Water Maser Emission in IC 342

arXiv:astro-ph/0201449v1 28 Jan 2002 [email protected] ugsigta hr ih eafml fa e unex- yet as of family 2001), a al. H be et ‘weak’ (Hagiwara might plored origin the there nuclear that 1987; that of shown is al. suggesting 51 been et M has in Ho than it maser more however, (see for Recently, enigma 51 decade. an the a M remained of 1988) and Kasuga nature Henkel & 253 The Nakai 1978; 1996). NGC al. Brouillet in et & Huchtmeier kilomasers Baudry e.g. 1986; 33, see al. M 82, (for et M formation and star of- 10, mass masers amplifying high IC weaker of gas The locations ambient jet. mark by the ten of or emission jet, continuum nuclear the the the near of material surface entrained galaxies, from host tori, their of circumnuclear region from nuclear the from arise sources fSyet2adLNRgalaxies. LINER and 2 Seyfert of stoi uioiiso xrglci 2GzH GHz 22 extragalactic of luminosities Isotropic Introduction 1. esn:teesucsalwu opnon ie fmassive of sites pinpoint to us of allow number sources a interest, these for obvious reasons: important of also are (Braatz are masers infrared kilomasers nuclear non-nuclear the While in 1997). bright al. are et that galaxies nearby in on nLNR n efr s n pt 00L 6000 to H luminous up most the and that obser- show available, Interferometric 2s) when vations, 2226-184). TXS Seyfert in and ‘gigamaser’ (the LINERs in found ta.19;Bat ta.19) rm10 from 1996), Koekemoer al. 1987; al. et et Braatz Ho 1995; e.g. (see al. range et huge a span eni u aay o1 0 L 500 – 10 to Galaxy) masers our intense most in the seen with consistent ‘kilomasers’ (the edopitrqet to requests offprint Send wl eisre yhn later) hand by inserted be (will Astrophysics & Astronomy xrglci H Extragalactic mn otenglxe ihIA on orefluxes source point IRAS with galaxies northern among omn region forming Abstract. date Accepted / date Received Hügel dem Auf für Radioastronomie, Max-Planck-Institut uioiyo 10 of luminosity hc orsod oabihns eprtr of temperature brightness a to corresponds which eiicn fte8k s km 8 the of reminiscent oadMffi2aeas eotd iliga5 tw a of yielding alignment reported, chance also a are of 2 terms Maffei in toward explained is fading rapid e words. Key ISM 2 h eeto f2 H ae ao msinfo C32i rep is 342 IC from emission vapor water GHz 22 of detection The ore oae ertencerengine nuclear the near located sources O aais niiul C32 ae aais starburst Galaxies: – 2 Maffei 342, IC individual: Galaxies: 2 aesaepeeetal detected preferentially are masers O ∼ − 10–15 2 L ⊙ ( .Tarchi A. ′′ D − aucitno. manuscript eto h ulu,cnit fasnl . ms km 0.5 single a of consists nucleus, the of west ae ae msini C342 IC in emission maser Water 18Mc.I h ievraiiyi nrni,temsrsi maser the intrinsic, is variability time the If Mpc). =1.8 1 .Trh,e-mail: Tarchi, A. : ue ae nOinK.Avlct hf f1k s km 1 of shift velocity A Orion-KL. in maser super ⊙ 1 te‘megamasers’, (the .Henkel C. , − 2 2 − masers O σ 0L 10 1 pe ii f2 J o hne pcn f10 ms km 1.05 of spacing channel a for mJy 25 of limit upper .B Peck B. A. , ∼ > 2 O 10 ⊙ ⊙ S 9 100 .Telnwdh uioiy n ai aiblt are variability rapid and luminosity, linewidth, The K. h ulwdht afpwrbawdhws40 was beamwidth power half to width full The 2258Gz a bevdwt h 0- eecp of Effelsberg telescope at 100-m MPIfR the with the observed was GHz) 22.23508 on n h airto ro sepce ob omore no be ac- 10 to into expected taken is error were than calibration elevation the of and function count Gain a 1988). al. as et Mauersberger variations see fluxes, (for W3(OH) of of bevtoswr are u nada emswitching 2 beam of throw dual beam a a in with out mode carried were observations of temperatures system ∼ provided receiver HEMT channel .Teosrainadiaeprocessing Effelsberg image and observation The 2. In observations. 2001). our al. of et results Meier Schulz the 2000; 2001; report al. Turner we et following & Henkel the Meier 1994; Hurt 2000; Ho 1991; al. & Turner et Turner & 1993; Hurt al. ra- et (e.g. and infrared, emission molecular, continuum promi- strong exhibit dio and which of bars both nuclear spiral 2, nent nearby Maffei and the 342 observed IC therefore galaxies Greenhill have We (e.g. 1993). velocity al. radial et and motion measure- de- proper complementary to of ments through and these distances studies, of true motion vectors termine proper velocity VLBI the through measure regions to formation, star a-lnkGslshf (MPG). of Max-Planck-Gesellschaft behalf on (MPIfR) für Radioastronomie Planck-Institut 9 -32 on Germany Bonn, D-53121 69, 1 µ 0 nami embihns eprtr cl.The scale. temperature brightness beam main a on K 200 ′′ ∼ m h 0- eecp tEesegi prtdb h Max- the by operated is Effelsberg at telescope 100-m The . es oeua lus bevtosa 2GHz 22 at Observations clouds. molecular dense o z lxclbainwsotie ymeasurements by obtained was calibration Flux Hz. 1 > ± 1 n .M Menten M. K. and , 0.Tepitn cuaywsawy etrthan better always was accuracy pointing The 10%. 0J o1% h ae,ascae ihastar a with associated maser, The 16%. to Jy 50 h 6 The S:mlcls–msr ai lines: Radio – masers – molecules ISM: – − 1 iefaueadrahsa isotropic an reaches and feature wide − 16 1 re,riigtedtcinrate detection the raising orted, ihntowesadsubsequent and weeks two within − eis ze 5 23 1 ∼ < oadI 4 n ae 2. Maffei and 342 IC toward ieo H of line 1 1 . ′ 5 n wthn frequency switching a and × 10 16 2 m( cm rs frequency: (rest O ∼ < − . mas) 0.5 1 . ′′ dual A . the 2 A. Tarchi et al.: Water maser emission in IC 342

VLA On May 12, 2001, IC 342 was observed with the are smaller and the velocity of the line is blue-shifted by Very Large Array2 (VLA) in its B configuration. We ∼1kms−1. Unfortunately, no emission above ∼30mJy (3σ observed with a single IF using full polarimetric infor- level; 0.08 km s−1 channel spacing) was detected in the mation. The source 0538+498 was used as flux calibra- 22 GHz VLA B-array data taken on May 12. This fading tor. The nearby point source 0224+671 was used for by at least a factor of 3 within 5 days implies a size scale phase and bandpass calibration. ‘Referenced pointing’ was of ∼< 1.5 × 1016 cm (∼< 900AU) or ∼< 0.5 mas at a distance performed during the observations (for details, refer to of 1.8 Mpc (McCall et al. 1989; see also Sect. 4.3); the cor- http://www.aoc.nrao.edu/vla/html/refpt.shtml). responding brightness temperature is ∼>109 K. The most The data were Fourier-transformed using natural recent spectrum was obtained on June 22 at Effelsberg weighting to produce a 2048 × 2048 × 103 spectral line (Fig. 1d). Confirming the VLA result, no maser signal was data cube, encompassing the central 100′′ of IC 342. The seen above 30mJy (3σ; channel spacing: 1.05kms−1). 103 channels used, out of the 128 observed, cover a range Offset positions, on-source integration times, Gaussian −1 −1 in velocity of ∼8kms centered at 16kms LSR (the fit parameters, and H2O luminosities assuming isotropic velocity of the line detected at Effelsberg). No continuum emission (see e.g. Henkel et al. 1998) are given in Table 3. subtraction was needed. The data were deconvolved using the CLEAN algorithm (Högbom 1974). The restor- ing beam is 0′′. 4 × 0′′. 3 and the rms noise per channel is a) April 3 (0,0) ∼10mJy, consistent with the expected thermal noise.

3. Results b) April 22 (-10,0) 3.1. Time variability Our single-dish observations towards Maﬀei 2 yielded no detection, with a 5σ upper limit of 25 mJy (channel c) May 7 (-10,0) spacing: 1.05 km s−1; velocity range: –250kms−1

Table 1. H2O in IC 342: observations details and line parameters

a) b) c) d) e) Obs.Date (∆α, ∆δ) Tel. tobs BW Vel. Res. Speak VLSR ∆V1/2 Luminosity ′′ ′′ −1 −1 ( , ) (min) (MHz) (kms ) (Jy) (kms ) (L⊙)

02 Apr 2001f) (0,0) EFF 180 40 1.05 – 16.16 (0.03) – (8.3±0.6)×10−3 03 Apr 2001 (–40,0) EFF 30 20 0.07 0.07±0.03 16.15 (0.09) 0.61 (0.28) (3.2±1.0)×10−3 03 Apr 2001 (–20,–20) EFF 20 ” ” <0.07 <3×10−3 03 Apr 2001 (–20,0) EFF 30 ” ” 0.18±0.03 16.08 (0.03) 0.49 (0.06) (6.5±0.7)×10−3 03 Apr 2001 (–20,20) EFF 30 ” ” 0.10±0.03 16.07 (0.06) 0.53 (0.13) (3.9±0.8)×10−3 03 Apr 2001 (0,–20) EFF 30 ” ” 0.06±0.02 16.06 (0.08) 0.46 (0.14) (2.0±0.6)×10−3 03 Apr 2001 (0,0) EFF 30 ” ” 0.17±0.02 16.14 (0.02) 0.58 (0.05) (7.3±0.6)×10−3 03 Apr 2001 (0,20) EFF 30 ” ” 0.06±0.04 16.15 (0.06) 0.40 (0.20) (1.8±0.6)×10−3 03 Apr 2001 (20,0) EFF 30 ” ” 0.04±0.02 16.14 (0.13) 0.64 (0.22) (1.9±0.7)×10−3 22 Apr 2001 (–10,0) EFF 8 ” ” 0.30±0.06 16.11 (0.04) 0.54 (0.08) (12.0±1.5)×10−3 07 May 2001 (–10,0) EFF 15 ” ” 0.17±0.05 14.98 (0.08) 0.81 (0.18) (10.3±1.9)×10−3 12 May 2001 VLA 120 0.781 0.08 <0.03 <1.2×10−3 22 Jun 2001 (–10,0)+(0,0) EFF 35 40 1.05 <0.03 <1.7×10−3

h m s ◦ ′ ′′ a) The position offset is relative to α2000 = 03 46 48.6 and δ2000 = +68 05 46 b) tobs (Effelsberg 22GHz H2O integration time) includes time for on- and off-source integration c) Peak fluxes calculated from integrated line intensities divided by linewidths (both obtained from Gaussian fits using the data reduction software package ‘CLASS’); if no signal is detected, 3σ limits are given for a channel spacing of 0.53kms−1 (Apr. 3), 0.08kms−1 (May 12), and 1.05 km s−1 (June 22). Since the data taken on April 3 were used for the computation of the position of the maser, where only (presumably negligible) relative calibration errors are relevant, stated errors do not include absolute calibration errors (±10%; see Sect.2). d) LSR = Local Standard of Rest × −1 × 2 e) Obtained via [LH2O/L⊙] = 0.023 [ S dV /Jy km s ] [D/Mpc] , D = 1.8Mpc. Corresponding luminosity limits refer to ∆V1/2 = −1 R 0.55 km s , the approximate width of the H2O feature. Errors do not include the estimated calibration uncertainty of ±10% (see Sect.2 and footnote c). f) Velocity resolution too coarse to determine meaningful values for the peak flux density and linewidth

compact H ii region W3(OH) as pointing source slightly 4.1. Galactic versus extragalactic H2O emission degraded this pointing accuracy because of the presence of a strong H O maser 6′′ to the west (W3(H O); e.g. Reid 2 2 IC 342 is located at a galactic longitude of 138◦ and lati- et al. 1995). The error introduced by the non-negligible tude 10◦, i.e. behind the northern outskirts of the Perseus flux contribution of the maser is σ = 1′′. 2. W3(OH) arm, the Cam OB1 association, and a Local Arm of the Absolute pointing accuracy: When moving the Milky Way (e.g. Digel et al. 1996; Dame et al. 2001). Local Effelsberg antenna from W3(OH) to IC 342, there may be Standard of Rest (LSR) velocities of the Perseus arm range an additional pointing offset caused by the change in az- from –60 to –30kms−1, consistent with the maser features imuth and elevation. On April 3, relatively large telescope ′′ ′′ reported by Huchtmeier et al. (1978), but significantly re- movements led to a pointing shift of 2 in azimuth and 4 moved from the radial velocity of the maser reported here. in elevation. Considering the improvement in weather con- For the Cam OB1 association and the Local Arm, Digel ditions when subsequently mapping IC 342, and account- et al. (1996) find velocities up to –5 and +7 km s−1, still ing for the much smaller distance between target (IC 342) below the measured maser velocity. Furthermore, only a and pointing source (W3(OH)), we estimate the absolute ′′ few of the hundreds of known galactic H2O masers have pointing error to be σapoi = 2. 5. been detected at bII∼10◦ (e.g. Valdettaro et al. 2001). All Spectral noise: For data taken during a single night, of these arguments support an extragalactic origin of the relative calibration should be excellent. The relevant fac- maser. In addition, the IC 342 isovelocity contour at VLSR tor affecting line intensity ratios is thus noise. Assuming ∼ 16kms−1, obtained from CO interferometric data, is ′′ a Gaussian beamshape (beamwidth: 40 ), resulting peak consistent with a maser location 10 – 15′′ west of its nu- and integrated H2O flux densities were calculated for var- cleus (e.g. Lo et al. 1984; Sakamoto et al. 1999). This ious maser positions. Agreement with the observed inten- agrees with the position determined in Sect. 3.2. We there- sity limit at the (–20′′,–20′′) position was taken as an ad- fore conclude that the H2O source is associated with the ditional check. The resulting positional error is σmap = central region of IC 342 (for an illustration, see Fig. 3). 4′′. The assumption that all errors can be statisti- cally added yields a total positional uncertainty of 4.2. Search for an optical counterpart σ = σ2 + σ2 + σ2 + σ2 ∼ 5′′. tot q rpoi apoi W3(OH) map Arising from the central region of IC 342, but being dis- 4. Discussion placed from the nucleus, the H2O maser is likely associated with a prominent star forming region. 4 A. Tarchi et al.: Water maser emission in IC 342

4.3. A comparison with other maser sources

Although seen almost face-on (inclination i∼25◦; e.g. Sage & Solomon 1991), IC 342 is believed to resemble the Milky Way in many ways. At the commonly assumed D ∼ 2Mpc (but see Krismer et al. 1995 for a larger distance), the giant molecular clouds near the center of IC 342 have similar linear sizes to the Sgr A and Sgr B2 clouds in our Galaxy. The inner 400 pc of IC 342 and the Galaxy have almost the same far infrared luminosity and the 2µm luminosities indicate similar stellar masses (Downes et al. 1992). Does this similarity extend to the 22 GHz maser emission? While there are a number of H2O maser sources in * the nuclear region of the Milky Way (Güsten & Downes 1983; Taylor et al. 1993; Yusef-Zadeh & Mehringer 1995), the emission is dominated by Sgr B2 (e.g. Waak & Mayer 1974; Morris 1976; Genzel et al. 1976; Elmegreen et al. 1980; Kobayashi et al. 1989). Like the H2O maser in IC 342, Sgr B2 is displaced from the galactic nucleus. The integrated luminosity of its numerous spatial components ∼ × −3 is LH2O 5 10 L⊙, again not inconsistent with the data from IC 342. The lineshapes are, however, quite different: While Sgr B2 is characterized by a variety of narrow components that have flux densities of a few hundred Jy, the velocity component we have detected in IC 342 is stronger than any related feature by at least an order of magnitude. Occasionally, prominent galactic star forming regions show narrow (∼0.5 km s−1) flaring components that are 12 J Fig. 3. A map of integrated CO = 1–0 emission from exceptionally bright. Such flares were observed in W 31 A the central region of IC 342 (Meier et al. 2000). The as- and W49 (Liljeström et al. 1989; Lekht et al. 1995). The terisk indicates the location of the water maser with the prototype of these flares is the 8kms−1 super maser in circle marking the positional uncertainty (Sect. 3.2). The Orion-KL (e.g. Garay et al. 1989). During several months, letters label the five giant molecular clouds identified by the narrow highly linearly polarized maser reached flux Downes et al. (1992). densities in excess of 5 MJy. Surpassing the flux of any other velocity component by more than an order of ∼ magnitude and reaching a peak luminosity of LH2O Fig. 4 shows an optical B-band image of the central re- −2 10 L⊙, the feature seems to be similar to that seen gion of IC 342, taken from the XDSS4. The maser emitting in IC 342. There is another even more impressive nar- region (white circle) coincides with an arc-like structure row flaring maser, which dwarfed neighboring features: extending E-S and is associated with a chain of sources the –324kms−1 component in the nearby irregular galaxy that appear to be H ii regions. The presence of optically IC10 (Becker et al. 1993; Argon et al. 1994; Baan & bright regions, likely sites of star formation activity, close Haschick 1994). This prominent flare, reaching a luminos- to the maser position is confirmed by an Hα (λ6563 A)˚ ity of ∼1L⊙ (∼100 Jy; D ∼ 0.8Mpc), lasted for a few HST WFPC2 picture5 (see also Meier et al. 2000). years. As unpublished spectra from Effelsberg show, the 4 The Digitized Sky Surveys were produced at the Space lineshape of the maser also resembles that of the flaring Telescope Institute under U.S. Government grant NAGW- feature in Orion-KL. 2166. The images of these surveys are based on photographic While it is not farfetched to assume that the flares in data obtained using the Oschin Schmidt Telescope on Palomar IC 342, IC 10, and Orion-KL are caused by the same physi- Mountain and the UK Schmidt Telescope. The plates were pro- cal phenomenon (but see Argon et al. 1994), the actual sit- cessed into the present compressed digital form with the per- uation remains unclear. For Orion-KL, observational con- mission of these institutions. 5 straints are quite strict. There is no significant background Some of the data mentioned in this paper were obtained continuum source and the high intensity seems to suggest from the Multimission Archive at the Space Telescope Institute (MAST). STScI is operated by the Association of Universities saturated emission and a collisional pump (e.g. Garay et for Research in Astronomy, Inc. under NASA contract NAS al. 1989). The small linewidth, however, appears to be 5-26555. Support for MAST for non-HST data is provided by incompatible with such a scenario (Nedoluha & Watson the NASA Office of Space Science via grant NAG5-7584 and 1991). Solving this apparent inconsistency requires either by other grants and contracts. very small beaming angles, which are readily obtained by A. Tarchi et al.: Water maser emission in IC 342 5 a chance alignment of two masing clouds (e.g. Deguchi & should be added to the list). There is a total of 44 galax- Watson 1989; Elitzur et al. 1991), or the inclusion of effi- ies, two ultraluminous galaxies at intermediate distances cient photon scattering, which can account for linewidths (NGC3690 and Arp220) and 42 nearby sources (V < ∼0.5 km s−1 even in the case of saturation (e.g. Elitzur 3000kms−1). Among these, seven (16%) are known to 1990). contain H2O masers in their nuclear region. Two of these While luminosity, linewidth, and flux variations are contain megamasers (NGC 1068 and NGC 3079), two are reminiscent of the 8 km s−1 super maser in Orion-KL, our possibly nuclear kilomasers (NGC 253 and M 51; for de- H2O profiles show a significant velocity shift between April tails, see Sect.1), and three are associated with prominent 22 and May 7 (Figs. 1b and c) that has not been seenin the sites of star formation (IC 10, IC 342, M 82). Among the Orion-KL flaring component. The shift, ∼1kms−1 (Table subsample of 19 sources with 100µm fluxes in excess of 1; see also Sect. 3.1), is too large to be explained by a 100Jy, five were so far detected in H2O, yielding a de- change in the relative intensities of the three main hyper- tection rate in excess of 20%. Since few deep integrations fine components (see e.g. Fiebig & Güsten 1989). A change have been obtained toward these sources, more H2O de- with time in the peak velocity could originate from a vari- tections can be expected from this promising sample in ation in the relative intensity of different emitting regions the near future. leading to the swapping-over of velocity peaks (see e.g. Wu et al. 1999). Such intensity fluctuations have been found to be sometimes “correlated” with observed time scales ranging from some days up to weeks (intrinsic time scales are more difficult to deduce because they strongly depend N on the degree of saturation of the masers; see e.g. Genzel

& Downes 1977; Rowland & Cohen 1986). Time scales E equal or greater than some months are also predicted for maser flux density variations (distinct from flares) due to refractive interstellar scintillations (Simonetti et al. 1993). * However, a simpler model for explaining both the flare and the shift is, in our opinion, that the latter has a kine- matic origin. Adopting the previously outlined scenario of a chance alignment of two masing clouds along the line-of- sight, motion of the foreground relative to the background cloud along the plane of the sky and velocity structure in the foreground cloud could explain the observed data. A velocity gradient in the foreground cloud would then first shift the line velocity; once velocities are reached that are not matched within the background cloud, the flux density of the maser rapidly drops. Assuming that the distance between these clouds is ≪1.8 Mpc and that their Fig. 4. XDSS B-band image of the central region of relative velocity in the plane of the sky is at the order of IC 342. The asterisk indicates the water maser emitting 100kms−1, this implies a cloud velocity gradient of up to position. The radius of the circle indicates its positional 1kms−1/AU during the time the source was monitored. error as outlined in Sect. 3.2. An almost identical scenario was proposed by Boboltz et al. (1998) to account for the velocity shift of the flaring water maser component at –66kms−1 in W49N. The similarity in velocity shift (∼ 0.5 km s−1) and time scale 5. Conclusions (58 days) of the event in W49N w.r.t the one in IC 342, Having detected water vapor emission in the galaxy indicates a common origin. IC 342, we have obtained the following main results:

′′ 4.4. Extragalactic maser detection rates 1. The maser arises from a location 10–15 to the west of the center of the galaxy. It is thus a maser associ- While typical searches for extragalactic maser sources ated with a powerful star forming region at a projected have only yielded detection rates between zero (e.g. Henkel distance of ∼100pc (D = 1.8 Mpc) from the nucleus. −2 −1 et al. 1998) and a few percent (e.g. Henkel et al. 1984; 2. Luminosity (10 L⊙), linewidth (0.5 km s ), and Braatz et al. 1996), there exists one sample with de- rapid flux density variations are reminiscent of the flar- tection rates >10%: These are the northern (δ > –30◦) ing 8 km s−1 super maser feature in Orion-KL. It seems extragalactic IRAS (Infrared Astronomy Satellite) point that we have observed an exceptional outburst of a sources with 100µm fluxes in excess of 50Jy (for a source maser that is usually well below the detection thresh- list, see Henkel et al. 1986; Maffei 2 with S100µm ∼ 200Jy hold. 6 A. Tarchi et al.: Water maser emission in IC 342

3. Time variability, if intrinsic, yields a maser size of Ho P.T.P., Martin R.N., Henkel C., 1987, ApJ 320, 663 ∼< 1.5 × 1016 cm−3 (∼<0.5 mas), and a brightness tem- Högbom J.A., 1974, A&AS 15, 417 perature of ∼>109 K. Huchtmeier W., Witzel A., Kühr H., et al., 1978, A&A 64, L21 4. A velocity shift of 1 km s−1 within 16 days and a sub- Hurt R.L., Turner, J.L., 1991, ApJ 377, 434 sequent decrease in flux density by at least a factor Hurt R.L., Turner J.L., Ho P.T.P., Martin R.N., 1993, ApJ of three within 5 days can be explained by a chance 404, 602 Kobayashi H., Ishiguro M., Chikada Y., Ukita N., et al. 1989, alignment of two dense molecular clouds along the line- PASJ 41, 141 of-sight, both of them associated with IC 342. Koekemoer A.M., Henkel C., Greenhill L.J., et al., 1995, Nat 5. Among galaxies with IRAS point source flux densi- 378, 697 ties of S100µm > 50 Jy, 16% are now known to contain Krismer M., Tully R.B., Gioa M., 1995, AJ 110, 1584 masers in their inner regions. Since deep H2O integra- Lekht E.E., Mendoza-Torres E., Sorochenko R.L., 1995, ApJ tions have been obtained towards only a few of them, 443, 222 more detections from this sample can be expected in Liljeström T., Mattila K., Toriseva M., Anttila R., 1989, A&AS the near future. 79, 19 Lo K.-Y., Berge G.L., Claussen M.J., et al., 1984, ApJ 282, Acknowledgements. We wish to thank P. Diamond for criti- L59 cally reading the manuscript and Nikolaus Neininger for useful Mauersberger R., Wilson T.L., Henkel C., 1988, A&A 201, 123 discussion. We are also endebted to the operators at the 100-m McCall M.L., 1989, AJ 97, 1341 telescope, and to Michael Rupen and the NRAO analysts, for Meier D.S., Turner J.L., Hurt R.L., 2000, ApJ 531, 200 their cheerful assistance with the observations. Meier D.S., Turner J.L., 2001, ApJ 551, 687 Morris M., 1976, ApJ 210, 100 Nakai N., Kasuga T., 1988, PASJ 40, 139 References Nedoluha G.E., Watson W.D., 1991, ApJ 367, L63 Argon A.L., Greenhill L.J., Moran J.M., Reid M.J., et al. 1994, Reid M.J., Argon A.L., Masson C.R., Menten K.M. et al., 1995, ApJ 422, 586 ApJ 443, 238 Baan W.A., Haschick A.D., 1994, ApJ 424, L33 Rowland P.R., Cohen R.J., 1986, MNRAS 220, 233 Baudry A., Brouillet N., 1996, A&A 316, 188 Sage L.J., Solomon P.M., 1991, ApJ 380, 392 Becker R., Henkel C., Wilson T.L., Wouterloot J.G.A., 1993, Sakamoto K., Okumura S.K., Ishizuki S., et al., 1999, ApJS A&A 268, 483 124, 403 Becklin E.E., Gatley I., Matthews K., Neugebauer G., et al. Schulz A., Güsten R., Köster B., Krause D., 2001, A&A 371, 1980, ApJ 236, 441 25 Boboltz D.A., Simonetti J.H., Dennison B., Diamond P.J., et Simonetti J.H., Diamond P.J., Uphoff J.A., Boboltz D., al. 1998, ApJ 509, 256 Dennison B., 1993, in Astrophysical Masers, Lecture Notes Braatz J.A., Wilson A.S., Henkel C., 1996, ApJS 106, 51 in Physics 412, eds. Clegg A.W. & Nedoluha G.E., Springer Braatz J.A., Wilson A.S., Henkel C., 1997, ApJS 110, 321 Verlag, Berlin, p. 311 Crosthwaite L.P., Turner J.L., Ho P.T.P., 2000, AJ 119, 1720 Taylor G.B., Morris M., Schulman E., 1993, AJ 106, 1978 Dame T.M., Hartmann D., Thaddeus P., 2001, ApJ 547, 792 Turner J.L., Ho P.T.P., 1994, ApJ 421, 122 Deguchi S., Watson W.D., 1989, ApJ 340, L17 Valdettaro R., Palla F., Brand J., Cesaroni R., et al., 2001, Digel, S.W., Lyder, D.A., Philbrick, A.J., Puche, D., Thaddeus, A&A 368, 845 P., 1996, ApJ 458, 561 van der Kruit P.C., 1973, A&A 29, 249 Downes D., Radford S.J.E., Guilloteau S., Guélin M., et al., Waak J.A., Mayer J.A., 1974, ApJ 189, 67 1992, A&A 262, 424 Wu Y.-F., Shi J.-R., Wang J.-Z., Jiang D.-R., Huang X.-Y., Elitzur M., 1990, ApJ 350, L7 1999, ChA&A 23, 454 Elitzur M., McKee C.F., Hollenbach D.J., 1991, ApJ 367, 333 Yusef-Zadeh F., Mehringer D.M., 1995, ApJ 452, L17 Elmegreen B.G., Genzel R., Moran J.M., Reid M.J., Walker R.C., 1980, ApJ 241, 1007 Fiebig D., Güsten R., 1989, A&A 214, 333 Garay G., Moran J.M., Haschick A.D., 1989, Apj 338, 244 Genzel R., Downes D., 1977, A&AS 30, 145 Genzel R., Downes D., Bieging J., 1976, MNRAS 177, 101 Greenhill L.J., Moran J.M., Reid M.J., Menten K.M., Hirabatashi H., 1993, ApJ 406, 482 Güsten R., Downes D., 1983, A&A 117, 343 Hagiwara Y., Henkel C., Menten K., Nakai N., 2001, ApJ 560, L37 Henkel C., Güsten R., Downes D., Thum C., Wilson T.L., Biermann P., 1984, A&A 141, L1 Henkel C., Wouterloot J.G.A., Bally J., 1986, A&A 155, 193 Henkel C., Wang Y.P., Falcke H., Wilson A.S., Braatz J.A., 1998, A&A 335, 463 Henkel C., Mauersberger R., Peck A.B., Falcke H., Hagiwara Y., 2000, A&A 361, L45 A. Tarchi et al.: Water maser emission in IC 342 7 Dec_offset (arcsec)

RA_offset (arcsec) h m s Fig. 2. H2O spectra obtained toward the central region of IC 342. Positions are oﬀsets relative to α2000 = 03 46 48.6 ◦ ′ ′′ ′′ and δ2000 = +68 05 46 . Note that the spacing between two individual spectra (20 ) is approximately half of the size of the 100-m Eﬀelsberg telescope beam at 22GHz (40′′). Averaging four contiguous channels, the spectra have been smoothed to a spacing of 0.26 kms−1.