CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy. V. Chemical composition gradients as a function of the galactocentric radius

TL;DR

CHEMOUT investigates molecular composition gradients in the Milky Way's outer disk ($R_{GC}\sim9-24$ kpc) using IRAM 30 m spectroscopy at 3 and 2 mm for 35 sources and NIKA2 dust maps to derive $N_{H_2}$. The study focuses on HCN, HCO+, c-C3H2, H13CO+, HCO, and SO, updating H2CO and CH3OH abundances with revised $N_{H_2}$, and finds that most $X_{mol}$ scale with [C/H] up to $\sim$24 kpc while SO declines more steeply and H13CO+ less steeply than their isotopic or elemental counterparts. Line widths decrease with $R_{GC}$, suggesting a more quiescent outer environment, and the results imply comparable or higher molecule-formation efficiency in the outer Galaxy despite lower metallicity. These findings constrain chemical models for low-metallicity star-forming regions and highlight the need for precise kinetic temperatures and larger samples to refine metallicity-driven gradients.

Abstract

The outer Galaxy is characterized by a lower metallicity than regions near the Sun, suggesting differences in the formation and survival of molecules in star-forming regions. To understand chemical evolution across the Milky Way, deriving molecular abundances in star-forming regions in the outer Galaxy is essential for refining models of sub-Solar metallicity environments. We analyzed IRAM 30 m observations at 3 and 2 mm toward 35 sources at Galactocentric distances of 9$-$24 kpc, within the "CHEMical complexity in star-forming regions of the outer Galaxy" (CHEMOUT) project. We focused on species with the highest detection rates (i.e., HCN, HCO$^+$, c-C$_3$H$_2$, H$^{13}$CO$^+$, HCO, SO) and searched for trends in column densities, abundances, and line widths with Galactocentric distance. Abundances for H$_2$CO and CH$_3$OH were updated using H$_2$ column densities from new NIKA2 dust maps. Fractional abundances relative to H$_2$ of most species (HCN, HCO$^+$, c-C$_3$H$_2$, HCO, H$_2$CO, CH$_3$OH) scale at most with the elemental carbon abundance ([C/H]) up to $\sim$24 kpc. SO shows a steeper gradient than sulfur abundance ([S/H]), while H$^{13}$CO$^+$ shows a shallower gradient than [$^{13}$C/H]. Gas turbulence, inferred from line widths, decreases with Galactocentric distance, suggesting a more quiescent environment in the outer Galaxy with respect to the inner Galaxy. In the outer Galaxy, the formation efficiency of most molecules, following the parent element availability, is comparable or higher (e.g., for H$^{13}$CO$^+$) than in the local Galaxy, whereas SO forms less efficiently. These results have significant implications for chemical models of the outermost star-forming regions and for understanding molecule formation under lower metallicity conditions.

Figures (16)

• Figure 1: Galactocentric gradients of fractional abundances with respect to H$_2$, $X_\text{mol}$. The plot shows the trends for HCN (a), HCO$^+$(b), c-C$_3$H$_2$(c), H$^{13}$CO$^+$(d), HCO (e), SO (f), H$_2$CO (g), and CH$_3$OH (h) as a function of the galactocentric radius ($R_\text{GC}$). The light blue data represent the abundances calculated using a constant gas-to-dust ratio ($\gamma=100$), while the dark blue data illustrate those estimated using a non-constant gas-to-dust ratio giannetti2017galactocentric. For both datasets, the linear regression results are shown as the light blue and dark blue lines, respectively. The $1\sigma$ error bars over the slope of the gradients are displayed for each fit. The upper-limit values are represented with triangles. The gradients of the elemental abundances of carbon (C and $^{13}$C), nitrogen (N), oxygen (O), and sulfur (S), as reported by mendez2022gradients, are plotted as dashed lines. The product of the parent elements of the species is represented, in each subplot, by a dash–dotted black line. All elemental trends are plotted with a vertical offset to align with the starting point of the molecular gradients derived using $\gamma(R_\text{GC})$, to facilitate comparison of their slopes with the linear fit of the molecular abundances. In the upper-right side of each subplot, the Pearson correlation coefficient, $\rho$, is shown (estimated only for the abundances estimated using the non-constant gas-to-dust ratio).
• Figure 2: Galactocentric gradients of line widths, FWHM. The plot shows the trends for HCN (a), HCO$^+$(b), c-C$_3$H$_2$(c), H$^{13}$CO$^+$(d), HCO (e), SO (f), H$_2$CO (g), and CH$_3$OH (h) as a function of the galactocentric radius ($R_\text{GC}$). The linear fit results are shown as dark blue lines. The $1\sigma$ error bars over the slope of the gradients are displayed for each fit. In the upper right side of each subplot, the Pearson correlation coefficient, $\rho$, is shown.
• Figure 3: Galactocentric gradients of molecular ratios. The plot shows the trend of the ratios between CO molecule products (i.e., HCO$^+$, H$^{13}$CO$^+$, HCO, CH$_3$OH, and H$_2$CO) and c-C$_3$H$_2$, as a function of galactocentric radius ($R_\text{GC}$). The $1\sigma$ error bars over the slope of the gradients are displayed for each molecular fit. The upper limit values are represented with triangles. The linear fit results are shown as the dark blue lines. The $1\sigma$ error bars over the slope of the gradients are displayed for each fit. In the upper right side of each subplot, the Pearson correlation coefficient, $\rho$, is shown.
• Figure 4: Preliminary reduced NIKA2 images (color-scale in mJy/beam) at 2.0 mm (150 GHz) and 1.2 mm (260 GHz) of 8 out of the 31 CHEMOUT sources observed (Fontani et al. in prep.). The contours start from the 5$\sigma$ rms level in each map. The NIKA2 angular resolution is shown in the bottom-left corner of each frame. The black circles on the maps represent the 28$"$ beam, centered on the position targeted for the molecular line observations analyzed in this paper.
• Figure 5: Same as Fig. \ref{['fig:NIKA2-1']} for 8 additional targets.