Emilio Casuso

A new chemo-evolutionary population synthesis model for early-type galaxies .2. Observations and results

We present the results of applying a new chemo-evolutionary stellar population model, developed in a previous paper, to new high-quality observational data of the nuclear regions of two representative elliptical galaxies and the bulge of the Sombrero galaxy. Here we fit in detail ~20 absorption lines and six optical and near-infrared colors, following two approaches: fitting a single-age, single-metallicity model and fitting our full chemical evolutionary model. We find that all the iron lines are weaker than the best-fitting models predict, indicating that the iron abundance is anomalous and deficient. We also find that the Ca I index at 4227 A is much lower than predicted by the models. We can obtain good fits for all the other lines and observed colors with models of old and metal-rich stellar populations and can show that the observed radial gradients are due to metallicity decreasing outward. We find that good fits are obtained both with fully evolutionary models and with single-age, single-metallicity models. This is due to the fact that in the evolutionary model more than 80% of the stars form within 1.5 Gyr after the formation of the galaxies. The fact that slightly better fits are obtained with evolutionary models indicates that these galaxies contain a small spread in metallicity.

The revised Mg-2 index as a metallicity indicator for stellar systems : Giant elliptical galaxies and bulges

We provide a new calibration of the metallicity dependence of the Mg-2 index in stellar systems. It is based on new isochrones by the Padova group and the latest stellar spectral libraries. Since the Mg-2 line is one of the strongest absorption lines in the optical spectrum, it is often used to determine metallicities of composite stellar systems assuming a certain age. This paper allows one to do this, with an uncertainty of about 0.1 in log Z, for an age range between 8 and 17 Gyr. We estimate that the uncertainties of the current Mg, calibrations in the literature amount to another 0.15 in log Z. Using our calibration we find, under conservative assumptions, that the large majority of elliptical galaxies, and also many bulges of spiral galaxies, must have Mg abundances larger than solar; this conclusion depends very little on their star formation history. If [Mg/Fe] = 0, the same can be said for total metallicities. If we assume that [Mg/Fe] = 0.45, only about 38% of the ellipticals must have metallicities (Z) larger than solar.

An improved evolutionary model for stellar population applied to NGC 2903

We outline the revised physical assumption which have been incorporated into our previously published evolutionary model for stellar population synthesis in galaxies. Applying the present version of the model to well-defined zones on the nucleus and along a spiral arm of NGC 2903 we show that the star formation rate has increased during the lifetime of the galaxy, steadily but only slightly in the arm, and spectacularly in the nucleus, where there is current starburst activity. We also infer that considerable gaseous inflow has occurred in the arm during the lifetime of the disc.

Application of a new evolutionary model for stellar populations to photometric maps of M32

We have developed a new model of evolutionary synthesis of stellar population, from a working cyclical approximation to a general solution for the mass and time dependence of the ‘birth function’ of stars. The general solution enables us to introduce a functionG(t): the ratio of gas injected into a defined region to that which forms stars in the same time interval, at timet. This function, by relating directly the output of dying stars with the formation of the next generation, allows us to make analytical approximations (in the absence of more accurate numerical information) to the macroscopic evolution of stellar populations in a well-defined zone of a galaxy.The model has been initially applied to 4 zones of the dwarf elliptical M32 (NGC 221) where, usingU, B, V from the literature, and our own photometric maps inJ andK, we obtained the following results: (a) Star formation in the most recent 108 years is lower by a factor 3–4 in a zone 68 arc sec (200 pc) from the nucleus than in the nuclear zone. (b) The metallicity appears to be a little lower in a zone diametrically away from the parent galaxy M31 than in a zone towards M31 at the same galactocentric distance (c) TheM/L ratio is a factor 2 higher at 200 pc from the nucleus towards M31, than in the nuclear zone. (d) A high concentration of mass is evident in the nuclear zone. (e) The estimated age of all the zones is of the same order,≲1.5×1010 yr.

The G-dwarf problem: A tentative analytical approach, relaxing instantaneous recycling

Starting from a general solution for the birth functionB(m, t) of stars described in detail in Casusoet al. (1989), we have obtained a first-order analytical approximation to this function as a function of metallicityZ. Using this, we obtained a fit to the observational curve compiled by Tinsley (1980) for the cumulative function of stars with metallicity lower than a given value in the solar neighbourhood. In addition, using the same expression, with its numerical fit to previous data, we obtain a good fit to the differential distributions of stars at low metallicity given in the review by Pagel (1987), given a bifurcation in the birth function at low values ofZ, which would correspond to two distinct epochs of onset of star formation. The analysis gives an infall of gas towards the solar neighbourhood up to the epoch of metallicityZ=6.7×10−3 with a correspondingly increased star formation rate, which subsequently stabilized, and another similar inflow up toZ=1.2×10−3, followed again by a steady star formation rate for largerZ. Although the assumptions made are still relatively crude, and the numbers should be considered tentative, the flexibility of the model in handling the problem is that we wish to show here.

Gas Flows, Star Formation and Galaxy Evolution

In the first part of this article we show how observations of th e chemical evo- lution of the Galaxy: G- and K-dwarf numbers as functions of metallicity, and abundances of the light elements, D, Li, Be and B, in both stars and the inter- stellar medium (ISM), lead to the conclusion that metal poor H i gas has been accreting to the Galactic disc during the whole of its lifeti me, and is accreting today at a measurable rate, � 2M⊙ per year across the full disc. Estimates of the local star formation rate (SFR) using methods based on stellar activity, support this picture. The best fits to all these data are for models whe re the accretion rate is constant, or slowly rising with epoch. We explain here how this conclusion, for a galaxy in a small bound group, is not in conflict with grap hs such as the Madau plot, which show that the universal SFR has declined steadily from z = 1 to the present day. We also show that a model in which disc galaxies in general evolve by accreting major clouds of low metallicity gas from their surroundings can explain many observations, notably that the SFR for whole galaxies tends to show obvious variability, and fractionally more for earl y than for late types, and yields lower dark to baryonic matter ratios for large disc galaxies than for dwarfs. In the second part of the article we use NGC 1530 as a template object, showing from Fabry-Perot observations of its Hemission how strong shear in this strongly barred galaxy acts to inhibit star formation, while compression acts to stimulate it.

The Evolution of the Light Elements, Be and B (also Li), in the Galaxy

We consider the evolution of the light elements, especially beryllium and boron but also lithium, in the Galaxy as derived from observations within 1kpc of the Sun. The interest in Li has much to do with the evaluation of the universal baryon abundance via primordial nucleosynthesis, but the difficulties of interpretation have led to the need to understand Li synthesis within the Galaxy, and this entails understanding many processes, both stellar and interstellar. In the case of Be, and B although measurable abundances produced in the primaeval fireball were predicted in certain models, these are largely (but not totally) discounted observationally. However understanding the evolution of Be and B as tracers of Galactic chemical evolution is important. While most experts in nucleosynthesis have concentrated on the linear relation between B/Be and Fe, (or O) in the Galactic halo, and taken disc evolution rather for granted, we show that it is vital to use a valid chemical evolution model for the disc to explain the observations. We present such a model, and emphasize its implications for the infall of low metallicity gas to the disc as the driving element in star formation during the whole disc lifetime.

Chaotic Behaviour of the Temporal Evolution of Star Formation Rate in the Solar Neighborhood

We use statistical data giving the number of stars formed as a function of the age, assigned using the observed deterministic relation between chromospheric activity and age, to show that the temporal behaviour of star formation in the solar neighbourhood must have been chaotic during time intervals of less than 1 Gyr. We also offer evidence that systems of gas and stars typified by that in the solar vicinity show behaviour analogous to that of the logistic map.

New spectrophotometric population synthesis models of ellipticals and early type spirals

We have developed a model for calculating colours and absorption line indices for composite stellar systems. The model can synthesize observations for single age, single metallicity stellar populations, but can also incorporate chemical evolution, following the evolution of a galaxy from an initial gas cloud to the present time. We have obtained accurate observations in a number of colours and line indices of three well-studied galaxies: NGC 3379, NGC 4472 and NGC 4594, and have used these to calibrate our models.