ABSTRACT
We present the first detection of extragalactic submillimeter H2O maser in the 321 GHz transition toward the center of Circinus galaxy, the nearby Type 2 Seyfert using the Atacama Large Millimeter/Submillimeter Array. We find that Doppler features of the detected 321 GHz H2O maser straddle the systemic velocity of the galaxy as seen in the spectrum of the known 22 GHz H2O maser in the galaxy. By comparing the velocities of the maser features in both transitions, it can be deduced that the 321 GHz maser occurs in a region similar to that of the 22 GHz maser, where the sub-parsec-scale distribution of the 22 GHz maser was revealed by earlier very long baseline interferometry observations. The detected maser features remain unresolved at the synthesized beam of ∼0
66 (∼15 pc) and coincide with the 321 GHz continuum peak within small uncertainties. We also present a tentative detection of the highest velocity feature (redshifts up to ∼635 km s−1) in the galaxy. If this high-velocity feature arises from a Keplerian rotating disk well established in this galaxy, it is located at a radius of ∼0.018 pc (∼1.2 × 105 Schwarzschild radii), which might probe molecular material closest to the central engine.
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1. INTRODUCTION
The H2O masers in the 616–523 transition (rest frequency (νrest) = 22.23508 GHz) in active galactic nuclei (AGNs), which we call megamasers, have been detected in ∼150 galaxies, most of which are detected exclusively toward narrow-line AGNs having a Seyfert 1.8–2.0 or LINER nucleus (e.g., Kondratko et al. 2006; Hagiwara et al. 2003b; Hagiwara 2007). Some of these megamasers show spectra giving evidence for emission from edge-on rotating disks on scales of sub-parsecs as measured in NGC 4258 with very long baseline interferometry (VLBI) observations at milliarcsecond angular resolution (e.g., Miyoshi et al. 1995; Hagiwara et al. 2001; Kuo et al. 2011). According to theoretical studies, the 22 GHz H2O maser is inverted in a wider range of physical conditions at a kinetic temperature of Tk = 200–2000 K, hydrogen densities of n(H2) = 108–1010 cm−3, and H2O densities of n(H2O) = 103–105 cm−3, which largely excites submillimeter H2O maser transitions such as the 321 GHz (1029–936) and 325 GHz (515–422) transitions (Deguchi 1977; Neufeld & Melnick 1990, 1991; Yates et al. 1997). These studies have shown that the 321 GHz maser is even strongly excited under the more restricted physical conditions of Tk > 1000 K. The upper state energy of the 321 GHz transition is higher than the 22 GHz transition (Eu/k = 1862 K for the 321 GHz transition and Eu/k = 644 K for the 22 GHz transition).
The first discovery of extragalactic submillimeter H2O masers was achieved by Humphreys et al. (2005), in which the detection of H2O masers in the 183 GHz (313–220) and 439 GHz (643–550) transitions was reported in the nuclear region of NGC 3079, the Type 2 Seyfert/LINER galaxy, although the detection of the 439 GHz maser was tentative. The H2O emission in the 183 GHz transition was also detected toward the luminous infrared galaxy Arp220 (Cernicharo et al. 2006). However, the detected emission could be thermal emission.
The Circinus galaxy at a distance of 4.2 ± 0.8 Mpc is a nearby spiral lying behind the Galactic plane (Freeman et al. 1977), exhibiting evidence of the co-existence of starbursts occurring within a nuclear molecular ring in CO emission (Curran et al. 2008). The galaxy hosts a Type 2 Seyfert nucleus having a luminous (∼1042 erg s−1) hard X-ray source obscured by a Compton-thick medium (Matt et al. 1996). The adopted systemic velocity of the galaxy is 433 km s−1 (de Vaucouleurs et al. 1991; Braatz et al. 2003). (The optical velocity definition is adopted throughout this article and the velocities are calculated with respect to the local standard of rest (LSR).)
Bright 22 GHz H2O maser emission in the galaxy was first reported by Gardner & Whiteoak (1982) and the known highly variable flux density of the maser is in the range of ∼5 to ∼40 Jy (e.g., Greenhill et al. 1997; Braatz et al. 2003; McCallum et al. 2005). The velocity range of highly Doppler-shifted (high-velocity) maser features detected to date is ∼50 to ∼900 km s−1 (Greenhill et al. 2003b). Follow-up VLBI observations of the 22 GHz maser at the milliarcsecond resolution revealed that the maser traces a warped edge-on disk that implies a sub-Keplerian rotation with a radius of ∼0.1–0.4 pc at the maximum rotation velocity of ∼270 km s−1 around a nucleus and outflowing components distributed near the disk with the velocity of ∼160 km s−1 (Greenhill et al. 2003a). The Circinus galaxy, already known to show the strongest flux density of the 22 GHz H2O maser among the H2O megamasers, could show the 321 GHz H2O emission as H2O masers in both the 22 and 321 GHz transitions have been detected in the same Galactic sources (Menten et al. 1990; Menten & Melnick 1991).
In this work, we present the observation of the 321 GHz H2O maser emission in Circinus galaxy, obtained using Atacama Large Millimeter/submillimeter Array (ALMA) in the Cycle 0 program.
2. OBSERVATIONS AND DATA REDUCTION
We observed H2O emission in the 1029–936 transition (νrest = 321.226 GHz) toward the Circinus galaxy on 2012 June 3 using ALMA as part of a search for extragalactic submillimeter H2O masers. Observations lasted for 49 minutes (approximately 25 minutes on-source) with 18 antennas. The tracking center of the galaxy was α(J2000) = 14h13m09
906, δ(J2000) = −65°20'20468. These observations were made with a single dual polarization spectral window (1.875 GHz bandwidth), divided into 3840 spectral channels, yielding spectral resolutions of 488.3 kHz or 0.458 km s−1 at the observed frequency of 319.4 GHz. A resultant total velocity coverage is ∼1760 km s−1. The 1.875 GHz spectral window was centered near the galaxy's systemic velocity of 433 km s−1. Phase-referencing observations were conducted by switching to a phase-referencing source, J 1329−5608, the position of which is ≈9° away from the target source. Data calibration was performed using the Common AstronomySoftware Applications (CASA). Amplitude calibration was performed using observations of Titan, and the bandpass correction was made with 3C 279. Flux-density errors of 10% are conservatively adopted, based on the capabilities of Cycle 0 observations in Band 7. Imaging was performed using CASA in natural weighting, for which the synthesized beam size was 066 × 051 (P.A. = −17°). The image analysis was performed using both the CASA and the Astronomical Image Processing Software. After the phase and amplitude calibrations, the 321 GHz continuum emission of the Circinus was subtracted from the spectral line visibilities using line-free channels prior to the imaging and CLEAN deconvolution of line emission. The line emission in the Circinus galaxy was thus separated out from the continuum emission. The rms noise level of spectral line maps in natural weight was ∼9–11 mJy beam−1 per spectral channel, depending on each frequency channel. The rms of the continuum map was ∼0.8 mJy beam−1.
3. RESULTS
The 321 GHz H2O maser in the Circinus galaxy was searched within the nominal field of view of 18'', centered on the phase-tracking center of the galaxy and in the velocity range of VLSR = −430 to +1290 km s−1, covering the total velocity range of known 22 GHz H2O maser measured in earlier single-dish observations (Gardner & Whiteoak 1982; Nakai et al. 1995; Greenhill et al. 1997; Braatz et al. 2003; McCallum et al. 2005). We detected the 321 GHz H2O line emission, the peak flux density of which is 131 mJy beam−1 at VLSR = 531.1 km s−1 with a signal-to-noise ratio (S/N) of >10 (Figure 1) and the total flux density is ∼350 mJy. The total integrated intensity estimated from all the detected H2O emission is ∼14 Jy km s−1. The apparent luminosity of the emission is ∼5 L☉, assuming isotropic radiation of the emission. The detected H2O emission can be divided into two groups, one of which is blueshifted emission of VLSR = ∼255 to ∼275 km s−1 peaking at VLSR = 274.4 km s−1 and the other is redshifted emission lying from VLSR = ∼470 to ∼670 km s−1. The line shapes of these Doppler-shifted features are narrow, similar to the H2O maser features seen in the 22 GHz single-dish spectra (Figure 2). The redshifted emission is composed of broad features, narrow-line features peaking at VLSR = 477.1, 511.4, 530.6, 531.1, 541.6, 547.1, 559.0, 587.8, 598.8, 602.4, 618.0 km s−1, and a minor peak at VLSR = 668.7 km s−1 (denoted by an arrow in Figure 2). Figure 3 displays a highly redshifted feature tentatively detected in ∼3σ level centered at VLSR = 1069.1 km s−1. One may suggest the possible detection of a feature peaking at VLSR = 1129.7 km s−1(<3σ), which must be confirmed in further measurements.
Figure 1. Spectrum of the 321 GHz H2O maser from the center of the Circinus galaxy at the spectral resolution of 488.3 kHz or 0.458 km s−1, obtained using 18 antennas on 2012 June 3 in the Cycle 0 ALMA program. The total velocity range covered in the spectrum is VLSR = −300.0–1250.0 km s−1, outside of which no significant emission was detected. The LSR systemic velocity, 433 km s−1 is denoted by an arrow. The amplitude scale is in mJy beam−1.
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Figure 2. Comparison of the H2O spectra between lines in the 321 GHz and 22 GHz transition toward the center of the Circinus galaxy. The 22 GHz spectra were obtained with the Deep Space Network (DSN) 70 m antenna located at Tidbinbilla (S. Horiuchi et al., in preparation). The observations of the Tidbinbilla 70 m were made on 2012 September 7 at the spectral resolution of 31.25 kHz or 0.42 km s−1 with an rms of ∼2.5 mJy and the 321 GHz spectra were on 2012 June 3. Minor features (VLSR = 206.4, 370.9, and 668.7 km s−1) are denoted by arrows. The flux density scales of the 22 GHz spectra shown in the figures are rather tentative and require detailed analysis (S. Horiuchi et al., in preparation). Upper left: blueshifted velocity range (200–400 km s−1) of the 321 GHz maser spectrum obtained from Figure 1. Lower left: spectrum of the blueshifted maser obtained at Tidbinbilla. Upper right: redshifted velocity range (450–725 km s−1) of the spectrum from Figure 1. Lower right: spectrum of the redshifted maser obtained at Tidbinbilla.
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Figure 3. Redshifted 321 GHz maser spectrum: a highly redshifted feature tentatively detected (∼3σ) peaking at VLSR = 1069 km s−1 and a possible detection of a feature at VLSR = 1129.7 km s−1. Top: amplitude of the spectrum between VLSR = 1050 and 1150 km s−1. Bottom: corresponding phase. The phases of the 1069 km s−1 feature are indicated by a circle.
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Standard image High-resolution imageIn Figure 2, we made a comparison of the 321 GHz H2O spectra with 22 GHz spectra obtained on 2012 September 7 using the Deep Space Network 70 m antenna at Tidbinbilla. The known strong variability of the 22 GHz maser in this galaxy makes it difficult to precisely compare the velocities of each feature between the 22 GHz and 321 GHz lines. In our 321 GHz observation, no feature is detected at or near the systemic velocity, consistent with the fact that earlier observations detected no distinct systemic feature of the 22 GHz maser in the galaxy (e.g., Greenhill et al. 2003b; Braatz et al. 2003). It should be also remarked that the asymmetry between the blueshifted and redshifted emission is observed as in the cases of other H2O megamasers such as NGC 4258. Redshifted features are more numerous and are significantly more intense than the blueshifted features (e.g., Maoz & McKee 1998). Thus, the 321 GHz emission in the galaxy largely overlap Doppler features appearing in the 22 GHz maser spectra obtained in the earlier single-dish observations. The Gaussian-fitted position (relative to the phase-referencing source) of the redshifted peak emission at 531.6 km s−1 is α(2000): 14h13m0995 ± 0.01, δ(2000): −65°20'2092 ± 0.02. The position of the blueshifted peak at 274.2 km s−1 is α(2000): 14h13m0996 ± 0.03, δ(2000): −65°20'2089 ± 0.03. The relative positions of other redshifted and blueshifted features coincide with these positions within uncertainties of fitting errors and these features remain unresolved at the angular resolution of ∼066 or 14.6 pc.
The 321 GHz submillimeter continuum was also detected in the center of the galaxy with an S/N of >50 (Figure 4), the peak position of which is α(2000): 14h13m09.96 ± 0.01, δ(2000): −65°20'20.87'' ± 0.01. The detected nuclear continuum with the total flux density of 55 mJy was partly resolved in the northeast direction accompanying an elongated substructure in the north (Figure 4). According to the ALMA Cycle 0 capabilities, the positional errors are ∼10% of the synthesize beam, 066/10 or 0066 in our observation, which seems to be the most dominant source of an error. Approximate relative positional errors between the H2O emission and the continuum peak, represented by the synthesized beam size divided by 2 × S/N (e.g., Hagiwara et al. 2003a), are ∼007 or 1.5 pc. Consequently, the relative positions between the maser and continuum emission peak are consistent within these errors. Note that the positions of the blueshifted and redshifted maser peaks at VLSR = 274.2 and 531.6 km s−1 are offset from the optical nucleus at α(2000): 14h13m09.950 ± 0.335, δ(2000): −65°20'21.20'' ± 0.25 (primary from NED) by −028 to − 031 in declination, although these offsets are almost within errors.
Figure 4. Integrated intensity image (contours) of the 321 GHz H2O maser overlaid the 321 GHz continuum image (gray scale) observed with the ALMA. The peak integrated intensity of the H2O maser and the peak flux density of the continuum are 13.6 Jy km s−1 beam−1 and 40.6 mJy beam−1, respectively. Contour levels and gray scales (scales are from 2.7 mJy (∼3σ) to 30 mJy) are denoted in the image. The synthesized beam (FWHM) is plotted in the left bottom corner.
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The resolved 321 GHz continuum emission shows an elongated structure (Figure 4), the direction of which is similar to that of a molecular or atomic gas disk at an inclination of ∼60°–70° (Curran et al. 2008).
4. DISCUSSION
4.1. Origin of the 321 GHz H2O and Submillimeter Continuum Emission
The synthesized beam size of our observation was too large to determine if the detected 321 GHz H2O emission is thermal emission or maser, from the value of brightness temperature. Given that the 321 GHz line features are confined within a diameter of ≲14 pc and the line shapes are narrow (<0.45 km s−1), it is most likely that the H2O emission detected in the Circinus galaxy is due to maser amplification.
To investigate the nature of the 321 GHz continuum, more information such as a spectral index would be required. We can speculate about the possibility of thermal emission from dust as the continuum shows an extended part in the general direction of a galactic-scale disk in a dusty environment, while a central nuclear continuum could contain the contribution of free–free emission.
Since the peak position of the continuum coincides with that of the maser, the maser could be enhanced through the thick dust lane in our line of sight. The optical studies revealed that the central 15 pc of the galaxy is dominated by the AGN luminosity, with star formation contributing only 2% (Sosa-Brito et al. 2001). The 321 GHz maser in the Circinus galaxy could be associated with AGN activity. Because of the relatively smaller maser luminosity, we still cannot still rule out that the maser originates in star-forming activity. The distinct velocity gradient of the 321 GHz maser is not detected throughout our analysis of the first moment map produced from the spectral-line cube containing features lying at VLSR = 256–670 km s−1, suggesting that the maser emission is neither spatially nor kinematically resolved in our observation on scales of 10 pc, while, according to the earlier VLBI observations, the 22 GHz maser in the galaxy was resolved on scales of ≈1 pc (Greenhill et al. 2003a; McCallum et al. 2009), implying that a better angular resolution is required better by at least one order of magnitude than that of our observation. The future VLBI mapping of the 321 GHz maser at milliarcsecond resolution should be able to clarify sub-parsec-scale kinematics of the galaxy by measuring distributions of each maser spot.
4.2. Comparison of 321 GHz H2O Emission with 22 GHz H2O Maser
In Figure 2, one can compare the spectra of the 321 GHz maser emission with the 22 GHz maser emission, both of which were observed at approximately the same time in 2012 June and September. The velocity ranges of masers at both transitions overlap to some extent and some of the prominent maser features peaked at or near the same velocities (see Figure 2). However, the velocity span of the 321 GHz maser is smaller than that of the 22 GHz maser due to the spectrum having a broader velocity coverage (Greenhill et al. 2003b); high-velocity features of the 22 GHz transition span from ∼50 km s−1 to ∼900 km s−1 having the most Doppler-shifted velocity of 460 km s−1 with respect to the systemic velocity (Greenhill et al. 2003b), while those of the 321 GHz transition from ∼270 km s−1 to ∼670 km s−1 in our observation, excepting the minor detection. This might be explained by the fact that the 321 GHz maser is present in more physically restricted regions in higher temperature and density as predicted in theoretical studies (Neufeld & Melnick 1990; Yates et al. 1997), which should be examined by resolving the spatial distribution of the maser at higher angular resolution. However, with the comparable sensitivity of the earlier 22 GHz maser spectra, the total velocity range of the 321 GHz maser emission might be as broad as that of the 22 GHz maser. In the case of the Cepheus A star-forming regions, the 321 GHz and 22 GHz masers were observed within the region of ∼1'', in which spatial coincidence between the 321 GHz and 22 GHz masers was not found and the 321 GHz maser is believed to be tracing warmer gas than the 22 GHz maser (Patel et al. 2007). We note that the 321 GHz maser in evolved stars is highly variable, compared with 22 GHz and 325 GHz masers (Yates et al. 1996). Thus, the study of the intensity variability of the 321 GHz maser in the galaxy is of great interest.
From Figure 2, it is difficult to find one-to-one correspondence of each velocity feature between the two transitions. However, the overall velocity spread of the 321 GHz maser features looks similar to that of 22 GHz masers that are believed to trace a masing disk rotating around a central engine having 1.7 × 106 M☉ enclosed mass and non-disk maser components from bipolar outflows lying within about 1 pc from the edge-on disk (Greenhill et al. 2003a). At the angular resolution on our map, it is not possible to distinguish only from their velocities whether or not each of the detected 321 GHz maser features is coming from the disk or non-disk outflows.
4.3. Possible Detection of High-velocity Gas in the Circinus
The known highest velocity maser feature in Circinus galaxy is redshifted by ∼460 km s−1 from the systemic velocity (Greenhill et al. 2003b), while the blueshifted counterpart was not detected in our 321 GHz spectra. Assuming that the 1069 km s−1 feature (redshifts up to ∼635 km s−1) is real, it is located at a radius of ∼0.018 pc (∼1.2 × 105 Schwarzschild radii for a 1.7 × 106 M☉ black hole) from the central engine, by calculation based on a Keplerian (v ∝ r−0.5) rotation disk model in Greenhill et al. (2003a, 2003b). This high-velocity feature could probe the innermost part of a molecular gas disk in the galaxy. The submillimeter maser is potentially a powerful tool for exploring regions which cannot be probed with the 22 GHz maser.
The interpretation of the origin of the H2O maser in the Circinus galaxy is not straightforward. Nevertheless, our discovery of the 321 GHz H2O maser in Circinus galaxy will be useful as a basis for future detailed studies of circumnuclear gas in AGNs. The detected 321 GHz and well-studied 22 GHz traditions are from ortho-H2O. Using these two different maser transitions will enable us to place new constraints on radiative transfer models (Humphreys 2007) to determine physical conditions of molecular gas in the inner sub-parsecs of AGNs. Moreover, the H2O lines at 22 and 321 GHz show higher atmospheric transmission than those at 183 and 325 GHz, which is advantage for interferometric observations. Ultimately, we hope to measure the angular distribution of submillimeter H2O masers including those of the 321 GHz or other transitions on the basis of currently on-going efforts in developments of VLBI at submillimeter wavelengths.
This Letter makes use of the following ALMA data: 2011.0.00121.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The Tidbinbilla 70 m telescope is part of the NASA Deep Space Network and is operated by CSIRO. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Finally, we thank the anonymous referee for helpful suggestions that significantly improved the article.