The CO-to-H2 Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies

Abstract: We present kiloparsec (kpc) spatial resolution maps of the CO-to-H2 conversion factor (alpha_co) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for alpha_co and DGR by assuming that the DGR is approximately constant on kpc scales. With this assumption, we can combine maps of dust mass surface density, CO integrated intensity and HI column density to solve for both alpha_co and 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, 12CO J=(2-1) maps from the IRAM 30m large program HERACLES and HI 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 12CO J=(1-0) scale and show using literature measurements that variations in the line ratio do not effect our results. In total, we derive 782 individual solutions for alpha_co and DGR. On average, alpha_co = 3.1 Msun 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 in the central kpc---a factor of ~2 below the galaxy mean, on average. In some cases, the central alpha_co value can be factors of 5 to 10 below the standard Milky Way (MW) value of alpha_co,MW =4.4 Msun pc^-2 (K km s^-1)^-1. While for alpha_co we find only weak correlations with metallicity, 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.

• The paper determines an average CO-to-H2 conversion factor of 3.1 M⊙ pc⁻² (K km s⁻¹)⁻¹, slightly below the Milky Way standard.
• It identifies a flat radial profile with a 0.3 dex decrease in central regions, emphasizing localized environmental effects on molecular gas.
• The study finds a near-linear correlation between dust-to-gas ratio and metallicity, supporting metallicity-based models for gas properties.

The CO-to-H$_2$ Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies

The paper presented by Sandstrom et al. provides a comprehensive analysis of the CO-to-H$_2$ conversion factor ($\alpha_{\rm CO}$) and dust-to-gas ratio (DGR) in 26 nearby star-forming galaxies. The researchers employ spatially resolved maps at kiloparsec scales to independently derive $\alpha_{\rm CO}$ and DGR, leveraging recent advancements in observational data from key programs like KINGFISH, HERACLES, and THINGS.

Key Findings:

1. Average CO-to-H$_2$ Conversion Factor: The study finds a mean $\alpha_{\rm CO}$ value of approximately 3.1 M$_\odot$ pc$^{-2}$ (K km s$^{-1}$)$^{-1}$ for the sample, which is slightly lower than the standard Milky Way value of 4.4 M$_\odot$ pc$^{-2}$ (K km s$^{-1}$)$^{-1}$. The standard deviation of $\alpha_{\rm CO}$ across the sample lines of sight is 0.38 dex, indicating considerable uniformity within individual galaxies but some variability among different galaxies.
2. Radial Profile of $\alpha_{\rm CO}$: The conversion factor generally exhibits a flat profile as a function of galactocentric radius, with a notable decrease in $\alpha_{\rm CO}$ in the central regions of most observed galaxies. This decrease is on average around 0.3 dex lower than the galaxy mean, implying that central environments significantly differ from disk environments in molecular gas properties.
3. Dust-to-Gas Ratio: DGR is observed to correlate well with metallicity in the sample, indicating a near-linear relationship. This finding supports the notion that a constant fraction of metals is locked in dust, reinforcing the use of metallicity as a proxy for DGR in models.
4. Implications for Galaxy Modeling: The results indicate that for integrated galaxy studies, a single $\alpha_{\rm CO}$ value is suitable for the main disk, while decreasing $\alpha_{\rm CO}$ should be considered for central kpc regions, especially for starburst or AGN-affected systems.

Implications and Future Directions:

The study's findings contribute significantly to our understanding of molecular gas properties across different galactic environments. The observed uniformity of $\alpha_{\rm CO}$ across galactic disks, contrasted with its decline in central regions, suggests localized factors, such as pressure or star-formation intensity, strongly influence conversion factors. The results also lend support to models predicting the existence of CO-dark molecular gas at lower metallicities in nearby galaxies.

For future developments, high-resolution observations, likely from facilities like ALMA, should target the internal dynamics and molecular excitation conditions of central regions to unravel the influencing factors for the lower $\alpha_{\rm CO}$ values. Additionally, expanding this analysis to lower metallicity systems would provide extensive insights into how molecular gas properties evolve with varying galactic chemistry.

This work lays crucial groundwork for both theoretical models of galaxy evolution and the interpretation of high-redshift observations, where assumptions about $\alpha_{\rm CO}$ and DGR significantly affect inferred molecular gas masses.