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Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33
The Astrophysical Journal (2003)
  • Sarah J.U. Higdon, Cornell University
  • J. L. Higdon, Cornell University
  • J. M. van der Hulst, University of Groningen
  • G. J. Stacey, Cornell University
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 FIRLWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35 K  T  49 K. The [C ii] 158 lm line flux is 0.2%–0.7% FIRLWS, which is typical of values seen (0.1%–1% FIR) in the nuclei of star-forming galaxies. The [C ii]/FIRLWS 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 lm 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 h in 103:1 cm3, an average incident far-ultraviolet flux (in units of the local interstellar value) h i G0 ¼ 102:4, a gas temperature T 200 K, and an AV 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. h i G0 is at the lower end of the range observed in samples of spiral and starburst galaxies (2:2  log G0  5), and log h i ðnÞ sits comfortably in the middle of the observed range (1:8  log n  4:2).
Keywords
  • Galaxies,
  • Individual,
  • M33,
  • ISM,
  • H ii regions,
  • Infrared,
  • ISM Stars,
  • Formation
Publication Date
July 20, 2003
DOI
10.1086/375623
Citation Information
Sarah J.U. Higdon, J. L. Higdon, J. M. van der Hulst and G. J. Stacey. "Infrared Space Observatory Long-Wavelength Spectrometer Spectroscopy of Star-forming Regions in M33" The Astrophysical Journal Vol. 592 Iss. 1 (2003) p. 161 - 171
Available at: http://works.bepress.com/sarah_higdon/34/