Star formation in giant extragalactic H II regions

Abstract

We have compared the B V R H? photometry of a sample of giant extragalactic H?II regions with an evolutionary population synthesis model, after correcting for extinction by dust. The basic diagnostic diagrams used are the ratio of Lyman continuum luminosity to blue band luminosity, denoted by log(Q(H?)/L_B), plotted against the B–V and V–R colours. The results of the study are summarized below.

The stellar component experiences a lower amount of extinction compared to the gaseous component. There is evidence for a significant escape of Lyman continuum photons from at least some nebulae. A majority of the regions have undergone more than one burst of star formation during the last 10 Myr. About 10? M? of gas is converted into stars during each burst of star formation.

The evaluation of the IMF slope and upper mass cutoff becomes difficult unless accurate allowance is made for the first three effects. We discuss these results below in the context of future studies aimed at better modelling of these regions.

1. Improvements in the Optical Data Our photometric data was essentially restricted to B, V, R, and H? bands. The detection of the red supergiant population demonstrates the importance of including the I band in the photometry. We used published slit spectroscopic information on the Balmer decrement and the [O?III]/H? ratio because of difficulties in reaching useful signal-to-noise levels with the 1?m telescope from which a major fraction of the data was obtained. Aperture photometry in these bands is desirable so that measurements representative of the entire cluster can be derived. Because these emission lines lie near the central wavelengths of the B and V bands, either a specially designed broadband filter or a calibrated combination of B and V band data is needed to subtract the in?band continuum. Additionally, slit spectroscopy or area spectroscopy will help estimate spatial fluctuations of these quantities within a region.

We highlighted the importance of background subtraction in Chapter III. The galaxy background was carefully estimated in this study by choosing a nearby region where the contribution from the GEHR is negligible. A continuum model of the entire galaxy may provide a better background estimate. Multi?band observations are required to effectively separate the GEHR emission from the galaxy background of older stars. Improving sensitivity in the optical region will also contribute significantly to understanding these regions. For example:

Measurements of low-ionization zones near GEHRs can reveal champagne flows. Improved continuum sensitivity can help avoid enlarging apertures to include nearby sources. Deeper measurements would allow individual study of faint regions and provide better constraints on models and extinction in the youngest regions—particularly those that may show evidence of a high upper mass cutoff.

We found a population older than ~6 Myr coexisting with the younger population in the majority of GEHRs. This evolutionary stage is characterized by the appearance of red supergiants. Spectroscopically, these can be identified by the near?infrared Ca?II triplet absorption. Observations of this feature will help directly estimate the number of red supergiants.

Recently, considerable interest has been focused on the early evolution of starbursts when Wolf–Rayet (WR) stars form from stars with initial masses above ~40 M?. These stars harden the ionizing radiation field, but uncertainties remain due to the complexities of WR model atmospheres. H?II regions containing WR stars-known as “warmers”-display higher excitation spectra characterized by:

enhanced [O?III] lines enhanced [N?II] lines presence of the He?II ?4686 Å emission line

Spectroscopic measurements of these lines, along with photometry in the He?II band, will be valuable in estimating the contribution of WR stars.