High-sensitivity maps of the large-scale structure of atomic hydrogen in the
starburst dwarf galaxy NGC 1569 show evidence for an HI cloud with a mass of
7*10**6 M_sun, at a projected distance of 5 kpc from the parent galaxy. This
cloud may be a condensation in a low-column-density HI halo or a companion
galaxy/HI-cloud. NGC 1569 and its companion are connected by a low surface
brightness HI bridge. At the edge of NGC1569, the HI bridge coincides with
H_alpha arcs, also detected in soft X-rays.Comment: 5 pages, 4 figures, 1 tabl

Israel, F. P.

Stil, J. M.

English

arXiv

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Stil, J.M.

Israel, F.P.

Leiden University Scholary Publications

Astron. Astrophys. 337, 64–68 (1998) ASTRONOMYANDASTROPHYSICSA low-mass HI companion of NGC 1569?J.M. Stil and F.P. IsraelLeiden Observatory, PO Box 9513, 2300 RA Leiden, The NetherlandsReceived 4 March 1998 / Accepted 3 June 1998Abstract. High-sensitivity maps of the large-scale structure ofatomic hydrogen in the starburst dwarf galaxy NGC 1569 showevidence for an HI cloud with a mass of7·106M, at a projecteddistance of 5 kpc from the parent galaxy. This cloud may be acondensation in a low-column-density HI halo or a companiongalaxy/HI-cloud. NGC 1569 and its companion are connectedby a low surface brightness HI bridge. At the edge of NGC1569,the HI bridge coincides with Hα arcs, also detected in soft X-rays.Key words: ISM: atoms – galaxies: individual: NGC 1569 –galaxies: interactions – galaxies: irregular – galaxies: kinemat-ics and dynamics – galaxies: starburst1. IntroductionNGC 1569 is a small Im type galaxy at a distance of2.2 ±0.6 Mpc (Israel 1988), and a probable member of the IC 342group. Due to its low galactic latitude, the members of thisgroup are heavily obscured. The number of known membersis growing rapidly with the discovery of Dwingeloo 1 (Kraan-Korteweg et al. 1994), Dwingeloo 2 (Burton et al. 1996), MB 1and 2 (McCall & Buta 1995) and MB 3 (McCall & Buta 1997).With a diameter of 1.85 kpc, NGC 1569 is dominated by theaftermath of a burst of star formation (e.g. Israel & de Bruyn1988, Vallenari & Bomans 1996). It suffers an outflow of gas(Waller 1991, Heckman et al. 1995 and references therein) andcontains three very luminous, compact star clusters and severalbright HII regions (Waller 1991).The nearest neighbour to NGC 1569 is the dwarf irregulargalaxy UGCA 92 at a projected distance of 60 kpc (Karachent-sev & Tikhonov 1993), at approximately the same radial veloc-ity (vhel = −99 kms−1), and with an HI mass of7 · 107M(de Vaucouleurs et al. 1991). Emissionline-dominated dwarfgalaxies such as NGC 1569 often, if not always, have nearbycompanions (Taylor et al. 1995, but see Taylor et al. 1996). Thepair NGC 1569/UGCA 92 fits the selection criteria and overallproperties given by Taylor et al. (1995), but the orbital timescaleSend offprint requests to: J.M. StilCorrespondence to: stil@strw.leidenuniv.nlof this pair seems far too long to explain the recent starburst inNGC 1569 in terms of interaction with UGCA 92.The overall HI distribution consists of a high-column-den-sity ridge with three peaks and an ‘arm’ extending to the west(Reakes 1980, Israel & Van Driel 1990). Channel maps pre-sented by Reakes (1980) also show weak emission south-eastof the bright HI ridge, likewise visible in the low resolution mapof Israel & Van Driel (1990). In this paper, we concentrate onthe nature of this emission and compare it with the extended HIhaloes observed in the dwarf galaxies IC 10 (Shostak & Skill-man 1989) and NGC4449 (Bajaja et al. 1994). A more completenalysis of the HI emission of NGC1569 is deferred to a forth-coming paper.2. Observations and data reductionHI in NGC 1569 was observed with the WSRT during four 12-hour runs between November 1989 and January 1990. A totalof 160 baselines ranging from 36 m to 2772 m were sampledat increments of 18 m. Because of missing short interferom-eter spacings, structures larger than a few arcminutes becomeincreasingly diluted. However, the HI emission in individualvelocity channels generally does not extend that far, so that theimportance of this effect is minor.The 2.5 MHz band was sampled by 128 independent fre-quency channels of 4.12 km s−1 width. The Hanning-smoothedvelocity resolution is 8.2 km s−1. Maps at27′′ and60′′ resolu-tion were produced by applying Gaussian weight functions of843 m and 390 m width in the UV plane. The continuum wassubtracted in each pixel by fitting a first-order polynomial to 48line-free channels at both edges of the frequency band. In eachchannel map, the area containing line emission was identifiedmanually and CLEANed down to half the r.m.s. noise. The rmsnoise in each27′′ channel map is 1.1 mJy per beam. The channelmaps have not been corrected for primary-beam attenuation.3. Results and analysisIn the velocity range−160 km s−1 to−120 km s−1, the channelmaps show emission from NGC 1569 itself, and also from asecond feature atα = 4h27m20s, δ = 64o43′.0, best identifiedin thevhel = −152 andvhel = −135km s−1 maps in Fig. 1.J.M. Stil & F.P. Israel: A low-mass HI companion of NGC 1569? 65Fig. 1. Velocity maps at27′′ resolution, summed over four channels each. Heliocentric velocities marked are those of the third channel in eachset. Contours are at -10.5, 10.5 (3σ), 17.5, 24.5, 31.5, 38.5, 77.0 and 154.0 mJy per beam. The last panel also contains Galactic foregroundemission present in a velocity range of about 10kms−1.Although the emission is weak (4σ), we consider it real. First, itslocation coincides with emission in Reakes’s independent maps.Second, we eliminated the possibility of solar interference byexcluding all hour angles with the sun above the horizon. Wealso extracted the line profile in the direction of the feature fromthe raw data, i.e. before continuum subtraction and cleaning. Inboth cases, the emission persisted. We will refer to this featureas NGC 1569-HI. There is further weak emission between NGC1569 and NGC 1569-HI in the velocity range−130 km s−1 to−90 km s−1 (also visible in Reakes’s−100 km s−1 channelmap.A second filamentary structure is visible in the channelmaps with velocities -86 and -69km s−1 nearα = 4h25m30s,δ = 64o44′. It appears as a westward extension of the brightestemission. It is considerably closer to NGC 1569 than NGC 1569-HI and always connected to the brightest emission. For this rea-son, we treat it as a part of NGC 1569 and refer to it as the‘western arm’. This feature was first noticed by Reakes (1980).A map of the total HI column density is shown inFig. 2a and b, together with line profiles summed over selectedregions. The emission of NGC 1569 is dominated by the high-column-density ridge and the western arm. NGC 1569-HI is8′east of the centre of NGC 1569, and has a FWHM velocity widthof 50 km s−1. The spatially-integrated emission of NGC 1569-HI and of the bridge over the velocity range−190 km s−1 to−60 km s−1, is 12.4Jy km s−1, corrected for 16% primary-beam attenuation. The line flux of NGC 1569-HI itself is 6.3Jy km s−1, half of the total. If they are at a distance of 2.2 Mpc,the masses of NGC 1569-HI and of the bridge are7 · 106 Meach.On the side of NGC 1569, the apparent connection withthe bridge is located on the southern side of the galaxy. This66 J.M. Stil & F.P. Israel: A low-mass HI companion of NGC 1569?-300 -200 -100 0 100-1000100-300 -200 -100 0 100-60-40-2002040-300 -200 -100 0 100-100-50050-300 -200 -100 0 100-40-2002040Fig. 2a and b. HI column density map of NGC1569. Contours are at -1 (dashed), 1, 2, 4, 6, 11, 16, 21, ...·1020 HI cm−2. The luminous starclusters A and B in the centre of NGC1569 are marked by dots. Thick lines mark the brightestHα mission,Hα arcs and the northern limit offilaments in a deep image by Hunter et al. (1993). The dashed curve shows the path of the position-velocity diagram of Fig. 3. The synthesizedbeam is shown as a circle in the lower right corner.b Line profiles of the areas labeled 1 through 4 ina, defined by the2 · 1020 cm−2 contour.The line profile centroid shifts from -130km s−1 to -80km s−1 along the HI bridge. Much narrower negative features are sidelobes of MilkyWay foreground emission (see Fig. 1 and Fig. 3).Fig. 3. Position-velocity map along the HI bridge, the ridge, andthe arm extracted from the60′′ resolution data cube. Specific posi-tions mentioned in the text have been idicated. Contour levels : -10,-5,5,10,15,20,25,50,100,150,200,250 mJy/beam per 4.12km s−1 chan-nel. Negative contours are drawn as dashed lines.region coincides with the location of two concentricHα shellsdescribed by Waller (1991).Fig. 3 shows a position velocity map along the curved dashedline in Fig. 2a and b. Going westwards from NGC 1569-HI,‘bridge’ emission occurs at progressively less negative veloci-ties. The rotation of NGC 1569 occurs in theoppositesense.We have scanned the red and blue POSS II plates on theLeiden ASTROSCAN, but failed to find an optical counterpartof NGC 1569-HI. The detection limit on the blue plates for asmooth disk of1′ diameter should be about 26 mag arcsec−2;foreground stellar confusion may raise this somewhat. With 2.3magnitudes foreground extinction (Israel 1988), we arrive at alimit of about 23.5 mag arcsec−2. As this is close to the mediansurface brightness of DDO dwarf galaxies (de Vaucouleurs etal. 1981), the optical non-detection of NGC 1569-HI thereforeallows it to be a low-surface-brightness dwarf galaxy with aforeground extinction similar to that of NGC 1569, an inter-galactic HI cloud without any stars, or a Galactic high-velocitycloud.The central velocity of NGC 1569-HI is−130 ± 5 km s−1,which is in the extreme blue wing of the line-of-sight Galactic HIline profile (Hartmann & Burton 1997). The Galactic foregroundis clearly present atv > −70 km s−1 throughout the field ofview, but with a velocity width less than10 km s−1. However,we find no emission other than that of NGC 1569 and NGC1569-HI at any velocityvhel < −120 km s−1, even at60′′resolution. We have summed the spectra of background pointsources to look for absorption (Fig. 4). The3 σ upper limitto the optical depth per channel is 0.12 in the velocity range−300 km s−1 to −30 km s−1. The corresponding upper limitto the HI column-density per channel of an extended componentwith spin temperatureTs, is NHI < 9.6 · 1019(Ts/100K).4. DiscussionWe first consider whether a Galactic origin is consistent withboth the large velocity range of the eastern emission and itssmall angular size. The high-velocity-cloud complex closest toNGC 1569 is complex H, at a distance of 5 degrees (Wakker1991) and is in the relevant velocity range of−120 kms−1 to−140 kms−1. In Table 1 we compare the properties of NGCJ.M. Stil & F.P. Israel: A low-mass HI companion of NGC 1569? 67Fig. 4. Spectrum of background continuum sources normalized to themean of 91 mJy between−300 kms−1 and−60 kms−1; rms fluc-tuations are 4%. The brightest continuum source contributes approxi-mately 50% of the flux. Baselines shorter than 300 m have been ex-cluded in order to suppress foreground emission. No velocity smooth-ing has been applied.1569-HI to those of HVC cores, the dwarf galaxy M81 dwA,and a sample of dwarf galaxies.No distinction can be made from the column density. How-ever, the linewidth of NGC 1569-HI is much higher than ex-pected for small-scale structure in the Galactic foreground. Thetimescale for free expansion to double its present size is short,2 · 104dkpc year, wheredkpc is the distance in kpc. Togetherwith the lack of other isolated HI clouds in the field of view, theapparent connection (bridge) with NGC 1569, and the relativelyhigh contrast with the surroundings, leads us to conclude thatNGC 1569-HI is not a condensation in the Galactic foreground.Although NGC 1569 shows the signs of a global outflowof gas (see e.g. Heckman et al. 1995), the mass of NGC 1569-HI and the bridge (7 · 106 M each) as well as their locationprojected along the major axis all rule out that they result fromthis outflow.Extended, low-column-density HI haloes have been detec-ted in a number of late type galaxies (Huchtmeier & Richter1982). NGC 1569-HI and the western arm are possibly highdensity regions in a similar halo. The only evidence for ex-tended emission with a column densityNHI ∼ 1020cm−2 fromthe present data is the emission in the north-west of NGC 1569.Its possible counterpart, symmetric in position and in velocitywith respect to the centre of NGC 1569 should appear in thechannel maps near−60km s−1. Negative sidelobes of Galacticforeground emission inhibit the detection of an extended, low-surface-brightness component (see e.g. Fig. 3). NGC 1569-HIcannot be identified with the symmetric counterpart of the ex-tended emission in the northwest, because its velocity is80km s−1 lower. An extended HI halo is sometimes inferred fromthe difference in flux measured by an interferometer and a sin-gle dish radio telescope. However, single dish observations areheavily contaminated by the large column of Galactic HI. Fur-thermore, there is no evidence for an HI halo in the maps pre-sented by Reakes (1980).Theapparentretrograde rotation of NGC 1569-HI does notexclude a prograde orbit, e.g. as a part of an extended HI disk.High resolution observations of the haloes of IC 10 (Shostak& Skillman 1989) and NGC 4449 (Bajaja et al. 1994) show areversal of the velocity field. Such a reversal can be the result ofa warped HI disk bending through the plane of the sky. However,the warp angle must then be larger than the inclination angle,which is60o for NGC 1569 (Israel 1988). There is no indicationfrom the HI distribution that such an extreme warp exists inNGC 1569.As it is unlikely that NGC 1569-HI is an extension of therotating disk of NGC 1569, it is either primordial gas whichhas not yet fallen into NGC 1569, or a separate object in aclose encounter with NGC 1569. The difference between theseinterpretations is mainly semantic. No definite choice can bemade since there is no optical identification of NGC 1569-HI.However, some general remarks can be made.If NGC 1569-HI is a part of an HI halo which settles intothe inner disk of NGC 1569 on a cosmic timescale, its smalldistance to NGC 1569 suggests that its is slowly spiraling in-wards. This implies a relatively circular orbit, as opposed to aparabolic or hyperbolic orbit expected for an encounter. Assum-ing a circular orbit, a lower limit to the mass of NGC 1569 canbe derived. As the projected distance,d, and radial velocity dif-ference∆v are lower limits to the actual distance and relativevelocity, the minimum mass of the system if it is to be bound,must beMmin ≈ G−1d∆v2 = 2 · 109M. This lower limit,which is absolute for a circular orbit, is an order of magnitudehigher than the estimateddynamicalmass of NGC 1569 within ar dius of 1.3 kpc, where the rotation velocity is maximal (Reakes1980). The lower limit to the total mass results in a strong upperlimit to the HI mass to total mass ratioMHI/Mtotal < 0.04within 5 kpc, as opposed to the observedMHI/Mtotal = 0.33within a radius of 1.3 kpc. The upper limit for the HI mass tototal mass ratio is comparable to the ratio in dark matter dom-inated dwarf irregular galaxies such as DDO 154 (Carignan &Beaulieu 1989) at the last measured point of the rotation curve.However, the HI mass to total mass ratio is relatively constantwith radius because the HI partial rotation curve mimics the totalrotation curve to a high accuracy (Carignan & Beaulieu 1989,Broeils 1992). The combinationMHI/Mtotal = 0.33 within1.3 kpc andMHI/Mtotal < 0.04 within 5 kpc, is exceptional.This contradiction is solved if the system is not gravitationallybound.The velocity difference between NGC 1569 and NGC 1569-HI is 40km s−1, their projected separation is 5 kpc, and their HImass ratio is about 0.06 (excluding the bridge). Correspondingvalues for other pairs of dwarf galaxies studied by Taylor etal. (1995) are50 km s−1, 30 kpc, and 0.2 respectively. Thus,68 J.M. Stil & F.P. Israel: A low-mass HI companion of NGC 1569?Table 1. Physical parameters of NGC 1569-HI compared with HVCcores and low-mass dwarf galaxies.N1569HIa HVC coreb M81 dwAc LSY dw.ddMpc 2.2 dMpc 3.25 0.76 - 5DHI (kpc) 2.6 × 1.3 0.6 dMpc 1.9 × 1.3 0.8 - 3.4NHI,p 3.5 · 1020 3 · 1020 4 · 1020 2.5 - 42∆v 50 5 19 20 - 36µB > 23.5 - 24.4 23.4 - 25.2Note :dMpc : distance in Mpc;DHI : HI diameter (kpc);NHI,p : peak HIcolumn density in1020 HIcm−2; ∆v : HI FWHM line width (kms−1);µB : mean surface brightness in B (mag arcsec−2); References:a.Thiswork; b. Wakker & Schwarz (1991);c. Sargent et al. (1983);d. rangeof the sample of 9 faint dwarfs in Lo et al. (1993).NGC 1569 and NGC 1569-HI appear to be a close, but not anexceptional pair.The virial mass of NGC 1569-HI isMvir ∼ RHI∆v2/G =6·108 M. If the HI mass ratio is representative for the total massratio, the ratio of the tidal radius to their actual separation is 0.4.Judging from their projected separation, NGC 1569-HI almostfills its tidal radius. Tidal forces exerted by NGC 1569 may thusexplain the HI bridge. However, the western arm is unlikely tobe the result of tidal forces exerted by NGC 1569-HI given thesmall mass and the apparent retrograde orbit of NGC 1569-HI.This leaves the western arm unexplained. If both are part of anouter halo which is not in equilibrium, as was suggested forIC 10, this problem does not exist.It is evident from Fig. 3 that the velocity structure of NGC1569-HI and the bridge is well ordered. It is therefore a coherentstructure and not a chance superposition of multiple clouds.The timescale on which this structure evolves is in the orderd/∆v = 108years. It is therefore a transient feature. Shostak& Skillman (1989) found a structure similar to the HI bridgeand a disturbed location where this “plume” meets the disk ofIC 10. This was used as an argument in favour of interaction. Asimilar argument can be made for NGC 1569.The present data do not unequivocally show that the bridge isfalling into NGC 1569, but we may note some indications thatthis is so. First, the presumed contact region exhibits chaotickinematics (see Heckman et al. 1995 forHα and our forth-coming paper for HI kinematics). Second, theHα arcs and theextended X-ray feature described by Heckman et al. (1995) arefound in the same direction. However,Hα filaments are com-mon in starburst galaxies, while a galactic wind is still needed toobtain the high Mach number required to heat the gas to X-raytemperatures. Finally, we note that the centre of the starburst isoffset to the south from the high column density HI ridge, i.e.towards the bridge. It is tempting to speculate about a possibleconnection between recent bursts of star formation and infallingmaterial.Acknowledgements.We wish to thank R. Sancisi, W.B. Burton and P.Van der Werf for their useful comments about this paper and E. Deulfor his help with the astroscan.ReferencesBajaja E., Huchtmeier W.K., Klein U., 1994, A&A 285,385Broeils A.H., 1992, A&A 256,19.Burton W.B., Verheijen M.A.W., Kraan-Korteweg, R.C. et al., 1996,A&A 309,687.Carignan C., Beaulieu S., 1989, ApJ 347,760.Della Ceca R., Griffiths R.E., Heckman T.H., Mackenty J.W., 1996,ApJ 469,662de Vaucouleurs G., de Vaucouleurs A., Buta R., 1981, AJ 86,1429de Vaucouleurs G., de Vaucouleurs A., Corwin H.G. et al., 1991, ThirdReference Catalogue, Springer Verlag, N.Y.Hartmann D., Burton W.B., 1997, Atlas of galactic neutral hydrogen,Cambridge University PressHeckman T.M., Dahlem M. 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A low-mass HI companion of NGC 1569?

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