
project and ground-based magnetometer measurements from the MM100 array to
relate SW and IMF measurements to the occurrence of Pc3 activity. Multiple
linear regression and artificial neural network models are used in iterative
processes in order to identify sets of SW-based input parameters, which
optimally reproduce a set of Pc3 activity data. The inclusion of SW density
in the parameter set significantly improves the models. Not only the density
itself, but other density related parameters, such as the dynamic pressure of
the SW, or the standoff distance of the magnetopause work equally well in the
model. The disappearance of Pc3s during low-density events can have at least
four reasons according to the existing upstream wave theory: 1. Pausing the
ion-cyclotron resonance that generates the upstream ultra low frequency waves
in the absence of protons, 2. Weakening of the bow shock that implies less
efficient reflection, 3. The SW becomes sub-Alfvénic and hence it is not
able to sweep back the waves propagating upstream with the Alfvén-speed,
and 4. The increase of the standoff distance of the magnetopause (and of the
bow shock). Although the models cannot account for the lack of Pc3s during
intervals when the SW density is extremely low, the resulting sets of optimal
model inputs support the generation of mid latitude Pc3 activity
predominantly through upstream waves
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ELAV links paused Pol II to alternative polyadenylation in the Drosophila nervious system
Alternative polyadenylation (APA) has been implicated in a variety of developmental and disease processes. A particularly dramatic form of APA occurs in the developing nervous system of flies and mammals, whereby various developmental genes undergo coordinate 3' UTR extension. In Drosophila, the RNA-binding protein ELAV inhibits RNA processing at proximal polyadenylation sites, thereby fostering the formation of exceptionally long 3' UTRs. Here, we present evidence that paused Pol II promotes recruitment of ELAV to extended genes. Replacing promoters of extended genes with heterologous promoters blocks normal 3' extension in the nervous system, while extension-associated promoters can induce 3' extension in ectopic tissues expressing ELAV. Computational analyses suggest that promoter regions of extended genes tend to contain paused Pol II and associated cis-regulatory elements such as GAGA. ChIP-seq assays identify ELAV in the promoter regions of extended genes. Our study provides evidence for a regulatory link between promoter-proximal pausing and APA
Empirically modelled Pc3 activity based on solar wind parameters
It is known that under certain solar wind (SW)/interplanetary magnetic field (IMF) conditions (e.g. high SW speed, low cone angle) the occurrence of ground-level Pc3–4 pulsations is more likely. In this paper we demonstrate that in the event of anomalously low SW particle density, Pc3 activity is extremely low regardless of otherwise favourable SW speed and cone angle. We re-investigate the SW control of Pc3 pulsation activity through a statistical analysis and two empirical models with emphasis on the influence of SW density on Pc3 activity. We utilise SW and IMF measurements from the OMNI project and ground-based magnetometer measurements from the MM100 array to relate SW and IMF measurements to the occurrence of Pc3 activity. Multiple linear regression and artificial neural network models are used in iterative processes in order to identify sets of SW-based input parameters, which optimally reproduce a set of Pc3 activity data. The inclusion of SW density in the parameter set significantly improves the models. Not only the density itself, but other density related parameters, such as the dynamic pressure of the SW, or the standoff distance of the magnetopause work equally well in the model. The disappearance of Pc3s during low-density events can have at least four reasons according to the existing upstream wave theory: 1. Pausing the ion-cyclotron resonance that generates the upstream ultra low frequency waves in the absence of protons, 2. Weakening of the bow shock that implies less efficient reflection, 3. The SW becomes sub-Alfvénic and hence it is not able to sweep back the waves propagating upstream with the Alfvén-speed, and 4. The increase of the standoff distance of the magnetopause (and of the bow shock). Although the models cannot account for the lack of Pc3s during intervals when the SW density is extremely low, the resulting sets of optimal model inputs support the generation of mid latitude Pc3 activity predominantly through upstream waves
A solar-wind-driven empirical model of Pc3 wave activity at a mid-latitude location
In this paper we describe the development of two empirical models of Pc3 wave activity observed at a ground station. The models are tasked to predict pulsation intensity at Tihany, Hungary, from the OMNI solar wind data set at 5 min time resolution. One model is based on artificial neural networks and the other on multiple linear regression. Input
parameters to the models are iteratively selected from a larger set of candidate inputs. The optimal set of inputs are solar wind speed, interplanetary magnetic field orientation (via cone angle), proton density and solar zenith angle (representing local time). Solar wind measurements are shifted in time with respect to Pc3 data to account for the propagation time of ULF perturbations from upstream of the bow shock. Both models achieve correlation of about 70 % between measured and predicted Pc3 wave intensity. The timescales at which the most important solar wind parameters influence pulsation intensity are calculated for the first time. We show that solar wind speed influences pulsation intensity at much longer timescales (about 2 days) than cone angle (about 1 h)
Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies
We present the results of a search for clusters in dwarf irregular galaxies
in the Virgo and Fornax Cluster using HST WFPC2 snapshot data. The galaxy
sample includes 28 galaxies, 11 of which are confirmed members of the Virgo and
Fornax clusters. In the 11 confirmed members, we detect 237 cluster candidates
and determine their V magnitudes, V-I colors and core radii. After statistical
subtraction of background galaxies and foreground stars, most of the cluster
candidates have V-I colors of -0.2 and 1.4, V magnitudes lying between 20 and
25th magnitude and core radii between 0 and 6 pc. Using H-alpha observations,
we find that 26% of the blue cluster candidates are most likely HII regions.
The rest of the cluster candidates are most likely massive (>10^4 Msol) young
and old clusters. A comparison between the red cluster candidates in our sample
and the Milky Way globular clusters shows that they have similar luminosity
distributions, but that the red cluster candidates typically have larger core
radii. Assuming that the red cluster candidates are in fact globular clusters,
we derive specific frequencies (S_N) ranging from ~0-9 for the galaxies.
Although the values are uncertain, seven of the galaxies appear to have
specific frequencies greater than 2. These values are more typical of
ellipticals and nucleated dwarf ellipticals than they are of spirals or Local
Group dwarf irregulars.Comment: 46 pages, 14 figures, 3 tables, accepted by AJ. Higher quality PS
version of entire paper available at
http://www.astro.washington.edu/seth/dirr_gcs.htm
Numerical simulations of a field emitter-based extractor gauge for pressure measurements in cryogenic vacuum systems
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