489 research outputs found
Postnatal Developmental Trends in Membrane Excitability and BK Channel Function in the Rodent Hippocampus
This dissertation outlines the postnatal development of excitability as well as expression and function of BK potassium channels in hippocampal neurons. I used patch clamp electrophysiology to measure how neuronal action potential waveforms and action potential firing frequencies change in early development, and how pharmacological blockade of BK channels affects these properties in hippocampal neurons. I also describe how the protein expression of the BK channel pore-forming α subunit and mRNA expression of different variants of the pore forming α subunit and auxiliary beta-4 subunit changes with development. I demonstrate in both cultured rat hippocampal neurons across the first seven postnatal days and in putative mouse hippocampal pyramidal neurons from postnatal day four to fifteen in acutely prepared slices that maturation brings changes in action potential kinetics and large increases in the capacity for high frequency firing. In the cultured neurons I demonstrate that during the first postnatal week, the contribution of BK channels to action potential repolarization decreases but the channel’s role in maintaining high-frequency firing increases. This change in the timing of BK channel activity in the action potential waveform is accompanied by a slowing of BK current onset measured by a decreasing effect of BK current blockade on potassium current rise time. Additionally, I demonstrated in pyramidal neurons in mouse brain slices that there is a loss of BK channel contribution to action potential repolarization between postnatal day four and fifteen. These changes in the BK channel’s role in neuronal action potential firing are accompanied by large increases in the expression of α subunit protein measured by Western blot and by large increases in mRNA transcript expression, measured by RT-qPCR, of both the α and beta-4 subunits in the hippocampus. The rise in beta-4 subunit expression can explain the apparent slowing of BK channel activation through development. I investigated whether BK channel blockade in neonatal brain slices could attenuate hyperexcitability in a model of seizure activity as BK channels in immature neurons share properties of BK channel variants implicated in epilepsy but found no evidence to support this hypothesis
Hubble Space Telescope Images of Stephan's Quintet: Star Cluster Formation in a Compact Group Environment
Analysis of Hubble Space Telescope/Wide Field Planetary Camera 2 images of
Stephan's Quintet, Hickson Compact Group 92, yielded 115 candidate star
clusters (with V-I < 1.5). Unlike in merger remants, the cluster candidates in
Stephan's Quintet are not clustered in the inner regions of the galaxies; they
are spread over the debris and surrounding area. Specifically, these sources
are located in the long sweeping tail and spiral arms of NGC 7319, in the tidal
debris of NGC 7318B/A, and in the intragroup starburst region north of these
galaxies. Analysis of the colors of the clusters indicates several distinct
epochs of star formation that appear to trace the complex history of dynamical
interactions in this compact group.Comment: 24 pages, 21 figures (13 PostScript and 8 JPEG), LaTeX (uses
aastexug.sty), accepted for publication in the Astronomical Journal (July
2001). Full-resolution PostScript figures available at
http://www.astro.psu.edu/users/gallsc/sq/figs.tar.g
The Luminosity Function of Galaxies in Compact Groups
From R-band images of 39 Hickson compact groups (HCGs), we use galaxy counts
to determine a luminosity function extending to M_R=-14.0, approximately two
magnitudes deeper than previous compact group luminosity functions. We find
that a single Schechter function is a poor fit to the data, so we fit a
composite function consisting of separate Schechter functions for the bright
and faint galaxies. The bright end is best fit with M^*=-21.6 and alpha=-0.52
and the faint end with M^*=-16.1 and alpha=-1.17. The decreasing bright end
slope implies a deficit of intermediate luminosity galaxies in our sample of
HCGs and the faint end slope is slightly steeper than that reported for earlier
HCG luminosity functions. Furthermore, luminosity functions of subsets of our
sample reveal more substantial dwarf populations for groups with x-ray halos,
groups with tidal dwarf candidates, and groups with a dominant elliptical or
lenticular galaxy. Collectively, these results support the hypothesis that
within compact groups, the initial dwarf galaxy population is replenished by
"subsequent generations" formed in the tidal debris of giant galaxy
interactions.Comment: 26 pages, to be published in The Astrophysical Journal, 8 greyscale
plates (figures 1 and 2) can be retrieved at
http://www.astro.psu.edu/users/sdh/pubs.htm
Prompt optical observations of GRB050319 with the Swift UVOT
The UVOT telescope on the Swift observatory has detected optical afterglow
emission from GRB 050319. The flux declines with a power law slope of alpha =
-0.57 between the start of observations some 230 seconds after the burst onset
(90s after the burst trigger) until it faded below the sensitivity threshold of
the instrument after ~5 x 10^4s. There is no evidence for the rapidly declining
component in the early light curve that is seen at the same time in the X-ray
band. The afterglow is not detected in UVOT shortward of the B-band, suggesting
a redshift of about 3.5. The optical V-band emission lies on the extension of
the X-ray spectrum, with an optical to X-ray slope of beta = -0.8. The
relatively flat decay rate of the burst suggests that the central engine
continues to inject energy into the fireball for as long as a few x 10^4s after
the burst.Comment: 16 pages, 4 figures, accepted by Ap
From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies
Using V and I images obtained with WFPC2/HST, we investigate compact stellar
structures within tidal tails. Six regions of tidal debris in the four classic
``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921,
and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how
the star formation depends upon the local and global physical conditions. These
mergers sample a range of stages in the evolutionary sequence and tails with
and without embedded tidal dwarf galaxies. The six tails are found to contain a
variety of stellar structures, with sizes ranging from those of globular
clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure
the luminosities and colors of the star clusters. NGC 3256 is found to have a
large population of blue clusters (0.2 < V-I < 0.9), particularly in its
Western tail, similar to those found in the inner region of the merger. In
contrast, NGC 4038/39 has no clusters in the observed region of the tail, only
less luminous point sources likely to be individual stars. A significant
cluster population is clearly associated with the prominent tidal dwarf
candidates in the eastern and western tails of NGC 7252. The cluster-rich
Western tail of NGC 3256 is not distinguished from the others by its dynamical
age or by its total HI mass. However, the mergers that have few clusters in the
tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent
tidal dwarfs. We speculate that star formation in tidal tails may manifest
itself either in small structures like clusters along the tail or in large
structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the
highest star formation rate of the four mergers studied, which may contribute
to the high number of star clusters in its tidal tails.Comment: Accepted to Astronomical Journal. 34 pages including 15 figures and 4
table
Ultraviolet, Optical, and X-Ray Observations of the Type Ia Supernova 2005am with Swift
We present ultraviolet and optical light curves in six broadband filters and
grism spectra obtained by Swift's Ultraviolet/Optical Telescope for the Type Ia
supernova SN2005am. The data were collected beginning about four days before
the B-band maximum, with excellent coverage of the rapid decline phase and
later observations extending out to 69 days after the peak. The optical and
near UV light curve match well those of SN1992A. The other UV observations
constitute the first set of light curves shorter than 2500 Angstroms and allow
us to compare the light curve evolution in three UV bands. The UV behavior of
this and other low redshift supernovae can be used to constrain theories of
progenitor evolution or to interpret optical light curves of high redshift
supernovae. Using Swift's X-Ray Telescope, we also report the upper limit to
SN2005am's X-ray luminosity to be 1.77 x 10^40 erg s^-1 in the 0.3--10 keV
range from 58,117 s of exposure time.Comment: 15 pages, including 3 figures and 2 tables, submitted to
Astrophysical Journa
Swift-UVOT detection of GRB 050318
We present observations of GRB 050318 by the Ultra-Violet and Optical
Telescope (UVOT) on-board the Swift observatory. The data are the first
detections of a Gamma Ray Burst (GRB) afterglow decay by the UVOT instrument,
launched specifically to open a new window on these transient sources. We
showcase UVOTs ability to provide multi-color photometry and the advantages of
combining UVOT data with simultaneous and contemporaneous observations from the
high-energy detectors on the Swift spacecraft. Multiple filters covering
1,800-6,000 Angstroms reveal a red source with spectral slope steeper than the
simultaneous X-ray continuum. Spectral fits indicate that the UVOT colors are
consistent with dust extinction by systems at z = 1.2037 and z = 1.4436,
redshifts where absorption systems have been pre-identified. However, the data
can be most-easily reproduced with models containing a foreground system of
neutral gas redshifted by z = 2.8 +/- 0.3. For both of the above scenarios,
spectral and decay slopes are, for the most part, consistent with fireball
expansion into a uniform medium, provided a cooling break occurs between the
energy ranges of the UVOT and Swifts X-ray instrumentation.Comment: 15 pages, 4 figures, ApJ Letters, in pres
- …