Technical News

The ALMA 1.3 mm receiver (Band 6) has been integrated with the ALMA cryostat as part of the Prototype System Integration task currently underway. The receiver itself is integrated into a "cartridge assembly", a cylindrical structure into which the receiver, elements of the local oscillator and other electronics associated with a particular ALMA band are assembled. The individual cartridges are then mounted into the ALMA cryostat which lies in the antenna focal plane. The integration of cartridge and cryostat is a key task of the ALMA Front End Integration Center which is being built at the NRAO Technology Center in Charlottesville. In somewhat less than a year’s time, several other cartridges will also be mounted in the cryostat, which will then be deployed on one of the prototype antennas at the ALMA Test Facility (ATF) at the Very Large Array site in New Mexico.

The first eight receivers in any ALMA Band are the pre-production receivers. Design improvements will result from the initial tests of cartridges incorporating these receivers before the final production run begins. The initial tests of the Band 6 receiver show that it is the most sensitive ever constructed. The receiver specification requires single-sideband receiver temperature below 83K over 80 percent of the band (211-275 GHz), and a receiver temperature less than 138K over the remaining 20 percent of the band. The first Band 6 receiver performs substantially better than these specifications.

Sensitive receiver performance is a key element for reaching ALMA science goals. For example, one of the Design Reference Science Plan (DRSP) projects (1.1.1) ( http://www.strw.leidenuniv.nl/~alma/drsp.html) proposes a medium-scale (4' × 4') deep survey of the IR background, and the population of dusty, star-forming galaxies at high redshift thought to provide the background radiation. A significant fraction (~50 percent) of star formation in the cosmos occurs in galaxies that are heavily obscured by dust; this fraction may rise with redshift, possibly corresponding to the formation of spheroidal galaxies in active starbursts. One highly uncertain aspect of the study of sub-millimeter galaxies is their redshift distribution. Optical redshifts remain problematic for the majority of such sources, and can be misleading due to possible mis-identifications.

With the ALMA sensitivity at 1.3 mm, the DRSP survey can identify several hundred sources in eighty hours of integration time to a detection limit 5-10 times more sensitive than continuum surveys made by large bolometer cameras (though the cameras will cover much wider fields of view they will be limited by their larger beamsize to a sensitivity limit such that adjacent sources blend into each other). ALMA will explore a range of star formation rates much lower than those of currently detected sub-mm sources. For example, at a proposed sensitivity level of 0.1 mJy, the survey should be able to find between 100 and 300 continuum sources, depending on the applicable cosmological model, with 30 to 90 sources brighter than 0.4 mJy.

A further phase of this proposed research investigates the molecular gas content of the same field of galaxies. The whole Band 6 frequency coverage can be covered in 8 tunings of the receivers. One could achieve a continuum sensitivity (averaged over the total 64 GHz) of 50 µJy at 5-sigma all over the 4'× 4' field. Such a field requires about 90 pointings at each frequency tuning (70 at 210 GHz, 110 at 270 GHz). An integration time of two hours per pointing (all tunings included) is required to reach this sensitivity level, leading to a total time of eight days to perform the program.

The continuum part of this survey duplicates Part 1: all sources detected in Part 1 will be detected in the second. However, this part also allows detection of the CO lines from the sources. Because of the overlap, it is conceivable to perform this second part with some angular resolution. For this exercise, we assume an angular resolution of 0.4", already better than that of the best images obtained so far. The continuum brightness sensitivity corresponds to 1 mK (1-sigma) at this angular resolution.

Statistics on the strength of the CO lines redshifted to such frequencies are still too scarce to predict a detection rate. The brightness sensitivity of the instrument is then 0.05 K (1-sigma) for 50 km/s resolution. This is well below the typical line strength of CO in nearby galaxies. Even accounting for additional beam dilution, we would thus expect a large number of detections. The highest redshift objects may escape detection, since the high-J CO lines may not be significantly excited at the temperature of the emitting gas. However, at the expected sensitivities, one could perhaps start to detect species other than CO such as HCN, which is typically ten times weaker. For galaxies with z > 2, multiple CO lines would fall within the spectral window.

Conducted as described above, the second part of the survey would allow one to obtain sizes of most of the sources detected in Part 1, as well as dynamical masses for the brighter objects. Lensing correction for the brightest sources may also be possible, as the angular resolution will help in building a first-order lens model for the brightest sources.

ALMA Board Meeting

The ALMA Board met in Pasadena, California during a series of meetings which included an ALMA Management Advisory Committee meeting. At this meeting, the Board approved Version A of the ALMA Operations Plan. The Board also heard a report from Jean Turner, Chairman of the ALMA Science Advisory Committee (ASAC), which had met 24 and 25 February in Garching, Germany. Discussions centered on a request by the ALMA Board to consider the scientific impact of options for rebaselining ALMA. Rebaselining is the process of adjusting the ALMA budget to account for changes over the years, and is driven in large part by large increases in commodity prices, particularly steel and oil. The resultant cost increases to ALMA must be accounted; options for realizing cost reductions were presented to the ASAC for discussion. The most sweeping change is to adjust the number of antennas. In the bilateral ALMA project, 64 antennas are specified to achieve the Level One science goals: Detection of a CO or [C II] transition in a galaxy of Milky Way luminosity at a redshift z =3 within a day’s integration, imaging of a protoplanetary disk in the nearest molecular cloud to determine its structure and kinematics, and production of high dynamic range high-fidelity images. The effect on these science goals was discussed by the ASAC and a report was written in consultation with the ALMA North American Science Advisory Committee. This report was presented to the ALMA Board in April and is available at http://www.alma.nrao.edu/committees/ASAC/asacreport_2005feb.pdf. The report concludes that ALMA science is driven by sensitivity and image quality. Sensitivity, measured by integration time, varies as the square of the number of antennas. Image quality varies by the square of the number of baselines. The Board approved the ASAC recommendation that L. Testi succeed J. Turner as its Chairperson. C. Wilson will become Vice Chair.

H. A. Wootten

North American ALMA Science Center

An ALMA workshop is being planned to discuss the new suite of ultrawide bandwidth sub-millimeter spectrometers now under development for several telescopes. The main goals of such a workshop titled: From Z - Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies would be to familiarize the community with these instruments’ capabilities, highlight the most important scientific questions about dusty high-redshift galaxies that they will be able to address in the near-term, and discuss how observing programs can be designed to optimize synergy with ALMA and the EVLA on longer timescales. This would be a small workshop (~60 participants) held in Charlottesville, Virginia on January 13-14th 2006, following the January meeting of the American Astronomical Society (AAS) in Washington D.C. The ANASAC is spearheading this effort and a Scientific Organizing Committee has begun planning for the workshop. Further information is available at http://www.cv.nrao.edu/naasc/zmachines/ or via email to zmachines@nrao.edu

H. A. Wootten