Interstellar Nitrogen Isotope Ratios: New NH₃ Data from the Galactic Center out to the Perseus Arm

Abstract

Our aim is to measure the interstellar 14N/15N ratio across the Galaxy, to establish a standard data set on interstellar ammonia isotope ratios, and to provide new constraints on the Galactic chemical evolution. The (J, K) = (1, 1), (2, 2), and (3, 3) lines of 14NH3 and 15NH3 were observed with the Shanghai Tianma 65 m radio telescope (TMRT) and the Effelsberg 100 m telescope toward a large sample of 210 sources. One hundred fourty-one of these sources were detected by the TMRT in 14NH3. Eight of them were also detected in 15NH3. For 10 of the 36 sources with strong NH3 emission, the Effelsberg 100 m telescope successfully detected their 15NH3(1, 1) lines, including 3 sources (G081.7522, W51D, and Orion-KL) with detections by the TMRT telescope. Thus, a total of 15 sources are detected in both the 14NH3 and 15NH3 lines. Line and physical parameters for these 15 sources are derived, including optical depths, rotation and kinetic temperatures, and total column densities. 14N/15N isotope ratios were determined from the 14NH3/15NH3 abundance ratios. The isotope ratios obtained from both telescopes agree for a given source within the uncertainties, and no dependence on heliocentric distance and kinetic temperature is seen. 14N/15N ratios tend to increase with galactocentric distance, confirming a radial nitrogen isotope gradient. This is consistent with results from recent Galactic chemical model calculations, including the impact of superasymptotic giant branch stars and novae.

Properties of dense molecular gas along the major axis of M 82

“…Figure 6 shows estimated 14 N/ 15 N ratios as a function of galactocentric distance. If we adopt an assumption of constant 12 C/ 13 C abundance ratio of 89, 14 N/ 15 N abundance ratios tend to increase with galactocentric distance, which is consistent with the 14 N/ 15 N gradient found in the Milky Way (Dahmen et al 1995;Adande & Ziurys 2012;Romano & Matteucci 2003;Romano et al 2017;Chen et al 2021).…”

“…This ratio measured in two starbursts, NGC 4945 (∼200-400, Henkel et al 2018) and ULIRG Arp 220 (440 +140 −82 , Wang et al 2014), are higher than those found in M 82 (∼ 80 − −250, see Figure 6). In our Milky Way, the 14 N/ 15 N ratios distribute in a large range (100-600, Li et al 2017), with a possible positive gradient from the center to out disk (Chen et al 2021).…”

Properties of Dense Molecular Gas along the Major Axis of M82

“…Figure 6 shows estimated 14 N/ 15 N ratios as a function of galactocentric distance. If we adopt an assumption of a constant 12 C/ 13 C abundance ratio of 89, 14 N/ 15 N abundance ratios tend to increase with galactocentric distance, which is consistent with the 14 N/ 15 N gradient found in the Milky Way (Dahmen et al 1995;Romano & Matteucci 2003;Adande & Ziurys 2012;Romano et al 2017;Chen et al 2021).…”

“…This ratio measured in two starbursts, NGC 4945 (∼200-400, Henkel et al 2018) and ULIRG Arp 220 (440 +140 −82 , Wang et al 2014), is higher than found in M82 (∼80-250, see Figure 6). In our Milky Way, the 14 N/ 15 N ratio is distributed in a large range (100-600, Li et al 2017), with a possible positive gradient from the center to the outskirts (Chen et al 2021).…”

Direct measurements of carbon and sulfur isotope ratios in the Milky Way

“…We are performing systematic observational studies on isotope ratios in the Milky Way, including 12 C/ 13 C (Yan et al 2019), 14 N/ 15 N (Chen et al 2021), 18 O/ 17 O (Zhang et al 2015(Zhang et al , 2020a(Zhang et al , 2020bLi et al 2016), and 32 S/ 34 S (Yu et al 2020). We have thus performed a more systematic study on CS and its isotopologs toward 110 high-mass star-forming regions (HMSFRs).…”