7. MOLECULAR CENSUS EVOLUTION

To date, a total of 54 molecular species and 23 isotopologues have been claimed to be detected in the extragalactic interestellar medium. Table 3 shows a similar table to that presented in Martín et al. (2011) which became outdated by the recent detection of SO⁺, l-C₃H, l-C₃H₂, H₂CCN, H₂CCO, C₄H, CH₃NH, and CH₃CHO in the spectral scan towards the absorption system PKS 1830-211 (see Sect. 5). It is definitely worth having a look on how the census of extragalactic molecular detections has evolved as a function of time. Fig. 1 shows the evolution with time of the cumulative number of molecular species detected in the extragalactic ISM. Since the first molecular detection 40 years ago (OH in absorption, Weliachew 1971) there has been a steady increase in the number of molecular detections. However, we can note two periods in which the rate of detections has been quicken. First, mostly in the period between 1985 and 1990, the increase was due to the availability of sensitive radio facilities, and in particular to the large collecting area of the IRAM 30m telescope. Only in the last few years, from 2003 up until now, spectral line scans have played the dominant role in terms of detection rate. The advent of ALMA, with a significant increase in the collecting area of about an order of magnitud compared to the largest current facilities, will be the next step in sensitivity. In the next decade such increase in sensitivity will allow us to match the number of extragalactic molecular detections to the number of detected species in the Galactic ISM.

Table 3. Census of extragalactic molecular species and isotopologues detected

Notes: Updated Table from Martín et al. (2011) with the new detections in the absorption system PKS 1830-211 by Muller et al. (2011).

Figure 1. Histogram shows the number of extragalactic molecular detections per year for both the main (in red) and rarer (in green) isotopologues. The connected circles shows the cumulative number of detections.