Eduardo Amores and colleagues presented a new web-based tool called GALExtin, which can be used to determine the interstellar extinction in the milky way. Their study recently appeared in Arxiv.
In a broad range of astronomical research, estimates of interstellar extinction are essential. Several maps and models have been published of the large scale interstellar extinction in the Galaxy. However, these maps and models have been developed in different programming languages, with different user interfaces and input/output formats, which makes using and comparing results from these maps and models difficult.
Now, Eduardo B. Amôres and colleagues addressed this issue by developing a tool called GALExtin that estimates interstellar extinction based on both 3D models/maps and 2D maps available. The user only needs to provide a list with coordinates (and distance) and to choose a model/map. GALExtin will then provide an output list with extinction estimates. It can be implemented in any other portal or model that requires interstellar extinction estimates.
How it works?
GALExtin works with two layers. The first one is a client that provides a web form to be filled out by the users, e.g. data such as coordinates and distance for the 3D extinction. Alternatively, the user can insert a list of coordinates (Galactic or Equatorial) with distances. It is also necessary to select the coordinate system and the desired model/map.
In the HTML code, the embedded PHP program accesses an SQL table to attribute a number to the process, unique identification for each run of GALExtin, which is also used to assign and manipulate input and output files names.
Once PHP receives a process number and the information passed through HTML, it calls an IDL program that manages the extinction computation. It verifies the number of input lines. Its primary task consists of calling the routines that compute the interstellar extinction for each chosen model/map.
In the final step, the routine returns the extinction estimates to the main program. The output is displayed in the web form if a single direction is given, or written to a file if a file with coordinates was given. For either Galactic or Equatorial coordinates, the values must be given in decimal degrees.
They also validated their tool by comparing the results of GALExtin with the ones obtained with the original software of the models, dustmaps, as well as with the values obtained using the Vizier−CDS in the case of some maps. You can refer their paper to explore these details. They are also planning to include several other excellent extinction maps and models in GALExtin.
Reference: Eduardo B. Amores, Ricardo M. Jesus, Andre Moitinho, Vladan Arsenijevic, Ronaldo S. Levenhagen, Douglas J. Marshall, Leandro O. Kerber, Roseli Kunzel, Rodrigo A. Moura, “GALExtin: An alternative online tool to determine the interstellar extinction in the Milky Way”, Arxiv, pp. 1-12, 2021. https://arxiv.org/abs/2108.00561
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Between the stars in our Milky Way, vast amounts of tiny dust grains are floating aimlessly around. They form the building blocks of new stars and planets. But we still don’t know what elements exactly are available to form planets like Earth. A research team at SRON led by Elisa Costantini has now matched observations from X-ray telescopes with data from synchrotron facilities to create a map of interstellar grains in the Milky Way.
If our Galaxy shrank to the point where stars have the size of marbles, there would still be about a thousand kilometers between each of them. So it is safe to say that galaxies consist mostly of empty space. Still, this space is not as empty as you might think. It is filled with the so-called interstellar medium. For the larger part this is made up of tenuous gas, but around one percent is in the form of tiny grains of about 0.1 micron—one thousandth the width of a human hair.
These grains are formed during the life cycle of stars. A star, and the planets around it, is formed by a collapsing cloud of gas and dust. When the star evolves toward the end of its life, it expels a good fraction of its mass in the surrounding medium, creating new material for dust formation. If the star ends its life with a supernova explosion, it will further enrich the environment with even more gas and dust. This in turn will eventually constitute new building blocks for stars and planets. As Carl Sagan said: “we are made of stardust”. But what elements exactly are available in the interstellar medium to form planets like Earth is still unknown.
The interstellar dust research group at SRON Netherlands Institute for Space Research led by Elisa Costantini has now studied the interstellar grains in our Milky Way using X-ray radiation. They could, for the first time, explore the properties of the dust in central regions of the Galaxy and found that those grains are consistently made of a glassy silicate: olivine, which is a compound of magnesium, iron, silicon and oxygen. The interaction with stellar radiation and cosmic rays melted these grains to form little glassy irregular spheres. When moving further out to more diffuse regions away from the Galactic center, the team found clues for the presence of a larger variety in dust composition. This may give rise to diversified planetary systems. It might even be that our planetary system is the exception rather than the norm.
Costantini: ‘Our solar system was formed in the outer regions of the Galaxy and is the result of a complex sequence of events, including nearby supernova explosions. It remains an open question what is the right environment to form planetary systems and which of these events are vital to form a planet where life can flourish.’
To get to their results, Costantini and her group matched observations from X-ray telescopes and synchrotron facilities. They used the latter to characterize the features that interstellar dust analogues like silicates, oxides and sulfates produce in X-rays. Then they compared these to the astronomical data to find the best matches. Observing several lines of sight allowed them to explore different environments of the Milky Way.
The research team used using the synchrotron facilities Soleil-LUCIA beamline, the Dubble-ESRF beamline and the Titan electron microscope at the University of Cadiz. On the astronomical side, they used the X-ray observatories XMM-Newton (ESA) and Chandra (NASA). The exploitation of these experimental data and application to astronomical data has been supported by an NWO-VIDI grant number: 639.042.525 (PI: E. Costantini).
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
The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by UCL and University of Oxford researchers.
For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.
The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.
In the study, published in the Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar – that is, they revolve around the galaxy at the same rate as the bar’s spin.
These stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter’s Trojan and Greek asteroids, which orbit Jupiter’s Lagrange points (ahead and behind Jupiter). If the bar’s spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period matched to that of the bar’s spin.
The researchers found that the stars in the stream carry a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.
Using this data, the team inferred that the bar – made up of billions of stars and trillions of solar masses – had slowed down its spin by at least 24% since it first formed.
Co-author Dr Ralph Schoenrich (UCL Physics & Astronomy) said: “Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.
“The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.
“Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”
Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: “Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.
“Our finding also poses a major problem for alternative gravity theories – as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”
The Milky Way, like other galaxies, is thought to be embedded in a ‘halo’ of dark matter that extends well beyond its visible edge.
Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.
Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein’s theory of general relativity.
The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar. The bar rotates in the same direction as the galaxy.
The research received support from the Royal Society, the Takenaka Scholarship Foundation, and the Science and Technology Facilities Council (STFC).
An artist’s conception of the Milky Way. Source: Wikimedia Commons. Credit: Pablo Carlos Budassi.
Reference: Rimpei Chiba, Ralph Schönrich, Tree-ring structure of Galactic bar resonance, Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 2, August 2021, Pages 2412–2426, https://doi.org/10.1093/mnras/stab1094
At the center of our Milky Way lies a supermassive black hole (SMBH) called Sagittarius A* (SgrA*). Supermassive black holes reside at the centers of most galaxies, and when they actively accrete gas and dust onto their surrounding hot disks and environments they radiate across the electromagnetic spectrum. The mass of SgrA* is about four million solar-masses, much smaller than the billions of solar-mass SMBHs seen in some galaxies. However it is relatively close by, only about twenty-five thousand light-years distant, and this proximity provides astronomers with unique opportunities to probe the properties of SMBHs.
Sag A* has been monitored at radio wavelengths since its discovery in the 1950’s. Variability was first reported in the radio in 1984, and subsequent infrared, submillimeter, and X-ray observations confirmed variability and found that it often flares. Monitoring programs have concluded that on average Sgr A* is accreting material at a very low rate, only a few hundredths of an Earth-mass per year. The fascination with SgrA*’s variability has a practical diagnostic reason too: changes in emission are a measure of the dimensions of the region, set by the time for light to travel across it. Flares have been measured that doubled in strength in less than forty-seven seconds, for example, a time that corresponds to a distance about as small as this black hole’s fundamental event horizon size (light cannot escape from within this boundary). These conclusions are in agreement with size inferences made with radio and near infrared interferometry.
CfA astronomers Steve Willner, Giovanni Fazio, Mark Gurwell, Joe Hora, and Howard Smith have been studying the infrared variability of SgrA* with the IRAC camera on Spitzer, combined with simultaneous X-ray and submillimeter variability with Chandra and the Submillimeter Array. They recently teamed with colleagues to analyze and model a comprehensive set of X-ray, near infrared, and submillimeter observations taken by multiple groups over several decades. The statistical modeling examines the relative timing of flare events and the frequency and duration of variability at each of the different wavelengths. The astronomers conclude that the variable emission probably arises predominantly from a region about twice the size of the event horizon, and that the same related physical activity is often producing the multiple events seen at different wavelengths. The quantitative models also imply the presence of a dense plasma of electrons along with a modestly strong magnetic field. These conclusions are the first to show that a simple physical model can explain most of the features of Sgr A*’s variability and the correlations between the X-ray, IR, and submillimeter emission, but many puzzles still remain including the origin of the strongest infrared flares and the reason for the long timescale of variability seen in the submillimeter.
Featured image: A schematic image of one stage of accretion around the supermassive black hole in the Milky Way’s center. Material flows into a spherical region around the black hole with a magnetic field; subsequent compression and expansion of the hot gas produces the infrared and submillimeter emission while scattering produces the X-ray emission. A new paper examines a comprehensive set of multiwavelength, multi-epoch data and presents a relatively simple physical model that can explain most of the variable features. Witzel et al. 2021
Reference: “Rapid Variability of Sgr A* across the Electromagnetic Spectrum,” G. Witzel, G. Martinez, S. P. Willner, E. E. Becklin, H. Boyce, T. Do, A. Eckart, G. G. Fazio, A. Ghez, M. A. Gurwell, D. Haggard, R. Herrero-Illana, J. L. Hora, Z. Li, J. Liu, N. Marchili, Mark R. Morris, Howard A. Smith, M. Subroweit, and J. A. Zensus, The Astrophysical Journal Supplement Series 2021 (in press).
Researchers from the University of Arizona have detected organic molecules in planetary nebulae, the aftermaths of dying stars, and in the far reaches of the Milky Way, which have been deemed too cold and too removed from the galactic center to support such chemistries. They present their findings at the 238th Meeting of the American Astronomical Society, or AAS, held virtually from June 7-9.
A team led by Lucy Ziurys at the University of Arizona reports observations of organic molecules in planetary nebulae in unprecedented detail and spatial resolution. Using the Atacama Large Millimeter Array, or ALMA, Ziurys and her team observed radio emissions from hydrogen cyanide (HCN), formyl ion (HCO+) and carbon monoxide (CO) in five planetary nebulae: M2-48, M1-7, M3-28, K3-45 and K3-58.
The emission from these molecules was found to outline the shapes of planetary nebulae, which previously had only been observed in visible light. In some cases, molecular signatures revealed previously unseen features. The high resolution of one arcsecond, equivalent to a dime viewed from 2.5 miles away, resulted in striking images of the nebulae, clearly showing the complex geometries of the dense, ejected material with bars, lobes and arcs never clearly observed before.
Planetary nebulae are bright objects, produced when stars of a certain type reach the end of their evolution. Most stars in our galaxy, including the sun, are expected to end their lives this way. As the dying star sheds large amounts of its mass into space and becomes a white dwarf, it usually emits strong ultraviolet radiation. This radiation was long thought to break up any molecules hurled into the interstellar medium from the dying star and reduce them to atoms.
Detection of organic molecules in planetary nebulae in recent years have shown that this is not the case, however, and the observations presented here further support the idea that planetary nebula serve as critical sources that seed the interstellar medium with molecules that serve as the raw ingredients in the formation of new stars and planets. Planetary nebulae are thought to provide 90% of the material in the interstellar medium, with supernovae adding the remaining 10%.
“It was thought that molecular clouds which would give rise to new stellar systems would have to start from scratch and form these molecules from atoms,” said Ziurys, a Regent’s Professor of Chemistry and Astronomy at UArizona. “But if the process starts with molecules instead, it could dramatically accelerate chemical evolution in nascent star systems.”
Ziurys and her team believe the shape shifting behavior in the nebulae’s geometry may be driven by certain processes involved in nucleosynthesis, in other words, the forging of new elements inside a star.
“It tells us that in a dying star, which is spherical until its final phase, some very interesting dynamics occurs once it goes through the planetary nebula stage, which changes that spherical shape,” Ziurys said. “These stars just lose their mass, and so there’s really no mechanism for them to all of a sudden become bipolar or even quadrupolar.”
According to the researchers, one possible explanation could be helium flashes, which originate in a hot, convective shell around the core of a dying star and could possibly provide a source of explosive nuclear synthesis away from the star’s center, resulting in the very complex shapes seen in some nebulae.
“This could probably distort the spherical shape because a helium flash can explode through the poles of a star, where it will be directed by magnetic fields, and that will have an effect on the shape of the nebula that will form around it,” she said.
According to Ziurys, many planetary nebulae are something of an enigma, because they evolved from spherical stars but then gave rise to bipolar or even quadrupolar structures.
“It’s been a puzzle to astronomers as to how you go from a spherical geometry into these multipolar geometries,” she said. “The molecules we observed trace the polar geometries beautifully, and so we’re hoping that this is going to give us some insight into the shaping of planetary nebulae.”
In a second presentation, Lilia Koelemay, a doctoral student in Ziurys’ research group, will report on the discovery of organic molecules in the outskirts of the Milky Way, more than twice as far from the galactic center than what is known as the Galactic Habitable Zone, or GHZ.
The Milky Way’s GHZ, which includes the solar system, is a region considered to have favorable conditions for the formation of life. It is thought to extend to only up to 10 kiloparsecs, or about 32,600 light-years, from the galactic center.
Using the UArizona ARO 12-Meter Telescope on Kitt Peak near Tucson, Arizona, Koelemay, Ziurys and team searched 20 molecular clouds in the Milky Way’s Cygnus arms for signature emission spectra of methanol, a basic organic molecule. At a mere 20 Kelvin, these clouds are typically extremely cold and far from the galactic center, at a distance of 13 to 23.5 kiloparsecs. The team detected methanol in all 20 clouds.
According to Koelemay, the detection of these organic molecules at the galactic edge may imply that organic chemistry is still prevalent at the outer reaches of the galaxy, and the GHZ may extend much further from the galactic center than the current established boundary.
“Scientists have wondered about the extent of organic chemistry in our galaxy for a long time, and it was always thought that not too far beyond our sun, we’re not going to see a lot of organic molecules,” Koelemay said. “The widely held assumption was that in the outskirts of our galaxy the chemistry necessary to form organics just doesn’t occur.”
That conclusion was partly based on the supposed dearth of organic molecules in the outer reaches of the galaxy, according to the researchers. The notion of the galactic habitable zone is based on the idea that for habitable conditions to exist where life can evolve, a planetary system can’t be too close to the galactic center with its extremely high density of stars and intense radiation, and it can’t be too far out, because there would not be enough elements critical for life, such as oxygen, carbon and nitrogen.
The observations were made possible by a new 2-millimeter wavelength receiver with unprecedented sensitivity. Developed in a collaboration between Ziurys, Gene Lauria, an engineer at Steward Observatory, and the National Radio Astronomy Observatory, the receiver allows for detection of molecular emission lines in a wavelength bandwidth radio astronomers in the US could not access for many years.
“Without this new instrument, these observations would have taken hundreds of hours, which is not feasible,” Ziurys said. “With this new capability, we expect to dramatically open our observation window and detect molecules in other regions of our galaxy previously thought to be devoid of such chemistry.”
Recently, Koelemay has begun to look for other molecules besides methanol, such as methyl cyanide, organic molecules with ring structures and others that contain functional groups known to be crucial building blocks for biomolecules. Discoveries of these molecules in the interstellar medium have attracted much interest, as many researchers deem them promising candidates for the emergence of life. When organic molecules are present in emerging planetary systems, they can condense onto the surfaces of asteroids, which then deliver them to nascent planets, where they could potentially jump-start the evolution of life.
“We’re finding these species way on the outskirts of the galaxy, and the abundance doesn’t even drop off 10 kiloparsecs from the solar system, where the chemistry necessary for building the molecules necessary for life just wasn’t believed to occur,” said Ziurys, Koelemay’s adviser and a co-author on the report. “The fact that they’re there expands the prospects of habitable planets forming far beyond what has been considered the habitable zone is extremely exciting.”
Researchers from the University of Arizona will present findings from radio-astronomical observations of organic molecules at the 238th Meeting of the American Astronomical Society, or AAS, during a press conference titled “Molecules in Strange Places” at the 238th AAS Meeting on Tuesday, June 8, at 12:15 p.m. EDT.
It’s hard to see more than a handful of stars from Princeton University, because the lights from New York City, Princeton and Philadelphia prevent our sky from ever getting pitch black, but stargazers who get into more rural areas can see hundreds of naked-eye stars — and a few smudgy objects, too.
The biggest smudge is the Milky Way itself, the billions of stars that make up our spiral galaxy, which we see edge-on. The smaller smudges don’t mean that you need glasses, but that you’re seeing tightly packed groups of stars. One of the best-known of these “clouds” or “clusters” — groups of stars that travel together — is the Pleiades, also known as the Seven Sisters. Clusters are stellar nurseries where thousands of stars are born from clouds of gas and dust and then disperse across the Milky Way.
For centuries, scientists have speculated about whether these clusters always form tight clumps like the Pleiades, spread over only a few dozen lightyears.
“We call them ‘open clusters’ — the ‘open’ part refers to the expectation that these things formed in a much denser group that then dispersed,” said Luke Bouma, a graduate student in astrophysical sciences at Princeton and the lead author on an upcoming paper published by the American Astronomical Society. “But we never thought we’d be able to find the stars that were lost.”
Then, two years ago, a machine-learning algorithm using data from the Gaia satellite identified that many far-flung stars are moving at the same speed and direction and could therefore be part of the same open cluster — but as more of a stream or a string than a clump.
Now, a team of astrophysicists led by Bouma can confirm that one of these streams of stars, NGC 2516, also known as the Southern Beehive, extends at least 1,600 light-years — 500 parsecs — from tip to tip. To an Earth-based stargazer, that would look as big as 40 full moons, side by side, stretching across the sky.
“Gaia data let us trace the process of star cluster formation and dissolution in unprecedented detail — but to complete the picture, we need independently estimated ages,” said Lynne Hillenbrand, a 1989 Princeton alumna and a professor of astronomy at Caltech, who was not involved in this research. “Bouma’s paper brings together several different methods to consistently age-date stars at both the core and the outer reaches of this cluster.”
“In retrospect, the existence of this large stellar stream is not too surprising,” said Bouma, who recently won the prestigious 51 Pegasus b Fellowship. One interpretation could be that a cluster starts as a tight clump that expands through time to form “tidal tails” stretching in front of it and behind it, as it moves through the Milky Way.
“The broader implication is that there are bound to be other enormous open clusters like this,” he said. “The visible part of the cluster, where we can easily see the stars close together, may be only a small part of a much, much larger stream.”
“I have seen the Southern Beehive many times through a pair of binoculars under the dark skies of Chile,” said Gáspár Bakos, a professor of astrophysical sciences and the director of Princeton’s program in planets and life, who was a co-author on the paper. “The cluster nicely fits the view of the binoculars, because its apparent size in the sky is something like the tip of my thumb at arm’s length. It is curious to know, thanks to Luke’s research, that the cluster actually spans an area as big as my entire palm held toward the sky.”
Bouma and his colleagues used data from the Transiting Exoplanet Survey Satellite (TESS) to precisely measure the rotation rates of stars that the Gaia study had assigned to NGC 2516. The researchers demonstrated that many stars with similar masses are all spinning at (or very near) the same rate, confirming that they were born in the same stellar nursery.
Bouma has spent years developing the tools to measure a star’s rotation so that he can calculate its age, a technique called gyrochronology (from the Greek words for “spin” and “time”). Our sun, which at the age of 4.6 billion years old is in its sedate middle age, rotates once every 27 days. The stars Bouma measured in NGC 2516 are rotating 10 times faster than our sun, because they are so much younger. Those stars are barely out of their infancy, only about 150 million years old.
“In addition to expanding our knowledge of this and other star clusters, Luke has given us an expanded list of young stars that we can search for planets,” said Joshua Winn, Bouma’s adviser and co-author and a professor of astrophysical sciences. “Finding planets around young stars will help us understand how planetary systems form and change with time.”
“What’s so surprising about this work — what’s so exciting — is that we confirmed that Gaia, because it really precisely measures the positions and the motions of stars, can find these ‘needles in the haystack’ of the Milky Way,” Bouma said. “Gaia can identify all the stars that are moving in the same direction, at the same rate. And we don’t have to just trust the machine learning algorithm saying that they’re related — we can verify it with TESS data, using our gyrochronological technique.”
This open cluster also has an intriguing connection with Greek mythology, Bouma said. “In the southern night sky, NGC 2516 is near a constellation called the Argo Navis, which was the boat sailed by Jason and the Argonauts to obtain the golden fleece.” He added with a smile: “Jason and the Argonauts are sailing on a stream of stars made by the open cluster NGC 2516.”
The research team also included Joel Hartman, a research scholar in astrophysical sciences, and Jason Curtis, a postdoctoral researcher at Columbia University.
“Rotation and Lithium Confirmation of a 500 Parsec Halo for the Open Cluster NGC 2516,” by L. G. Bouma, J. L. Curtis, J. D. Hartman, J. N. Winn and G. Á. Bakos, has been submitted to the journals of the American Astronomical Society (AAS) and will be shared with the media at the 238th AAS meeting on Monday, June 7. The research was supported by the TESS Guest Investigator Program (G04032). L.G.B. was also supported by a Charlotte Elizabeth Procter Fellowship from Princeton University. This study was based in part on observations at Cerro Tololo Inter-American Observatory at the National Science Foundation’s NOIRLab (NOIRLab Prop. ID 2020A-0146; 2020B-0029 PI: L. Bouma), which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This paper includes data collected by the TESS mission, which are publicly available from the Mikulski Archive for Space Telescopes (MAST). Funding for the TESS mission is provided by NASA’s Science Mission directorate.
An international team of scientists led from the Centre for Astrobiology (CAB, CSIC-INTA), with participation from the Instituto de Astrofísica de Canarias (IAC), has used the Gran Telescopio Canarias (GTC) to study a representative sample of galaxies, both disc and spheroidal, in a deep sky zone in the constellation of the Great Bear to characterize the properties of the stellar populations of galactic bulges. The researchers have been able to determine the mode of formation and development of these galactic structures. The results of this study were recently published in The Astrophysical Journal.
The researchers focused their study on massive disc and spheroidal galaxies, using imaging data from the Hubble Space Telescope and spectroscopic data from the SHARDS (Survey for High-z Absorption Red and Dead Sources) project, a programme of observations over the complete GOODS-N (Great Observatories Origins Deep Survey – North) region through 25 different filters taken with the OSIRIS instrument on the Gran Telescopio Canarias (GTC), the largest optical and infrared telescope in the world, at the Roque de los Muchachos Observatory (Garafía, La Palma, Canary Islands).
Analysis of the data allowed the researchers to discover something unexpected: the bulges of the disc galaxies were formed in two waves. One third of the bulges in disc galaxies were formed at redshift 6.2, which corresponds to an early epoch in the Universe, when it was only 5% of its present age, around 900 million years old. “These bulges are the relics of the first structures formed in the Universe, which we have found hidden in local disc galaxies”, explains Luca Costantin, a researcher at the CAB within a programme of Attracting Talent of the Community of Madrid, and the first author on the paper.
But in contrast, almost two thirds of the bulges observed show a mean value of redshift of around 1.3, which means that they were formed much more recently, corresponding to an age of four thousand million years, or almost 35% of the age of the Universe.
A peculiar characteristic which permits the distinction between the two waves is that the central bulges of the first wave, the older bulges, are more compact and dense than those formed in the second, more recent wave. In addition, the data from the spheroidal galaxies in the sample show a mean redshift value of 1.1, which suggests that they formed in the same general time as the bulges of the second wave.
For Jairo Méndez Abreu, a researcher at the University of Granada (UGR) and a co-author of the article, who was formerly a Severo Ochoa postdoctoral researcher at the IAC, “the idea behind the technique used to observe the stars in the central bulge is fairly simple, but it has not been possible to apply it until the recent development of methods which have allowed us to separate the light from the stars in the central bulge from those in the disc, to be specific the GASP2D and C2D algorithms, which we have developed recently and which have enabled us to achieve unprecedented accuracy”.
Another important result of the study is that the two waves of bulge formation differ not only in terms of the ages of their stars, but also in terms of their star formation rates. The data indicate that the stars in the bulges of the first wave formed quickly, on timescales of typically 200 million year. On the contrary, a significant fraction of the stars in the bulges of the second wave required formation times five times longer, some thousand million years.
“We have found that the Universe has two ways of forming the central zones of galaxies like our own: starting early and performing very quickly, or taking time to start, but finally forming a large number of stars in what we know as the bulge”, comments Pablo G. Pérez González, a researcher at the CAB, and Principal Investigator of the SHARDS project, which gave essential data for this study. In the words of Antonio Cabrera, the Head of Science Operations at the GTC, “SHARDS is a perfect example of what is possible due to the combination of the huge collecting capacity of the GTC and the extraordinary conditions at the Roque de los Muchachos Observatory, to produce 180 hours of data with such excellent image quality, essential for the detection of the objects analysed here”.
As described by Paola Dimauro, a researcher at the National Observatory of Brazil and a co-author of this article, “this study has allowed us to explore the morphological evolution and the history of the assembly of the structural components of the galaxies, analagous to archaeological studies, analysing the information encoded in the millions of stars of each galaxy. The interesting point was to find that not all the structures were formed at the same time, or in the same way”.
The results of this study have allowed the observers to establish a curious parallel between the formation and the evolution through time of the disc galaxies studies and the creation and development of a large city during the centuries. Just as we find that some large cities have historic centres, which are older and house the oldest buildings in cluttered narrow streets, the results of this work suggest that some of the centres of massive disc galaxies harbour some of the oldest spheroids formed in the Universe, which have continued to acquire material, forming discs more slowly, the new city outskirts in our analogy.
The Gran Telescopio Canarias and the Observatories of the Instituto de Astrofísica de Canarias (IAC) form part of the network of Singular Scientific and Technical Infrastructures (ICTS) of Spain.
Featured image: An example of a nearby spiral galaxy, M81, where the bulge and the disc are easily identified. Credit: NASA/JPL-Caltech/ESA/Harvard-Smithsonian CfA.
Becerra-Vergara and colleagues in their recent paper, explored the possibility of an alternative nature of Sagittarius A*. By using the astrometry data of 17 best-resolved S-stars, they suggested that, instead of a supermassive black hole, it is a dark compact object having a dense core of darkinos, which are neural massive dark matter fermions. Their study recently appeared in the journal Monthly Notices of the Royal Astronomical Society: Letters.
Sagittarius A* is a bright and very compact astronomical radio source at the Galactic Center of the Milky Way. It has been long thought by scientific community that it is a supermassive black hole. This inference on the nature of Sgr A* mainly comes from the nearly Keplerian orbits of tens of stars belonging to the S-star cluster, whose motions are well described by geodesics in the Schwarzschild spacetime geometry.
The most important S-cluster member is S2 which, with an orbital period of about 16 yr and a pericenter of about 1500 Schwarzschild radii, has the most-compact orbit around Sgr A. The S2 orbit data have allowed to test General Relativity predictions such as the relativistic redshift and precession. But, astrophysicists were also confronted with a problem: they could not able to explain G2 motion, which is a gas cloud. This cloud reached so close to Sagittarius A* that it should have been destroyed or pulled in by the black hole. But, instead, it continued on its way, unharmed.
In this view, Becerra-Vergara and colleagues now explored the possibility of an alternative nature of Sgr A based on fermionic DM profile predicted by the Ruffini-Argüelles-Rueda (RAR) model.
They suggested, the reason G2 was able to survive its journey past Sagittarius A*, was because Sagittarius A* is not a black hole, instead, it is a compact object made up of dark matter.
To come to this conclusion, they ran a simulation of the Milky Way, in which Sagittarius A* was replaced by dark compact object composed of darkinos. They showed for the first time that, this dark compact object at the Galactic center can explain the G2 motion and the dynamics of the S-stars with similar (and some cases better) accuracy compared to a central BH model.
“Our results strengthen the alternative nature of Sgr A* as a dense quantum core of darkinos superseding the central massive BH scenario.”
In addition, an interesting fact they found is that, this dense core of darkinos also explains the rotation curves of the Milky Way. For particle masses ∼ 100 keV, the core radius shrinks from 0.4 mpc to a few Schwarzschild radii, so the gravitational potential produced by a central BH of mass MBH and a (RAR) DM core of mass, Mc = MBH, practically coincide for r ≳ 10 GM_BH/c². Therefore, the dynamics of baryonic matter at these scales should not differ much in the two scenarios. In simple terms, you can say, this dark compact object would have very similar characteristics compared to a black hole.
Reference: E A Becerra-Vergara, C R Argüelles, A Krut, J A Rueda, R Ruffini, Hinting a dark matter nature of Sgr A* via the S-stars, Monthly Notices of the Royal Astronomical Society: Letters, 2021;, slab051, https://doi.org/10.1093/mnrasl/slab051
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