Broadband continuum images of the edge-on galaxy NGC 891 at 850 and 450 microns are presented. These images are qualitatively very similar to the 1300 micron and CO images obtained by others. With respect to the 850 micron continuum, CO is strongest in the socalled `molecular ring', and weakest at the largest radii sampled. Inside the molecular ring, the CO/850 micron ratio is somewhat less than in the ring, but higher than in the remainder of the disk. The integrated far-infrared emission from NGC 891 is dominated by small particles shortwards of 100 microns. Longwards of 100 micron, the emission can be equally well fitted by a single population of large dust grains at 21 K, or a bimodal population of grains at temperatures of 18 K and 27 K. The circumnuclear disk is at a temperature of at least 50 K, and probably much higher

Israel, F P

Tilanus, R P J

Van der Werf, P P

arXiv

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Israel, F.P.

Werf, P.P. van der

Tilanus, R.P.J.

Leiden University Scholary Publications

Astron. Astrophys. 344, L83–L86 (1999) ASTRONOMYANDASTROPHYSICSLetter to the EditorSubmillimetre maps of the edge-on galaxy NGC 891F.P. Israel1, P.P. van der Werf1, and R.P.J. Tilanus2,31 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands2 Joint Astronomy Centre, 660 N. A’ohoku Pl., Hilo, Hawaii, 96720, USA3 Netherlands Foundation for Research in Astronomy, P.O. Box 2, 7990 AA Dwingeloo, The NetherlandsReceived 20 April 1998 / Accepted 17 March 1999Abstract. Broadband continuum images of the edge-on galaxyNGC 891 at 850µm and 450µm are presented. These imagesare very similar to the 1300µm and CO line images. With re-spect to the 850µm continuum, CO is strongest in the socalledmolecular ring’, and weakest at the largest radii sampled. Insidethe molecular ring, the CO/850µm ratio is somewhat less thanin the ring, but higher than in the remainder of the disk. Theintegrated infrared emission from NGC 891 is dominated bysmall particles shortwards of 100µm. Longwards of 100µm,the emission can be equally well fitted by a single populationof large dust grains at 21 K, or a bimodal population of grainsat temperatures of 18 K and 27 K. The circumnuclear disk is ata temperature of at least 50 K, and probably much higher.Key words: galaxies: active – galaxies: individual: NGC 891 –galaxies: ISM – galaxies: nuclei – infrared: galaxies1. IntroductionNGC 891 is a bright, non-interacting spiral galaxy (SA(s)b: DeVaucouleurs et al. 1976). It is very nearly edge-on (i > 88◦,PA = 23◦; Sofue& Nakai 1993). NGC 891 is one of the majormembers of the NGC 1023 group and its distance is estimated tobe 9.5 Mpc forH0 = 75 km s−1 Mpc−1, making for a scale of22′′/kpc or 46 pc/′′. Many authors have remarked on the apparentsimilarity between NGC 891 and the Milky Way. The interstellarmedium in the disk of NGC 891 has been the subject of severalstudies, notably radio continuum (Allen et al. 1978; Sukumar& Allen 1991), HI (Sancisi& Allen 1979; Rupen 1991), dust(Howk&Savage 1997); CO (Garcı́a-Burillo et al. 1992; Scovilleet al. 1993; Sofue& Nakai 1993; Garćıa-Burillo& Guélin 1995;Sakamoto et al. 1997) and CI (Gerin& Phillips 1997). Far-infrared dust emission was observed by IRAS (see Wainscoatet al. 1987; Rice et al. 1988), and atλ = 1.3 mm by Gúelinet al. 1993. In this paper we present further observations ofthe submillimeter dust emission from NGC 891 with the newSCUBA submillimeter continuum array detector.Send offprint requests to: F.P. IsraelCorrespondence to: israel@strw.leidenuniv.nl2. ObservationsThe SCUBA 850 and 450µm images were obtained in 1997September on the JCMT1. SCUBA employs two arrays ofcooled bolometers, each covering a field of about2′.3. The re-spective filters used have central frequencies of 347 GHz and677 GHz respectively; both filters have a bandwidth of 30 GHz.We observed three fields on NGC 891 (center, northeast andsouthwest) in socalled ‘jiggle-mode’ in order to cover the wholegalaxy. Chopping distance was 2′ perpendicular to the majoraxis of the galaxy. Total integration time for the three fields was128 minutes. Reduction was performed in the standard manner.Photometric calibration was achieved by skydip analysis andmapping of the standard star CRL 618. More complete detailsof the instrument as well as observing and reduction methodscan be found in Holland et al. (1998) and Jenness et al. (1998),as well as on the world-wide web (www.jach.hawaii.edu). Theresulting images are shown in Fig. 1. Especially in the 450µmimage, residual noise is seen at the SCUBA field edges; thisshould be discounted.3. Results3.1. Total submillimetre spectrum and dust contentGuélin et al. (1993) have fitted two dust components at 15 Kand 30 K to their 1.3 mm flux and the IRAS 100µm flux. Fromour maps, we have determined integrated flux-densitiesS850 =4.8±0.6 Jy andS450 = 39± 7 Jy. The observations through the850µm filter include emission from theJ=3–2 12CO line. Insimilar beams, the first three CO transitions occur in intensityratios of 1:0.75:0.4 (see Garcı́a-Burillo et al. 1992; Gerin&Phillips 1997). From this, we estimate an integratedJ=3–2 COcontribution of 0.22 Jy to the measured 850µm flux-density.Corrected for this contribution, the dust continuum emissionbecomesS850 ≈ 4.6 Jy. TheJ=6–5 12CO transition is at theedge of the 450µm filter; we expect its contribution to the ob-1 The James Clerk Maxwell Telescope is operated by the Joint As-tronomy Centre on behalf of the Particle Physics and Astronomy Re-search Council of the United Kingdom, the Netherlands Organisationfor Scientific Research and the National Research Council of CanadaLETTERL84 F.P. Israel et al.: Submillimetre maps of the edge-on galaxy NGC 891Fig. 1. Submillimeter contin-uum images at 13′′ resolution ofNGC 891 at 850µm (top left) and450µm (top right). Contour levelsare 15, 30, 50, 75, 100, 125, 150,175, 200, 250 and 300 mJy/beam(850µm), 500, 750, 1000, 1250,1500, 1750, 2000, 2250 and 2500mJy/beam (450µm). For compar-ison, the interferometricJ=1–012CO map obtained by Scovilleet al. (1993) is included atbottomleft, and an optical image atbottomright.served flux to be negligible. WithS1300 = 0.73 Jy (Gúelin et al.1993), and the IRAS flux densitiesS12 = 6 Jy,S25 = 7 Jy,S60= 80 Jy andS100 = 196 Jy (Wainscoat et al. 1987; Rice et al.1988; Young et al. 1989), the infrared/submillimeter spectrumof NGC 891 is now well-determined.Assuming a dust emissivity proportional toλ−2, we havefitted these flux-densities with various dust temperatures.(i). Longwards of 100µm, a good fit is obtained with asingledust temperatureTd ≈ 21 K. However, this fit leaves the mid-infrared fluxes and most of the 60µm flux to be explained. Theinterstellar dust models by D́esert, Boulanger& Puget (1990)suggest that virtually all of the 12µm flux and about 50% of the25µm flux is attributable to PAH particles. This implies a ratioF25/F60 = 0.1 for the remaining very-small grain (VSG) compo-nent which suggests an average VSG size somewhat larger thanin the Solar Neighbourhood, i.e. about 10 nm instead of 7 nm(see D́esert et al. 1993). We have used Eq. (2) from Guélin et al.(1993) to calculate the total hydrogen mass associated with theradiating dust (assumingb(Z/Z = 1.6) and findMH = 3×109M.(ii). Next, we assume that the 12–1300µm spectrum is describedby the dust model of D́esert et al. (1990), with big-grain dustLETTERF.P. Israel et al.: Submillimetre maps of the edge-on galaxy NGC 891 L85Fig. 2.Radial distribution of 850µm(left), J=1–0 12CO (center) emis-sion in NGC 891 and the ratio of ob-served CO/850µm emission (right).Profiles show emission at 13′′ reso-lution along the major axis.radiating at two different temperatures. With dust temperaturesTwd = 28±2 K andTcd = 18±1 K, close to the values assumed byGuélin et al. (1993), calculated fluxes reproduce the observedvalues to better than 20% over the full wavelength range. At850µm, we find flux-densities of 1.2 Jy (VSG), 0.55±0.15 Jy(warm dust) and 3.1±0.2 Jy (cold dust). From this, we obtainMH(warm) = 3.3 ± 1.1 × 108 M andMH(cold) = 3.6 ±0.4×109 M, or a total massMrmH(total) = 3.9±0.3×109M, slightly less than that estimated by Guélin et al. (1993).The ratioMH(warm)/MH(cold) = 0.1 ± 0.04 is higher thansurmised by Gúelin et al. (1993).Because the dust emission is a strong function of tempera-ture, the temperatures are relatively well-determined, depend-ing on the choice of one or two temperature components. Asis clear from the above, the choice of temperature model hasrelatrively little effect on the derived mass. Following Guélinet al. (1993), we adoptM(HI) = 2.5 × 109 M so thatM(H2) = 1.4 × 109 M. this yields a conversion factorX = N(H2)/ICO ≈ 7×1019 mol cm−2 ( K km s−1)−1, some-what less than found by Guélin et al. (1993) and Gerin& Phillips(1997) and substantially less than the canonical value. Assum-ing M(HI)tot = 3.7 × 109 (Allen & Sancisi, 1979; Rupen1991) and all H2 to be sampled, the overall ratio of molecular toatomic hydrogen is 0.38, the molecular gas fraction is 0.20 andwithin R = 16.5 kpc, the total gas fraction, including helium,is 5%. The first two values are similar to those found for theMilky Way and for dwarf irregular galaxies (Israel 1997).3.2. Radial distributionsThe structure of CO and dust continuum emission along themajor axis can be roughly described by three components: thevery compact central source at (a circumnuclear disk – Garcı́a-Burillo et al. 1992; Scoville et al. 1993; Sofue& Nakai 1993),a ‘molecular ring’ betweenR = 40′′ andR = 120′′ (2–6 kpc)and the extended disk traceable out toR = 200′′ (9 kpc). Giventhe edge-on configuration of NGC 891, the various peaks inthe emission are most likely spiral arms seen tangentially. Thisincludes the bright inner peaks which may well be due to brightspiral arms encircling the bulge rather than a ring.As Fig. 1 shows, the 850µm (and 450µm) continuum imageis virtually identical to theJ=1–0 CO image. This is in linewith the close resemblance of the 1.3 mm to theJ=2–112COdistribution (Gúelin et al. 1993) and indeed to the3P1 −3 P0CI distribution (Gerin& Phillips 1997) and the 6 cm continuumemission (see Allen et al. 1978). This is further illustrated bythe radial distributions shown in Fig. 2. Fig. 2c showsJ=1–0CO/850µm emission ratios of 1500 for the central peak and2000 for the molecular ring peaks (or inner spiral arm tangentialpoints), dropping to 1300 for the inner region inside the ‘ring’.Although decreasing signal-to-noise ratios introduce in-creasingly large relative errors in the outer disk ratios, thereappears to be a steady decrease in this ratio, in the outer diskdropping to 70% of the value atR = ±2′, just outside the molec-ular ring. The run of the 450/850/1300µm intensities with radiusprovides no support for significant dust temperature gradients.We propose that the high ratio of the molecular ring primarilyreflects a higher CO surface filling factor. If the inner spiral armscontain a rich population of dense CO clouds, this will quite nat-urally produce a relatively high CO filling factor (hence a highratio in Fig. 2c) at the arm tangential points. High line-of-sightCO optical depths are suggested by the12CO/13CO ratio of 2.5for the ring (Sakamoto et al. 1997).The decrease of the CO/850µm ratio with increasing dis-tance to the center parallels the radial drop in CO emissiv-ity shown by Scoville et al. (1993) and the even steeper de-crease of13CO emission (Sakamoto et al. 1997), resulting in a12CO/13CO gradient increasing from 5 just outside the molecu-lar ring to about 15 in the outer disk. The latter interpret this asa steep radial decrease of the dense gas fraction, so that we mayexpect the effects of photo-dissociation processes to increase ra-dially. This appears to be confirmed by the observed major axisdistributions of [CI] (Gerin& Phillips 1997) and [CII] (Maddenet al. 1994).The central region is quite different. Virtually all major axisprofiles exhibit a pronounced dip inside the molecular ring, ontowhich the compact emission from the circumnuclear disk is su-LETTERL86 F.P. Israel et al.: Submillimetre maps of the edge-on galaxy NGC 891perposed. This suggests that the regioninside the molecularring, i.e. the region withinR = 2 kpc corresponding to the bulgeof NGC 891, containslittle interstellar material except for thecircumnuclear disk of radius 600 pc. The emission observed to-wards the center mostly represents material in the molecular ringwith a smaller contribution from the outer disk. The cavity ap-pears to be filled by very hot (T = 3.6×106 K) X-ray-emittinggas, extending into the molecular ring (Bregman& Pildis 1994).West of the nucleus, an X-ray jet feature is seen. This, and thepresence of a nuclear disk suggests that NGC 891 has a mildlyactive nucleus. We have attempted to separate the nuclear con-tinuum emission peak from the more extended emission. We findfluxesS1300 = 55 mJy,S850 = 230 mJy andS450 = 2200 mJy.Corrected for CO line contributions (Garcı́a-Burillo et al. 1992;Gerin& Phillips 1997), these reduce to 40, 190 and 2200 mJyrespectively. Although these fluxes are relatively uncertain, theydefine a spectrum considerably steeper than that of the galaxyas a whole. If all dust radiates at the same temperature, we findTd > 50 K, at least twice as high as the whole galaxy underthe same assumption. A similar temperature (Tk > 40 K wasderived by Sakamoto et al. (1997) for the molecular gas in thenuclear disk.4. ConclusionsWe have mapped the submillimetre emission from the edge-ongalaxy NGC 891 at wavelengths of 850µm and 450µm. Theappearance of both maps is very similar to those obtained byothers at 1300µm and in the12CO line. The integrated infraredspectrum of NGC 891 shortwards of 100µm is dominated byemission from PAH particles and very small dust grains. Long-wards of 100µm emission from large dust grains dominates.The emission spectrum between 100µm and 1300µm can befitted equally well by a single population of grains at a temper-ature of 20 K, or a bimodal population of grains at temperaturesof 18 K and 27 K. In either case, we find a total hydrogen massof the order of 3×109 M, somewhat less than estimated byGuélin et al. (1993), but close enough to be consistent. As aconsequence, we confirm their conclusion that the CO-to-H2conversion ratio in NGC 891 is substantially less (by a factor of2–4) than the canonical values frequently used.The distribution of the various ISM components as afunction of distance to the center of the galaxy of these ISMcomponents can be described as roughly gaussian, with a strongcentral peak and several lesser peaks in the disk superposed.The central peak originates in a compact circumnuclear disk(R = 15′′ or 0.7 kpc; see Garcı́a-Burillo et al. 1992; Scovilleet al. 1993). The compact circumnuclear disk is embedded insoft X-ray emission and is at a temperature of at least 40 K,and probably significantly higher. The lesser peaks probablymark spiral arms seen tangentially. Although the submillimetrecontinuum emission from dust and the line emissions from COand CI show very similar distributions, their intensity ratiosvary across the galaxy. Towards the central region CO andespecially CI are relatively weak compared to the dust emissionat 850µm. In the socalled ‘molecular ring’ we find the reverse:CO and CI are relatively strong. At greater distances to thecenter, the CO to dust emission ratio decreases, whereas theCI to dust emission ratio increases. This behaviour may beexplained by an ‘empty’ bulge region, by greater CO andCI surface filling factors in tangential spiral arms, and by anincrease in photo-dissociation effects at greater radial distancesdue to an increasing paucity of dense molecular clouds.ReferencesAllen R.J., Baldwin J.E., Sancisi R., 1978 A&A62, 397Bregman J.N., Pildis R.A., 1994 ApJ420, 570De Vaucouleurs G., De Vaucouleurs A., Corwin H.C., 1976,SecondReference Catalogue of Bright Galaxies, University of Texas Press,Austin (RC2)Désert F.-X., Boulanger F., Puget J.L., 1990 A&A237, 215García-Burillo S., Gúelin M., Cernicharo J., Dahlem M., 1992A&A266, 21García-Burillo S., Gúelin M., 1995 A&A299, 657Gerin M., Phillips T.G., 1997 ESA-IRAM Conference on the FIRSTMission, in pressGuélin M., Zylka R., Mezger P.G., et al. 1993 A&A279, L37Holland W.S., Cunningham C.R., Gear W.K., et al. 1998SPIE, in pressHowk J.C., Savage B.D., 1997 AJ114 246Israel F.P., 1997 A&A328, 471Jenness T., Lightfoot J.F., 1998ASP Conf. Series***, in pressRice W., Lonsdale C.J., Soifer B.T., et al. 1988 ApJS68, 91Rupen M.P., 1991 AJ102, 48Sakamoto S., Handa T., Sofue Y., Honma M., Sorai K., 1997 ApJ475,134Sancisi R., Allen R.J., 1979 A&A74, 73Scoville N.Z., Thakkar D., Carlstrom J.E., Sargent A.I., 1993, ApJ404,L59Sofue Y., Nakai N., 1993 PASJ45, 139Sukumar S., Allen R.J., 1991 ApJ382, 100Wainscoat R.J., de Jong T., Wesselius P.R., 1987 A&A181, 225Young J.S., Xie S., Kenney J.D.P., Rice W.L., ApJS70, 699LETTER

Submillimetre maps of the edge-on galaxy NGC 891

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