1,595 research outputs found
Star Formation Timescales and the Schmidt Law
We offer a simple parameterization of the rate of star formation in galaxies.
In this new approach, we make explicit and decouple the timescales associated
(a) with disruptive effects the star formation event itself, from (b) the
timescales associated with the cloud assembly and collapse mechanisms leading
up to star formation. The star formation law in near-by galaxies, as measured
on sub-kiloparsec scales, has recently been shown by Bigiel et al. to be
distinctly non-linear in its dependence on total gas density. Our
parameterization of the spatially resolved Schmidt-Sanduleak relation naturally
accommodates that dependence. The parameterized form of the relation is rho_* ~
epsilon x rho_g/(tau_s + rho_g ^{-n}), where rho_g is the gas density, epsilon
is the efficiency of converting gas into stars, and rho_g^{-n} captures the
physics of cloud collapse. Accordingly at high gas densities quiescent star
formation is predicted to progress as rho_* ~ rho_g, while at low gas densities
rho_* ~ rho_g^{1+n}, as is now generally observed. A variable efficiency in
locally converting gas into stars as well as the unknown plane thickness
variations from galaxy to galaxy, and radially within a given galaxy, can
readily account for the empirical scatter in the observed (surface density
rather than volume density) relations, and also plausibly account for the noted
upturn in the relation at very high apparent projected column densities.Comment: Accepted to the Astrophysical Joirnal (Letters); 10 pages, 1 figure;
Revised caption is now fully readable. One reference correcte
Regulation of TGF-β1-Induced Pro-Apoptotic Signaling by Growth Factor Receptors and Extracellular Matrix Receptor Integrins in the Liver
Hepatocellular carcinoma (HCC) often arises from chronically diseased livers. Persistent liver inflammation causes the accumulation of excessive extracellular matrix (ECM) proteins and impairs the liver function, finally leading to the development of HCC. A pleiotropic cytokine, transforming growth factor (TGF)-β1, plays critical roles throughout the process of fibrogenesis and hepatocarcinogenesis. In the liver, TGF-β1 inhibits the proliferation of hepatocytes and stimulates the production of ECM from hepatic stellate cells (HSCs) to maintain tissue homeostasis. During disease progression, both growth factors/cytokines and the ECM alter the TGF-β1 signals by modifying the phosphorylation of Smad proteins at their C-terminal and linker regions. TGF-β1 stimulates the expression of integrins, cellular receptors for ECM, along with an increase in ECM accumulation. The activation of integrins by the ECM modulates the response to TGF-β1 in hepatic cells, resulting in their resistance to TGF-β1-induced growth suppression in hepatocytes and the sustained production of ECM proteins in activated HSCs/myofibroblasts. Both growth factor receptors and integrins modify the expression and/or functions of the downstream effectors of TGF-β1, resulting in the escape of hepatocytes from TGF-β1-induced apoptosis. Recent studies have revealed that the alterations of Smad phosphorylation that occur as the results of the crosstalk between TGF-β1, growth factors and integrins could change the nature of TGF-β1 signals from tumor suppression to promotion. Therefore, the modification of Smad phosphorylation could be an attractive target for the prevention and/or treatment of HCC
Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations
In this paper we investigate scaling relations between star formation rates
and molecular gas masses for both local Galactic clouds and a sample of
external galaxies. We specifically consider relations between the star
formation rates and measurements of dense, as well as total, molecular gas
masses. We argue that there is a fundamental empirical scaling relation that
directly connects the local star formation process with that operating globally
within galaxies. Specifically, the total star formation rate in a molecular
cloud or galaxy is linearly proportional to the mass of dense gas within the
cloud or galaxy. This simple relation, first documented in previous studies,
holds over a span of mass covering nearly nine orders of magnitude and
indicates that the rate of star formation is directly controlled by the amount
of dense molecular gas that can be assembled within a star formation complex.
We further show that the star formation rates and total molecular masses,
characterizing both local clouds and galaxies, are correlated over similarly
large scales of mass and can be described by a family of linear star formation
scaling laws, parameterized by , the fraction of dense gas contained
within the clouds or galaxies. That is, the underlying star formation scaling
law is always linear for clouds and galaxies with the same dense gas fraction.
These considerations provide a single unified framework for understanding the
relation between the standard (non-linear) extragalactic Schmidt-Kennicutt
scaling law, that is typically derived from CO observations of the gas, and the
linear star formation scaling law derived from HCN observations of the dense
gas.Comment: 14 pages + 2 figures. Accepted for publication in ApJ 16 December
201
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