Empirical Population Synthesis Research Articles

Stellar ages and metallicities of nearby elliptical galaxies

Stellar ages and metallicities are crucial for understanding the formation and evolution of elliptical galaxies. However, due to the age--metallicity degeneracy, it is hard to measure these two parameters accurately with broad-band photometry. In this paper, we observed high-resolution spectra for a sample of 20 nearby elliptical galaxies (EGs) with the NAOC 2.16 m telescope, and determined stellar ages and metallicities by using the empirical population synthesis and Lick/IDS index system methods. We found that stellar ages from these two methods are consistent with each other for purely old EGs; however, stellar metallicities show a zeropoint offset of 0.5 Z⊙. Our results confirm that stellar populations in low-density environment galaxies are more diverse compared to their high-density counterparts. We also investigated the element abundance--galaxy mass relation for nearby elliptical galaxies.

Stellar population history of Seyfert 2 nuclei

We present the results of a new stellar population synthesis code which mixes both evolutionary and empirical population synthesis techniques. The code decomposes a given galaxy spectrum into a base of high resolution spectra from the recent evolutionary synthesis models of Bruzual & Charlot (2003). The main novelty is that now we synthesize the whole Fλ spectrum, rather than a handful of absorption lines and colors.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The evolution of stars and gas in starburst galaxies

In systems undergoing starbursts the evolution of the young stellar population is expected to drive changes in the emission-line properties. This evolution is usually studied theoretically, with a combination of evolutionary synthesis models for the spectral energy distribution of starbursts and photoionization calculations. In this paper we present a more empirical approach to this issue. We apply empirical population synthesis techniques to samples of starburst and H ii galaxies in order to measure their evolutionary state and correlate the results with their emission-line properties. A couple of useful tools are introduced that greatly facilitate the interpretation of the synthesis: (1) an evolutionary diagram, the axes of which are the strengths of the young, intermediate age and old components of the stellar population mix; and (2) the mean age of stars associated with the starburst, . These tools are tested with grids of theoretical galaxy spectra and found to work very well even when only a small number of observed properties (absorption-line equivalent widths and continuum colours) is used in the synthesis. Starburst nuclei and H ii galaxies are found to lie on a well-defined sequence in the evolutionary diagram. Using the empirically defined mean starburst age in conjunction with emission-line data, we have verified that the equivalent widths of Hβ and [O iii] decrease for increasing . The same evolutionary trend was identified for line ratios indicative of the gas excitation, although no clear trend was identified for metal-rich systems. All these results are in excellent agreement with long-known, but little tested, theoretical expectations.

Measuring the evolution of H II and starburst galaxies

The evolution of starbursts and their young stellar populations is usually studied using evolutionary population synthesis and photo-ionization calculations. We give a more empirical approach to this issue by appling empirical population synthesis techniques to samples of starburst and H ii galaxies, to measure their evolutionary state and correlate the results with their emission line properties. The results are presented using an evolutionary diagram whose axis are the strengths of the young, intermediate and old age components of the population mix. We also recover the long known prediction that the WHβ and the gas extinction decrease as the starburst ages.

A probabilistic formulation for empirical population synthesis: sampling methods and tests

We present a probabilistic formulation of the classical problem of synthesizing spectral properties of a galaxy using a base of star clusters. The problem consists of estimating the population vector x, composed by the contributions of n_star base elements to the integrated spectrum of a galaxy, and the extinction A_V, given a set of absorption line equivalent widths and continuum colors. The formalism is applied to the n_star = 12 base defined by Schmidt etal and subsequently used in several studies. The 13-D parameter space is explored with a Markov chain Monte Carlo sampling scheme based on the Metropolis algorithm, which produces a smooth and efficient mapping of the P(x,A_V) probability distribution. This version of Empirical Population Synthesis is used to investigate the ability to recover the detailed history of star-formation and chemical evolution using this spectral base. This is studied as a function of (1) the magnitude of the measurement errors and (2) the set of observables used in the synthesis. Only for extremely high S/N all 12 base proportions can be accurately recovered, though the observables are very precisely reproduced for any S/N. Furthermore, the individual mean x components are biased in the sense that components which carry a large fraction of the light tend to share their contribution preferably among components of same age. This compensation effect is linked to noise-induced linear dependences in the base, which very effectively redistribute the likelihood in x-space. The age distribution, however, can be satisfactorily recovered for realistic data quality. (abridged)

Stellar populations in shell galaxies

view Abstract Citations (23) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Populations in Shell Galaxies McGaugh, Stacy S. ; Bothun, Gregory D. Abstract UBV surface photometry of the shell galaxies Arp 230, NGC 7010, and Arp 223 is presented. We find all to be the result of mergers. In Arp 230, the merger of two spirals induced a burst of star formation which has declined in strength since the collision. The remnant seems likely to become elliptical. NGC 7010 has very red shells, perhaps redder than the galaxy itself. Evolution of the stellar content of these shells may be important. Arp 223 has shell colors which are slightly bluer than the main body of the galaxy, consistent with an origin in an Sb. Our data indicate that the timescale since the interaction is typicaly ~ 1 Gyr. Moreover, those ellipticals that have been formed by mergers should have complex stellar populations with a component formed in the collision. This extra component might be detectable by the methods of empirical population synthesis, at least to some age limit, making it possible to investigate the relative numbers of ellipticals formed in this manner by methods other than morphology. Publication: The Astronomical Journal Pub Date: October 1990 DOI: 10.1086/115580 Bibcode: 1990AJ....100.1073M Keywords: Galactic Structure; Star Distribution; Star Formation; Astronomical Photometry; Color-Color Diagram; Ultraviolet Radiation; Astronomy; GALAXIES: INTERACTIONS; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (9) NED (3)

Nonuniqueness of galactic population sythesis models - A reexamination of M31

The empirical population synthesis problem for galaxies is reexamined as a linear program, and a nonuniqueness theorem is presented which allows the prediction of model behavior in the presence of photometric errors. Models of M31 based on O'Connell's spectrophotometry are next examined. The main sequence turnoff point, population metallicity, and relative evolved star to main sequence contributions are shown to be ambiguous as a result of photometric errors and the effects of intrinsic strength anomalies (Taylor and Kellman).