... galaxies

Based on results collected at the European Southern Observatory, La Silla, Chile and Onsala Space Observatory, Sweden.

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... (ULIRGs)

Where the far infrared luminosities of $L_{\rm FIR}>10^{11}~L_{\odot}$ give FIR to molecular gas luminosity ratios which are an order of magnitude higher than for normal spiral galaxies Soifer et al. 1984; Sanders et al. 1986; Solomon & Sage 1988).

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... SEST

The Swedish-ESO Sub-millimetre Telescope is operated jointly by ESO and the Swedish National Facility for Radio Astronomy, Onsala Space Observatory, Chalmers University of Technology.

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...

$I_{\rm mb}=I_{\rm A}^{*}/\eta_{\rm mb}$, where $I_{\rm A}^{*}$ is the velocity integrated Rayleigh-Jeans antenna temperature above the atmosphere and $\eta _{\rm mb}$ is the measured main-beam efficiency of the telescope.

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...

Again Sy refers to the Seyfert type but here and in Table 4 we 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 1.5 constitute type 1 Seyferts and types 1.8, 1.9 and 2 constitute type 2 Seyferts.

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... OSO)

Assuming a Hubble parameter of H₀=75 km s^-1 Mpc^-1. This value is used in all other size estimates throughout the paper.

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... CO

From now on, unless otherwise stated, when CO is written it refers to the $1\rightarrow 0$ transition.

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... all

Since the HCN may possibly be more confined than the CO; within 1 kpc (the HPBW at $v\approx1000$ km s^-1) (Downes et al. 1992; Nguyen et al. 1992; Henkel et al. 1994; Tacconi et al. 1996).

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... range

From normal spirals to ULIRGs (SDR92; Helfer & Blitz 1993).

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... luminosity)

We achieve the same value using only galaxies with  km s^-1, thus again indicating that the CO may be centralised.

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... gas

For example, in NGC 1068 the HCN/CO intensity ratio is 0.6 in the central few hundred pc (Helfer & Blitz 1995)cf. $\approx0.2$ within 1.2 kpc (this work) and 0.1 out to 2.2 kpc (Helfer & Blitz 1993).

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... obscuration

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).

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... around

Assuming that the molecular hydrogen mass, $M({\rm H}_2) [M_{\odot}] = 4.6 L_{\rm CO}$ [ ${\rm K ~km~s}^{-1}~{\rm pc}^2]$ (Scoville & Sanders 1987).

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... spirals

Since this ratio of $\frac{L_{\rm FIR}}{L_{\rm CO}}~(\rm Seyferts)\approx2\frac{\rm L_{FIR}}{L_{\rm CO}}~(\rm normal ~spirals)$ is based on the whole sample, and not just the distant FIR bright Seyferts, it is somewhat less than that determined by Eq. (3).

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...$L_{\rm FIR}\sim10^{11}-10^{12}~L_{\odot}$)

The extreme cases; Mrk 231, Mrk 273 and Arp 220, which are located in the $L_{\rm FIR}\sim10^{12}~L_{\odot}\approx70M({\rm H}_2)~M_{\odot}$ range, are also considered to be ULIRGs (e.g. SDR92; Downes & Solomon 1998).

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... cores

Estimating the CO $2\rightarrow 1$ source sizes from the radio continuum (e.g. Allen 1992; Bajaja et al. 1995), we find that the CO emission seems to be sub-thermally excited, which would suggest a less dense cloud component in addition to that traced by the HCN. This is discussed further in Curran (2000b).

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... luminosity

Over a factor of two according to the linear fit from $L_{\rm FIR}\sim10^{10}-10^{12}~L_{\odot}$.

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... Seyferts

In fact $\sim10\%$ of ULIRGs are considered to be type 1 Seyferts or IR QSOs (Deng et al. 1997) and up to $\approx80\%$ may be radio quiet AGNs (i.e. Seyferts and LINERs) (Wu et al. 1998) and up to $\approx30\%$ may be powered by AGNs in general (Genzel et al. 1998).

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