Tag Archives: Astronomy

Franco Battiato and Astronomy (Astronomy)

The great Sicilian singer-songwriter passed away in the night between 17 and 18 May, after a long illness. His death leaves Italy and the whole world orphans of a visionary artist who has often been inspired by the sky and astronomy in his songs. We propose some verses of his lyrics, to remember him with us

The night between 17 and 18 May 2021 brought us sad and unfortunately long overdue news.

The death of Franco Battiato leaves Italy and the whole world orphans of a visionary artist who has often been inspired by the sky and astronomy in his songs. Through the unrivaled evocativeness of music and poetry, Battiato has allowed us for entire decades, and will do so for many generations to come, to travel to distant worlds and free ourselves – even if for those few moments – from the gravity of every existence.

In Battiato’s mysticism, astronomy played a non-secondary role, as demonstrated by the lyrics of some unforgettable songs.

“I know the laws of the world, and I will give them to you I will
overcome the gravitational currents
Space and light so as not to make you grow old”

These lofty love verses – whose lightness is worthy of Calvino’s “American Lessons” – are contained in the text of ” La Cura “, a masterpiece of music and poetry that punctually and incredibly moves us despite having listened to it hundreds of times. , perhaps because it focuses entirely on giving and not on asking and receiving. While remaining utopia, these verses refer above all to the fact that the world is governed by laws that have nothing to do with the courts but which are those of physics and what philosophers have always called “necessity”.

Space and light cannot fail to lead us straight to Albert Einstein, at the speed of light, to the concept of spacetime. In real life, spacetime represents the simple inevitability of the consummation of our lives but – as we can read on the site of the Italian Amateur Astronomers Union which collects astronomical quotes in music – if we imagined traveling at relativistic speeds, that is, close to that of light, the time would actually stop. And Battiato, like any passionate lover, would be able to stop time with love.

And, again on the same site, we find the quote from ” The birds “, another masterpiece that brings up the laws of physics:

“Flying birds fly
in the space between the clouds
with the rules assigned
to this part of the universe
to our solar system”

Battiato here sensed something extraordinary, namely that – despite astrophysicists from all over the world strive every day to find constants and laws that are the same for everyone – these laws of physics, which we have already found in “The Cure” as referring to the “world ”, They are probably not even the same for the entire universe, even if we are familiar with those of the Solar System. If this were not the case, we would have long ago solved the “dark matter” puzzle that is missing when astronomers observe excessive speeds of visible matter in distant stars and galaxies.

And it is to ours, instead, of the galaxy that the song, beautiful but not very well known, ” Via Lattea ” is dedicated , contained in an album whose title is once again turned to the sky, indeed to the thousand skies of ” Worlds far away ” .

In this song we imagine the interstellar travels that we as a human species will probably never do. However, listening to it, we can imagine the trips to the Moon and Mars on which we are really working in these years. The text is so beautiful that we offer it all:

“We got up when it was not yet dawn
Ready to transship
Inside an artificial satellite
That led us quickly
To the gates of Sirius
Where an experimental crew was
preparing for the
long journey.
Provincials of the Ursa Minor
Conquering interstellar spaces
And dressed in light gray
So as not to get lost.

We followed certain routes diagonally
Inside the Milky Way.

An Impression Center captain
Overwhelmed by exhaustion
He was soon sent into exile.
I was preparing for the
long journey
… in which you get lost.

We followed certain routes diagonally
Inside the Milky Way.”

But the history of astronomy also finds space in Battiato’s texts. In at least one case, in fact, we find an almost hidden historical quotation, of those that if we do not go into it, only mysterious evocations remain in our imagination. For example, in the song ” Permanent center of gravity “, a title that is already quite explicit, the ” Euclidean Jesuits dressed as bonzes to enter the court of the emperor of the Ming dynasty ” are not an artistic invention but a real group of religious led by Father Matteo Ricci who, on the threshold of 1600, before arriving at the emperor’s court in Beijing, spent about twenty years spreading Christianity but above all Western mathematical-scientific and astronomical thought, integrating it with Confucianism.

The accuracy of the Jesuits and their Euclidean calculations for the prediction of eclipses then led them to direct the imperial astronomical office from 1650 until almost the end of 1700, carrying out a profound revision of the Chinese calendar, the importance of which was fundamental for the taking of any imperial decision given that Chinese thought was historically steeped in astrological superstition.

A main belt asteroid discovered in 1997 by Francesco Manca and Pietro Sicoli at the Sormano observatory near Bergamo was also dedicated to Franco Battiato . Since 2003, the surname of the great Catania artist has been added to the small asteroid, also known as the small planet 18556 .

A more in-depth research could take us even further but today is the day of sadness and remembrance, we must therefore be content with remembering Franco Battiato while respecting the spaces and times that are granted to us.

Battiato really knew the laws of the world, at least those that in all of us are able to open the doors of the mind and heart without intermediaries.

And he gave it to us.

Featured image: Franco Battiato in concert at Teatro Circo Price, Madrid (2013). Credits: Juan Lupión

Provided by INAF

LAMOST Helps Gaia Achieve Millimagnitude Photometry Precision (Astronomy)

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has helped Gaia achieve millimagnitude (mmag) precision in photometry, according to a study led by researchers from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) and Beijing Normal University (BNU).  

Their study was published in The Astrophysical Journal

If you look at the sky on a clear, starry night, you may notice that Aldebran is relatively red and Rigel is blue. Why? The answer stems from their intrinsic physical properties. Precisely measuring magnitudes and colors helps to explain such phenomena. 

The ESA Gaia satellite is well known for its remarkable capacity for astrometric observation. It delivers the most precise photometric data ever, with much higher quality than ground-based telescopes. 

However, in order for Gaia to cover a wide magnitude range from 6 to 22 magnitude (mag), different observation modes have been adopted for stars of different brightness. In addition, photometric data in the G, BP, and RP bands come from different instruments and CCDs. Therefore, magnitude and color-dependent systematic errors exist. 

The researchers combined two datasets, Gaia Data Release 2 (DR2) and LAMOST Data Release 5 (DR5), then selected samples of high data quality and low extinction.

Color residuals versus G magnitudes before (left) and after corrections (right). (Image by NIU Zexi) 

The samples comprised 779,691 main-sequence stars and 71,952 red giant branch stars. Stars from 13.3 to 13.7 mag in the G band were selected as the control sample in order to establish the empirical relationship between intrinsic colors and physical parameters deduced from LAMOST. 

“Applying the relationship to the whole sample, the differences between the observed colors and model-determining colors at different G magnitudes represent the color correction terms,” said Prof. LIU Jifeng from NAOC. 

With the help of LAMOST’s massive stellar spectrum, the researchers used a spectroscopy-based stellar color regression method to correct systematic errors that are magnitude- and color-dependent. The relationship between the intrinsic colors and physical parameters of stars represents the key focus of this method. 

By using a sample of about 500,000 stars from LAMOST and Gaia, color correction curves for the F/G/K stars are derived. “With an unprecedented precision of about 1 mmag, systematic trends in the G magnitude are revealed for both G-RP and BP-RP colors in detail,” said Prof. YUAN Haibo from BNU, the corresponding author of the study. 

“Our work could be beneficial to studies where a high-precision color-color diagram is required, including the estimation of Gaia photometric metallicities, detection of peculiar objects, discrimination between binaries and single stars, and so on,” said NIU Zexi, Ph.D. candidate from NAOC and lead author of the study. 

In addition, another study by the same team on color corrections for Gaia Data Early Release 3 (EDR3) was published in The Astrophysical Journal Letters

Featured image: Schematic illustration of the spectroscopy-based stellar color regression method. (Image by NIU Zexi) 

Reference: Zexi Niu, Haibo Yuan, and Jifeng Liu, “Correction to the Photometric Colors of the Gaia Data Release 2 with the Stellar Color Regression Method”, The Astrophysical Journal, 909(1), 2021. Link to paper

Provided by Chinese Academy of Sciences

Not Just For Finding Planets: Exoplanet-hunter TESS Telescope Spots Bright Gamma-ray Burst (Astronomy)

NASA has a long tradition of unexpected discoveries, and the space program’s TESS mission is no different. SMU astrophysicist and her team have discovered a particularly bright gamma-ray burst using a NASA telescope designed to find exoplanets – those occurring outside our solar system – particularly those that might be able to support life.

It’s the first time a gamma-ray burst has been found this way. 

Gamma-ray bursts are the brightest explosions in the universe, typically associated with the collapse of a massive star and the birth of a black hole. They can produce as much radioactive energy as the sun will release during its entire 10-billion-year existence.

Krista Lynne Smith, an assistant professor of physics at Southern Methodist University, and her team confirmed the blast – called GRB 191016A – happened on Oct. 16 and also determined its location and duration. A study on the discovery has been published in The Astrophysical Journal.  

“Our findings prove this TESS telescope is useful not just for finding new planets, but also for high-energy astrophysics,” said Smith, who specializes in using satellites like TESS (Transiting Exoplanet Survey Satellite) to study supermassive black holes and gas that surrounds them. Such studies shed light on the behavior of matter in the deeply warped spacetime around black holes and the processes by which black holes emit powerful jets into their host galaxies.

Smith calculated that GRB 191016A had a peak magnitude of 15.1, which means it was 10,000 times fainter than the faintest stars we can see with the naked eyes. 

That may sound quite dim, but the faintness has to do with how far away the burst occurred. It is estimated that light from GRB 191016A’s galaxy had been travelling 11.7 billion years before becoming visible in the TESS telescope.

Most gamma ray bursts are dimmer – closer to 160,000 times fainter than the faintest stars. 

The burst reached its peak brightness sometime between 1,000 and 2,600 seconds, then faded gradually until it fell below the ability of TESS to detect it some 7000 seconds after it first went off.

How SMU and a team of exoplanet specialists confirmed the burst

This gamma-ray burst was first detected by a NASA’s satellite called Swift-BAT, which was built to find these bursts. But because GRB 191016A occurred too close to the moon, the Swift-BAT couldn’t do the necessary follow-up it normally would have to learn more about it until hours later.

NASA’s TESS happened to be looking at that same part of the sky.That was sheer luck, as TESS turns its attention to a new strip of the sky every month. 

While exoplanet researchers at a ground-base for TESS could tell right away that a gamma-ray burst had happened, it would be months before they got any data from the TESS satellite on it. But since their focus was on new planets, these researchers asked if any other scientists at a TESS conference in Sydney, Australia was interested in doing more digging on the blast.

Smith was one of the few high-energy astrophysics specialists there at that time and quickly volunteered.

“The TESS satellite has a lot of potential for high-energy applications, and this was too good an example to pass up,” she said. High-energy astrophysics studies the behavior of matter and energy in extreme environments, including the regions around black holes, powerful relativistic jets, and explosions like gamma ray bursts.

TESS is an optical telescope that collects light curves on everything in its field of view, every half hour. Light curves are a graph of light intensity of a celestial object or region as a function of time. Smith analyzed three of these light curves to be able to determine how bright the burst was.

She also used data from ground-based observatories and the Swift gamma ray satellite to determine the burst’s distance and other qualities about it. 

“Because the burst reached its peak brightness later and had a peak brightness that was higher than most bursts, it allowed the TESS telescope to make multiple observations before the burst faded below the telescope’s detection limit,” Smith said. “We’ve provided the only space-based optical follow-up on this exceptional burst.”

Reference: Krista Lynne Smith, Ryan Ridden-Harper et al., “GRB 191016A: A Long Gamma-Ray Burst Detected by TESS”, The Astrophysical Journal, 911(1), 2021. Link to paper

Provided by SMU

This Deep Learning Model Turns Blurred Galactic Images Into Clearer Ones (Astronomy)

Gan and colleagues proposed a new deep learning method called generative adversarial networks (GAN’s) to turn blurred galactic images into clearer ones.

Classification of galactic morphologies has long been a critical task in extragalactic astronomy, not only because global galactic morphologies such as bulge-to-disk-ratios and spiral arm shapes can have fossil information of the galaxy formation, but also because the detailed statistical studies of galactic properties for each category can provide insights into the formation processes of different types of galaxies. Galaxy classification schemes proposed in previous pioneering works have long been used as standard tools in many observational and theoretical studies of galaxy formation and evolution. These days, galaxy classification is also done by non-professional astronomers such as the Galaxy Zoo project, in which a large number of galaxy images (> 106) from SDSS are provided for citizen science.

Galaxy classification has always been done by the human eye and will be done in future works. More recently, however, this process has begun to be automated by applying machine learning algorithms to actual observational data. For example, convolutional neural networks (CNNs) have been used in the automated classification of galactic morphologies for many galaxies . Galaxy classification using these deep learning algorithms has been successfully done for a large number (> 106 ) of images from large ground-based telescopes such as the Subaru 8m telescope. Such quick automated classification is now considered to be the primary (and possibly only) way to classify a vast number of galaxies from ongoing and future surveys of galaxies such as LSTT and EUCLID.

One of the potential problems in classifying galaxy images from ground-based telescopes is that the images can be severely blurred owing to the seeing effects of the sky. Fine structures of galaxies, such as bars, spiral arms, and rings, is used to classify and quantify galaxies, such structures can be much less visible in galaxy images from ground-based telescopes, in particular, for distant galaxies. Thus, if this optical blurring due to sky seeing can be removed by applying machine learning algorithms to real galaxy images, it will provide significant benefits both to professional astronomers and non-professional ones who are working on the Galaxy Zoo project.

Now, Gan and colleagues developed a new GANbased model that can convert blurred ground-based Subaru Telescope images of galaxies into clear HST-like galaxy images.

“Galaxy images from the HST do not have such problems as seeing effects because atmospheric distortion due to light travelling through the turbulent atmosphere is not a problem in these observations by a space telescope.”

— told Gan, lead author of the study

In the present study, they manifested that using an existing deep learning method called generative adversarial networks (GANs), they can eliminate seeing effects, effectively resulting in an image similar to an image taken by the HST. Using their first of its kind machine learning-based deblurring technique on space images, they obtained up to 18% improvement in terms of CW-SSIM (Complex Wavelet Structural Similarity Index) score when comparing the Subaru-HST pair versus SeeingGAN-HST pair (refer fig 1 below).

“With this model, we can generate HST-like images from relatively less capable telescopes in very less time, making space exploration more accessible to the broader astronomy community. Furthermore, this model can be used not only in professional morphological classification studies of galaxies but in all citizen science for galaxy classifications.

— told Gan, lead author of the study

There are several scientific merits of their SeeingGAN in astronomical studies. First, astronomers can see the internal fine structures of galaxies such as spiral arms, tidal tails, and massive clumps more clearly, which can be difficult to see in optical images of distant galaxies from ground-based telescopes. These generated clearer images by SeeingGAN would assist astronomers to classify galaxies better and discover new internal structures of distant galaxies which otherwise could be difficult to find in original blurred images. For example, it could be possible that distant galaxies classified as S0s with no spirals in original blurred images are indeed spiral galaxies in the debarred images by SeeingGAN. This can influence the redshift evolution of S0 fraction in groups and clusters, discussed in many recent papers. Also, SeeingGAN can be used for citizen science projects for galaxy classification by the public, e.g., the Galaxy Zoo project. If galaxy images in these projects are blurred (more challenging to classify galaxies), then the deblurred images generated by SeeingGAN can be easily used for the public galaxy classification instead of the original image. The speed at which SeeingGAN can convert blurred images to deblurred ones is rapid, it is not tricky for SeeingGAN to generate a massive number of deblurred galaxy images.

Figure 1. Sample results produced by SeeingGAN. The images are listed in the order of HST, Subaru, SeeingGAN prediction. The SeeingGAN result is obtained by predicting the results from the Subaru image. The CW-SSIM value is obtained by comparing the said image and the HST image, a higher CW-SSIM value indicates that the image is more similar to the HST image. © Gan et al.

As shown in Fig. 1, the deblurred images are clearer than the original Subaru images, however, some of them are not dramatically clearer as the HST counterparts. Hence, their future study investigates whether different CNN architectures, larger numbers of image pairs, and model parameters of SeeingGAN can improve the performance of SeeingGAN. Since the present study has proposed one example of SeeingGAN for a limited number of Subaru-HST image pairs, it is worth a try for Gan et al. to investigate different architectures of GAN for a much larger number of image pairs.

They plan to use the large number (a million) of Subaru Hyper Suprime-Cam and HST images to test new architectures of SeeingGAN for its better performance. It might be essential for them to use galaxy images from other optical telescopes (e.g., VLT) to confirm that SeeingGAN can be developed from different combinations of ground-based and space telescopes. Although they have focused exclusively on galaxy images in optical wavelengths, it might be an interesting future study to use galaxy images at other wavelengths from space telescopes (e.g., JWST) to develop new SeeingGAN.

Featured image: One enlarged sample result predicted by SeeingGAN. The predicted image is obtained by feeding the Subaru 8.2m telescope’s image into SeeingGAN. The resultant image has a higher CW-SSIM score, which indicates a better similarity to the HST image. © Gan et al.

Reference: Fang Kai Gan, Kenji Bekki, Abdolhosein Hashemizadeh, “SeeingGAN: Galactic image deblurring with deep learning for better morphological classification of galaxies”, pp. 1-11, ArXiv, 2021. https://arxiv.org/abs/2103.09711

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

Scientists Sketch Aged Star System Using Over a Century of Observations (Planetary Science)

Astronomers have painted their best picture yet of an RV Tauri variable, a rare type of stellar binary where two stars – one approaching the end of its life – orbit within a sprawling disk of dust. Their 130-year dataset spans the widest range of light yet collected for one of these systems, from radio to X-rays.

“There are only about 300 known RV Tauri variables in the Milky Way galaxy,” said Laura Vega, a recent doctoral recipient at Vanderbilt University in Nashville, Tennessee. “We focused our study on the second brightest, named U Monocerotis, which is now the first of these systems from which X-rays have been detected.”

A paper describing the findings, led by Vega, was published in The Astrophysical Journal.

Video: Two stars orbit each other within an enormous dusty disk in the U Monocerotis system, illustrated here. When the stars are farthest from each other, they funnel material from the disk’s inner edge. At this time, the primary star is slightly obscured by the disk from our perspective. The primary star, a yellow supergiant, expands and contracts. The smaller secondary star is thought to maintain its own disk of material, which likely powers an outflow of gas that emits X-rays.Credits: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)

The system, called U Mon for short, lies around 3,600 light-years away in the constellation Monoceros. Its two stars circle each other about every six and a half years on an orbit tipped about 75 degrees from our perspective.

The primary star, an elderly yellow supergiant, has around twice the Sun’s mass but has billowed to 100 times the Sun’s size. A tug of war between pressure and temperature in its atmosphere causes it to regularly expand and contract, and these pulsations create predictable brightness changes with alternating deep and shallow dips in light – a hallmark of RV Tauri systems. Scientists know less about the companion star, but they think it’s of similar mass and much younger than the primary.

The cool disk around both stars is composed of gas and dust ejected by the primary star as it evolved. Using radio observations from the Submillimeter Array on Maunakea, Hawai’i, Vega’s team estimated that the disk is around 51 billion miles (82 billion kilometers) across. The binary orbits inside a central gap that the scientists think is comparable to the distance between the two stars at their maximum separation, when they’re about 540 million miles (870 million kilometers) apart.

This infographic shows U Mon’s components to scale. Credits: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)

When the stars are farthest from each other, they’re roughly aligned with our line of sight. The disk partially obscures the primary and creates another predictable fluctuation in the system’s light. Vega and her colleagues think this is when one or both stars interact with the disk’s inner edge, siphoning off streams of gas and dust. They suggest that the companion star funnels the gas into its own disk, which heats up and generates an X-ray-emitting outflow of gas. This model could explain X-rays detected in 2016 by the European Space Agency’s XMM-Newton satellite.

“The XMM observations make U Mon the first RV Tauri variable detected in X-rays,” said Kim Weaver, the XMM U.S. project scientist and an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s exciting to see ground- and space-based multiwavelength measurements come together to give us new insights into a long-studied system.”

In their analysis of U Mon, Vega’s team also incorporated 130 years of visible light observations.

The earliest available measurement of the system, collected on Dec. 25, 1888, came from the archives of the American Association of Variable Star Observers (AAVSO), an international network of amateur and professional astronomers headquartered in Cambridge, Massachusetts. AAVSO provided additional historical measurements ranging from the mid-1940s to the present.

The researchers also used archived images cataloged by the Digital Access to a Sky Century @ Harvard (DASCH), a program at the Harvard College Observatory in Cambridge dedicated to digitizing astronomical images from glass photographic plates made by ground-based telescopes between the 1880s and 1990s.

On May 12, 1948, astronomers at Boyden Observatory in Bloemfontein, South Africa, captured a portion of the sky containing U Monocerotis (left, circled) on a glass photographic plate. The logbook entry (right) for the observation reads: Gusty S wind. H.A. [Hour Angle] should be 2 02 W. Credits: Harvard College Observatory, Photographic Glass Plate Collection. Used with permission.

U Mon’s light varies both because the primary star pulsates and because the disk partially obscures it every 6.5 years or so. The combined AAVSO and DASCH data allowed Vega and her colleagues to spot an even longer cycle, where the system’s brightness rises and falls about every 60 years. They think a warp or clump in the disk, located about as far from the binary as Neptune is from the Sun, causes this extra variation as it orbits.

Vega completed her analysis of the U Mon system as a NASA Harriett G. Jenkins Predoctoral Fellow, a program funded by the NASA Office of STEM Engagement’s Minority University Research and Education Project.

“For her doctoral dissertation, Laura used this historical dataset to detect a characteristic that would otherwise appear only once in an astronomer’s career,” said co-author Rodolfo Montez Jr., an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, also in Cambridge. “It’s a testament to how our knowledge of the universe builds over time.”

Co-author Keivan Stassun, an expert in star formation and Vega’s doctoral advisor at Vanderbilt, notes that this evolved system has many features and behaviors in common with newly formed binaries. Both are embedded in disks of gas and dust, pull material from those disks, and produce outflows of gas. And in both cases, the disks can form warps or clumps. In young binaries, those might signal the beginnings of planet formation.

“We still have questions about the feature in U Mon’s disk, which may be answered by future radio observations,” Stassun said. “But otherwise, many of the same characteristics are there. It’s fascinating how closely these two binary life stages mirror each other.”

Banner image: U Mon’s primary star, an elderly yellow supergiant, has around twice the Sun’s mass but has billowed to 100 times the Sun’s size. Scientists know less about the companion, the blue star in the background of this illustration, but they think it’s of similar mass and much younger than the primary. Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)

Reference: Laura D. Vega, Keivan G. Stassun, Rodolfo Montez Jr., Tomasz Kamiński, Laurence Sabin, Eric M. Schlegel, Wouter H. T. Vlemmings, Joel H. Kastner, Sofia Ramstedt, and Patricia T. Boyd, “Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk”, The Astrophysical Journal, Volume 909, Number 2, 2021. https://iopscience.iop.org/article/10.3847/1538-4357/abe302

Provided by NASA

What Determines The Structure of Short Gamma-ray Burst Jets? (Astronomy)

Urrutia and colleagues in their recently published paper in arxiv, presented numerical simulations of relativistic jets associated with short gamma ray bursts (SGRBs). They explored different initial jet structures, duration and luminosities and followed the jet interaction with the merger remnant wind and computed its final structure at distances ≥ 10¹¹ cm from the central engine. They showed that the final jet structure, as well as the resulting afterglow emission, depend strongly on the initial structure of the jet, its luminosity and duration. While the initial structure at the jet is preserved for long-lasting SGRBs, it is strongly modified for jets barely making their way through the wind. To read more download pdf given below.

Featured image: Kinetic luminosity and Lorentz factor profiles for the structured jets implemented in the numerical simulations. The top-hat jet (solid line) has a cut-off angle of 𝜃𝑗 = 10° . Gaussian (dashed line) and power-law jet (dotted line) profiles present extended wings moving with Newtonian velocity. © Urrutia et al.

Reference: Gerardo Urrutia, Fabio De Colle, Ariadna Murguia-Berthier, Enrico Ramirez-Ruiz, “What determines the structure of short gamma-ray burst jets?”, ArXiv, pp. 1-9, 2021.

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

Peeping Milky Way’s Merging History: Reconstructing the Cetus Stream (Astronomy)

Around the Milky Way, there are many river-like structures made up of stars. They are called stellar streams. How these stellar streams formed remains unclear. 

Researchers led by Prof. ZHAO Gang and Dr. CHANG Jiang from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) reproduced the formation process of the newly discovered Cetus stream in a computer.

The study was published in The Astrophysical Journal

“Stellar streams are the remnants of dwarf satellite galaxies that are swallowed by the Milky Way, but have not been fully digested,” said Dr. CHANG, the first author of the study. “The accretion process is not that the Milky Way swallowed the dwarf galaxy in one bite, but it peeled the dwarf galaxy layer by layer from outside to inside by tidal stripping, just like peeling an onion. The stripped stars distributed in their original orbits, and they formed a river-like structure, that is, a stellar stream.” 

The Milky Way galaxy grows by constantly devouring dwarf satellite galaxies, which is called the galaxy merger. Through the study of the merging history of the Milky Way, we can know how the Milky Way formed and evolved. 

In their previous study, the researchers discovered the Cetus stream based on the observational data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also known as Guoshoujing Telescope) Survey and the Sloan Digital Sky Survey.

Now, they reconstructed the formation history of this stellar stream in the supercomputer through a series of high-resolution dynamics numerical simulations, and provided a simple portrait of the Cetus Stream progenitor before being swallowed by the Milky Way. 

“Our work shows how the Milky Way slowly peeled apart and swallowed a dwarf galaxy with a mass of about 20 million times of the sun over a period of 5 billion years,” said Prof. ZHAO, the co-corresponding author of the study.

In satellite galaxies, there always remains a core structure composed of relatively dense stars. Some researchers put forward the hypothesis that the globular star cluster NGC 5824 is a core structure associated with the Cetus Stream. But in this work, the researchers overturned this hypothesis through detailed numerical simulations. 

“The globular cluster NGC 5824 is not the remnant core structure corresponding to the Cetus stream, because the dynamic feature is not correct,” Dr. CHANG said. “But we found that there is a strong correlation between the two. NGC 5824 should be a globular cluster in the Cetus stream progenitor galaxy.” 

The distribution of stellar streams is usually throughout the sky. While LAMOST helped to discover the Cetus stream in the northern sky, the researchers also found the candidate counterpart of the Cetus stream in the southern sky, that is, the Palca stream.

“There are a large number of merging relics in the Milky Way similar to the Cetus stream,” said Prof. ZHAO. “They compose a treasure house for studying the structure and formation history of the Milky Way, which helps us to better understand how galaxies in the universe have formed and evolved.”

Featured image: The simulated process of Cetus Stream progenitor merging with the Milky Way. (Image by CHANG Jiang)

Reference: Jiang Chang, Zhen Yuan et al., “Is NGC 5824 the Core of the Progenitor of the Cetus Stream?”, Astrophysical Journal, 905(2), 2021. https://iopscience.iop.org/article/10.3847/1538-4357/abc338

Provided by Chinese Academy of Sciences

Sudden Death in the Universe – The Agony of a Massive Dusty Galaxy as Seen By its Blue Companion (Astronomy)

Heavily dust-obscured ultramassive star-forming galaxies in the early Universe contributed significantly to the cosmic star formation rate. But, how did such objects manage to build up their stellar masses at a relatively short time? Were they once starburst galaxies or were they gradually forming stars, exhausting their hydrogen reservoirs?

In the last two decades, astronomers have gained massive knowledge of how galaxies form and evolve. This advancement in the field of extragalactic astronomy was coupled with the rapid development of the instrumentation aspect of astronomy, which led to accumulating a large amount of observations of galaxies at different epochs of the lifetime of the Universe. With giant multiwavelength datasets, we realized that galaxies are not just entities in space and time. They are more like living organisms: They are born, they get old and then they die! Such evolution of a lifetime of galaxies is governed by the amount by which they form stars. Logically, and since hydrogen is the main ingredient of star formation, galaxies will build up their stellar population until they exhaust their hydrogen reservoir.

Years worth of observations have suggested that when the Universe was adolescent, galaxies formed more stars. Specifically, when the Universe was only 3 Billion years old, that is 10 Billion years ago, galaxies were the most efficient in turning their gas content into stars. This epoch of the Universe is called the cosmic noon.

To make things even more ambiguous, it turns out that some of the galaxies during the cosmic noon were even more massive than old galaxies like our own, the Milky Way. This has made the field of extragalactic astronomy one of the most active themes of research, since it involves interdisciplinary fields such as physics, chemistry and analytical modeling. And given the amount of good quality data, it became possible to trace these ultramassive galaxies when the Universe was younger, and this led to a lot of natural questions such as: how did these galaxies manage to become so massive at an early time? Were they once exhibiting an over-to-normal star formation activity, or were they very efficient in turning their hydrogen into stars?

To answer these questions, and to investigate the nature of these ultramassive objects, an international team of researchers under the leadership of Mahmoud Hamed, along with his PhD supervisor dr. hab. Katarzyna Malek (Astrophysics Division of the NCBJ), and with close collaboration with dr. Laure Ciesla and dr. Matthieu Béthermin (both from the astrophysics laboratory of Marseille – LAM), shed a new light on the physical processes involved in the interstellar medium in these “giants”. To better understand the nature of such heavily dust-obscured galaxies, they detected an interesting system of two galaxies at the epoch of the cosmic noon, and analyzed it with different wavelengths observations in order to constrain their underlying physics and chemistry. This system consists of one ultramassive galaxy and its satellite galaxy, which we called Astarte and Adonis respectively, as the Phoenician gods.

Astarte is not only ultramassive, but it is also ultra-dusty – it is very bright in the infrared spectrum, which is a thermal radiation emitted by the dust in the interstellar medium. In fact, Astarte is so dusty that it is almost not visible in the shorter wavelengths at ultraviolet and visible light. This is a common feature of dust behavior in galaxies, it absorbs shorter wavelength photons which are typically the same size of the dust grains on average.

Adonis is less dusty and is not bright in the longer wavelengths of infrared bands, and together with Astarte, it forms an interesting system of opposites, which can reveal a lot about their evolution and can possibly answer the puzzle of how these massive galaxies managed to become more massive than their local environments in very short timescale.

The ultramassive Astarte was observed with the Atacama Large Millimeter Array (ALMA), as part of an observation program (PI: Béthermin). ALMA can observe cold dust and emission from excited molecules in the interstellar medium. With this observation program, we detected the emission coming from the carbon monoxide (CO) from the molecular clouds of Astarte. With this CO emission, it was possible to estimate the mass of hydrogen in that galaxy, based on conversion ratios that are already established in galaxies of the local Universe, and from our knowledge of the expected abundance ratios in the interstellar medium of such dusty galaxies like Astarte.

The various wavelengths through which Astarte and Adonis were detected helped in modeling the total spectrum coming from them. This in turn allowed us to constrain the physical properties of the system, such as how many stars do these galaxies have and at what rate new stars are being born. The surprising element of this study was that the estimated rate at which Astarte is forming stars is too high, much higher than what can be explained from its hydrogen reservoir. In fact, if Astarte continues to form stars at this rate, it will exhaust all its gas in the molecular clouds in the next 220 million years, which is rather short compared to the timescales that we deal with in the Universe. Adonis on the other hand, is forming too many stars for its mass, this is what we commonly refer to as a strong starburst.

One important conclusion of this study was that the ultramassive Astarte, which is more massive than our old and mature Milky Way, is dying; Astarte is turning its hydrogen into stars by inertia from its past when it was once a starburst too, like its neighbouring Adonis. This indeed motivates the quest to understand how very massive galaxies form and evolve, and how they deplete their resources while converting them efficiently into stars.

Featured image: The two galaxies as seen with the VISTA telescope, with ALMA detection of Astarte. © NCBJ

Publication: M. Hamed, L. Ciesla, M. Béthermin, K. Małek, E. Daddi, M. T. Sargent, R. Gobat “Multiwavelength dissection of a massive heavily dust-obscured galaxy and its blue companion at z~2”, Astronomy & Astrophysics, 2021. https://www.aanda.org/articles/aa/full_html/2021/02/aa39577-20/aa39577-20.html DOI: https://doi.org/10.1051/0004-6361/202039577

Provided by NCBJ

Supercomputer Turns Back Cosmic Clock (Astronomy)

Astronomers have tested a method for reconstructing the state of the early Universe by applying it to 4000 simulated universes using the ATERUI II supercomputer at the National Astronomical Observatory of Japan (NAOJ). They found that together with new observations the method can set better constraints on inflation, one of the most enigmatic events in the history of the Universe. The method can shorten the observation time required to distinguish between various inflation theories.

Just after the Universe came into existence 13.8 billion years ago, it suddenly increased more than a trillion, trillion times in size, in less than a trillionth of a trillionth of a microsecond; but no one knows how or why. This sudden “inflation,” is one of the most important mysteries in modern astronomy. Inflation should have created primordial density fluctuations which would have affected the distribution of where galaxies developed. Thus, mapping the distribution of galaxies can rule out models for inflation which don’t match the observed data.

However, processes other than inflation also impact galaxy distribution, making it difficult to derive information about inflation directly from observations of the large-scale structure of the Universe, the cosmic web comprised of countless galaxies. In particular, the gravitationally driven growth of groups of galaxies can obscure the primordial density fluctuations.

A research team led by Masato Shirasaki, an assistant professor at NAOJ and the Institute of Statistical Mathematics, thought to apply a “reconstruction method” to turn back the clock and remove the gravitational effects from the large-scale structure. They used ATERUI II, the world’s fastest supercomputer dedicated to astronomy simulations, to create 4000 simulated universes and evolve them through gravitationally driven growth. They then applied this method to see how well it reconstructed the starting state of the simulations. The team found that their method can correct for the gravitational effects and improve the constraints on primordial density fluctuations.

“We found that this method is very effective,” says Shirasaki. “Using this method, we can verify of the inflation theories with roughly one tenth the amount of data. This method can shorten the required observing time in upcoming galaxy survey missions such as SuMIRe by NAOJ’s Subaru Telescope.”

These results appeared as Masato Shirasaki et. al. “Constraining Primordial Non-Gaussianity with Post-reconstructed Galaxy Bispectrum in Redshift Space,” in Physical Review D on January 4, 2021.

Featured image: Schematic diagram of the evolution of the Universe from the inflation (left) to the present (right). The “reconstruction method” winds back the evolution from right to left on this illustration to reproduce the primordial density fluctuations from the current galaxy distribution. (Credit: The Institute of Statistical Mathematics) 

Provided by NAOJ