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Search Results: 1 - 10 of 602480 matches for " R. A. Jimenez "
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High-z objects and cold-dark-matter cosmogonies: the case of 53W091
A. Kashlinsky,R. Jimenez
Physics , 1996,
Abstract: The recently discovered high redshift galaxy, 53W091, with accurate age measurements (Dunlop et al 1996) provides a measure of the small-scale power of the primordial density field and, as we show, a crucial test of the inflation-inspired models. It allows $\Omega$=1 cosmologies only for low values of $H_0$, but then pushes formation of that galaxy to redshift much greater than allowed for by the cold-dark-matter density field. Cold-dark-matter (CDM) models with cosmological constant ($\Lambda$) and low $\Omega$ would decrease the redshift at which this galaxy has collapsed. However, in CDM models decreasing $\Omega$ suppresses the small scale power in the density field and this effect turns out to be dominant. We estimate the mass of the galaxy and show that it represents a very rare and unlikely event in the density field of such models. Similar problems would occur in other modifications of the CDM cosmogonies.
Modified Cold-Dark-Matter models in light of 53W091 an old galaxy at high z
A. Kashlinsky,R. Jimenez
Physics , 1996, DOI: 10.1086/310439
Abstract: The epoch of galaxy formation provides an important additional test of cosmological theories. Cold-dark-matter (CDM) models with cosmological constant ($\Lambda$) are designed to account for the observed excess power in galaxy distribution, but at the same time suppress the power on small scales pushing galaxy formation to recent epochs. We point out that the recently discovered high redshift galaxy, 53W091, with accurate age measurements (Dunlop et al 1996) provides a particularly important test of these models. In the flat $\Lambda$-dominated Universe, the redshift of formation of 53W091 decreases with decreasing $\Omega$. However, in the modified CDM models decreasing $\Omega$ suppresses the small scale power in the density field and this effect turns out to be dominant. We estimate the mass of the galaxy and show that it represents a very rare and unlikely event in the density field of such models. Similar problems would occur in other modifications of the CDM cosmogonies.
The role of star formation in the Tully-Fisher law
A. Heavens,R. Jimenez
Physics , 1999, DOI: 10.1046/j.1365-8711.1999.02448.x
Abstract: We investigate the influence of the star formation rate on the Tully-Fisher relation. We find that a simple model which combines the empirically-determined star-formation rate with the expected properties of galaxy halos provides a remarkably good fit to the absolute magnitude-rotation speed correlation. We find that the power-law nature, its slope, normalisation and scatter, are all readily accounted for if the Universe has a low density parameter, with or without a cosmological constant and disks are assembled at $z \sim 1 - 1.5$. Moreover, this agreement is found simultaneously in 4 wavebands. An Einstein-de Sitter Universe produces disks which are too faint unless the disks are assembled at $z\sim 0.5$. The scatter in the relation is due to a combination of the expected range of spin parameters of the halos and the range of formation redshifts. The source of the scatter opens up possibilities of a better galaxy distance indicator, if spectroscopic observations of globular clusters can be used to determine the halo rotation.
Galaxy Evolution, Deep Galaxy Counts and the Near-IR Cosmic Infrared Background
R. Jimenez,A. Kashlinsky
Physics , 1998, DOI: 10.1086/306648
Abstract: Accurate synthetic models of stellar populations are constructed and used in evolutionary models of stellar populations in forming galaxies. Following their formation, the late type galaxies are assumed to follow the Schmidt law for star formation, while early type galaxies are normalized to the present-day fundamental plane relations assumed to mimic the metallicity variations along their luminosity sequence. We then compute predictions of these models for the observational data at early epochs for various cosmological parameters $\Omega, \Omega_\Lambda$ and $H_0$. We find good match to the metallicity data from the damped $L_\alpha$ systems and the evolution of the luminosity density out to $z\simeq 1$. Likewise, our models provide good fits for low values of $\Omega$ to the deep number counts of galaxies in all bands where data is available; this is done without assuming existence of extra populations of galaxies at high $z$. Our models also match the data on the redshift distribution of galaxy counts in $B$ and $K$ bands. We compute the predicted mean levels and angular distribution of the cosmic infrared background produced from the early evolution of galaxies. The predicted fluxes and fluctuations are still below the current observational limits, but not by a large factor. Finally, we find that the recent detection of the diffuse extragalactic light in the visible bands requires for our models high redshift of galaxy formation, $z_f \geq$(3-4); otherwise the produced flux of the extragalactic light at optical bands exceeds the current observational limits.
On the solar origin of the signal at 220.7microHz: A possible component of a g mode?
A. Jimenez,R. A. Garcia
Physics , 2009, DOI: 10.1088/0067-0049/184/2/288
Abstract: Gravity modes in the Sun have been the object of a long and difficult search in recent decades. Thanks to the data accumulated with the last generation of instruments (BiSON, GONG and three helioseismic instruments aboard SoHO), scientists have been able to find signatures of their presence. However, the individual detection of such modes remains evasive. In this article, we study the signal at 220.7 microHz which is a peak that is present in most of the helioseismic data of the last 10 years. This signal has already been identified as being one component of a g-mode candidate detected in the GOLF Doppler velocity signal. The nature of this peak is studied in particular using the VIRGO/SPM instrument aboard SoHO. First we analyse all the available instrumental data of VIRGO and SoHO (housekeeping) to reject any possible instrumental origin. No relation was found, implying that the signal has a solar origin. Using Monte Carlo simulations, we find, with more than 99% confidence level, that the signal found in VIRGO/SPM is very unlikely to be due to pure noise.
Differences of the Solar Magnetic Activity Signature in Velocity and Intensity Helioseismic Observations
D. Salabert,R. A. Garcia,A. Jimenez
Physics , 2013,
Abstract: The high-quality, full-disk helioseismic observations continuously collected by the spectrophotometer GOLF and the three photometers VIRGO/SPMs onboard the SoHO spacecraft for 17 years now (since April 11, 1996, apart from the SoHO "vacations") are absolutely unique for the study of the interior of the Sun and its variability with magnetic activity. Here, we look at the differences in the low-degree oscillation p-mode frequencies between radial velocity and intensity measurements taking into account all the known features of the p-mode profiles (e.g., the opposite peak asymmetry), and of the power spectrum (e.g., the presence of the higher degrees l= 4 and 5 in the signal). We show that the intensity frequencies are higher than the velocity frequencies during the solar cycle with a clear temporal dependence. The response between the individual angular degrees is also different. Time delays are observed between the temporal variations in GOLF and VIRGO frequencies. Such analysis is important in order to put new constraints and to better understand the mechanisms responsible for the temporal variations of the oscillation frequencies with the solar magnetic activity as well as their height dependences in the solar atmosphere. It is also important for the study of the stellar magnetic activity using asteroseismic data.
Helioseismic inferences of the solar cycles 23 and 24: GOLF and VIRGO observations
D. Salabert,R. A. Garcia,A. Jimenez
Physics , 2014,
Abstract: The Sun-as-a-star helioseismic spectrophotometer GOLF and photometer VIRGO instruments onboard the SoHO spacecraft are collecting high-quality, continuous data since April 1996. We analyze here these unique datasets in order to investigate the peculiar and weak on-going solar cycle 24. As this cycle 24 is reaching its maximum, we compare its rising phase with the rising phase of the previous solar cycle 23.
Supersymmetric Electroweak Renormaliztion of the Z-Width in the MSSM (II)
D. Garcia,R. A. Jimenez,J. Sola
Physics , 1994, DOI: 10.1016/0370-2693(94)00407-X
Abstract: We address the computation of \Gamma_Z and of the intriguing quantity R_b in the MSSM including full treatment of the Higgs sector. For a pseudoscalar Higgs mass m_{A^0}>70\,GeV and CDF limits on m_t, the bounds on R_b at 1\sigma level leave no room to the MSSM to solve the `R_b crisis' for any combination of the parameters, not even admitting the possibility of a light chargino and a light stop of O(50) GeV; however, for m_t not restricted by CDF, a `tangential' solution exists in the window 2<\tan\beta<10 with a light chargino and stop. In contrast, for a pseudoscalar mass 40\,GeVm_t/m_b. Our general conclusion is that, if there is a `R_b crisis' at all, its solution within the MSSM has to do more with the peculiar structure of the SUSY Higgs sector rather than with the spectrum of genuine supersymmetric particles. In view of the range predicted for m_{A^0}, LEP 200 should be able to definitely settle down this question.
Supersymmetric Electroweak Renormalization of the Z-Width in the MSSM (I)
D. Garcia,R. A. Jimenez,J. Sola
Physics , 1994, DOI: 10.1016/0370-2693(95)00031-F
Abstract: Within the framework of the MSSM, we compute the complete set of electroweak one-loop supersymmetric quantum effects on the width $\Gamma_Z$ of the $Z$-boson in the on-shell renormalization scheme. Numerical analyses of the corrections to the various partial widths into leptons and quarks are presented. On general grounds, the average size of the electroweak SUSY corrections to $\Gamma_Z$ may well saturate the level of the present theoretical uncertainties, even if considering the full supersymmetric spectrum lying in the neighbourhood of the unaccessible LEP 200 range. Remarkably enough, for the present values of the top quark mass, the electroweak SUSY effects could be, globally, very close or even bigger than the electroweak SM corrections, but opposite in sign. Therefore, in the absence of theoretical errors, there are large regions of parameter space where one could find that, effectively, the electroweak SM corrections are ``missing'', or even having the ``wrong'' sign. This should be helpful in discriminating between the SM and the MSSM. However, an accurate prediction of the electroweak quantum effects on $\Gamma_Z$ will only be possible, if $\Delta r$ and $\alpha_s$ are pinned down in the future with enough precision.
The extended minimum of solar cycle 23 as seen by radial velocity (GOLF, GONG) and intensity (VIRGO) helioseismic instruments
D. Salabert,R. A. Garcia,P. L. Palle,S. J. Jimenez-Reyes,A. Jimenez
Physics , 2010,
Abstract: We present an analysis of the variability of the solar oscillation spectrum during solar cycle 23 and its extended minimum. We use simultaneous observations of the low-degree solar p modes collected by the space-based, Sun-as-a-star GOLF (radial velocity) and VIRGO (intensity) instruments, and by the ground-based, multi-site network GONG. We investigate in particular the response of the p-mode eigenfrequencies to the observed peculiar deep solar minimum of surface activity of 2007-2009 as compared with the previous solar cycle 23. We study the different temporal variations of the p-mode frequencies with individual angular degrees.
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