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Register a new userAccurate parameter estimation for star formation history in galaxies using SDSS spectra
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To further our knowledge of the complex physical process of galaxy formation, it is essential that we characterize the formation and evolution of large databases of galaxies. The spectral synthesis STARLIGHT code of Cid Fernandes et al. (2004) was designed for this purpose. Results of STARLIGHT are highly dependent on the choice of input basis of simple stellar population (SSP) spectra. Speed of the code, which uses random walks through the parameter space, scales as the square of the number of basis spectra, making it computationally necessary to choose a small number of SSPs that are coarsely sampled in age and metallicity. In this paper, we develop methods based on diffusion map (Lafon & Lee, 2006) that, for the first time, choose appropriate bases of prototype SSP spectra from a large set of SSP spectra designed to approximate the continuous grid of age and metallicity of SSPs of which galaxies are truly composed. We show that our techniques achieve better accuracy of physical parameter estimation for simulated galaxies. Specifically, we show that our methods significantly decrease the age-metallicity degeneracy that is common in galaxy population synthesis methods. We analyze a sample of 3046 galaxies in SDSS DR6 and compare the parameter estimates obtained from different basis choices.
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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.
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.
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 star formation history (SFH) in spheroidal and disk dominated galaxies with masses from 10$^9$ to 10$^{12}$ M$_odot$. We derive the half mass radius, and we find that galaxies are on average 15% more compact in mass than in light. HMR/HLR decreases with increasing mass for disk galaxies, but is almost constant in spheroidal galaxies. We find that the galaxy-averaged stellar population age, stellar extinction, and $mu$ are well represented by their values at 1 HLR. Negative radial gradients of the stellar population ages support an inside-out formation. The larger inner age gradients occur in the most massive disk galaxies that have the most prominent bulges; shallower age gradients are obtained in spheroids of similar mass. Disk and spheroidal galaxies show negative $mu$ gradients that steepen with stellar mass. In spheroidal galaxies $mu$ saturates at a critical value that is independent of the galaxy mass. Thus, all the massive spheroidal galaxies have similar local $mu$ at the same radius (in HLR units). The SFH of the regions beyond 1 HLR are well correlated with their local $mu$, and follow the same relation as the galaxy-averaged age and $mu$; suggesting that local stellar mass surface density preserves the SFH of disks. The SFH of bulges are, however, more fundamentally related to the total stellar mass, since the radial structure of the stellar age changes with galaxy mass even though all the spheroid dominated galaxies have similar radial structure in $mu$. Thus, galaxy mass is a more fundamental property in spheroidal systems while the local stellar mass surface density is more important in disks.
We present the first results of a pilot study aimed at understanding the influence of bars on the evolution of galaxy discs through the study of their stellar content. We examine here the kinematics, star formation history, mass-weighted, luminosity-weighted, and single stellar population (SSP) equivalent ages and metallicities for four galaxies ranging from lenticulars to late-type spirals. The data employed extends to 2-3 disc scalelengths, with S/N(A)>50. Several techniques are explored to derive star formation histories and SSP-equivalent parameters, each of which are shown to be compatible. We demostrate that the age-metallicity degeneracy is highly reduced by using spectral fitting techniques --instead of indices-- to derive these parameters. We found that the majority of the stellar mass in our sample is composed of old (~10 Gyr) stars. This is true in the bulge and the disc region, even beyond two disc scalelengths. In the bulge region, we find that the young, dynamically cold, structures produced by the presence of the bar (e.g., nuclear discs or rings) are responsible for shaping the bulges age and metallicity gradients. In the disc region, a larger fraction of young stars is present in the external parts of the disc compared with the inner disc. The disc growth is, therefore, compatible with a moderate inside-out formation scenario, where the luminosity weighted age changes from ~10 Gyrs in the centre, to ~4 Gyrs at two disc scalelengths, depending upon the galaxy. For two galaxies, we compare the metallicity and age gradients of the disc major axis with that of the bar, finding very important differences. In particular, the stellar population of the bar is more similar to the bulge than to the disc, indicating that, at least in those two galaxies, bars formed long ago and have survived to the present day. (abridged)
We examine the past and current work on the star formation (SF) histories of dwarf galaxies in cosmological hydrodynamic simulations. The results obtained from different numerical methods are still somewhat mixed, but the differences are understandable if we consider the numerical and resolution effects. It remains a challenge to simulate the episodic nature of SF history in dwarf galaxies at late times within the cosmological context of a cold dark matter model. More work is needed to solve the mysteries of SF history of dwarf galaxies employing large-scale hydrodynamic simulations on the next generation of supercomputers.
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