Michael A. Dopita

Compact H II Regions: What Lies Within?

This paper presents both stellar mass and H ii region diagnostics based on dusty, radiation-pressure-dominated photoionization models for compact and ultracompact H ii regions, and compares these with observational constraints. These models successfully reproduce the observed relationship between the density and the thickness of the ionized layer. The absorption of ionizing photons in the dusty ionized plasma makes denser ionized regions thinner than simple photoionization models would predict, improving the fit with the observations. The models provideagoodfittoobserveddiagnosticplotsinvolvingratiosofinfraredemissionlines,allaccessiblewiththeIRS instrument of theSpitzer Space Telescope. These give the effective temperature to an accuracy of about 2500 K and themassoftheionizingstartoaprecisionofabout � 30%.TheSiv/Siiiratioissensitivetoforegroundextinctionas well as to stellar effective temperature or mass. From this ratio, we determine that the mean extinction to observed compact H iiregions is typically AV � 30 mag. The electron temperature depends on the chemical abundances, the pressure, and theeffective temperature ofthe excitingstar. We usethesemodelsto rederivethe slope oftheGalactic abundance gradient, with the result that d log (O/H)/dRG ¼ 0:06 � 0:01 dex kpc � 1 , bringing the Galactic abundance gradient derived from compact H ii regions into closer agreement with those based on other techniques. The shape of the far-IR SED of compact H ii regions can be used to constrain the mean pressure or density in the H ii region. The Spitzer MIPS instrument should be very helpful in this regard. Subject headings: circumstellarmatter — dust,extinction — galaxies:starburst — Hiiregions — infrared:ISM — techniques: spectroscopic

Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: III. Emission Line Diagnostics of Ensembles of H II Regions

We build, as far as theory will permit, self-consistent model H II regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual H II regions in galaxies or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual H II regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter that depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why H II regions with low abundances have high excitation and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low-pressure H II regions. We present line ratios (at both optical and IR wavelengths) that provide reliable abundance diagnostics for both single H II regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual H II regions.

Local analogs of high-redshift galaxies: Interstellar medium conditions

Local analog galaxies play an important role in understanding the properties of highredshift galaxies. We present a method to select a type of local analog that closely resembles the ionized interstellar medium conditions in high-redshift galaxies. These galaxies are selected based on their locations in the [O III]/Hβ versus [N II]/Hα nebular emission-line diagnostic diagram. The ionization parameters and electron densities in these analogs are comparable to those in z 2 − 3 galaxies, but higher than those in normal SDSS galaxies by 0.6 dex and 0.9 dex, respectively. We find that the high sSFR and SFR surface density can enhance the electron densities and the ionization parameters, but still cannot fully explain the difference in ISM condition between nearby galaxies and the local analogs/high-redshift galaxies.

Properties of SN1978K from multi-wavelength observations

We update the light curves from the X-ray, optical, and radio bandpasses which we have assembled over the past decade, and present two observations in the ultraviolet using the Hubble Space Telescope Faint Object Spectrograph. The HRI X-ray light curve is constant within the errors over the entire observation period which is confirmed by ASCA GIS data obtained in 1993 and 1995. In the UV, we detected the Mg II doublet at 2800 A and a line at ∼3190 A attributed to He I 3187 at SN1978K’s position. The optical light curve is formally constant within the errors, although a slight upward trend may be present. The radio light curve continues its steep decline. The longer time span of our radio observations compared to previous studies shows that SN1978K belongs in the class of highly X-ray and radio-luminous supernovae. The Mg II doublet flux ratio implies the quantity of line optical depth times density is ∼1014 cm−3. The emission site must lie in the shocked gas.