Friday, July 20, 2012

ALMA Opens its Eyes to the Sky

High in the Atacama Desert of northern Chile a new astronomical instrument is under construction. It is the Atacama Large Millimeter/Submillimeter (mm/submm) Array, aka ALMA. With its large number of antennas, its location in one of the driest places on the Earth, and at an altitude of 5000m (16,500 feet), ALMA represents a huge step forward for millimeter/submm astronomy, with promises of fantastic science to come. But why build such an expensive observatory (ALMA is the most expensive ground based instrument ever built) at such an inhospitable place (high altitude and extremely dry), and, how is it important for CANDELS?

ALMA antennas at the Chajnantor Plateau. Photo credit: Babak Tafreshi

First a few definitions: astronomers use the term 'radio' when dealing with electromagnetic radiation with wavelengths from hundreds of meters to less than a millimeter (submm). It has nothing to do with actually listening, like we listen to a normal radio, but with detecting this electromagnetic radiation using sensitive receivers. In this blog I will use the terms 'radio', 'mm' and 'submm' to refer to this type of emission - and the instruments used to detect it. Wavelengths are given in millimeters (mm), sometimes referred to as 'submm' when it's slightly less than 1mm, and in microns (a millionth of a meter).

If you could view the sky with eyes that were sensitive to millimeter and submillimeter wavelengths, it would look very different from what we see with the eyes we have. Instead of warm and hot celestial objects, such as stars and ionized gases, you would see dark and cold interstellar clouds, dusty disks around young stars, faint cirrus clouds permeating our Galaxy, and the occasional point-like quasar. The interstellar dust grains are heated by stellar light that is re-radiated at far-infrared and submm wavelengths. The characteristic dust temperature is in the range 20-40K and the molecular gas that permeates interstellar space can be even colder than that. Your ‘submm eyes’ would pick up the light from thousands of spectral lines, mainly coming from rotational transitions of simple – and some not quite so simple – molecules. Most of the light would be diffuse and spread out over the sky, but a few point-like sources would also be seen. These are synchrotron radiating Active Galactic Nuclei (AGNs) - the main part of the emission is, however, thermal in character.

As a matter of fact, your eyes wouldn’t actually see much of anything, even if they were sensitive to these wavelengths. The reason for this is two-fold; First, there isn’t as much energy in the mm/submm light as there is at optical wavelengths. A photon with a wavelength of 1mm has about 2,200 times less energy than one at 0.45 microns (visible light). Second, the angular resolution provided by the eye would be very limited indeed. The small diameter of the eye’s pupil would make the sky look blurry, providing an angular resolution roughly 1,000 times worse than what they do at optical wavelengths. You would have a hard time to pick out even the Moon or the Sun.

These degrading effects associated with observing the universe at longer wavelengths translate to more conventional telescopes as well; a radio telescope has to be bigger, in fact a lot bigger, than an optical telescope in order to achieve a similar performance. From an engineering point of view it is easier to build a large radio telescope than to build a corresponding optical telescope, but gravity and money limit how big they can really be. To circumvent these limitations, radio astronomers have long used the technique of interferometry, where the light from several telescopes is combined to improve angular resolution, and, in most cases, improve sensitivity. The use of several radio telescopes, or 'antennas', as an interferometer is referred to as aperture synthesis. Rather than go into details here, I recommend wikipedia's general overview of astronomical interferometers.

In fact, the first radio interferometer observations were done back in 1946, using a single antenna that happened to look out over the ocean. With this setup, the rising sun reflecting off the ocean surface creates interference fringes with the light directly reaching the receiver. The results showed that solar radiation at radio wavelengths during a flare is co-located with the corresponding sunspot and active area. These sea-cliff interferometers could create baselines up to ~200m.

As an aside, interference from single antennas looking towards the horizon were first identified during WWII when trying to detect approaching aircraft along the British coast and was correctly interpreted as an ‘interferometric’ effect.

ALMA antennas being tested at the Operations Support Facility. Photo: T. Wiklind

The latest radio interferometer is currently being built high in the Chilean Andes, quiet far from any ocean. This is of course the Atacama Large Millimeter/submillimeter Array, or ALMA; a project with a history going back more than 25 years. ALMA is a collaborative effort of three partners: North America, Europe, and East Asia. ALMA represents a huge step forward in terms of both sensitivity and angular resolution at mm and submm wavelengths, being about 10-1000 times more sensitive than existing instruments (depending on the wavelength). When completed, ALMA will consist of 66 high precision antennas located on the Chajnantor Plateau at an altitude of 5000m (16,500 feet). The antennas are divided into a main array, made up of fifty 12m telescopes, and the ALMA Compact Array (ACA) with twelve 7m and four 12m antennas. The total telescope area will correspond to a single dish with a diameter of 91m.

ALMA Operations Support Facility. Photo: T. Wiklind
ALMA produces both images and spectroscopic data with a wavelength coverage currently ranging from 3mm to 430 microns. When all planned receivers are installed, the wavelength coverage will range from 10mm to 320 microns. The antennas will be spread out over the Chajnantor Plateau with the longest baselines reaching 15km. This will allow an angular resolution of 5 milli-arcseconds for the most extended configuration and the shortest wavelength. This resolution would make it possible for ALMA to view NASA's left-over Lunar Rovers on the Moon (at least in principle – the Rovers don't radiate well at mm/submm wavelengths).

The Salar de Atacama, looking up towards the Chajnantor plateau.
Although the construction of ALMA will not be over until late 2013, scientific observations have already started. The deadline for the second call for Early Science Proposals was July 12, 2012 and resulted in more than 1,000 proposals. During the Early Science phase, ALMA will operate with fewer antennas than it will have when completed, but it already represents the best sensitivity and the highest angular resolution available at these wavelengths – which explains the intense interest among astronomers in using ALMA. Cycle 1 observations (ALMA science started with Cycle 0 in 2011) will be done with a minimum of 32 12m antennas, allow the use of a limited ALMA Compact Array (ACA), but do not include all the receiver bands that will eventually be installed. The introduction of the ACA in this cycle represents a big step forward. ALMA is extraordinarily sensitive and has an enormous angular resolution, but the penalty is that it loses sensitivity to structures more extended than a few arceconds (again, wavelength dependent). The reason is that the 12m antennas cannot be placed close together, lest they start shadowing each other. This means that the ‘inner’ part of the synthesized telescope is missing. By using the compact array, with the smaller 7m antennas, it is possible to fill the hole and allow ALMA to image large- as well as small-scale structure.
Sunset at the Operations Support Facility. Photo: T. Wiklind

So what can ALMA do for CANDELS aficionados? The science specification for ALMA states that it shall be able to detect a galaxy like the Milky Way at a redshift z=3 in the lines of carbon monoxide (CO is the second most common molecule in the Universe – the most common is molecular hydrogen but it does not radiate effectively since it is close to perfectly symmetric) or [CII] (singly ionized carbon atom) in less than 24 hours – remember that ALMA was conceived at a time when very little was known about the high redshift universe. The full ALMA will certainly be able to do this, but only with a few hours to spare. Fortunately, galaxies at high redshift are now believed to be considerably more gas-rich than their present day counterparts making them even easier to detect at these wavelengths, and it will in fact be possible to observe a lot more than a single galaxy per day.

An example of a high-redshift observation, done as part of the ALMA Science Verification program, is the far-infrared luminous quasar BR1202-0725. This quasar has a redshift z=4.7, meaning that the photons we observe from this object were emitted 12.4 billion years ago, just 1.3 billion years after the Big Bang. BR1202-0725 was observed with seventeen 12m antennas for a total on-source time of 25 minutes. The receivers were tuned to the atomic fine-structure line [CII] with a rest-frame wavelength of 158 microns, here redshifted to 900 microns. Two sources are detected in the [CII] line, the quasar and a nearby galaxy just 3.8” to the NW of the quasar. A third component 2.5” to the SW is seen in the continuum emission. The third component is not seen in the [CII] line and was previously unknown. It is presently not clear that the SW source is associated with the two other sources – but the proximity suggests it is. We will know as soon as we get additional data on this new object.
BR1202-0725, Top: ALMA continuum (green contours) and HST images in 3 different bands. The bright source seen in the HST images is the quasar. The submm sources seen to the NW and SW of the quasar have weak optical emission associated with them. Bottom: Spectra of the [CII] line from the quasar and the NW source. Notice that the NW source has a very broad line profile. From Wagg et al. 2012, ApJ 752, L30.
While ALMA provides unprecedented sensitivity for line observations, it really excels when it comes to continuum emission. The state-of-the-art receivers, broad bandwidth, and the high-and-dry location all conspire to provide a high sensitivity. On top of that, the Universe is very kind to submm observers; at a fixed observed wavelength, the distance dimming is offset by the increase in restframe flux. This leads to a sensitivity that is almost independent of distance in the redshift range z=1–10 (a redshift z=10 corresponds to a time only 480 million years after the Big Bang). This statement is true when observing at a relatively long wavelength (850 microns) and for dust grains characterized by a temperature 40K or lower. Already in Cycle 1 (the second call for proposals), it is possible to detect far-infrared luminosities of about 1011 Lo (that is 100 billion times the Sun's luminosity, a rather modest luminosity for star forming galaxies) at redshifts up to z~10 in less than 10 minutes of integration time.

The observed flux at two fixed wavelengths (850 and 450 microns) as a function of redshift. The total far-infrared luminosity is set to 1012 Lo and the flux is shown for several different dust temperatures. Figure ripped from one of my powerpoint presentations.

One thing that ALMA is not, however, is a survey instrument. The field of view (FoV) is given by the field of view of an individual antenna. For a 12m antenna, the FoV - aka as the ‘primary beam’ – ranges from ~62” at a wavelength of 3mm (100GHz) to 8.5” at 420 microns (720GHz), that corresponds to 0.8 – 0.016 square arcminutes, respectively. As a comparison, seen from the Earth, the Moon covers more than 2800 square arcmintes). ALMA therefore needs targets to point at and this is where the CANDELS fields become an invaluable source for ALMA targets. The GOODS-S, UDS, and COSMOS fields can be observed with ALMA, while the EGS and GOODS-N is outside the reach of ALMA.

To summarize, ALMA is already an existing scientific facility, providing the highest sensitivity and angular resolution at millimeter and submm wavelengths. The sensitivity is such that sub-L* galaxies (fainter than the most common galaxies) can be detected with a modest investment of telescope time. ALMA produces a data set that contains both spatial and spectral information. That is, is gives information on the velocity of the emission as well as an image of its distribution. ALMA has a very limited field of view and therefore needs targets to point at. This is where the synergy between ALMA and projects like CANDELS shine.

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