Saturday, March 11, 2023

3912 - GALAXY - history how they are created?

 

-   3912  - GALAXY  -  history how they are created?   Astronomers know that galaxies grow over time through mergers with other galaxies. We can see it happening in our galaxy. The Milky Way is slowly absorbing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.


------------  3912  -     GALAXY  -  history how they are created?

-    Astronomers have found evidence of an ancient mass migration of stars into another galaxy. They spotted over 7,000 stars in Andromeda (M31), our nearest neighbor, that merged into the galaxy about two billion years ago.

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-    The James Webb Space Telescope's main scientific objectives is to look back in time to the Universe’s earliest galaxies to understand how they’ve grown and evolved into what they are today.   Galaxies like M31 and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history.

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-    These new observations of Andromeda and the inward migration of stars comes from the “Dark Energy Spectroscopic Instrument” (DESI.) It was built to measure the effect dark energy has on the expansion of the Universe. It does that by gathering optical spectra on tens of millions of objects, mostly galaxies and quasars, and then constructing a 3D map of the results.

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-   DESI is similar to the more well-known “Gaia” spacecraft. Gaia has an ambitious goal to precisely map the positions and motions of billions of stars in the Milky Way. Gaia data led to a wealth of discoveries about our own galaxy. But it’s confined to mapping stars in the Milky Way.

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-   DESI has given astronomers a partial map of the stars in Andromeda for the first time. And that map, including the motions of nearly 7,500 stars in the inner halo of the Andromeda Galaxy, is revealing their history.

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-   DESI shows that about two billion years ago, another galaxy merged with Andromeda. The positions and motions of about 7,500 stars DESI measured reveal that they came from another galaxy.

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-    Our new observations of the Milky Way’s nearest large galactic neighbor, the Andromeda Galaxy, reveal evidence of a galactic immigration event in exquisite detail.  Although the night sky may seem unchanging, the Universe is a dynamic place. Galaxies like M31 and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history.

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-    The Milky Way experienced a similar merger between 8 to 10 billion years ago. Most of the stars in our galaxy’s halo originated in a different galaxy and joined the Milky Way as a result of the ancient merger.

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-    Our emerging picture is that the history of the Andromeda Galaxy is similar to that of our own Galaxy, the Milky Way. The inner halos of both galaxies are dominated by a single immigration event.

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-    The expected observational signatures of galactic migration include debris streams, shells, rings, and plumes, the expected outcomes of merger interactions between large galaxies and their companions.

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-    We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf and Western Shelf regions.  The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 billion years ago.

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-    Though the positions and velocities of the 7500 stars play a major role in these findings, so did stellar metallicity. The high-metallicity stars in all of the sub-structures stemming from the merger. We find significant numbers of metal-rich stars across all of the detected substructures, suggesting that the progenitor galaxy had an extended star formation history, one perhaps more representative of more massive galaxies.

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-    The results stem from DESI’s ability to gather spectra from 5,000 objects simultaneously. This complex instrument is the most powerful multi-object survey spectrograph in the world and can reconfigure its 5,000 separate focal planes in only two minutes as it slews between targets.

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-    It was designed to measure the spectra of over 40 billion distant galaxies and quasars to map the large-scale structure of the Universe and how dark energy fuels its expansion. Along the way, it’s showing us how galaxies merge over time.  In only a few hours of observing time, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.

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-   Over the course of billions of years, galaxies grow and evolve by forging new stars and merging with other galaxies through aptly named “galactic immigration” events. Astronomers try to uncover the histories of these immigration events by studying the motions of individual stars throughout a galaxy and its extended halo of stars and dark matter. Such cosmic archaeology, however, has only been possible in our own galaxy, the Milky Way, until now.

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-    By measuring the motions of nearly 7,500 stars in the inner halo of the Andromeda Galaxy, also known as Messier 31 (M31), the team discovered telltale patterns in the positions and motions of stars that revealed how these stars began their lives as part of another galaxy that merged with M31 about 2 billion years ago. While such patterns have long been predicted by theory, they have never been seen with such clarity in any galaxy.

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-    To trace the history of migration in M31, the team turned to DESI. DESI was constructed to map tens of millions of galaxies and quasars in the nearby Universe in order to measure the effect of dark energy on the expansion of the Universe.

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-    It is the most powerful multi-object survey spectrograph in the world, and is capable of measuring the spectra of more than 100,000 galaxies a night.

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-   Even though the Mayall Telescope was completed 50 years ago (it achieved first light in 1973), it remains a world-class astronomical facility thanks to continued upgrades and state-of-the-art instrumentation.

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-    The team now plans to use the unparalleled capabilities of DESI and the Mayall Telescope to explore more of M31’s outlying stars, with the aim of revealing its structure and immigration history in unprecedented detail.

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-  It’s amazing that we can look out at the sky and read billions of years of another galaxy’s history as written in the motions of its stars, each star tells part of the story.   Who knows what new discoveries await!

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                   March 10, 2023      GALAXY  -  history how they are created?        3912                                                                                                                         

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--------------------- ---  Saturday, March 11, 2023  ---------------------------

 

 

 

 

         

 

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