Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium

S. Malhotra, M. J. Kaufman, D. Hollenbach, G. Helou, R. H. Rubin, J. Brauher, D. Dale, N. Y. Lu, S. Lord, G. Stacey, A. Contursi, D. A. Hunter, H. Dinerstein

Research output: Contribution to journalArticle

250 Citations (Scopus)

Abstract

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fv(60 μm)/Fv(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fv(60 μm)/Fv(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fv(60 μm)/Fv(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ na, where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

Original languageEnglish (US)
Pages (from-to)766-786
Number of pages21
JournalAstrophysical Journal
Volume561
Issue number2 PART 1
DOIs
StatePublished - Nov 10 2001
Externally publishedYes

Fingerprint

infrared spectroscopy
galaxies
dust
gas
stars
photodissociation
heating
cooling
ionized gases
optical depth
continuums
scaling
Infrared Space Observatory (ISO)
surface temperature
spectrometer
observatory
active galaxies
physical property
temperature
physical conditions

Keywords

  • Galaxies: ISM
  • H II regions
  • ISM: atoms
  • ISM: general
  • ISM: lines and bands
  • Radiation mechanisms: thermal

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Malhotra, S., Kaufman, M. J., Hollenbach, D., Helou, G., Rubin, R. H., Brauher, J., ... Dinerstein, H. (2001). Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium. Astrophysical Journal, 561(2 PART 1), 766-786. https://doi.org/10.1086/323046

Far-infrared spectroscopy of normal galaxies : Physical conditions in the interstellar medium. / Malhotra, S.; Kaufman, M. J.; Hollenbach, D.; Helou, G.; Rubin, R. H.; Brauher, J.; Dale, D.; Lu, N. Y.; Lord, S.; Stacey, G.; Contursi, A.; Hunter, D. A.; Dinerstein, H.

In: Astrophysical Journal, Vol. 561, No. 2 PART 1, 10.11.2001, p. 766-786.

Research output: Contribution to journalArticle

Malhotra, S, Kaufman, MJ, Hollenbach, D, Helou, G, Rubin, RH, Brauher, J, Dale, D, Lu, NY, Lord, S, Stacey, G, Contursi, A, Hunter, DA & Dinerstein, H 2001, 'Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium', Astrophysical Journal, vol. 561, no. 2 PART 1, pp. 766-786. https://doi.org/10.1086/323046
Malhotra S, Kaufman MJ, Hollenbach D, Helou G, Rubin RH, Brauher J et al. Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium. Astrophysical Journal. 2001 Nov 10;561(2 PART 1):766-786. https://doi.org/10.1086/323046
Malhotra, S. ; Kaufman, M. J. ; Hollenbach, D. ; Helou, G. ; Rubin, R. H. ; Brauher, J. ; Dale, D. ; Lu, N. Y. ; Lord, S. ; Stacey, G. ; Contursi, A. ; Hunter, D. A. ; Dinerstein, H. / Far-infrared spectroscopy of normal galaxies : Physical conditions in the interstellar medium. In: Astrophysical Journal. 2001 ; Vol. 561, No. 2 PART 1. pp. 766-786.
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T1 - Far-infrared spectroscopy of normal galaxies

T2 - Physical conditions in the interstellar medium

AU - Malhotra, S.

AU - Kaufman, M. J.

AU - Hollenbach, D.

AU - Helou, G.

AU - Rubin, R. H.

AU - Brauher, J.

AU - Dale, D.

AU - Lu, N. Y.

AU - Lord, S.

AU - Stacey, G.

AU - Contursi, A.

AU - Hunter, D. A.

AU - Dinerstein, H.

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N2 - The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fv(60 μm)/Fv(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fv(60 μm)/Fv(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fv(60 μm)/Fv(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ na, where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

AB - The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fv(60 μm)/Fv(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fv(60 μm)/Fv(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fv(60 μm)/Fv(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ na, where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

KW - Galaxies: ISM

KW - H II regions

KW - ISM: atoms

KW - ISM: general

KW - ISM: lines and bands

KW - Radiation mechanisms: thermal

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