A&A 368, 824-834 (2001)
S. J. Curran1,2 - A. G. Polatidis1 - S. Aalto1 - R. S. Booth1
1 - Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
2 - European Southern Observatory, Casilla 19001, Santiago 19, Chile
Received 19 December 2000 / Accepted 12 January 2001
In this paper we present large-scale maps of the transitions of CO and HCN in a sample of 7 near-by Seyfert galaxies. These are the first large-scale maps of CO in NGCs 2273 and 5033 and of HCN in all of the sample. From the maps we tabulate the integrated intensities at various extents and calculate the corresponding luminosities. The CO luminosities are compared with those of Young et al. (1995). It is our intention to use these results in conjunction with those of a distant Seyfert sample, in Paper II (Curran et al. 2001b), in order to analyse the differences in molecular gas luminosities and distributions, not only between the full sample and other galaxy types, but between the near-by and distant samples. Immediately, from the observational data we see that the HCN is distributed beyond the central beam (1 kpc) in much of the sample. We consider this to be an important result which we will discuss further in Paper II.
Key words: galaxies: Seyfert-galaxies: abundances-galaxies: ISM
From a survey of the transitions of CO and HCN in the central positions of 20 Seyfert galaxies, we (Curran et al. 2000) found a HCN/CO luminosity ratio of 1/6 for the "distant'' sample in which the telescope beam-width exceeds 10 kpc (galaxies with a recessional velocity of km s-1), i.e. a global ratio similar to that of ultra-luminous infrared galaxies (ULIRGs) and over 10 times the ratio for normal spiral galaxies. This relatively high abundance of HCN, which traces dense molecular gas, in comparison to the bulk component of the gas, as traced by the CO, suggests a significant presence of molecular hydrogen densities in excess of 104 cm-3 in the Seyfert sample. The relative luminosities of CO, HCN and the far infrared (FIR) radiation lead to the conclusion , from which the excess far infrared flux may arise from an active galactic nucleus in addition to the star-burst activity. Furthermore, some questions were raised about the differences in the molecular gas luminosities and distributions between the distant and the near-by samples (see Paper II). In order to resolve this issue, in this work we present maps of the near-by sources, NGCs 1068, 1365, 2273, 5033 and 6814 (in conjunction with the two other near-by Seyferts, visible from SEST; the Circinus galaxy and NGC 4945), and give the global luminosity values obtained from these, the results of which will be discussed in the forthcoming Paper II.
The Northern sources in the sample were observed over one continuous 7 day session in
September 2000 with the 20 m telescope at Onsala Space Observatory
(OSO). Both the CO
were observed with the SIS 100 GHz receiver. The
backend was a filter-bank with a bandwidth of 512 MHz and a channel
separation of 1 MHz. We used a similar dual-beam switching as the SEST
observations and obtained similar pointing errors. The weather was
good (typical system temperatures were around 250 to 400 K at 89 GHz
and 450 to 600 K at 115 GHz) and, as with the SEST data, only linear
baselines were removed. The beam sizes and efficiences for the two telescopes
at various transitions are given in Table 1.
The observational results are summarised in Table 2 and the maps are shown in
Figs. 1 to 6, where the intensity scale is
and velocities are shown relative to local standard of rest. Note that although the SEST maps are shown at
their observed spacings, the global integrated intensities (Table 2), like the OSO observations, are calculated for a map spacing of one
beam (as in Table 1).
|Figure 1: Top: CO in NGC 1068 over the full mapped region. The spacing is 44'' and the velocity resolution is 10 km s-1. Bottom: HCN in NGC 1068 over the full mapped region. The spacing is 57'' and the velocity resolution is 40 km s-1|
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|Figure 2: Top: CO in NGC 1365 over the full mapped region. The spacing is 44'' and the velocity resolution is 10 km s-1. Bottom: HCN in NGC 1365 over of the region mapped. The spacing is 34'' and the velocity resolution is 40 km s-1|
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|Figure 3: Top: CO in NGC 2273 over the full mapped region. The spacing is 30'' and the velocity resolution is 80 km s-1. There were no HCN detections in this galaxy. Bottom: HCN in NGC 4945 over of the mapped region. The spacing is 20'' and the velocity resolution is 80 km s-1|
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|Figure 4: Top: CO in NGC 5033 over the full mapped region. The spacing is 30'' and the velocity resolution is 80 km s-1. Bottom: HCN in NGC 5033 over the full mapped region. The spacing is 44'' and the velocity resolution is 80 km s-1. The emission at ( -44'',44'') could correspond to that in position ( -30'',30'') of the CO map|
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|Figure 5: Top: CO in Circinus over of the mapped region. The antenna temperature increment is 0.1 K, the spacing is 20'' and the velocity increment, of 10 km s-1 resolution, is 500 km s-1. Bottom: HCN in Circinus over the full mapped region. The spacing is 30'' and the velocity resolution is 100 km s-1|
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|Figure 6: Top: CO in NGC 6814 over the full mapped region. The spacing is 44'' and the velocity resolution is 10 km s-1. Bottom: HCN in NGC 6814 over the full mapped region. The spacing is 57'' and the velocity resolution is 100 km s-1|
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|NGC 1068||SEST||9/00 & 11/00|
|NGC 4945||SEST||12/98 & 6/00|
|Circinus||SEST||6/99, 4/00 & 6-7/00|
In Table 3 the luminosities are summmarised. These are
calculated by multiplying the average integrated intensity by the area
of the map (e.g. as in Sandqvist et al. 1995). Since there may be some
ambiguity due to how much area is actually mapped, we constrain
the value by multiplying the global integrated intensity at single beam spacing over the
estimated source size, which gives a similar result as calculated by the average integrated intensity
over the mapped area.
Our value of the global CO luminosity (Table 3) is comparable with that Planesas et al. (1989) who get, also from single dish observations, K km s-1 kpc2 over a partial map spanning (cf. Fig. 1). Both results are consistent with those of Young et al. (1995), see Sect. 4.
Concerning HCN, note that the central integrated intensity is similar to that obtained previously at Onsala ( K km s-1, Curran et al. 2000), although, due to the larger SEST beam, the central luminosity is slightly higher (cf. K km s-1 kpc2).
Our CO values agree well with those previously published by Sandqvist et al. (1995); 1.7 103 K km s-1 kpc2 over the central beam and 5.3 103 K km s-1 kpc2 at 20'' spacing over .
Again the central HCN result agrees well with our previous measurement of K km s-1 kpc2 (Curran et al. 2000).
We are not aware of any large-scale maps of either CO or HCN in this galaxy. The central CO value agrees well with that of K km s-1 kpc2(Curran et al. 2000).
We adopt a distance of 3.7 Mpc (Mauersberger et al. 1996) (although distance estimates to NGC 4945 vary somewhat, e.g. 6.7 Mpc according to Dahlem et al. 1993, Forbes & Norris 1998). The choice of value has no effect on the relative luminosities (i.e. ratios). Note that while the central CO integrated intensity compares well with that obtained by Mauersberger et al. (1996) (450 K km s-1), the global value from this data is around 3 times greater than actually published by Dahlem et al.(1993) ( K km s-1), Table 2.
The central HCN integrated intensity compares well with that of Curran et al. (2001b) ( K km s-1).
Again we are not aware of any large-scale maps, although both the CO and HCN luminosities agree with those previously determined, i.e. and K km s-1 kpc2, respectively (Curran et al. 2000).
Note that due to the relatively close proximity and vastness of this galaxy, we may be far from having global CO values and that the "structure'' in our map may be due to some structure within the galaxy, e.g. spiral arms (Thean et al. 1997 and references therein).
The new values agree well with those from the maps of 1988 and 1993 (see Curran et al. 1998), i.e. 0.3 103 K km s-1 kpc2 for both the CO and transitions.
Concerning the HCN, the central integrated intensity agrees (within uncertainties) with that of Curran et al. (2001b), i.e. K km s-1. Note that (blue-shifted) HCN may also be detected at -60'' along the minor axis, which corresponds to the (approaching) NW molecular outflow (Curran et al. 1999) and where a tentative detection of HCO+ has been made (see Curran 2000a).
With no large-scale maps available, again the CO luminosity agrees with that previously obtained, i.e. K km s-1 kpc2, although due to the weak emission (a central integrated intensity of 0.5 K km s-1, Curran et al. 2000), no HCN was detected over the time allocated to map this molecule.
Note that for NGC 1068 the Young et al. (1995) observation is over a diameter of 84'' and hence captures most of the flux (Fig. 1). This can be seen in their value of K km s-1, cf. ours of 133 K km s-1 for the global integrated intensity.
NGC 2273 is only observed over a diameter of 30'', although it appears that most of the flux is sampled, K km s-1, cf. our value of 4 K km s-1 for the global integrated intensity.
NGC 5033 is observed over a diameter of 96'' and our map (Fig. 4) suggests that there may be emission beyond this.
NGC 6814 is over a diameter of 78'' which is a similar region to where we find emission (Fig. 6).
For completeness, our previous value for NGC 7469 (Curran et al. 2000), was correct as the source is distant (v=4900 km s-1) and so the flux contained within the central beam (diameter 10 kpc) determines the global luminosity. Thus with the exception of NGC 2273, the remaining results are at least consistent with those of Young et al. (1995).
We see that our results are mostly in agreement with those previously
published (where these exist) and by using the full map (global)
values of these in conjunction with those of the distant sample
(Curran et al. 2000, Table 5),
in Paper II we will analyse these results and draw our conclusions.
We wish to thank the referee R. Antonucci for his prompt and helpful comments. Also, Lars E. B. Johansson for his advice, Francisco Azagra at SEST for staying up to monitor the impromtu SEST observations of NGC 1068, Aage Sandqvist for his NGC 1365 data as well as Felipe Mac-Auliffe and Saul Vidal at La Silla for getting the Dahlem et al. (1993) data from 9-track tape, and Susanne Hüttemeister for her permission to use it. This research made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.