ALMA Basics

ALMA is an instrument composed of 50 dishes of 12 meters each, located on the Chajnantor plain at 5000m altitude. Initially, it is sensitive to wavelengths in the range 400 μm to 3 mm (84 to 720 GHz). The dishes can be moved around, in order to form arrays with different distributions of baseline lengths. More extended arrays will give high spatial resolution, more compact arrays give better sensitivity for extended sources. In addition to the main array, there is the Atacama Compact Array (ACA), consisting of twelve 7-m antennas and four 12-m antennas. This array will mostly stay in a fixed configuration and is used to image large scale structures that are not well sampled by the main ALMA array.

The design of ALMA was driven by three key science goals:

1. The ability to detect spectral line emission from CO or CII in a normal galaxy like the Milky Way at a redshift of z=3, in less than 24 hours,
2. The ability to image the gas kinematics in protostars and in protoplanetary disks around young Sunlike stars in the nearest molecular clouds (150 pc),
3. The ability to provide precise high dynamic range images at an angular resolution of 0.1 arcsec.

ALMA delivers data cubes, of which the third axis is frequency. In this sense, the final data products are very much like that of an integral field unit with up to a million spaxels. An observer interested in continuum images may remove spectral regions with confusing line emission and then collapse the cube along the spectral axis. One interested in line emission may observe with a higher spectral resolution and subtract the continuum from the cube.

Proposing and observing

The ALMA users will interact with ALMA through the ALMA Regional Centers (ARCs). For European users, there is an ARC located at ESO, with various nodes distributed throughout Europe. Proposals will be submitted to the ARCs (Phase I) using the ALMA Observing Tool (OT). Successful applicants will have to prepare observing procedures (Phase II), also accomplished using the OT, which will be executed by the observatory staff. Astronomers will not visit the telescope to conduct their observations, mainly because projects will be executed depending to a large extent on the prevailing weather conditions which are of course not foreseeable in advance. The data will be pipeline processed and delivered to the user. For more details see Policies and Procedures.

Interferometry

An interferometer is an instrument for sampling the visibility function, which is the Fourier transform of the sky brightness distribution. This visibility function V(u,v) is measured as a function of position in the u-v plane. The coordinates u and v simply describe the vectorial separation between each pair of interferometer elements measured in wavelengths, as seen from the source.

In order to obtain images, the raw visibility data need to be Fourier transformed. When ALMA is operational, this imaging step, as well as various calibration steps, will be done in the data reduction pipeline. Thus, fully calibrated data cubes will be delivered to the user. However, the imaging (and subsequent deconvolution) is a non-unique procedure, so users may want to redo these steps to optimize the data products for their scientific objectives. The Common Astronomy Software Applications package (CASA) is currently under development for this purpose.