ALMA

Most of the matter in the universe is cold. Not the dramatic cold of deep space, but the scientifically critical cold of molecular clouds where stars are born, protoplanetary disks where planets assemble, dusty envelopes around dying stars, and the reservoirs of gas that fuel galaxies. This cold matter radiates not in visible light or X-rays but at millimeter and submillimeter wavelengths, in the spectral territory between infrared and radio. For most of the history of astronomy, this wavelength range was nearly inaccessible: the instruments were crude, the atmosphere absorbed most of the signal, and the angular resolution was too poor to reveal meaningful structure. The Atacama Large Millimeter/submillimeter Array changed that. ALMA is the most powerful telescope ever built for observing the cold universe, and it has transformed every field it touches.

Why Millimeter Wavelengths

Millimeter and submillimeter astronomy occupies a unique scientific niche. At these wavelengths (roughly 0.3 to 10 millimeters, corresponding to frequencies of 30 to 950 GHz), the universe reveals phenomena invisible at other wavelengths.

Thermal dust emission: Interstellar dust grains, heated to temperatures of 10-50 Kelvin by ambient starlight, emit thermal radiation that peaks in the far-infrared and extends into the submillimeter. Dust is mixed with gas throughout the interstellar medium, so dust emission traces the distribution of cold gas, the raw material of star and planet formation. Submillimeter observations of dust are the primary tool for measuring the masses of molecular clouds, protoplanetary disks, and the gas reservoirs of distant galaxies.

Molecular line emission: Molecules in cold gas emit or absorb photons at specific frequencies corresponding to transitions between rotational energy levels. These rotational transitions fall predominantly in the millimeter and submillimeter bands. Carbon monoxide (CO) is the most commonly observed molecule (it is the second most abundant molecule in the universe after H2, and unlike H2 it has a permanent dipole moment and thus produces easily detectable emission), but ALMA can observe hundreds of molecular species: water, formaldehyde, methanol, hydrogen cyanide, complex organic molecules, and prebiotic chemistry building blocks like glycolaldehyde (a simple sugar). Molecular line observations provide not just the distribution of gas but its temperature, density, velocity, and chemical composition.

The high-redshift universe: The cosmic microwave background peaks at about 1 millimeter. Dust emission from distant galaxies, redshifted by cosmic expansion, is shifted into the millimeter bands observable by ALMA. Heavily dust-obscured galaxies at high redshifts (z > 2), which are nearly invisible at optical wavelengths because their starlight is absorbed by dust and re-radiated in the infrared, are among ALMA's most important targets. These dusty star-forming galaxies produce stars at prodigious rates (hundreds to thousands of solar masses per year) and represent a major phase of galaxy evolution that optical surveys systematically miss.

Earth's atmosphere absorbs strongly at many millimeter and submillimeter wavelengths, primarily due to water vapor. Observing at these wavelengths requires a site that is extremely high (above most of the atmosphere) and extremely dry (minimal water vapor). This requirement drove the choice of ALMA's location.

The Site: Chajnantor

ALMA is located on the Llano de Chajnantor, a plateau at 5,058 meters elevation in the Atacama Desert of northern Chile. The Atacama is one of the driest places on Earth; some weather stations in the desert have never recorded rainfall. At Chajnantor's altitude, the precipitable water vapor column (the total amount of atmospheric water vapor above the site, expressed as the depth of liquid water it would produce if condensed) is frequently below 1 millimeter, and occasionally below 0.5 millimeters. These are among the lowest values found at any accessible location on Earth, and they are essential for submillimeter transparency: at sea level, atmospheric water vapor absorption renders most of ALMA's observing bands opaque.

The altitude exacts a human cost. At 5,058 meters, the atmospheric pressure is roughly half that at sea level, and the oxygen partial pressure is correspondingly reduced. Working at the Array Operations Site requires acclimatization, and sustained activity is physically demanding. The Operations Support Facility (OSF), at 2,900 meters, serves as the base for staff and the correlator facility. Astronomers and engineers commute to the high site for maintenance and do not remain overnight.

The Array: 66 Antennas, One Telescope

ALMA is an interferometer: an array of individual antennas whose signals are combined electronically to synthesize a single telescope with angular resolution determined by the maximum separation between antennas, not by the diameter of any individual dish.

The array consists of 66 high-precision antennas: fifty 12-meter antennas in the main array, twelve 7-meter antennas and four 12-meter antennas in the Atacama Compact Array (ACA, also called the Morita Array). The main array antennas can be repositioned on 192 concrete pads using custom-built antenna transporters (each transporter carries a 115-ton antenna at walking speed), allowing the array to be reconfigured between compact configurations (baselines of roughly 150 meters, for sensitivity to extended emission) and extended configurations (baselines up to 16 kilometers, for maximum angular resolution).

In its most extended configuration, ALMA achieves angular resolution as fine as 5 milliarcseconds at its highest operating frequencies, sharper than Hubble and comparable to VLBI at centimeter wavelengths. This resolution is sufficient to resolve the orbital motion of gas around protostars, distinguish individual clumps in protoplanetary disks, and map the detailed structure of molecular emission in nearby galaxies.

The antennas are manufactured by three different consortia (contributing to the partnership between ESO, the National Science Foundation, and the National Institutes of Natural Sciences of Japan that funds and operates ALMA) but are designed to meet identical specifications: surface accuracy of 25 micrometers (essential for observing at wavelengths as short as 0.3 millimeters), pointing accuracy of 0.6 arcseconds, and operation in winds up to 9 meters per second.

The signals from all antennas are brought via fiber optic cables to the ALMA correlator, a special-purpose supercomputer at the OSF that cross-correlates the signals from every pair of antennas simultaneously. With 66 antennas, there are 2,145 unique antenna pairs (baselines), each producing a visibility measurement. The correlator performs roughly 17 quadrillion operations per second, making it one of the most powerful special-purpose computers ever built.

Protoplanetary Disks: Watching Planets Form

ALMA's most iconic scientific contribution is the direct imaging of protoplanetary disks with sufficient resolution to see the structures carved by forming planets. The first and most famous of these images is HL Tauri, observed during ALMA's Long Baseline Campaign in 2014.

HL Tauri is a young star (less than one million years old) in the Taurus star-forming region, roughly 450 light-years from Earth. ALMA's image at 1.3 millimeters revealed the thermal dust emission from the star's protoplanetary disk in extraordinary detail: a series of concentric bright rings and dark gaps, extending from a few astronomical units to roughly 100 AU from the star. The gaps are widely interpreted as evidence for planets forming within the disk, their gravitational influence clearing lanes in the dust distribution.

The HL Tauri image was transformative because the star is so young. Planet formation theories had generally assumed that the process of clearing gaps in a disk took millions of years; HL Tauri showed that significant disk structuring occurs within the first million years of a star's life, far earlier than expected. The image became one of the most reproduced astronomical images of the decade and catalyzed a new era of disk structure studies.

Subsequent ALMA surveys have revealed that disk substructure, rings, gaps, spirals, asymmetries, and crescents, is ubiquitous. The DSHARP (Disk Substructures at High Angular Resolution Project) survey (2018) imaged 20 nearby protoplanetary disks at 5 AU resolution, finding that essentially every disk showed structure. The diversity of structures observed (some disks have many narrow rings, others show broad gaps, others display asymmetric crescents or spiral arms) suggests a diversity of planet formation histories.

ALMA observations of gas (through molecular line emission, particularly CO and its isotopologues) complement the dust images. Gas observations reveal velocity fields that can indicate the gravitational influence of unseen planets (through deviations from Keplerian rotation), measure disk masses and temperatures, and identify chemical gradients that affect the composition of forming planets.

Star Formation

ALMA observes the full sequence of star formation, from molecular cloud cores on the verge of collapse to actively accreting protostars to pre-main-sequence stars with clearing disks.

Molecular cloud cores, the dense (10^4-10^6 molecules per cubic centimeter), cold (10 K), compact (0.01-0.1 parsec) regions that collapse to form individual stars or small stellar systems, are best observed at millimeter wavelengths through thermal dust continuum emission and molecular line emission. ALMA resolves these cores in nearby star-forming regions, measuring their masses, temperatures, velocity fields (infall signatures, rotation, turbulence), and magnetic field structures (through polarized dust emission, which traces the magnetic field geometry).

Protostars, still deeply embedded in infalling envelopes, drive bipolar outflows and jets that sweep up surrounding material. ALMA resolves the launching regions of these outflows, measuring the velocity structure of the molecular gas being accelerated by the protostellar jet. These observations constrain the mechanisms by which protostars shed angular momentum (a necessary process, since the angular momentum of the collapsing core is far too large for the protostar to absorb).

ALMA observations of astrochemistry in star-forming regions have detected complex organic molecules, including methyl formate, dimethyl ether, glycolaldehyde, and tentative detections of amino acid precursors, in the gas surrounding protostars. These molecules form on dust grain surfaces and are released into the gas phase as the protostellar luminosity heats the surrounding material. The chemical inventory of the protostellar environment determines the starting conditions for planetary chemistry, connecting interstellar chemistry to the molecular composition of comets and the volatile inventories of forming planets.

Galaxy Evolution and the Distant Universe

ALMA has transformed the study of galaxies at high redshifts by detecting their dust and molecular gas emission, providing measurements of gas masses, star formation rates, and kinematics that are inaccessible at optical wavelengths for dust-obscured systems.

Submillimeter galaxies (SMGs), first detected by SCUBA in the late 1990s but unresolved and poorly characterized, have been studied individually by ALMA with arc-second resolution. Many turn out to be massive merging systems with gas reservoirs of 10^10-10^11 solar masses, forming stars at rates of 100-1000 solar masses per year. Their contribution to the cosmic star formation rate density at z = 2-4 is comparable to that of optically selected galaxies, meaning that roughly half of all star formation in the universe's most active epoch was hidden by dust and missed by optical surveys.

ALMA has also detected molecular gas in galaxies in the epoch of reionization (z > 6), within the first billion years of cosmic history. Detections of [CII] (the 158-micrometer fine-structure line of ionized carbon, redshifted to ALMA's bands at z > 4) and CO in these galaxies demonstrate that massive gas reservoirs and heavy-element enrichment were already in place remarkably early, constraining models of early galaxy assembly.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) conducted a blind survey of molecular gas in a cosmological volume, providing an unbiased census of the cold gas content of galaxies across cosmic time. The results show that the molecular gas fraction of galaxies peaks at z ~ 1-3, coinciding with the peak of cosmic star formation, and declines toward the present day as galaxies consume their gas reservoirs.

Event Horizon Telescope: ALMA as the Anchor

ALMA's most visible contribution to public awareness came through its role as the most sensitive station in the Event Horizon Telescope (EHT) network. The EHT is a Very Long Baseline Interferometry (VLBI) array that links millimeter-wavelength telescopes across the globe to achieve angular resolution sufficient to image the shadow of a supermassive black hole's event horizon.

ALMA's contribution to the EHT is disproportionately important because its collecting area (equivalent to a single dish roughly 85 meters in diameter) exceeds that of all other EHT stations combined. In VLBI, the sensitivity of each baseline is proportional to the geometric mean of the collecting areas of the two stations. ALMA's enormous collecting area therefore dramatically improves the sensitivity of every baseline that includes it, making it the anchor of the EHT array.

The EHT's 2017 observations, which produced the first image of the shadow of the supermassive black hole in M87 (published April 2019) and the image of Sagittarius A* at the center of the Milky Way (published May 2022), would not have been possible without ALMA's participation. The images confirmed the predictions of general relativity for the appearance of a black hole's event horizon and provided measurements of the black hole masses and spin orientations.

The Partnership

ALMA is the most expensive ground-based astronomical facility ever built, with a total cost exceeding $1.4 billion. It is operated as a partnership among three organizations: ESO (representing its European member states), the U.S. National Science Foundation (through the National Radio Astronomy Observatory), and the National Institutes of Natural Sciences of Japan (through the National Astronomical Observatory of Japan). Chile, as the host country, receives guaranteed observing time.

Observing time is allocated through a competitive proposal process, with proposals evaluated by international review panels. Demand exceeds available time by roughly a factor of five. All data becomes publicly available through the ALMA Science Archive after a 12-month proprietary period.