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The CO-to-H₂ conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies

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Abstract
We present similar to kiloparsec spatial resolution maps of the CO-to-H-2 conversion factor (alpha(CO)) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for alpha(CO) and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both alpha(CO) and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, (CO)-C-12 J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our alpha(CO) results on the more typically used (CO)-C-12 J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for alpha(CO) and the DGR. On average, alpha(CO) = 3.1M(circle dot) pc(-2) (K km s(-1))(-1) for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of alpha(CO) as a function of galactocentric radius. However, most galaxies exhibit a lower alpha(CO) value in the central kiloparsec-a factor of similar to 2 below the galaxy mean, on average. In some cases, the central alpha(CO) value can be factors of 5-10 below the standard Milky Way (MW) value of alpha(CO,MW) = 4.4 M-circle dot pc(-2) (K km s(-1))(-1). While for alpha(CO) we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate alpha(CO) for studies of nearby galaxies.
Keywords
GIANT MOLECULAR CLOUDS, POLYCYCLIC AROMATIC-HYDROCARBONS, VIOLENT, STAR-FORMATION, GAMMA-RAY EMISSION, SPIRAL GALAXIES, GALACTIC-CENTER, DWARF GALAXIES, X-FACTOR, PHOTODISSOCIATION REGIONS, INFRARED-EMISSION, dust, extinction, galaxies: ISM, infrared: ISM, ISM: molecules

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MLA
Sandstrom, KM, et al. “The CO-to-H₂ Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies.” ASTROPHYSICAL JOURNAL, vol. 777, no. 1, 2013.
APA
Sandstrom, K., Leroy, A., Walter, F., Bolatto, A., Croxall, K., Draine, B., … Vigroux, L. (2013). The CO-to-H₂ conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies. ASTROPHYSICAL JOURNAL, 777(1).
Chicago author-date
Sandstrom, KM, AK Leroy, F Walter, AD Bolatto, KV Croxall, BT Draine, CD Wilson, et al. 2013. “The CO-to-H₂ Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies.” ASTROPHYSICAL JOURNAL 777 (1).
Chicago author-date (all authors)
Sandstrom, KM, AK Leroy, F Walter, AD Bolatto, KV Croxall, BT Draine, CD Wilson, M Wolfire, D Calzetti, RC Kennicutt, G Aniano, J Donovan Meyer, A Usero, F Bigiel, E Brinks, WJG de Blok, A Crocker, D Dale, CW Engelbracht, M Galametz, B Groves, LK Hunt, J Koda, K Kreckel, H Linz, Sharon Meidt van der Wel, E Pellegrini, H-W Rix, H Roussel, E Schinnerer, A Schruba, K-F Schuster, R Skibba, T van der Laan, P Appleton, L Armus, B Brandl, K Gordon, J Hinz, O Krause, E Montiel, M Sauvage, A Schmiedeke, JDT Smith, and L Vigroux. 2013. “The CO-to-H₂ Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies.” ASTROPHYSICAL JOURNAL 777 (1).
Vancouver
1.
Sandstrom K, Leroy A, Walter F, Bolatto A, Croxall K, Draine B, et al. The CO-to-H₂ conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies. ASTROPHYSICAL JOURNAL. 2013;777(1).
IEEE
[1]
K. Sandstrom et al., “The CO-to-H₂ conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies,” ASTROPHYSICAL JOURNAL, vol. 777, no. 1, 2013.
@article{8622810,
  abstract     = {We present similar to kiloparsec spatial resolution maps of the CO-to-H-2 conversion factor (alpha(CO)) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for alpha(CO) and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both alpha(CO) and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, (CO)-C-12 J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our alpha(CO) results on the more typically used (CO)-C-12 J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for alpha(CO) and the DGR. On average, alpha(CO) = 3.1M(circle dot) pc(-2) (K km s(-1))(-1) for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of alpha(CO) as a function of galactocentric radius. However, most galaxies exhibit a lower alpha(CO) value in the central kiloparsec-a factor of similar to 2 below the galaxy mean, on average. In some cases, the central alpha(CO) value can be factors of 5-10 below the standard Milky Way (MW) value of alpha(CO,MW) = 4.4 M-circle dot pc(-2) (K km s(-1))(-1). While for alpha(CO) we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate alpha(CO) for studies of nearby galaxies.},
  articleno    = {5},
  author       = {Sandstrom, KM and Leroy, AK and Walter, F and Bolatto, AD and Croxall, KV and Draine, BT and Wilson, CD and Wolfire, M and Calzetti, D and Kennicutt, RC and Aniano, G and Donovan Meyer, J and Usero, A and Bigiel, F and Brinks, E and de Blok, WJG and Crocker, A and Dale, D and Engelbracht, CW and Galametz, M and Groves, B and Hunt, LK and Koda, J and Kreckel, K and Linz, H and van der Wel, Sharon Meidt and Pellegrini, E and Rix, H-W and Roussel, H and Schinnerer, E and Schruba, A and Schuster, K-F and Skibba, R and van der Laan, T and Appleton, P and Armus, L and Brandl, B and Gordon, K and Hinz, J and Krause, O and Montiel, E and Sauvage, M and Schmiedeke, A and Smith, JDT and Vigroux, L},
  issn         = {0004-637X},
  journal      = {ASTROPHYSICAL JOURNAL},
  keywords     = {GIANT MOLECULAR CLOUDS,POLYCYCLIC AROMATIC-HYDROCARBONS,VIOLENT,STAR-FORMATION,GAMMA-RAY EMISSION,SPIRAL GALAXIES,GALACTIC-CENTER,DWARF GALAXIES,X-FACTOR,PHOTODISSOCIATION REGIONS,INFRARED-EMISSION,dust,extinction,galaxies: ISM,infrared: ISM,ISM: molecules},
  language     = {eng},
  number       = {1},
  pages        = {33},
  title        = {The CO-to-H₂ conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies},
  url          = {http://dx.doi.org/10.1088/0004-637X/777/1/5},
  volume       = {777},
  year         = {2013},
}

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