arXiv subjects

R. Jimenez

Publications and source records attributed to R. Jimenez.

25 records · Page 2Linked to original sources

arXiv1998

Galaxy Evolution, Deep Galaxy Counts and the Near-IR Cosmic Infrared Background

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 $Ω, Ω_Λ$ and $H_0$. We find good match to the metallicity data from the damped $L_α$ systems and the evolution of the luminosity density out to $z\simeq 1$. Likewise, our models provide good fits for low values of $Ω$ 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.

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arXiv1998

Old high-redshift galaxies and primordial density fluctuation spectra

We have discovered a population of extremely red galaxies at $z\simeq 1.5$ which have apparent stellar ages of $\gs 3$ Gyr, based on detailed spectroscopy in the rest-frame ultraviolet. In order for galaxies to have existed at the high collapse redshifts indicated by these ages, there must be a minimum level of power in the density fluctuation spectrum on galaxy scales. This paper compares the required power with that inferred from other high-redshift populations. If the collapse redshifts for the old red galaxies are in the range $z_c\simeq 6$ -- 8, there is general agreement between the various tracers on the required inhomogeneity on 1-Mpc scales. This level of small-scale power requires the Lyman-limit galaxies to be approximately $ν\simeq 3.0$ fluctuations, implying a very large bias parameter $b\simeq 6$. The high collapse redshifts of the red galaxies as deduced from gravitational collapse provides independent support for the ages estimated from their stellar populations. Such early-forming galaxies are rare, and their contribution to the cosmological stellar density is consistent with an extrapolation to higher redshifts of the star-formation rate measured at $z<5$; there is no evidence for a general era of spheroid formation at extreme redshifts.

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arXiv1997

The ages and distances of globular clusters with the luminosity function method: the case of M5 and M55

We present new age and distance determinations for the Galactic Globular Clusters M55 and M5, using the luminosity function method (Jimenez & Padoan 1996, Padoan & Jimenez 1997). We find an age of $11.8 \pm 1.5$ Gyr for M55 and $11.1 \pm 0.7$ Gyr for M5. This confirms previous results (Jimenez et al. 1996, Sandquist et al. 1996) and allows to conclude that the oldest stars in the Universe are not older than 14 Gyr. We also find $m-M=14.13 \pm 0.11$ for M55, and $m-M=14.49 \pm 0.06$ for M5. These values agree with the ones obtained using the tip of the red giant branch (Jimenez et al. 1996) and the sub-dwarf fitting method (Sandquist et al. 1996).

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arXiv1996

High-z objects and cold-dark-matter cosmogonies: the case of 53W091

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 $Ω$=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 ($Λ$) and low $Ω$ would decrease the redshift at which this galaxy has collapsed. However, in CDM models decreasing $Ω$ 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.

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arXiv1996

Modified Cold-Dark-Matter models in light of 53W091 an old galaxy at high z

The epoch of galaxy formation provides an important additional test of cosmological theories. Cold-dark-matter (CDM) models with cosmological constant ($Λ$) 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 $Λ$-dominated Universe, the redshift of formation of 53W091 decreases with decreasing $Ω$. However, in the modified CDM models decreasing $Ω$ 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.

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arXiv1996

Ages of globular clusters: a new approach

We have applied a new method to analyze the horizontal branch (HB) morphology in relation to the distribution of stars near the red giant branch (RGB) tip for the globular clusters M22, M5, M68, M107, M72, M92, M3 and 47 Tuc. This new method permits determination of the cluster ages to greater accuracy than conventional isochrone fitting. Using the method in conjunction with our new high-quality photometric data for RGB and HB stars in the first five of these clusters, we discuss the origins of the spread in color on the HB and its relation to the `second parameter' problem. The oldest clusters in our sample are found to have relatively low ages ($13.5 \pm 2$ Gyr). A 1$σ$ uncertainty in each of the parameters of mass and helium content combined with the effects of helium diffusion gives a lower limit for the age of the oldest clusters of 9.7 Gyr.

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arXiv1995

Stellar evolution with mass loss - comparison of numerical and semi-analytical computations.

We present here results of stellar evolution calculations that include the latest advances in radiative opacities and neutrino cooling, and discuss on the basis of these models how the internal stellar structure responds to mass-loss from the stellar surface. This problem has particular importance for the development of semi-analytical algorithms for effi cient calculation of synthetic stellar populations with realistic (and hence complex) mass-loss scenarios. We therefore compare our numerical results with test calculations based on a semi-analytical stellar evolution method developed by us. Although small, but important, differences between results from the two methods are revealed, the evolutionary tracks in the HR-diagram predicted by the two approaches are almost identical.

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