A new facility in Puerto Rico - the "Alpha" system that upgrades an existing radio telescope - may lead to a breakthrough in astronomy. Its main purpose is a high-quality measurement of the "mass distribution function" of the neutral hydrogen among the galaxies, a fact that has not yet been measured in its entirety when it comes to hydrogen-poor galaxies
In a natural crater on the island of Puerto Rico, a long-term study began in February 2005 with the aim of discovering the fate of hydrogen in the universe. The research utilizes new capabilities of the world's largest radio wave telescope to establish the hydrogen mass distribution function in different types of galaxies. Other results that are expected from this research also concern the distribution of galaxies in the universe, from the environment closest to us to distances of 800 million light years from us, the nature of the "dark" galaxies that contain few stars, and more.
Although the basis of the research is the information received from the universe in the field of radio radiation, there is a crucial importance in combining data collected in different spectrum areas: starting with visible light photographs, through infrared and ultraviolet information, and ending with the addition of radio maps in other areas and X-ray measurements. In this article we will describe the new facility that enables the breakthrough in conducting the research, and we will stand up for the expected consequences for the science of astronomy from the final results.
Astronomical information is collected nowadays in different ways. Most readers are probably familiar with the wonderful images of the "Hubble" space telescope; These photos were taken from space, from the environment close to the Earth, several hundreds of kilometers above its surface. The photographs are of excellent sharpness, and some even show areas of radiation that cannot be photographed through the Earth's atmosphere. But not all astronomical information is in the field of visible light. Nowadays, scientists are assisted by various types of information, starting with high-energy X-rays and gamma rays and ending with long-wavelength radio radiation, where the energy of each photon is extremely low. Here we will deal with the latter type of information. The possibility of receiving radiation from space in the field of radio became clear only after the invention of the radio, and in fact about thirty years later - when the use of short-wavelength radio waves for communication purposes began. It turns out that radio waves with different wavelengths behave differently when they are transmitted from the Earth's surface, because around the Earth, as part of the upper atmosphere, there is a layer of ionized atoms known as the "ionosphere". From the earth it is possible to study only radiation that is able to penetrate the ionosphere.
The basics of radio astronomy
While the atmosphere we know is made up mostly of neutral molecules and atoms, the ionosphere is made up of plasma - an ionized substance, which is made up of negative and positive electric charges and conducts electricity, therefore it behaves in many ways as if it were metallic. As we know, metallic surfaces reflect electromagnetic waves, including radio radiation. The "metallic" behavior of the ionosphere layer causes radio waves to be reflected from it without being able to go out into space.
However, the reflectivity of the ionosphere depends on the wavelength of the radiation: long-wave radio waves are reflected, while short-wave radiation manages to pass through the ionosphere into space. The boundary that differentiates between a "short" and a "long" wave is a few meters; Radiation with a shorter wavelength flows into space while radiation with a wavelength exceeding a few meters is reflected back towards the Earth. Since the Earth itself conducts electricity, short-wave radio waves are reflected from the ground as they are from the ionosphere. Therefore, long-wave radio waves are received from broadcast stations located on the other side of the globe, while television broadcasts from distant stations cannot be received unless using cables or communication satellites.
The radio waves created by the inventors of the method at the beginning of the radio era had a long wavelength of tens of meters. These waves are reflected well from the ionosphere and from the surface of the earth, so the radio inventors were able to transmit information to the most distant reception stations with their help. At the end of the second decade of the 20th century, radio engineers succeeded in creating short radio waves as well. Radio waves with a short wavelength have a high frequency, so more information can be transmitted through them (in a given period of time) than with long waves.
Among the first uses of short waves were attempts to operate communication transmissions, and for this purpose elaborate antennas were built. One of the engineers involved in this was the American Carl Jansky, who worked at Bell Laboratories. Jansky studied natural radio sources that could disrupt the quality of reception. The antenna and receiver used by Yansky were sensitive to radiation with a wavelength of 14.5 meters. Yansky found that the source of some of the interference is signals from space, and not from Earth. He identified the Milky Way region as the source of the radio radiation that interfered with wireless communications. This discovery, that celestial bodies transmit information to us in the field of radio waves, was surprising and opened a new field of research, which grew stronger after World War II.
Another American, Grote Reber, who was a radio enthusiast, heard about Jansky's discovery that the Milky Way transmits radio waves. He built an antenna in his yard in the shape of a huge dish for those days, whose diameter was 10 meters, and measured with it the intensity of radiation at a wavelength of 1.9 meters, coming from different directions of the sky. Part of the radiation could be attributed to emission from hot matter (thermal radiation), but another part of the radiation was only explained in the middle of the 20th century, when the Russian-Jewish scientist Ginzburg showed that fast particles moving in a magnetic field emit radio radiation (synchrotron radiation).
Spin's story
In the years of World War II, the young Dutch doctoral student Henrik van der Hulst studied theoretical aspects of hydrogen atoms derived from quantum theory at the observatory of the city of Leiden in the Netherlands under the guidance of Prof. Jan Oort. The hydrogen atom is the simplest of the atoms: it consists of a heavy particle charged with a positive electric charge (proton) in the nucleus and a light electron charged with a negative electric charge that travels around it. The electron can be in different energy states when it is attached to the nucleus - in a sense this can be compared to different distances between the electron and the nucleus.
In the ground state, where the energy is the lowest of all states, the electron is "closest" to the nucleus. Moving away to other energy states requires energy, which must be invested by swallowing a photon (a particle of electromagnetic radiation). When an electron in an atom jumps from a higher energy level to a lower one, a photon is emitted. Van der Holst showed that even when the single electron of the hydrogen atom is in the ground state, i.e. when it is in the state closest to the nucleus, it can still have two different energy states.
The energy states that van der Holst found are related to a property known as spin, as if the electron and the proton were spinning wheels that constantly rotate around themselves without stopping. From a quantum point of view, van der Holst showed, the electron's axis of rotation can be in the same direction as that of the proton, or it can be exactly opposite to it. Van der Holst showed that when the directions of rotation of the proton in the nucleus and the electron around it are opposite, the energy state of the atom is lower than if the directions of rotation are the same. The energy difference between the two states, of parallel or antiparallel spins, is tiny.
Thirty weak photons
In a hydrogen atom, an electron falling from the third energy level per second causes the emission of a photon in the visible light range, which is red in color and has a wavelength of 656.3 nanometers. This photon is part of a spectral line that astronomers call H-alpha. The energy carried by this photon is 1.88 electron-volts. The energy of one H-alpha photon is indeed low, but it is infinitely greater than that of the photon released when the electron in the hydrogen atom changes its spin direction from parallel to anti-parallel to the spin of the proton in the nucleus. This energy transition causes the emission of a photon whose wavelength is 21 cm, a wavelength 320,000 times longer than that of an H-alpha photon.
These are, therefore, extremely "weak" photons in terms of energy. Astronomers are interested in these photons because they teach about hydrogen, the most common element in nature, and also because such long wavelengths are not affected by the interstellar dust, which is very common between the stars. The short wavelength, relative to other radiations in the radio field, also ensures that the ionosphere will not negatively affect the passage of the signals to the researchers' receivers.
The radio transmissions of the neutral hydrogen atoms in space are likened to a whistle whose pitch and intensity can be measured. The pitch is the transmission frequency (or wavelength). Let's remember that this is a spectral line, that is, a transmission whose frequency is precisely determined by atomic parameters. A deviation from this frequency can only occur if there is a relative velocity between the source of the transmission and the receiver, due to the Doppler effect.
Through careful mapping of the strength of the signals and the location from which they come, and in particular in the field of the 21 cm radiation of hydrogen, much can be learned about the universe. With the help of these measurements, the astronomers were able to show that part of the material that makes up the galaxies is "dark matter": it contributes to the force of gravity but has no signature in the visible light field. The important information obtained from the measurements of the 21 cm line is about the total amount of hydrogen in a certain galaxy. This measurement, which depends on the strength of the signal coming from a certain galaxy and the range of velocities between the galaxies where the hydrogen emission is measured, shows how much raw material for star formation there is in the galaxy.
In the second half of the 20th century, the process of star formation was deciphered. These are born inside huge gas clouds, which contain far more material than is needed to form a single star. The gas in these nebulae is mostly hydrogen, about a quarter of which is helium, and a much smaller amount of other substances. The gas clouds can be compressed due to external forces or an internal force within the content. The external spur to the cloud compression could be a shock wave from a nearby exploding star. Another external perturbation can be a strong interstellar wind - stars of various types emit huge amounts of matter, moving at high speed.
These interstellar winds are able to compact interstellar matter. The intrinsic drive of a protostellar nebula to contract could be due to the self-gravity of the nebular material. Every part of the nebula attracts every other part and consequently the cloud of interstellar matter tends to shrink and compact. Against this aspiration works the internal pressure, caused by the temperature of the cloud material.
Normally, clouds of matter are in equilibrium, but the emission of energy into space can upset this balance. The energy is emitted in the form of radiation in the far infrared range, suitable for a body whose temperature is several tens of degrees Kelvin. With the compression, the cloud can turn into stars, meaning that the density and temperature in its center will reach levels that allow nuclear fusion of the hydrogen into helium. In this situation, a new star will be born in the center of the cloud, the densest and hottest place.
The initial condition for this process to occur is that in the particular place in the universe there will be a sufficient amount of hydrogen, the raw material for the entire process. The measured parameter that determines if the condition is fulfilled is the total amount of hydrogen in a certain galaxy, which is the result of the reception of the radio signals of the neutral hydrogen in the 21 cm spectral line.
Most observations of the neutral hydrogen in the universe have so far been made with radio telescopes that measure the signal coming from only one location. This is because a single radio receiver was placed at the center of the parabolic dish that concentrates the radio signals, as Grote Reber did already in 1937. The exceptions here are the radio synthesis telescopes, which are capable of mapping an entire area, but their sensitivity is relatively low.
step by step
A first step to develop a radio telescope that is based on a single dish but allows the placement of several receivers at the focus of the antenna was made in Australia. Australia invested a lot of money in the development of the field of radio astronomy, and this brought its scientists to a world leading position. The breakthrough was in a device built more than 40 years ago, the Parkes radio telescope located in the New South Wales region and 64 meters in diameter.
The astronomers who study the universe with the help of this observatory decided to develop a device that combines several radio receivers in one device. Each receiver is extremely sensitive and collects the radiation from the sky into its own special detector. The innovation is that each receiver in the new facility is directed towards a section of sky that is slightly different from the other receivers, so that the connection of the information achieved the result of "taking a picture" in the field of radio radiation.
Due to the technical limitations of the size of the "waveguides", which transfer the radio radiation from the focus of the dish antenna to the detector itself, it was possible to install only 13 receivers, each of which is fed with radiation through a long tube, facing the telescope dish. This tube, which carries the waves from place to place, is called a "waveguide".
Each tube leading the radio radiation towards a detector is "illuminated" by a section of sky that is about 14 minutes of arc wide, that is, about a quarter of a degree. Hence, the radio camera of the Parks telescope is able to simultaneously "photograph" an area of the sky with a diameter of one degree, when only 13 pixels (image units) create the information. The image of the universe that is obtained in this way is necessarily blurry, and the sensitivity to weak radio signals is also not very high, due to the relatively small diameter of the dish antenna, only 64 meters.
The astronomers of the Arecibo Radio Observatory worked to improve the situation significantly. The limitation there, compared to the Parkes radio telescope, is that the receiving plate is fixed in the ground, since it is built inside a natural crater. The set of receivers hangs above the plate at a height of about 170 meters, weighs like a small ship (!), and is held by extremely strong steel cables. A narrow suspension bridge brings the shelter farm island in the center of the plate from a high level located on the edge of the plate. There is also a cable car, which is intended for transferring equipment and technicians who maintain the shelters.
Ionosphere research facility
The Arecibo dish was originally built, in 1963, as a facility for ionospheric research. The research was carried out for the US Air Force by Cornell University, but from the very beginning it was clear that the dish would be used for radio astronomy. In 1970, control of the facility passed from the Air Force to the American Academy of Sciences. The original surface of the plate was not solid at all, but was a metal mesh, which stretched over the natural crater and was supported by steel cables.
In the mid-70s, the plate underwent an upgrade and the grid surface was replaced by a surface built from 38,788 perforated aluminum plates. The transition from a grid with wide "holes" to metal plates with relatively small holes allowed operation at relatively small wavelengths, up to 3 cm. The diameter of the plate is 305 meters, its lowest part is 51 meters below the edge of the plate, and the receiver systems are installed as mentioned at a height of almost 170 meters above the bottom of the plate.
In the second upgrade of Arecibo, between 1992 and 1997, one of the "houses" placed in the center of the dish was replaced and contained antennas and receivers in a dome known as the "Gregorian device" and a high network wall was installed outside the receiving dish. Both devices are designed to reduce the background noise picked up by the radio telescope. The mesh wall reduces the ground that the receivers installed in the center of the antenna "see", because it only reflects the (cold) sky back to the receivers.
The universe is in a hurry
The Gregorian device is built as a system of two additional mirrors, one 23 meters in diameter and the other 90 meters in diameter. The mirrors are installed together with the receivers inside the domed structure: the height of this structure is as high as a six-story building and its weight is about XNUMX tons. The return of the electromagnetic radiation from these two additional mirrors ensures that only the transmissions coming from the piece of sky being studied will reach the receivers. Nevertheless, human transmissions have an effect on the research frequencies, as will become clear later.
In 2004, another upgrade was made to the Arecibo radio observatory by installing a receiver system similar to the one operating in Parkes, Australia. The system was built in the same Australian laboratory that built the "radio camera" with 13 pixels. Because of the limitations of the Arecibo plate, only seven receivers could be installed in this case.
high sensitivity
The advantage, compared to the Parkes radio telescope, is much greater sensitivity due to the 25 times greater light gathering capacity of the Arecibo giant dish compared to Parkes. At the same time, this size of the dish means that each receiver will see a smaller part of the sky, which means a much better angular resolution of about three minutes-of-arc, four times that of Parkes. The new system was named "Alpha", the initials of the "Arecibo L-band Feed Array" (Arecibo L-band Feed Array), where the name "L-band" is reserved for the radio field where the frequencies are of the order of a billion oscillations per second (about 1000 mega- Hertz) of the electromagnetic wave, and there appears the natural radiation of the 21 cm line of neutral hydrogen.
The astronomers interested in radio observations at a wavelength of 21 cm towards deep space gathered in Arecibo in the summer of 2004 to determine the ways of research with the new instrument (the author of the article participated in these discussions). It was decided to propose a number of studies that utilized the abilities inherent in the new system in the study of the extragalactic universe. The main question they faced was "What is the amount of hydrogen in the universe?" Apparently, this question should have been decided many years ago, since the observations of the 21 cm line began. However, it turns out that this is a difficult problem, because when the hydrogen is not "connected" to the galaxy and is part of it, it becomes ionized under the influence of the background radiation, which originates from hot stars, the radiation emitted from the environment of giant black holes that absorb matter (quasars), and shock waves in the intergalactic medium.
These factors "heat" the hydrogen and tear the single electron above each of the atoms. In the absence of this electron, the hydrogen atom is nothing more than a proton and a free electron, therefore the typical radio radiation, which has a wavelength of 21 cm, cannot be emitted.
You can usually find clouds of neutral hydrogen inside light-emitting galaxies, i.e. celestial bodies where stars already exist. Hydrogen is found within the collection of stars or in its immediate vicinity, as a shell of gas clouds around a luminous galaxy. Nevertheless, in the last decade evidence has been found for the existence of regions where there is a lot of neutral hydrogen, but from which not much starlight emanates. It is possible that these places are sites of "lazy galaxies", which do not produce stars at the same rate as other galaxies and therefore are not bright enough. The first galaxy of this type was found by chance in 1987 and it turned out to be huge, but of extremely low brightness even though it has a large amount of hydrogen. Since then, more examples have been found and it has even been hypothesized that there are many dark galaxies, which look like faint specks in sky photographs, but their hydrogen content is particularly large. If this is indeed true, then it will be possible to find a large part of the baryonic matter of the universe.
Study of objects outside the Milky Way
The main goal of the extragalactic observations using alpha is a high-quality measurement of the "mass distribution function" of the neutral hydrogen among the galaxies. This means measuring the prevalence of the various amounts of hydrogen in the population of galaxies: such and such percentages of neutral hydrogen clouds containing less than a million solar masses, such and such with a mass between one million and ten million solar masses, etc. Today this function is reasonably defined only for the large masses, of more than a hundred million solar masses of neutral hydrogen, but there may be a considerable population of relatively hydrogen-poor galaxies with low luminosities, so they are not studied. The high sensitivity of Alpha's array of receivers, together with the large reception area of the Arecibo dish, combine to enable the detection of relatively small amounts of hydrogen, which are suitable for dwarf galaxies.
The main part of the extragalactic study with Alpha will focus on sky scanning. The telescope will be placed in a fixed position and the sky will pass in front of it at the rate of rotation of the earth. According to the plan, two passes will be made over the same stretch of sky several months apart. At each pass, a heavenly source will be seen for about 12 seconds by the Alpha system; This allows reaching the required sampling depth while providing the possibility of eliminating interference. Observations in the field of radio suffer from interference by terrestrial and space radio transmissions.
Associated studies, also in the extragalactic field, are aimed at deepening the mapping of hydrogen in certain regions. For example, one area of interest is the closest rich galaxy cluster to us, which looks towards Virgo. Thousands of galaxies were identified in this cluster, mostly dwarfs, and in many of them the amount of hydrogen was even measured. The scan of the Virgo cluster with the Alpha system will deepen the sample to low masses of hydrogen, suitable for tiny galaxies. Among the dwarf galaxies, a special type are those called "irregular" (irregular), because they do not have a defined shape. The large regular galaxies are of the "elliptical" type, which are shaped like a flattened sphere, or "spiral", which have a linear shape of arms. The irregulars make up a small part of the entire population of galaxies, but their special characteristic is large amounts of hydrogen and/or relatively young stars.
In the last two decades, studies were carried out at Tel Aviv University's Weiss Observatory, which operates at the Ramon Observatory, to characterize the processes of star formation in dwarf galaxies of the "disordered" type. It turned out that both in the pale dwarfs and in the brightness the formation of stars is not a continuous process, but occurs in strong bursts of formation. A burst of star formation can last only millions of years, or tens of millions of years, and billions of years can pass between one burst and the next.
Irregular dwarf groups
The research at the Weiss Observatory focused on four groups of irregular dwarf galaxies: those with high brightness and those that are pale, and also those with a lot of hydrogen and those with almost no hydrogen. The first part of the study was carried out with galaxies in the Virgo cluster. The sequel, which is now taking place, investigates galaxies that are not members of a galaxy cluster but are very distant from any other galaxy.
While for galaxies in a cluster one can rely, as a first approximation, on a similar position between one galaxy and another, for a galaxy that is distant from any other galaxy there is no choice but to measure its speed (to determine its distance from us, due to the expansion of the universe). This is possible for many galaxies only if they are bright, so the sample tends to contain only galaxies of this type and lacks faint galaxies. To complete the sample with pale galaxies, with a lot of neutral hydrogen or with little, the researchers would prefer to use the collection of galaxies that will be detected by alpha with the Arecibo radio telescope.
Apart from this study, the expected results from the more in-depth study of the galaxy cluster in Virgo, of galaxy clusters, and more, will help in understanding the processes of star formation in galaxies. The focus on dwarf galaxies should provide important information about how the first galaxies in the universe were formed, where the star formation process took place for the first time. Dwarf galaxies billions of years ago joined together to produce large galaxies, such as our Milky Way. In fact, this process continues to occur now.
Our galaxy "swallows" a dwarf galaxy whose stars were distinguished as a separate population among the other stars towards Sagittarius. Further evidence of "galactic cannibalism" abounds among other galaxies. Even at greater distances from us, fragments of galaxies can be seen, which should in the future form one large galaxy; This can be seen in the images of the space telescope. Combining the neutral hydrogen data with information in the visible light field (for example, those collected at the Weiss Observatory), will form a data base for comparison with computational models of the development of star populations. In this way, it will be possible to learn whether even in the pale galaxies the star formation process occurs in an explosive, short way, followed by a long period of silence. These data will help to understand the evolution of the universe, the place of the radiation of stars later in the process of galaxy formation, and more.
