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Open Access
Issue
A&A
Volume 686, June 2024
Article Number A205
Number of page(s) 14
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202348931
Published online 13 June 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model.

Open Access funding provided by Max Planck Society.

1 Introduction

The first maser was obtained from a source of ammonia (NH3) molecules by Charles H. Townes and his group members in the laboratory (Gordon et al. 1954, 1955). Various maser species were discovered in the interstellar medium (ISM), such as hydroxyl (OH; Weaver et al. 1965), water (H2O; Cheung et al. 1969), and methanol (CH3OH; Barrett et al. 1971). Although thermal emission from NH3 was discovered in 1968 by Cheung et al., the first NH3 maser in the ISM was detected 14 yr later, in the (J, K) = (3,3) metastable (J = K) line toward the massive star-forming region W33 (Wilson et al. 1982). The first highly excited non-metastable (J > K) ammonia masers were detected by Madden et al. (1986) in the (J, K) = (9,6) and (6,3) lines. So far, a total of 34 NH3 inversion transitions (∆K = 0 and ∆J = 0) have been identified as masers in the ISM (see Yan et al. 2022b, and references therein). Even in its rare isotopolog 15NH3, maser emission was detected in the (3,3) (Mauersberger et al. 1986), (4,3), and (4,4) transitions (Schilke et al. 1991) but only toward the high-mass star-forming region (HMSFR) NGC 7538. The ammonia transitions identified as masers in the ISM are summarized in Table A.1.

Ammonia masers are rare in the ISM compared to other maser species; that is, those of OH, H2O, and CH3OH. Over the last five decades after the first detection of astronomical masers, numerous successful maser surveys were carried out in different molecules and led to thousands of detections in the Milky Way. They targeted, for example, OH masers (e.g., te Lintel Hekkert et al. 1989; Lewis 1994; Hu et al. 1994; Sevenster et al. 1997; Caswell 1998; Wolak et al. 2012; Beuther et al. 2019), CH3OH masers (e.g., Menten 1991; Caswell et al. 1993; Xu et al. 2009; Caswell 2009; Hu et al. 2016; Breen et al. 2016; Yang et al. 2017, 2019, 2020, 2023; Lu et al. 2019; Nguyen et al. 2022), and H2O masers (e.g., Genzel & Downes 1979; Cesaroni et al. 1988; Menten et al. 1990; Wouterloot et al. 1993; Palagi et al. 1993; Wang et al. 2006; Urquhart et al. 2011; Breen & Ellingsen 2011; Motogi et al. 2011; Walsh et al. 2014; Xi et al. 2015; Titmarsh et al. 2016; Svoboda et al. 2016; Kim et al. 2018; Ladeyschikov et al. 2022a). All of these are collected and can be easily accessed from the online database, Maserdb1 (Sobolev et al. 2019; Ladeyschikov et al. 2019, 2022b).

So far, ammonia maser lines have only been detected in 32 sources. Among them, metastable NH3 masers are quite common and have been detected in 22 different regions. Nonmetastable (J > K) ammonia masers have been found in 14 objects (Yan et al. 2022a,b). Only four sources host both metastable and non-metastable NH3 masers. These are the HMSFRs DR 21 (Guilloteau et al. 1983; Madden et al. 1986; Mangum & Wootten 1994; Gaume et al. 1996), W51 (Madden et al. 1986; Mauersberger et al. 1987; Zhang & Ho 1995; Henkel et al. 2013), NGC 6334 (Kraemer & Jackson 1995; Beuther et al. 2007; Walsh et al. 2007), and Sgr B2(M) (Mills et al. 2018; Yan et al. 2022a). In Table A.2, we summarize the sources that are known to host ammonia masers. The metastable NH3 (3,3) masers are thought to be collisionally pumped (e.g., Walmsley & Ungerechts 1983; Flower et al. 1990; Mangum & Wootten 1994; Zhang & Ho 1995; Zhang et al. 1999; McEwen et al. 2016). Pumping scenarios of other NH3 transitions are still speculative. High-angular-resolution data show that the excitation of nonmetastable NH3 (9,6) masers in W51, Cepheus A, G34.26+0.15, and the Sgr B2 complex may be related to shocks by outflows or by the expansion of ultracompact (UC) H II regions (Pratap et al. 1991; Yan et al. 2022b,a). Furthermore, the NH3 (9,6) maser stands out as being the strongest and most variable one in W51-IRS2. Its variability is comparable to the H2O masers in the region (e.g., Henkel et al. 2013).

There exists no systematic survey to search for ammonia masers so far. Therefore, we selected 119 HMSFRs with high NH3 column densities (NNH3 ≥ 1015.5${N_{{\rm{N}}{{\rm{H}}_3}}} \ge {\rm{ }}{10^{15.5}}$ cm−2) that are known to host water masers from previous K-band surveys. The sample is mainly based on the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL, Schuller et al. 2009) catalogs (Wienen et al. 2012, 2018), and the Red MSX Source survey (RMS, Urquhart et al. 2011). Our sample is listed in Table A.3 together with the systemic local standard of rest (LSR) velocities and the source beam-averaged NH3 column densities, which are based on ammonia (1,1), (2,2), and (3,3) thermal emission in previous studies with typical beam sizes of order 32″ at Green Bank and 38″ at Effelsberg. We performed a K-band line survey with the 100-m Effelsberg telescope of this source sample with the motivations: (1) to search for NH3 maser lines, and (2) to search for the higher metastable ammonia transitions; in other words, the NH3 (4,4) to (7,7) lines with excitation levels up to 500 K above the ground state to reveal so-far poorly studied warm gas components in the HMSFR sources. We also obtained H2O and CH3OH maser spectra simultaneously as well as quasi-thermal lines from other molecules. The data allow us to perform a new K-band spectral classification of massive star-forming clumps, uniquely complementing other surveys.

In this paper, we report the discovery of numerous non-metastable NH3 and two probable metastable NH3 maser sources in the Milky Way. Results for the high-excitation metastable ammonia thermal transitions, the non-metastable ammonia thermal lines, and the H2O and CH3OH maser spectra will be published in future papers.

thumbnail Fig. 1

Comparison of velocities and line widths of ammonia maser lines to the intrinsic line widths of ammonia (J, K) = (1,1) thermal emission. The number of data points is substantially larger than the number of newly detected sources due to the occasional presence of more than one maser line in a given source and more than one velocity component in a specific maser line. δV = VLSR(maser) − VLSR(NH3(1,1)) is the deviation from velocities of the NH3 maser lines to those of NH3 (1,1) thermal emission. δWidth = ∆V1/2 (NH3 (1,1))-∆V1/2(maser) refers to the difference between the line widths of ammonia maser lines and the intrinsic line widths of an individual NH3 hyperfine structure component. The dashed green lines indicate positions with zero deviation. The metastable NH3 transitions are marked in blue. Among the non-metastable NH3 lines, ortho-NH3 is presented in black and para-NH3 is given in red.

2 Observations and data reduction

The K-band line survey was performed with the 100-meter Effelsberg telescope2 in November 2022 and in February, April, May, and July 2023. The S14mm double-beam secondary focus receiver was employed to simultaneously cover the entire K-band frequency range; that is, 18.0–26.0 GHz. The receiver band was divided into four 2.5 GHz-wide subbands with the frequency ranges of 18.0–20.5 GHz, 19.9–22.4 GHz, 21.6–24.1 GHz, and 23.5–26.0 GHz. Each subband has 65536 channels, providing a channel width of 38.1 kHz, changing from ~0.62 km s−1 at 18.5 GHz to ~0.44 km s−1 at 26.0 GHz. The observations were performed in position-switching mode with the off position 10′ in azimuth away from the source. The half power beam width (HPBW) was 49 × 18.5/ν (GHz) arcseconds; that is, 49″ at 18.5 GHz, the frequency of the NH3 (9,6) line. A high-spectral-resolution backend with 65 536 channels and a bandwidth of 300 MHz was employed to measure the new NH3 (9,6) maser sources, providing a channel width of 0.07 km s−1 at 18.5 GHz. Pointing and focus calibrations were done at the beginning of the observations, during sunset and sunrise, as well as every 2 h toward NGC 7027. The calibrator was measured between elevations of 30 and 60 degrees. The pointing was checked using nearby quasars prior to on-source integrations. The elevations of our targets were in a range of 15–55 degrees. The system temperatures were 60–130 K on a main-beam brightness temperature, TMB, scale.

We used the GILDAS/CLASS3 package (Pety 2005) to reduce the spectral line data. The data were split into small frequency intervals with bandwidths of 300 MHz and were calibrated based on continuum cross scans of NGC 7027 (Winkel et al. 2012), whose flux density was adopted from Ott et al. (1994). The TMB/S ratios are 1.95 K Jy−1, 1.73 K Jy−1, and 1.68 K Jy−1 at 18.5 GHz, 22.2 GHz, and 24.0 GHz, respectively. Calibration uncertainties were estimated to be ±10%. All of the NH3 lines covered in our observations were measured simultaneously, which ensures a good relative calibration.

thumbnail Fig. 2

NH3 (9,6) spectra observed at different epochs toward nine sources. The dashed red lines indicate the systemic velocities of the sources.

3 Results and discussion

We detected 15 new ammonia maser sources, resulting in a detection rate of ~13%. These maser lines, including NH3 (J, K) = (5,4), (6,4), (6,5), (7,7), (7,6), (8,6), (9,6), (9,8), (10,8), (11,9), and possibly the (3,3) line arise from energy levels of 342 K, 513 K, 465 K, 537 K, 606 K, 834 K, 1090 K, 942 K, 1226 K, 1449 K, and 122 K above the ground state. The maser line parameters obtained by Gaussian fits are listed in Table A.4. The masers are identified in three different ways: (1) narrow line widths compared to those of ammonia (J, K) = (1,1) thermal emission, (2) blueshifted or redshifted velocities with respect to the sources’ systemic LSR velocities, and (3) flux density variations. The first method can easily be verified by comparing the widths of putative maser lines with simultaneously observed quasi-thermal transitions.

Figure 1 shows the comparison of line widths of ammonia maser lines to the intrinsic line widths of hyperfine components of the (J, K) = (1,1) thermal emission as well as the difference between the systemic velocities of the sources and the maser velocities. We derived the intrinsic line width by using the hyperfine fitting in CLASS for the NH3 (1,1) line. The line profiles of the NH3 (1,1) thermal emission and maser transitions are presented in Fig. B.1. All of these maser lines have narrower features than the NH3 (1,1) thermal emission. This further confirms their maser nature. Furthermore, their velocities are shifted with respect to the sources’ systemic LSR velocities, by at least 0.7 km s−1, and reaching up to 30 km s−1. This is similar to recent discoveries of non-metastable NH3 masers with δV ~ 10 km s−1 in Cep A, δV ~ 4 km s−1 in G34.26+0.15, and δV in a range of 0.3 km s−1 to 24 km s−1 toward the Sgr B2 complex (Yan et al. 2022b,a). Fourteen of the new ammonia maser sources contain non-metastable ammonia masers, which doubles the number of non-metastable ammonia maser detections in our Galaxy. Among the non-metastable ammonia maser lines, larger velocity distributions are found in the ortho-NH3 (K = 3n) transitions than in the para-NH3 (K ≠ 3n) ones. The velocity range of para-NH3 masers is limited within ±5 km s−1 with respect to the sources’ systemic velocities, marked in red in Fig. 1. This is enlarged to about ±30 km s−1 for ortho-NH3 masers.

Among the 14 non-metastable ammonia maser sources, NH3 (9,6) masers are the most common and are detected in nine objects. The spectra of NH3 (9,6) masers are shown in Fig. 2. The observations at different epochs indicate that the flux densities of these NH3 (9,6) masers vary by at least 50% over timescales of several months. Even within two days, variations were observed in G031.41+0.30 (hereafter G031.41), similar to that detected by Henkel et al. (2013) toward W51-IRS2. The exception is G030.21-0.19 (hereafter G030.21): its NH3 (9,6) flux density stays constant for 20 days. In order to increase the signal-to-noise ratios (S/Ns) of NH3 (9,6) spectra toward G030.21, we averaged all three measurements at different epochs. The spectra and fitting results are presented in Fig. 2 and in Table A.4, respectively. Five targets, G010.47+0.03 (hereafter G010.47), G012.21-0.10 (hereafter G012.21), G019.61-0.23 (hereafter G019.612), G031.41, and G111.53+0.76 (hereafter G111.53), were also observed in the high-spectral-resolution mode (Fig. 2). These data show that the NH3 (9,6) masers contain narrow components with line widths smaller than 1.0 km s−1 and some even below the channel width of the wide band spectra (Sect. 2); that is, smaller than 0.6 km s−1. In eight sources, G010.47, G012.21, G019.612, G030.21, G030.60+0.18 (hereafter G030.60), G030.79+0.20 (hereafter G030.79), G043.79-0.13 (hereafter G043.79), and G111.53, we detect only NH3 (9,6) masers. Five of these eight targets show only blueshifted (9,6) features, and the other three only have redshifted features. G031.41, the ninth object, is unique in that it hosts both blueshifted and redshifted NH3 (9,6) maser components.

The NH3 (11,9) transition with an energy level of 1449 K above the ground state, the highest value in our maser sample, was also detected toward G031.41 (Fig. 3). Its velocity is consistent with a redshifted NH3 (9,6) maser feature and its flux density decreased from November 2020 to May 2023 by 63%. In addition, NH3 (11,9) masers were also detected in G029.95-0.02 (hereafter G029.95) and G030.70-0.07 (hereafter G030.70). In G029.95, NH3 (8,6) and (7,7) masers were also detected (Fig. 4). The flux density of the NH3 (7,7) maser increased by 55% in three days, while the NH3 (8,6) and (11,9) maser lines show no obvious variability. Toward G030.70, we also detected NH3 (7,6) and (8,6) masers. Their spectra are shown in Fig. 5. The NH3 (7,6) maser stays constant for 20 days. During this time interval, NH3 (8,6) and (11,9) masers initially showed the same trend; their flux densities are roughly the same from April 30 to May 4, 2023 but then decrease between May 4 and 19. Among the eleven sources mentioned above, G029.95 hosts both ortho- and para-NH3 masers and the other ten sources only host ortho-NH3 masers.

Three targets, G024.79+0.08 (hereafter G024.79), G032.74-0.08 (hereafter G032.74), and G035.19-0.74 (hereafter G035.19), only host para-NH3 masers (Fig. 6). Toward G024.79, we only detected an NH3 (5,4) maser. Toward G032.74, four transitions were identified as masers: the NH3 (5,4), (6,4), (9,8), and (10,8) lines. Two NH3 maser lines, (6,4) and (6,5), were detected in G035.19. Variations in the flux densities of the NH3 (5,4) maser in G024.79, the NH3 (10,8) maser in G032.74, and the NH3 (6,5) maser in G035.19 were also observed. These variations amount to 24% or more, while the NH3 (1,1) thermal emissions stay constant at the same time; thus, the maser variability appears to be significant.

The frequencies of the NH3 (1,1), (2,2) and (3,3) transitions are within a range of only 200 MHz. The peak flux density ratios of NH3 (3,3)/(1,1) and (3,3)/(2,2) toward G048.98-0.30 (hereafter G048.98) are ~0.83 and ~I.I4, respectively, based on previous 100-m Green Bank Telescope (GBT) observations in 2010 (Urquhart et al. 2011), but these are ~1.57 and ~2.22 in our measurements. However, the ratios of NH3 (1,1)/(2,2), which are ~1.38 and ~ 1.42, remain consistent, within the uncertainties due to noise in these two datasets. This indicates that the NH3 (3,3) emission in G048.98 has become stronger and likely shows a maser nature. The spectra of NH3 (1,1), (2,2), and (3,3) toward G048.98 are presented in Fig. 7.

In Fig. 8, we compare the velocity ranges of NH3 and H2O masers. The velocity range of NH3 masers is always smaller than that of H2O masers in each source. There are 12 objects, 80% of our maser sample, for which the velocities of their brightest NH3 maser feature are similar to that of H2O masers within + 10 km s−1. For the remaining three targets, G030.79, G029.95, and G030.70, the differences between velocities of their brightest NH3 maser feature and brightest H2O maser feature are large: 11 km s−1, 19 km s−1, and 22 km s−1, respectively.

With the single-dish observations at Effelsberg, the sizes of ammonia masers, the spatial distributions of ammonia masers and thermal emissions cannot be accurately determined. This also excludes realistic determinations of kinetic temperatures, densities, column densities, and estimates of the ~10 µm infrared irradiation, potentially causing significant populations of vibrationally excited NH3, right at the maser spots. So far, the NH3 maser pumping processes under debate are collisional excitation, radiative excitation, and radiative excitation combined with infrared line overlap (e.g., Walmsley & Ungerechts 1983; Madden et al. 1986; Brown & Cragg 1991; Wilson & Schilke 1993; Henkel et al. 2013). Velocity offsets with respect to systemic velocities may suggest emission associated with outflows or disks (Yan et al. 2022a). Higher-angular-resolution observations are mandatory and proposed to provide precise physical positions and sizes of the newly detected ammonia masers. This will lead to a deeper comprehension of the NH3 maser phenomenon and its connection to sites of massive star formation.

thumbnail Fig. 3

NH3 spectra toward G031.41. The dashed red lines indicate the systemic velocity.
thumbnail Fig. 4

NH3 (7,7), (8,6), and (11,9) spectra toward G029.95. The dashed red lines indicate the systemic velocity.
thumbnail Fig. 5

NH3 (7,6), (8,6), and (11,9) spectra toward G030.70. The dashed red lines indicate the systemic velocity.
thumbnail Fig. 6

NH3 (5,4) spectra toward G024.79, NH3 (6,4) and (6,5) spectra toward G035.19, and NH3 (5,4), (6,4), (9,8), and (10,8) spectra toward G032.74. The dashed red lines indicate the systemic velocities of the sources.
thumbnail Fig. 7

NH3 (1,1), (2,2), and (3,3) spectra toward G048.98. The dashed red lines indicate the systemic velocity.
thumbnail Fig. 8

Comparison of the velocity ranges of NH3 and H2O masers. Dots indicate the velocities of bright NH3 and H2O maser features. Error bars show the velocity ranges in detected NH3 and H2O masers. The dashed line marks locations where the velocity of NH3 equals that of a water maser. The green region shows the ± 10 km s−1 zone.

4 Summary

We report the discovery of at least 14 and likely 15 new ammonia maser sources in the Milky Way, based on our K-bmd line survey with the 100-meter Effelsberg telescope. Our total sample consists of 119 sources exhibiting 22 GHz H2O maser emission, and thus yields a detection rate in excess of 10%. Fourteen of the newly detected masers are encountered in non-metastable inversion transitions and this doubles the number of non-metastable NH3 masers in our Galaxy. Metastable ammonia masers are also detected in one or two sources: an NH3 (7,7) maser in G029.95 and likely an NH3 (3,3) maser in G048.98. Narrow line widths compared to those of ammonia (J, K) = (1,1) thermal emission, as well as variations in flux density, indicate their maser nature. All of the NH3 masers in our detections have blueshifted or redshifted velocities with respect to the source systemic LSR velocities.

Acknowledgements

The authors want to thank the anonymous referee for providing useful comments, which have improved the quality of the paper. Y.T.Y. is a member of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. Y.T.Y. thanks the China Scholarship Council (CSC) for the financial support. We would like to thank the staff at the Effelsberg telescope for their help provided during the observations.

Appendix A Tables

Table A.1

Catalog of ammonia transitions that have been identified as masers in the ISM.

Table A.2

Catalog of sources hosting ammonia masers.

Table A.3

Our sample of 119 observed sources.

Table A.4

New ammonia masers.

Appendix B Figures

thumbnail Fig. B.1

Line profiles of NH3 (1,1) thermal emission and maser transitions in 14 sources. The dashed red lines indicate the systemic velocities of the sources.

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2

Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg.

All Tables

Table A.1

Catalog of ammonia transitions that have been identified as masers in the ISM.

In the text
Table A.2

Catalog of sources hosting ammonia masers.

In the text
Table A.3

Our sample of 119 observed sources.

In the text
Table A.4

New ammonia masers.

In the text

All Figures

thumbnail Fig. 1

Comparison of velocities and line widths of ammonia maser lines to the intrinsic line widths of ammonia (J, K) = (1,1) thermal emission. The number of data points is substantially larger than the number of newly detected sources due to the occasional presence of more than one maser line in a given source and more than one velocity component in a specific maser line. δV = VLSR(maser) − VLSR(NH3(1,1)) is the deviation from velocities of the NH3 maser lines to those of NH3 (1,1) thermal emission. δWidth = ∆V1/2 (NH3 (1,1))-∆V1/2(maser) refers to the difference between the line widths of ammonia maser lines and the intrinsic line widths of an individual NH3 hyperfine structure component. The dashed green lines indicate positions with zero deviation. The metastable NH3 transitions are marked in blue. Among the non-metastable NH3 lines, ortho-NH3 is presented in black and para-NH3 is given in red.
In the text
thumbnail Fig. 2

NH3 (9,6) spectra observed at different epochs toward nine sources. The dashed red lines indicate the systemic velocities of the sources.
In the text
thumbnail Fig. 3

NH3 spectra toward G031.41. The dashed red lines indicate the systemic velocity.
In the text
thumbnail Fig. 4

NH3 (7,7), (8,6), and (11,9) spectra toward G029.95. The dashed red lines indicate the systemic velocity.
In the text
thumbnail Fig. 5

NH3 (7,6), (8,6), and (11,9) spectra toward G030.70. The dashed red lines indicate the systemic velocity.
In the text
thumbnail Fig. 6

NH3 (5,4) spectra toward G024.79, NH3 (6,4) and (6,5) spectra toward G035.19, and NH3 (5,4), (6,4), (9,8), and (10,8) spectra toward G032.74. The dashed red lines indicate the systemic velocities of the sources.
In the text
thumbnail Fig. 7

NH3 (1,1), (2,2), and (3,3) spectra toward G048.98. The dashed red lines indicate the systemic velocity.
In the text
thumbnail Fig. 8

Comparison of the velocity ranges of NH3 and H2O masers. Dots indicate the velocities of bright NH3 and H2O maser features. Error bars show the velocity ranges in detected NH3 and H2O masers. The dashed line marks locations where the velocity of NH3 equals that of a water maser. The green region shows the ± 10 km s−1 zone.
In the text
thumbnail Fig. B.1

Line profiles of NH3 (1,1) thermal emission and maser transitions in 14 sources. The dashed red lines indicate the systemic velocities of the sources.
In the text

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