Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33

Abstract

We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer (LWS) far-infrared (FIR) spectra of the nucleus and six giant H II regions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133). The seven fine-structure lines observed in the FIR are used to model the H II and photodissociation regions (PDRs). There is no observed trend in the FIR properties, observed with the LWS, as a function of galactic radius or metallicity. The cold neutral medium (CNM) is the main reservoir for the atomic gas, containing between 60% and 95% of the gas. The FIR_LWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35K<=T<=49 K. The [C II] 158 μm line flux is 0.2%-0.7% FIR_LWS, which is typical of values seen (0.1%-1% FIR) in the nuclei of star-forming galaxies. The [C II]/FIR_LWS ratio peaks at the nucleus and is fairly constant across the rest of the sample. Massive star formation is traced by the intensity of the [O III] 88 μm line. The emission from the observed FIR lines that arise solely from H II regions can be modeled as a single component with a given oxygen and nitrogen abundance, effective temperature, density, and ionizing flux. There is no need for an extended low-density component (ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emission that arises from the H II regions and/or PDRs is not well constrained, but typically 5%-50% arises in the H II regions, 10%-35% from the CNM, and the bulk of the emission (40%-90%) in the PDRs. The average PDR in this sample has a gas density ~10^3.1 cm^-3, an average incident far-ultraviolet flux (in units of the local interstellar value) 0>=10^2.4, a gas temperature T~200 K, and an A_V~10 through the clouds. NGC 604 has 40% of the atomic gas residing in the PDRs, while the rest have a much smaller fraction, ~5%-15%. The PDRs are similar to those found in other star-forming galaxies such as Centaurus A. 0> is at the lower end of the range observed in samples of spiral and starburst galaxies (2.2<=logG₀<=5), and sits comfortably in the middle of the observed range (1.8<=logn<=4.2).