This ordinary gas allows us to draw the most beautiful "Galactic Picture"

(This article was first published in "Science Academy")

In the past 10 years, China's "Galactic Picture" has completed the first phase of the survey of CO and its isotopes with one telescope and 40 people. Recently, the "Galactic Picture" has started the second phase of the survey, which is expected to last 10 years.

Composite image of CO/¹³CO/C¹⁸O molecular radiation in the millimeter wave band (above) and optical image (below) of the local sky area of ​​315 square degrees in the northern galactic disk. The images are from the "Galactic Scroll" millimeter wave survey and the Pan-STARRS optical survey. ￨ Image source: The upper image is from the author, and the lower image is from Aladin Sky Atlas

So what is a sky survey? What exactly does the "Galactic Scroll" do? This article will take you to find out as a person who has experienced it firsthand.

Sky survey is actually a "census" work

Astronomy is a discipline characterized by observation and discovery. Untargeted blind sky survey is a systematic observation method that scans the scannable area of ​​the sky block by block without distinction, just like a "census" of celestial bodies. It is a basic way to discover unknown celestial bodies. The inclusive data and unexpected discoveries can meet the diverse needs of scientists. The sky survey mode is also conducive to maximizing the observation efficiency of the telescope, which can be said to be an efficient operation mode that can benefit the entire astronomical community.

Professor J. Ostriker, a theoretical astrophysicist, former head of the Department of Astronomy and Physics and vice president of Princeton University, summed up the importance of sky surveys: "Sky surveys are not the most important part of astronomy, but they are also indispensable."

Surveys aren't the most important thing in astronomy—they're the only thing. —— J. Ostrker

Sky surveys in various wavelengths have gone through rounds of internal competition, with resolution, sensitivity, and then scope. Millimeter-wave and submillimeter-wave (high-frequency radio) astronomy, as the youngest wavelength, has also gone through multiple rounds of competition in its short 50-year history.

The embryonic stage: the discovery of interstellar CO

CO is the most powerful molecular gas and the most common molecule in interstellar space. It was not discovered until 1970 by Wilson, Jeffets and Penzias in the constellation Orion[1]. Coincidentally, the first and third authors were the two engineers who won the Nobel Prize for their accidental discovery of microwave background radiation. CO isotope molecules ¹³CO and C¹⁸O were also discovered by them in 1971[2].

This combination of three spectral lines can be described as a "golden pair" that reveals the physical properties of molecular gas, such as temperature and density, and the chemical properties. From CO→¹³CO→C¹⁸O, the molecular abundance gradually decreases, the emission range gradually becomes smaller, and the spectral line intensity gradually weakens. CO can be observed in the outer cloud where most of the mass of the molecular cloud is concentrated, the isotope molecule C¹⁸O can be observed in the denser area inside the molecular cloud, and the isotope molecule ¹³CO is between the two. At the same time, the abundance ratio between them can reflect the material cycle feedback that accompanies the formation and evolution of stars under different environments [3]. This combination is beyond the reach of other spectral lines.

After the discovery of the CO molecule, many early low-frequency radio astronomers who were engaged in HI-21cm atomic gas surveys became pioneers of CO surveys. Telescopes around the world working in the 3 mm band (about 110 GHz) have also carried out a large number of CO surveys, and even many telescopes or receiver systems have been built from scratch to achieve this survey goal.

The era of widespread CO surveys

Two review articles in 1991 and 2015 comprehensively summarized the Milky Way survey projects since the discovery of the CO molecule [4,5]. Don’t be shocked by the project numbers in the figure. There are 59 of them! Pay attention! This is the key point!

A summary of CO survey projects completed internationally as of 2015 [5]. The length and width of the rectangles indicate the coverage of the galactic longitude and latitude, respectively, and the grayscale indicates the sensitivity. Red: ¹³CO survey, black: ¹²CO survey.

"The Founder" (1970-1980)

The 36-foot (about 11 meters) millimeter-wave telescope (NRAO-11m) of the National Radio Astronomy Observatory of the United States was the telescope that discovered the CO molecule and was the first to conduct CO surveys. From 1970 to 1980, it was almost the only one in the field of CO surveys. As many as ten surveys (numbered 1-6, 8-10, and 16) were conducted with it. Although the scale of the survey is at best a "mini simplified version" survey in today's view, and the data quality is also slightly rough, it is extremely pioneering. People's initial understanding of the distribution of molecular gas in the inner disk of the Milky Way, the rotation curve, and the CO isotope abundance ratio are all based on these surveys.

The era of two carriages running side by side (1980-2010)

Around 1975, the Harvard-Smithsonian Center for Astrophysics' twin 1.2-meter millimeter telescopes (CfA-1.2m) in the northern and southern hemispheres and the five-university observatory 14-meter millimeter telescope (FCRAO-14m) in the United States were completed and put into use. Driven by these two carriages, CO sky surveys entered the "highway".

More than ten sky surveys have been carried out using the CfA-1.2m telescope. If all these surveys are spliced ​​together, the survey almost covers the galactic plane from longitude l = -180° to +180°, and latitude ±35° (No. 48). The large sky coverage rate (20%) is its biggest advantage. Therefore, it provides important observational data for us to study the large-scale structure of the Milky Way. However, due to its low spatial resolution, there is still a large deviation in the detection of cloud samples, and those clouds with small mass or angular scale cannot be well detected.

There are 7 sky surveys using the FCRAO-14m telescope, the most influential of which are Mass-SB (No. 20), OGS (No. 41) and GRS (No. 53). As the aperture is 10 times higher than that of 1.2 meters, these surveys have better spatial resolution and sensitivity, but the spatial coverage has become smaller.

It is worth mentioning that the OGS and GRS surveys use an upgraded multi-pixel focal plane array receiver system, which improves observation efficiency and significantly improves the sensitivity and sampling completeness of the surveys compared to the Mass-SB survey. Taking advantage of the higher spatial resolution and sensitivity of these survey data, important progress has been made in the study of the density and velocity structure inside the Milky Way molecular clouds. However, its small spatial coverage also limits its contribution to the study of the structure of the Milky Way gas disk.

Don't forget they have

In addition to these two carriages, there were some important telescopes during this period that were also the main force of the CO sky survey at that time, including the 7-meter aperture of Bell Laboratories in the United States (Bell-7m for short) and the 4-meter aperture millimeter-wave radio telescope of Nagoya University in Japan (NANTEN-4m for short). They have made achievements in revealing the large-scale structure of the central region of the Milky Way and the distribution of super bubbles in the Milky Way.

China's CO survey: "Good things come late"

Readers may have a lot of questions at this point. We have been digging deep into the history of CO surveys for 40 years, but we have not seen any CO surveys in China in the 21st century. Is it that we don’t have telescopes that can observe CO? Or have other telescopes already completed their surveys, and ours are no longer useful?

The high technical threshold is a major reason why it is difficult to qualify for the competition. my country's 13.7-meter millimeter-wave telescope, located deep in the Gobi Desert at an altitude of more than 3,000 meters in the Qaidam Basin of the Qinghai-Tibet Plateau, was initially built in 1990, but the 3mm-band semiconductor receiver at the back end of the telescope did not pass the engineering acceptance until 1996. Since then, it has only started to work in the millimeter-wave band. It is the only telescope in my country that can observe CO, and it is of medium caliber among telescopes of the same frequency band internationally. However, before 2010, it was still in the era of single-beam receivers, and its small field of view was a major constraint on its inability to conduct sky surveys. In the era of single-beam reception, unbiased large-area sky surveys were almost the unique skill of small-aperture telescopes.

Another reality is that previous sky surveys were far from perfect and had obvious "biased" problems. Either the resolution and sensitivity were not high enough, or the sky area covered was not wide enough. In addition, the survey of the three-line combination of CO,¹³CO and C¹⁸O was still blank. And this three-line combination is just the golden pair for tracing the properties of molecular gases. It is better to retreat and make a net than to envy the fish by the abyss. Those who did not catch the previous rounds of CO surveys are ready to go.

Taking advantage of the rapid development of China's economy and science and technology, China has also begun to emerge in this field.

Reaching new heights: sky surveys that simultaneously receive multiple molecular spectral lines

The 13.7-meter millimeter-wave telescope of Qinghai Observatory under the bright starry sky and Milky Way ￨ Photo source: Riding a donkey to Tibet

“If you want to do your work well, you must first sharpen your tools.” At the end of 2010, my country successfully developed a 9-beam sideband-separated superconducting imaging spectrometer and successfully applied it to the 13.7-meter millimeter-wave telescope[6]. This upgrade expanded the previous one-eye to nine eyes to observe the starry sky at the same time, and the field of view increased by 9 times; the sideband separation technology combined with the clever intermediate frequency setting made it possible for the three spectral lines of CO,¹³CO and C¹⁸O, which differed by up to 6 GHz in frequency, to be simultaneously received by the 1 GHz bandwidth spectrometer. In addition, the application of the rapid scanning observation mode has greatly improved the observation efficiency of the telescope. Overall, these upgrades have increased the observation efficiency by nearly 60 times compared with the past. It is these continuously improved technologies that have given the 13.7-meter telescope a new ability to conduct large-area rapid surveys. The new multi-spectral line combination also enables it to explore the large-scale distribution and properties of molecular gas in interstellar space in an unusual way.

Phase I of the Milky Way Image Survey (MWISP) project was officially launched in September 2011, conducting an unbiased blind survey of CO, ¹³CO and C¹⁸O in the observable sky area of ​​±5° in the northern galactic plane. After 10 years of hard work by about 40 people, especially the operators of the 13.7-meter telescope, a beautiful Milky Way color map was finally completed!

The arrival of the millimeter-wave color image era has brought the CO sky survey to a new level. A sky survey database with both beauty and wisdom and no obvious bias has been established.

Travel back in time to the era of "black and white TV"

Partial image of the Milky Way color map obtained by the “Galactic Scroll” - a color map of the typical giant molecular cloud complex W3 (about 2.5 deg²). ￨ Image source: author; data are from the Milky Way Scroll, and the publicly released archival data of the OGS and CfA 1.2-meter sky surveys

As the saying goes, great minds think alike. Internationally, Australia's Mopra-22m and Japan's National Radio Astronomy Observatory's NRO-45m millimeter-wave telescope followed closely and joined the CO triple-line survey camp around 2013.

Which CO survey company is the best?

It is important to find the right position, both in life and in sky surveys! Only by recognizing these things can we better play to our strengths while avoiding or making up for our weaknesses, and better explore the scientific value of data. In order to clarify these issues, we compared the Milky Way Scroll with the 10 most influential sky surveys in the world.

Sensitivity is the sensitivity per unit area at the same spectral resolution and at the ¹²CO frequency. The larger the circle, the higher the sensitivity.

Spatial resolution and spatial coverage; right: velocity resolution and velocity coverage; the sensitivity per unit area is indicated by the size of the circle, the larger the circle, the higher the sensitivity. The cross marks the survey with incomplete sampling in space. (Image source: author)

The characteristics of each survey are clearly shown on paper. There is no doubt that the biggest advantage of the CfA-1.2m survey is its large coverage of the sky area; the biggest advantage of the FUGIN survey of the NRO-45m telescope is its high spatial resolution;

The “Galactic Scroll” survey is relatively balanced in all aspects, and has remarkable advantages and features:

High sensitivity

Multi-line probe

Complete sampling of large sky areas

High spectral resolution and wide velocity coverage

The end is also a new beginning

In the new round of competition, the MWISP data quality is superior and performs well in all aspects. A series of scientific discoveries have been made using the data accumulated in the early stage of the survey. From the discovery of a spiral arm structure farthest from the center of the Milky Way to the revelation of the properties of the Milky Way's molecular thick disk, from the establishment of a complete molecular cloud sample to the discovery of statistical laws of the physical and chemical properties of molecular clouds, from the identification of large-scale molecular inward/outward flow candidates to the search for evidence of SNR interactions between molecular clouds and HII regions, these new results are changing our understanding of the large-scale structure of the Milky Way and interstellar molecular clouds. We can expect more discoveries to be made from the subsequent more systematic analysis of MWISP data.

So has the road of sky surveying come to an end, and is it time to rest? Of course not, there is still room for improvement in many aspects of sky surveying.

Although the total flux detected by MWISP has increased significantly with the improvement of its sensitivity (reaching 1.6 times that of the CO survey of the CfA1.2m telescope and the OGS survey of the FCRAO-14m telescope), the flux detection completeness of MWISP at the current sensitivity is roughly estimated by extrapolation and interpolation to be only 58% on average, which means that there are still some leaks. Moreover, the degree of flux loss increases rapidly with the distance. For example, in the Outer Scutum-Centaurus Arm (OSC) at the edge of the Milky Way, the ratio of the flux detected by MWISP to that detected by the OGS and CfA surveys is 7.4 and 43.8 respectively, and the flux completeness of MWISP in this spiral arm is only 32% [7]. It is not difficult to understand that the current survey's detection capability of the Milky Way is still quite limited.

The CO continuum radiation obtained by the Planck satellite [8], the solid and dashed lines indicate the range of the “Galactic Panorama” Phase I and Phase II surveys.

In addition, the "Galactic Picture Scroll" Phase I sky area covers the galactic latitude of ±5°, which is still very limited. In order to peek into the vast Milky Way and cover a wider area of ​​the sky, the "Galactic Picture Scroll" Phase II sky survey with a galactic latitude of ±10° came into being and was launched on September 1, 2021.

A new dream has set sail, and it is expected that the CO molecular spectrum survey will reach a higher level in the next decade. But even so, these surveys only cover a small part of the sky, and there are more areas that need to be carefully surveyed, such as the familiar Orion, Taurus, Ophiuchus... Based on the current technical status, they still cannot be effectively covered and are far beyond the survey range.

There is no end to scientific pursuits, nor to technological demands. Sky surveys are always on the way; larger sky area coverage, higher sensitivity, higher resolution, and greater efficiency... are the eternal pursuits of sky surveys.

References:

[1] Wilson, RW, Jefferts, KB, Penzias, AA, 1970, ApJL, 161, 43

[2] Penzias, AA, Jefferts, KB, Wilson, RW, 1971, ApJ, 165, 229

[3] Microwave Spectral Line Diagnosis in Astrophysics, edited by Zeng Qin, Mao Ruiqing, and Pei Chunchuan, Science and Technology Press of China

[4] Combes, F. 1991, ARA&A, 29, 195

[5] Heyer, M., & Dame, TM, 2015, ARA&A, 53, 583

[6] Shan, WL, Yang, J., Shi, SC, et al. 2012, ITTST, 2, 593

[7] Sun, Y., Yang, J., Yan, Q.-Z. et al. 2021, ApJS, 256, 32

[8] Planck Collaboration, 2014, A&A, 571, A13

About the Author

Sun Yan

Associate researcher at Purple Mountain Observatory, Chinese Academy of Sciences, member of Youth Innovation Promotion Association of Chinese Academy of Sciences, and core member of the “Galactic Panorama” sky survey.

Yang Ji

Researcher at the Purple Mountain Observatory of the Chinese Academy of Sciences and chief investigator of the "Galactic Panorama" survey.

Rotating Editor-in-Chief: Ji Jianghui

Editor: Wang Kechao