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Water Maser Emission in IC 342

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Water Maser Emission in IC 342 A&A 385, 1049–1055 (2002) Astronomy DOI: 10.1051/0004-6361:20020199 & c ESO 2002 Astrophysics Water maser emission in IC 342 A. Tarchi, C. Henkel, A. B. Peck, and K. M. Menten Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany Received 19 September 2001 / Accepted 4 February 2002 Abstract. The detection of 22 GHz water vapor emission from IC 342 is reported, raising the detection rate among northern galaxies with IRAS point source fluxes S100 µm > 50 Jy to 16%. The maser, associated with a star forming region ∼10–1500 west of the nucleus, consists of a single 0.5 km s−1 wide feature and reaches an isotropic −2 16 luminosity of 10 L (D =1.8 Mpc). If the time variability is intrinsic, the maser size is ∼<1.5×10 cm (∼<0.5 mas) which corresponds to a brightness temperature of ∼>109 K. The linewidth, luminosity, and rapid variability are reminiscent of the 8 km s−1 super maser in Orion-KL. A velocity shift of 1 km s−1 within two weeks and subsequent rapid fading is explained in terms of a chance alignment of two dense molecular clouds. Observations at 22 GHz toward Maffei 2 are also reported, yielding a 5σ upper limit of 25 mJy for a channel spacing of 1.05 km s−1. Key words. galaxies: individual: IC 342, Maffei 2 – galaxies: starburst – ISM: molecules – masers – radio lines: ISM 1. Introduction star formation, to measure the velocity vectors of these regions through VLBI proper motion studies, and to de- Isotropic luminosities of extragalactic 22 GHz H2O masers termine true distances through complementary measure- span a huge range (see e.g. Ho et al. 1987; Koekemoer et al. ments of proper motion and radial velocity (e.g. Greenhill −2− 1995; Braatz et al. 1996), from 10 10 L (the “kilo- et al. 1993). We have therefore observed the nearby spiral masers” consistent with the most intense masers seen in galaxies IC 342 and Maffei 2, both of which exhibit promi- our Galaxy) to 10–500 L (the “megamasers”, found in nent nuclear bars and strong molecular, infrared, and ra- LINERs and Seyfert 2s) and up to 6000 L (the “giga- dio continuum emission (e.g. Hurt & Turner 1991; Hurt maser” in TXS 2226-184). Interferometric observations, et al. 1993; Turner & Ho 1994; Henkel et al. 2000; Meier when available, show that the most luminous H2O sources et al. 2000; Meier & Turner 2001; Schulz et al. 2001). In arise from the nuclear region of their host galaxies, from the following we report the results of our observations. circumnuclear tori, from entrained material near the sur- face of the nuclear jet, or by ambient gas amplifying the continuum emission of the jet. The weaker masers often 2. The observation and image processing mark locations of high mass star formation (for M 33, − IC 10, and M 82, see e.g. Huchtmeier et al. 1978; Henkel Effelsberg The 616 523 line of H2O (rest frequency: et al. 1986; Baudry & Brouillet 1996). The nature of the 22.23508GHz) was observed with the 100-m telescope of the MPIfR at Effelsberg1 toward IC 342 and Maffei 2. kilomasers in NGC 253 and M 51 (see Ho et al. 1987; 00 Nakai & Kasuga 1988) remained an enigma for more than The full width to half power beamwidth was 40 . A dual a decade. Recently, however, it has been shown that the channel HEMT receiver provided system temperatures of ∼ maser in M 51 is of nuclear origin (Hagiwara et al. 2001), 200 K on a main beam brightness temperature scale. The observations were carried out in a dual beam switching suggesting that there might be a family of as yet unex- 0 plored “weak” H O sources located near the nuclear en- mode with a beam throw of 2 and a switching frequency 2 ∼ gine of Seyfert 2 and LINER galaxies. of 1 Hz. Flux calibration was obtained by measurements of W3(OH) (for fluxes, see Mauersberger et al. 1988). Extragalactic H O masers are preferentially detected 2 Gain variations as a function of elevation were taken into in nearby galaxies that are bright in the infrared (Braatz account and the calibration error is expected to be no et al. 1997). While nuclear masers are of obvious interest, more than 10%. The pointing accuracy was always bet- non-nuclear kilomasers are also important for a number of 00 ter than 10 . reasons: these sources allow us to pinpoint sites of massive 1 The 100-m telescope at Effelsberg is operated by the Max- Send offprint requests to:A.Tarchi, Planck-Institut f¨ur Radioastronomie (MPIfR) on behalf of the e-mail: [email protected] Max-Planck-Gesellschaft (MPG). Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020199 1050 A. Tarchi et al.: Water maser emission in IC 342 VLA On May 12, 2001, IC 342 was observed with the a) April 3 (0,0) Very Large Array2 (VLA) in its B configuration. We observed with a single IF using full polarimetric infor- mation. The source 0538+498 was used as flux calibra- tor. The nearby point source 0224+671 was used for b) April 22 (-10,0) phase and bandpass calibration. “Referenced pointing” was performed during the observations (for details, refer to http://www.aoc.nrao.edu/vla/html/refpt.shtml). The data were Fourier-transformed using natural c) May 7 (-10,0) weighting to produce a 2048 × 2048 × 103 spectral line data cube, encompassing the central 10000 of IC 342. The Flux density (Jy) 103 channels used, out of the 128 observed, cover a range in velocity of ∼8kms−1 centered at 16 km s−1 LSR (the d) June 22 (-10,0) velocity of the line detected at Effelsberg). No continuum subtraction was needed. The data were deconvolved us- ing the CLEAN algorithm (H¨ogbom 1974). The restor- ing beam is 000. 4 × 000. 3 and the rms noise per channel is ∼10 mJy, consistent with the expected thermal noise. Velocity (km/s) Fig. 1. The H2O maser feature observed with high velocity 3. Results resolution (channel spacings after smoothing four contiguous channels are 0.26 km s−1)ona) April 3, b) April 22, c) May 7, 3.1. Time variability and d) June 22. The first spectrum has been taken at the (000, 000) position, the others at (–1000,000) relative to the position Our single-dish observations towards Maffei 2 yielded given in footnote “a” of Table 1. The dashed line indicates −1 no detection, with a 5σ upper limit of 25 mJy (chan- VLSR =16kms . nel spacing: 1.05 km s−1; velocity range: –250 km s−1 < −1 V<+230 km s ; epoch: Apr 3, 2001; position: α2000 = h m s ◦ 0 00 are smaller and the velocity of the line is blue-shifted by 02 41 55.2, δ2000 =+59 36 11 ). ∼ −1 ∼ On April 2, 2001, we obtained the first definite de- 1kms . Unfortunately, no emission above 30 mJy (3σ −1 tection of water vapor emission in IC 3423 (see also level; 0.08 km s channel spacing) was detected in the Sect. 4.1). During the next night the detection was con- 22 GHz VLA B-array data taken on May 12. This fading firmed with a velocity resolution sufficient to resolve the by at least a factor of 3 within 5 days implies a size scale of ∼<1.5 × 1016 cm (∼<900 AU) or ∼<0.5 mas at a distance of line profile (Fig. 1a). The detected feature lies at VLSR = 16 km s−1 and has a linewidth of ∼0.5 km s−1;onApril2, 1.8 Mpc (McCall et al. 1989; see also Sect. 4.3); the cor- − > 9 no other component was seen at velocities –175 km s 1 < responding brightness temperature is ∼10 K. The most V<+310 km s−1 (channel spacing: 1.05 km s−1;5σ noise recent spectrum was obtained on June 22 at Effelsberg level: 16 mJy). A high line intensity (∼100 mJy) and good (Fig. 1d). Confirming the VLA result, no maser signal was −1 weather conditions allowed us to map the emitting region. seen above 30 mJy (3σ; channel spacing: 1.05 km s ). Figure 2 shows the spectra taken towards six positions Offset positions, on-source integration times, Gaussian near the center of the galaxy. On April 22, observations fit parameters, and H2O luminosities assuming isotropic were performed towards the offset position (−1000,000), emission (see e.g. Henkel et al. 1998) are given in Table 3.1. close to the peak position inferred by the map. The flux density had almost doubled (compare Figs. 1a and b) 3.2. Maser position which is more than the expected 20% increase for a point source located ∼1000 off our previously observed Fitting a synthetic Gaussian to the data taken on April 3 (000,000) position. Further measurements were performed (see Table 1 and Fig. 2), i.e. minimizing the sum of the dif- on May 7 (Fig. 1c). The peak and integrated intensities ference squared between calculated and observed peak and integrated flux densities, we can obtain an accurate posi- 2 h m s The National Radio Astronomy Observatory is a facility of tion of the emitting region: α2000 =03 46 46.3, δ2000 = the National Science Foundation operated under cooperative +68◦0504600.Thisis1300 to the west of our center position agreement by Associated Universities, Inc.
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