ALMA Explores The Cold Universe, Part 1

With the moon set against the Milky Way and the galactic center in this stunning panoramic view of the Chajnantor plateau, the antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) appear silhouetted against a breathtaking starry night sky. Image credit to European Southern Observatory (ESO) Photo Ambassador Babak Tafreshi, founder and leader of “The World At Night”, an international project to produce and exhibit a collection of stunning photographs and time-lapse videos of the world’s landmarks with a backdrop of the most beautiful celestial vistas

Often, during discussions of astronomy, stars and the expanding universe, images of  exploding stars, swirling galaxies, hot gas, solar flares and black holes are conjured up, aspects of a “hot” universe. Rarely is the “cold” universe discussed and, up until recently, for good reason: we had very limited ways of observing it.

alma_telescope_altitude
Location and relative altitude of the ALMA site

Enter the recently operational ALMA observatory. At an altitude of 5 Km, approximately 1/2 the elevation of Mt. Everest, it is located on the high Atacama desert plains of the Chajnantor Plateau in northern Chile. ALMA, an acronym for Atacama Large Millimeter/submillimeter Array, is located atop the high Chilean desert plateau with the pristine air ranking among the driest on the planet, an ideal location to establish such a facility. The facility is an international collaboration between Europe, the United States, Canada, several countries from East Asia and the Republic of Chile.

How is this observatory different from any other, after all we’ve been observing the universe for over 400 years since the time of Galileo with an ever increasing variety of instrumentation, increasing in size, scope, power and diversity? So, just how does a telescope help us “see” the universe and, more specifically, how does this telescope help us see the universe?

Up until last month, the electromagnetic spectrum provided the only portal through which we could “see” or “observe” the universe around us. With the detection of gravity waves, that has changed and a separate article about that can be found here. A telescope is simply a device that collects electromagnetic radiation and focuses that energy to a point, be it optical, in the case of our eyes or a conventional telescope, or a radio telescope or any other instrument that is used to collect and focus electromagnetic radiation in a given region of the spectrum. Depending on what region of the spectrum you’re observing in, the technology will be different, but the fundamental principles will always be the same. For example, in the case of a radio telescope, radio energy is collected by a radio “antenna” (usually a parabolic dish of varying proportions), focused to a point by the dish and analyzed by an electronic receiver located at the focus of that dish instead of an eyepiece and our eye or a camera as in the case of a conventional telescope.

Any telescope in the sense that “telescope” is understood to be such a device, to collect, focus and analyze electromagnetic radiation, the larger the collecting surface, the better; simply put, a larger collecting surface will always collect more energy than a smaller surface, hence producing a brighter image in the case of a conventional telescope.  The second, equally important aspect of a telescope’s performance is its resolving power, something else directly related to its aperture. Simply stated, the larger the telescope is in diameter, the better its resolution. This aspect of a telescope’s performance, the inverse relationship between telescope aperture (diameter) and fine detail resolution is linked to the wave nature of light (electromagnetic radiation – light, heat, UV, x-rays, etc). In both aspects of performance, bigger is always better; more collection area = a brighter image and finer detail.

5 of the 54 12 meter ALMA telescopes

The ALMA “Observatory” consists of a total 66 telescopes, 54 12-meter and 12 7-meter diameter telescopes integrated together into what is known as an Interferometer Array. In this sense, there is a kinship between the ALMA facility and the twin LIGO facilities that detected gravity waves, albeit with completely different designs and operational parameters. The LIGO facilities each incorporate 2, 4 km long optical interferometers to “magnify” the minute amplitude of the detected gravity wave while the ALMA configuration links together the individual output signals of each 12 or 7 meter telescope in a millimeter/submillimeter interferometer array.

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One of the 54 12 meter telescopes following delivery as it was undergoing commissioning, testing and setup at ALMA

One of the benefits of an interferometric configuration is “aperture synthesis”, where the greatest separation of any of the telescopes in the array becomes the effective (synthesized) aperture (diameter) of a single, virtual telescope. As we discussed above, bigger is always better and aperture synthesis in the case of ALMA has allowed the virtual telescope so created to achieve resolutions exceeding the Hubble Space Telescope by an order of magnitude (a factor of 10) in their operational wavebands. The telescopes were manufactured by Mitsubishi Electric Corporation with the surface of each dish accurate to within 25 micrometers (25 ppm) or about 1/25 the width of a sheet of paper. Each of the 12 meter telescopes has a resolution of 11 arc-seconds at a wavelength of 500 microns (0.5 mm, a submillimeter wavelength in the operational range of the telescopes) or the ability to resolve a golf ball at a distance of 1 kilometer. When all 66 telescopes operate as a single, integrated instrument it becomes the most powerful astronomical facility in existence.

Region of the Electromagnetic spectrum observed by the ALMA compared to other wavebands and the very narrow “visible” region of the spectrum

Each “telescope” of the ALMA “Observatory” observes the universe in the millimeter and submillimeter regions of the electromagnetic spectrum. This is the region between Infrared and the Radio regions, in a wavelength region shorter than the “radar” or “microwave” regions. In this realm, much of the energy is attenuated (absorbed or reduced) by the water vapor in the earth’s atmosphere, hence the necessity for a clear, dry location. By comparison, our eyes, and optical telescopes, “see” the universe in the “visible” region of the electromagnetic spectrum.

At 5 km in altitude, much of the earth’s water vapor is below the observing site; that combined with the extraordinarily clear, transparent air of the Atacama Desert Plateau makes this an ideal site to observe in this wavelength region.

The observatory began commissioning the telescopes in January, 2010 and by the summer of 2011 there were sufficient telescopes operational during the extensive program of testing prior to the Early Science phase of the facility’s construction for the first images to be acquired. By October, 2012, 43 of the 66 telescopes had been delivered and by March, 2013 all 66 telescopes had been delivered, commissioned and setup allowing full science operations to commence at that time.

The Antennae Galaxies are a pair of distorted colliding spiral galaxies about 70 million light-years away. This view combines ALMA observations, made in two different wavelength ranges during the observatory’s early testing phase with visible-light observations from the Hubble Space Telescope. When the full ALMA array is completed, its vision will be up to ten times sharper in its operational wavebands than HST. Most of the ALMA test observations used to create this image were made using only twelve antennas working together, used for the first science observations, far fewer than the 66 that could be integrated together in the full array. While the visible light aspects of the image, shown here in blue, reveal newborn stars in the galaxies, ALMA’s view shows us something that cannot be seen at those wavelengths: clouds of dense cold gas from which new stars form. The ALMA observations, depicted in red, pink and yellow, were made at specific millimeter and submillimeter wavelengths (ALMA bands 3 and 7), tuned to detect carbon monoxide molecules in otherwise invisible hydrogen clouds where new stars are forming. Massive concentrations of gas are found not only in the hearts of the two galaxies but also in the chaotic region where they are colliding. Here, the total amount of gas is billions of times the mass of the Sun, a rich reservoir of material for future generations of stars. Observations like these will be vital in helping us understand how galaxy collisions can trigger the birth of new stars.

In Part II, we will discus the targets of ALMA, the physics behind them, their importance and how a deeper understanding of the dynamics and processes involved will lead to a more complete picture of their role in stellar and planetary formation and interactions between galaxies on intergalactic scales. We will also discuss some of the exciting observing programs currently underway at ALMA.

Imagination is more important than knowledge585px-Albert_Einstein_signature_1934(invert)
An index of all articles in this blog can be found here.

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