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Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies

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Abstract
We present a study of ionized gas, polycyclic aromatic hydrocarbons (PAHs), and H-2 emission in the halos of three edge-on galaxies, NGC 891, NGC 5775, and NGC 3044, based on 10-20 mu m Spitzer Space Telescope spectra. The [Ne III]/[Ne II] ratio, an excellent measure of radiation hardness, rises with z in the halo of NGC 891. It is also higher in the halo of NGC 5775 than in the disk. NGC 3044 presents a more confusing situation. To explain the [Ne III]/[Ne II] as well as optical line ratio behavior in NGC 891, we carry out a simple exploration of parameter space with CLOUDY, which indicates a large increase in radiation temperature with height. Illustrative examples of physical models using a Monte Carlo radiative transfer code show that the rising neon ratio may be explained by adding a vertically extended, hot stellar source to a thin disk of massive stars. However, several other sources of hard spectra may be relevant. PAH features have scale heights of 430-530 pc in NGC 891 and 720-1080 pc in NGC 5775, suggesting they can be transported by disk-halo flows. Within NGC 891 and NGC 5775, scale heights are similar for all PAHs. For NGC 891, the scale heights exceed that of 8 mu m emission, indicating a transition from more ionized to more neutral PAHs with height. Most PAH equivalent widths are higher in the halos. H(2)17.03 mu m emission with scale heights of 550-580 pc in NGC 891 and 850 pc in NGC 5775 suggests a molecular component in a surprisingly thick layer.
Keywords
POLYCYCLIC AROMATIC-HYDROCARBONS, HIGH-VELOCITY CLOUDS, FINE-STRUCTURE, LINES, SPITZER-IRS SPECTROSCOPY, MULTIPHASE GASEOUS HALOS, HOT SUBDWARF, STARS, DIGITAL-SKY-SURVEY, NGC 891, SPIRAL GALAXIES, H-ALPHA, galaxies: individual (NGC 891, NGC 5775, NGC 3044), galaxies: ISM, galaxies: spiral, methods: numerical

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Citation

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MLA
Rand, Richard J., et al. “Infrared Spectroscopy of the Diffuse Ionized Halos of Edge-on Galaxies.” ASTROPHYSICAL JOURNAL, vol. 728, no. 2, 2011.
APA
Rand, R. J., Wood, K., Benjamin, R. A., & van der Wel, S. M. (2011). Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies. ASTROPHYSICAL JOURNAL, 728(2).
Chicago author-date
Rand, Richard J, Kenneth Wood, Robert A Benjamin, and Sharon Meidt van der Wel. 2011. “Infrared Spectroscopy of the Diffuse Ionized Halos of Edge-on Galaxies.” ASTROPHYSICAL JOURNAL 728 (2).
Chicago author-date (all authors)
Rand, Richard J, Kenneth Wood, Robert A Benjamin, and Sharon Meidt van der Wel. 2011. “Infrared Spectroscopy of the Diffuse Ionized Halos of Edge-on Galaxies.” ASTROPHYSICAL JOURNAL 728 (2).
Vancouver
1.
Rand RJ, Wood K, Benjamin RA, van der Wel SM. Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies. ASTROPHYSICAL JOURNAL. 2011;728(2).
IEEE
[1]
R. J. Rand, K. Wood, R. A. Benjamin, and S. M. van der Wel, “Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies,” ASTROPHYSICAL JOURNAL, vol. 728, no. 2, 2011.
@article{8622877,
  abstract     = {We present a study of ionized gas, polycyclic aromatic hydrocarbons (PAHs), and H-2 emission in the halos of three edge-on galaxies, NGC 891, NGC 5775, and NGC 3044, based on 10-20 mu m Spitzer Space Telescope spectra. The [Ne III]/[Ne II] ratio, an excellent measure of radiation hardness, rises with z in the halo of NGC 891. It is also higher in the halo of NGC 5775 than in the disk. NGC 3044 presents a more confusing situation. To explain the [Ne III]/[Ne II] as well as optical line ratio behavior in NGC 891, we carry out a simple exploration of parameter space with CLOUDY, which indicates a large increase in radiation temperature with height. Illustrative examples of physical models using a Monte Carlo radiative transfer code show that the rising neon ratio may be explained by adding a vertically extended, hot stellar source to a thin disk of massive stars. However, several other sources of hard spectra may be relevant. PAH features have scale heights of 430-530 pc in NGC 891 and 720-1080 pc in NGC 5775, suggesting they can be transported by disk-halo flows. Within NGC 891 and NGC 5775, scale heights are similar for all PAHs. For NGC 891, the scale heights exceed that of 8 mu m emission, indicating a transition from more ionized to more neutral PAHs with height. Most PAH equivalent widths are higher in the halos. H(2)17.03 mu m emission with scale heights of 550-580 pc in NGC 891 and 850 pc in NGC 5775 suggests a molecular component in a surprisingly thick layer.},
  articleno    = {163},
  author       = {Rand, Richard J and Wood, Kenneth and Benjamin, Robert A and van der Wel, Sharon Meidt},
  issn         = {0004-637X},
  journal      = {ASTROPHYSICAL JOURNAL},
  keywords     = {POLYCYCLIC AROMATIC-HYDROCARBONS,HIGH-VELOCITY CLOUDS,FINE-STRUCTURE,LINES,SPITZER-IRS SPECTROSCOPY,MULTIPHASE GASEOUS HALOS,HOT SUBDWARF,STARS,DIGITAL-SKY-SURVEY,NGC 891,SPIRAL GALAXIES,H-ALPHA,galaxies: individual (NGC 891,NGC 5775,NGC 3044),galaxies: ISM,galaxies: spiral,methods: numerical},
  language     = {eng},
  number       = {2},
  pages        = {22},
  title        = {Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies},
  url          = {http://dx.doi.org/10.1088/0004-637X/728/2/163},
  volume       = {728},
  year         = {2011},
}

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