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  • richardmitnick 8:31 am on June 4, 2020 Permalink | Reply
    Tags: "Hubble Makes Surprising Find in the Early Universe", A European team of astronomers have found no evidence of the first generation of stars, , , , , Deep Space Quest Doesn't Find the First Stars Pushing Back the Timeline of the Universe's Evolution., Hubble Frontier Fields program, , Population III stars, The galaxy cluster MACS J0416   

    From NASA/ESA Hubble Telescope: “Hubble Makes Surprising Find in the Early Universe” 

    NASA/ESA Hubble Telescope


    From NASA/ESA Hubble Telescope

    June 03, 2020

    Bethany Downer
    ESA/Hubble, Garching, Germany
    bethany.downer@partner.eso.org

    Rachana Bhatawdekar
    European Space Agency / ESTEC, Noordwijk, The Netherlands
    rachana.bhatawdekar@esa.int

    Ray Villard
    Space Telescope Science Institute, Baltimore, Maryland
    410-338-4514
    villard@stsci.edu

    1
    This artist’s impression presents the early universe.
    About This Image
    New results from the NASA/ESA Hubble Space Telescope suggest the formation of the first stars and galaxies in the early universe took place sooner than previously thought. A European team of astronomers have found no evidence of the first generation of stars, known as Population III stars, when the universe was less than one billion years old. Credits: ESA/Hubble, M. Kornmesser, and NASA

    2
    About This Image
    This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416. This is one of six galaxy clusters being studied by the Hubble Frontier Fields program, which together have produced the deepest images of gravitational lensing ever made. Scientists used intracluster light (visible in blue) to study the distribution of dark matter within the cluster. Credits: NASA, ESA, and M. Montes (University of New South Wales)


    Right after the Universe started with the Big Bang, the cosmos was dark. Only the first stars created millions of years later brought light. These first stars and their radiation drastically changed the Universe during what is known as the epoch of reionisation. This Hubblecasts talks about this important time, what Hubble has shown us so far, the open questions and what we can expect from future missions.

    Credit:

    Directed by: Bethany Downer
    Visual design and editing: Martin Kornmesser
    Written by: Laura Hiscott
    Narration: Sara Mendes da Costa
    Images: NASA, ESA
    Videos: NASA, ESA, NASA/GSFC, ESO/L. Calçada, M. Kornmesser
    Music: Johan B. Monell (www.johanmonell.com)
    Web and technical support: Bethany Downer and Raquel Yumi Shida
    Executive producer: Lars Lindberg Christensen

    Summary
    Deep Space Quest Doesn’t Find the First Stars, Pushing Back the Timeline of the Universe’s Evolution.

    In Greek mythology the first deities born from the universe’s origin in “the Chaos,” created a race of Titans. The powerful Titans were eventually superseded by the gods of Olympus. In modern cosmology, the stellar equivalent of the legendary Titans are so-called Population III stars, that would have been the very first stars born after the big bang. These hypothetical stars are as elusive as the Titans. Unlike the stars of today—like our Sun (that contains heavier elements, such as oxygen, nitrogen, carbon and iron)—the Population III stars would have been solely made out of the few primordial elements first forged in the seething crucible of the big bang. Much more massive and brighter than our Sun, they would have defiantly blazed as lords over the inky void of the newborn universe.

    A team of European researchers, led by Rachana Bhatawdekar of the European Space Agency, set out to find the elusive first-generation stars by probing from about 500 million to 1 billion years after the big bang. In their quest they used observations from Hubble, NASA’s Spitzer Space Telescope, and the ground-based Very Large Telescope of the European Southern Observatory.

    NASA/Spitzer Infrared Telescope. No longer in service.

    ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
    •KUEYEN (UT2; The Moon ),
    •MELIPAL (UT3; The Southern Cross ), and
    •YEPUN (UT4; Venus – as evening star).
    elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

    They used the gravitational lensing power of a massive foreground galaxy cluster (that acts as a giant magnifying lens in space) to find brightened images of far more distant background galaxies 10 to 100 times fainter than any previously observed.

    Gravitational Lensing

    Gravitational Lensing NASA/ESA

    Unfortunately, the team found no evidence of these first-generation Population III stars in this cosmic time interval they explored. These results are nevertheless important because they show that galaxies must have formed even earlier after the big bang than previously thought.

    New results from the NASA/ESA Hubble Space Telescope suggest the formation of the first stars and galaxies in the early universe took place sooner than previously thought. A European team of astronomers have found no evidence of the first generation of stars, known as Population III stars, as far back as when the universe was just 500 million years old.

    The exploration of the very first galaxies remains a significant challenge in modern astronomy. We do not know when or how the first stars and galaxies in the universe formed. These questions can be addressed with the Hubble Space Telescope through deep imaging observations. Hubble allows astronomers to view the universe back to within 500 million years of the big bang.

    A team of European researchers, led by Rachana Bhatawdekar of the European Space Agency, set out to study the first generation of stars in the early universe. Known as Population III stars, these stars were forged from the primordial material that emerged from the big bang. Population III stars must have been made solely out of hydrogen, helium and lithium, the only elements that existed before processes in the cores of these stars could create heavier elements, such as oxygen, nitrogen, carbon and iron.

    Bhatawdekar and her team probed the early universe from about 500 million to 1 billion years after the big bang by studying the cluster MACS J0416 and its parallel field with the Hubble Space Telescope (with supporting data from NASA’s Spitzer Space Telescope and the ground-based Very Large Telescope of the European Southern Observatory). “We found no evidence of these first-generation Population III stars in this cosmic time interval,” said Bhatawdekar of the new results.

    The result was achieved using the Hubble Space Telescope’s Wide Field Camera 3 and Advanced Camera for Surveys, as part of the Hubble Frontier Fields program.

    NASA/ESA Hubble WFC3

    NASA Hubble Advanced Camera for Surveys

    This program (which observed six distant galaxy clusters from 2012 to 2017) produced the deepest observations ever made of galaxy clusters and the galaxies located behind them which were magnified by the gravitational lensing effect, thereby revealing galaxies 10 to 100 times fainter than any previously observed. The masses of foreground galaxy clusters are large enough to bend and magnify the light from the more distant objects behind them. This allows Hubble to use these cosmic magnifying glasses to study objects that are beyond its nominal operational capabilities.

    Bhatawdekar and her team developed a new technique that removes the light from the bright foreground galaxies that constitute these gravitational lenses. This allowed them to discover galaxies with lower masses than ever previously observed with Hubble, at a distance corresponding to when the universe was less than a billion years old. At this point in cosmic time, the lack of evidence for exotic stellar populations and the identification of many low-mass galaxies supports the suggestion that these galaxies are the most likely candidates for the reionization of the universe. This period of reionization in the early universe is when the neutral intergalactic medium was ionized by the first stars and galaxies.

    “These results have profound astrophysical consequences as they show that galaxies must have formed much earlier than we thought,” said Bhatawdekar. “This also strongly supports the idea that low-mass/faint galaxies in the early universe are responsible for reionization.”

    These results also suggest that the earliest formation of stars and galaxies occurred much earlier than can be probed with the Hubble Space Telescope. This leaves an exciting area of further research for the upcoming NASA/ESA/CSA James Webb Space Telescope — to study the universe’s earliest galaxies.

    These results are based on a previous 2019 paper [MNRAS] by Bhatawdekar et al., and a paper that will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society (MNRAS). These results are also being presented at a press conference during the 236th meeting of American Astronomical Society.

    See the full article here .


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    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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  • richardmitnick 1:54 pm on February 13, 2019 Permalink | Reply
    Tags: , , , , , , Population III stars,   

    From Medium: “Here’s what I Zwicky 18 can tell us about the first stars in the universe” 

    From Medium

    Feb 10, 2019
    Graham Doskoch

    A blue dwarf galaxy only 59 million light-years away may harbor cousins of the mysterious Population III stars.

    1
    A Hubble Space Telescope image of I Zwicky 18 shows gas illuminated by young blue stars. Image credit: NASA/ ESA/A. Aloisi.

    The first stars in the universe were unlike any we can see today. Known to astronomers as Population III stars, they were large, massive, and composed almost entirely of hydrogen and helium. Population III stars were important because they enriched the interstellar medium with metals — all the elements heavier than hydrogen and helium — and participated in reionization, an event a few hundred million years after the Big Bang that made the universe more transparent.

    Finding Population III stars could confirm important parts of our theories of cosmology and stellar evolution. However, they should all be gone from the Milky Way by now, having exploded as supernova long ago. We can look into the distant universe to search for them at high redshifts — and indeed, the James Webb Space Telescope will do just that — but detecting individual stars at that distance is beyond our current capabilities. So far, telescopes have turned up nothing.

    Recent observations of a nearby dwarf galaxy named I Zwicky 18, however, have given us some hope. Only 59 million light-years away, the galaxy seems to contain clouds of hydrogen that are nearly metal-free. What’s more, it’s undergoing a burst of star formation that might be producing stars very similar to Population III stars. If we could learn more about this galaxy, it could provide us with clues as to what the earliest stars and galaxies in the universe were like.

    Is the current wave of star formation the first?

    2
    The initial HI observations of I Zwicky 18 used the radio interferometer at Westerbork, in the Netherlands. Image credit: Wikipedia user Onderwijsgek, under the Creative Commons Attribution-Share Alike 2.5 Netherlands license.

    One of the first studies to draw attention to the possibility that I Zwicky 18 is forming Population III-analog stars was by Lequex & Viallefond 1980. They supplemented existing optical observations of HII regions — clouds of ionized gas that host young, hot, massive stars — with studies of HI regions via the 21-cm emission line, a key tool for mapping neutral hydrogen. They were trying to figure out if the current round of massive star formation in the dwarf galaxy is its first, or if it had been preceded by other events, polluting the hydrogen clouds with metals.

    Their radio observations with the Westerbork Synthesis Radio Telescope [above] found a total HI mass of about 70 million solar masses in six separate regions, three of which remained unresolved. They were unable to connect individual components to the maps of HII regions, but radial velocity measurements of the clouds found that the total mass of the galaxy was much greater by about a factor of ten, suggesting that some other sort of mass was present.

    There were two possibilities: either the unseen mass was molecular hydrogen — which would not emit 21-cm radiation — or there was a dim population of older stars. The molecular hydrogen hypothesis couldn’t be ruled out, but the idea of an as-yet unseen group of stars was attractive. For one thing, the HI clouds appeared quite similar to the primordial clouds needed for galaxy formation. If these HI regions were actually primordial, then these dim stars could have supported them against gravitational collapse for billions of years.

    2
    Figure 5, Lequex & Viallefond 1980. A map of the HI regions in the galaxy show that three (labeled 1, 2 and 5) are large enough to be resolved, while the others are point sources. Regions 1, 4 and 5 are the most massive.

    A picture began to emerge. Comparison of Lyman continuum emission with far-ultraviolet emission indicated that the burst of star formation must have begun about a few million years ago, likely due to the collision of several hydrogen clouds. Before this, there would have been formation of dim red stars on a smaller scale, but not enough to enrich the galaxy more than low observed oxygen abundances suggested. Therefore, the stars forming in I Zwicky 18 should indeed be very close to Population III stars.

    What sort of stars are we dealing with?

    3
    Figure 1, Kehrig et al. 2015. A composite (hydrogen alpha + UV + r’-band) image of luminous knots in the dwarf galaxy that show intense helium emission.

    The idea caught on over the next few decades, and astronomers became interested in determining the nature of these young stars. One group (Kehrig et al. 2015 The Astrophysical Letters) was particularly interested in determining what type of massive stars could best explain the He II λ4686 line, an indicator of hard radiation and hot stars ionizing material in HII star-forming regions. There were a couple possible culprits:

    Early-type Wolf-Rayet stars, which are thought to be responsible for much of the He II λ4686 emission in star-forming galaxies.

    Shocks and x-ray binaries, which have also been found in extragalactic HII regions.

    Extremely metal-poor O stars, or — going one step further — entirely metal-free O stars, similar to Population III stars.

    The group ruled out the Wolf-Rayet stars quickly. Key signatures of metal-poor carbon Wolf-Rayet stars were clearly evident in the spectra, but the inferred number based on the C IV λ1550 line was too small to account for all of the helium emission. Similarly, the x-ray binary possibility was discarded because the sole x-ray binary found was too dim by a factor of 100.

    4
    Figure 2, Kehrig et al. 2015. A region of high Hα and He II λ4686 emission shows little overlap with [OI] λ6300 emission and low [S II] contrast, ruling out the possibility of x-ray shocks.

    However, a group of maybe a dozen or so metal-free stars of a hundred solar masses or more could successful reproduce the observed He II λ4686 line. There are pockets of gas near a knot in the northwest edge of the galaxy that are devoid of metals and would provide a suitable environment for these stars to form, although there are also likely chemically-enriched stars there, too. Certain models of extremely high-mass (~300 solar masses) offer an alternative to these metal-free stars, but in light of the previous observations, the metal-free models remain enticing.

    For the time being, our telescopes can’t detect Population III stars. Until they do, we can still learn a lot about the early universe by studying blue compact dwarf galaxies like I Zwicky 18. Low-redshift, metal-free analogs of the first stars in the universe are close enough for us to study today. The most metal-poor galaxy in the universe is a good place to start.

    See the full article here .

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    About Medium

    Medium is an online publishing platform developed by Evan Williams, and launched in August 2012. It is owned by A Medium Corporation. The platform is an example of social journalism, having a hybrid collection of amateur and professional people and publications, or exclusive blogs or publishers on Medium, and is regularly regarded as a blog host.

    Williams developed Medium as a way to publish writings and documents longer than Twitter’s 140-character (now 280-character) maximum.

     
  • richardmitnick 12:37 pm on April 6, 2016 Permalink | Reply
    Tags: , , , Population III stars   

    From astrobites: “Where are the Sun’s grandparents?” 

    Astrobites bloc

    Astrobites

    Apr 6, 2016
    Ingrid Pelisoli

    Title: Population III stars around the Milky Way
    Authors: Yutaka Komiya, Takuma Suda, Masayuki Y. Fujimoto

    First author’s institution: Research Center for the Early Universe, University of Tokyo

    Status: accepted by ApJ

    1
    Figure 1: NGC 1300, a barred spiral galaxy. You can notice that there’s a huge difference between the colors in the central region and on the spiral arms. Image Credit: Hubble Heritage Team, ESA, NASA.

    NASA/ESA Hubble Telescope.
    NASA/ESA Hubble Telescope

    When one looks at an image of a spiral galaxy such as NGC 1300 on Figure 1, the difference in color between the central region called the bulge and spiral arms strikes the eye. The bulge has an yellowish color, while the spiral arms are blue. This actually reflects the existence of different stellar populations in each region. The spiral arms are sites of active star formation and their luminosities are dominated by young hot O and B stars, which are bluish. Astronomers call these young stars Population I stars, and our Sun is one of them. Besides being young, they are also metallic, since they formed out of metal-enriched materials from the preceding stellar generations. On the other hand, stellar formation has more or less ceased in the bulge, so the yellow light is caused by evolved stars such as red giants. These evolved stars belong to the Population II, which is less metallic than its successors, the Population I stars. They do, however, contain some measurable percentage of metals. Since the early Universe contained only hydrogen and helium (and a bit of lithium and beryllium, if you want to be thorough), where did the metals come from? As with all heavier elements, they must have been synthesized by stars. In this case, by the first stars to ever form, which were composed only of hydrogen and helium. These stars with null metallicity are, by definition, Population III stars. But no Population III stars have been seen yet. Where are they hiding?

    Are the Sun’s Grandparents still around?

    Since Population III stars are thought to have formed billions of years ago, it’s fair to ask if they have already evolved off the main sequence, i.e. had the heavier ones exploded as supernovae and the less massive ones became white dwarfs? Well, most of them. But Population III stars with masses lower than 0.8 Msun would still be identifiable as Population III, because low-mass stars take longer to evolve off the main-sequence. Once a star reaches the asymptotic giant branch, however, convection brings heavier elements from the core to the upper layers (in a process known as dredge-up), so we cannot determine its original metallicity from this point on. And we’re faced with yet another problem: we’re not even sure if Population III stars with less than 0.8 Msun ever existed. Population III stars should form out of very pristine clouds of H and He, and these pure clouds are less likely to fragment into tiny pieces to form low mass stars. So Population III stars tend to be very massive and unlikely to have masses as low as 0.8 Msun. The bottom line is we don’t know if Population III stars less massive than 0.8 Msun even formed, let alone if they are still observable! Assuming that they did form, the authors estimated how likely we are to observe them around the Milky Way.

    Where should we be looking?

    To find Population III stars, it’s best to look in the Milky Way halo. This is because Population III stars in the inner region of the Galaxy can be contaminated by tiny percentages of metals from accretion and so be unidentifiable as Population III stars. Some objects which could fit this profile were already found, the so-called hyper or ultra metal-poor stars (like the famous “star that should not exist”). So how did Population III stars maintain their true identity and remain metal-free? They have to escape from the cloud in which they formed. There are two ways to achieve this: gravitational slingshots due to multi-body interactions, and ejection of the secondary star (in this case, our Population III star) from a binary when the primary star explodes as a supernova. The authors studied only the latter case, which is slightly easier to model. The escape probability will depend on lots of parameters, including the poorly known initial mass function (IMF) of population III stars, so the authors studied many different scenarios, such as different types of IMF, formation redshifs, and mass ratio distribution of binaries. As you can see in Figure 2, the escape probability can be as high as 0.3, or 30 %.

    2
    Escape probability of the low-mass Population III star (when its binary companion explodes as a supernova) as a function of the mass of the primordial mini-halo in which it formed. Mmd is the median mass of the adopted IMF, z is the formation redshift, and p, which is zero unless otherwise indicated, is the power-law index for the mass ratio distribution of binaries.

    The authors used the escape probabilities to estimate the number of escaped stars in each scenario. They computed that up to 3800 population III stars might have survived to nowadays, with up to 170 managing to escape the dark matter halos in which they formed. But to find these escapees, we must know where to look, so the authors went one step further and estimated the spacial distribution of the escaped Population III stars in the Galaxy. In order to do that, they assumed that the dark matter halos where the first stars formed undergo a spherical collapse to form the Milky Way. They then computed the orbit of each escaped star formed in this model and looked at the distribution of their galactocentric distances.

    3
    Figure 3: Fraction of escaped population III stars compared to extremely metal poor stars for giants (red) or subgiants (blue) for three magnitude bins. Squares are the results for their fiducial set of parameters, while circles consider an optimistic set.

    The authors finally estimated the likelihood of observing an escaped Population III star by comparing the number of extremely metal-poor stars in their model with the number of escaped, metal-free Population III stars. As you can see on Figure 3, escaped Population III stars are just a tiny fraction compared to extremely metal-poor stars, which are already very rare, so it’s highly unlikely any Population III star will be observed. Still, the authors concluded that a survey of the outskirts of the Milky Way halo with a large enough volume, such as 10 times that of the Hamburg/ESO Survey, might do the trick, and that might be doable in the near future. In the mean time, stay tuned for more news on these ancient stars!

    Population III stars around the Milky Way

    Science team:
    Yutaka Komiya, Takuma Suda, Masayuki Y. Fujimoto
    Affiliations:
    1 Research Center for the Early Universe, University of Tokyo,
    Hongo 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
    2 Department of Cosmoscience, Hokkaido University, Sapporo,
    Hokkaido 060-0810, Japan
    3 Department of Engineering, Hokkai-Gakuen University, Sapporo,
    Hokkaido 062-8605, Japan

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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