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A&A
Volume 666, October 2022
Article Number L15
Number of page(s) 16
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202245024
Published online 19 October 2022

© Y. T. Yan et al. 2022

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.

This Open access funding provided by Max Planck Society.

1. Introduction

Since their discovery in the (J, K) = (3,3) metastable (J = K) line toward the high-mass star-forming region (HMSFR) W33 (Wilson et al. 1982), sources emitting maser emission in the ammomia molecule (NH3) have attracted much attention. To date, metastable ammonia maser lines have been detected in 22 HMSFRs (see Yan et al. 2022, and references therein), while non-metastable (J >  K) ammonia masers have only been found in ten sources. The NH3 (6,3), (7,4), (8,5), and (9,6) maser lines, which will be discussed below, arise from energy levels of 551 K, 713 K, 892 K, and 1089 K above the ground state, respectively. These four maser transitions have only been detected, respectively, in three (NGC 7538, W51, and Sgr B2(N)), in two (W51 and Sgr B2(N)), in two (W51 and Sgr B2(N)), and in seven (W51, NGC7538, W49, DR21 (OH), Sgr B2(N), Cep A and G34.26+0.15) HMSFRs (Madden et al. 1986; Henkel et al. 2013; Mei et al. 2020; Yan et al. 2022). The NH3 (10,7) line, also observed by us, connects states as high as 1303 K above the ground state and has so far only been classified as a maser in W51 (Henkel et al. 2013). Among all the abovementioned regions of massive star formation, Sgr B2 hosts a particularly high number of active sites of star formation.

Sgr B2 is located at a projected distance of ∼100 pc from Sgr A* (Reid et al. 2009), the compact radio source associated with the supermassive black hole in the Galactic center at a distance of 8.178 ± 0.013stat. ± 0.022sys. kpc (GRAVITY Collaboration 2019). This region is normally divided into three high-mass star-forming cores: Sgr B2(N), Sgr B2(M), and Sgr B2(S) (see our Fig. 1 for the locations of these). In total, Sgr B2 contains more than 50 H II regions, most of which are ultracompact H II regions (UCH II) with diameters smaller than 0.1 pc (Gaume et al. 1995; De Pree et al. 1998, 2014, 2015; Lazio & Cordes 2008; Ginsburg et al. 2018; Meng et al. 2019, 2022; Nguyen et al. 2021). In Sgr B2 (N) and (M), several deeply embedded high-mass young stellar objects are surrounded by dense, hot molecular cores with an exceedingly rich chemistry that gives rise to a plethora of lines from numerous complex organic molecules (e.g., Belloche et al. 2008, 2013, 2022). Detected molecular maser species are OH (Gaume & Claussen 1990; Caswell et al. 2013; Cotton & Yusef-Zadeh 2016), H2O (Reid et al. 1988; McGrath et al. 2004; Walsh et al. 2014), SiO (Morita et al. 1992; Zapata et al. 2009), H2CO (Mehringer et al. 1994; Hoffman et al. 2007; Lu et al. 2019), CH3OH class I (Mehringer & Menten 1997; Cotton & Yusef-Zadeh 2016), and class II (Caswell 1996; Rickert et al. 2019; Lu et al. 2019), and NH3 (Martín-Pintado et al. 1999; Mills et al. 2018; Mei et al. 2020). In the case of NH3, an ammonia maser in the metastable (3,3) transition was only detected in the southern part of Sgr B2(S) (Martín-Pintado et al. 1999). Another metastable ammonia maser, in the (2,2) line, was found toward SgrB2(M) (Mills et al. 2018). Recently, 18 ammonia non-metastable maser lines at frequencies of 13.0–24.0 GHz were detected toward Sgr B2(N) with the Shanghai 65-m Tianma radio telescope with characteristic beam sizes of 54 × 18.5/ν(GHz) arcseconds (Mei et al. 2020).

thumbnail Fig. 1.

JVLA 1.6-cm continuum map of Sgr B2, shown in gray. The synthesized beam is 0 . ″ 22 × 0 . ″ 08 $ 0{{\overset{\prime\prime}{.}}}22 \times 0{{\overset{\prime\prime}{.}}}08 $, PA = − 10 . ° 61 $ -10{{\overset{\circ}{.}}}61 $.

In this Letter, we report the discovery of NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) masers in Sgr B2(M) and Sgr B2(N), an NH3 (6,3) maser in Sgr B2(NS), as well as NH3 (7,4), (9,6), and (10,7) masers in Sgr B2(S). All of these increase the number of (6,3), (7,4), (8,5), (9,6), and (10,7) maser detections in our Galaxy, respectively, from three to six, two to four, two to three, seven to nine, and one to four. Observations with the Effelsberg 100-m telescope and the Karl G. Jansky Very Large Array (JVLA) are presented in Sect. 2. Results are described in Sect. 3. A comparison of the positions of the different ammonia masers with those of other relevant tracers of the interstellar medium is presented in Sect. 4. Our main results are summarized in Sect. 5.

2. Observations and data reduction

2.1. Effelsberg observations and data reduction

The NH3 (9,6) and (10,7) lines were observed toward Sgr B2 with the 100-m Effelsberg telescope1 at 12 epochs in January 2020, February and August 2021, as well as in March, May, June, July, and August 2022. The observations were performed in position switching mode. The off position was 30′ in azimuth away from the source. An S14mm double beam secondary focus receiver was employed. The half power beam width (HPBW) is 49 × 18.5/ν(GHz) arcseconds, that is 49″ at 18.5 GHz, the frequency of the NH3 (9,6) line. Before August 2021, the spectrometer covered 2 GHz with a channel width of 38.1 kHz, corresponding to ∼0.62 km s−1 at 18.5 GHz. From August 2021, a high spectral resolution backend with 65 536 channels and a bandwidth of 300 MHz was employed, providing a channel width of 0.07 km s−1 at 18.5 GHz. Pointing was checked every hour using NGC 7027. Focus calibrations were done at the beginning of the observations, and during sunset and sunrise, toward NGC 7027. The calibrator was measured between elevations of 30 and 56 deg. The elevation on target was about 10 deg, requiring only minimal (≤2%) elevation-dependent flux density corrections. The system temperatures were 140–220 K in a main-beam brightness temperature, TMB, scale. The flux density was calibrated assuming a TMB/S ratio of 1.95 K Jy−1, derived from continuum cross scans of NGC 7027 (its flux density was adopted from Ott et al. 1994). Calibration uncertainties were estimated to be ∼10%.

We used the GILDAS/CLASS2 package (Pety et al. 2005) to reduce the spectral line data. A first-order polynomial was subtracted from each spectrum for baseline removal.

2.2. JVLA observations and data reduction

Observations of six NH3 lines, the (5,1), (6,3), (7,4), (8,5), (9,6), and (10,7) transitions (Table 1), toward Sgr B2 were made on 5 March 2022 with the JVLA of the National Radio Astronomy Observatory3 (NRAO) in the A-configuration (project ID: 22A-106, PI: Yaoting Yan). Eight-bit samplers were used to perform the observations. For the NH3 (9,6) and (10,7) line observations, we used two subbands with the eight-bit samplers covering a bandwidth of 16 MHz with full polarization, eight recirculations, and four baseline board pairs (BLBPs) to provide a velocity range of 260 km s−1 with a channel spacing of 0.13 km s−1. Four additional subbands of bandwidth 16 MHz were used to cover the NH3 (5,1), (6,3), (7,4), and (8,5) lines. The remaining ten subbands of the eight-bit sampler with a bandwidth of 128 MHz were used to measure the continuum emission between 18 and 20 GHz. The primary beam of the JVLA antennas is 150″ (FWHM) at 18.5 GHz, covering all prominent star-forming regions in Sgr B2 simultaneously. 3C 286 was used as a calibrator for pointing, flux density, bandpass, and polarization (Perley & Butler 2013). J1745−2900 served as our gain calibrator during the observations. The on-source time was 30 min toward Sgr B2.

Table 1.

Summary of the JVLA observations.

A total of 27 antennas were employed for the observations. Data from two antennas were lost due to technical issues. The data from the remaining 25 antennas were reduced through the Common Astronomy Software Applications package (CASA4; McMullin et al. 2007). We calibrated the data with the JVLA CASA calibration pipeline using CASA 6.2.1. The results were obtained after flagging and recalibrating data that contained artifacts. We inspected the phase, amplitude, and bandpass variations of the calibrated visibility data to search for additional artifacts before imaging. Then, the uvcontsub task in CASA was used to separate the calibrated visibilities into two parts: one with line-only data and the other with the line-free continuum data. The tclean task with a cell size of 0 . ″ $ {{\overset{\prime\prime}{.}}} $02 and Briggs weighting with robust = 0.5 was used to produce the images of spectral line and continuum emission. All of the images were corrected for primary beam response. The synthesized beams and the rms noises in a channel image for the observed lines are listed in Table 1. For the 1.6 cm (18–20 GHz) continuum emission, the synthesized beam is 0 . ″ 22 × 0 . ″ 08 $ 0{{\overset{\prime\prime}{.}}}22 \times 0{{\overset{\prime\prime}{.}}}08 $ at PA = − 10 . ° 61 $ -10{{\overset{\circ}{.}}}61 $. The typical absolute astrometric accuracy of the JVLA is ∼10% of the synthesized beam5. The flux density scale calibration accuracy is estimated to be within 15%.

The maser spots were identified in two different ways and then cross-checked. First, we searched for masers using eyes in channel maps with a velocity spacing of 0.5 km s−1. Second, we used an automated source extraction code (SEC; Murugeshan 2015; Nguyen 2015) running in CASA 5.4 to find the maser features. Emission with signal-to-noise ratios (S/Ns) larger than six identified in this way was considered to be a real detection. A detailed description of the SEC code can be found in Sect. 3.1 of Nguyen et al. (2022).

3. Results

In January 2020, with the Effelsberg 100-m telescope, we observed two strong NH3 (9,6) maser features with velocities of ∼77 km s−1 and ∼84 km s−1, and a weaker one at ∼72 km s−1, toward the equatorial position α J 2000 = 17 h 47 m 20 . s 8 $ \alpha_{\mathrm{J2000}} = 17^{\mathrm{h}}47^{\mathrm{m}}20{{\overset{\text{s}}{.}}}8 $, and δ J 2000 = − 28 ° 23 ′ 32 . ″ 1 $ \delta_{\mathrm{J2000}} = -28{\circ}23 {\prime}32{{\overset{\prime\prime}{.}}}1 $, which is offset by ( + 3 . ″ 96 $ +3{{\overset{\prime\prime}{.}}}96 $, − 26 . ″ 19 $ -26{{\overset{\prime\prime}{.}}}19 $) from Sgr B2(M). In addition, maser emission at ∼77 km s−1 was also found in the non-metastable para-NH3 (10,7) transition. The NH3 (9,6) and (10,7) maser spectra are shown in Fig. A.1. In February 2021, we extended our observations to 20 positions to cover, in a fully sampled way, an area of ∼5.0 square arc minutes surrounding Sgr B2(M) with a spacing of 25″, half the beam size. (9,6) maser emission was found to be quite widespread in Sgr B2, not only residing in Sgr B2(M), but also in Sgr B2(N) and Sgr B2(S), while (10,7) masers were detected in a more limited region comprising Sgr B2(M) and Sgr B2(N). The maps of NH3 (9,6) and (10,7) spectra are presented in Figs. A.2 and A.3.

Effelsberg monitoring observations spanning 19 months show that the NH3 (9,6) maser at VLSR = 72.5 km s−1 toward Sgr B2(N) became weaker from February to August in 2021 and was not detectable from March 2022 on 3σ levels of 0.12 Jy with a 0.07 km s−1 channel width. A weaker (9,6) feature at VLSR = 63.8 km s−1 was detected in March 2022. The (9,6) maser spectra from Sgr B2(N) are presented in Fig. A.4. NH3 (9,6) line parameters obtained by Gaussian fits are listed in Table B.1. An NH3 (10,7) maser was detected at a different velocity of 82.0 km s−1 toward Sgr B2(N). Its flux density was increasing from February to August 2021 but was decreasing from March to August 2022. In Table B.2, NH3 (10,7) line parameters obtained from Gaussian fits are presented.

Toward Sgr B2(M), NH3 (9,6) maser emission at VLSR = 72.5 km s−1 became stronger between February 2021 and March 2022, then weakened until 2022 August. Higher spectral resolution data since March 2022 show the NH3 (9,6) emission to be composed of three different components. The NH3 (10,7) maser in Sgr B2(M) has a velocity offset with respect to the (9,6) maser, with a velocity of VLSR = 70.0 km s−1. The flux density in the Effelsberg beam remained constant within the uncertainties during the 19 months. Spectra are shown in Fig. A.5 and line parameters obtained by Gaussian fits are listed in Tables B.3 and B.4.

The 1.6 cm continuum, derived from our JVLA A-configuration measurements, is shown in Fig. 1. A total of 22 known compact H II regions (Gaume et al. 1995; De Pree et al. 1998, 2014, 2015) were detected by our observations. The locations and sizes of these sources, derived with the imfit task in CASA, are consistent with previous results from 7 mm continuum measurements (De Pree et al. 2015). Details are given in Table B.5.

The JVLA has a better angular resolution compared to the Effelsberg 100-m single dish and those data reveal 18 maser spots in the NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) transitions. We did not find any emission in the NH3 (5,1) line from Sgr B2. The 3σ upper limit for the NH3 (5,1) line is 9.96 mJy beam−1 (about 1800 K) for a channel width of 0.12 km s−1. The JVLA NH3 (9,6) and (10,7) line profiles toward Sgr B2(M), extracted from an Effelsberg-beam-size region (FWHM, 49″), are shown in Fig. A.5. From the similarity of the flux density obtained at Effelsberg and the JVLA, measured in March 2022, we conclude that there is no “missing spacing” flux density in the JVLA data, that is, emission on angular scales larger than defined by the shortest JVLA baseline. NH3 (6,3) masers arise from four different locations, named 63A, 63B, 63C, and 63D. NH3 (7,4), (8,5), (9,6), and (10,7) masers are detected toward three, two, four, and five spots, respectively. Positions and spectral parameters of these masers are listed in Table B.6. The detected isolated maser spots are distributed over a 16″ × 93″ area in Sgr B2, corresponding to 0.6 × 3.7 pc. Details related to the individual sources are given below.

Sgr B2(N). Among the 18 maser spots detected in Sgr B2, 50% are located in Sgr B2(N). These are seen in the NH3 (6,3) transition toward 63A and 63B, in the (7,4) transition toward 74A, in the (8,5) line toward 85A, in the (9,6) transition toward 96A and 96B, as well as in the (10,7) line toward 107A, 107B, and 107C. Four of these sources, 85A, 96A, 96B, and 107C, are close to the UCH II region K2 (see Fig. 2). The two maser spots, 85A and 107C, share the same position and have similar velocity distributions. Three maser spots, 63A, 63B, and 74A, surround the compact continuum source K3. The two maser spots, 63A and 74A, share, within the uncertainties, the same position and have similar velocity distributions. One maser spot, 107A, originates from a region offset by ( − 0 . ″ 61 ± 0 . ″ 02 $ -0{{\overset{\prime\prime}{.}}}61\pm0{{\overset{\prime\prime}{.}}}02 $, + 0 . ″ 47 ± 0 . ″ 01 $ +0{{\overset{\prime\prime}{.}}}47\pm0{{\overset{\prime\prime}{.}}}01 $) from the continuum source K7. The second maser spot in the (10,7) line, 107B, has the highest brightness temperature. The lower limit is no less than 6 × 105 K and originates from a region without any centimeter continuum source. Spectra from these nine maser spots are presented in Fig. 3.

thumbnail Fig. 2.

JVLA 1.6 cm continuum map of Sgr B2(N), shown by the gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is α J 2000 = 17 h 47 m 19 . s 883 $ \alpha_{\mathrm{J2000}} = 17^{\mathrm{h}}47^{\mathrm{m}}19{{\overset{\text{s}}{.}}}883 $, and δ J 2000 = − 28 ° 22 ′ 18 . ″ 412 $ \delta_{\mathrm{J2000}} = -28{\circ}22{\prime}18{{\overset{\prime\prime}{.}}}412 $, the peak position of the continuum source K2. The crosses, thin diamonds, diamonds, stars, and circles show the positions of NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004), class II CH3OH (Caswell 1996; Lu et al. 2019), H2CO (Mehringer et al. 1994; Hoffman et al. 2007; Lu et al. 2019), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, pentagons, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of maser spots. Red crosses mark the positions of the hot cores Sgr B2(N1), N2, and N3, taken from the 3 mm imaging line survey “Exploring Molecular Complexity with ALMA” (EMoCA, Bonfand et al. 2017). The systemic velocities of the hot cores N1, N2, and N3 are VLSR = 64 km s−1, VLSR = 74 km s−1 and VLSR = 74 km s−1, respectively (Bonfand et al. 2017).

thumbnail Fig. 3.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(N). The dashed red lines indicate the systemic velocities of the associated hot cores. VLSR = 64 km s−1 for N1, and VLSR = 74 km s−1 for N2 and N3 (Bonfand et al. 2017). Main beam brightness temperature scales are presented on the left hand side of the profiles.

Sgr B2(NS). Only one maser spot, 63C, in the NH3 (6,3) transition was detected in this region. It is the strongest among all the detected (6,3) masers in Sgr B2 and is at a position with an offset of ( − 0 . ″ 09 ± 0 . ″ 02 $ -0{{\overset{\prime\prime}{.}}}09\pm0{{\overset{\prime\prime}{.}}}02 $, + 0 . ″ 01 ± 0 . ″ 01 $ +0{{\overset{\prime\prime}{.}}}01\pm0{{\overset{\prime\prime}{.}}}01 $) from the H II2 region, Z10.24 (see Fig. A.6 and Table B.6). The spectrum is shown in Fig. A.7.

Sgr B2(M). We detected five maser spots in this area (see Fig. A.8). Three of them, 63D, 74B, and 96C, arise from similar positions close to the UCH II region F10.39. These three maser features are distributed in the same velocity range, from 70.0 km s−1 to 70.4 km s−1, while the (6,3) maser, 63D, extends spectroscopically down to the lower velocity of 68.0 km s−1. Spectra are presented in Fig. A.9. Another two maser spots, 85B and 107D, in the (8,5) and (10,7) transitions are located in regions close to the UCH II region F10.39.

Sgr B2(S). Three maser spots, 74C, 96D, and 107E, in the NH3 (7,4), (9,6), and (10,7) transitions, are detected in this region. These are close to each other and are found slightly off the head of the cometary UCH II source (Fig. A.10). Spectra are shown in Fig. A.11.

4. Discussion

As shown in Figs. A.4 and A.5, and claimed in Sect. 3, our JVLA data of NH3 (9,6) and (10,7) lines are not affected by missing flux. All of the detected maser spots are spatially unresolved, so the derived brightness temperatures are lower limits. Nevertheless, lower limits to the brightness temperature are > 3000 K and reach 6 × 105 K (107B). A comparison with the NH3 (6,3), (7,4), (8,5), and (9,6) observations toward Sgr B2(N), using the TMRT 65-m telescope in March 2020 (Mei et al. 2020), reveals that almost all maser spots in our JVLA data are new detections at different velocities, with the exception of 74A. This strongly suggests substantial variations of the NH3 (6,3), (8,5), and (9,6) masers since March 2020.

Maser spots from other molecules, H2O (McGrath et al. 2004), class II CH3OH (Caswell 1996; Hu et al. 2016; Lu et al. 2019), H2CO (Mehringer et al. 1994; Hoffman et al. 2007; Lu et al. 2019), and OH (Gaume & Claussen 1990), are presented in Figs. 2, A.6, A.8, and A.10. The class I CH3OH masers at 44 GHz toward the Sgr B2(M) and N regions from Mehringer & Menten (1997) are outside the scales of Figs. 2, A.6, and A.8, and are therefore not shown. The SiO maser (Morita et al. 1992; Zapata et al. 2009) and the metastable NH3 (2,2) masers in Sgr B2(M) (Mills et al. 2018) originate from a region around the radio object F3, which is more than two arcseconds north of source F10.39. These sources are, therefore, also not shown in Fig. A.8. The ammonia (3,3) masers are located in an area more than 15 arcsec south of Sgr B2(S) (Martín-Pintado et al. 1999) and outside the region shown in Fig. A.10. It is noteworthy that they are not spatially related to any (6,3) maser component, another transition within the same K = 3 ladder. Our detected non-metastable NH3 masers and previously detected metastable NH3 masers arise from different regions. This indicates that these types of ammonia masers are excited in different ways.

There are no apparent space and velocity correlations between our detected non-metastable NH3 maser spots and other molecular masers. The locations of the hot cores Sgr B2(N1), N2 and N3 in Sgr B2(N), as well as Sgr B2(N5) in Sgr B2(NS), derived from the 3 mm imaging line survey “Exploring Molecular Complexity with ALMA” (EMoCA, Bonfand et al. 2017), are shown in Figs. 2 and A.6. A bipolar outflow in an east-west direction was found around the UCH II region K2, also known as Sgr B2(N1) (Higuchi et al. 2015; Bonfand et al. 2017). Bipolar outflows are also observed in a north-south direction and a northeast-southwest direction in the hot cores of Sgr B2 (N3) and N5, respectively (Bonfand et al. 2017). There is a class II CH3OH maser spot and an H2O maser spot close to the hot core Sgr B2(N3). Seven NH3 maser spots, 63A, 63B, 74A, 85A, 96A, 96B, and 107C, are close to the hot core Sgr B2(N1). 107A and 107B originate from regions near the hot cores Sgr B2(N2) and N3, respectively. 63C arises from an area close to the hot core Sgr B2(N5). None of the NH3 masers are spatially coincident with the hot cores in projection. The redshifts seen in 63B, 96A, and 107B, as well as blueshifts seen in 63A, 63C, 74A, 85A, 96B, 107A, and 107C with respect to the systemic velocities of the associated hot cores, may suggest that these maser spots are related to the outflows. The line profiles from these ten maser spots are shown in Figs. 3 and A.7. Bonfand et al. (2017) did not find any sign of an outflow around Sgr B2(N2), while our maser spot 107A, with a blueshifted velocity of VLSR = 65.5 km s−1 (Vsys = 74.0 km s−1), could indicate the presence of an outflow.

All detected NH3 maser spots in Sgr B2(M) show redshifted velocities with respect to the systemic velocity of Sgr B2(M), VLSR = 62 km s−1 (Belloche et al. 2013). This supports the suggestion that NH3 masers in Sgr B2(M) are also related to outflows.

Toward Sgr B2(S), the ammonia maser spots 74C, 96D, and 107E also have redshifted velocities with respect to the systemic velocity of Sgr B2(S), VLSR = 60 km s−1 (Meng et al. 2022), which may indicate that this emission takes part in outflows. Several hot cores were identified by Jeff et al. (in prep.) in Sgr B2(S) in ALMA data (project code: 2017.1.00114.S, PI: A. Ginsburg). No hot cores were found to be close to the ammonia masers. This indicates that the presence of hot, dense gas alone is not sufficient to excite these masers. The ammonia masers detected in Sgr B2(S) are close to the head of the cometary UCH II region, similar to the NH3 (9,6) maser M1 in G34.26+0.15 (Yan et al. 2022). The maser spots in Sgr B2(S) show almost twice the angular distance compared to M1, with an offset of ( + 0 . ″ 36 ± 0 . ″ 13 $ +0{{\overset{\prime\prime}{.}}}36\pm0{{\overset{\prime\prime}{.}}}13 $, − 0 . ″ 07 ± 0 . ″ 11 $ -0{{\overset{\prime\prime}{.}}}07\pm0{{\overset{\prime\prime}{.}}}11 $) in G34.26+0.15. In view of the different distances of G34.26+0.15 (D ∼ 3.3 kpc, Yan et al. 2022) and Sgr B2 (8.2 kpc see Sect. 1), the linear distance is even five times larger in Sgr B2(S), that is 0.03 pc. The velocity difference between the masers and the cometary UCH II regions is ten times higher in Sgr B2(S) than in G34.26+0.15 (ΔVSgrB2(S) ≥ 17.4 km s−1 and ΔVG34.26 ∼ −1.3 km s−1). That indicates that the cometary UCH II region in Sgr B2(S) is more active than the one in G34.26+0.15.

Overall, the detected non-metastable ammonia masers in Sgr B2 are consistent with the discussion on pumping scenarios in Yan et al. (2022). Therefore, we speculate that the detected NH3 masers in the non-metastable (6,3), (7,4), (8,5), (9,6), and (10,7) transitions in Sgr B2(N), Sgr B2(NS), Sgr B2(M), and Sgr B2(S) appear to be associated with shocks caused either by outflows or UCH II expansion.

5. Summary

We report the discovery of NH3 non-metastable (6,3), (7,4), (8,5), (9,6), and (10,7) masers in Sgr B2(M) and Sgr B2(N), an NH3 (6,3) maser in Sgr B2(NS), as well as NH3 (7,4), (9,6), and (10,7) masers in Sgr B2(S). High angular resolution data from the JVLA A-configuration reveal 18 maser spots. Nine maser spots arise from Sgr B2(N), one from Sgr B2(NS), five from Sgr B2(M), and three originate in Sgr B2(S). All of these increase the number of (6,3), (7,4), (8,5), (9,6), and (10,7) maser detections in our Galaxy from three to six, two to four, two to three, seven to nine, and one to four. Compared to the Effelsberg 100-m telescope data, the JVLA data indicate no missing flux. The detected maser spots are not resolved by our JVLA observations. Lower limits to the brightness temperature are > 3000 K and reach up to 6 × 105 K, manifesting their maser nature. Long-term Effelsberg monitoring (19 months) indicates that the intensities of the (9,6) masers in Sgr B2(M), as well as the (9,6) and (10,7) masers in Sgr B2(N), show noticeable variations. However, the (10,7) maser in Sgr B2(M) is stable. While the NH3 masers all arise near hot cores, there are many hot cores that do not exhibit NH3 maser emission. All of these non-metastable ammonia maser lines show redshifted or blueshifted features that may be related to outflows or UCH II expansion.

1

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

3

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.

Acknowledgments

The authors thank the anonymous referee for the useful comments that improve the manuscript. We thank Chris De Pree for providing the 7 mm continuum images of Sgr B2(M) and Sgr B2(N). 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) and the Max-Planck-Institut für Radioastronomie (MPIfR) for the financial support. Y.T.Y. also thanks his fiancee, Siqi Guo, for her support during this pandemic period. We would like to thank the staff at the Effelsberg telescope for their help provided during the observations. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the staff of the JVLA, especially Tony Perreault and Drew Medlin, for their assistance with the observations and data reduction.

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Appendix A: Figures

thumbnail Fig. A.1.

NH3 (9,6) and (10,7) maser lines from the 100-m telescope at Effelsberg toward a region south of Sgr B2(M), at αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $8, δJ2000 = −28°23′32 . ″ $ {{\overset{\prime\prime}{.}}} $1. The three red dashed lines indicate the three different velocity components at VLSR = ∼72 km s−1, ∼77 km s−1, and ∼84 km s−1.

thumbnail Fig. A.2.

NH3 (9,6) line profiles observed with the Effelsberg 100-m telescope over the region of Sgr B2. The black spectra show the original flux density scales. The red, blue, and cyan spectra are presented after multiplying the flux densities by factors of three, ten, and fifteen, respectively.

thumbnail Fig. A.3.

NH3 (10,7) spectra observed with the Effelsberg 100-m telescope toward Sgr B2. The black spectra show the original flux density scales, while the red spectra are presented after multiplying the flux densities by a factor of three.

thumbnail Fig. A.4.

Effelsberg 100-meter telescope and JVLA A-configuration spectra from NH3 (9,6) and (10,7) transition lines at eight epochs toward Sgr B2(N), after subtracting a first-order polynomial baseline. The JVLA spectra are extracted over a region of radius 35″ centered at Sgr B2(N).

thumbnail Fig. A.5.

Effelsberg 100-meter telescope and JVLA A-configuration spectra from NH3 (9,6) and (10,7) transition lines at 12 epochs toward Sgr B2(M), after subtracting a first-order polynomial baseline. The JVLA spectra are extracted from the Effelsberg beam (FWHM, 49″) sized region.

thumbnail Fig. A.6.

JVLA 1.6 cm continuum map of Sgr B2(NS) presented as gray shaded area and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $043, and δJ2000 = −28°22′41 . ″ $ {{\overset{\prime\prime}{.}}} $143, the peak position of continuum source Z10.24. The yellow cross shows the position of NH3 (6,3) emission. H2O (McGrath et al. 2004), class II CH3OH (Caswell 1996; Hu et al. 2016; Lu et al. 2019), H2CO (Hoffman et al. 2007), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, pentagons, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The red cross marks the position of the hot core, Sgr B2(N5), taken from the 3 mm imaging line survey EMoCA (Bonfand et al. 2017). The systemic velocity of the hot core, N5, is VLSR = 60 km s−1 (Bonfand et al. 2017).

thumbnail Fig. A.7.

JVLA A-configuration spectrum of the NH3 (6,3) transition line toward Sgr B2(NS). The systemic velocity of the associated hot core, VLSR = 60 km s−1 in N5 (Bonfand et al. 2017), is indicated by the dashed red line.

thumbnail Fig. A.8.

JVLA 1.6 cm continuum map of Sgr B2(M) presented as gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $197, and δJ2000 = −28°23′06 . ″ $ {{\overset{\prime\prime}{.}}} $484, the peak position of continuum source F10.39. The cross, thin diamond, diamond, circle and star show the positions of NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004), class II CH3OH (Lu et al. 2019), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The systemic velocity of Sgr B2(M) is VLSR = 62 km s−1 (Belloche et al. 2013).

thumbnail Fig. A.9.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(M). The systemic velocity of Sgr B2(M), VLSR = 62 km s−1 (Belloche et al. 2013), is indicated by the dashed red line.

thumbnail Fig. A.10.

JVLA 1.6 cm continuum map of Sgr B2(S) presented as gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $472, and δJ2000 = −28°23′45 . ″ $ {{\overset{\prime\prime}{.}}} $120, the peak position of the continuum source. The thin diamond, star, and circle show the positions of NH3 (7,4), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004) and OH (Gaume & Claussen 1990) masers are presented as triangles and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The systemic velocity of Sgr B2(S) is VLSR = 60 km s−1 (Meng et al. 2022).

thumbnail Fig. A.11.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(S). The systemic velocity of Sgr B2(S), VLSR = 60 km s−1 (Meng et al. 2022), is indicated by the dashed red line.

Appendix B: Tables

Table B.1.

Summary of NH3 (9,6) maser observations toward Sgr B2(N)

Table B.2.

Summary of NH3 (10,7) maser observations toward Sgr B2(N)

Table B.3.

Summary of NH3 (9,6) maser observations toward Sgr B2(M)

Table B.4.

Summary of NH3 (10,7) maser observations toward Sgr B2(M)

Table B.5.

1.6 cm JVLA flux densities of individual continuum sources

Table B.6.

NH3 maser positions in Sgr B2, derived from the JVLA observations.

All Tables

Table 1.

Summary of the JVLA observations.

In the text
Table B.1.

Summary of NH3 (9,6) maser observations toward Sgr B2(N)

In the text
Table B.2.

Summary of NH3 (10,7) maser observations toward Sgr B2(N)

In the text
Table B.3.

Summary of NH3 (9,6) maser observations toward Sgr B2(M)

In the text
Table B.4.

Summary of NH3 (10,7) maser observations toward Sgr B2(M)

In the text
Table B.5.

1.6 cm JVLA flux densities of individual continuum sources

In the text
Table B.6.

NH3 maser positions in Sgr B2, derived from the JVLA observations.

In the text

All Figures

thumbnail Fig. 1.

JVLA 1.6-cm continuum map of Sgr B2, shown in gray. The synthesized beam is 0 . ″ 22 × 0 . ″ 08 $ 0{{\overset{\prime\prime}{.}}}22 \times 0{{\overset{\prime\prime}{.}}}08 $, PA = − 10 . ° 61 $ -10{{\overset{\circ}{.}}}61 $.
In the text
thumbnail Fig. 2.

JVLA 1.6 cm continuum map of Sgr B2(N), shown by the gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is α J 2000 = 17 h 47 m 19 . s 883 $ \alpha_{\mathrm{J2000}} = 17^{\mathrm{h}}47^{\mathrm{m}}19{{\overset{\text{s}}{.}}}883 $, and δ J 2000 = − 28 ° 22 ′ 18 . ″ 412 $ \delta_{\mathrm{J2000}} = -28{\circ}22{\prime}18{{\overset{\prime\prime}{.}}}412 $, the peak position of the continuum source K2. The crosses, thin diamonds, diamonds, stars, and circles show the positions of NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004), class II CH3OH (Caswell 1996; Lu et al. 2019), H2CO (Mehringer et al. 1994; Hoffman et al. 2007; Lu et al. 2019), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, pentagons, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of maser spots. Red crosses mark the positions of the hot cores Sgr B2(N1), N2, and N3, taken from the 3 mm imaging line survey “Exploring Molecular Complexity with ALMA” (EMoCA, Bonfand et al. 2017). The systemic velocities of the hot cores N1, N2, and N3 are VLSR = 64 km s−1, VLSR = 74 km s−1 and VLSR = 74 km s−1, respectively (Bonfand et al. 2017).
In the text
thumbnail Fig. 3.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(N). The dashed red lines indicate the systemic velocities of the associated hot cores. VLSR = 64 km s−1 for N1, and VLSR = 74 km s−1 for N2 and N3 (Bonfand et al. 2017). Main beam brightness temperature scales are presented on the left hand side of the profiles.
In the text
thumbnail Fig. A.1.

NH3 (9,6) and (10,7) maser lines from the 100-m telescope at Effelsberg toward a region south of Sgr B2(M), at αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $8, δJ2000 = −28°23′32 . ″ $ {{\overset{\prime\prime}{.}}} $1. The three red dashed lines indicate the three different velocity components at VLSR = ∼72 km s−1, ∼77 km s−1, and ∼84 km s−1.
In the text
thumbnail Fig. A.2.

NH3 (9,6) line profiles observed with the Effelsberg 100-m telescope over the region of Sgr B2. The black spectra show the original flux density scales. The red, blue, and cyan spectra are presented after multiplying the flux densities by factors of three, ten, and fifteen, respectively.
In the text
thumbnail Fig. A.3.

NH3 (10,7) spectra observed with the Effelsberg 100-m telescope toward Sgr B2. The black spectra show the original flux density scales, while the red spectra are presented after multiplying the flux densities by a factor of three.
In the text
thumbnail Fig. A.4.

Effelsberg 100-meter telescope and JVLA A-configuration spectra from NH3 (9,6) and (10,7) transition lines at eight epochs toward Sgr B2(N), after subtracting a first-order polynomial baseline. The JVLA spectra are extracted over a region of radius 35″ centered at Sgr B2(N).
In the text
thumbnail Fig. A.5.

Effelsberg 100-meter telescope and JVLA A-configuration spectra from NH3 (9,6) and (10,7) transition lines at 12 epochs toward Sgr B2(M), after subtracting a first-order polynomial baseline. The JVLA spectra are extracted from the Effelsberg beam (FWHM, 49″) sized region.
In the text
thumbnail Fig. A.6.

JVLA 1.6 cm continuum map of Sgr B2(NS) presented as gray shaded area and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $043, and δJ2000 = −28°22′41 . ″ $ {{\overset{\prime\prime}{.}}} $143, the peak position of continuum source Z10.24. The yellow cross shows the position of NH3 (6,3) emission. H2O (McGrath et al. 2004), class II CH3OH (Caswell 1996; Hu et al. 2016; Lu et al. 2019), H2CO (Hoffman et al. 2007), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, pentagons, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The red cross marks the position of the hot core, Sgr B2(N5), taken from the 3 mm imaging line survey EMoCA (Bonfand et al. 2017). The systemic velocity of the hot core, N5, is VLSR = 60 km s−1 (Bonfand et al. 2017).
In the text
thumbnail Fig. A.7.

JVLA A-configuration spectrum of the NH3 (6,3) transition line toward Sgr B2(NS). The systemic velocity of the associated hot core, VLSR = 60 km s−1 in N5 (Bonfand et al. 2017), is indicated by the dashed red line.
In the text
thumbnail Fig. A.8.

JVLA 1.6 cm continuum map of Sgr B2(M) presented as gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $197, and δJ2000 = −28°23′06 . ″ $ {{\overset{\prime\prime}{.}}} $484, the peak position of continuum source F10.39. The cross, thin diamond, diamond, circle and star show the positions of NH3 (6,3), (7,4), (8,5), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004), class II CH3OH (Lu et al. 2019), and OH (Gaume & Claussen 1990) masers are presented as triangles, squares, and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The systemic velocity of Sgr B2(M) is VLSR = 62 km s−1 (Belloche et al. 2013).
In the text
thumbnail Fig. A.9.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(M). The systemic velocity of Sgr B2(M), VLSR = 62 km s−1 (Belloche et al. 2013), is indicated by the dashed red line.
In the text
thumbnail Fig. A.10.

JVLA 1.6 cm continuum map of Sgr B2(S) presented as gray shaded areas and black contours with levels of 5, 10, 30, and 50 × 0.2 mJy beam−1. The reference position is αJ2000 = 17h47m20 . s $ {{\overset{\text{s}}{.}}} $472, and δJ2000 = −28°23′45 . ″ $ {{\overset{\prime\prime}{.}}} $120, the peak position of the continuum source. The thin diamond, star, and circle show the positions of NH3 (7,4), (9,6), and (10,7) emissions. H2O (McGrath et al. 2004) and OH (Gaume & Claussen 1990) masers are presented as triangles and hexagons, respectively. The color bar indicates the velocity range (VLSR) of the maser spots. The systemic velocity of Sgr B2(S) is VLSR = 60 km s−1 (Meng et al. 2022).
In the text
thumbnail Fig. A.11.

JVLA A-configuration spectra of NH3 transition lines toward Sgr B2(S). The systemic velocity of Sgr B2(S), VLSR = 60 km s−1 (Meng et al. 2022), is indicated by the dashed red line.
In the text

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