9,226 research outputs found
Central Star Formation in Pseudobulges and Classical Bulges
I use Spitzer 3.6-8.0 \mu m color profiles to compare the radial structure of
star formation in pseudobulges and classical bulges. Pseudobulges are
``bulges'' which form through secular evolution, rather than mergers. In this
study, pseudobulges are identified using the presence of disk-like structure in
the center of the galaxy (nuclear spiral, nuclear bar, and/or high ellipticity
in bulge); classical bulges are those galaxy bulges with smooth isophotes which
are round compared to the outer disk, and show no disky structure in their
bulge. I show that galaxies structurally identified as having pseudobulges have
higher central star formation rates than those of classical bulges. Further, I
also show that galaxies identified as having classical bulges have remarkably
regular star formation profiles. The color profiles of galaxies with classical
bulges show a star forming outer disk with a sharp change, consistent with a
decline in star formation rates, toward the center of the galaxy. Classical
bulges have a nearly constant inner profile (r < 1.5 kpc) that is similar to
elliptical galaxies. Pseudobulges in general show no such transition in star
formation properties from the outer disk to the central pseudobulge. Thus I
conclude that pseudobulges and classical bulges do in fact form their stars via
different mechanisms. Further, this adds to the evidence that classical bulges
form most of their stars in fast episodic bursts, in a similar fashion to
elliptical galaxies; whereas, pseudobulges form stars from longer lasting
secular processes.Comment: accepted to ApJ Letter
Low Hubble Constant from Type Ia Supernovae by van den Bergh's Method
An interesting way to calibrate the absolute magnitudes of remote Type Ia
supernovae (SNe Ia) that are well out in the Hubble flow, and thus determine
the value of the Hubble constant, H_0, has been introduced by van den Bergh.
His approach relies on calculations of the peak absolute magnitudes and
broad--band colors for SN Ia explosion models. It does not require any
corrections for extinction by interstellar dust, and no SNe Ia are excluded on
grounds of peculiarity. Within the last few years distances have been
determined to the parent galaxies of six SNe Ia by means of Cepheid variables.
Cepheid--based distances also have become available for three other SNe Ia if
one is willing to use the distance to a galaxy in the same group in lieu of the
distance to the parent galaxy itself. Here we determine the value of H_0 in a
way that is analogous to that of van den Bergh, but now using Cepheid--based
distances instead of calculated light curves. We obtain H_0 = 55 km/s/Mpc. This
value, with Lambda=0 and Omega=1, corresponds to a cosmic expansion time of 12
Gyr, which is consistent with several recent determinations of the ages of
globular clusters.Comment: Latex, 4 pages, 1 table, 1 figure, Submitted to Nature March 28,
1996. PostScript version available at http://www.nhn.ou.edu/~nugent
Automorphisms of real Lie algebras of dimension five or less
The Lie algebra version of the Krull-Schmidt Theorem is formulated and proved. This leads to a method for constructing the automorphisms of a direct sum of Lie algebras from the automorphisms of its indecomposable components. For finite-dimensional Lie algebras, there is a well-known algorithm for finding such components, so the theorem considerably simplifies the problem of classifying the automorphism groups. We illustrate this by classifying the automorphisms of all indecomposable real Lie algebras of dimension five or less. Our results are presented very concisely, in tabular form
Distinct Examples of Immunologic Analysis in Molecular and Cellular Biology: Small Nuclear Ribonucleoproteins and the T Cell Receptor
Since the discovery (Tan and Kunkel, 1966; Mattioli and Peichlin, 1971; Sharp, et al., 1972) of human SLE autoantibodies directed against RNA-protein complexes in eucaryotic nuclei, a great deal of interest has been generated regarding both the relationships of antibody specificities to disease and the biological roles of the recognized antigens. These antibodies are some of the many autoantibody species which have been described in systemic lupus erythematosus (SLE) as well as other rheimatic disease conditions. A common feature of many of these reactivities is the apparent nuclear localization of antigen. Study of the antibodies as well as these antigens has led to formation of a particularly unique bridge between clinical medicine and basic science questions in cellular biology and metabolism. The spectrum of diseases which present with antibodies directed against cellular constituents is vast although certain features of these conditions imply common aspects which may, furthermore, be of some diagnostic use
The Origin of Meteorites: Space Erosion and Cosmic Radiation Ages
particles in the asteroidal belt, coupled with the fact that there is a high mass cut-off for chondrites at about 1000 kg, is sufficient to explain the absence of chondrites with radiation ages greater than 55 x 10^ years. Then if one postulates continuous creation of meteorites by asteroidal collisions, the effect of space erosion is to shift the measured ages toward lower values and to destroy a certain portion of meteorites as a function of their time in space. The total effect, as estimated with several simple but plausible erosion rates, is to duplicate quite nicely the observed shape of the radiation age spectrum. Thus space erosion is not the sole factor in determining the radiation age of a chondrite, but is a factor which grows in importance as the age increases, becoming the sole factor at 55 x 10 years. The model is in agreement with the postulate of a distinct bronzite producing collision 4 million years ago, whose effect is observed on top of the background of continuous collision* It is therefore suggested that both stone and iron meteorites are created by collisional processes in the asteroidal belt
Heuristic Spike Sorting Tuner (HSST), a framework to determine optimal parameter selection for a generic spike sorting algorithm
Extracellular microelectrodes frequently record neural activity from more than one neuron in the vicinity of the electrode. The process of labeling each recorded spike waveform with the identity of its source neuron is called spike sorting and is often approached from an abstracted statistical perspective. However, these approaches do not consider neurophysiological realities and may ignore important features that could improve the accuracy of these methods. Further, standard algorithms typically require selection of at least one free parameter, which can have significant effects on the quality of the output. We describe a Heuristic Spike Sorting Tuner (HSST) that determines the optimal choice of the free parameters for a given spike sorting algorithm based on the neurophysiological qualification of unit isolation and signal discrimination. A set of heuristic metrics are used to score the output of a spike sorting algorithm over a range of free parameters resulting in optimal sorting quality. We demonstrate that these metrics can be used to tune parameters in several spike sorting algorithms. The HSST algorithm shows robustness to variations in signal to noise ratio, number and relative size of units per channel. Moreover, the HSST algorithm is computationally efficient, operates unsupervised, and is parallelizable for batch processing
The Impact of MITF on Melanoma Development – News from Bench and Bedside
Summary In the current issue, two articles highlight the impact of MITF on melanoma development. In the first, Lister et al. (2013) reveal in vivo proof of MITF directly regulating tumor development in BRAFV600E melanomas. In the second, Sturm et al. (2013) present a clinical trial that emphasizes the importance of the recently discovered E318K MITF germline mutation in patients with multiple primary melanomas
DNA bending and the curious case of Fos/Jun
DNA bending has been implicated as an important regulatory mechanism in several processes involving protein—DNA interactions. Various methods for examining intrinsic and protein-induced DNA bending may lead to different conclusions. For the Fos and Jun transcription factors, this has resulted in controversy over whether these factors significantly bend DNA at all
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