Population Synthesis for Emission‐Line GalaxiesAuthor(s):  Kong XuSource: Publications of the Astronomical Society of the Pacific, Vol. 112, No. 777 (November2000), pp. 1502-1503Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/317696 .

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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 112 :1502È1503, 2000 November2000. The Astronomical Society of the PaciÐc. All rights reserved. Printed in U.S.A.(

Dissertation Summary

Population Synthesis for Emission-Line GalaxiesKONG XU

Center for Astrophysics, University of Science and Technology of China, 230026 Hefei, PeopleÏs Republic of China ; xkong=mail.ustc.edu.cnThesis work conducted at Center for Astrophysics, University of Science and Technology of China

Ph.D. thesis directed by Cheng Fu-zhen ; Ph.D. degree awarded 2000 July

Received 2000 July 9 ; accepted 2000 July 10

To understand the star formation history, energy source,and internal dust distribution of galaxies, we need to inves-tigate their stellar populations and to derive their mainparameters, such as metallicity and age. This study plays animportant role in our understanding of how galaxies formand evolve. Nearby emission-line galaxies, such as bluecompact dwarf galaxies (BCDs), blue compact galaxies(BCGs), and starburst galaxies (SBs), are ideal laboratoriesfor such studies because their proximity allows us toexamine in detail issues which are important for inter-preting galaxy behavior with time, and therefore with red-shift. Besides, their low level of evolution (low metallicityand high gas content) makes these systems the most similarto primeval galaxies, and thus the most useful to infer theprimordial galaxy conditions and big bang nucleosynthesisproducts. In addition, BCDs and BCGs are considered tobe the local candidates for the faint blue galaxy populationseen at large look-back times, i.e., high redshifts, and canpossibly yield some insight into the galactic starburst phe-nomenon believed to account for this new galaxy popu-lation. Understanding how star formation proceeds innearby emission-line galaxies is then fundamental for astro-physical and cosmological issues.

In order to study the stellar components, star formationhistory, energy source, and internal dust distribution in bluecompact dwarf galaxies, blue compact galaxies, and star-burst galaxies, we developed and applied the populationsynthesis techniques using the spectral library of star clus-ters (H. R. Schmitt, E. Bica, & M. G. Pastoriza 1996,MNRAS, 278, 965) to the spectra of 11 emission-line gal-axies (ELGs). All the spectral observations were made withthe 2.16 m telescope of Beijing Astronomical Observatory(BAO). From the results of population synthesis, we knownot only the stellar components and star formation historybut also the metallicity of ELGs (X. Kong & F. Z. Cheng1999, A&A, 351, 477). The results indicate that the nuclei ofthe BCGs are dominated by young components and thestar-forming process is still ongoing. The maximum metal-licity of the stellar population is The[Z/Z

_]\ [0.5.

nucleus of the BCDs, on the other hand, is dominated bythe intermediate-age component. The metallicity has, atmost, reached up to the solar value. The young component

is not so important as in the BCGs, but it is still not negligi-ble. For all BCGs and BCDs, the old population in therange is important ; the very metal rich com-[Z/Z

_]¹ 0.0

ponent provides quite a small contribution in these galaxies.Their stellar populations suggest that they are old galaxieswith intermittent star formation history. The nucleus of SBsis dominated by young components ; the contribution fromthe old-age component is very small. A good match can beachieved between the synthesized and observed spectrum ofELGs. It suggests that the stellar radiation is an importantenergy source for ELGs. We have also derived the internalreddening for the stellar population by the population syn-thesis method and the internal reddening for the emittinggas clouds by the Balmer line ratio. The former is signiÐ-cantly smaller than the latter for ELGs. A model of clumpyforeground dust, with di†erent covering factors for the gasand stars, can explain the di†erence. Combining the internalreddening value and the stellar population, we havedecreased the e†ect from the internal reddening and under-lying stellar absorption and accurately measured mostemission lines for each ELG. Using these emission lines, wehave attempted to identify the ionizing mechanism ofELGs. The ionizing mechanism for these emission-lineregions of ELGs is typical of photoionization by OB stars.Based on the stellar population of ELGs, we suggest adetailed evolutionary link among di†erent dwarf galaxies atlast : dI ] SB] BCD] BCG] dE.

To study the stellar population properties for somebright H II regions and the nucleus of M81, we have selected10 bright H II regions and the nucleus of M81 from theBeijing-Arizona-Taiwan-Connecticut (BATC) multicolorsurvey sample and obtained the spectra from the 2.16 mtelescope of the BAO. We applied the population synthesistechniques using the spectral library of star clusters to thesespectra. For the nucleus of M81, the results of populationsynthesis suggest that the contribution from the old andmetal-poor stellar population is important. For the brightH II regions, the present star formation rate is very high, andthe metallicity is di†erent for di†erent H II regions. Usingthe spectra, we have calculated the internal reddening valuefor the emission-line regions at last.

Using the data obtained from the BATC multicolor sky

1502

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POPULATION SYNTHESIS FOR EMISSION-LINE GALAXIES 1503

survey and evolutionary population synthesis models, wedeveloped a method that can be used to study two-dimensional stellar population properties for nearbyface-on galaxies. We apply this method to the nearby spiralgalaxy M81, and we have, for the Ðrst time, obtained atwo-dimensional distribution of age, metallicity, and inter-nal reddening. This method opens up a new Ðeld of studyfor the multicolor sky survey data (X. Kong et al. 2000, AJ,119, 2745). Using the extensive grid of the simple stellarpopulation covering a wide range of metallicity and age, wecalculated the colors and color indices for 13 colors in theBATC intermediate-band Ðlter system. We Ðnd that someof them can be used to resolve the age and metallicitydegeneracy, which enables us to obtain two-dimensionalmaps of the metallicity, interstellar reddening, and age ofM81. Using these colors and color indices, we quantify therelative chemical abundance, age, and reddening distribu-tions for di†erent components of M81. We Ðnd that themetallicity of M81 is about Z\ 0.03 with no signiÐcantdi†erence over the whole galaxy. In contrast, an age gra-dient is found between stellar populations of the centralregions and of the bulge and disk regions of M81: the stellarpopulation in its central regions is older than 8 Gyr whilethe disk stars are considerably younger, D2 Gyr. We alsogive the reddening distribution in M81. Some dust lanes arefound in the galaxy bulge region, and the reddening in theouter disk is higher than that in the central regions.

In order to understand how sensitive our method is todi†erent assumptions about star formation history and dif-ferent stellar population synthesis models, we have studiedan exponential star formation history and compared theresults obtained from di†erent evolution population synthe-

sis models (C. Leitherer et al. 1996, PASP, 108, 996). WeÐnd that although the precise values of age, metallicity, andinterstellar reddening are di†erent, the general trend of themetallicity, age, and reddening distributions is similar.Finally, we have compared the internal reddening andmetallicity maps of M81 with previous studies. We Ðnd thatthe agreements are generally good. In addition, we Ðnd thatthe properties for the bright H II regions and other partsmay be di†erent. The results of M81 presented here illus-trate that our method and observational data provide anefficient way to study the distribution of metallicity, age,and interstellar reddening for nearby face-on galaxies.

The evolutionary population synthesis technique is avery efficient method to infer the stellar components forgalaxies, but the problem is the degeneracy of the e†ectsfrom variations in age and in metallicity. To resolve thisdegeneracy, we apply a di†erent sophisticated statisticaltechnique, principal component analysis (X. Kong & F. Z.Cheng 2000, Chinese Phys. Lett., in press), to the spectralindex sample of simple stellar population with metallicitiesand age. We want to extract some spectral indices that canbe used to determine the age of the stellar population.Besides the well-known age-sensitive index Hb, we also Ðndsome other spectral indices have great potential to deter-mine the age of stellar population, such as G4300, Fe4383,

and Mg b. In addition, we Ðnd that these spectralC24668,indicesÏ sensitivity to age depends on the metallicity of asimple stellar population : Hb and G4300 are more suited todetermine the age of low-metallicity stellar population, and

and Mgb are more suited to the high-metallicityC24668stellar population. It is the Ðrst time that this method hasbeen used for selecting the age-sensitive spectral indices.

2000 PASP, 112 :1502È1503

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