Isochrone Synthesis Research Articles

Luminosity Density of Galaxies and Cosmic Star Formation Rate from Λ Cold Dark Matter Hydrodynamical Simulations

We compute the cosmic star formation rate (SFR) and the rest-frame comoving luminosity density in various passbands as a function of redshift using large-scale ΛCDM hydrodynamical simulations, with the aim of understanding their behavior as a function of redshift. To calculate the luminosity density of galaxies, we use an updated isochrone synthesis model that takes metallicity variations into account. The computed SFR and the UV luminosity density have a steep rise from z = 0 to 1, a moderate plateau between z = 1-3, and a gradual decrease beyond z = 3. The raw calculated results are significantly above the observed luminosity density, which can be explained either by dust extinction or the possibly inappropriate input parameters of the simulation. We model the dust extinction by introducing a parameter f, the fraction of the total stellar luminosity (not galaxy population) that is heavily obscured and thus only appears in the far-infrared to submillimeter wavelength range. When we correct our input parameters (baryon mass-density and the yield of metals) to the current best estimate and apply dust extinction with f = 0.65, the resulting luminosity density fits various observations reasonably well, including the present stellar mass density, the local B-band galaxy luminosity density, and the FIR-to-submillimeter extragalactic background. Our result is consistent with the picture that ~2/3 of the total stellar emission is heavily obscured by dust and observed only in the FIR. The rest of the emission is only moderately obscured, which can be observed in the optical to near-IR wavelength range. We also argue that the steep falloff of the SFR from z = 1 to 0 is partly due to the shock-heating of the universe at late times, which produces gas that is too hot to easily condense into star-forming regions.

Colors andK‐Band Counts of Extremely Faint Field Galaxies,

We combine deep K-band (Keck) with V- and I-band (NTT) observations of two high-Galactic latitude fields, surveying a total of ~2 sq. arcmin. The K-band galaxy counts continue to rise above K=22, reaching surface densities of few x 10^5 per sq. degree. The slope for the counts is (d log(N) per mag per sq. degree) = 0.23 +/- 0.02 between K=18-23, consistent with other deep K surveys. The numbers of galaxies in each mag bin is about two times greater than the galaxy counts of Djorgovski et al. (1995). The optical and near infrared magnitudes of all objects detected in the V+I+K image are discussed in the context of grids of isochrone synthesis galaxy evolutionary models (Bruzual & Charlot 1993, 1995). The colors of most of the observed galaxies are consistent with a population drawn from a broad redshift distribution. A few galaxies at K=19-20 are red in both colors (V-I>3; I-K>2), consistent with being early-type galaxies having undergone a burst of star formation at z>5 and viewed at z~1. At K>20, we find ~8 ``red outlier'' galaxies with I-K>4 and V-I<2.5, whose colors are difficult to mimic by a single evolving or non-evolving stellar population at any redshift. They are likely either low-metallicity, dusty dwarf galaxies, or old galaxies at high redshift (z>1.2). Their surface density is several per square arcminute, which is so high that they are probably common objects of low luminosity $L<L_*$.

Population Synthesis Models

Increasingly complex population synthesis models have been developed during the last few years. Among the different authors working in the field, these groups have been particularly active: Arimoto &amp; Yoshii (1987), Guiderdoni &amp; Rocca-Volmerange (1987), Buzzoni (1989), Fritze-v. Alvensleben &amp; Gerhard (1994), Bressan, Chiosi, &amp; Fagotto (1994), Bruzual &amp; Chariot (1993, 1995). The basic astrophysical ingredients used in these models are the stellar evolutionary tracks and the stellar spectral libraries (either empirical or theoretical). The computational algorithms are different in each set of models, but, one way or the other, these models depend on the same adjustable parametric functions: (1) the stellar initial mass function (IMF); (2) the star formation rate (SFR); and (3) the rate of chemical enrichment (in some models Z = Z⊙ = constant). Most of the recent codes use the isochrone synthesis algorithm, which allows to compute the evolution of a simple stellar population (SSP), and from it, by a convolution integral, the properties of more complex composite stellar populations (CSP).

Population Synthesis and the Ultraviolet Spectrum of Elliptical Galaxies

view Abstract Citations (34) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Population Synthesis and the Ultraviolet Spectrum of Elliptical Galaxies Magris C., Gladis ; Bruzual A., Gustavo Abstract We enlarge the library of stellar evolutionary tracks compiled by Charlot & Bruzual to include a more detailed treatment of the post-asymptotic giant branch (PAGB) evolution of solar metallicity stars. The resulting set of tracks depends on the assumed relation M∫(Mi) between the zero-age main-sequence (ZAMS) mass of a star, Mi, and its mass M∫ at the end of the AGB. The isochrone synthesis spectral evolution code of Bruzual & Charlot has been used to study the possible stellar sources of the UV flux observed in elliptical galaxies, exploring the effects on the models of assuming different M∫(Mi) relations. We adopt the semiempirical relation determined by Weidemann to build the models used in this investigation. The main conclusions of our work do not depend on the adopted M∫(Mi). The UV spectra of M32, NGC 4472, NGC 3379, and NGC 4649 are examined. We conclude that whereas NGC 4472 and NGC 3379 have a UV spectrum which is consistent with that produced by a passively evolving old stellar population of solar metallicity, M32 shows definite spectral evidence that a recent major event of star formation took place in this galaxy close to 4 Gyr ago. The spectrum of NGC 4649 cannot be understood unless we assume that star formation (in minor amounts) has been going on in this galaxy until recent epochs, or that a UV bright stellar component is missing in our synthesis. Our models provide a lower limit to the total UV output from stellar populations dominated by solar metallicity stars. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173295 Bibcode: 1993ApJ...417..102M Keywords: GALAXIES: EVOLUTION; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; ULTRAVIOLET: GALAXIES full text sources ADS | data products SIMBAD (4) MAST (1)

Spectral evolution of stellar populations using isochrone synthesis

We combine the photometric model of isochrone synthesis recently published by Charlot & Bruzual with an updated library of stellar spectra to predict the spectral evolution of stellar populations with solar metallicity. The library of spectra assembled here supersedes other existing libraries (Bruzual; Guiderdoni & Rocca-Volmerange; Buzzoni) by its spectral range (extreme ultraviolet to far-infrared), its complete coverage of the color-magnitude diagram, and its inclusion of observed near-infrared spectra out to 2.56 μm. Also, the spectra are distributed on the stellar evolutionary tracks using optical/near-infrared color calibrations, as an improvement over models that used a single color of the effective temperature of the stars alone

New Models for the Spectral Evolution of Galaxies

Bruzual &amp; Chariot (1991, BC91 hereafter) have supplemented the purely photometric isochrone synthesis models of Chariot &amp; Bruzual (1991) with an updated library of stellar spectra and evolutionary tracks in order to compute the spectral evolution of stellar populations. BC91 used the evolutionary tracks of Maeder &amp; Meynet (1989), with the main-sequence lifetimes for stars in the mass range 1.3 M⊙ ≤ m ≤ 2.5 M⊙, revised as prescribed by Maeder &amp; Meynet (1991). BC91 included an accurate description of the late evolution of low and intermediate-mass stars as hot nuclei of planetary nebulae and through the white dwarf cooling sequence. While stars in these stages contribute negligibly to the optical/near-IR light, the self-consistent model of BC91 for an initial burst of star formation can account for the UV excess observed in quiescent early-type galaxies. Models that do not include post-AGB stars require additional star formation to account for this UV excess.

Stellar population synthesis revisited

view Abstract Citations (335) References (66) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Population Synthesis Revisited Charlot, Stephane ; Bruzual A, Gustavo Abstract The isochrone synthesis method is introduced as a population synthesis method for galaxies. The method permits accurate distributions of stars in the theoretical color-magnitude diagram to be computed for any stellar population and thereby circumvent the problems faced by earlier techniques. A set of evolutionary tracks extending from the ZAMS up to the beginning of the white-dwarf cooling sequence is compiled to optimize the models. Comparison of model predictions with observational data leads to the conclusion that the isochrone synthesis can reproduce the observed colors of stellar populations of ages from a few tens of millions of yr to a Hubble time. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169608 Bibcode: 1991ApJ...367..126C Keywords: Astronomical Photometry; Asymptotic Giant Branch Stars; Color-Magnitude Diagram; Galactic Evolution; Stellar Evolution; Star Clusters; Stellar Color; Stellar Models; Ubv Spectra; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: PHOTOMETRY; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION full text sources ADS | data products SIMBAD (1)