ASTRONOMY & ASTROPHYSICS JANUARY II 2000, PAGE 193

SUPPLEMENT SERIES

Astron. Astrophys. Suppl. Ser. 141, 193–209 (2000)

Dense molecular gas in Seyfert galaxies?

S.J. Curran, S. Aalto, and R.S. Booth

Onsala Space Observatory, Chalmers University of Technology, S-439 92 Onsala, Swedene-mail: sjc@oso.chalmers.se

Received March 2; accepted October 22, 1999

Abstract. We have used the 20 m Onsala and 15 m SESTtelescopes to observe the CO J = 1 → 0 (simultaneouslywith CO J = 2 → 1 at SEST) and HCN J = 1 → 0 (si-multaneously with CS J = 3→ 2 at SEST) transitions in20 Seyfert galaxies. The sample consists of the 18 galaxiesdetected in CO J = 1→ 0 by Heckman et al. (1989) plus2 which were not detected in their survey. We have suc-cessfully detected all of the galaxies in the CO transitionand 13 in HCN of which NGCs 1667, 2273, 5033, 5135,6814 and Mrk 273 are new detections. For the galaxiesin which the beam-width exceeds ≈ 10 kpc we find thatLHCNLCO

≈ 1/6, i.e. a global ratio similar to that of ultra-luminous infrared galaxies and over 10 times the ratio fornormal spiral galaxies. This implies that LFIRLCO (LFIR ∼1011 L� Seyferts) ≈ 10LFIRLCO (normal spirals) and we be-lieve that the far infrared fluxes in our sample arise fromstar-burst activity, although we cannot rule out the pos-sibility of a contribution from active galactic nuclei.

Key words: galaxies: Seyfert–galaxies: abundances —galaxies: ISM

1. Introduction

Activity in the centres of galaxies ranges from extremelycompact active galactic nuclei (AGN) powered by mass ac-cretion onto a black hole (Begelman et al. 1984; Rees 1984)to extended star-bursts of more modest power. In someextreme ultra-luminous infrared galaxies (ULIRGs)1, e.g.

Send offprint requests to: S. Curran? Based on results collected at the European Southern

Observatory, La Silla, Chile and Onsala Space Observatory,Sweden.

1 Where the far infrared luminosities of LFIR > 1011 L� give

FIR to molecular gas luminosity ratios which are an order ofmagnitude higher than for normal spiral galaxies Soifer et al.1984; Sanders et al. 1986; Solomon & Sage 1988).

in Arp 220 the star-powered luminosity may rival that ofa compact AGN. Interest in the connection between thestar-burst phenomenon and the central AGN has beenstimulated by the fact that in galaxies with dominantAGNs, there may be significant amounts of circumnu-clear gas, e.g. NGC 1068 (e.g. Tacconi et al. 1994) andCentaurus A (e.g. Rydbeck et al. 1993). Gas fuels the star-forming activity and responds both physically and chemi-cally to the ultraviolet radiation and supernovae producedin a young population of massive stars. Dynamically,gas clouds are highly dissipative, relative to the starsand thus sink readily towards the centres of galacticgravitational potentials, possibly fuelling the black hole.Indeed, recent VLBI observations of molecular masers(e.g. NGC 4258) have revealed a very close connection be-tween molecules and accretion discs surrounding galacticblack holes Miyoshi et al. 1995. Also, recent interferomet-ric observations of the type 2 Seyfert galaxy NGC 1068have shown that the HCN gas is much more confined thanthe CO (e.g. Tacconi et al. 1996), and all interferometricstudies (e.g. Helfer & Blitz 1997) show the HCN to bemainly concentrated towards the centre. We are thereforeconfident that the HCN emission will trace gas in the nu-clear regions. In addition to this, Solomon et al. (1992)(hereafter SDR92) suggest, from their HCN observations,that (galactic) IR luminosities are a consequence of starformation, rather than of an AGN. In order to comparehow Seyferts differ from non-Seyferts in their gas content,we investigate the CO, HCN and infrared luminosities ofthe 18 galaxies detected in CO 1 → 0 by Heckman et al.(1989) with the NRAO 12 m. We chose to observe thesegalaxies because:

1. They form a good sample of Seyfert galaxies;2. To verify the results of Heckman et al. (1989), who

find higher molecular gas abundances in type 2 thanin type 1 Seyferts. This issue is addressed in Curran(2000a).

194 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Table 1. Beam sizes and efficiencies

Transition ν [GHz] HPBW [′′] ηmbOSO SEST OSO SEST

HCN 1→ 0 89 44 57 0.59 0.75CO 1→ 0 115 33 45 0.50 0.70CS 3→ 2 147 – 34 – 0.66CO 2→ 1 230 – 22 – 0.50

In addition to these results, in this paper we also presentthe CO 2 → 1 and CS 3 → 2 results from the Southernpart of this sample.

2. Observations

We have observed the CO 1→ 0 (115 GHz) and HCN 1→0 (88.6 GHz) lines in a sample of 20 Seyfert galaxies.The Southern sample was observed in December 1997 andOctober 1998 with the 15 m SEST2 at La Silla, Chile.Since the 100 GHz and 115 GHz can be used simultane-ously with the 150 GHz and the 230 GHz receivers, re-spectively, we also observed CS 3→ 2 (147 GHz) and CO2 → 1 (230 GHz). All receivers were tuned to the single-band mode and typical system temperatures, on the T ∗A-scale, were 150 K at 89 GHz, 200 to 300 K at 115 GHz,200 K at 150 GHz and 300 K at 230 GHz. The HPBWs are57′′, 45′′, 34′′, and 22′′, respectively. The backends wereacousto-optical spectrometers with 1440 channels and achannel width of 0.7 MHz. We used dual-beam switchingwith a throw of about 12′ in azimuth, with pointing er-rors being typically 3′′ rms on each axis. The intensity wascalibrated using the chopper-wheel method. For the firstobserving run the weather was excellent, and only the re-moval of linear baselines was required. However, two of thesources, NGCs 6814 and 7130, were within the sun-limitand, after the application for more time, were observedduring the second run under similarly good conditions.

The Northern sample was originally observed inFebruary 1998 with the 20 m telescope at Onsala SpaceObservatory (OSO). During this session, however, theweather was unfavourable and more time was awardedfor the following November. Both the CO 1 → 0 andHCN 1 → 0 transitions were observed with the SIS100 GHz receiver. The HPBWs are 33′′ and 44′′, respec-tively. The backend was a filter-bank with a bandwidthof 512 MHz and a channel separation of 1 MHz. Weused a similar dual-beam switching as the SEST obser-vations and obtained similar pointing errors. Althoughfar from ideal for most of the session, the weather wasa great improvement over the previous run, with HCN

2 The Swedish-ESO Sub-millimetre Telescope is operatedjointly by ESO and the Swedish National Facility for RadioAstronomy, Onsala Space Observatory, Chalmers Universityof Technology.

being observed when the weather was poorest. Typicalsystem temperatures were around 500 K for CO. Thisis considerably worse than the optimal 300 K, but giventhe improved receivers, is comparable with the systemtemperatures of Heckman et al. (1989). Some of theNorthern sources were re-observed in April 1999 when theexcellent weather gave system temperatures of ≈ 230 Kand ≈ 350 K for the HCN and CO lines, respectively. Aswith the SEST data, only linear baselines were removed.

3. Results

Upon comparison of the CO detections (Figs. 1 and 2)with those from previous surveys (Heckman et al. 1989;Maiolino et al. 1997; Papadopoulos & Seaquist 1998), wefind that, for the most part, we have similar results withthe following exceptions: CO 1 → 0 in NGC 1365 issimilar to that of Maiolino et al. (1997); Papadopoulos& Seaquist (1998), although more symmetric than thedetection of Heckman et al. (1989). Our 2 → 1 de-tection is not as symmetric as that of Papadopoulos &Seaquist (1998) and since the profile of Sandqvist et al.(1995) is also quite symmetrical we attribute our asym-metry to a pointing error. In NGC 1667 the profile isidentical in shape to that of Heckman et al. (1989) butreflected with the peak occurring at the minimum, asopposed maximum velocity. In the case of Maiolino et al.(1997); Papadopoulos & Seaquist (1998), the peak also oc-curs at the maximum although these are somewhat moresymmetric, indicating that we have a pointing error. NGC5033; our detection appears to be somewhat skewed com-pared with those of Heckman et al. (1989); Maiolino et al.(1997); Papadopoulos & Seaquist (1998), although of asimilar integrated intensity. NGC 5548; undetected byHeckman et al. (1989) and not observed by Papadopoulos& Seaquist (1998), our detection has a somewhat widerprofile than that of Maiolino et al. (1997). The results ofthe observations are summarised in Table 23, where theupper limits for HCN 1 → 0 and 3 → 2 are over thevelocity range of the CO 1→ 0 detections.

The luminosities according to the integrated intensityover the projected beam area are shown in Table 34.

Testing our global results, initially we obtained a fairlinear fit by plotting lnLCO against lnLHCN for the samplegalaxies with recessional velocities exceeding 4000 km s−1.This corresponds to a HPBW of ≈ 12 kpc (at SEST,

3 Imb = I∗A/ηmb, where I

∗A is the velocity integrated

Rayleigh-Jeans antenna temperature above the atmosphereand ηmb is the measured main-beam efficiency of the telescope.

4 Again Sy refers to the Seyfert type but here and in Table 4we use the classification scheme of Meurs & Wilson (1984);Edelson (1987); Osterbrock & Shaw (1988); Heckman et al.(1989); Maiolino & Rieke (1995) where types 1, 1.2 and 1.5constitute type 1 Seyferts and types 1.8, 1.9 and 2 constitutetype 2 Seyferts.

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 195

Fig. 1. The CO J = 1 → 0 results. Here and in Figs. 2, 3 and 4 the intensity scale is T ∗A and the velocity resolution is shownabove each scan

196 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 1. continued

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 197

Fig. 1. continued

198 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 1. continued

Table 2. The 18 Seyfert galaxies detected in CO 1→ 0 by Heckman et al. (1989) hbw+89 plus two non-detections (†, selected inorder to provide a more balanced sample of type 1 and type 2 Seyferts) which are observable from OSO and SEST (Teles.). Inthis table Sy refers to the (intermediate; Osterbrock 1981) Seyfert type and v is the heliocentric radial velocity (NASA/IPACExtragalactic Database). The final columns refer to the main-beam brightness temperature, Imb, [K km s

−1] measured forthe corresponding transition, where the results have been rounded to two figures (because of ηmb) and the errors and upperlimits are according to 1σ (defined by the noise). The “New” column indicates whether these are first time detections in theHCN 1→ 0 transition for these galaxies

Galaxy Sy v [km s−1] Teles. CO 1→ 0 CO 2→ 1 HCN 1→ 0 New CS 3→ 2

NGC 0034 2 5931 SEST 10(1) 18(1) 1.6(0.2) < 1NGC 0931 1.5 5001 OSO 0.5(0.4) – < 0.8 –NGC 1068 2 1134 OSO 86(3) – 11(1) –NGC 1365 1.8 1636 SEST 97(1) 110(1) 6.0(0.1) 6.3(0.3)NGC 1667 2 4547 OSO 15(1) – 3.5(0.5)

√–

UGC 03374/MCG 08-11-011 1.5 6141 OSO 2.8(0.3) – < 0.2 –NGC 2273 2 1840 OSO 3.2(0.3) – 0.5(0.3)

√–

Mrk 10/UGC 04013 1 8770 OSO 1.0(0.3) – < 0.3 –NGC 4593† 1 2698 SEST 1.7(0.4) 2.2(0.3) – –Mrk 231/UGC 08058 1 12651 OSO 10(1) – 1.0(0.2) –NGC 5033 1.9 875 OSO 32(2) – 1.7(0.3)

√–

Mrk 273/UGC 08696 2 11318 OSO 5(1) – 3(1)√

–NGC 5135 2 4112 SEST 18(1) 25.8(0.6) 0.65(0.07)

√< 1

NGC 5347 2 2336 OSO 1.3(0.5) – < 0.3 –NGC 5548† 1.5 5149 SEST 2.4(1.2) 7.0(0.4) – –Arp 220 2 5314 OSO 12(1) – 2.4(1.0) –NGC 6814 1.5 1563 SEST 7.2(0.1) 3.9(0.1) 0.45(0.08)

√< 1

NGC 7130/IC 5135 2 4842 SEST 17(1) 26(1) 0.7(0.1) < 1NGC 7172 2 2603 SEST 5.1(0.9) 7.3(0.3) – –NGC 7469 1.2 4889 OSO 22(1) – 1.7(0.4) –

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 199

Fig. 2. The CO J = 2→ 1 results

200 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 2. continued

Table 3. The observed luminosities of the sample. LCO 1→0, LCO 2→1, LHCN 1→0 and LCS 3→2 refer to the luminosity over theHPBW for each respective transition, again with 1σ errors and upper limits [×103 K km s−1 kpc2]. Like Heckman et al. (1989)we have calculated the physical beam area by using the distances given by heliocentric radial velocities (Table 2) and assumingthe same Hubble parameter as before, except in the case NGC 1365 where the assumed distance of 20 Mpc, to the Fornaxcluster, is used. The

∫LCO 1→0 refers to the global CO 1 → 0 luminosity [×103 K km s−1 kpc2]: aPlanesas, Gomez-Gonzalez

& Martin-Pintado (1989) (over a partial map spanning (120′′ × 120′′), bYoung et al. (1995), cSandqvist et al. (1995) (over204′′ × 164′′) and dBryant & Scoville (1996). Although apparently high, the CO luminosity in NGC 1068 (Young et al. 1995)is similar to that obtained from a large scale map of the Circinus galaxy (Curran 2000b; Curran et al. 2000). LFIR [10

10 L�]refers to the far infrared luminosity computed using the FIR flux (Lonsdale et al. 1985; Heckman et al. 1989). The CO 2 → 1(CO intensity ratios) and CS results are discussed in Curran (2000b)

Galaxy Sy LCO 1→0∫LCO 1→0 LCO 2→1 LHCN 1→0 LCS 3→2 LFIR

NGC 0034 2 2.3± 0.1 – 0.28± 0.01 0.56 ± 0.07 < 0.1 14.3NGC 0931 1 0.08± 0.04 – – < 0.1 – 2.1NGC 1068 2 0.42± 0.01 >∼ 3.8a/23± 8b – 0.09 ± 0.01 – 7.4NGC 1365 2 1.5± 0.1 5.3c 0.48 0.16 ± 0.03 0.063 ± 0.006 6.8NGC 1667 2 1.18± 0.08 – – 0.45 ± 0.06 – 4.2UGC 03374 1 0.40± 0.04 – – < 0.05 – 2.9NGC 2273 2 0.041 ± 0.003 0.38 ± 0.07b – 0.008 ± 0.004 – 0.66Mrk 10 1 0.3± 0.1 – – < 0.1 – 2.7NGC 4593 1 0.08± 0.02 – 0.025 ± 0.003 – – 0.79Mrk 231 1 6.0± 0.6 5d – 1.0 ± 0.2 – 128NGC 5033 2 0.093 ± 0.006 10± 3b – 0.014 ± 0.002 – 0.53Mrk 273 2 2.4± 0.4 – – 2.4 ± 0.8 – 73NGC 5135 2 2.0± 0.1 – 0.69± 0.02 0.11 ± 0.01 < 0.06 9.0NGC 5347 2 0.03± 0.01 – – < 0.01 – 0.28NGC 5548 1 0.4± 0.2 – 0.29± 0.02 – – 0.86Arp 220 2 2.6± 0.1 – – 0.4 ± 0.2 – 84NGC 6814 1 0.118 ± 0.002 0.38 ± 0.06b 0.015 0.011 ± 0.002 < 0.01 0.66NGC 7130 2 2.6± 0.2 – 0.96 0.16 ± 0.02 < 0.03 11.9NGC 7172 2 0.23± 0.04 – 0.19± 0.01 – – 1.2NGC 7469 1 2.00± 0.09 1.9± 0.3b – 0.25 ± 0.07 – 18.2

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 201

Fig. 3. The HCN J = 1→ 0 results

202 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 3. continued

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 203

Fig. 3. continued

204 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 4. The CS J = 3→ 2 results

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 205

Table 4. The approximate observed CO 1 → 0/HCN 1 → 0luminosity ratios

Galaxy Sy LCOLHCN Published intensity ratios

NGC 0034 2 4 –NGC 0931 1 >∼ 1 –NGC 1068 2 5 9 (Helfer & Blitz 1993)NGC 1365 2 9 –NGC 1667 2 3 –UGC 03374 1 >∼ 4 –NGC 2273 2 5 –Mrk 10 1 >∼ 3 –Mrk 231 1 6 4.2 (SDR92)NGC 5033 2 7 –Mrk 273 2 1 –NGC 5135 2 18 –NGC 5347 2 >∼ 3 –Arp 220 2 7 8 (SDR92)NGC 6814 1 11 –NGC 7130 2 16 13 (Aalto et al. 1995)NGC 7469 1 4 10 (Bryant 1997)

≈ 10 kpc at OSO)5, beyond which there is expected toexist little molecular gas (Maiolino et al. 1997). PlottinglogLCO against logLHCN for all of the sample, how-ever, Fig. 5, we see that we obtain the same linear re-lationship between the CO6 and HCN luminosities. From

Fig. 5. logLCO versus logLHCN (units as in Table 3, i.e. ×103 Kkm s−1 kpc2) for all the detections. From these we obtaina ratio of LCO/LHCN = 6

+5−3 and using only sources with

v >∼ 4000 km s−1, we obtain a ratio of LCO/LHCN = 6+7−3

the intercept of the log plot, we estimate the LHCN to5 Assuming a Hubble parameter of H0 = 75 km s

−1 Mpc−1.This value is used in all other size estimates throughout thepaper.

6 From now on, unless otherwise stated, when CO is writtenit refers to the 1→ 0 transition.

LCO ratio for the sample to be similar to the valuedetermined for the ULIRGs (SDR92; Bryant 1997). It isinteresting that this relationship holds true for both thedistant (v >∼ 4000 km s−1) and near-by (v ∼ 4000 km s−1), resulting in a lowerLHCN/LCO compared to the near-by galaxies. Typicallythe HCN/CO ratio in the disks of galaxies is greater than40 (e.g. Helfer & Blitz 1993; Kuno et al. 1995), however,here both near-by and distant sources obey the same LCO

7 Since the HCN may possibly be more confined than theCO; within 1 kpc (the HPBW at v ≈ 1000 km s−1) (Downeset al. 1992; Nguyen et al. 1992; Henkel et al. 1994; Tacconiet al. 1996).

206 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

to LHCN correlation, and so this may (partly) be an effectof a highly centralised molecular emission. From interfer-ometric studies of the CO distribution we know this to betrue for FIR luminous objects such as Mrk 231, Mrk 273,Arp 220 and NGC 7469, but also less FIR bright galaxiessuch as NGC 0034, NGC 1667 and NGC 7130 have verybright HCN with respect to their CO emission. It is possi-ble that we are missing CO emission at radii greater than6 kpc, thus getting an incorrect global LHCN/LCO ratio,but the more likely explanation is that the CO emissionis also highly concentrated in these galaxies. This confine-ment of the CO is another similarity between our sampleand ULIRGs (Scoville et al. 1991), as opposed to normalspirals where the HCN is much more centralised than theCO (Helfer & Blitz 1993).

Comparing the global with our measured CO luminosi-ties of the near-by sample (Table 3), it appears that theCO extends well beyond ≈ 3 kpc in the near-by sample(based on NGC 2273), while the HCN is expected to beconfined to within 1 kpc (Sect. 3), leading us to believethat we have sampled the majority of the HCN in thenear-by galaxies. Why should we have different CO distri-butions for the distant and near-by galaxies? Examiningthe FIR luminosities (Table 3), the mean values of the lu-minosities are LFIR ≈ 30 1010 L� for v > 4000 km s−1and LFIR ≈ 2 1010 L� for v < 4000 km s−1. In the caseof ULIRGs, the high FIR luminosity is an indicator ofa high CO concentration (Bryant 1997), and so perhapsthere exists a selection effect at play, in which our distantsources comprise mainly of galaxies suffering from littleCO contamination in the galactic disk.

The FIR/HCN luminosity plot, Fig. 6, shows a simi-lar correlation to that of SDR92, which holds over a largerange8 of LFIR and LHCN. It appears that, in our Seyfertsample, as in the ULIRGs, the FIR to HCN luminosity issimilar to that of normal spiral galaxies, i.e.LFIRLHCN

(Seyferts) ≈ LFIRLHCN

(normal spirals) (1)

and since normal spirals have LHCN/LCO ≈ 1/80(SDR92), from previously

LHCNLCO

(distant Seyferts) ≈ 10LHCNLCO

(normal spirals), (2)

and combining these equation givesLFIRLCO

(distant Seyferts) ≈ 10LFIRLCO

(normal spirals). (3)

As a further check, in order to account for the distancesand the extents in our sample, as SDR92, we plottedLFIR/LCO against LHCN/LCO, Fig. 7.

From this we can see that the normalisa-tion does not significantly alter the linear fit, al-though three of the sample deviate further fromthe line. In any case, from the fit we also find

8 From normal spirals to ULIRGs (SDR92; Helfer & Blitz1993).

Fig. 7. logLFIR [L�(K km s−1 pc2)−1] versus logLHCN nor-

malised by the CO luminosity for all the detections [no units]

LFIR ≈ 600LHCN L�(K km s−1 pc2)−1 (normalisedby the CO luminosity)9 which again agrees well with theresult of SDR92 (LFIR ≈ 750LHCN L�(K km s−1 pc2)−1).

With regard to what these results entail, we interpretthe high LHCN/LCO ratio in our sample of Seyferts aslargely an effect of a high degree of central concentration ofthe gas10. The steep central potential and high gas surfacedensities result in large gas pressures which could force thebulk of the molecular mass to reside in high density gas.This should be true regardless of what activity dominatesthe FIR emission from the galaxy. SDR92 suggest thatthe LHCN/LFIR correlation means that the HCN emissiontraces star forming cores, which are responsible for theIR emission, i.e. the ULIRGs are powered by star-bursts.For several of the Seyferts in our sample, however, at least50% of the FIR emission may come from the AGN activity,even if a star-burst also contributes to the total luminosity:Kohno et al. (1999) suggest that Seyferts with jets haveparticularly bright HCN emission and that the dense gas,which is a component of a large-scale obscuration11 con-fines the jet (Antonucci & Miller 1985; Wilson et al. 1988;Tadhunter & Tsvetanov 1989; Wilson & Tsvetanov 1994;Baker & Scoville 1998). Also, Bryant (1997); Kohno et al.(1999) discuss the connection between very high HCN/COratios and these nuclei and point out that Seyfert nucleiare associated with the lowest published HCN/CO ratios

9 We achieve the same value using only galaxies with v >∼4000 km s−1, thus again indicating that the CO may be cen-tralised.10 For example, in NGC 1068 the HCN/CO intensity ratiois 0.6 in the central few hundred pc (Helfer & Blitz 1995)cf. ≈ 0.2 within 1.2 kpc (this work) and 0.1 out to 2.2 kpc(Helfer & Blitz 1993).11 E.g. Shlosman, Begelman & Frank (1990); Friedli &Martinet (1993); Shaw et al. (1993); Wilson & Tsvetanov(1994); Maiolino & Rieke (1995); Fosbury et al. (1999);Conway (1999); Curran (2000a).

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 207

so far. The only similarity between these objects is thatthey host broad-line AGN, and similarly, in our sample,NGC 1667 and Mrk 273 both have extremely bright HCNwith respect to CO, yet they differ by over an order ofmagnitude in their FIR luminosities.

An LHCN/LCO ratio of 1/6 is at the lower end of whatis typically found even for galactic nuclei. For near-bygalaxies, standard HCN/CO ratios appear to range be-tween 1/15 and 1/5 on scales smaller than a kiloparsec(e.g. Helfer & Blitz 1995; Aalto et al. 1995), and so a highdegree of gas concentration may not be a sufficient expla-nation for the high ratio we observe. The “extra” brightHCN emission may be an additional effect of extreme gasexcitation and/or unusual abundance effects.

Heckman et al. (1989) state that the CO and far in-frared properties differ between the two Seyfert classes,although Seyferts as a class exhibit similar LCO to LFIRratios to non-Seyferts. In order to test this, we plot the logof the LCO/LFIR ratio versus the log of LFIR (Table 3),and find that, not unexpectedly, all of our detections lie inthe same range as Fig. 6 of Heckman et al. (1989), Fig. 8.From these results we calculate the mean ratio, at a 90%confidence level, to be

log(LCOLFIR

)= −8.0± 0.2 for v >∼ 4000 km s−1 and, (4)

log(LCOLFIR

)= −8.0± 0.1 for all the detections. (5)

cf. −7.76 for Seyferts and −7.84 for non-Seyferts(Heckman et al. 1989).

Fig. 8. logLCO/LFIR [K km s−1 kpc2 L−1� ] versus logLFIR [L�]

This is the same range over which normal spiral galax-ies are distributed (Young et al. 1984; Sanders & Mirabel1985; Stark et al. 1986; Young et al. 1986). If we referto Fig. 6c of Sanders & Mirabel (1996), our sample is

located at around12 LFIR ∼ 1011 L� ≈ 50M(H2) M� for star-burst galax-ies. So we find that for similar values of LFIR, that oursample has about double the LFIR/LCO ratio of normalspiral galaxies and a similar ratio to the more moderatestar-burst galaxies (LFIR ∼ 1011 − 1012 L�)14.

As seen from Fig. 8 and Fig. 6c of Sanders & Mirabel(1996), the Seyfert sample follows the same trend forLCO/LFIR to decrease over an order of magnitude as de-fined by normal spirals to ULIRGs, with the Seyferts beinglocated in between these two extremes. This result couldbe caused by either:

1. The FIR luminosity arising from young stars, in whichcase the HCN would be tracing dense star-formingcloud cores15.

2. The FIR luminosity arising from something other, i.e.an AGN.

In the first case we would expect the LFIR/LHCN ratioto be fairly independent of the FIR luminosity of thegalaxy and in the latter case we would expect LFIR/LHCNto increase, i.e. a further FIR contribution in additionto that from star formation coming into play. Plottingthe LHCN/LFIR ratio versus LFIR (in a similar mannerto Fig. 8), Fig. 9, we see that the HCN/FIR ratio maywell decrease with the FIR luminosity16 thus indicatingthat the latter scenario may be the case, although withthese statistics this is far from conclusive. In support ofthe non-AGN argument, a constant (on average) value ofLHCN/LFIR may be feasible (although the least squareslinear fit does give a non-zero gradient, Fig. 9).

This is supported by Figs. 6 and 7 where a simpleline does provide a good fit; a decreasing LHCN/LFIRratio would demand a (slight on a log−log plot) curvein these figures. Also from Figs. 6 and 7, there appearsto be no FIR to HCN excess cf. normal gas rich galax-ies and ULIRGs (SDR92), although their fit may besignificantly affected by the high values of LFIR for12 Assuming that the molecular hydrogen mass,M(H2)[M�] = 4.6LCO [K km s

−1 pc2] (Scoville & Sanders1987).13 Since this ratio of LFIRLCO (Seyferts) ≈ 2

LFIRLCO

(normal spirals)is based on the whole sample, and not just the distant FIRbright Seyferts, it is somewhat less than that determined byEq. (3).14 The extreme cases; Mrk 231, Mrk 273 and Arp 220, whichare located in the LFIR ∼ 1012 L� ≈ 70M(H2) M� range, arealso considered to be ULIRGs (e.g. SDR92; Downes & Solomon1998).15 Estimating the CO 2→ 1 source sizes from the radio con-tinuum (e.g. Allen 1992; Bajaja et al. 1995) we find that theCO emission seems to be sub-thermally excited, which wouldsuggest a less dense cloud component in addition to that tracedby the HCN. This is discussed further in Curran (2000b).16 Over a factor of two according to the linear fit fromLFIR ∼ 1010 − 1012 L�.

208 S.J. Curran et al.: Dense molecular gas in Seyfert galaxies

Fig. 9. logLHCN/LFIR [K km s−1 kpc2 L−1� ] versus logLFIR

[L�]. The least squares linear fit is shown

Mrk 231 and Arp 220 which are also considered to beSeyferts17, and there does remain the possibility thatthe HCN may be associated with the gas obscuring theAGN rather than dense star-forming cores. The lowCS 3 → 2/HCN 1 → 0 luminosity ratios (< 0.5) inthe (Southern) sample may support this result (Curran2000b).

A similar correlation for both Seyfert and star-burstgalaxies is also found between LFIR and LHα (Gu et al.1997), and this as well as other Seyfert samples, whichutilise FIR luminosities (e.g. in comparison with Lblue;Whittle 1992; Gu et al. 1999 and the radio continuum;Roy et al. 1998), lead to the conclusion that the FIR fluxis thermal in origin for most Seyfert galaxies.

5. Summary

We have detected CO 1→ 0 in all and HCN 1→ 0 in 13 ofthe 20 Seyfert galaxies observed. The detections includesix new detections in HCN for NGCs 1667, 2273, 5033,Mrk 273, NGC 5135 and NGC 6814. Similar to ULIRGs,for the distant sources we find a global HCN to CO lu-minosity ratio of ≈ 1/6 which is an order of magnitudegreater than in normal spiral galaxies and, again as in thecase of ULIRGs (Scoville et al. 1991), this implies that theCO appears to be confined to the nuclear region (otherwisean even lower luminosity ratio is obtained). The CO 1→0HCN 1→0intensity ratios obtained are significantly lower than thosein moderate infrared luminosity (LFIR < 1011 L�) galax-ies. Also, perhaps because of the centralisation, our sampledoes not appear to suffer from the same selection effect at17 In fact ∼ 10% of ULIRGs are considered to be type 1Seyferts or IR QSOs (Deng et al. 1997) and up to ≈ 80% maybe radio quiet AGNs (i.e. Seyferts and LINERs) (Wu et al.1998) and up to ≈ 30% may be powered by AGNs in general(Genzel et al. 1998).

play, due to the difficulty in detecting HCN in such distantgalaxies, cf. CO 1→0HCN 1→0 > 20 at velocities >∼ 4000 km s−1for moderate infrared luminosity galaxies (Bryant 1997),although a selection effect may be responsible for the highCO/HCN ratios here.

We also find that the HCN to FIR luminosity ratiois similar to that for normal spiral galaxies through toULIRGs. This result implies that there is no excess in thefar infrared continuum (which could be due to an AGN) inour sample, although if the denser gas tracers form part ofthe obscuration rather than star-forming clouds, the FIRto HCN ratio would imply an additional FIR source. Inany case, as the results stand there is no overwhelmingevidence to invoke a contribution from an AGN.

Acknowledgements. We are grateful to the anonymous ref-eree for his helpful comments and we would like to thankPer Bergman, Lars E.B. Johansson and the SEST andOnsala telescope operators. This research has made useof the NASA/IPAC Extragalactic Database (NED) whichis operated by the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under contract with the NationalAeronautics and Space Administration.

References

Aalto S., Booth R.S., Black J.H., Johansson L.E.B., 1995, A&A300, 369

Allen R.J., 1992, ApJ 399, 573Antonucci R.R.J., Miller J.S., 1985, ApJ 297, 621Bajaja E., Wielebinski R., Reuter H.P., Harnett J.I., Hummel

E., 1995, A&AS 114, 147Baker A.J., Scoville N.Z., 1998, Am. Astron. Soc. Meet. 192,

3605Begelman M.C., Blandford R.D., Rees M.J., 1984, Rev. Mod.

Phys. 56, 255Bryant P.M., Scoville N.Z., 1996, ApJ 457, 678Bryant P.M., 1997, Ph.D. Thesis. California Institute of

TechnologyConway J., 1999, HI Absorption from a Circumnuclear TORUS

in the Hidden Quaser Cygnus A. In: Carilli C., Radford S.,Menton K., Langston G. (eds.). Highly Redshifted RadioLines, ASP Conf. Ser., p. 259

Curran S.J., Koribalski B., Haynes R.F., Krause M., JohanssonL.E.B., 2000, A&A (in preparation)

Curran S.J., 2000a, A&A (in preparation)Curran S.J., 2000b, Ph.D. Thesis. Chalmers University of

Technology (in preparation)Deng Z.G., Xia X.Y., Wu H., 1997, in IAU Symposia, p. 110Downes D., Solomon P.M., 1998, ApJ 507, 615Downes D., Radford S.J.E., Guilloteau S., et al., 1992, A&A

262, 424Edelson R.A., 1987, ApJ 313, 651Fosbury R.A.E., Vernet J., Villar-Martin M., et al., 1999,

Optical Continuum Structure of Cygnus A. In: Best P.,Lehnert M. (eds.). KNAW colloqium on: The Most DistantRadio Galaxies. Reidel, Amsterdam (in press)

Friedli D., Martinet L., 1993, A&A 277, 27Genzel R., Lutz D., Sturm E., et al., 1998, ApJ 498, 579

S.J. Curran et al.: Dense molecular gas in Seyfert galaxies 209

Gu Q.S., Huang J.H., Su H., Shang Z.H., 1997, A&A 319, 92Gu Q.S., Huang J.H., Ji L., 1999, Astrophys. Space Sci. 260,

389Heckman T.M., Blitz L., Wilson A.S., Armus L., Miley G.K.,

1989, ApJ 342, 735Helfer T.T., Blitz L., 1993, ApJ 419, 86Helfer T.T., Blitz L., 1995, ApJ 450, 90Helfer T.T., Blitz L., 1997, ApJ 478, 233Henkel C., Whiteoak J.B., Mauersberger R., 1994, A&A 284,

17Kohno K., Kawabe R., Vila-Vilaró B., 1999, NMA Survey of

CO and HCN Emission from Nearby Active Galaxies. In:The Proceedings of the 3rd Cologne-Zermatt Symposium,“The Physics and Chemistry of the Interstellar Medium”,p. 2251

Kuno N., Nakai N., Handa T., Sofue Y., 1995, PASJ 47, 745Lonsdale C.J., Helou G., Good J.C., Rice W., 1985, Cataloged

Galaxies and Quasars Observed in the IRAS Survey. JetPropulsion Laboratory, Pasadena

Maiolino R., Rieke G.H., 1995, ApJ 454, 95Maiolino R., Ruiz M., Rieke G.H., Papadopoulos P., 1997, ApJ

485, 552Meurs E.J.A., Wilson A.S., 1984, A&A 136, 227Miyoshi M., Moran J., Herrnstein J., et al., 1995, Nat 373, 127Nguyen Q., Jackson J.M., Henkel C., Truong B., Mauersberger

R., 1992, ApJ 399, 521Osterbrock D.E., Shaw R.A., 1988, ApJ 327, 89Osterbrock D.E., 1981, ApJ 249, 462Papadopoulos P.P., Seaquist E.R., 1998, ApJ 492, 521Planesas P., Gomez-Gonzalez J., Martin-Pintado J., 1989,

A&A 216, 1Rees M.J., 1984, ARA&A 22, 471Roy A.L., Norris R.P., Kesteven M.J., Troup E.R., Reynolds

J.E., 1998, MNRAS 301, 1019

Rydbeck G., Wiklind T., Cameron M., et al., 1993, A&A 270,L13

Sanders D.B., Mirabel I.F., 1985, ApJ 298, L31Sanders D.B., Mirabel I.F., 1996, Ann. Rev. Astr. Ap. 34, 749Sanders D.B., Scoville N.Z., Young J.S., et al., 1986, ApJ 305,

L45Sandqvist A., Jörsäter S., Lindblad P., 1995, A&A 295, 585Scoville N.Z., Sanders D.B., 1987, H2 in the Galaxy. In:

Hollenbach D.J., Jr. H.A.T. (eds.), Interstellar Processes.Reidel, Dordrecht, p. 21

Scoville N.Z., Sargent A.I., Sanders D.B., Soifer B.T., 1991,ApJ 366, L5

Shaw M.A., Combes F., Axon D.J., Wright G.S., 1993, A&A273, 31

Shlosman I., Begelman M.C., Frank J., 1990, Nat 345, 679Soifer B.T., Rowan-Robinson M., Houck J.R., et al., 1984, ApJ

278, L71Solomon P.M., Sage L.J., 1988, ApJ 334, 613Solomon P.M., Downes D., Radford S.J.E., 1992, ApJ 387, L55Stark A.A., Knapp G.R., Bally J., et al., 1986, ApJ 310, 660Tacconi L.J., Genzel R., Blietz M., et al., 1994, ApJ 426, L77Tacconi L.J., Gallimore J.F., Schinnerer E., Genzel R., Downes

D., 1996, BAAS 189, 906Tadhunter C., Tsvetanov Z., 1989, Nat 341, 422Whittle M., 1992, ApJ 387, 121Wilson A.S., Tsvetanov Z.I., 1994, AJ 107, 1227Wilson A.S., Ward M.J., Haniff C.A., 1988, ApJ 334, 121Wu H., Zou Z.L., Xia X.Y., Deng Z.G., 1998, A&AS 132, 181Young J.S., Kenney J., Lord S.D., Schloerb F.P., 1984, ApJ

287, L65Young J.S., Schloerb F.P., Kenney J.D., Lord S.D., 1986, ApJ

304, 443Young J.S., Xie S., Tacconi L., et al., 1995, ApJS 98, 219