The Physical Drivers and Observational Tracers of CO-to-H₂ Conversion Factor Variations in Nearby Barred Galaxy Centers

Abstract

The CO-to-H2 conversion factor (α CO) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower α CO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner ∼2 kpc of NGC 3627 and NGC 4321 tracing 12CO, 13CO, and C18O lines on ∼100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of α CO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T k ≳ 100 K and density n H 2 > 10 3 cm−3. Assuming a CO-to-H2 abundance of 3 × 10−4, we derive 4–15 times lower α CO than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of α CO with low-J 12CO optical depths (τ CO), as well as an anticorrelation with T k. The τ CO correlation explains most of the α CO variation in the three galaxy centers, whereas changes in T k influence α CO to second order. Overall, the observed line width and 12CO/13CO 2–1 line ratio correlate with τ CO variation in these centers, and thus they are useful observational indicators for α CO variation. We also test current simulation-based α CO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.

Evidence of a Cloud–Cloud Collision from Overshooting Gas in the Galactic Center

“…Lower values of X CO have been observed in the bars and bar lanes of other spiral galaxies (e.g., Meier & Turner 2001;Bolatto et al 2013;Teng et al 2022Teng et al , 2023Sormani et al 2023). Our results are consistent with and offer additional explanations for these results.…”

Star Formation Efficiency in Nearby Galaxies Revealed with a New CO-to-H2 Conversion Factor Prescription

“…In those environments, α CO can be 5-15 times lower than the Galactic disk value (Ackermann et al 2012;Sandstrom et al 2013;Israel 2020;Teng et al 2022Teng et al , 2023den Brok et al 2023). The lower α CO in galaxy centers is likely driven by CO emissivity variations due to higher excitation and/or stronger dynamical effects, such as turbulence or inflowing gas (Narayanan et al 2012;Papadopoulos et al 2012;B13;Gong et al 2020;Teng et al 2023). These effects may also explain the low α CO seen in mergers or (ultra)luminous infrared galaxies (U/LIRGs; Downes & Solomon 1998;Krieger et al 2017;Sliwa et al 2017;Cicone et al 2018;Herrero-Illana et al 2019).…”

“…Thanks to the high resolution and sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA), CO (isotopologue) observations are now routinely possible at cloud scales in nearby galaxies (e.g., Leroy et al 2021;Davis et al 2022;Koda et al 2023;Williams et al 2023). In particular, recent studies modeling multi-CO isotopologues in nearby galaxy centers have revealed that CO opacity is the dominant driver of α CO variations (Israel 2020;Teng et al 2022Teng et al , 2023. This strong dependence of α CO on CO opacity further leads to a clear anticorrelation between α CO and the observed line width at ∼100 pc scales in barred galaxy centers (Teng et al 2023, hereafter T23).…”

“…The impacts of α CO variations on both SFE and cloud evolutionary timescale estimates are particularly critical in galaxy centers (Leroy et al 2013;Utomo et al 2017;Muraoka et al 2019;Ellison et al 2020b;Pessa et al 2021;Maeda et al 2023;Sun et al 2023). In those environments, α CO can be 5-15 times lower than the Galactic disk value (Ackermann et al 2012;Sandstrom et al 2013;Israel 2020;Teng et al 2022Teng et al , 2023den Brok et al 2023). The lower α CO in galaxy centers is likely driven by CO emissivity variations due to higher excitation and/or stronger dynamical effects, such as turbulence or inflowing gas (Narayanan et al 2012;Papadopoulos et al 2012;B13;Gong et al 2020;Teng et al 2023).…”

“…Recent studies have proposed various types of α CO prescription, depending on metallicity, stellar mass surface density, SFR, SFE, and/or CO line-related properties (Genzel et al 2012;Narayanan et al 2012;B13;Hunt et al 2015;Amorín et al 2016;Accurso et al 2017;Renaud et al 2019;Gong et al 2020;Madden et al 2020;Ramambason et al 2023). However, establishing a reliable α CO calibration remains a challenge, because it requires α CO measurements covering a sufficient sample of galaxies spanning a broad range of molecular gas physical and dynamical conditions, and the two most realistic ways to measure α CO in nearby galaxies are via dust emission (which is typically restricted to kiloparsec resolutions; Israel 1997;Leroy et al 2011;Sandstrom et al 2013;Schruba et al 2017;Pitts & Barnes 2021;Chiang et al 2023;den Brok et al 2023;Yasuda et al 2023) or multi-CO isotopologue observations (which are expensive at cloud scales; Sliwa et al 2017;Cormier et al 2018;Israel 2020;Sharda et al 2022;Teng et al 2022Teng et al , 2023.…”