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Astron. Astrophys. 347, 37-46 (1999)

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4. Discussion

4.1. Physical conditions of the gas

The previous maps reveal two main features for the molecular gas (also RD97): a) Two intense arcs in both 12CO lines and with little HCN; b) A nuclear ring or spiral with strong HCN and 13CO(1-0). The radio continuum maps show a distribution similar to HCN, an intense ring and weak arcs.

To discuss these results more quantitatively, we chose five regions in the nuclear disk of NGC 1530, and calculated the various line ratios for these regions. Fig. 1 shows these regions on the 12CO(1-0) map. Table 2 gives their coordinates and the line ratios, along with the intensity of 12CO(1-0) and fluxes from the cm maps convolved to a [FORMULA] gaussian for a better signal to noise ratio. We now relate the kinematics and the physical conditions of the gas in these regions.


[TABLE]

Table 2. Line ratios and cm fluxes in five regions of the center of NGC 1530.
Notes:
a) Coordinates (J2000): [FORMULA]; [FORMULA]
b) 12CO(1-0) integrated intensity
c) Line ratios 12CO(2-1)/12CO(1-0), 12CO(1-0)/13CO(1-0) and 12CO(1-0)/HCN(1-0)
d) 20 cm, 6 cm fluxes in [FORMULA] diameter lobes


Regions 2, 3 and 5 correspond to local maxima of 12CO(1-0) and 12CO(2-1) emission. Region 1 is a local maximum of 12CO(2-1) and HCN(1-0) while region 4 corresponds to a local minimum of 12CO(1-0) and 12CO(2-1). These regions are typical of the differing conditions in the nuclear disk. We have compared the location of these five regions with a CO(1-0) velocity map from RD97. Regions 1 and 2 correspond to a nearly circular rotation of the gas, as they are at the contact point between the arcs and the nuclear ring. They are close to the dynamical center, at a radius of 0.7 kpc. Regions 3 and 4, at radii of 1.7 kpc, correspond to transition points in the kinematics, a transition between infall motions (in the CO arcs) and circular rotation (in the ring). Region 5 is further out in the southwest arc, at a 3 kpc radius. The motion of this region has an infall component of [FORMULA]. This infall motion is associated with the density wave of the arc (RD97).

The molecular ratios seem normal in the nuclear disk of NGC 1530. We compare the central kpc of NGC 1530 to the center of the spiral galaxy IC342 which probably contains a weak bar (see Downes et al. 1992, Wright et al. 1993). IC 342 is one of the rare galaxies which has been observed in several different molecules with interferometers. IC 342 shows straight regular 12CO(1-0) lanes (Wright et al. 1993, their Fig. 2b) that are curved near the nucleus, similarly to NGC 1530. The lanes emit in 13CO(1-0) (Wright et al. 1993, their Fig. 2b) but not in HCN(1-0) (Downes et al. 1992, their Fig. 1); in these two transitions one finds five 50 pc diameter clumps at the places where the lanes curve toward the nucleus. These clumps are not prominent in 12CO(1-0). [FORMULA] is 4.4 in these clumps and [FORMULA] in the CO lanes. Similarly, [FORMULA] is 7 in the clouds and [FORMULA] in the CO lanes. Therefore the results seem similar for both galaxies, with ratios [FORMULA] and [FORMULA] lower near the nucleus ([FORMULA] pc for IC342, [FORMULA] pc for NGC 1530). The places where the CO lanes become a nuclear ring or spiral show large emissivity in 13CO(1-0) and HCN(1-0), which can be interpreted as the sign of a high concentration of dense gas (see, e.g., Downes et al. 1992, Mauersberger & Henkel 1993).

We supposed that the 12CO and 13CO transitions are emitted by the same masses of gas. Therefore we could run escape probability models, to reproduce the observed line ratios of CO. We assumed the following values for the abundance ratios: [12CO]/[H2][FORMULA] and [12CO]/[13CO][FORMULA]. We assumed a velocity gradient of [FORMULA]km s-1 pc-1. Table 3 displays the results of the best fitting model. We deduced from this model the expected H2 density and kinetic temperature of the emitting gas for each region. Then we could derive the theoretical brightness temperature and obtain a filling factor by computing the ratio [FORMULA]. We computed also for each one of these 5 regions the CO to H2 conversion factor, using the formula [FORMULA](CO(1-0)), in units of [FORMULA] (K [FORMULA] pc2)-1 (Radford et al. 1991). This formula is valid for an ensemble of virialized molecular clouds, which is the case here.


[TABLE]

Table 3. Results from an escape probability analysis for the regions of Table 2.
Notes:
a) H2 density
b) Model kinetic temperature
c) Observed 12CO(1-0) brightness temperature
d) Model 12CO(1-0) brightness temperature
e) Area filling factor of molecular clouds
f) Conversion factor (in [FORMULA] (K [FORMULA] pc2)-1) computed from [FORMULA](CO(1-0))


The derived kinetic temperatures are standard values for the molecular gas, between 20 and 90 K. This kinetic temperature is weakly constrained by the escape probability calculations, and the error bars of the measured ratios do not allow a precise derivation. The H2 density is more tightly constrained. The gas density is greater near the center ([FORMULA] cm-3) than in the arms (2 to [FORMULA]). Region 4 has a greater density than the equivalent region 3. Regions with greater density (1, 2, 4) show a stronger HCN emission than the ones with a lower density, confirming the association of HCN emission with dense gas (Mauersberger & Henkel 1993). The derived filling factors are loosely constrained, as are the theoretical CO(1-0) brightness temperatures, depending on the kinetic temperatures. The average value is [FORMULA]. These low values reveal the clumpy nature of the molecular gas, contained in many small molecular clouds, unresolved with the [FORMULA]pc beam of the interferometer. The derived values for the conversion factor have a average of [FORMULA] [FORMULA] (K [FORMULA] pc2)-1. This is lower than the average value for the giant molecular clouds of our own galaxy ([FORMULA] [FORMULA] (K [FORMULA] pc2)-1, Sage & Isbell 1991). Region 5 has a conversion factor significantly lower than the others, possibly due to different excitation conditions for the molecular transitions.

We found in the central kiloparsec of NGC 1530 a [FORMULA] ratio about three times lower than the standard galactic value of Sage & Isbell (1991). It is unlikely that this conversion factor is universal. For the inner 14 kpc diameter region of NGC 891, Guélin et al. (1993) found a ratio 3 times lower than the standard galactic value. For the innermost 11 kpc of M51, Guélin et al. (1995) found a ratio 4 times lower than the standard galactic value. Our results are similar to the results found by Guélin et al. (1993, 1995). In the inner 1200 pc of the Milky Way, Dahmen et al. (1998) found a factor of 10 discrepancy relative to the standard Galactic value. We do not find comparable results in the inner kiloparsec of NGC 1530.

4.2. Star formation and dense gas

Helou et al. (1985) found a proportionality between the far infrared flux and the 20 cm flux for disks of spiral galaxies. This proportionality indicates that the global star formation rate and the global 20 cm flux are linearly correlated. We assumed a linear correlation between the local star formation rate and the local 20 cm flux density. Thus for each of the 5 regions we computed its local star formation rate [FORMULA] in a [FORMULA] beam by the following formula, with [FORMULA]mJy (total 20 cm flux from NGC 1530, from Wunderlich et al. 1987, Condon et al. 1996) and [FORMULA] yr-1

[EQUATION]

To compare with the dense gas distribution, we calculated the amount of H2 gas in dense phase present in each one of these five regions. We used for that the relation existing between the mass of dense gas and the velocity integrated HCN intensity. This relation is derived from HCN radiative transfer solutions (Solomon et al. 1992). It can be written as [FORMULA] (K [FORMULA] pc2)-1, where [FORMULA] is the mass of H2 at a density [FORMULA]cm-3 as traced by HCN. From the HCN fluxes of Fig. 1 and this relation, we computed the molecular gas masses of Table 4. The comparison of [FORMULA] and [FORMULA] show that the star formation rate seems correlated with the amount of available dense gas. The region 4 shows a higher [FORMULA] and a higher amount of dense gas than regions 3 and 5, even though region 4 is included in an arc, like the other two regions. Region 5 shows a very low level of star formation, consistent with its cm wavelength spectral index, -1.2 (see Table 2), which indicates the absence of thermal component in the cm emission.


[TABLE]

Table 4. Star formation rates and masses of dense gas (n(H2)[FORMULA] cm-3) in the five regions of Table 2.
Notes:
a) Local star formation rates from 20 cm flux in a [FORMULA] beam
b) Dense gas masses from HCN integrated fluxes in a [FORMULA] beam from [FORMULA] (K [FORMULA] pc2)-1


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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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