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A determination of the cosmic microwave background temperature using Galactic molecules.

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Title: A determination of the cosmic microwave background temperature using Galactic molecules.
Authors: Cooke, Ryan1,2 (AUTHOR), Welsh, Louise3,4,5 (AUTHOR)
Source: Monthly Notices of the Royal Astronomical Society. Jan2025, Vol. 536 Issue 2, p1980-1999. 20p.
Subject Terms: *MOLECULAR clouds, *ABSORPTION spectra, *MILKY Way, *RADICALS (Chemistry), *THERMOMETERS
Abstract: We report a new, reliable determination of the cyano radical (CN) excitation temperature of diffuse molecular clouds in the Milky Way, based on ultrahigh spectral resolution observations. Our determination is based on CN |$B^{2}\Sigma ^{+}-X^{2}\Sigma ^{+}$| (0, 0) vibronic band absorption spectra seen along the lines of sight to eight bright Galactic stars. Our analysis is conducted blind, and we account for multiple sources of systematic uncertainty. Like previous studies, our excitation temperature measures exhibit an intrinsic scatter that exceeds the quoted uncertainties. Accounting for this scatter, we derive a 3 per cent determination of the typical CN excitation temperature, |$T_{01}=2.769^{+0.084}_{-0.072}~{\rm K}$|⁠ , which is consistent with the direct determination of the cosmic microwave background (CMB) temperature. We also perform a single joint fit to all sightlines simultaneously, and find that our data can be fit with an excitation temperature |$T_{01}=2.725\pm 0.015~{\rm K}$| – a 0.55 per cent measure that is consistent with the CMB temperature. We propose a future observational strategy to reduce systematic uncertainties and firmly test the limitations of using CN as a CMB thermometer. [ABSTRACT FROM AUTHOR]
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  Label: Title
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  Data: A determination of the cosmic microwave background temperature using Galactic molecules.
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  Data: <searchLink fieldCode="AR" term="%22Cooke%2C+Ryan%22">Cooke, Ryan</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Welsh%2C+Louise%22">Welsh, Louise</searchLink><relatesTo>3,4,5</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Monthly+Notices+of+the+Royal+Astronomical+Society%22">Monthly Notices of the Royal Astronomical Society</searchLink>. Jan2025, Vol. 536 Issue 2, p1980-1999. 20p.
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  Data: *<searchLink fieldCode="DE" term="%22MOLECULAR+clouds%22">MOLECULAR clouds</searchLink><br />*<searchLink fieldCode="DE" term="%22ABSORPTION+spectra%22">ABSORPTION spectra</searchLink><br />*<searchLink fieldCode="DE" term="%22MILKY+Way%22">MILKY Way</searchLink><br />*<searchLink fieldCode="DE" term="%22RADICALS+%28Chemistry%29%22">RADICALS (Chemistry)</searchLink><br />*<searchLink fieldCode="DE" term="%22THERMOMETERS%22">THERMOMETERS</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: We report a new, reliable determination of the cyano radical (CN) excitation temperature of diffuse molecular clouds in the Milky Way, based on ultrahigh spectral resolution observations. Our determination is based on CN |$B^{2}\Sigma ^{+}-X^{2}\Sigma ^{+}$| (0, 0) vibronic band absorption spectra seen along the lines of sight to eight bright Galactic stars. Our analysis is conducted blind, and we account for multiple sources of systematic uncertainty. Like previous studies, our excitation temperature measures exhibit an intrinsic scatter that exceeds the quoted uncertainties. Accounting for this scatter, we derive a 3 per cent determination of the typical CN excitation temperature, |$T_{01}=2.769^{+0.084}_{-0.072}~{\rm K}$|⁠ , which is consistent with the direct determination of the cosmic microwave background (CMB) temperature. We also perform a single joint fit to all sightlines simultaneously, and find that our data can be fit with an excitation temperature |$T_{01}=2.725\pm 0.015~{\rm K}$| – a 0.55 per cent measure that is consistent with the CMB temperature. We propose a future observational strategy to reduce systematic uncertainties and firmly test the limitations of using CN as a CMB thermometer. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Monthly Notices of the Royal Astronomical Society is the property of Oxford University Press / USA and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
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    Identifiers:
      – Type: doi
        Value: 10.1093/mnras/stae2738
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 20
        StartPage: 1980
    Subjects:
      – SubjectFull: MOLECULAR clouds
        Type: general
      – SubjectFull: ABSORPTION spectra
        Type: general
      – SubjectFull: MILKY Way
        Type: general
      – SubjectFull: RADICALS (Chemistry)
        Type: general
      – SubjectFull: THERMOMETERS
        Type: general
    Titles:
      – TitleFull: A determination of the cosmic microwave background temperature using Galactic molecules.
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            NameFull: Cooke, Ryan
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            NameFull: Welsh, Louise
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            – D: 15
              M: 01
              Text: Jan2025
              Type: published
              Y: 2025
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              Value: 536
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              Value: 2
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            – TitleFull: Monthly Notices of the Royal Astronomical Society
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