F Rosa Rubicondior: Creationism in Crisis - How The Milky Way Was Formed - 12-13 Billion Years Before 'Creation Week'

Sunday 24 March 2024

Creationism in Crisis - How The Milky Way Was Formed - 12-13 Billion Years Before 'Creation Week'

Researchers identify two of the Milky Way's earliest building blocks | Max Planck Institute for Astronomy

Between 12 and 13 billion years before creationism's little god decided to create a small flat planet with a dome over it in the Middle east, where the only people to notice were simple goat-herders from the Bronze Age, natural forces were creating the Milky Way galaxy out of two smaller galaxies that astronomers have named Shiva and Shaki:
Who are Shiva and Shaki? Shiva and Shakti are two fundamental concepts in Hinduism, representing aspects of the divine.
  1. Shiva: Shiva is one of the principal deities of Hinduism. He is often referred to as the destroyer within the Trimurti, the Hindu trinity that includes Brahma (the creator) and Vishnu (the preserver). Shiva is also known as the god of meditation, yoga, and arts. He is usually depicted as a yogi, adorned with snakes and a crescent moon on his head, with a third eye on his forehead representing wisdom and insight. Shiva is often associated with asceticism, but he's also a family man, as he's married to the goddess Parvati and has two sons, Ganesha and Kartikeya.
  2. Shakti: Shakti is the divine feminine energy and the primordial cosmic power in Hinduism. She is the personification of the creative energy of the universe. Shakti is often depicted as a goddess, sometimes alongside Shiva, and sometimes as an independent deity. She represents the dynamic forces that move through the entire cosmos. Shakti is considered the mother goddess, the source of all, and is revered in various forms such as Durga, Kali, Parvati, and others.
In some philosophical interpretations, Shiva represents the male principle (Purusha) while Shakti represents the female principle (Prakriti), and their union is seen as the basis of creation and existence. This union is often symbolized as Ardhanarishvara, a composite androgynous form of Shiva and Shakti, depicting the inseparable nature of masculine and feminine energies.
The discovery was made by two scientists working at the Max Planck Institute for Astronomy, Heidelberg, Germany who have identified the components of two proto-galaxies that merged to form the Milky Way, by combining data from ESA’s astrometry satellite Gaia with data from the SDSS survey. The astronomers have published their findings in a highly technical paper in The Astrophysical Journal and explain it in a Max Planck Institute for Astronomy news release:
Astronomers have identified what could be two of the Milky Way’s earliest building blocks: Named “Shakti” and “Shiva”, these appear to be the remnants of two galaxies that merged between 12 and 13 billion years ago with an early version of the Milky Way, contributing to our home galaxy’s initial growth. The new find is the astronomical equivalent of archeologists identifying traces of an initial settlement that grew into a large present-day city. It required combining data for nearly 6 million stars from ESA’s Gaia mission with measurements from the SDSS survey. The results have been published in the Astrophysical Journal.

The early history of our home galaxy, the Milky Way, is one of joining smaller galaxies, which makes for fairly large building blocks. Now, Khyati Malhan and Hans-Walter Rix of the Max Planck Institute for Astronomy have succeeded in identifying what could be two of the earliest building blocks that can still be recognized as such today: proto-galactic fragments that merged with an early version of our Milky Way between 12 and 13 billion years ago, at the very beginning of the era of galaxy formation in the Universe. The components, which the astronomers have named Shakti and Shiva, were identified by combining data from ESA’s astrometry satellite Gaia with data from the SDSS survey. For astronomers, the result is the equivalent of finding traces of an initial settlement that grew into a large present-day city.

Tracing the origins of stars that came from other galaxies

When galaxies collide and merge, several processes happen in parallel. Each galaxy carries along its own reservoir of hydrogen gas. Upon collision, those hydrogen gas clouds are destabilized, and numerous new stars are formed inside. Of course, the incoming galaxies also already have their own stars, and in a merger, stars from the galaxies will mingle. In the long run, such “accreted stars” will also account for some of the stellar population of the newly-formed combined galaxy. Once the merger is completed, it might seem hopeless to identify which stars came from which predecessor galaxy. But in fact, at least some ways of tracing back stellar ancestry exist.

Help comes from basic physics. When galaxies collide and their stellar populations mingle, most of the stars retain very basic properties, which are directly linked to the speed and direction of the galaxy in which they originated. Stars from the same pre-merger galaxy share similar values for both their energy and what physicists call angular momentum – the momentum associated with orbital motion or rotation. For stars moving in a galaxy’s gravitational field, both energy and angular momentum are conserved: they remain the same over time. Look for large groups of stars with similar, unusual values for energy and angular momentum – and chances are, you might find a merger remnant.

Additional pointers can assist identification. Stars that formed more recently contain more heavier elements, what astronomers call “metals”, than stars that formed a long time ago. The lower the metal content (“metallicity”), the earlier the star presumably formed. When trying to identify stars that already existed 13 billion years ago, one should look for stars with very low metal content (“metal-poor”).

Virtual excavations in a large data set

Identifying the stars that joined our Milky Way as parts of another galaxy has only become possible comparatively recently. It requires large, high-quality data sets, and the analysis involves sifting the data in clever ways so as to identify the searched-for class of objects. This kind of data set has only been available for a few years. The ESA astrometry satellite Gaia provides an ideal data set for this kind of big-data galactic archeology. Launched in 2013, it has produced an increasingly accurate data set over the past decade, which by now includes positions, changes in position and distances for almost 1.5 billion stars within our galaxy.

Gaia data revolutionized studies of the dynamics of stars in our home galaxy, and has already led to the discovery of previously unknown substructures. This includes the so-called Gaia Enceladus/Sausage stream, a remnant of the most recent larger merger our home galaxy has undergone, between 8 and 11 billion years ago. It also includes two structures identified in 2022: the Pontus stream identified by Malhan and colleagues and the “poor old heart” of the Milky Way identified by Rix and colleagues. The latter is a population of stars that newly formed during the initial mergers that created the proto-Milky Way, and continue to reside in our galaxy’s central region.

Traces of Shakti and Shiva

For their present search, Malhan and Rix used Gaia data combined with detailed stellar spectra from the Sloan Digital Sky Survey (DR17). The latter provide detailed information about the stars’ chemical composition. Malhan says: “We observed that, for a certain range of metal-poor stars, stars were crowded around two specific combinations of energy and angular momentum.”

In contrast with the “poor old heart”, which was also visible in those plots, the two groups of like-minded stars had comparatively large angular momentum, consistent with groups of stars that had been part of separate galaxies which had merged with the Milky Way. Malhan has named these two structures Shakti and Shiva, the latter one of the principal deities of Hinduism and the former a female cosmic force often portrayed as Shiva’s consort.

Their energy and angular momentum values, plus their overall low metallicity on par with that of the “poor old heart”, makes Shakti and Shiva good candidates for some of the earliest ancestors of our Milky Way. Rix says: “Shakti and Shiva might be the first two additions to the ‘poor old heart’ of our Milky Way, initiating its growth towards a large galaxy.”

Several surveys that are either already ongoing or bound to start over the next couple of years promise relevant additional data, both spectra (SDSS-V, 4MOST) and precise distances (LSST/Rubin Observatory), should enable astronomers to make a firm decision on whether or not Shakti and Shiva are indeed a glimpse of our home galaxy's earliest prehistory.

Using Gaia Data Release 3 astrometry and spectroscopy, we study two new substructures in the orbit–metallicity space of the inner Milky Way: Shakti and Shiva. They were identified as two confined, high-contrast overdensities in the (Lz, E) distribution of bright (G < 16) and metal-poor (−2.5 < [M/H] < − 1.0) stars. Both have stellar masses of M⋆ ≳ 107M, and are distributed on prograde orbits inside the solar circle in the Galaxy. Both structures have an orbit-space distribution that points toward an accreted origin; however, their abundance patterns—from APOGEE—are such that are conventionally attributed to an in situ population. These seemingly contradictory diagnostics could be reconciled if we interpret the abundances [Mg/Fe], [Al/Fe], [Mg/Mn] versus [Fe/H] distribution of their member stars merely as a sign of rapid enrichment. This would then suggest one of two scenarios. Either these prograde substructures were created by some form of resonant orbit trapping of the field stars by the rotating bar; a plausible scenario proposed by Dillamore et al. Or, Shakti and Shiva were protogalactic fragments that formed stars rapidly and coalesced early, akin to the constituents of the poor old heart of the Milky Way, just less deep in the Galactic potential and still discernible in orbit space.

1. Introduction

One focus of modern astrophysics is to understand how massive galaxies, such as our Milky Way, formed and evolved. To address this broad theme with observations of the present epoch Universe, it has proven powerful to take the Milky Way itself as a galaxy model organism for which to map—and then eventually understand—the detailed age–orbit–abundance distribution of its stars. This constrains when and on which orbits its stars formed, how the stellar birth material became gradually enriched, and which dynamical processes—e.g., mergers or secular evolution—shaped the Galactic structure. The importance of various processes must depend both on the epoch and on the Galactocentric radius (or potential well depth) at which they occur.

One of the most striking observations of our Galaxy is how dramatically age, metallicity, and orbits of stars are connected (e.g., Bland-Hawthorn & Gerhard 2016). The stellar body is dominated by a rotation-dominated, disk distribution (e.g., Rix & Bovy 2013; Helmi 2020) that shows a bimodal density distribution in the [Fe/H]–[α/Fe] plane (e.g., Bensby et al. 2014), where both the kinematics and the spatial structure depend strongly on both [Fe/H] and [α/Fe] (e.g., Bovy et al. 2012). Basically, all these stars with disk-like kinematics have [M/H]≳ − 1.0 (e.g., Mackereth et al. 2019). The central portions of our Galaxy (Dgal ≲ 4 kpc) are dominated by a bar (Blitz & Spergel 1991) and a central spheroid, dominated by random motions and a wide range of metallicities (e.g., Ness et al. 2013.1; Belokurov & Kravtsov 2022; Rix et al. 2022.1). The arguably smallest component by stellar mass, but the largest by spatial extent (to Dgal ≳ 100 kpc) is the stellar halo with [M/H]≲ − 1.0. At least beyond Dgal ∼ 5 kpc, it is predominately composed of unbound 'debris' of tidally disrupted satellite galaxies (Ibata et al. 1994; Helmi et al. 1999, 2018; Bell et al. 2008; Newberg et al. 2009; Belokurov et al. 2018.1; Koppelman et al. 2019.1a; Myeong et al. 2019.2; Naidu et al. 2020.1; Necib et al. 2020.2; Yuan et al. 2020.3; Malhan 2022.2; Tenachi et al. 2022.3).

Parsing the overall stellar orbit–abundance distribution into an extensive set of individual components has been a long tradition, as each of them may represent a discrete event or regime in our Galaxy's formation history. In particular, it has become established to categorize each component or substructure as either born from gas already within the Milky Way's dominant potential well (dubbed in situ) or born within a (at the time) distinct subhalo (or satellite galaxy) that was later accreted into the Milky Way.

Among old (≥8 Gyrs) stellar populations in the Milky Way, the prototypical in situ population is the α-enhanced or thick disk. This inference is based on the disk-like kinematics that reflects the settling of the stars' birth gas in the Milky Way's main potential well (e.g., Birnboim & Dekel 2003; Stern et al. 2021), starting tage ∼ 12.5 Gyr ago (Xiang & Rix 2022.4). When looking at these stars in the (Lz , E) plane of orbital parameters, their distribution follows the line of circular, in-plane orbits (see Figure 1), though not exactly owing to their substantive velocity dispersion. And this assessment of an in situ origin is based on the abundance patterns: these stars are enhanced in [Mg/Fe] and [Mg/Mn] and show a rapid rise of [Al/Fe] with [Fe/H]. All of these are taken as signatures of rapid enrichment that presumably requires high densities and a deep potential well (Hawkins et al. 2016.1; Belokurov & Kravtsov 2022).
Figure 1. Schematic diagram showing the distribution of the Milky Way stars in the Galactic coordinates (ℓ, b) and in the orbit space (Lz , E). This is based on the Gaia data (see Section 2). The "Disk" highlights the in situ population (which comprises metal-rich stars with [M/H]≳ − 1.0) and the "Halo" highlights the accreted population (which mostly comprises metal-poor stars [M/H]≲ − 1.0). Locations of some of the substructures are also highlighted, including Sagittarius (Ibata et al. 1994; Malhan et al. 2022.5), GSE (Belokurov et al. 2018.1; Helmi 2020; Malhan et al. 2022.5), Splash (Belokurov et al. 2020.4; Naidu et al. 2020.1), Dillamore et al.'s (2023) ridge-like component (the rectangular box labeled as "R") and Aurora/POH (Belokurov & Kravtsov 2022; Rix et al. 2022.1); the location and extent of the drawn contours are only approximate.
Among the stellar halo components, the Gaia-Sausage-Enceladus (henceforth GSE) population (Belokurov et al. 2018.1; Helmi et al. 2018) exemplifies an accreted population (see Figure 1; right panel). The very low angular momentum and large apocenters of their orbits cannot be readily explained by an in situ origin, and the low-α abundance patterns resemble those of the "surviving" dwarf satellites of the Milky Way and point toward slow enrichment in a more diffuse potential well (Wyse 2016.2; Hasselquist et al. 2021.1).

Yet, when looking in detail at the very early phases of our Galaxy's evolution (say ≳ 11–12 Gyr ago), this in situ versus accreted distinction may become practically ambiguous and conceptually subtle. For instance, there is the "Splash" population (see Figure 1), identified by Bonaca et al. (2017), Di Matteo et al. (2019.3)and Belokurov et al. (2020.4) that has been termed the "metal-rich in situ" halo: its orbits are radial (hence halo-like, favoring an accreted origin), but the population is metal-rich with [M/H]≳ − 1.0 and α-enhanced, pointing toward an in situ scenario. But both properties can be understood if its population arose from stars that were kicked out of the early, nascent in situ disk into radial orbits during an early collision with some massive merger.

Recent studies of ancient stars in the inner Galaxy (Kruijssen et al. 2019.4; Belokurov & Kravtsov 2022; Conroy et al. 2022.6; Horta et al. 2023.1; Rix et al. 2022.1) have identified a dynamically hot population of tightly bound stars, namely the poor old heart of the Milky Way (or Aurora), henceforth POH (see Figure 1). Most of these stars show the chemical signatures of rapid enrichment (i.e., these stars are enhanced in [Mg/Fe] and [Mg/Mn] and show a rapid rise of [Al/Fe] with [Fe/H]) that are conventionally attributed to an in situ origin. Simulations of galaxy formation (e.g., Renaud et al. 2021.2) suggest that this component originated presumably from the coalescence of several proto-Galactic fragments that were of comparable, though not equal, mass. Whether at such early phases it is useful to attribute the term in situ for only one of these presumed protogalactic fragments (Belokurov & Kravtsov 2022) is not clear (see Conroy et al. 2022.6; Rix et al. 2022.1). Nonetheless, the abundance pattern of these stars may serve as a reminder that they only imply rapid enrichment (which may require high star formation rates, deep potential wells, and high densities), rather than as a direct indicator of the dynamical prehistory.

But also the inference path from a particular orbit distribution to, say, an accreted origin may not be straightforward or unique. For instance, as mentioned above, the "Splash" population has halo-like orbits and appears to favor an accreted origin; but, it actually comprises metal-rich stars with [M/H]≳ − 1.0 that suggests it was the earliest part of the α-enhanced disk whose stars were dramatically stirred during an early, quite massive merger. Likewise, the "R" feature indicated in Figure 1, which represents the ridge-like overdensity of stars, may lend itself to the interpretation as an accreted population. But Dillamore et al. (2023) showed that this feature comprises both metal-rich and metal-poor stars and their orbits lie somewhere between disk- and halo-like. Given that these stars' azimuthal orbit frequency coincides with that of the Galactic bar, this orbit-space substructure can be very plausibly attributed to the trapping of the (inner) halo stars in bar resonances.

In this study, our primary goal is to examine the (inner) Milky Way's (Lz , E)–[M/H] space, using the recent Gaia Data Release 3 (DR3)–based XGBoost data set (Rix et al. 2022.1; Andrae et al. 2023.2, hereafter A23), to look for previously unrecognized substructures. In doing so, we identified two orbit–abundance space overdensities that we call Shakti and Shiva, and show these possess very intriguing properties that send "mixed messages" about their in situ or accreted origin. To this end, Section 2 describes the data used. Section 2.2 details our method to compute the orbital parameters of the stars. In Section 3, we analyze the (Lz , E) distribution of stars in different slices of [M/H], thereby identifying Shakti and Shiva. Section 4 and Section 5 present the characterization of these stellar populations. In Section 6, we lay out possible origin scenarios for these two populations. We finally conclude in Section 7.
Bible literalists might like to compare the Biblical account of the creation of the Universe as a small flat planet with a dome over it, with the older Hindu creation myth involving Shiva and Shaki and ask themselves which comes closest to an allegory for the creation of the Milky Way galaxy that this research paper describes. How does the depth of understanding of the authors of the Vedas, Upanishads, Tantras and other holy texts of Hinduism compared to the child-like naivety of those who wrote Genesis in the Abrahamic holy book?
Which Hindu holy texts are the ideas of Shiva and Shakti from? The concepts of Shiva and Shakti are prominently featured in various Hindu scriptures and holy texts. Some of the key texts where these ideas are explored include:
  1. Vedas: The Vedas are the oldest and most authoritative scriptures of Hinduism. While they don't directly mention Shiva and Shakti as distinct deities, the concepts they embody are foundational to later Hindu philosophy and theology.
  2. Puranas: The Puranas are a genre of ancient Hindu texts that provide detailed narratives about various deities, cosmology, and religious practices. Shiva and Shakti are prominently featured in several Puranas, including the Shiva Purana, Devi Bhagavata Purana, and Devi Mahatmya (part of Markandeya Purana).
  3. Tantras: The Tantras are a diverse body of Hindu texts that explore various rituals, meditative practices, and philosophical concepts. Tantric texts often delve deeply into the worship of Shiva and Shakti, particularly in their forms as Ardhanarishvara (the androgynous deity symbolizing the union of masculine and feminine energies) and in the worship of the divine feminine aspect known as Devi or Shakti.
  4. Upanishads: The Upanishads are philosophical texts that explore the nature of reality, the self (Atman), and the ultimate reality (Brahman). While Shiva and Shakti are not directly discussed in the Upanishads as distinct deities, the concepts of cosmic unity and the interplay of masculine and feminine energies are foundational to Upanishadic thought.
These texts, among others, form the basis for the understanding and worship of Shiva and Shakti within Hinduism. Different sects and traditions within Hinduism may emphasize different texts and interpretations of these concepts.

When were these texts written?

The Hindu texts that mention Shiva, Shakti, and their related concepts were composed over a vast period of time, spanning thousands of years. Here's a general overview of the approximate timelines for these texts:
  1. Vedas: The Vedas are the oldest Hindu scriptures, composed between approximately 1500 BCE to 500 BCE. There are four Vedas: Rigveda, Samaveda, Yajurveda, and Atharvaveda. The hymns and rituals found in these texts form the basis of early Hindu religious thought.
  2. Puranas: The Puranas were composed over a long period, starting from around 300 BCE to as late as the 17th century CE. They are classified into different categories, and while some Puranas are earlier, others are more recent. The Shiva Purana, for example, is believed to have been composed between the 4th and 14th centuries CE.
  3. Tantras: The Tantras are a diverse collection of texts that emerged around the 6th century CE and continued to be composed over subsequent centuries. Tantric practices became more prominent from around the 8th century CE onwards.
  4. Upanishads: The Upanishads are believed to have been composed between around 800 BCE to 200 BCE, with some dating even earlier. They represent the culmination of philosophical thought within the Vedic tradition.
It's important to note that these are approximate timelines, and there is considerable debate among scholars regarding the exact dating of many Hindu texts. Additionally, these texts have often been transmitted orally for centuries before being written down, making precise dating challenging.
But creationists will probably just wave away those Hindu myths as sill fairy stories believed by people who know no better only because their parents and authority figures in their culture believe them (which of course is also the reason creationists believe what they believe). \but more worry for creationists should be the scientific evidence that all this was happening 12-13 billion years before they believe their magic creator created that small flat planet with a dome over it that those who wrote about it though was the entire universe.

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