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تسجيل مستخدم جديدThe History of Star Formation in Galaxies
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If we are to develop a comprehensive and predictive theory of galaxy formation and evolution, it is essential that we obtain an accurate assessment of how and when galaxies assemble their stellar populations, and how this assembly varies with environment. There is strong observational support for the hierarchical assembly of galaxies, but our insight into this assembly comes from sifting through the resolved field populations of the surviving galaxies we see today, in order to reconstruct their star formation histories, chemical evolution, and kinematics. To obtain the detailed distribution of stellar ages and metallicities over the entire life of a galaxy, one needs multi-band photometry reaching solar-luminosity main sequence stars. The Hubble Space Telescope can obtain such data in the low-density regions of Local Group galaxies. To perform these essential studies for a fair sample of the Local Universe, we will require observational capabilities that allow us to extend the study of resolved stellar populations to much larger galaxy samples that span the full range of galaxy morphologies, while also enabling the study of the more crowded regions of relatively nearby galaxies. With such capabilities in hand, we will reveal the detailed history of star formation and chemical evolution in the universe.
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We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute som40%+/- 17% of M32s
mass, while 55%+/-21% of M32s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are <Age>=6.8+/-1.5 Gyr and <[M/H]>=-0.01+/-0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H]sim-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining sim3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This contradicts spectroscopic studies that show a steep age gradient from M32s center to 1re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with sim95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; sim30%+/-7.5% of its mass is in a 5-8 Gyr old population, and sim65%+/-9% of the mass is in a 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are <Age>=9.2+/-1.2 Gyr and <[M/H]>=-0.10+/-0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31s disk at F2 (sim1 disk scale length) to be 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31s disk.
We study the radial structure of the stellar mass surface density ($mu$) and stellar population age as a function of the total stellar mass and morphology for a sample of 107 galaxies from the CALIFA survey. We use the fossil record to recover the st
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Context. There are typically two different approaches to infer the mass formation history (MFH) of a given galaxy from its luminosity in different bands. Non-parametric methods are known for their flexibility and accuracy, while parametric models are
more computationally efficient. Aims. In this work we propose an alternative that combines the advantages of both techniques, based on a polynomial expansion around the present time. Methods. In our approach, the MFH is decomposed through an orthonormal basis of N polynomia in lookback time. To test the proposed framework, synthetic observations are generated from models based on common analytical approximations (exponential, delayed-tau and Gaussian star formation histories). A normalized distance is used to measure the quality of the fit, and the input MFH are compared with the polynomial reconstructions both at the present time as well as through cosmic evolution. Results. The observed luminosities are reproduced with an accuracy of around 10 per cent for a constant star formation rate (N=1) and better for higher-order polynomia. Our method provides good results on the reconstruction of the total stellar mass, star formation rate and even its first derivative for smooth star formation histories, but it has difficulties in reproducing variations on short timescales and/or star formation histories peaking at the earliest times of the Universe. Conclusions. The polynomial expansion appears to be a promising alternative to other analytical functions used in parametric methods, combining both speed and flexibility.
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