J/AJ/169/18 Modeled lines intensities of CO isotopes in M51 (den Brok+, 2025)
CO isotopologue-derived molecular gas conditions and CO-to-H.
den Brok J., Jimenez-Donaire M.J., Leroy A., Schinnerer E., Bigiel F.,
Pety J., Petitpas G., Usero A., Teng Y.-H., Humire P., Koch E.W.,
Rosolowsky E., Sandstrom K., Liu D., Zhang Q., Stuber S., Chevance M.,
Dale D.A., Eibensteiner C., Galic I., Glover S.C.O., Pan H.-A.,
Querejeta M., Smith R.J., Williams T.G., Wilner D.J., Zhang V.
<Astron. J., 169, 18 (2025)>
=2025AJ....169...18D 2025AJ....169...18D
ADC_Keywords: Radio lines; Interferometry; Models; Galaxies, radio;
Molecular clouds; Carbon monoxide; Spectroscopy
Keywords: Spiral galaxies ; Giant molecular clouds ; Radio interferometry ;
Radio spectroscopy ; Millimeter-wave spectroscopy
Abstract:
Over the past decade, several millimeter interferometer programs have
mapped the nearby star-forming galaxy M51 at a spatial resolution of
≤170 pc. This study combines observations from three major programs:
the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and
the Surveying the Whirlpool at Arcseconds with NOEMA. The data set
includes the (1-0) and (2-1) rotational transitions of 12CO, 13CO, and
C18O isotopologues. The observations cover the r<3kpc region,
including the center and part of the disk, thereby ensuring strong
detections of the weaker 13CO and C18O lines. All observations are
convolved in this analysis to an angular resolution of 4",
corresponding to a physical scale of 170pc. We investigate empirical
line ratio relations and quantitatively evaluate molecular gas
conditions such as temperature, density, and the CO-to-H2 conversion
factor (αCO). We employ two approaches to study the molecular
gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to
analytically determine the CO column density and αCO, and (ii)
using non-LTE modeling with RADEX to fit physical conditions to
observed CO isotopologue intensities. We find that the αCO
values in the center and along the inner spiral arm are ∼0.5dex (LTE)
and 0.1dex (non-LTE) below the Milky Way inner disk value. The average
non-LTE αCO is 2.4±0.5M☉.pc-2.(K.km/s)-1. While both
methods show dispersion due to underlying assumptions, the scatter is
larger for LTE-derived values. This study underscores the necessity
for robust CO line modeling to accurately constrain the molecular
interstellar medium's physical and chemical conditions in nearby
galaxies.
Description:
Here, we use observations of the (1-0) and (2-1) rotational
transitions of 12CO, 13CO, and C18O from two different millimeter
interferometers.
The Plateau de Bure Interferometer (PdBI) Arcsecond Whirlpool Survey
(PAWS) observed the 12CO(1-0) line emission at 1" (∼40pc) resolution
across the center and a large part of the disk of M51
(Schinnerer+, 2013ApJ...779...42S 2013ApJ...779...42S). The observations were taken from
on the Pic de Bure in the French Alps, where the PdBI is located, and
have been short-spacing corrected using IRAM 30m single-dish data,
taken from the Pico Veleta in the Spanish Sierra Nevada where the IRAM
telescope resides.
The Surveying the Whirlpool at Arcseconds with NOEMA (SWAN) is an IRAM
large program. Observations were carried out using the Northern
Extended Millimeter Array (NOEMA), located on the Pic de Bure in the
French Alps and IRAM 30m single dish for short-spacing corrections.
The observations targeted several molecular lines in the 3mm line
window within the central 5x7kpc of M51. The spectral coverage of the
program includes the 1-0 transitions of 13CO and C18O. The data
have a native angular resolution of ∼2.5" (∼100pc).
As part of a Submillimeter Array (SMA) Large Program, the molecular
disk of M51 was observed in three configurations: subcompact (SUB),
compact (COM), and extended (EXT). The observations were split into 14
different tracks over the course of 2017. While the SUB configuration
consists of 147 pointings, covering the entire molecular disk of M51,
the observations in COM consist of 55 pointings covering the same
footprint as the PAWS data. The SMA observations include the
12CO(2-1), 13CO(2-1), and C18O(2-1) lines. For the subsequent
analysis, we combine the observations in COM and SUB configurations,
limiting the footprint to the overlapping FOV (which is equivalent to
the PAWS FOV). Furthermore, the SMA observations have been
short-spacing corrected using IRAM 30m observations from the CLAWS
large program.
Objects:
------------------------------------------------------
RA (2000) DE Designation(s)
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13 29 52.69 +47 11 42.9 M51 = NAME Whirlpool Galaxy
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table7.dat 119 1152480 RADEX single-zone modeled line intensities
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See also:
J/A+A/432/369 : Leiden Atomic and Molecular Database (LAMDA) (Schoeier+, 2005)
J/AJ/136/2782 : Star formation efficiency in nearby galaxies (Leroy+, 2008)
J/A+A/549/A39 : A 3mm molecular line survey of NGC1068 (Aladro+, 2013)
J/AJ/146/150 : Velocity dispersions of 12 nearby galaxies (Caldu-Primo+, 2013)
J/ApJ/808/42 : CHemical Abundances of Spirals (CHAOS) II. M51 (Croxall+, 2015)
J/ApJ/846/71 : M51 ISM structures from the CO maps of PAWS (Leroy+, 2017)
J/ApJ/835/217 : Density distributions for mm-wave line ratios (Leroy+, 2017)
J/ApJS/234/24 : VLA 33GHz obs. of star-forming regions (Murphy+, 2018)
J/ApJ/880/127 : EMPIRE: IRAM 30m dense gas survey (Jimenez-Donaire+, 2019)
J/A+A/625/A19 : 33 GHz continuum map of M51 (Querejeta+, 2019)
J/ApJ/893/96 : CHAOS IV: NGC3184 LBT obs. & 3 other gal. abund. (Berg+, 2020)
J/A+A/635/A131 : 12CO/13CO ratio in 126 nearby galaxy centers (Israel, 2020)
J/ApJS/248/25 : SFRS: 3-33GHz VLA obs. of star-forming regions (Linden+, 2020)
J/ApJS/257/21 : VERTICO: Virgo EnviRonment Traced In CO survey (Brown+, 2021)
J/ApJ/927/149 : Low-J CO line ratios for 152 CO map pairs (Leroy+, 2022)
J/A+A/679/A112 : California and Orion A clouds emission lines (Tafalla+, 2023)
Byte-by-byte Description of file: table7.dat
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Bytes Format Units Label Explanations
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1- 5 F5.1 K Tkin [4/100] Kinetic temperature
7- 14 F8.1 cm-3 n(H2) [100/1e5] H2 volume density
16- 25 E10.4 cm-2.km-1.s N(CO)/dv [5e14/5e18] CO column density per
line width dv (1)
27- 28 I2 --- [12CO/13CO] [20/80] CO isotopologue abundance
ratio
30- 49 F20.18 --- bff [0.01/1] Beam filling factor, φ
51- 61 E11.4 --- tau12CO [-0.09/3875] Optical depth, 12CO
(1-0)
63- 73 E11.4 --- tau13CO [-0.1/186.5] Optical depth, 13CO
(1-0)
75- 85 E11.4 K.km/s I-12CO(1-0) [-1.5e-13/1535] Simulated RADEX
intensity for the (1-0) rotational
transition of 12CO
87- 96 E10.4 K.km/s I-12CO(2-1) [0/1506] Simulated RADEX intensity
for the (2-1) rotational transition
of 12CO
98- 108 E11.4 K.km/s I-13CO(1-0) [-28/937] Simulated RADEX intensity
for the (1-0) rotational transition
of 13CO
110- 119 E10.4 K.km/s I-13CO(2-1) [4.8e-6/1362] Simulated RADEX
intensity for the (2-1) rotational
transition of 13CO
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Note (1): We fix the 12CO line width, dv, to 15 km/s.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 21-Oct-2025