J/AJ/170/8 Sightlines candidates for observing ISM CII absorption (Clark+, 2025)
Measuring interstellar Carbon abundance via 158um [C II] absorption with SOFIA.
A potential detection and proof-of-concept for depletion studies with future
far-IR facilities
Clark C.J.R., Roman-Duval J.C., Madden S.C., Mertens M., Murray C.E.,
Stutzki J., Tarantino E., Tchernyshyov K.
<Astron. J., 170, 8 (2025)>
=2025AJ....170....8C 2025AJ....170....8C
ADC_Keywords: Abundances; Infrared sources; Interstellar medium; Opacities
Keywords: Far infrared astronomy ; Interstellar line absorption ;
Interstellar abundances ; Astronomical techniques ;
Infrared telescopes ; Space telescopes
Abstract:
Carbon plays key roles in the interstellar medium (ISM)-as a
constituent of dust, as the carrier of the dominant far-infrared (FIR)
cooling line, and as a component of various important molecules. But
despite this, there are very few measurements of the abundance and
depletion of carbon in the diffuse ISM. As with other elements, these
measurements are traditionally performed in the ultraviolet. But for
carbon, such measurements are extremely difficult, and fewer than 20
have been reported in the literature to date. Here, we present a novel
method of measuring the abundance and depletion of carbon in the
diffuse ISM: by observing absorption of the 158um [C II] line in
the FIR. We present a catalog of 432 candidate sightlines that use
bright nearby galaxies as background sources, and predict the [C II]
absorption expected toward each. We conducted a pilot study using
SOFIA, targeting sightlines toward the galaxies IC 342 and Circinus.
We report a potential detection of Galactic [C II] absorption along
the IC 342 sightline, although it requires disentangling [C II]
emission from IC 342 itself. The Circinus sightline had an
insufficiently stable instrumental baseline to allow a detection. This
SOFIA study informs the prospects for [C II] absorption measurements
with future facilities. To that end, we explore the potential for four
proposed future FIR telescopes-PRIMA, FIRSST, SALTUS, and Origins-to
detect [C II] absorption. We find that all four facilities would be
able to detect [C II] absorption along a significant number of
sightlines.
Description:
In this work, we present the novel methodology of using FIR-bright
background galaxies to detect foreground 158um [C II] absorption by
the cold Galactic ISM. By observing [C II] absorption against the
continuum of bright background sources, it should be possible to make
a clean detection of the absorption profile, without the confusion of
self-absorption.
We designed a pilot study to test the implementation of the [C II]
absorption method using SOFIA. For this study, we searched for
extragalactic background sources with sufficiently bright FIR dust
continuum emission, along sightlines that should have sufficiently
strong Galactic 158um [C II] absorption, to be detectable.
To identify possible sightlines for observation, we started by
performing a systematic analysis of every nearby (<40Mpc) galaxy
observed using the 160um band of the Photodetector Array Camera and
Spectrometer (PACS) instrument aboard the Herschel Space Observatory.
Then, to estimate the continuum surface brightness that would be
observed by each telescope, we convolved the Herschel-PACS 160um
observations to the resolution that would be achieved by each
telescope at that wavelength. The future facilities are PRIMA, FIRSST,
SALTUS, and Origins.
The PRobe far-Infrared Mission for Astrophysics (PRIMA) is a mission
concept that has been down-selected as the FIR candidate for NASA's
astrophysics probe solicitation. PRIMA would have a 2-3m primary
mirror and a potential medium-resolution mode, the Far-Infrared
Enhanced Survey Spectrometer (FIRESS), with spectroscopic resolution
of R=4400 at 158um.
The Far-Infrared Spectroscopy Space Telescope (FIRSST) is a mission
concept that was developed for NASA's astrophysics probe solicitation.
The FIRSST would have a ∼2m primary mirror and its Heterodyne
Spectroscopy Instrument (HSI) would have a resolving power of R>105,
providing at least 3km/s velocity resolution.
The Single Aperture Large Telescope for Universe Studies (SALTUS) is a
mission concept that was developed for NASA's astrophysics probe
solicitation. Its High Resolution Receiver (HiRX) would provide a
resolving power of R=106-107, providing 0.3km/s velocity
resolution. The 14m primary mirror would provide 2.8" resolution at
158um.
The Origins Space Telescope is a mission concept that was submitted to
Astro2020, as one of the New Great Observatories. The high-resolution
mode of the Origins Survey Spectrometer (OSS) would have a resolving
power of R=104.5 at 157.7um, providing 10km/s velocity resolution.
The 5.9m primary mirror of Origins would give a resolution of 6.7".
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 91 402 Catalog of our candidate sightlines for observing
Milky Way [CII] absorption
table4.dat 189 402 *Predicted observation parameters for proposed
future FIR facilities
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Note on table4.dat: Facilities as follows:
FIRSST = The Far-Infrared Spectroscopy Space Telescope with the Heterodyne
Spectroscopy Instrument (HSI)
PRIMA = The PRobe far-Infrared Mission for Astrophysics with the Far-Infrared
Enhanced Survey Spectrometer (FIRESS)
SALTUS = The Single Aperture Large Telescope for Universe Studies with the
High Resolution Receiver (HiRX)
Origins = The Origins Space Telescope with the Origins Survey Spectrometer
(OSS)
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See also:
B/simbad : Simbad objects catalogue (M.Wenger 2000)
J/ApJ/700/1299 : Gas-phase element depletions in the ISM (Jenkins, 2009)
J/MNRAS/394/1857 : H2/HI ratio in galaxies (Obreschkow+, 2009)
J/ApJ/734/65 : CI radial velocities with HST/STIS (Jenkins+, 2011)
J/ApJ/744/20 : S4MC project: 6 star forming regions PAHs (Sandstrom+, 2012)
J/ApJ/774/68 : [CII]157.7um feature in LIRGs (Diaz-Santos+, 2013)
J/A+A/571/A53 : 4 IRDC data cubes (Beuther+, 2014)
J/MNRAS/449/2274 : Characterization of Herschel SPIRE FTS (Hopwood+, 2015)
J/ApJ/801/127 : 3.6um, 4.5um, B and V light curves of NGC 6418 (Vazquez+, 2015)
J/A+A/594/A116 : HI4PI spectra and column density maps (HI4PI team+, 2016)
J/A+A/586/A37 : M33 center and BCLMP302 [CII] spectroscopy (Mookerjea+, 2016)
J/ApJ/845/96 : [CII] emission in the ISM of 20 nearby galaxies (Croxall+, 2017)
J/ApJS/233/22 : xCOLD GASS catalog (Saintonge+, 2017)
J/MNRAS/474/2094 : Inferring probabilistic stellar rot. periods (Angus+, 2018)
J/A+A/620/A112 : Template SED of DustPedia galaxies (Bianchi+, 2018)
J/A+A/609/A37 : 875 nearby galaxies multiwavelength photometry (Clark+, 2018)
J/A+A/636/A16 : M43, Horsehead, MonR2, M17SW [CII]158um spect. (Guevara+, 2020)
J/ApJ/895/38 : MIR extinction toward Cyg OB2-12 ISM (Hensley+, 2020)
J/ApJ/915/92 : SOFIA CII and NII obs. of M101 and NGC6946 (Tarantino+, 2021)
J/A+A/659/A36 : Self-absorption in RCW 120 (Kabanovic+, 2022)
J/A+A/680/A4 : IRAM NOEMA CO(1-0) cubes for IC342 (Querejeta+, 2023)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Name Name
13- 19 F7.3 deg RAdeg Right Ascension, sight line (J2000) (1)
21- 27 F7.3 deg DEdeg Declination, sight line (J2000) (1)
29- 36 E8.2 cm-2 NH [7e19/5.5e21] Total (HI+H2) hydrogen column
density, cgs
38- 42 F5.3 --- fmol [0/0.2] Molecular gas fraction, Eq. 4
44- 51 E8.2 cm-2 NCII [1.5e16/1.1e18] Predicted column density of
CII, cgs (2)
53- 57 F5.3 km/s tauCIIdv [0.1/7.3] Predicted, integrated line
opacity (2)
59- 63 F5.3 --- tauCIIpeak [0.003/0.35] Predicted line opacity at
velocity where peak absorption occurs (2)
65- 70 F6.2 Jy Fcont [0.03/313.7] 160 um flux density of the
background source, measured within the
defined aperture
72- 78 F7.3 Jy dFcont [-39.8/-0.001] Reduction in flux density
expected at the wavelength absorption is
greatest (2)
80- 85 F6.1 --- CIIratio [0.1/2599] Predicted ratio between Milky
Way [CII] emission and absorption for the
sightline (2)
87- 91 F5.1 % SubErr [0/100] Median error on the sky subtraction
(3)
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Note (1): The coordinates are of the FIR-brightest point used for the
sightline, as per Section 2.2.1; this is not necessarily the
exact center of the background galaxy.
Note (2): These values are all predictions.
Note (3): SubErr is the median error on the sky subtraction,
estimated from our IRAS simulations, using the Gaussian Process
Regression method.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Name Name
13- 33 F21.19 --- tauCIIpeak-F [0.002/0.341] Predicted FIRSST line
opacity at peak (1)
35- 55 E21.19 s IntTime-F [9.8e-5/157660] Predicted FIRSST
integration time (2)
57- 79 E23.19 --- tauCIIpeak-P [-3.7e-14/0.1] Predicted PRIMA line
opacity at peak (1)
81- 101 E21.19 s IntTime-P [1e-4/185801] Predicted PRIMA integration
time (2)
103- 123 F21.19 --- tauCIIpeak-S [3e-3/0.345] Predicted SALTUS line
opacity at peak (1)
125- 145 E21.19 s IntTime-S [7.2e-6/11605] Predicted SALTUS
integration time (2)
147- 167 F21.19 --- tauCIIpeak-O [1e-3/0.299] Predicted Origins line
opacity at peak (1)
169- 189 E21.19 s IntTime-O [4.6e-8/74.2] Predicted Origins
integration time (2)
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Note (1): Predicted opacity of the [CII] absorption feature, within
the velocity resolution element where absorption peaks.
Note (2): Predicted integration time necessary to detect the [CII]
absorption feature at S/N=10.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 24-Mar-2026