2,234 research outputs found
Spectral gradients in central cluster galaxies: further evidence of star formation in cooling flows
We have obtained radial gradients in the spectral features D4000 and Mg2 for
a sample of 11 central cluster galaxies (CCGs). The new data strongly confirm
the correlations between line-strength indices and the cooling flow phenomenon
found in our earlier study. We find that such correlations depend on the
presence and characteristics of emission lines in the inner regions of the
CCGs. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2
plane, with a neat segregation depending on emission-line types and blue
morphology. This sequence can be modelled, using stellar population models with
a normal IMF, by a recent burst of star formation. In CCGs with emission lines,
the gradients in the spectral indices are flat or positive inside the
emission-line regions, suggesting the presence of young stars. Outside the
emission-line regions, and in cooling flow galaxies without emission lines,
gradients are negative and consistent with those measured in CCGs in clusters
without cooling flows and giant elliptical galaxies. Index gradients measured
exclusively in the emission-line region correlate with mass deposition rate. We
have also estimated the radial profiles of the mass transformed into new stars
which are remarkably parallel to the radial behaviour of the mass deposition
rate. A large fraction (probably most) of the cooling flow gas accreted into
the emission-line region is converted into stars. We discuss the evolutionary
sequence suggested by McNamara (1997), in which radio triggered star formation
bursts take place several times during the lifetime of the cooling flow. This
scenario is consistent with the available observations.Comment: 19 pages, 18 PostScript figures, accepted for publication in MNRA
Galaxy Quenching from Cosmic Web Detachment
We propose the Cosmic Web Detachment (CWD) model, a framework to interpret
the star-formation history of galaxies in a cosmological context. The CWD model
unifies several starvation mechanisms known to disrupt or stop star formation
into one single physical framework. Galaxies begin accreting star-forming gas
at early times via a network of primordial filaments, simply related to the
pattern of density fluctuations in the initial conditions. But when
shell-crossing occurs on intergalactic scales, this pattern is disrupted, and
the galaxy detaches from its primordial filaments, ending the accretion of cold
gas. We argue that CWD encompasses known external processes halting star
formation, such as harassment, strangulation and starvation. On top of these
external processes, internal feedback processes such as AGN contribute to stop
in star formation as well.
By explicitly pointing out the non-linear nature of CWD events we introduce a
simple formalism to identify CWD events in N-body simulations. With it we
reproduce and explain, in the context of CWD, several observations including
downsizing, the cosmic star formation rate history, the galaxy mass-color
diagram and the dependence of the fraction of red galaxies with mass and local
density.Comment: 20 pages, accepted for publication in OJA. High-res version:
http://skysrv.pha.jhu.edu/~miguel/Papers/CWD/ms.pd
The Tully-Fisher relation of distant field galaxies
We examine the evolution of the Tully-Fisher relation (TFR) using a sample of
89 field spirals, with 0.1 < z < 1, for which we have measured confident
rotation velocities (Vrot). By plotting the residuals from the local TFR versus
redshift, or alternatively fitting the TFR to our data in several redshift
bins, we find evidence that luminous spiral galaxies are increasingly offset
from the local TFR with redshift, reaching a brightening of -1.0+-0.5 mag, for
a given Vrot, by approximately z = 1. Since selection effects would generally
increase the fraction of intrinsically-bright galaxies at higher redshifts, we
argue that the observed evolution is probably an upper limit.
Previous studies have used an observed correlation between the TFR residuals
and Vrot to argue that low mass galaxies have evolved significantly more than
those with higher mass. However, we demonstrate that such a correlation may
exist purely due to an intrinsic coupling between the Vrot scatter and TFR
residuals, acting in combination with the TFR scatter and restrictions on the
magnitude range of the data, and therefore it does not necessarily indicate a
physical difference in the evolution of galaxies with different Vrot.
Finally, if we interpret the luminosity evolution derived from the TFR as due
to the evolution of the star formation rate (SFR) in these luminous spiral
galaxies, we find that SFR(z) is proportional to (1+z)^(1.7+-1.1), slower than
commonly derived for the overall field galaxy population. This suggests that
the rapid evolution in the SFR density of the universe observed since
approximately z = 1 is not driven by the evolution of the SFR in individual
bright spiral galaxies. (Abridged.)Comment: 14 pages, 10 figures, accepted by MNRA
Maximum power, ecological function and efficiency of an irreversible Carnot cycle. A cost and effectiveness optimization
In this work we include, for the Carnot cycle, irreversibilities of linear
finite rate of heat transferences between the heat engine and its reservoirs,
heat leak between the reservoirs and internal dissipations of the working
fluid. A first optimization of the power output, the efficiency and ecological
function of an irreversible Carnot cycle, with respect to: internal temperature
ratio, time ratio for the heat exchange and the allocation ratio of the heat
exchangers; is performed. For the second and third optimizations, the optimum
values for the time ratio and internal temperature ratio are substituted into
the equation of power and, then, the optimizations with respect to the cost and
effectiveness ratio of the heat exchangers are performed. Finally, a criterion
of partial optimization for the class of irreversible Carnot engines is herein
presented.Comment: 17 pages, 4 figures. Submitted to Energy Convers. Manag
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