Evolutionary synthesis of stellar populations: a modular tool

A new tool for the evolutionary synthesis of stellar populations is presented, which is based on three independent matrices, giving respectively (1) the fuel consumption during each evolutionary phase as a function of stellar mass, (2) the typical temperatures and gravities during such phases, and (3) the colours and bolometric corrections as functions of gravity and temperature. The modular structure of the code allows one easily to assess the impact on the synthetic spectral energy distribution of the various assumptions and model ingredients, such as, for example, uncertainties in stellar evolutionary models, the mixing length, the temperature distribution of horizontal branch stars, asymptotic giant branch mass loss, and colour–temperature transformations. The so-called 'AGB phase transition' in Magellanic Cloud clusters is used to calibrate the contribution of the thermally pulsing asymptotic giant branch phase to the synthetic integrated luminosity. As an illustrative example, solar-metallicity ( Y  = 0.27, Z  = 0.02) models, with ages ranging between 30 Myr and 15 Gyr and various choices for the slope of the initial mass function, are presented. Synthetic broad-band colours and the luminosity contributions of the various evolutionary stages are compared with Large Magellanic Cloud and Galactic globular cluster data. In all these cases, a good agreement is found. Finally, the evolution is presented of stellar mass-to-light ratios in the bolometric and U B V R K passbands, in which the contribution of stellar remnants is accounted for.     

A theoretical analysis of the systematic errors in the red clump distance to the Large Magellanic Cloud (LMC)

We present a detailed analysis of the uncertainty on the theoretical population corrections to the Large Magellanic Cloud (LMC) red clump (RC) absolute magnitude, by employing a population synthesis algorithm to simulate theoretically the photometric and spectroscopic properties of RC stars, under various assumptions concerning the LMC star formation rate (SFR) and age–metallicity relationship (AMR). A comparison of the outcome of our simulations with observations of evolved low- to intermediate-mass stars in the LMC allows one to select the combinations of SFR and AMR that bracket the real LMC star formation history, and to estimate the systematic error on the associated RC population corrections.
The most accurate estimate of the LMC distance modulus from the RC method (adopting the OGLE-II reddening maps for the LMC) is obtained from the K -band magnitude, and provides  ( m − M )_0,LMC= 18.47 ± 0.01(random)^+0.05_−0.06(systematic)  . Distances obtained from the I band, or from the multicolour RC technique which determines at the same time reddening and distance, both agree (albeit with a slightly larger error bar) with this value.     

Multiple stellar populations in the Sextans dwarf spheroidal galaxy?

We present wide-field     multiband ( BVI ) CCD photometry (down to     of the very low surface brightness dwarf spheroidal galaxy Sextans. In the derived colour–magnitude diagrams we find evidence suggesting the presence of multiple stellar populations in this dwarf spheroidal. In particular, we discover (i) a blue horizontal branch tail that appears to lie on a brighter sequence with respect to the prominent red horizontal branch and the RR Lyrae stars, very similar to what was found by Majewski et al. for the Sculptor dwarf spheroidal, (ii) hints of a bimodal distribution in colour of the red giant branch stars, (iii) a double red giant branch bump. All of these features suggest that (at least) two components are present in the old stellar population of this galaxy: the main one with     and a minor component around     . The similarity to the Sculptor case may indicate that multiple star formation episodes are also common in the most nearby dwarf spheroidals that ceased their star formation activity at very early epochs.     

Star formation in the LMC: Comparative CCD observations of young stellar populations in two giant molecular clouds

This work deals with a CCD imaging study at optical and near‐infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color‐magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently “starless” cloud has so far formed only low‐luminosity, low‐mass stars (fainter than m_Ks ∽ 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low‐mass and luminous high‐mass stars. The surface density excess oflow‐luminosity stars (∽2 per square arcmin) in the “starless” cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star‐forming cloud. The difference may be explained theoretically by the gravo‐turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

On a systematic bias in surface brightness fluctuations based distances due to gravitational microlensing

The effect of gravitational microlensing on the determination of extragalactic distances using the surface brightness fluctuations (SBF) technique is considered and a method to calculate SBF amplitudes in the presence of microlensing is presented. With a simple approximation for the magnification power spectrum at low optical depth, the correction to the SBF-based luminosity distance is calculated. The results suggest the effect can be safely neglected at present but may become important for SBF-based Hubble diagrams at luminosity distances of about 1 Gpc and beyond.     

The star formation histories of two northern LMC fields

Ground-based UBV photometry of two fields in the northern disc of the Large Magellanic Cloud (LMC) is presented. A distance modulus of ( m − M )₀=18.41±0.04 and an extinction of A _V =0.30±0.05 have been calculated for these fields. The measurable star formation history of the LMC began no more than 12 Gyr ago with a strong star‐forming episode with [Fe/H]=−1.63±0.10 that accounted for approximately half (by mass) of the total star formation of the LMC in the first 3 Gyr. The data do not give accurate star formation rates during intermediate ages, but there appears to have been a recent increase in the star formation rate in these fields, beginning approximately 2.5 Gyr ago, with the current metallicity in the region being [Fe/H]=−0.38±0.10. The two fields have had very similar star formation rates until 200 Myr ago, at which point one shows a large increase.     

The chemical evolution of dwarf spheroidal galaxies: dissecting the inner regions and their stellar populations

Using three-dimensional hydrodynamical simulations of isolated dwarf spheroidal galaxies (dSphs), we undertake an analysis of the chemical properties of their inner regions, identifying the respective roles played by Type Ia supernovae (SNe Ia) and Type II supernovae (SNe II). The effect of inhomogeneous pollution from SNe Ia is shown to be prominent within two core radii, with the stars forming therein amounting to ∼20 per cent of the total. These stars are relatively iron-rich and α-element depleted compared to the stars forming in the rest of the galaxy. At odds with the projected stellar velocity dispersion radial profile, the actual three-dimensional one shows a depression in the central region, where the most metal-rich (i.e. [Fe/H]-rich) stars are partly segregated. This naturally results in two different stellar populations, with an anticorrelation between [Fe/H] and velocity dispersion, in the same sense as that observed in the Sculptor and Fornax dSphs. Because the most iron-rich stars in our model are also the most α depleted, a natural prediction and test of our model is that the same radial segregation effects should exist between [α/Fe] and velocity dispersion.