Astron. Astrophys. 324, 41-50 (1997)

7. Discussion

We have searched for dense molecular gas in three galaxies, NGC 3049, 5430 and 6764, which share the following common properties. They are the sites of very young (  5 Myr) starbursts, as evidenced by the presence of Wolf-Rayet stars and by our age determinations, they have a large far-infrared flux, they are barred and highly inclined.

HCN has been detected in NGC 5430 and 6764, CS in the latter galaxy only. The HCN/FIR luminosity ratios of the two galaxies appear to be normal, confirming the trend noticed by Helfer & Blitz (1993) for normal and starburst galaxies. The upper limit to that ratio for NGC 3049 is also consistent with that trend. The measured intensities of CS or upper limits are also normal relative to that of CO and HCN in the same galaxies, when compared to other surveys of CS in external galaxies. If large amounts of dense molecular gas are required for star formation in burst mode, they no longer exist 5 Myr after the burst has started, presumably because they have been used up to form stars and/or ionized by the intense radiation emanating from massive hot stars.

We also find that the HCN/CO integrated intensity ratios are rather low relative to the mean value found in other surveys of HCN in external galaxies. This is due to the fact that our three galaxies have an unusually high CO integrated intensity relative to their FIR flux, compared to other galaxies, including ones with young starbursts. In other words, this higher than usual CO intensity is not a general property of galaxies with young starbursts.

The standard CO-N (H₂) conversion factor overestimates the amount of molecular hydrogen by a factor 2 or 3 in our three Wolf-Rayet galaxies. But again, our galaxies have unusual CO intensities. This standard factor is thus probably valid for starburst galaxies in a statistical way, with a large uncertainty for individual estimates.

Can the optical properties of our galaxies provide an explanation for their molecular line properties? The central star formation rates in the three galaxies, estimated by the luminosity of the H line, are not unusual among starburst galaxies, because they are all proportional (with the right factor) to the global star formation rates estimated by the FIR luminosities. The FIR luminosity and that of H in the center are highest in NGC 5430 and lowest in NGC 3049, reflecting the pecking order for the luminosities in the molecular lines. This is another confirmation that the latter are not unusual in very young starbursts.

We note that NGC 5430, the galaxy with the oldest starburst, also has a circum-nuclear ring. Its presence is consistent with the fact that most of the bar is in differential rotation (Sect. 4.1), as such rings form where the rotation becomes differential (Lesch et al. 1990). The timescale of ring formation (a few 10⁸ yr) is much larger than the age of the central starburst (6.0 Myr) of NGC 5430, and of the young star clusters (some less than 10 Myr) recently detected in other circum-nuclear rings by the Hubble Space Telescope (Maoz et al. 1996). The absence of such a structure in NGC 3049 and 6764 should thus not be attributed to the fact that their central starbursts are younger, but to dynamical properties, as their bars appear to be rigidly rotating.

One interesting property of NGC 5430 revealed in the present paper which deserves to be explored further is the fact that the CO and HI velocity profiles are asymmetric in opposite ways; there is relatively less HI where there is more CO. One possible reason for this asymmetry is a more efficient conversion of HI to H₂ in the Wolf-Rayet region which may explain the existence of the young starburst at that end of the bar. A detailed comparison of the relative distributions of HI and CO in starburst galaxies would certainly lead to a better understanding of the transformation of the gas before and during starbursts.

Finally, we point out that the linewidths of the CO lines are correlated to the central starburst ages of the three galaxies. This is a general property of young starbursts, which has been discovered by us (Contini et al. 1997).

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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