We report the first results of an ongoing survey at 22 GHz with the 100-m
Effelsberg telescope to search for water maser emission in bright IRAS sources.
We have detected water vapor emission in IC 342. The maser, associated with a
star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5
km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8
Mpc). Our detection raises the detection rate among northern galaxies with IRAS
point source fluxes S(100micron) > 50 Jy to 16%.Comment: 6 pages, 3 figures. Proceeding of the conference "SRT: the impact of
  large antennas on Radio Astronomy and Space Science", Cagliari, Italy, 7-10
  Nov. 2001, Eds. Porceddu et al. Needs srt_style.st

Henkel, C.

Menten, K. M.

Moscadelli, L.

Peck, A. B.

Tarchi, A.

English

arXiv

We report the first results of an ongoing survey at 22 GHz with the 100-m Effelsberg telescope to search for water maser emission in bright IRAS sources. We have detected water vapor emission in IC 342. The maser, associated with a star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8 Mpc). Our detection raises the detection rate among northern galaxies with IRAS point source fluxes S(100micron) > 50 Jy to 16%

TARCHI, ANDREA

MOSCADELLI, Luca

OA@INAF - Istituto Nazionale di Astrofisica

2002Publication Year2023-01-03T11:39:38ZAcceptance in OA@INAFObservations of extragalactic masers in bright IRAS sourcesTitleTARCHI, ANDREA; Henkel, C.; Peck, A. B.; MOSCADELLI, Luca; Menten, K. M.Authorshttp://hdl.handle.net/20.500.12386/32825HandlearxivSeriesarXiv:astro-ph/0206053v1 4 Jun 2002SRT: the impact of large antennas on Radioastronomy and Space ScienceSRT Conference Proceedings, Vol. 1, 2002Editors: TBDObservations of extragalactic H2O masers in bright IRASsourcesA. Tarchi1, C. Henkel1, A. B. Peck1, L. Moscadelli2 and K. M. Menten11 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121Bonn, Germany2 Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada54, 09012 Capoterra (CA), ItalyAbstract. We report the rst results of an ongoing survey at 22GHzwith the 100-m Eelsberg telescope to search for water maser emission inbright IRAS sources. We have detected water vapor emission in IC 342.The maser, associated with a star forming region ∼10–15′′ west of the nu-cleus, consists of a single 0.5 km s−1 wide feature and reaches an isotropicluminosity of 10−2 L⊙ (D=1.8Mpc). Our detection raises the detectionrate among northern galaxies with IRAS point source uxes S100µm >50 Jy to 16%.1. IntroductionTo date, luminous extragalactic H2O masers can be grouped into three classes:those tracing accretion disks (e.g. NGC 4258); those in which the emission iseither the result of an interaction between the radio jet and an encroachingmolecular cloud or an accidental overlap, along the line-of-sight, between a warmdense molecular cloud and the radio continuum of the jet (e.g. NGC 1052 andMrk 348); those related to prominent sites of star formation, such as the onesobserved in M 33 (the earliest known extragalactic H2O masers), and later alsoin IC 10.Extragalactic H2Omasers are preferentially detected in nearby galaxies thatare bright in the infrared ([4]). While nuclear masers are of obvious interest, non-nuclear masers are also important for a number of reasons: these sources allowus to pinpoint sites of massive star formation, to measure the velocity vectorsof these regions through VLBI proper motion studies, and to determine truedistances through complementary measurements of proper motion and radialvelocity (e.g. [6]).We have therefore observed the nearby spiral galaxies IC 342 and Maei 2,both of which exhibit prominent nuclear bars and strong molecular, infrared,12and radio continuum emission. In the following we report the results of ourobservations.2. Water maser observations of Maei 2 and IC 3422.1. The observation and image processingThe 616 − 523 line of H2O (rest frequency: 22.23508 GHz) was observed withthe 100-m telescope of the MPIfR at Eelsberg1 toward Maei 2 and IC 342.The full width to half power beamwidth was 40′′. The observations were carriedout in a dual beam switching mode with a beam throw of 2′ and a switchingfrequency of ∼1Hz. The pointing accuracy was always better than 10′′.On May 12, 2001, IC 342 was observed with the Very Large Array2 (VLA)in its B conguration. The 103 channels used, out of the 128 observed, cover arange in velocity of ∼8 km s−1 centered at 16 km s−1 LSR (the velocity of the linedetected at Eelsberg). No continuum subtraction was needed. The data weredeconvolved using the CLEAN algorithm ([10]). The restoring beam is 0.′′4×0.′′3and the rms noise per channel is ∼10mJy, consistent with the expected thermalnoise.2.2. ResultsOur single-dish observations towards Maei 2 yielded no detection, with a 5σupper limit of 25mJy (channel spacing: 1.05 km s−1; velocity range: –250 km s−1< V < +230 km s−1; epoch: Apr 3, 2001; position: α2000 = 02h 41m 55.s2, δ2000= +59◦ 36′ 11′′).On April 2, 2001, we obtained the rst denite detection of water vaporemission in IC 342. During the next night the detection was conrmed with avelocity resolution sucient to resolve the line prole (Fig. 1a). The detectedfeature lies at VLSR = 16 km s−1 and has a linewidth of ∼0.5 km s−1; on April2, no other component was seen at velocities –175 km s−1 < V < +310 km s−1(channel spacing: 1.05 km s−1; 5σ noise level: 16mJy). A high line intensity(∼100mJy) and good weather conditions allowed us to map the emitting re-gion (Fig. 2). Further measurements were performed on April 22 and May 7(Fig. 1b&1c). It is noticeable a fading in the peak and integrated intensitiesand a blue-shift in the velocity of the line by ∼1 km s−1. Unfortunately, noemission above ∼30mJy (3σ level; 0.08 km s−1 channel spacing) was detected inthe 22 GHz VLA B-array data taken on May 12. This fading by at least a factorof 3 within 5 days implies a size scale of∼< 1.5×1016 cm (∼< 900AU) or∼< 0.5mas1The 100-m telescope at Eelsberg is operated by the Max-Planck-Institut für Radioastronomie(MPIfR) on behalf of the Max-Planck-Gesellschaft (MPG).2The National Radio Astronomy Observatory is a facility of the National Science Foundationoperated under cooperative agreement by Associated Universities, Inc.3b) April 22c) May 7d) June 22(0,0)(-10,0)(-10,0)(-10,0)a) April 3Velocity (km/s)Flux density (Jy)Figure 1. The H2O maser feature observed with high velocity reso-lution (channel spacings after smoothing four contiguous channels are0.26 kms−1) on a) April 3, b) April 22, c) May 7, and d) June 22.The rst spectrum has been taken at the (0′′,0′′) position, the othersat (–10′′,0′′) relative to the position given in footnote ‘a’ of Table 1.The dashed line indicates VLSR = 16 km s−1.RA_offset (arcsec)Dec_offset (arcsec)Figure 2. H2O spectra obtained toward the central region ofIC 342. Positions are osets relative to α2000 = 03h46m48.s6 andδ2000 = +68◦05′46′′. Note that the spacing between two individualspectra (20′′) is approximately half of the size of the 100-m Eelsbergtelescope beam at 22GHz (40′′). Averaging four contiguous channels,the spectra have been smoothed to a spacing of 0.26 km s−1.4*ENFigure 3. XDSS B-band image of the central region of IC 342. Theasterisk indicates the water maser emitting position. The radius of thecircle indicates its positional error as outlined in Sect. 3.2.at a distance of 1.8Mpc ([13]; see also Sect. 4.3); the corresponding brightnesstemperature is∼>109 K. The most recent spectrum was obtained on June 22 atEelsberg (Fig. 1d). Conrming the VLA result, no maser signal was seen above30mJy (3σ; channel spacing: 1.05 km s−1).Fitting a synthetic Gaussian to the data taken on April 3 (see Fig. 2), i.e.minimizing the sum of the dierence squared between calculated and observedpeak and integrated ux densities, we can obtain an accurate position of theemitting region: α2000 = 03h 46m 46.s3, δ2000 = +68◦ 05′46′′. This is 13′′ to thewest of our center position that coincides with the optical nucleus and the 2µmpeak [14]; [1]. The accuracy of the derived maser position has been estimatedto be ∼ 5′′.2.3. DiscussionArising from the central region of IC 342, but being displaced from the nucleus,the H2O maser is likely associated with a prominent star forming region.Fig. 3 shows an optical B-band image of the central region of IC 342, takenfrom the XDSS3. The maser emitting region (white circle) coincides with an3The Digitized Sky Surveys were produced at the Space Telescope Institute under U.S. Gov-ernment grant NAGW-2166. The images of these surveys are based on photographic dataobtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Tele-5arc-like structure extending E-S and is associated with a chain of sources thatappear to be HII regions.Occasionally, prominent galactic star forming regions show narrow (∼0.5km s−1) aring components that are exceptionally bright. Such ares were ob-served in W31A and W49 ([12]; [11]). The prototype of these ares is the8 kms−1 super maser in Orion-KL (e.g. [5]). During several months, the nar-row highly linearly polarized maser reached ux densities in excess of 5MJy.Surpassing the ux of any other velocity component by more than an order ofmagnitude and reaching a peak luminosity of LH2O ∼ 10−2 L⊙, the feature seemsto be similar to that seen in IC 342. While luminosity, linewidth, and ux varia-tions are reminiscent of the 8 kms−1 super maser in Orion-KL, our H2O prolesshow a signicant velocity shift between April 22 and May 7 (Figs. 1b and c)that has not been seen in the Orion-KL aring component. A simple model forexplaining both the are and the shift is, in our opinion, that the latter hasa kinematic origin. Adopting a scenario of a chance alignment of two masingclouds along the line-of-sight, motion of the foreground relative to the back-ground cloud along the plane of the sky and velocity structure in the foregroundcloud could explain the observed data. A velocity gradient in the foregroundcloud would then rst shift the line velocity; once velocities are reached that arenot matched within the background cloud, the ux density of the maser rapidlydrops. Assuming that the distance between these clouds is ≪1.8Mpc and thattheir relative velocity in the plane of the sky is at the order of 100 kms−1, thisimplies a cloud velocity gradient of up to 1 km s−1/AU during the time the sourcewas monitored.An almost identical scenario was proposed by [2] to account for the velocityshift of the aring water maser component at –66 km s−1 in W49N. The similarityin velocity shift (∼ 0.5 km s−1) and time scale (58 days) of the event in W49Nw.r.t the one in IC 342, indicates a common origin.3. Extragalactic maser detection ratesWhile typical searches for extragalactic maser sources have only yielded detec-tion rates between zero (e.g. [9]) and a few percent (e.g. [7]; [3]), there existsone sample with detection rates >10%: These are the northern (δ > –30◦)extragalactic IRAS (Infrared Astronomy Satellite) point sources with 100µmuxes in excess of 50 Jy (for a source list, see [8]; Maei 2 with S100µm ∼ 200 Jyshould be added to the list). There is a total of 44 galaxies, two ultralumi-nous galaxies at intermediate distances (NGC3690 and Arp 220) and 42 nearbysources (V < 3000 km s−1). Among these, seven (16%) are known to contain H2Oscope. The plates were processed into the present compressed digital form with the permissionof these institutions.6masers in their nuclear region. Two of these contain megamasers (NGC 1068and NGC 3079), two are possibly nuclear kilomasers (NGC 253 and M 51; fordetails, see Sect. 1), and three are associated with prominent sites of star for-mation (IC 10, IC 342, M 82). Among the subsample of 19 sources with 100µmuxes in excess of 100 Jy, ve were so far detected in H2O, yielding a detectionrate in excess of 20%. Since few deep integrations have been obtained towardthese sources, more H2O detections can be expected from this promising samplein the near future.Acknowledgments. We wish to thank Nikolaus Neininger for useful dis-cussion. We are also endebted to the operators at the 100-m telescope, and toMichael Rupen and the NRAO analysts, for their cheerful assistance with theobservations.References[1] Becklin, E.E., Gatley, I., Matthews, K., Neugebauer, G., Sellgren, K., etal. 1980, ApJ 236, 441[2] Boboltz, D.A., Simonetti, J.H., Dennison, B., Diamond, P.J., & Upho,J.A 1998, ApJ 509, 256[3] Braatz, J.A., Wilson, A.S., & Henkel, C. 1996, ApJS 106, 51[4] Braatz, J.A., Wilson, A.S., & Henkel, C. 1997, ApJS 110, 321[5] Garay, G., Moran, J.M., & Haschick, A.D. 1989, ApJ 338, 244[6] Greenhill, L.J., Moran, J.M., Reid, M.J., Menten, K.M., & Hirabatashi,H. 1993, ApJ 406, 482[7] Henkel, C., Güsten, R., Downes, D., Thum, C., Wilson, T.L., et al. 1984,A&A 141, L1[8] Henkel, C., Wouterloot, J.G.A., & Bally, J. 1986, A&A 155, 193[9] Henkel, C., Wang, Y.P., Falcke, H., Wilson, A.S., & Braatz, J.A. 1998,A&A 335, 463[10] Högbom, J.A. 1974, A&AS 15, 417[11] Lekht, E.E., Mendoza-Torres, E., & Sorochenko, R.L. 1995, ApJ 443, 222[12] Liljeström, T., Mattila, K., Toriseva, M., & Anttila, R. 1989, A&AS 79,19[13] McCall, M.L. 1989, AJ 97, 1341[14] van der Kruit, P.C. 1973, A&A 29, 249

Observations of extragalactic masers in bright IRAS sources

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