3. RECEIVER BANDWIDTH UPGRADES: ONGOING SURVEYS

At that time of the first line surveys described in previous section, the available instantaneous bandwidth in both SEST and the IRAM 30m telescopes was ~ 2 GHz. Even with such not inconsiderable bandwidths, spectral scans projects were still significantly expensive in terms of telescope time. To illustrate this, the surveys towards M 82 and NGC 253 each required more than one hundred hours of telescope time at the IRAM 30m telescope. However, in the last few years, major upgrades have been performed in all the mm and submm facilities around the globe. By the end of 2005, the MOPRA telescope commissioned the MOPS digital filterbank spectrometer which provided an ~ 8 GHz instantaneous bandwidth. Though unique at the time, most single-dish telescopes and interferometers catched up with similarly wide band receivers as shown in Table 1. Just recently, the Nobeyama 45m telescope started commissioning the SAM45 spectrometer, consisting of a copy of the ALMA/ACA correlator that allows a maximum of a 32 GHz bandwidth.

These major telescope upgrades have open the possibility of extending the exploration of wider spectral bands towards larger sample of extragalactic objects given the significantly reduced time required for such spectral scans. Consequently, a number of spectral line surveys have been carried out or are currently ongoing in almost every mm and submm facility. Table 2 summarizes the observational details of some of the most relevant line surveys carried out at mm and submm wavelenghts. Here we describe the main aims of theses surveys:

IRAM 30m .- Following the success of the 2mm line surveys of NGC 253 and M 82, a series of deep line survey follow up at 3mm are currently ongoing and near completion. This project sample consist of 6 galaxies, including two young starbursts (M 83 and NGC 253), two evolved starbursts (M 82 and M 51), the active galactic nucleus (AGN) prototype NGC 1068 and the ULIRG Arp 220. A total of 36 molecular species are detected in the covered band. This study aims to set the basis of a chemical classification of the extragalactic ISM in the central few hundred parsecs of nearby bright galaxies. The new instrumentation at the IRAM 30m telescope reduced the time required for each spectral scan by almost an order of magnitude, down to 10 hours per source.

Nobeyama 45m .- Three line surveys in the 3 mm atmospheric band had just been finished at the 45 m facility making use of the recently upgraded spectrometer (Poster 2.69). With more than a dozen molecular species detected, these surveys will be able to establish the differences between the chemistry in starburst galaxies (NGC 253 and IC 342) and the AGN dominated NGC 1068. Thus the main motivation of this work is to study the chemical fingerprints of the AGN influence on the surrounding molecular gas. The relatively small (18") beam of the 45 m telescope is smaller than the starburst ring in NGC 1068, allowing the observation of the chemical composition towards the AGN without contamination from the molecular material in starburst.

APEX 12m .- The spectral scans towards NGC 4945, NGC 253 and Arp 220 with the APEX telescope (Poster 2.83) are exploiting the capabilities of the FLASH receiver at APEX which processes an 8 GHz instantaneous bandwidth. These surveys are exploring the higher energy transitions in the 0.9 mm atmospheric window. A total of 16 molecular species are detected at the highest frequencies ever surveyed outside the Galaxy. Additionally, NGC 253 has been surveyed in the 1.3 mm atmospheric window from 185 to 275 GHz (Poster 2.83). These APEX scans complete the spectral coverage towards NGC 253 over a total band of 274 GHz between 3.5 and 0.8 mm (see Table 2).

SMA .- The Submillimeter Array was used to carry out the first interferometric extragalactic unbiased survey (see Section 6). The large width of the emission lines in Arp 220 together with the strong line confusion in this object, made it necessary to use the stable baselines achieved with the interferometer. A total of 70 spectral features from 15 molecular species and 6 isotopologues were identified. The survey was partially confusion limited, with an average of 1.8 lines per GHz. This spectral scan was the first carried out towards a ULIRG at a distance of 70 Mpc, an order of magnitude farther than nearby starburst such as NGC 253 and M 82. Some of the highlights of this study are summarized in Section 6.