2,542 research outputs found
Constraints on the Merger Models of Elliptical Galaxies from their Globular Cluster Systems
The discovery of proto-globular cluster candidates in many current-day
mergers allows us to better understand the possible effects of a merger event
on the globular cluster system of a galaxy, and to foresee the properties of
the end-product. By comparing these expectations to the properties of globular
cluster systems of today's elliptical galaxies we can constrain merger models.
The observational data indicate that i) every gaseous merger induces the
formation of new star clusters, ii) the number of new clusters formed in such a
merger increases with the gas content of the progenitor galaxies.
Low-luminosity (about M_V>-21), disky ellipticals are generally thought to be
the result of a gaseous merger. As such, new globular clusters are expected to
form but have not been detected to date. We investigate various reasons for the
non-detection of sub-populations in low-luminosity ellipticals, i.e. absence of
an old population, absence of a new population, destruction of one of the
populations, and finally, an age-metallicity conspiracy that allows old and new
globular clusters to appear indistinguishable at the present epoch. All of
these possibilities lead us to a similar conclusion, namely that low-luminosity
ellipticals did not form recently (z<1) in a gas-rich merger, and might not
have formed in a major merger of stellar systems at all. High-luminosity
ellipticals do reveal globular cluster sub-populations. However, it is
difficult to account for the two populations in terms of mergers alone, and in
particular, we can rule out scenarios in which the second sub-population is the
product of a recent, gas-poor merger.Comment: 11 pages (MNRAS style, two columns, including 2 figures, mn.sty
included), accepted for publication in the MNRAS, also available at
http://www.ucolick.org/~mkissle
Is demagnetization an efficient optimization method?
Demagnetization, commonly employed to study ferromagnets, has been proposed
as the basis for an optimization tool, a method to find the ground state of a
disordered system. Here we present a detailed comparison between the ground
state and the demagnetized state in the random field Ising model, combing exact
results in and numerical solutions in . We show that there are
important differences between the two states that persist in the thermodynamic
limit and thus conclude that AC demagnetization is not an efficient
optimization method.Comment: 2 pages, 1 figur
Noise-induced memory in extended excitable systems
We describe a form of memory exhibited by extended excitable systems driven
by stochastic fluctuations. Under such conditions, the system self-organizes
into a state characterized by power-law correlations thus retaining long-term
memory of previous states. The exponents are robust and model-independent. We
discuss novel implications of these results for the functioning of cortical
neurons as well as for networks of neurons.Comment: 4 pages, latex + 5 eps figure
Immune Response Modulation by Tumor-Secreted Glycosphingolipids
Although originally considered merely structural components of cellular membranes, glycosphingolipids (GSL) arenow recognized as having critical effects on cellular physiology, including proliferation, differentiation, viraltransformation and ontogenesis. In addition, a vast majority of human cancers have modified GSL compositioncompared to parental normal cells. These modifications may contribute to both tumor survival and exert strikingeffects on anti-tumor immunity. In this review, we discuss mechanisms of immune modulation by tumor-secreted GSL.Fil: Lardone, Ricardo Dante. John Wayne Cancer Institute at Providence Saint John’s Health Center. Santa Monica; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Química Biológica de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Centro de Investigaciones en Química Biológica de Córdoba; ArgentinaFil: Cely, Ingrid. John Wayne Cancer Institute at Providence Saint John’s Health Center. Santa Monica; Estados UnidosFil: Sieling, Peter A.. John Wayne Cancer Institute at Providence Saint John’s Health Center. Santa Monica; Estados UnidosFil: Lee, Delphine. John Wayne Cancer Institute at Providence Saint John’s Health Center. Santa Monica; Estados Unido
The Elliptical Galaxy formerly known as the Local Group: Merging the Globular Cluster Systems
Prompted by a new catalogue of M31 globular clusters, we have collected
together individual metallicity values for globular clusters in the Local
Group. Although we briefly describe the globular cluster systems of the
individual Local Group galaxies, the main thrust of our paper is to examine the
collective properties. In this way we are simulating the dissipationless merger
of the Local Group, into presumably an elliptical galaxy. Such a merger is
dominated by the Milky Way and M31, which appear to be fairly typical examples
of globular cluster systems of spiral galaxies.
The Local Group `Elliptical' has about 700 +/- 125 globular clusters, with a
luminosity function resembling the `universal' one. The metallicity
distribution has peaks at [Fe/H] ~ -1.55 and -0.64 with a metal-poor to
metal-rich ratio of 2.5:1. The specific frequency of the Local Group Elliptical
is initially about 1 but rises to about 3, when the young stellar populations
fade and the galaxy resembles an old elliptical. The metallicity distribution
and stellar population corrected specific frequency are similar to that of some
known early type galaxies. Based on our results, we briefly speculate on the
origin of globular cluster systems in galaxies.Comment: 22 pages, Latex, 4 figures, 5 tables, submitted to A &
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