Dazhi Zhou
Since different isotopes are formed in different mass stars, isotope ratio (in particular 13C/18O but also other isotopes involving N, Si and S, etc) becomes an index of the relative contribution of the nucleosynthesis of different mass stars with high sensitivity and it recently has been proposed that isotope ratio can be used to indicate a top-heavy initial mass function in distant starburst galaxies. However, the same effect can also be produced by a very young starburst and has not been considered by detailed analysis. To check whether age plays a role, we use ALMA data and analyze 14 super star clusters in the nearby starburst galaxy NGC253 with well-determined ages from SINFONI data to find the correlation between the clusters age and the isotope ratios. The isotope ratios were derived from molecular isotopologue ratios. For the estimation, we have tested several different isotopologues, examined their opacity, and also considered the chemical effect (e.g. chemical fractionation, selective photodissociation, etc). Ages of the SSCs are derived by Starburst99 software with a pixel by pixel analysis. Different correlations are found in different analyses. We conclude that young starburst can indeed play a role in the extreme isotope ratio. But detailed analysis and modelling are still needed.
Based on observations and the chemical evolution model (Romano et al. 2017), Zhang et al. (2018) proposed that the low 13C/18O reveals a top-heavy IMF and there are more massive stars in the distant starburst galaxies than in normal galaxies. Their model predicts that the top-heavy IMF should have an extremely low 13C/18O compared to other IMFs since 13C comes from low mass and intermediate-mass stars and 18O is produced by the nucleosynthesis of high mass stars. The observation shows a negative correlation between 13C/18O and the total infrared luminosity, which means for lower isotope ratio, the star formation rate is higher. However, they do not rule out that the same effect is also possible to be produced by a very young starburst.
In order to determine whether the young starburst has the contribution, we compare a number of super star clusters (SSCs) with well-determined ages to the corresponding isotope ratios.
We use ALMA data and 13CO/C18O in 1-0, 2-1, and 3-2 transitions separately to trace the relative variation of 13C and 18O abundance because 13CO and C18O share a similar frequency, critical density and upper energy level. Although it is possible that in some regions, optically thin 13CO is no longer valid, it is believed that it would not affect the conclusion of this project. H13CO+ and HC18O+ are also used, but large uncertainties are expected because of their low abundances.
The equivalent width of the Brγ line, which can be simply regarded as the ratio of the Brγ line to the K band continuum line, is used for the age estimation. Because Brγ comes from young star formation and K continuum stands for the older medium, the equivalent width can be treated as a function of age. We use SINFONI data and Starburst99 model with a zero-age instantaneous starburst with a fixed mass of 10^6 solar mass to derive the ages.
Correlations are found from both 14 SSCs (Leroy et al. 2018) and the pixel by pixel analysis. However, a linear relation is not observed and they show opposite results. From the result obtained by CO isotopologues, as for SSCs, the relation is negative. As for the whole map, the correlation is positive. The HCO+ results are also different. The correlation in SSCs is positive and for the pixel by pixel analysis, it is negative. The difference between CO and HCO+ can be explained by the different gas that they trace. The distinction between 14 SSCs and pixel by pixel plot may come from the very young ages of the SSCs and the possible existence of old stars.
From the result, we know that very young clusters could be also a possible explanation of the very low isotope ratio, but we still cannot conclude that whether such a low ratio is caused by ages instead of the top-heavy IMF. To confirm our result, detailed modelling of enrichment and chemistry is needed.