The interstellar medium of star-forming irregular galaxies: The view with ISO

Deidre A. Hunter, Michael Kaufman, David J. Hollenbach, Robert H. Rubin, Sangeeta Malhotra, Daniel A. Dale, James R. Brauher, Nancy A. Silbermann, George Helou, Alessandra Contursi, Steven D. Lord

Research output: Contribution to journalArticle

60 Citations (Scopus)

Abstract

We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 μm that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 μm that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] λ158 μm and [O I] λ63 μm, important coolants of photodissociation regions (PDRs), and [O III] λ188 μm and [N II] λ122 μm, which come from ionized gas. [O I] λ145 μm and [O III] λ52 μm were measured in one galaxy as well. These data are combined with PDR and H II region models to deduce properties of the interstellar medium of these galaxies. We find a decrease in PAH emission in our irregulars relative to small grain, FIR, and Hα emissions for increasing FIR color temperature, which we interpret as an increase in the radiation field due to star formation resulting in a decrease in PAH emission. The f15/f ratio is constant for our irregulars, and we suggest that the 15 μm emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possibly Lyα photons trapped in H II regions. The low f15/f ratio, as well as the high f[C II]/f15 ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer dust grains being, on average, near heating sources. We find that, as in spirals, a large fraction of the [C II] emission comes from PDRs. This is partly a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that produce the 6.75 μm emission and the PAHs that heat the PDR are the same, then the much higher f[C II]/f6.75 ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in spirals. Alternatively, the carrier of the 6.75 μm feature tracks, but contributes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 μm feature. The high f[O III]/f[C II] ratio may indicate a smaller solid angle of optically thick PDRs outside the H II regions compared to spirals. The very high L[C II]/LCO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irregulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spirals; the third irregular has properties like those in typical spirals. We deduce the presence of several molecular clouds in each galaxy with masses much larger than typical GMCs.

Original languageEnglish (US)
Pages (from-to)121-145
Number of pages25
JournalAstrophysical Journal
Volume553
Issue number1 PART 1
DOIs
StatePublished - May 20 2001
Externally publishedYes

Fingerprint

irregular galaxies
polycyclic aromatic hydrocarbons
photodissociation
PAH
stars
galaxies
H II regions
dust
heat
radiation distribution
heating
transient heating
stellar radiation
photons
coolants
ionized gases
molecular clouds
ultraviolet radiation
star formation
infrared spectroscopy

Keywords

  • Galaxies : Irregular
  • Galaxies : ISM
  • Galaxies : Stellar content
  • Galaxies: Individual (IC 10, IC 4662, NGC 1156, NGC 1569, NGC 2366, NGC 6822)
  • Infrared: Galaxies

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Hunter, D. A., Kaufman, M., Hollenbach, D. J., Rubin, R. H., Malhotra, S., Dale, D. A., ... Lord, S. D. (2001). The interstellar medium of star-forming irregular galaxies: The view with ISO. Astrophysical Journal, 553(1 PART 1), 121-145. https://doi.org/10.1086/320654

The interstellar medium of star-forming irregular galaxies : The view with ISO. / Hunter, Deidre A.; Kaufman, Michael; Hollenbach, David J.; Rubin, Robert H.; Malhotra, Sangeeta; Dale, Daniel A.; Brauher, James R.; Silbermann, Nancy A.; Helou, George; Contursi, Alessandra; Lord, Steven D.

In: Astrophysical Journal, Vol. 553, No. 1 PART 1, 20.05.2001, p. 121-145.

Research output: Contribution to journalArticle

Hunter, DA, Kaufman, M, Hollenbach, DJ, Rubin, RH, Malhotra, S, Dale, DA, Brauher, JR, Silbermann, NA, Helou, G, Contursi, A & Lord, SD 2001, 'The interstellar medium of star-forming irregular galaxies: The view with ISO', Astrophysical Journal, vol. 553, no. 1 PART 1, pp. 121-145. https://doi.org/10.1086/320654
Hunter DA, Kaufman M, Hollenbach DJ, Rubin RH, Malhotra S, Dale DA et al. The interstellar medium of star-forming irregular galaxies: The view with ISO. Astrophysical Journal. 2001 May 20;553(1 PART 1):121-145. https://doi.org/10.1086/320654
Hunter, Deidre A. ; Kaufman, Michael ; Hollenbach, David J. ; Rubin, Robert H. ; Malhotra, Sangeeta ; Dale, Daniel A. ; Brauher, James R. ; Silbermann, Nancy A. ; Helou, George ; Contursi, Alessandra ; Lord, Steven D. / The interstellar medium of star-forming irregular galaxies : The view with ISO. In: Astrophysical Journal. 2001 ; Vol. 553, No. 1 PART 1. pp. 121-145.
@article{817ded04ca484037b3bc41f9c913fabc,
title = "The interstellar medium of star-forming irregular galaxies: The view with ISO",
abstract = "We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 μm that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 μm that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] λ158 μm and [O I] λ63 μm, important coolants of photodissociation regions (PDRs), and [O III] λ188 μm and [N II] λ122 μm, which come from ionized gas. [O I] λ145 μm and [O III] λ52 μm were measured in one galaxy as well. These data are combined with PDR and H II region models to deduce properties of the interstellar medium of these galaxies. We find a decrease in PAH emission in our irregulars relative to small grain, FIR, and Hα emissions for increasing FIR color temperature, which we interpret as an increase in the radiation field due to star formation resulting in a decrease in PAH emission. The f15/fHα ratio is constant for our irregulars, and we suggest that the 15 μm emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possibly Lyα photons trapped in H II regions. The low f15/fHα ratio, as well as the high f[C II]/f15 ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer dust grains being, on average, near heating sources. We find that, as in spirals, a large fraction of the [C II] emission comes from PDRs. This is partly a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that produce the 6.75 μm emission and the PAHs that heat the PDR are the same, then the much higher f[C II]/f6.75 ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in spirals. Alternatively, the carrier of the 6.75 μm feature tracks, but contributes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 μm feature. The high f[O III]/f[C II] ratio may indicate a smaller solid angle of optically thick PDRs outside the H II regions compared to spirals. The very high L[C II]/LCO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irregulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spirals; the third irregular has properties like those in typical spirals. We deduce the presence of several molecular clouds in each galaxy with masses much larger than typical GMCs.",
keywords = "Galaxies : Irregular, Galaxies : ISM, Galaxies : Stellar content, Galaxies: Individual (IC 10, IC 4662, NGC 1156, NGC 1569, NGC 2366, NGC 6822), Infrared: Galaxies",
author = "Hunter, {Deidre A.} and Michael Kaufman and Hollenbach, {David J.} and Rubin, {Robert H.} and Sangeeta Malhotra and Dale, {Daniel A.} and Brauher, {James R.} and Silbermann, {Nancy A.} and George Helou and Alessandra Contursi and Lord, {Steven D.}",
year = "2001",
month = "5",
day = "20",
doi = "10.1086/320654",
language = "English (US)",
volume = "553",
pages = "121--145",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1 PART 1",

}

TY - JOUR

T1 - The interstellar medium of star-forming irregular galaxies

T2 - The view with ISO

AU - Hunter, Deidre A.

AU - Kaufman, Michael

AU - Hollenbach, David J.

AU - Rubin, Robert H.

AU - Malhotra, Sangeeta

AU - Dale, Daniel A.

AU - Brauher, James R.

AU - Silbermann, Nancy A.

AU - Helou, George

AU - Contursi, Alessandra

AU - Lord, Steven D.

PY - 2001/5/20

Y1 - 2001/5/20

N2 - We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 μm that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 μm that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] λ158 μm and [O I] λ63 μm, important coolants of photodissociation regions (PDRs), and [O III] λ188 μm and [N II] λ122 μm, which come from ionized gas. [O I] λ145 μm and [O III] λ52 μm were measured in one galaxy as well. These data are combined with PDR and H II region models to deduce properties of the interstellar medium of these galaxies. We find a decrease in PAH emission in our irregulars relative to small grain, FIR, and Hα emissions for increasing FIR color temperature, which we interpret as an increase in the radiation field due to star formation resulting in a decrease in PAH emission. The f15/fHα ratio is constant for our irregulars, and we suggest that the 15 μm emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possibly Lyα photons trapped in H II regions. The low f15/fHα ratio, as well as the high f[C II]/f15 ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer dust grains being, on average, near heating sources. We find that, as in spirals, a large fraction of the [C II] emission comes from PDRs. This is partly a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that produce the 6.75 μm emission and the PAHs that heat the PDR are the same, then the much higher f[C II]/f6.75 ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in spirals. Alternatively, the carrier of the 6.75 μm feature tracks, but contributes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 μm feature. The high f[O III]/f[C II] ratio may indicate a smaller solid angle of optically thick PDRs outside the H II regions compared to spirals. The very high L[C II]/LCO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irregulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spirals; the third irregular has properties like those in typical spirals. We deduce the presence of several molecular clouds in each galaxy with masses much larger than typical GMCs.

AB - We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 μm that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 μm that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] λ158 μm and [O I] λ63 μm, important coolants of photodissociation regions (PDRs), and [O III] λ188 μm and [N II] λ122 μm, which come from ionized gas. [O I] λ145 μm and [O III] λ52 μm were measured in one galaxy as well. These data are combined with PDR and H II region models to deduce properties of the interstellar medium of these galaxies. We find a decrease in PAH emission in our irregulars relative to small grain, FIR, and Hα emissions for increasing FIR color temperature, which we interpret as an increase in the radiation field due to star formation resulting in a decrease in PAH emission. The f15/fHα ratio is constant for our irregulars, and we suggest that the 15 μm emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possibly Lyα photons trapped in H II regions. The low f15/fHα ratio, as well as the high f[C II]/f15 ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer dust grains being, on average, near heating sources. We find that, as in spirals, a large fraction of the [C II] emission comes from PDRs. This is partly a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that produce the 6.75 μm emission and the PAHs that heat the PDR are the same, then the much higher f[C II]/f6.75 ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in spirals. Alternatively, the carrier of the 6.75 μm feature tracks, but contributes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 μm feature. The high f[O III]/f[C II] ratio may indicate a smaller solid angle of optically thick PDRs outside the H II regions compared to spirals. The very high L[C II]/LCO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irregulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spirals; the third irregular has properties like those in typical spirals. We deduce the presence of several molecular clouds in each galaxy with masses much larger than typical GMCs.

KW - Galaxies : Irregular

KW - Galaxies : ISM

KW - Galaxies : Stellar content

KW - Galaxies: Individual (IC 10, IC 4662, NGC 1156, NGC 1569, NGC 2366, NGC 6822)

KW - Infrared: Galaxies

UR - http://www.scopus.com/inward/record.url?scp=0035918669&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0035918669&partnerID=8YFLogxK

U2 - 10.1086/320654

DO - 10.1086/320654

M3 - Article

AN - SCOPUS:0035918669

VL - 553

SP - 121

EP - 145

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1 PART 1

ER -