Observations of extragalactic masers in bright IRAS sources

a r X

i

v

:a

s t

r o

-

p

h

/

2

6

53

v

1

4

J u

n

2

2

SRT:the impact of large antennas on Radioastronomy and Space Science SRT Conference Proceedings,Vol.1,2002Editors:TBD Observations of extragalactic H 2O masers in bright IRAS sources A.Tarchi 1,C.Henkel 1,A.B.Peck 1,L.Moscadelli 2and K.M.Menten 11Max-Planck-Institut f¨u r Radioastronomie,Auf dem H¨u gel 69,D-53121Bonn,Germany 2Osservatorio Astronomico di Cagliari,Loc.Poggio dei Pini,Strada 54,09012Capoterra (CA),Italy Abstract.We report the ?rst results of an ongoing survey at 22GHz with the 100-m E?elsberg 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′′west of the nu-cleus,consists of a single 0.5km s ?1wide feature and reaches an isotropic luminosity of 10?2L ⊙(D =1.8Mpc).Our detection raises the detection rate among northern galaxies with IRAS point source ?uxes S 100μm >50Jy to 16%.1.Introduction To date,luminous extragalactic H 2O masers can be grouped into three classes:those tracing accretion disks (e.g.NGC 4258);those in which the emission is either the result of an interaction between the radio jet and an encroaching molecular cloud or an accidental overlap,along the line-of-sight,between a warm

dense molecular cloud and the radio continuum of the jet (e.g.NGC 1052and Mrk 348);those related to prominent sites of star formation,such as the ones observed in M 33(the earliest known extragalactic H 2O masers),and later also in IC 10.

Extragalactic H 2O masers are preferentially detected in nearby galaxies that are 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 allow us to pinpoint sites of massive star formation,to measure the velocity vectors of these regions through VLBI proper motion studies,and to determine true distances through complementary measurements of proper motion and radial velocity (e.g.[6]).

We have therefore observed the nearby spiral galaxies IC 342and Ma?ei 2,both of which exhibit prominent nuclear bars and strong molecular,infrared,

1

2

and radio continuum emission.In the following we report the results of our observations.

2.Water maser observations of Ma?ei2and IC342

2.1.The observation and image processing

The616?523line of H2O(rest frequency:22.23508GHz)was observed with the100-m telescope of the MPIfR at E?elsberg1toward Ma?ei2and IC342. The full width to half power beamwidth was40′′.The observations were carried out in a dual beam switching mode with a beam throw of2′and a switching frequency of～1Hz.The pointing accuracy was always better than10′′.

On May12,2001,IC342was observed with the Very Large Array2(VLA) in its B con?guration.The103channels used,out of the128observed,cover a range in velocity of～8km s?1centered at16km s?1LSR(the velocity of the line detected at E?elsberg).No continuum subtraction was needed.The data were deconvolved using the CLEAN algorithm([10]).The restoring beam is0.′′4×0.′′3 and the rms noise per channel is～10mJy,consistent with the expected thermal noise.

2.2.Results

Our single-dish observations towards Ma?ei2yielded no detection,with a5σupper limit of25mJy(channel spacing:1.05km s?1;velocity range:–250km s?1

On April2,2001,we obtained the?rst de?nite detection of water vapor emission in IC342.During the next night the detection was con?rmed with a velocity resolution su?cient to resolve the line pro?le(Fig.1a).The detected feature lies at V LSR=16km s?1and has a linewidth of～0.5km s?1;on April 2,no other component was seen at velocities–175km s?1

3

F l u x d e n s i t y (J y )Figure 1.The H 2O maser feature observed with high velocity reso-lution (channel spacings after smoothing four contiguous channels are 0.26km s ?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 others at (–10′′,0′′)relative to the position given in footnote ‘a’of Table 1.The dashed line indicates V LSR =16km s ?1.

RA_offset (arcsec)

D e c _o f f s e t (a r c s e c )

Figure 2.H 2O spectra obtained toward the central region of IC 342.Positions are o?sets relative to α2000=03h 46m 48.s 6and δ2000=+68?05′46′′.Note that the spacing between two individual spectra (20′′)is approximately half of the size of the 100-m E?elsberg telescope beam at 22GHz (40′′).Averaging four contiguous channels,the spectra have been smoothed to a spacing of 0.26km s ?1.

4

N

E

*

Figure3.XDSS B-band image of the central region of IC342.The

asterisk indicates the water maser emitting position.The radius of the

circle indicates its positional error as outlined in Sect.3.2.

at a distance of1.8Mpc([13];see also Sect.4.3);the corresponding brightness temperature is～>109K.The most recent spectrum was obtained on June22at E?elsberg(Fig.1d).Con?rming the VLA result,no maser signal was seen above 30mJy(3σ;channel spacing:1.05km s?1).

Fitting a synthetic Gaussian to the data taken on April3(see Fig.2),i.e. minimizing the sum of the di?erence squared between calculated and observed peak and integrated?ux densities,we can obtain an accurate position of the emitting region:α2000=03h46m46.s3,δ2000=+68?05′46′′.This is13′′to the west of our center position that coincides with the optical nucleus and the2μm peak[14];[1].The accuracy of the derived maser position has been estimated to be～5′′.

2.3.Discussion

Arising from the central region of IC342,but being displaced from the nucleus, the H2O maser is likely associated with a prominent star forming region.

Fig.3shows an optical B-band image of the central region of IC342,taken from the XDSS3.The maser emitting region(white circle)coincides with an

5 arc-like structure extending E-S and is associated with a chain of sources that appear to be HII regions.

Occasionally,prominent galactic star forming regions show narrow(～0.5 km 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 the 8km s?1super maser in Orion-KL(e.g.[5]).During several months,the nar-row highly linearly polarized maser reached?ux densities in excess of5MJy. Surpassing the?ux of any other velocity component by more than an order of

magnitude and reaching a peak luminosity of L H

2O

～10?2L⊙,the feature seems

to be similar to that seen in IC342.While luminosity,linewidth,and?ux varia-tions are reminiscent of the8km s?1super maser in Orion-KL,our H2O pro?les show a signi?cant velocity shift between April22and May7(Figs.1b and c) that has not been seen in the Orion-KL?aring component.A simple model for explaining both the?are and the shift is,in our opinion,that the latter has a kinematic origin.Adopting a scenario of a chance alignment of two masing clouds 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 foreground cloud could explain the observed data.A velocity gradient in the foreground cloud would then?rst shift the line velocity;once velocities are reached that are not matched within the background cloud,the?ux density of the maser rapidly drops.Assuming that the distance between these clouds is?1.8Mpc and that their relative velocity in the plane of the sky is at the order of100km s?1,this implies a cloud velocity gradient of up to1km s?1/AU during the time the source was monitored.

An almost identical scenario was proposed by[2]to account for the velocity shift of the?aring water maser component at–66km s?1in W49N.The similarity in velocity shift(～0.5km s?1)and time scale(58days)of the event in W49N w.r.t the one in IC342,indicates a common origin.

3.Extragalactic maser detection rates

While 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 exists one sample with detection rates>10%:These are the northern(δ>–30?) extragalactic IRAS(Infrared Astronomy Satellite)point sources with100μm ?uxes in excess of50Jy(for a source list,see[8];Ma?ei2with S100μm～200Jy should be added to the list).There is a total of44galaxies,two ultralumi-nous galaxies at intermediate distances(NGC3690and Arp220)and42nearby sources(V<3000km s?1).Among these,seven(16%)are known to contain H2O

6

masers in their nuclear region.Two of these contain megamasers(NGC1068 and NGC3079),two are possibly nuclear kilomasers(NGC253and M51;for details,see Sect.1),and three are associated with prominent sites of star for-mation(IC10,IC342,M82).Among the subsample of19sources with100μm ?uxes in excess of100Jy,?ve were so far detected in H2O,yielding a detection rate in excess of20%.Since few deep integrations have been obtained toward these sources,more H2O detections can be expected from this promising sample in the near future.

Acknowledgments.We wish to thank Nikolaus Neininger for useful dis-cussion.We are also endebted to the operators at the100-m telescope,and to Michael Rupen and the NRAO analysts,for their cheerful assistance with the observations.

References

[1]Becklin,E.E.,Gatley,I.,Matthews,K.,Neugebauer,G.,Sellgren,K.,et

al.1980,ApJ236,441

[2]Boboltz,D.A.,Simonetti,J.H.,Dennison,B.,Diamond,P.J.,&Upho?,

J.A1998,ApJ509,256

[3]Braatz,J.A.,Wilson,A.S.,&Henkel,C.1996,ApJS106,51

[4]Braatz,J.A.,Wilson,A.S.,&Henkel,C.1997,ApJS110,321

[5]Garay,G.,Moran,J.M.,&Haschick,A.D.1989,ApJ338,244

[6]Greenhill,L.J.,Moran,J.M.,Reid,M.J.,Menten,K.M.,&Hirabatashi,

H.1993,ApJ406,482

[7]Henkel,C.,G¨u sten,R.,Downes,D.,Thum,C.,Wilson,T.L.,et al.1984,

A&A141,L1

[8]Henkel,C.,Wouterloot,J.G.A.,&Bally,J.1986,A&A155,193

[9]Henkel,C.,Wang,Y.P.,Falcke,H.,Wilson,A.S.,&Braatz,J.A.1998,

A&A335,463

[10]H¨o gbom,J.A.1974,A&AS15,417

[11]Lekht,E.E.,Mendoza-Torres,E.,&Sorochenko,R.L.1995,ApJ443,222

[12]Liljestr¨o m,T.,Mattila,K.,Toriseva,M.,&Anttila,R.1989,A&AS79,

19

[13]McCall,M.L.1989,AJ97,1341

[14]van der Kruit,P.C.1973,A&A29,249