Diffuse Ionized Gas in the Anti-center of the Milky Way

Abstract

Using data from the LAMOST Medium-Resolution Spectroscopic Survey of Nebulae, we create a sample of 17,821 diffuse ionized gas (DIG) spectra in the anti-center region of the Milky Way, by excluding fibers in the directions of H II regions and supernova remnants. We then analyze the radial and vertical distributions of three line ratios ([N II]/H$\hat{I}\pm$, [S II]/H$\hat{I}\pm$, and [S II]/[N II]), as well as the oxygen abundance. [N II]/H$\hat{I}\pm$ and [S II]/H$\hat{I}\pm$ do not exhibit a consistent, monotonic decrease with increasing Galactocentric distance (R${}_{gal}$). Instead, they show enhancement within the interarm region, positioned between the Local Arm and the Perseus Arm. [S II]/[N II] has a radial gradient of 0.1415 $\pm$ 0.0646 kpc${}^{-1}$ for the inner disk (8.34 $<R_{gal}<$ 9.65 kpc), and remains nearly flat for the outer disk ($R_{gal}>$ 9.65 kpc). In the vertical direction, [N II]/H$\hat{I}\pm$, [S II]/H$\hat{I}\pm$, and [S II]/[N II] increase with increasing Galactic disk height ($|z|$) in both southern and northern disks. Based on the N2S2H$\hat{I}\pm$ method, which combines [S II]/[N II] and [N II]/H$\hat{I}\pm$, we estimate the oxygen abundance. The oxygen abundance exhibits a consistent radial gradient with R${}_{gal}$, featuring a slope of -0.0559 $\pm$ 0.0209 dex kpc${}^{-1}$ for the inner disk and a similar slope of -0.0429 $\pm$ 0.0599 dex kpc${}^{-1}$ for the outer disk. A single linear fitting to the entire disk yields a slope of -0.0317 $\pm$ 0.0124 dex kpc${}^{-1}$. In the vertical direction, the oxygen abundance decreases with increasing $|z|$ in both southern and northern disks.