arXiv subjects
Publications and source records attributed to Gustavo Bruzual.
We show that Montecarlo simulations of the TP-AGB stellar population in the LMC and SMC galaxies using the CB* models produce LF and color distributions that are in closer agreement with observations than those obtained with the BC03 and CB07 models. This is a progress report of work that will be published elsewhere.
We propose an extension of the GC3 streamer finding method of Johnston et al. 1996 that can be applied to the future Gaia database. The original method looks for streamers along great circles in the sky, our extension adds the kinematical restriction that velocity vectors should also be constrained to lie along these great circles, as seen by a Galactocentric observer. We show how to use these combined criteria starting from heliocentric observables. We test it by using the mock Gaia catalogue of Brown et al. 2005, which includes a realistic Galactic background and observational errors, but with the addition of detailed star formation histories for the simulated satellites. We investigate its success rate as a function of initial satellite luminosity, star formation history and orbit. We find that the inclusion of the kinematical restriction vastly enhances the contrast between a streamer and the background, even in the presence of observational errors, provided we use only data with good astrometric quality (fractional errors of 30 per cent or better). The global nature of the method diminishes the erasing effect of phase mixing and permits the recovery of merger events of reasonable dynamical age. Satellites with a star formation history different to that of the Galactic background are also better isolated. We find that satellites in the range of 10^8-10^9 Lsun can be recovered even for events as old as ~10 Gyr. Even satellites with 4-5x10^7 Lsun can be recovered for certain combinations of dynamical ages and orbits.
This work continues our efforts to calibrate model surface brightness luminosities for the study of unresolved stellar populations, through the comparison with data of Magellanic Cloud star clusters. We present here the relation between absolute K_s-band fluctuation magnitude and (V - I) integrated colour, using data from the 2MASS and DENIS surveys, and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. (2002). We find that intermediate-age to old clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the barM_{K_s} vs. (V - I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V - I_c)_0 < 1.25, ours holds in the interval -5 >= barM_{K_s} >= -9, 0.3 <= (V - I) <= 1.25. This implies, according to Bruzual & Charlot (2003) and Mouhcine & Lancon (2003) models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, there is a slight offset between the galaxies and the star clusters [the latter are ~ 0.7 mag fainter than the former at a given value of (V - I)], caused by the difference in metallicity of roughly a factor of two. The confrontation between models and galaxy data also suggests that galaxies with K_s fluctuation magnitudes that are brighter than predicted, given their (V - I) colour, might be explained in part by longer lifetimes of TP-AGB stars. (Abridged version.)
We develop a new method to constrain the star formation histories, dust attenuation and stellar masses of galaxies. It is based on two stellar absorption line indices, the 4000 Angstrom break strength and the Balmer absorption line index Hdelta_A. Together, these indices allow us to constrain the mean stellar ages of galaxies and the fractional stellar mass formed in bursts over the past few Gyr. A comparison with broad band photometry then yields estimates of dust attenuation and of stellar mass. We generate a large library of Monte Carlo realizations of different star formation histories, including starbursts of varying strength and a range of metallicities. We use this library to generate median likelihood estimates of burst mass fractions, dust attenuation strengths, stellar masses and stellar mass-to-light ratios for a sample of 122,208 galaxies drawn from the Sloan Digital Sky Survey. The typical 95% confidence range in our estimated stellar masses is +-40%. We study how the stellar mass-to-light ratios of galaxies vary as a function of absolute magnitude, concentration index and photometric pass-band and how dust attenuation varies as a function of absolute magnitude and 4000 Angstrom break strength. We also calculate how the total stellar mass of the present Universe is distributed over galaxies as a function of their mass, size, concentration, colour, burst mass fraction and surface mass density. We find that most of the stellar mass in the local Universe resides in galaxies that have stellar masses \~5\times 10^10 M_sol, half light radii ~3 kpc, and half- light surface mass densities ~10^9 M_sol/kpc^2. The distribution of D(4000) is strongly bimodal, showing a clear division between galaxies dominated by old stellar populations and galaxies with more recent star formation.
We present a statistical approach for estimating the age of Globular Clusters by measurement of the likelihood between the observed cluster sequences in the Colour--Magnitude Diagram and the synthetic cluster sequences computed from stellar evolutionary models. In the conventional isochrone fitting procedure, the age is estimated in a subjective way. Here, we calculate the degree of likelihood by applying a modern statistical estimator, the Saha estimator {\it W}, and the interval of confidence from $χ^{2}$ statistics. We apply this approach to sets of three different evolutionary models. Each of these sets consists of different chemical abundances, ages, input physics, etc. Based on our approach, we estimate the age of NGC 6397, M92 and M3. With a confidence level of 99%, we find that the best estimate of the age is 14.0 Gyr within the range of 13.8 to 14.4 Gyr for NGC 6397, 14.75 Gyr within the range of 14.50 to 15.40 Gyr for M92, and 16.0 Gyr within the range of 15.9 to 16.3 Gyr for M3.
We present a statistical approach for determining the age of Globular Clusters (GCs) that allows estimating the age derived from CMDs more accurately than the conventional methods of the isochrone fitting. We measure how closely a set of synthetic CMDs constructed from different evolutionary models resemble the observed ones by determining the likelihood using Saha and Chi-square statistics. The model which best matches the observational data, of a set of plausible ones, is the one with the highest value of the estimator. We apply this method to a set of three different evolutionary models presented by three different authors. Each of theses sets consists again of many different models of various chemical abundances, ages, input physics and there alike. We subsequently derive the age of GCs NGC 6397, M92 and M3. With a confidence level of 99%, we find that the best estimate of the age is 14.0 Gyrs within the range of 13.8 to 14.4 Gyrs for NGC 6397, 14.75 Gyrs within the range of 14.50 to 15.40 Gyrs for M92, and 16.0 Gyrs within the range of 15.9 to 16.3 Gyrs for M3.
Photometric properties of stellar populations that evolve according to chemical evolution models developed by Carigi, Colin, & Peimbert (1999) and Carigi & Peimbert (2001) are explored. Models explain the oxygen abundance and gas mass fraction of irregular galaxies NGC 1560, I Zw 18, NGC 2366, and a typical irregular galaxy. The photometric predictions help to narrow down the range of possible chemical evolution models. Observed colors of I Zw 18 imply an age of 10^8 - 10^9 yr for its dominant population, while 10^9 - 10^{10} yr is an older age preferred by the rest of the galaxy sample. Observed colors are in general, within errors, close to predicted. There is a tendency for the observed (B-V) color to imply higher metallicities at a fixed age than (U-B) (equivalently, (U-B) implies younger ages than (B-V)).
Synthetic spectra for single-aged stellar populations of metallicities [M/H] = -0.5, 0.0 and +0.5, ages = 3 to 17 Gyrs, and initial mass function exponents x = 0.1 to 2.0 were built in the wavelength range 6000-10200 Angstrons. For such we have employed the grid of synthetic spectra described in Schiavon & Barbuy (1999), computed for the stellar parameters 2500 <= Teff <= 6000 K, -0.5 <= log g <= 5.0, [M/H] = -0.5, 0.0 and +0.5, and [alpha/Fe] = 0.0, together with the isochrones by Bertelli et al. (1994) and Baraffe et al. (1998). The behavior of the features NaI8190, CaII8662, TiO6600 and FeH9900 in the integrated spectra of single stellar populations were studied in terms of metallicity, initial mass function and age variations. The main conclusions are that the NaI doublet is an IMF-sensitive feature, which is however sensitive also to metallicity and age, whereas TiO, CaII and FeH are very sensitive to metallicity and essentially insensitive to IMF and age.
Empirical evidence suggests that the stellar population in LBDS 53W091 is significantly younger than that in M32. Population synthesis models indicate that the age of LBDS 53W091 is in the range from 1 to 2 Gyr and depends on the specific model. Older ages require sub-solar metallicity models. The estimates of the age of the dominant population in M32 range from 3 to 5 Gyr and depend not only on the model but also on the SED of this galaxy used in the fits. The same models predict an age of 11 to 13 Gyr for the stars in a typical old E/S0 galaxy. A 1 to 2 Gyr old galaxy at z = 1.55 poses no problem for the Lambda = 0, Omega = 1 universe, as long as h <= 0.8. The most likely reason for the difference between our age estimate and the value of 3.5 Gyr derived by Dunlop et al. (1996), is the fact that these authors did not require that the population synthesis models that they used fitted simultaneously the UV break amplitudes and the observed $R-J, R-H$, and $R-K$ colors of this galaxy. As we will discuss elsewhere, a comparison of our results with those by Spinrad et al. (1997) shows that our conclusions still hold. The paper by Spinrad et al. was unknown to us at the time of this Conference.
Velocity dispersions $σ$ and Mg absorption line-strengths Mg$_b$ have been measured for a sample of 16 ellipticals in 3 clusters at a redshift of 0.37. Like local cluster ellipticals, these objects show a correlation between Mg$_b$ and $σ$. However, at any given $σ$, the mean Mg$_b$ of the ellipticals at $z=0.37$ is weaker than the mean Mg$_b$ of their local relatives in the Coma and Virgo clusters. The Mg$_b$ weakening is smallest for the most luminous ellipticals and larger for the fainter objects. This is unambiguous evidence for {\it small but significant passive evolution} of the stellar populations of elliptical galaxies with redshift. It requires that the bulk of the stars in cluster ellipticals has formed at $z>2$. The most luminous objects may even have formed at $z>4$. The Mg$_b-σ$ test is a very reliable estimator for the evolution of old stellar populations because it is virtually independent from the stellar initial mass function (IMF) and from the metallicities of the galaxies. Furthermore, the influence of selection effects is minimal.