DENSITY STRUCTURE OF THE GIANT HII REGION NGC 2363

ENRIQUE PÉREZ, ROSA GONZÁLEZ DELGADO and JOSÉ M. VÍLCHEZInstituto de Astrofísica de Andalucía, Aptdo. 3004, 18080 Granada, Spain

E-mail: {eperez;rosa;jvm}@iaa.es

Abstract. We perform a detailed measurement of the electron density along two slit position anglesin the bright, low metallicity extragalactic HII region NGC 2363. A comparison of the density struc-tures obtained with the two independent diagnostics given by [AIV]4711/4740 and [SII]6716/6731,show that they present both different absolute values and different radial dependencies, with the[AIV] densities reaching up to 1000 cm−3. We explore the implications for the computation of theHe abundance.

Keywords: ISM: HII regions, ISM: individual: NGC 2363

1. Introduction

Giant HII regions are bright tracers of massive star formation at extragalactic dis-tances. In this way, they are useful to understand the star forming processes and theevolution of the chemical abundances in galaxies. They also serve as checks forstellar atmosphere models, and as filters for the ultraviolet radiation that escapesfrom their stellar clusters and ionizes the surrounding interstellar and intergalacticmedia. One important application of HII region studies is the computation of prim-ordial He abundance, of paramount importance as a constraint for Big Bang nucle-osynthesis models. It is thus important that we do clearly understand the ionization,density, thermal and chemical structure of HII regions, if we are to rely heavily onthe inferences drawn from their study.

We know that giant HII regions can be quite structured and complex systems.Indeed they are the remains from the star formation process that occurred in a giantmolecular complex; we know that these complexes have a hierarchical distributionof densities and velocities. Soon after the onset of the star forming episode, themassive stars begin to modify the HII region structure by means of input energy inthe form of high energy ionizing photons and mechanical energy from their strongwinds. Thus HII regions are not the simple uniform objects that we are forced toconsider in order to extract some primary useful properties. The importance ofunderstanding the thermal and density structure of HII regions has been manifes-ted, for example, in the problem of temperatures fluctuations (e.g., see Peimbert inthese Proceedings). Contrary to the electron temperature, there have been relativelyfew detailed studies of the density structure of HII regions (e.g., Rubin, 1989;Castañeda et al., 1992; Franco et al., 2000). From numerical simulations, Rubin

Astrophysics and Space Science 277 (Suppl.): 83–86, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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(1989) concludes that when the density is not constant, there can be importantbiases in the compilation of chemical abundances. González Delgado et al. (1994,hereafter GD94) and Luridiana et al. (1999) have studied the effects of temperaturefluctuations in the bright nearby giant HII region NGC 2363. In this contribution wepresent preliminary results from a comparative study of two independent electrondensity diagnostics in NGC 2363.

2. Two Independent Diagnostics for the Electron Density

We have reanalysed the high quality long slit data studied by GD94. In GD94 weextracted a few 1D spectra by summing a number of spatial increments along theslit; here we make a pixel-by-pixel analysis of the same data, in order to extractmore detailed information about the density structure. For this purpose, we ana-lyse two independent density diagnostic emission line ratios, [SII]6716/6731 and[AIV]4711/4740. These two diagnostics are sensitive to approximately the sameelectron density range, but they map quite different parts of the ionization structure.

At the spectral resolution of the data, the [AIV] 4711 Å line is partially blendedwith HeI 4713 Å. We correct for the HeI 4713 Å contribution by modelling theratio HeI 4713/4471 based on the ratio HeI 7065/5876.

The resulting A+3 and S+ electron density structures are shown in Figure 1, forthe two main knots in NGC 2363, labelled A and B (cf. GD94). The full line isthe intensity of the emission line [AIV] 4711 Å, drawn as a reference; the dashedand dotted lines are the electron densities from [AIV] and [SII], respectively. The[AIV] density has a maximum of 800–1000 cm−3 at the brightest part of the knot,and decreases outwards. The [SII], however, does not appear related to the fluxdistribution; it has a constant value close to the low density limit in knot B, whilein knot A it rises from the low density limit in one side of the knot to about 400cm−3 at the other side.

3. Discussion

These distinct results from such different ions surely imply that there is a densitystratification in NGC 2363, with higher densities present in the high excitationregion as mapped by A+3, and significantly lower densities (a factor between 2and 10) in the low excitation region mapped by S+. More detailed results andmodelling will be presented elsewhere (Pérez et al., in preparation), but it is pos-sible to think of a straightforward scenario that can accomodate these results. Ifthere is a density structure with higher densities close to the ionization source, anddecreasing outwards, then the A+3 that is closer to this ionizing source will mapthe higher densities, while the S+ that will be present in a relatively narrow outerzone will map the lower density gas, and its integral along the line of sight will beapproximately constant if this region is sufficiently narrow.

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Figure 1. Top: Density structures along the slit crossing knot A at position angle 187◦. The full linerepresents the intensity of the emission line [AIV]4740Å, given for reference (right hand side axisscale, in arbitrary units). The long-dash line shows the electron density computed from the ratio[AIV]4711/4740, while the dotted line gives the density from the ratio [SII]6716/6731; for both ofthese the scale is shown on the left hand side axis. Bottom: Same as top, but for the slit crossingthrough knot B at a position angle 130◦.

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In this scenario, the He+ region will form in an intermediate zone between theA+3 and the S+, the details depending on the exact density structure and ionizingspectral energy distribution. It is possible to show that, in general, it will share morein common with the A+3 than with the S+ ionization zones. This implies that, atleast in NGC 2363, the computation of the helium abundance will have to take intoaccount these higher values of the density, that affect the helium level populations,and thus the line ratios. For example, at the electron temperature of NGC 2363,T e ∼ 16000, there is a factor of 7 increase in the collisional-to-recombinationratio for HeI 4471 Å at a density of 1000 cm−3 compared with a density of 100cm−3. This is potentially very dangerous for the low metallicity, high electrontemperature galaxies used for the computation of the primordial helium, becausethe collisional-to-recombination ratio increases faster than linearly with Te and Ne.We are working out a more elaborated analysis and modelling.

We do not know how common and steep are density structures in giant HII

regions, but the case of NGC 2363 is telling us a cautionary tale that we should bestriving to obtain significantly higher S/N spectra to understand their density andtemperature structures, that have been shown to bear important consequences forthe computation of e.g. the primordial helium abundance.

References

Castañeda, H.O., Vílchez, J.M. and Copetti, M.V.F.: 1992, A&A 260, 370.Franco, J., García-Barreto, J.A. and de la Fuente, E.: 2000, ApJ 544, 277.González Delgado, et al.: 1994, ApJ 437, 239.Luridiana, V., Peimbert, M. and Leitherer, C.: 1999.Rubin, R.H.: 1989, ApJS 69, 897.