Searcharxiv⌕ Search

arXiv subjects

Raul Jimenez

Publications and source records attributed to Raul Jimenez.

At least 217 records · Page 12Linked to original sources
arXiv2001

Constraining Cosmological Parameters Based on Relative Galaxy Ages

We propose to use relative galaxy ages as a means of constraining cosmological parameters. By measuring the age difference between two ensembles of old galaxies at somewhat different redshifts, one could determine the derivative of redshift with respect to cosmic time, dz/dt. At high redshifts, z=1-2, this measurement would constrain the equation-of-state of the dark energy, while at low redshifts, z< 0.2, it would determine the Hubble constant, H_0. The selected galaxies need to be passively-evolving on a time much longer than their age difference.

astro-ph↗
arXiv2001

Energetics of Gamma Ray Bursts

We determine the distribution of total energy emitted by gamma-ray bursts for bursts with fluences and distance information. Our core sample consists of eight bursts with BATSE spectra and spectroscopic redshifts. We extend this sample by adding four bursts with BATSE spectra and host galaxy R magnitudes. From these R magnitudes we calculate a redshift probability distribution; this method requires a model of the host galaxy population. From a sample of ten bursts with both spectroscopic redshifts and host galaxy R magnitudes (some do not have BATSE spectra) we find that the burst rate is proportional to the galaxy luminosity at the epoch of the burst. Assuming that the total energy emitted has a log-normal distribution, we find that the average emitted energy (assumed to be radiated isotropically) is $ = 1.3^{+1.2}_{-1.0} \times 10^{53}$ ergs (for H$_0$ = 65 km s$^{-1}$ Mpc$^{-1}$, $Ω_m=0.3$ and $Ω_Λ=0.7$); the distribution has a logarithmic width of $σ_γ=1.7^{+0.7}_{-0.3}$. The corresponding distribution of X-ray afterglow energy (for seven bursts) has $ = 4.0^{+1.6}_{-1.8} \times 10^{51}$ergs and $σ_X = 1.3^{+0.4}_{-0.3}$. For completeness, we also provide spectral fits for all bursts with BATSE spectra for which there were afterglow searches.

astro-ph↗
arXiv2001

Tests for primordial non-Gaussianity

We investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure (LSS), high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness.

astro-ph↗
arXiv2001

Galactosynthesis Predictions at High Redshift

We predict the Tully-Fisher (TF) and surface-brightness--magnitude relation for disk galaxies at z=3 and discuss the origin of these scaling relations and their scatter. We show that the variation of the TF relation with redshift can be a potentially powerful discriminator of galaxy-formation models. In particular, the TF relation at high redshift might be used to break parameter degeneracies among galactosynthesis models at z=0, as well as to constrain the redshift distribution of collapsing dark-matter halos, the star-formation history and baryon fraction in the disk, and the distribution of halo spins.

astro-ph↗
arXiv2001

Recovering Physical Parameters from Galaxy Spectra Using MOPED

We derive physical parameters of galaxies from their observed spectrum, using MOPED, the optimized data compression algorithm of Heavens, Jimenez & Lahav 2000. Here we concentrate on parametrising galaxy properties, and apply the method to the NGC galaxies in Kennicutt's spectral atlas. We focus on deriving the star formation history, metallicity and dust content of galaxies. The method is very fast, taking a few seconds of CPU time to estimate 17 parameters, and so specially suited to study of large data sets, such as the Anglo-Australian 2 degree field galaxy survey and the Sloan Digital Sky Survey. Without the power of MOPED, the recovery of star formation histories in these surveys would be impractical. In the Kennicutt atlas, we find that for the spheroidals a small recent burst of star formation is required to provide the best fit to the spectrum. There is clearly a need for theoretical stellar atmospheric models with spectral resolution better than 1Åif we are to extract all the rich information that large redshift surveys contain in their galaxy spectra.

astro-ph↗
arXiv2000

The Sun, stellar-population models, and the age estimation of high-redshift galaxies

Given sufficiently deep optical spectroscopy, the age estimation of high-redshif t ($z > 1$) galaxies has been claimed to be a relatively robust process (e.g. Dunlop et al. 1996) due to the fact that, for ages $< 5$Gyr, the near-ultraviolet light of a stellar population is expected to be dominated by `well-understood' main-sequence (MS) stars. Recently, however, the reliability of this process has been called into question by Yi et al (2000), who claim to have developed models in which the spectrum produced by the main sequence reddens much more rapidly than in the models of Jimenez et al (2000a), leading to much younger age estimates for the reddest known high-redshift ellipticals. In support of their revised age estimates, Yi et al cite the fact that their models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the solar spectrum is not reproduced by the Jimenez et al models until $\simeq 10$ Gyr. Here we confirm this discrepancy, but point out that this is in fact a {\it strength} of the Jimenez et al models and indicative of some flaw in the models of Yi et al (which, in effect, imply that the Sun will turn into a red giant any minute now). We have also explored the models of Worthey (1994) (which are known to differ greatly from those of Jimenez et al in the treatment of post-MS evolution) and find that the main-sequence component of Worthey's models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We conclude that either the models of Yi et al are not as main-sequence dominated at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these models is in error by a factor possibly as high as two. (abridged)

astro-ph↗
arXiv2000

Where are the absorbers towards Q2302+029?

We present images and spectroscopy of objects close to the sightline of Q2302+029 in order to search for galaxies responsible for the remarkable z=0.7 high-ionization absorption line system found by Jannuzi et al (1996). This system shows `normal' narrow OVI, NV, and CIV lines superimposed on broader (3,000-5,000 km/s wide), unsaturated absorption troughs some 56,000 km/s away from the QSO emission redshift (z=1.052). Despite reaching sensitivities sufficient to detect 1/10 L* galaxies in the optical and 1/20 L* in the infra-red, we are unable to detect any obvious bright galaxies which might be responsible for the absorption beyond ~ 6 h-1 kpc of the sightline. This supports the hypotheses that the absorption is either intrinsic to the QSO, or arises in intracluster gas. Adopting either explanation is problematic: in the first case, `associated' absorption at such high ejection velocities is hard to understand, and challenges the conventional discrimination between intrinsic and intervening absorbers; in the second case, the gas must reside in a ~ 40 h-1 Mpc long filament aligned along the line of sight in order to reproduce the broad absorption. Spectroscopy of objects beyond the immediate vicinity of the QSO sightline reveals a galaxy cluster at z=0.59, which coincides with strong Lya and more narrow high ionization lines in the quasar spectrum. Here too, the lack of galaxies at distances comparable to those found for, e.g., Lya-absorbing galaxies, suggests that the absorption may arise from intracluster gas unassociated with any individual galaxies.

astro-ph↗
arXiv2000

Galactosynthesis: Halo Histories, Star Formation, and Disks

We investigate the effects of a variety of ingredients that must enter into a realistic model for disk-galaxy formation, focusing primarily on the Tully-Fisher (TF) relation and its scatter in several wavebands. Our main findings are: (a) the slope, normalization, and scatter of the TF relation across various wavebands is determined {\em both} by halo properties and star formation in the disk; (b) TF scatter owes primarily to the spread in formation redshifts. The scatter can be measurably reduced by chemical evolution, and also in some cases by the weak anti-correlation between peak height and spin; (c) multi-wavelength constraints can be important in distinguishing between models which appear to fit the TF relation in I or K; (d) successful models seem to require that the bulk of disk formation cannot occur too early (z>2) or too late (z<0.5), and are inconsistent with high values of $Ω_0$; (e) a realistic model with the above ingredients can reasonably reproduce the observed z=0 TF relation in {\em all} bands (B, R, I, and K). It can also account for the z=1 B-band TF relation and yield rough agreement with the local B and K luminosity functions and B-band surface-brightness--magnitude relation. The remarkable agreement with observations suggests that the amount of gas that is expelled or poured into a disk galaxy must be small, and that the specific angular momentum of the baryons must roughly equal that of the halo; there is little room for angular momentum transfer. In an appendix we present analytic fits to stellar-population synthesis models.

astro-ph↗
arXiv2000

Massive Lossless Data Compression and Multiple Parameter Estimation from Galaxy Spectra

We present a method for radical linear compression of datasets where the data are dependent on some number $M$ of parameters. We show that, if the noise in the data is independent of the parameters, we can form $M$ linear combinations of the data which contain as much information about all the parameters as the entire dataset, in the sense that the Fisher information matrices are identical; i.e. the method is lossless. We explore how these compressed numbers fare when the noise is dependent on the parameters, and show that the method, although not precisely lossless, increases errors by a very modest factor. The method is general, but we illustrate it with a problem for which it is well-suited: galaxy spectra, whose data typically consist of $\sim 10^3$ fluxes, and whose properties are set by a handful of parameters such as age, brightness and a parametrised star formation history. The spectra are reduced to a small number of data, which are connected to the physical processes entering the problem. This data compression offers the possibility of a large increase in the speed of determining physical parameters. This is an important consideration as datasets of galaxy spectra reach $10^6$ in size, and the complexity of model spectra increases. In addition to this practical advantage, the compressed data may offer a classification scheme for galaxy spectra which is based rather directly on physical processes.

astro-ph↗
arXiv2000

The abundance of high-redshift objects as a probe of non-Gaussian initial conditions

The observed abundance of high-redshift galaxies and clusters contains precious information about the properties of the initial perturbations. We present a method to compute analytically the number density of objects as a function of mass and redshift for a range of physically motivated non-Gaussian models. In these models the non-Gaussianity can be dialed from zero and is assumed to be small. We compute the probability density function for the smoothed dark matter density field and we extend the Press and Schechter approach to mildly non-Gaussian density fields. The abundance of high-redshift objects can be directly related to the non-Gaussianity parameter and thus to the physical processes that generated deviations from the Gaussian behaviour. Even a skewness parameter of order 0.1 implies a dramatic change in the predicted abundance of $z\gap 1$ objects. Observations from NGST and X-ray satellites (XMM) can be used to accurately measure the amount of non-Gaussianity in the primordial density field.

astro-ph↗
arXiv1999

Primordial Proto-Galaxies

The first stars to form in protogalaxies must have primordial chemical composition. We refer to a protogalaxy that is forming stars of primordial composition, or very low metallicity ($Z\le 0.01 Z_{\odot}$), as ``Primordial Proto-Galaxy'' (PPG). PPGs contain little or no dust, and therefore their spectral energy distribution can be modelled from the rest-frame ultraviolet to the infrared without accounting for dust extinction and emission. We present the results of computing the photometric properties of high redshift PPGs at near-d-to-mid infrared wavelengths, that will soon be available with the new generation of infrared space telescopes, such as SIRTF and the NGST. We show that: i) PPGs at very high redshift (5<z<10) should be easily selected from deep near/mid IR surveys with a colour-colour criterion; ii) PPGs at redshift 5<z<10 can be detected at 8 $μ$m with the NGST, if they have a constant star formation rate of at least 100 M$_{\odot}/$yr; iii) once the redshift of a PPG photometric candidate is determined, its near-to-mid infrared colors should provide strong constraints on the stellar IMF at zero or very low metallicity.

astro-ph↗
arXiv1999

Proto-Galactic Starbursts at High Redshift

We have computed the evolving ultraviolet-millimeter spectral energy distribution (SED) produced by proto-galactic starbursts at high redshift, incorporating the chemical evolution of the interstellar medium in a consistent manner. Dust extinction is calculated in a novel way, that is not based on empirical calibrations of extinction curves, but rather on the lifetime of molecular clouds which delays the emergence of each successive generation of stars at ultraviolet wavelengths by typically 15 Myr. The predicted rest-frame far-infrared to millimeter-wave emission includes the calculation of molecular emission-line luminosities ($^{12}$CO and O$_{2}$ among other molecules) consistent with the evolving chemical abundances. Here we present details of this new model, along with the results of comparing its predictions with several high-redshift observables, namely: the ultraviolet SEDs of Lyman-limit galaxies, the high-redshift radiogalaxies 4C41.17 and 8C1435, the SCUBA sub-millimeter survey of the Hubble Deep Field (HDF), and the SEDs of intermediate-redshift elliptical galaxies. With our new reddening method, we are able to fit the spectrum of the Lyman-limit galaxy 1512-cB58, and we find an extinction of about 1.9 magnitudes at 1600 Å. The model also predicts that most Lyman-limit galaxies should have a value of $α$ inside that range, as it is observed. The 850 $μ$m flux density of a typical Lyman-limit galaxy is expected to be only $\simeq 0.5$ mJy, and therefore the optical counterparts of the most luminous sub-mm sources in the HDF (or any other currently feasible sub-mm survey) are unlikely to be Lyman-break galaxies.

astro-ph↗
arXiv1999

Overmerging and M/L ratios in phenomenological galaxy formation models

We show that the discrepancy between the Tully-Fisher relation and the luminosity function predicted by most phenomenological galaxy formation models is mainly due to overmerging of galaxy haloes. We have circumvented this overmerging problem, which is inherent in both the Press-Schechter formalism and dissipationless N-body simulations, by including a specific galaxy halo formation recipe into an otherwise standard N-body code. This numerical technique provides the merger trees which, together with simplified gas dynamics and star formation physics, constitute our implementation of a phenomenological galaxy formation model. Resolving the overmerging problem provides us with the means to match both the I-band Tully-Fisher relation and the B and K band luminosity functions within an EdS sCDM structure formation scenario. It also allows us to include models for chemical evolution and starbursts, which improves the match to observational data and renders the modelling more realistic. We show that the inclusion of chemical evolution into the modelling requires a significant fraction of stars to be formed in short bursts triggered by merging events.

astro-ph↗
arXiv1998

Premature dismissal of high-redshift elliptical galaxies

It has recently been argued that single-collapse high-redshift models for elliptical galaxy formation can be rejected because they predict large numbers of very red galaxies at intermediate redshifts which are not seen in deep optical-infrared surveys. We argue, however, that this conclusion is premature since, while much effort has been invested in refining the predictions of hierarchical CDM models, only very simplistic models have been used to study the evolution of galaxies in other cosmogonies (e.g. isocurvature models). We demonstrate that the use of a more realistic multi-zone chemo-dynamical single-collapse model, yields colours at intermediate redshifts which are much bluer than inferred from the one-zone model, and indeed are comparable to those predicted by hierarchical merging despite still allowing $> 90%$ of the final stellar mass of elliptical galaxies to be formed in the first Gyr of their evolution. We, therefore, conclude that the one-zone model should be avoided to predict the colours of high-redshift galaxies and that the use of realistic multi-zone models allows the existence of ellipticals at high redshift, being their dismissal premature.

astro-ph↗
arXiv1998

On the origin of Damped Lyman-alpha systems: a case for LSB galaxies?

We use a model of galaxy disk formation to explore the metallicities, dust content, and neutral-gas mass density of damped Lyman-$α$ (D\lya) absorbers. We find that the [Zn/H] abundance measurements of D\lya systems now available can be reproduced either by a population of low surface brightness (LSB) galaxies forming at redshifts $z > 3$, whose chemical contents evolve slowly with time and whose star formation rates are described by continuous bursts, or by high surface brightness (HSB) galaxies which form continuously over an interval of $z \sim 0.5-3$ (and no higher). Although, in reality, a mixture of galaxy types may be responsible for low-z D\lya systems, our models predict that HSB galaxies form more dust, more rapidly, than LSB galaxies, and that HSB galaxies may therefore obscure background QSOs and not give rise to D\lya lines, as suggested by other researchers. Significantly, we find that the rate at which HSB disks consume neutral gas is too fast to explain the observed evolution in the neutral gas mass density with redshift, and that the consumption of hydrogen by LSB galaxies better fits the data. This further suggests that LSB disks may dominate the D\lya population at high-redshift.

astro-ph↗
arXiv1998

Galaxy Formation and Evolution: Low Surface Brightness Galaxies

We investigate in detail the hypothesis that low surface brightness galaxies (LSB) differ from ordinary galaxies simply because they form in halos with large spin parameters. We compute star formation rates using the Schmidt law, assuming the same gas infall dependence on surface density as used in models of the Milky Way. We build stellar population models, predicting colours, spectra, and chemical abundances. We compare our predictions with observed values of metallicity and colours for LSB galaxies and find excellent agreement with all observables. In particular, integrated colours, colour gradients, surface brightness and metallicity match very well to the observed values of LSBs for models with ages larger than 7 Gyr and high values ($λ> 0.05$) for the spin parameter of the halos. We also compute the global star formation rate (SFR) in the Universe due to LSBs and show that it has a flatter evolution with redshift than the corresponding SFR for normal discs. We furthermore compare the evolution in redshift of $[Zn/H]$ for our models to those observed in Damped Lyman $α$ systems by \scite{Pettini+97} and show that Damped Lyman $α$ systems abundances are consistent with the predicted abundances at different radii for LSBs. Finally, we show how the required late redshift of collapse of the halo may constrain the power spectrum of fluctuations.

astro-ph↗
arXiv1997

Hipparcos and the Age of the Galactic Disc

We have used the Hipparcos satellite colour magnitude diagram to determine the age of the Galactic disc. We first measure the metallicities of Hipparcos giants using DDO photometry (Høg & Flynn 1997). With accurate metallicities and distances we compute the minimum age of the Galactic disc as $11\pm1$ Gyr using the colour of the reddest clump stars as a function of metallicity. In conjunction with the new ages derived for Globular Clusters using the same method (Jimenez et al 1996) we show that any delay between the formation of the halo and the disc was only 2-3 Gyr. Since disc galaxies may have been detected to redshifts as high as $z=3.15$ (Lu, Sargent and Barlow 1997) an age for our own Galactic disc of 11 Gyr favours open or non-zero $Λ$ cosmological models. We show that a Reimers mass-loss law is sufficient to explain the morphology of the red clump.

astro-ph↗