Relation between SF, H 2 and HI

In contrast to the sSFR, which spans a wide range, the SFE is very similar for all our galaxies, and there is no clear relation between the value of the SFE and other parameters such as their SFR, gas deficiency or morphology. Specifically, we can not see any relation between SFE and the HI content of the galaxies. It seems that HI is not necessary for SF to take place or, at least, that SF processes have the same efficiency in absence of HI, suggesting an universal SFE inde- pendent of the HI content of the galaxies. The SF processes going on in these galaxies with low HI will finish once their molecular gas content is exhausted, since there will be not an atomic gas reservoir to replenish it. On the other hand, the HI-rich galaxies (40c, 40d, 79d) show higher values for their sSFR, which suggests HI leads to a higher sSFR. These results found here from a detailed study of HCG40 and HCG79 are in agreement with those from the statistical analysis of the 20 HCGs in Chap. 2.3.6.

If we focus on theH2 content, we see that, while late-type galaxies in HCG40 and HCG79 show an excess ofH2, elliptical galaxies show high deficiencies and the value for the S0 galaxies is close to the average deficiency of our whole HCG sample. This seems to agree with the idea of the S0 galaxies in dense environments as the result of the loss of their spiral structure as a result of tidal stripping within the group (e.g. Moore et al. 1996).

Chapter 4

High resolution study of HCGs with the next generation of mm / submm interferometers:

ALMA

and how many brisk lads drying in the sun at Execution Dock?

and all for this same hurry and hurry and hurry Long John Silver- Treasure Island

The millimeter and submillimeter wavelength range, between centimeter radio wave- lengths and the infrared part of the electromagnetic spectrum, is one of the spectral windows not deeply explored yet, mostly due to the technical problems associated with the transmission of these waves in the atmosphere of the Earth. Nevertheless, as we have seen in the former chapters of this work, in this range we can find most of the information on the molecular rich- ness of the interstellar medium.

The comparison between the single dish CO measurements shown in Chap. 2 and the in- terferometric ones performed with OVRO, shown in Chap. 3 is a good example of the challenge implied in mm/submm interferometric observations. The sensitivity level achieved with the 30m radiotelescope is much higher than the one we got with OVRO. At this point, it is manda- tory to set some master lines on the work that should be performed in order to continue the study of the molecular gas content in HCGs, which must include high-resolution interferomet- ric observations.

This thesis includes two main topics: the study of the molecular gas in HCGs and the prepa- ration of observations of these type of objects with the Atacama Large Millimeter Array (ALMA).

This chapter reflects this interest in two ways: first, we perform a study of the time requirements to observe HCGs with current and future interferometers, with special focus on ALMA. In addi- tion, we present the results coming from our interest in the calibration of interferometers, and in particular our tests of the fast-switching method, a calibration technique which will be used in ALMA.

4.1 Observing HCGs with mm interferometers

In this chapter, we study the feasibility to observe the HCGs in our sample with different mm interferometers: the SubMillimeter Array (SMA), the Plateau de Bure interferometer (PdBi) and ALMA.

69

SMA, located at Mauna Kea summit (Hawaii), at 4080 meters over the sea level, is property of the Center for Astrophysics (Smithsonian Institution-Harvard University) and ASIAA (Academia Sinica-Institute for Astronomy and Astrophysics). It consists of eight 6-meters antennas that can be arranged into configurations with baselines up to 509 meters operating at frequencies from 180 to 700 GHz. We cannot observe the 115 GHz emission line with this instrument and, therefore, we have studied the feasibility of the SMA observations for the CO(2-1) emission line.

At this frequency, SMA has a field of view of 50^′′.

The PdB interferometer, property of IRAM, is located at the Plateau de Bure, at 2550 m alti- tude, in the French Alps. Consisting of six 15-meters antennas with baselines ranging up to 760 meters, it can observe the millimeter range, from 80 to 267 GHz. New receivers covering higher frequencies (from 277 to 371 GHz) have started their operation during this winter observing pe- riod. Unlike SMA, it can be tuned at 115 GHz, so we can study the CO(1-0) emission line, with a field of view of 40^′′.

Finally, ALMA is an interferometer which is currently being built by an international consor- tia between the European Southern Observatory (ESO), the National Radio Astronomy Obser- vatory (NRAO), the National Astronomical Observatory of Japan (NAOJ), the National Research Council of Canada, the Academia Sinica Institute of Astronomy & Astrophysics (ASIAA) of Tai- wan and the Republic of Chile. It is placed in Llano de Chajnantor, 5000 meters high, in the desert of Atacama, Chile. It will operate in the mm and submm range, from 0.3 to 9.6 mm, which corresponds to a frequency range of 31-950 GHz, although the lower frequency bands, below 84 GHz, are part of the future development plan. For the early science observations, the 3 mm, 1.3 mm, 0.8 mm and 0.4 mm bands will be available. After several changes from the ini- tial design, it will have at least 50 antennas, each with a diameter of 12 meters, with baselines ranging from 15 m to 18 km, and a compact sub-array (Atacama Compact Array, ACA), with four 12-meters and twelve 7-meters antennas. The field of view of the 12m antennas array at 115 GHz is 50^′′.

Right now, there are 9 antennas at the Array Operations Site (AOS, the place where the in- terferometer will operate), with which the commissioning and science verification procedures are being performed. ALMA will bring new opportunities to the astronomical community, of- fering a much higher sensitivity and resolution than current interferometers, in addition to the possibility of performing observations in the submm range.