It contains over 150 thousand compact objects, with star formation in progress or in power, the final catalog created with data from the Herschel space telescope as part of the Hi-Gal survey. Media Inaf interviewed Davide Elia of the National Institute of Astrophysics, first author of the article presenting the new results, published in Monthly Notices of the Royal Astronomical Society
At the beginning there were almost 101 thousand. Now there are more than 150 thousand. We are talking about the compact pre- and proto-stellar sources in our galaxy, the Milky Way, cataloged as part of the Herschel infrared Galactic Plane Survey , or Hi-Gal, a project led by researchers from the National Institute of Astrophysics based on observations made by the Herschel satellite of the European Space Agency (ESA).
The first version of the catalog , published in 2017, mainly included sources in the inner part of the Milky Way, observed looking in the direction of the galactic center from our position – the Sun is located halfway between the center and the periphery. Now the new catalog adds the view on the outer part of the galaxy , allowing us to study, for the first time, the distribution of these objects on a galactic scale in unprecedented detail. An article published in June in the Monthly Notices of the Royal Astronomical Society presents the content and early results of the scientific analysis of this massive dataset. Media Inafinterviewed the first author of the article, Davide Elia , a researcher at the National Institute of Astrophysics in Rome.
Doctor Elia, what is new in this work compared to the one published four years ago?
«This article presents the final catalog of all the compact sources, ie point-like or nearly point-like, which can be the site of star formation – in progress or in the future – identified by the Hi-Gal survey in the far infrared, between 70 and 500 microns. The survey was conducted in pieces: the first corresponded to the inner part of the Milky Way, about 140 degrees straddling the galactic center, which is the most densely populated region of the galaxy, also in terms of matter that can give rise to star formation. So we released the catalog for that part first. Now we have the complete survey of the galactic plane, on a slice of the sky 2 degrees wide in latitude and 360 degrees in longitude ».
What has changed since then?
«Numerically, the portion of the catalog presented previously continues to be preponderant, because it includes the central part of the galaxy, the most populated one. However, qualitatively we added an important piece of information because we observed the part of the outer galaxy, which is less populated – the ratio between the areas observed in the new study compared to the previous one is almost 2: 1, but the number of objects it is half present – but it has characteristics that can differ considerably from those of the inner galaxy ».
What can be found in the new catalog?
«First of all, the distance of over 150 thousand compact sources, which allows us to study their distribution in the galaxy. Then there are the physical properties that depend on distance: mass and brightness. If we see a source with a certain brightness from the ground, the estimate we give for its intrinsic brightness changes according to the distance we attribute to the source, and the same goes for the mass we calculate. And then there is the physical dimension: normally we look at these maps in 2D, but only if we know the distance can we estimate how physically extended an object is. Another important novelty is that, in these four years, there has been an impressive preparatory work on the estimation of distances, which has been refined compared to the 2017 catalog not only for the new 50 thousand objects but also for the previous 100 thousand.
This is a very large dataset . How complex is it to create a catalog of this size?
«Both the previous work and this one have characteristics of dataset grandeur and complexity. Furthermore, there is an additional complication for Herschel, a happy complication we could say: Herschel observed at five different wavelengths, and the same source does not necessarily appear in all five. It can appear in some yes and in others no, for example depending on its temperature. Even where it appears in multiple adjacent bands, it can look very different from band to band. The sky changes its appearance with the wavelength and this also applies to individual sources. Putting all this information together, necessary to bring out the physical characteristics of these objects, is a fairly complex preparatory work ».
So what is all this work for?
“We finally have the possibility of making a comparison between the inner part of the galaxy, the one inside an ideal circle corresponding to the orbit of the Sun around the center of the galaxy, and the outer part, from which almost all of the objects introduced come. ex novo in this version of the catalog. If the inner part of the galaxy is the most populated and most efficient in forming new stars, the outer part puts us in front of a series of other questions: for example, the metallicity is lower – in astronomical jargon, this means a lower abundance of chemical elements heavier than helium – which can determine a different behavior of the interstellar medium, compact objects and star formation ».
Measuring distances, a fundamental aspect in this work, is notoriously a thorny subject in astronomy. What method did you use to estimate the distances from your sources?
“The estimation of distances, which is by no means trivial, was presented in an article led by our colleagues from the Laboratoire d’Astrophysique de Marseille, to which our group also contributed substantially, and which came out shortly before our article. The method used is that of kinematic distances: spectroscopy is used, if available, and we start from the molecular lines emitted by a nebula, in particular from that part of the nebula corresponding to the compact region that interests us. We identify a line – this is not trivial too, because there could be several lines along the same line of sight – and then once that line has been identified we go to measure the Doppler effect ».
What does it mean?
“The frequency at which we measure a line is slightly offset from what we would expect to measure in a ground-based laboratory from a gas emitting inside a stationary instrument. Instead, since there are movements due to the rotation of the whole galaxy, and at each distance from the galactic center each object has its own peculiar rotation speed, we must invoke a model that describes the rotation of the galaxy and that tells us the distance for each object. which we observe in a certain direction and which has a certain relative speed with respect to us.
However, when we go to solve this equation, in the direction of the inner galaxy we have two solutions, it is an intrinsic geometric problem. We have to decide which of the two solutions to choose, which are generally radically different from each other, and to resolve this ambiguity we use secondary indications from other data sets. This, on the other hand, does not happen for objects in the outer galaxy, for a geometric question, so the distances measured for objects in the outer galaxy are not affected by this ambiguity “.
It seems like a very laborious process. Is that why four years passed between the two catalogs being published?
«Yes, the estimation of the distances has certainly complicated the job. The software prepared by the colleagues in Marseille is a big “machine” that does a lot of calculations, drawing from all the spectroscopic survey databases , comparing each source with the surroundings to extract the most likely line to associate with the source, and then calculate the distance, resolves the ambiguity related to the double solution, and also considers known catalogs of distances. The latest version of the distance catalog was produced in summer 2020. Only at that point were we able to consolidate the catalog of the physical properties of the sources and conclude its scientific analysis ».
What are the main results that you have extracted from the catalog?
“In addition to distance-dependent parameters, it is also convenient for us to discuss distance-independent quantities, such as temperature, which can be derived from the shape of the continuous spectrum and the position of the peak of this spectrum. Another distance-independent parameter is the ratio of brightness to mass, because both depend on distance equally. This ratio (L / M) is very important because it is an indicator of the evolutionary state of a source ».
What is meant by the evolutionary state of a source?
“Our sources are concentrations of gas and dust called clumps , which can form one or more stars, or are already forming them. The less evolved ones, with the same mass, have a lower brightness. As star formation progresses inside such an envelope, its brightness increases, while the mass remains nearly constant because only a small part of the clump collapses to form stars. If the brightness increases in the L / M ratio, the whole fraction increases. At a certain point, the stars that are forming begin to dissipate the surrounding matter, thanks to the pressure exerted by the radiation they emit. Therefore these clumpthey lose mass and therefore not only does the luminosity increase but the mass begins to decrease, and so the L / M ratio continues to grow. In this way, we can use the L / M ratio, which does not depend on distance, to draw up a sort of evolutionary ranking, from pre-stellar objects to those hosting star formation, up to those hosting even well-evolved, massive and which already heavily influence the surrounding environment “.
And what did you discover by drawing up this evolutionary ranking?
“An interesting result is that, by representing this magnitude as a function of the distance from the center of the galaxy to the periphery, we don’t notice any particular trends. This is interesting because the galaxy’s disk does not have a uniform distribution of objects, but has strong cloud densities near the spiral arms. We do not see any particular dependence of the average evolutionary values on the position of the spiral arms, so the passage of a spiral arm – which is a kind of wave that passes through the galaxy – does not seem to speed up the star formation process. More than anything else, it seems that the arms act as collectors: they are like density waves that “accumulate” more clouds of gas and more stars as they pass, but without shortening the time of star formation.
Were you surprised by this result?
“We actually expected the absence of an evolutionary trend from the 2017 article, but we didn’t have the coverage of the entire galaxy to affirm it with the same authority as today. There were also indications from other surveys , but with lower quality than Herschel’s. We think we have provided a rather clear observational constraint to the theories that intend to explain the link between the role of spiral arms and star formation ».
Have you noticed any other interesting aspects?
«Another interesting thing is that even the“ inter-arm ”areas , between one arm and the other, are not as depopulated as one might think. This is evident both from the simple analysis of the distances and when we then combine the distances with the evolutionary indicators. So we shouldn’t expect star formation only on the spiral arms, where surely there are more objects ».
How much other science is hidden in this catalog?
“There is still a lot of it. The numbers are large, so you can study from small to large, extract information either on individual regions or on the entire galaxy. And then we also included objects from the Far Outer Galaxy , which are located at distances over 40-45 thousand light-years from the center of the galaxy – but for us they are closer because we in turn are over 27 thousand light-years from the center. . We have identified a few hundred and these lend themselves to further studies for those who deal with this part of the extreme periphery of our galaxy ».
What kind of studies can be done with the new catalog?
«The catalog lists the physical properties of these objects and therefore leaves the community the opportunity to study them further starting from a very broad statistical base. For those who are interested, for example, in a piece of the galaxy, in a particular region, or those interested in all objects with temperatures higher than a certain value, or the most massive or farthest from the galactic center, etc. Furthermore, the catalog can be used, in its entirety, to characterize our galaxy in a single number – the star formation rate, or how much matter converts into stars in a year – to then compare it with distant galaxies. It is possible to study the link between compact sources and filaments, elongated structures in molecular clouds which, especially after Herschel, they are thought to play an important role in star formation. Our research group in Iaps is strongly committed to this matter. A series of selections can also be made for observations offollow-up , for example of a spectroscopic type, also by means of interferometers ».
What are the ideal tools to continue observing these sources?
«They are sources that Herschel, despite being a truly marvelous instrument, has observed with a resolution of from a few to a few tens of arc seconds. Today we have instruments such as Alma in the southern hemisphere and Noema in the northern hemisphere that allow observations to be made at a higher resolution and go below the second of arc. We can observe these objects that with Herschel appear as large blobs, to see if they really host a single star in formation or a small cluster of cores from which single stars are forming or will form.
A “son” of Hi-Gal is AlmaGal , a large project approved with Alma whose Principal Investigator is Sergio Molinari, who was also the principal investigatorby Hi-Gal: we have extracted a thousand sources from the 150 thousand of the catalog to study their structure in more detail thanks to the resolution of Alma. Clearly with Alma it would be unthinkable to observe 150,000 objects, so we have selected the best candidates for the formation of massive stars, in which we are particularly interested. The approach is always to study star formation in a statistical way in the entire galaxy by observing a variety of physical and environmental conditions in which star formation can take place. And in any case even a thousand springs are certainly not few! ».
Besides observations with large radio telescopes and interferometers like Alma, what else is there in the future of infrared astronomy after Herschel?
«At the frequencies of our catalog we are a bit stuck: Herschel observed from 70 to 500 microns, in this domain there was the perspective of Spica which unfortunately was set aside by ESA . For this catalog, in addition to Herschel, we also used photometric data at shorter wavelengths, between 20-25 microns, from previous missions such as Spitzer , Wise and Msx . In future this band will operate the instrument Miri on board JWST which should start in the fall and that will surely help you look at these wavelengths counterparties to our clumpof dust seen in the far infrared with Herschel. It will observe them with good resolution and sensitivity so if there are stars already formed inside these objects, it will be Jwst’s task to reveal these populations to us and therefore confirm whether or not the proto-stellar or pre-stellar nature of these objects we have. established on the basis of our far infrared data. The inheritance value that this catalog has is very important, also because there will be no facilities like Herschel in the coming decades. After all, even Spica, if it had been made, would have been too similar to Herschel in terms of capacity and wavelength range observed. We expect this catalog to become a reference point for a long time, a bit like the Iras catalog was of the Eighties, before the advent of Herschel ».
Featured image: The Herschel Space Telescope (2009-2013) observed the sky in the infrared, allowing us to get a fascinating glimpse into the early life stages of stars. Credits: ESA
To know more:
- Read the article in Monthly Notices of the Royal Astronomical Society “ The Hi-Gal compact source catalog – II. The 360◦ catalog of clump physical properties “by Davide Elia, M. Merello, S. Molinari, E. Schisano, A. Zavagno, D. Russeil, P. Mège, PG Martin, L. Olmi, M. Pestalozzi, R. Plume, SE Ragan, M. Benedettini, DJ Eden, TJT Moore, A. Noriega-Crespo, R. Paladini, P. Palmeirim, S. Pezzuto, GL Pilbratt, KLJ Rygl, P. Schilke, F. Strafella, JC Tan, A. Traficante, A. Baldeschi, J. Bally, AM di Giorgio, E. Fiorellino, SJ Liu, L. Piazzo and D. Polychroni
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